3 _CRT Display Design_A_

CRT Display Design

© 2000 Display Laboratories Inc.

Session 3Deflection &High Voltage

Deflection and High Voltage Design, Theory

Deflection and High Voltage Scanning Method

Stroke Raster

Vertical Deflection Horizontal Deflection

High Voltage Supply

Block DiagramHigh Voltage &Power Supply

Cathode Ray Tube

Vertical & HorizontalDeflection Amplifiers

Video Amplifier& Blanking

RGB

H+V

Sync.Sepr.

Focus andConvergence

Scanning Methods Stroke.

The beam is deflected through a desired path to form the image.

Smooth shapes. Limited coverage.

Raster. The beam is deflected in a fixed path

covering the entire display surface. Broken edges. Complete coverage.

Magnetic Deflection Two orthogonal electromagnet coils are used

to deflect the electron beam. One coil is used for horizontal positioning. The other for vertical position. Current through each coil determines position

of beam. They act like an optical lens. They are subject to similar distortions.

Stroke Display In the stroke character CRT the image is

painted by the electron beam. There is no raster. This type of CRT is often used for computer

aided design and other applications where line drawn images are preferred.

The effect is much like a pen plotter.

Raster Scan CRT Television sets and most computer monitors

are raster scanned. The electron beam scans the screen from left

to right and top to bottom to create a raster on the screen.

Characters are formed by changing the intensity of the beam at the required points on the raster.

Vertical Deflection Principles and Popular types Power Amplifiers Retrace Boosters Vertical Centering Inductive load conditions DAC control Vertical Linearity Fly Back Vertical Circuit

Horizontal Deflection Principles of fly-back scanning design Combined HV and deflection Geometry correction Deflection Power Separate deflection systems Multi - frequency operation Retrace detection Base drive circuits

High Voltage System Fly-back Split Diode Modulator Multipliers FBT construction and

operating principles Static and dynamic regulation Beam current

END Notes:

Vertical Deflection

Vertical Deflection Principles and Popular types Deflection Power Power Amplifiers Retrace Boosters Vertical Centering Inductive load conditions DAC control Vertical Linearity Fly Back Vertical Circuit

Principals of Progressive Scanned Rasters Also known as Non-Interlaced. By convention the scanning beam moves

from the top to the bottom of the screen. The vertical “sweep” rate is typically in the

range of 50 to 180Hz. Large area flicker can be eliminated for

most viewers at vertical sweep rates higher than 75Hz.

Interlaced Raster

Interlaced Pictures The benefit of an interlaced picture is that the

horizontal and video rate can be cut in half. This makes the video card in the computer much easier to build. The video amplifier and the horizontal deflection

circuits in the monitor are also simplified. An interlaced picture as used in television, works

well for pictures of flowers and trees or action shots.

Interlaced Pictures Generally an interlace picture is not

acceptable for data applications where the viewer is close to the picture.

An example of where interlace does not work well is the letter "E".

The vertical bar in the letter is drawn both in the odd and even fields and thus gets updated 60 times a second.

The three horizontal lines in the letter "E" reside in the odd field and only get drawn 30 times a second.

This makes the right side of the "E" flicker.

Deflection Power

Deflection Power The data sheets for this SONY yokes show

the deflection power for one of the yokes is 30.3 ohm-Amps.

For this yoke with a 13.6 ohm vertical the deflection current is given by:

Since the winding volumes are constant for

this type of yoke you can have more or fewer turns for more or less inductance and resistance but the current need to deflect the beam is given by these power factors.

22 22.26.13/3.30 AA AmpsPeak49.122.2

Vertical Power Amplifiers A vertical power amplifier is related to an

audio power amplifier. Audio amplifiers are voltage amplifiers (voltage in

voltage out). Vertical amplifiers are current amplifiers (voltage in

current out). Feedback comes from a current sensing point.

This is done because current is proportional to the amount of deflection.

Vertical Power Amplifier

Audio amplifier

Vertical amplifier

B+

B-

B+

B-

Vertical Power Amplifier Low noise is critical. Open loop unity gain needs

to extend to 1 - 10mhz. A small monitor may need

only ± 0.5 Amps p-p of vertical yoke current using a 12 volt supply.

Large color monitors may require ± 3 to 4 amps p-p and use a 35 – 50 volt supply during vertical trace and 70 – 100 volts during retrace.

Vertical Retrace Booster In order to obtain

sufficiently short fly-back times, a voltage greater than that required during scanning must be applied to the yoke.

During vertical retrace time a large voltage is needed across the yoke to cause a fast retrace.

A voltage doubler boosts the positive supply voltage only during vertical retrace.

Voltage Doublers The vertical power

amplifier can then run from a low supply voltage when little output voltage is needed.

And from a high supply voltage for the short time that a high output voltage is needed.

This results in 1/3 the power loss and 2 to 3 times faster retrace.

Voltage Doublers The top trace is the output voltage of the power amplifier. The second trace is the supply voltage.

Anti-ringing Resistor Many power amplifiers

have instability in the 1 to 3Mhz region.

An anti-ringing resistor & capacitor dampens out oscillations.

See the manufacture’s data sheet for proper values.

Generally the resistor is in the 1 to 5 ohm range.

Anti-ringing Resistor It is chosen to load down

the amplifier at the oscillation frequency.

The time constant for the RC is often in the .2 to 1 µS range.

The impedance of the capacitor, at the oscillation frequency, should be ½ to ¼ that of the resistor.

If the value of the capacitor is too large, the resistor and amplifier will get hot.

Vertical Damping In many vertical amplifier

designs a damping resistor is placed across the yoke.

One method to determine the resistor value is to select a power resistor in the 100 to 500 ohm range and adjust the value for best results.

As can be seen, too large a value of resistance leaves oscillation.

Too small of a value slows the amplifier.

Vertical Damping The second method of determining the

damping resistor value involves knowing the power amplifier’s gain/phase plot.

The gain and phase of the resistors, capacitors and yoke inductance must also be known and plotted on the same graph.

Watch for adequate gain and phase margin.

Vertical Design TermsThe power amplifier can not pull it’s output all the way to the supplies. From the data sheet there should be terms like ‘output saturation voltage to ground’ and ‘output saturation voltage to supply’ both measured at the peak yoke current. The total voltage lost to the power amplifier’s saturation effect is Vsat.

VsatH = 2.2V @ 3AVsatL = 0.9V @ 3A Vsat = VsatH + VsatLVsat = 3.1V

The voltage lost to resistive effects must be found. The yoke resistance is typically only 20% accurate. Current sense resistors are 5% accurate. Therefore use 1.2 Ry and 1.05 Rf to get the worst case values.

Iy p-p = 6A Vr=(1.2 Ry +1.05 Rf) X Iy p-p = (1.2 X 1 + 1.05 X 10) X 6

It takes voltage to get a change in current. The voltage needed is a function of yoke inductance X yoke current peak to peak divided by the scan time.

Vl = (Ly X Iy p-p)/Ts

Vc is the voltage due to the charge of capacitor Cd.

Vc = Ly X Ts / (8X Cd)

Vertical Centering Most vertical amplifiers are A.C.

coupled. The very nature of A.C. coupling

will cause the raster to be centered on the screen.

The video will be slightly low however.

This is because there are generally 0 to 3 blank lines before V sync and many blank lines after.

Any current pulled from the cold side of the deflection yoke to ground will cause the video to move up.

If more range is needed then the centering circuit must be built to push and pull.

One to two watts can easily be dissipated in the centering resistors.

Vertical Centering If the D.C. blocking capacitor is

extremely large, the cold side of the yoke should have no voltage movement.

You can’t afford a cap that large. Several volts of signal appear on

the cold side of the yoke because of the charging and discharging of the D.C. blocking capacitor and voltage across the current sense resistor.

This causes the centering current to change from top to bottom of the screen, causing a non linearity effect that is not corrected for on many monitors.

A more accurate circuit includes a current source in place of the resistors.

D.C. Vertical Amplifier If the vertical yoke is D.C.

coupled the vertical centering is handled within the vertical power amplifier.

Very slightly pulling up or down on either input of the power amplifier will cause a D.C. current to flow through the vertical yoke.

This will make the entire picture move to a new vertical position on the tube.

This saves the D.C. blocking capacitor and the power resistors.

A DAC can control the centering.

Ramp Generator The vertical oscillator and ramp generator are often combined in one circuit. . A timing capacitor is connected to pin 13. A current source charges C13 causing a ramp. The current source is controlled by pin 12. The presents of a V-sync pulse causes the ramp to reset and start over again.

Ramp Generator If no vertical sync is present then, when the ramp reaches 6.8 volts the I.C.

generates it's own sync. Any sync pulses that appear when the ramp is less than 5.2 volts will be

ignored. The quality of the timing capacitor on pin 13 is critical! This capacitor must have very good temperature stability.

(poly-carbonate is good and not too expensive)

Ramp Generator Here is a example for choosing

C13 and the resistors on pin 12. Known factors:

The vertical ramp starts at 2 volts and should end at 6 volts.

The p-p voltage is 4 volts. The timing capacitor is .1uF.

If the minimum vertical frequency is 41Hz then the capacitor has 24 milliseconds to ramp 4 volts.

4 volts x .1µF -------------- = 16.7 µAmps

24x10-3 sec.

Thus the current source must produce 16.7 µA at the lowest vertical frequency.

Ramp Generator If the maximum vertical frequency

is 125 Hz then the vertical time is only 8 milliseconds and the current source must deliver 50 uAmps.

.4 volts * .1uF.-------------- = 50 uAmps.

8 x10-3 sec. . The next step is to choose

resistors to connect to pin 12 that will deliver the 16.7 to 50 uAmps needed to cover the frequency range.

The voltage at pin 12 is 3.5 volts. The range of the VHOLD DAC is 0

to 5 volts. The average D.C. on the VSLOPE

DAC is 2.5 volts thus the voltage across R12C is 1.5 volts. (3.5-2.5).

Ramp Generator At the maximum vertical

frequency the VHOLD DAC will be at 0 volts. There will be 3.5 volts across R12A and R12B.

R12B R12C R12A -------- + -------- + -------- = 50 uAmps. 3.5v 1.5v 3.5v At the minimum vertical

frequency the VHOLD DAC will be at 5 volts. The voltage across R12A is -1.5 volts. (3.5-5).

R12B R12C R12A -------- + -------- - -------- = 16.7 uAmps. 3.5v 1.5v 1.5v

Vertical Linearity Vertical linearity is achieved by

adjusting the vertical ramp's slope many times down the screen.

The VSHAPE DAC has an A.C. wave form on it that modifies the slope of the ramp to cause linearity corrections.

At the maximum vertical frequency and with the [vertical shape at vertical frequency maximum] set to a value that causes a 4 volt p-p signal on the VSHAPE DAC set R12C for good linearity.

If the vertical frequency is reduced then the p-p voltage on the VSHAPE DAC will reduce. This is set by the [vertical shape at vertical frequency minimum] control.

Vertical Size The vertical size is controlled

by a variable gain amplifier. The voltage on pin 16 (0 to 5

volts) will adjust the size of the vertical ramp on pin 15 by +/-20%.

Fly Back Vertical Deflection

END Notes:

Horizontal Deflection

Horizontal Deflection Combined Deflection and High

Voltage Pin-cushioning Regulation

Separate Deflection Conventional Buck-down Bi-directional scan

Horizontal Deflection Principles of fly-back scanning design Combined HV and deflection Geometry correction Deflection Power Separate deflection systems Multi - frequency operation Retrace detection DC Centering circuits

Principles of fly-back Scanning

The most popular raster scanning circuit is the single ended Flyback.

It has a minimum of components and is energy efficient.

Primary geometry correction is made by C2 the ‘S’ correction capacitor.

High-Voltage can be easily generated from the large voltage pulse during retrace.

Simple H-Size control In monochrome monitors

typically the ratio of horizontal size to high voltage is adjusted by the addition of a size coil.

The size coil will change the effective inductance of the DY.

If the size coil changes the total inductance by 10% the deflection current will have the same 10% change while the high voltage will change by the square root of 10%.

Pin-Cushioning Transformers

In Color yokes the Horizontal size does not fit the screen. It is smaller in the center.

The pin-cushion transformer is a size coil that is electrically controlled by passing a current through the control winding.

Applying a parabolic current to the control winding will change the width to fill the screen.

The inductance changes by saturating the core material reducing the inductance.

Changing the size this way changes the high voltage also.


Magnetic Amplifiers. A mag-amp controlled

horizontal circuit has the addition of a ‘size coil’ that is controlled by current through a control winding.

The mag-amp adds inductance to the horizontal section like a size coil.

A control winding is used to vary the inductance.

The two windings (yoke current) and (control current) do not cross couple energy because of the way they are wound.

T1

Bsase Drive C1Fly Back cap

C2S cap

D1

Damper Diode

Q1H. Switch

DYH. Yoke

T2Fly Back Transformer

B+

L1 Mag Amp


A typical mag-amp transformer looks like this.

The large yoke current passes through a small number of turns of heavy wire.

The flux from the yoke current makes a loop with an air gap in it.

These two factors will give this coil little inductance and very high current handling ability.

The flux density is far from saturation.


The control current passes through many turns of small wire.

The flux path does not have an air gap.

The resulting coil will have high inductance and can be easily saturated by D.C. current.

The outer legs of the transformer are saturation (in whole or in part) by the control current.

Saturation in the outer legs causes a reduction in inductance in both coils.

Flux from the yoke winding is nulled in the two half's of the control winding.

Split Diode Modulation Before looking at the split diode

modulator we need to review the operation of a horizontal section.

Let the supply has a supply voltage of B+.

The voltage at the collector of Q1 is zero volts during trace and is a high voltage half sign wave during retrace.

The average voltage is the same as B+.

The average voltage across C2 is the same as B+.

Current in C2, DY, Q1 and D1 is typically ten times that flowing through T1.

It is important to remember that the “power supply” that delivers power to the DY is C2 not B+.

Split Diode Modulation The split diode modulator has

two horizontal sections, one above the other.

Think of the two horizontal sections as completely separate.

Transistors Q1A and Q1B can be combined into one transistor.

Q1A and Q1B are both open or closed at the same time.

A single transistor from the collector of Q1b to the emitter of Q1A will operate the same.

Split Diode Modulation To make discussion easier let the

two horizontal sections be of equal value. (L1=L2, C1=C3, C2=C4) These values are not typical. And for now remove L3 and V1.

The Flyback pulse is equal to the voltage across C1+C3.

The voltage across C2 + C4 = B+. The current through L1 comes

from voltage stored on C2, and the current through L2 comes from voltage stored on C4.

So far the current in the two inductors are equal.

Now add back in L3 and V1 with its voltage set to ½ B+.

Note that nothing changes!

Split Diode Modulation The voltage source V1 can set the

voltage across C4 from near zero volts to near B+.

The current through L2 is directly related to the voltage across C4.

Because the voltage across C4 + C2 = B+, the current through L1 is related to the supply voltage B+ minus V1.

If the current in L2 is dropped by 10% then the current in L1 must increase by 10%.

The sum of the two currents will remain constant.

To say that another way; the two Flyback pulses will change by +10% & -10% with the addition of the two remains constant.

Split Diode Modulation One of the two coils is the

deflection yoke and the other is a “dummy coil” or “modulation coil”.

V1 sets the size of the picture. B+ controls the high voltage and possible the picture size.

V1 can be a supply or an active load that pulls down.

The most efficient method is to make a switcher that pushes or pulls.

In this example the size PWM watches horizontal size while the HV PWM watches the high voltage.

Deflection Power

Deflection Power The data sheets for the SONY yoke show that

the deflection power for one of the yokes is 13.9mHA2.

The yoke inductance is about 100uH so:

13.9/0.1 = 139A2, or A = square root of 139 or about 11.77A peak.

Separating Deflection and HV

The high voltage and horizontal can be made in separate circuits.

This eliminates the interactions of high voltage load on horizontal size and size on high voltage.

The high voltage is monitored by a tap on the bleeder resistor inside the Flyback transformer.

The horizontal size can be monitored by numerous methods.

Conventional Horiz. Deflection

In conventional horizontal deflection systems the width of the raster is controlled by a variable power supply.

This supply is modulated with the correction pin and trap.

The supply is filtered with an Electrolytic Capacitor before it feeds the Horizontal section.

The supply voltage will range over a two or more range for multi-modes.

Power efficiency is poor in this analog supply.

Separate Horizontal Deflection

This horizontal section uses a PWM to set the horizontal size.

This allows for a wide frequency operating range with good efficiency.

Two DACs can be used with one setting the horizontal size and the other setting the pincushion and trap.

The frequency response of the output filter will effect the shape of pin waveform differently at different vertical rates in both of these types of amps.

Retrace Time and ‘S’ Correction

On large monitors or wide frequency range monitors two different retrace times are available.

The flyback time is set by the micro computer by selecting two different flyback capacitors.

At lower frequencies the longer retrace time is selected.

Different ‘S’ corrector capacitor values are selected by the micro computer.

At the highest frequency the smallest capacitors are selected.

“Fly Back” Caps

“S” Capacitors

Buck-Down Horizontal Size This horizontal section does

not have a horizontal power supply like most monitors.

A chopper is used to take the B+ supply and create the necessary power for the horizontal section.

This circuit has excellent response time because of the fast time constant of the supply filter.LC = L(T2+DY) x C2

Variable vertical rates do not effect correction wave shape.

PWM

T1


C2S cap

D1

Damper Diode

Q1H. Switch

DYH. Yoke


B+

D2

DIODE

Q2

Buck-Down Horizontal The duty cycle of the

chopper is controlled by a pulse width modulator (PWM).

Feed back can be measured to adjust the duty cycle to get the desired width from either of two places.

The peak current in the horizontal yoke.

The voltage on the horizontal ‘S’ capacitor C2.

I prefer current as it does not need to be compensated for horizontal frequency.

PWM

T1


C2S cap

D1

Damper Diode

Q1H. Switch

DYH. Yoke


B+

D2

DIODE

Q2

Buck-Down Horizontal Size Example: If the PWM is running at 50% duty

cycle then a square wave is fed into the gate of Q2.

The junction of Q2/D2 will be at B+ for 50% of the time and ground for 50% of the time.

The result is the top if the Flyback transformer will appear to be at ½ of the supply voltage.

The picture will be at about ½ the maximum size.

This method has a large range. The horizontal may be easily

turned off in power save modes.

PWM

T1


C2S cap

D1

Damper Diode

Q1H. Switch

DYH. Yoke


B+

D2

DIODE

Q2

Horizontal Losses Real world losses include:

Resistive Skin effect/ Eddie currents Semiconductor forward drops Switching Dielectric Magnetic

Each of these losses are caused be a different parameter and has been identified and minimized over the years.

Effects Not in the Schematic

The layout can add significant inductance and capacitance not shown in the schematic.

A common visible effect is left side ringing. It is caused by either of two mechanisms. The vertical and horizontal windings of the

yoke may cross couple inducing horizontal current in the vertical.

As this current decays the beam is deflected up and down.

This is the cause if characters & lines appear wavy.

Effects Not in the Schematic

The other source is the presence of high-frequency ringing in the horizontal yoke.

This will appear as fat and narrow characters but other wise straight horizontal lines.

The white lines are where the beam slows down.

The dark lines where the beam speeds up. Usually the components that cause and tune

this resonance are the Damper Diode, Flyback Cap and trace inductance.

Effects Not in the Schematic After retrace the damper diode

is forced into forward conduction.

This takes considerable time, on the order of several hundred nano-seconds.

During this time considerable reverse voltage (>50v) is formed across the retrace capacitor.

As the Damper conducts this energy resonates with the trace inductance.

This effect can be greatly reduced by adding a ferrite bead with high loss characteristics.

Retrace Detection Retrace detection is

necessary for proper phasing and blanking.

Because of the poor de-saturation characteristics and variable operating conditions of the horizontal transistor, detecting retrace time directly from the voltage across the horizontal is difficult.

After many years of trying to solve this DLabs has found a remarkably accurate and low-cost solution.

Retrace Detection By placing a ferrite bead in series with the fly-back cap to reduce

left-side ringing, a voltage pulse is generated when he current shifts from Q1 to C1.

This shift occurs in the fall time of the transistor. ~20-30nS. Then again as the current goes through zero in the middle of retrace. And then again at the end of trace as the damper conducts.

DC centering In the horizontal

section of a monitor, the yoke has DC on the cold end and a flyback pulse on the hot side.

The current ramps in a saw tooth fashion centering around zero.

If a parallel coil is added the current through the yoke current is not effected.

DC Centering With the addition of a

battery and limiting resistor a DC current is added to the current ramp.

A plus and minus supply gives full control of the DC offset current.

DC Centering The batteries or

power sources are created with small secondary windings.

The final circuit has no AC effect on the yoke but can cause a +/- current flow through the yoke.

D C Centering Fixed Variable with Pot Variable with inductor Electrically adjustable

END Notes:

H o r i z o n t a l Linearity

Horizontal Linearity Horizontal current in the yoke is given by the

equation:

At a glance it looks that the voltage and inductance are constants so the current should have a constant slope.

This is not entirely true. The yoke inductance is very constant. The effective voltage across that inductance is

not. There are several causes of this, each must be

considered individually.

TimeLVI */

Horizontal Linearity Primary Causes:

Yoke resistance. Deflection transistor saturation resistance. Damper Diode forward drop.

Secondary Causes: De-saturation and base off drive conditions. Ton and Vfoward characteristics. Trace/Flyback cap resonance Tube curvature (Inner Pin)

Yoke resistance Horizontal yoke resistance can

be measured or is given in the data sheet.

Yoke current times this resistance adds or subtracts from the effective voltage across the inductance.

This amounts to a saw-tooth or ramp from left to right reducing the effective ‘supply’ voltage.

The steady loss of voltage will show as a linear reduction in character width of the display.

Yoke resistance The Forward drop in the

horizontal output transistor is relatively linear with current.

The data sheet for the transistor shows the relation of Vforward vs. Iforward.

It is of relative low voltage and impedance.

This can be modeled as a small battery and resistor.

This effect is at least linear until de-saturation.

Yoke resistance The damper diode takes

effect on the left side as current and energy is discharged from the yoke.

Once the Diode is in hard conduction the V/I curve is linear and of low impedance.

This can be modeled as a small battery and resistor.

However at Ton it is a different story.

The extreme amount of energy change at the end of retrace causes complex distortions.

Semiconductor losses For 90% of the trace time

things are fairly well behaved. Basically the supply voltage is,

V – (Iyoke x Ryoke) - Vsat.

on the right side. And.

V + (Iyoke x Ryoke) + Vfoward.

on the left. This type of distortion is

efficiently compensated with a saturable inductor.

A Linearity coil is a special class of inductor whose inductance is dependent on the current flow through it.

Semiconductor losses As can be seen in these diagrams

the effective left side supply voltage is 50 volts + the diode drop + the voltage across the resistor.

This equals 54 volts. The supply voltage in the right side

is 50-3-1=46 volts. The linearity coil has high

inductance on the left side of the picture causing the left side to shrink.

The right side inductance is small thus causing the right side to appear to grow larger.

The voltage across the linearity coil should balance out the voltage across the semiconductors and resistance.

Horizontal Linearity The linearity coil is placed in the

yoke current path. Just like a size coil, inductance in series with the yoke will reduce the size of the picture.

This saturable coil will change inductance depending on the amplitude and direction of current flow.

At the start of a trace the linearity coil has an inductance of 20 percent of that of the yoke.

By the center of the trace, the linearity inductance has decreased to about 4 percent of the yoke where it remains for the rest of the trace.

Horizontal Linearity Adjust the bias magnet so the

right and left sides of the picture are the same size.

The effect of a linearity coil can be hard to measure.

A fast way to test a linearity coil is to add two turns of insulated wire around the coil.

Connect an oscilloscope to measure the voltage from the two turns.

When the coil saturates the voltage drops to near zero.

Voltage from two turns of wire added around the linearity coil.

Horizontal Linearity These six traces show different

amounts of bias magnet applied to a linearity coil.

The top trace shows no saturation.

The bottom trace indicates a saturated core for all current levels.

The third and fourth traces are typical.

If the coil is too small for the job there will be saturation on both side of the trace. This last condition (not shown) is hard to detect by measuring the screen with a ruler.

Voltage from two turns of wire added around the linearity coil.

Horizontal Linearity Trace A is the yoke voltage at

about 1000 volts peak to peak.

Trace B is the yoke current. Trace C is the voltage across

the total of all resistance in the horizontal loop.

Trace D is the voltage loss due to the semiconductors in the loop.

Horizontal Linearity Trace E is the voltage across

the S capacitor. Trace F is the voltage across

the linearity coil. The linearity coil should have

a waveform like the inverse of trace C+D.

Thus the loss seen in traces C+D+F should equal a straight line.

Horizontal Linearity Coil The shaded area covers a

family of possible curves that are obtainable by adjusting the magnet on a linearity coil.

The left side of the screen (represented by –3 amps) is where the most inductance is needed.

The right side ( +3 amps) has the least inductance.

Horizontal Linearity Coil If the magnet was removed

the natural inductance verses current curve is shown at the right.

The inductance is 80 μH for most of the graph.

The left side line shows the inductance if the core was removed from the linearity coil, leaving a air wound coil.

Common Linearity Coils Many simple monitors have a

saturable core glued to a magnet.

More advanced monitors have an adjustable magnet.

By rotating the magnet the saturation point can be moved.

It is very common to combine a fixed magnet and a adjustable magnet.

I have made linearity coils combining adjustable magnets and inductor.

These are great for finding the correct values in the lab.

Micro Controlled Linearity By the time multi-sync

monitors became popular design engineers were placing multiple linearity coils in the deflection circuits.

The problem is how to switch in the right coil.

Relays and FETs have been used with varying degrees of success.

If a monitor has two linearity coils then there is only really only two horizontal frequencies where the linearity is correct.

Micro Controlled Linearity Coil

Now with microprocessor controlled monitors, the design engineer has the option of building a linearity coil that can be adjusted with out the need of high power switches.

The coil has an infinite number of settings.

In the micro code the microprocessor will determine what linearity coil setting is best for a particular horizontal frequency.

Micro Controlled Linearity Coil

In this linearity coil the adjustable magnet is replaced with an elector magnet.

A small amplifier drives current into the control winding changing the saturation point.

Horizontal Linearity Primary Causes:

Yoke resistance. Deflection transistor saturation resistance. Damper Diode forward drop.

Secondary Causes: De-saturation and base off drive conditions. Ton and Vfoward Damper diode

characteristics. Trace/Flyback cap resonance Tube curvature (Inner Pin)

De-Saturation High Voltage deflection

transistors suffer from turn off anomalies.

These include storage delay. Current fall time. Saturation levels. These are all somewhat

interrelated and very dependent on Transistor type.

Individual Transistors will need to be evaluated for these conditions verses ease of use.

De-Saturation Screen Effects Right side dark line 2 - 3μS

from the end of the scan line and parabolic shaped.

This is due to over driving Ib2 to turn off the transistor.

The base is pulling the collector negative during this time.

The beam is being sped up during at this edge.

Proper base drive conditions must be used.

De-Saturation On Screen Effects

Right side brightening of the last 0.25 to 0.5 in. of raster.

The voltage across the transistor increases greatly as current in the collector region is swept clear.

This is transistor manufacturer process dependent.

Again each type of transistor will need to be evaluated for this characteristic.

Motorola made a line of transistors (switch mode III) that specified this performance and offered superior operation.

Ton and Vfoward Damper Diode

The operation of the Damper diode is critical to good left side screen performance.

Several factors should be considered.

Ton is the time required to establish forward current in the junction.

Slow turn on will cause the left edge raster to show dark.

There will be far to much voltage across the yoke for a few hundred nanoseconds. (1mm)

The speed that current starts will effect trace ringing as well.

Look for diodes that exhibit a fast but soft turn ON verses a avalanche turn ON characteristic.

Ton and Vfoward Damper Diode

The forward voltage of the damper will effect efficiency and linearity.

Some horizontal circuits (split diode) use two dampers in series.

This can increase losses and make the linearity more difficult to correct.

END Notes:

H o r i z ontal ‘S’ Correc t i o n

‘S’ Capacitor

The ‘S’ capacitors corrects outside versus center linearity in the horizontal scan.

The voltage on the ‘S’ cap has a parabola plus the DC horizontal supply.

Reducing the value of ‘S’ cap increases this parabola thus reducing the size of the outside characters and increasing the size of the center characters.

‘S’ Capacitor value: Too low: picture will be

squashed towards edges. Too high: picture will be

stretched towards edges.

‘S’ Capacitor By simply putting a capacitor

in series with the deflection coil, the saw-tooth waveform is modified into a slightly sine-wave shape.

This reduces the scanning speed near the edges where the yoke is more sensitive.

Generally the deflection angle of the electron beam and the yoke current are closely related.

T?


C2S cap

D1

Damper Diode

Q1H. Switch

DYH. Yoke


B+

L1

Size Coil

Deflection Angle .vs. ‘S’ Linearity

In this example an electron beam is deflected with nine different current values. (4,3,2,1,0,-1,-2,-3,-4 amps)

A current in the range of 0 to 1 amp causes the beam to move 4cm.

Current changing from 3 to 4 amps causes 6.5cm movement.

The yoke appears to be 1.5 times more sensitive at the edge of the picture.

4

5

5.5

6.5

4

5

5.5

6.5

“S” Capacitors

High Deflection Angles & Flat Tubes

The amount of ‘S’ correction needed is related to the flatness of the tube and the deflection angle.

If the yoke is at the radius of the curvature of the tube then no ‘S’ correction is needed.

As the yoke is pushed toward the face of the tube deflection angles get large.

This problem is compounded on very flat tubes.

Inner Pin-cushion Many CRTs, especially

flatter ones, need geometry correction that goes beyond simple ‘S’ correction.

Most tubes need inner pin-cushion correction, which is also called "dynamic ‘S’ correction".

Some tubes need more ‘S’ correction only at the extreme edges, this is called "higher-order ‘S’ correction".

“S” Capacitor Specifications Typ. Data sheet

END Notes:

Horizontal Base Drive

Using HV Transistors Power Loss Base Drive Circuits

Horizontal Transistor Power Loss

There are two kinds of power loss in the horizontal transistor.

DC Loss Collector Base

AC Loss Turn ON Loss Turn OFF Loss Current Tailing Dynamic Saturation Dynamic De-saturation

DC Loss in the Collector DC loss due to Collector

current (Ic) times the Collector-Emitter voltage while the transistor is closed (Vsat).

Base current (Ib) is needed to turn the transistor on.

Ic is the load current. Vsat is the Collector-

Emitter voltage when the transistor is on.

DC Loss in the Base DC loss is due to Base

current (Ib) times the Base-Emitter voltage while the transistor is closed (Vbe).

High voltage transistors like high voltage diodes have a large forward voltage drop.

The Base-Emitter on voltage may be near one volt.

Two amps of Base current combined with one volt of forward drop will result in two watts of heat.

Vsat Characteristics This graph shows the DC

saturation region of a typical high voltage transistor.

Collector current curves are shown for 1,2,3,4 and 5 amps.

Base current ranges from 30mA to 3A.

The Collector Emitter voltage is graphed over a 0.1 to 2 volt range.

The red curves are points where the gain is at 1,2,5,10 and 20. It is clear that it takes base current to keep the transistor closed.

VceIcP

Vsat Characteristics This transistor is not built to

work with a current gain of 20.

It struggles to handle more than 2A with a gain of 10.

If we are talking about DC losses it is clear that this transistor needs to be driven with a gain of 1 or 2.

AC Loss Here are four types of AC

losses found in bipolar power transistors.

The first two are the traditional turn on & turn off transition losses.

The second two relate to the transistor’s condition just after turn on and just before turn off.

Under AC conditions a transistor’s Vsat voltage is not what it is at DC.

At high speeds, dynamic saturation and dynamic de-saturation voltages become a major part of the heating.

AC Loss at turn ON Turn on losses are generally

known to happen during the crossover time when the Collector current rises from 10% to 90% while the collector voltage is falling.

The area under the curve is power loss.

This type of loss is not found in a CRT monitor horizontal switch.

Horizontal Flyback structures are zero-voltage turn on circuits.

VceIc

AC Loss at turn OFF Luckily there is a Flyback

capacitor across the Collector Emitter of Q1.

This capacitor limits the voltage rise time.

Hopefully the Vce of Q1 will not rise very far before the Collector current drops to zero.

There is a potential of large power loss during the time the collector current drops from 100% to 0% while the voltage is increasing.

AC Loss at turn OFF Tfall should be as short as possible! Most transistor data sheets do not

show enough details about this. This graph shows Ic Collector

current, Vbe Base turn off voltage, hfe current gain and Tf Collector current fall time.

It can be seen that Collector current has an effect on fall time.

Generally there is a valley in the fall time curve.

The current gain and Base turn off voltage also effects fall time.

Generally the higher the gain the faster the fall time.

This is in direct contradiction with what makes the DC losses low.

AC Loss at turn OFF Each and every power transistor

type acts differently. This graph shows time in s for

storage delay and collector fall time verses collector current under the condition of hfe=5 and Ib2 is twice Ib1.

Very typically the storage delay decreases with an increase in collector current.

The collector current fall time shows a valley at 7 to 8 amps.

This transistor was designed to operate at that current level.

AC Loss at turn OFF This graph shows the effect of Ib2

on storage delay and current fall time.

The collector current is fixed at 8 amps.

Gain is held at 5. This is the optimal value. The base current Ib1 is 1.6 amps. When Ib2 is very small the storage

delay is “forever”. With increasing Ib2 current the

delay is shortened. It is important to notice the

current fall time. There is a pronounced valley in

the current fall time curve. This transistor will run coolest

when Ib2=3 amps.

Current Tailing In this picture the collector

current in green has a rapid fall time of 150nS from 90% to 10%.

The problem is that the last 10% of the current takes 1S to drop to 0%.

In developing a good base drive circuit it is important to find a trade off between best 90%-10% current fall time and current tailing.

It is likely that if one is very good the other is poor.

Dynamic Saturation This type of loss is not found in CRT

monitor circuits because horizontal switches are turned on at zero volts.

This type of loss would be found in switching power supplies applications where a bipolar transistor is turned on under load.

Dynamic Saturation refers to the current turn area in the 90% to 100% area.

In high voltage power transistors it is very typical for the Vsat voltage to be high for the first µS after turn on.

Depending of the transistor, the Vce may be two or three volts at the 90% current point.

During a measurable time the Vce will drop to its DC level near 0.5 volts.

Dynamic De-Saturation High voltage transistors (1000

to 1500V) show very poor collector-emitter voltages during the storage delay time.

In a power supply this causes heating.

In a CRT monitor this causes linearity distortion specifically crushing at the right hand inch or two of the raster.

A typical 1200V transistor may have a Vce of 0.7V.

During the storage time the Vce slopes up to 6V before the collector tears open.

Dynamic De-Saturation The collector fall time is fast

but it does not start to fall until the collector voltage increases greatly.

At 100kHz the storage delay time (quite long on high-voltage parts) becomes a significant portion of the duty cycle.

Six volts and eight amps do not make for a happy transistor.

It is a good idea to minimize the storage delay time by either reducing the Ib1 or increasing Ib2.

Base Drive Duty Cycle & Storage Delay

The horizontal switch should never, never, ever be turned ON during the Flyback pulse.

It must be turned ON before the horizontal current crosses zero.

This sounds simple. In a multi scan monitor the scan frequency varies over a 3:1 or

more range (30khz to 90khz). The Flyback pulse probably has two different retrace times. The storage delay time has a positive temperature coefficient

and varies with the size of the picture.

Base Drive and PLL Timing Base current

must start after the Flyback pulse and before the deflection current crosses zero.

The job of the PLL is to turn the Transistor OFF by the storage delay time before the start of the Flyback pulse.

Base Drive at High Frequency

At high frequencies the Flyback time and the storage delay time become a major portion of the total time.

It often happens that the Base is turned on too soon.

Current flows at the end of the Flyback pulse.

This is a very dangerous condition!

Notice there is no current in the damper diode.

The base is pulled positive one volt.

The Base Collector diode acts as the damper diode.

Base Drive at Low Frequency

At very low deflection frequencies the Flyback and storage delay times are a smaller part of the total time.

It may happen that the base does not get turned ON in time.

Deflection current ramps to zero before the transistor is turned on.

A bright vertical bar appears in the image.

A strange looking pulse forms in the center of the Vce waveform.

There is a dead spot in the deflection current.

Transistor Failure Transistor failure during start up (and or) shutdown, or rapid

frequency changes, can often be traced to base drive conditions and timings.

The above drawings show bad conditions for the horizontal switch.

These conditions need to be looked for under high and low frequency operations and at temperature extremes.

If the retrace pulse has two or more speeds then you need to test at the switch over point.

Storage Delay Storage delay is to

transistors what reverse recovery is to diodes.

Signal source V1 is a fast square wave.

During the positive portion of the cycle current flows through the diode.

When the voltage drops negative reverse current flows for a short period of time ( Trr ).

Storage Delay When the base is at zero volts

no current flows. . The forward drop of the BC

diode is lower than the BE diode.

When base current flows the collector steers load current to ground (Emitter).

If the Collector Emitter voltage drops to about 1/3 the Base Emitter voltage there will be Base Collector current.

The Base Collector diode is slow, it has a long Trr.

Storage Delay If suddenly there is no Base

current: The collector voltage heads up. The Base Collector diode is slow

and sticky. Reverse recovery current flows

into the Base. The Base is pushed up (ON ). Internal Base current is just

enough to hold the collector low. The transistor is not allowed to

turn OFF. Eventually the energy stored in

the Base Collector diode runs down and stops the base current.

The transistor can then open.

Storage Delay Q2 supplies Base turn-ON

current (Ib1). . Q3 pulls Base current out of

the power transistor. (Ib2) . Ib2 current is much greater

than Ib1. Ib2 current is typically

slightly less that Ic. The Trr time is made short by

increasing Ib2 current.

Storage Delay Large high voltage power transistors have long pronounced

storage delay times. A 1000 volt, 10 amp., 150-watt transistor may take 3

microseconds to turn OFF even when the base is being pulled OFF with 5 amps.

It takes current and time to charge a battery. It takes time and current to charge up the Base Collector diode. The time verses charge curve appears to be exponential. In the case of the 1000 volt, 10 amp., transistor it took 1S to

charge to 50% and 9S more to charge to 95%. If the base was over driven for more than 10S little more charge

is added to the diode. If a power transistor is only slightly over driven then little current is

stored in the BC diode. If the base current is many times that needed to saturate the Vce

junction, then the BC diode will have a large charge.

Base Drive Modes There are two

common methods of controlling Base drive.

Voltage mode. Current mode.

Voltage Mode Voltage mode is simple to

understand. The secondary of T1 produces

a voltage, for example 5 volts.

Current flows through R1 into the Base of Q1.

Transistors are current amplifiers and are controlled by the Base current.

It can bee seen that the base current is controlled by the voltage across T1 and the value of R1 (a large power resistor).

Voltage Mode Note: During the left half of

horizontal trace the Collector voltage of Q1 will be negative.

Depending on the quality and speed of the damper diode it is very typical to find Base-Collector current for a short time after horizontal retrace.

By the 1/3 point horizontally the base current is gone but the Base is still negative.

When T1 firsts delivers power to Q1-Base it will find the base negative and more current will flow then planed.

Voltage Mode Transistors that have been

turned ON take negative Base current for a short period of time to turn OFF the transistor.

Generally the Base turn OFF current (Ib2) far exceeds the base turn on current (Ib1).

By adding a diode and resistor R2 the Ib2 current can be adjusted.

The shape or slope of Ib2 is controlled by the inductance in the base loop.

Resistor may be zero ohms or just the winding resistance of T1.

The coil L1 may be the leakage inductance of T1.

We will talk more about Ib1 and Ib2 current later on.

Current Mode In current mode the high

current components are not in the base loop.

Current is passed through T1 not voltage.

Current Mode Let D2 represent the B-E of a

power transistor. Lets start with a very simple

current source. Battery V1 is a one-volt

source. V3 is a square wave signal

source running at the horizontal frequency.

The inductance of L1 is very high.

This forces the circuit into ‘continuous’ mode (the current in L1 never drops to zero).

Current Mode When Q2 is ON current in L1

moves from 1.0 to 1.1 amps. When Q2 is OFF the inductor

kicks upward sending current through D2 starting at 1.1 amps and ramping down to 1.0 amp.

The slope (or delta) of the current is set by voltage (V1), the inductance (of L1) and the time (1/2 cycle).

Current Mode R5 and C1 have been added

to get more control of the currents.

Known: V3 = square wave 50%/50%

duty cycle. D2 Vf = 1.0 volts (high

voltage base junction). Q2 Rds on = 0 ohms (no

loss). The average voltage across

L1 must be zero. Inductance of L1 is high. Capacitance of C1 in high.

Current Mode Then: During the time when current

is flowing through D2 there is 1.0 volt across L1.

During the time when Q2 is ON there must be 1.0 volt in the opposite direction.

This forces the voltage across C1 to be 1.0 volts or one diode drop.

The average current flow can be found by measuring the voltage drop across R5.

V4 and R5 set the diode current D2.

Current Mode Now replace Coil L1 with

transformer T1. The total inductance of all 10

turns is the same of the coil L1. The transformer is tapped at 10%. The voltage at Q2 is ten times

that of D2. The current through D2 is 10

times that of Q2. The voltage on T1 is one volt per

turn. Capacitor C1 has 10 volts (10

diode drops) across it. With the current gain of T1, now

Q2 can be a much smaller transistor.

Common Base Drive Many base drive

transformers have turn rations of 10 to 20.

Isolation is a good idea. The emitter may not be at

ground. Even if Q1e is at ground it is

not the same ground as Q2. Current in the Base of Q1 is

very high and has a sharp edge.

This current should not pass through the ground plane.

Common Base Drive It is a good idea to keep the

secondary close to the Base of Q1.

The Ib2 turn OFF current needed for the Base of Q1 can be handled easier by Q2 now that there is a 10:1 turn ratio between the two transistors.

R6 and C2 form a primary snubber to protect Q2.

Resistor R7 and C3 form an anti-ringing circuit.

Increasing Base turn OFF Vbe

In a current base drive configuration the turn OFF voltage is minus one diode drop.

Typically -0.8 to -1.0 volts. Any configuration can increase

the turn OFF voltage by adding a “battery” in the base current loop.

In this example T1 is wound to produce +/- two diode drops of voltage.

During turn ON condition current flows through D1 and Q1be.

Increasing Base turn OFF Vbe

A diode drop of voltage is stored across C1. .

During turn OFF conditions the transformer produces negative two diode drops that adds with the voltage stored on C1.

The base is held at a negative three diode drops.

Diode D1 can be replaced with a power resistor.

The resistor is sized to develop a voltage across C1.

Proportional Current Drive This base drive circuit is designed to create a Ib1 waveform that ramps

up like the collector current in a CRT monitor application. This will store the minimum amount of energy on the base of Q1. The Ib2 waveform is a square wave not a ramp and is adjustable by T1

turns ratio. The negative base voltage is also set by T1 turns ratio.

Proportional Current Drive To turn ON Q1, turn ON Q2. Current flows from the 25 volt supply through Q2, T1 and into the Base of Q1. The current ramp starts out at zero amps and ramps up at a rate set by the

inductance of 50 turns on T1. Twenty-four volts appear across the 50-turn winding. Current only flows in the primary winding. Energy is stored on the core of T1.

Proportional Current Drive When Q2 turns OFF, the circuit goes into the storage delay mode. Current stops flowing in Q2. The + end of the 50-turn windings moves downward. The - end of the 10-turn winding moves upward. The Base of Q1 remains at about one volt.

Proportional Current Drive The first winding to find a load is the 10-turn winding. As its - end runs into D1. A current of 5 times Ib1 flows backwards out of Q1-Base through

D1, through the 10-turns of wire and around. This leaves about zero volts across the windings. Current does not run down with almost no voltage across the

winding.

Proportional Current Drive Shortly the stored energy on the base of Q1 is discharged. The + end of all windings head down until diode D2 catches on ground. The catch winding puts the stored energy from T1 back into the 25-volt

supply. This holds the Base of Q1 at -4.2 to -5 volts. Eventually the current decays to zero and the windings return to zero

volts.

Proportional Current Drive The Base Emitter capacitance remains charged to - 4 volts until Q2 is

turned ON to start the next cycle. The Ib1/Ib2 ratio is set by the ratio of 10 turns to the 50-turn primary. The negative base voltage is set by 10-turn and the 50-turn catch

winding. A snubber RC across Q2 is not shown. The table below shows the voltages during all three periods of

operation.

Auto Transformer Drive This is the old isolated base drive but it is not isolated. (run from –

30 volts). When the FET is ON current is pulled from ground, through C1/D1-3,

through T1,FET and to the –20.7 volt supply. This energy is stored on the core. When the FET is OFF T1 flies up, dumping the energy into Q1-base.

Auto Transformer Drive During IB2 the T1 turns ratio amplifies the current. During the OFF time the –2.1V supply and the turns ratio keep

Vbe very negative. You can move C1 & D1-3 to the base and connect the right end

of T1 to ground with very little change.

Proportional Base Drive Proportional Base Drive controls

the gain of Q1. Base current follows the

collector current. Storage delay is minimized by

not over driving Base current when Ic is low.

It has been decided by looking at the data sheet that Q1 will be operated with a fixed gain of ten.

The secondary of T1 is built with ten turns of wire from Base to Emitter.

The Emitter current passes through one single turn of wire.

Proportional Base Drive The 10:1 windings produce a

current transformer. Any current in the Emitter of Q1

will result in a Base current of 1/10 that amount.

The transistor is forced to operate with a current gain of 10.

The current stored on the primary of T1 needs only to be enough to get Q1 turned ON.

The real base current comes from the load.

There will be 1/10 diode drop of voltage across the one turn winding.

Proportional Base Drive In proportional base drive

transistor Q2 must work slightly harder to turn off Q1.

In this example I will pick some numbers.

The collector load current Ic=10 amps.

The Base on current Ib1=1amp as set by 10:1 turn ratio plus an additional 100ma that is supplied from the primary of T1 for a total of 1.1 amps.

The base turn OFF current during the storage delay time is Ib2=8 amps.

Proportional Base Drive During the storage delay time

of Q1 we need to pull 8 amps out of the base of Q1.

Through the 100:10 turns ratio, Q2 will see 800ma directly related to Ib2 current.

This current will only last 1 to 2 microseconds.

To reverse the transformer the one turn winding must also be reversed.

The 10 amps of Collector current through the 100:1 turn ratio places 100ma more load on Q2 but only during storage delay time.

Proportional Base Drive I found that some types of

transistors do not like Proportional Base Drive.

Looking back at my notes I did not try increasing the negative turn off voltage.

Next time………..

Baker Clamp Storage time is a severe

limitation to the speed performance of saturated switches.

One solution is to not allow the switch to saturate.

A Schottky clamping diode is connected from base to collector in a manner as to steal excess base current and hold the transistor just out of saturation.

The Schottky diode has a forward drop if less than that of the base-emitter junction of a silicon diode.

Baker Clamp Base current passes through

R3 into the base of Q1. If too much base current is

applied to Q1 then the Vce voltage will be low.

A low collector voltage will cause D1 to conduct away excess base current.

D1 parallels the BC diode and does not allow BC current.

Until recently high voltage Schottky diodes were not available.

Baker Clamp When Baker clamping high

voltage transistors a high voltage diode must be used.

The problem is that high voltage diodes have high forward voltage.

In our example the Baker clamping diode has a forward voltage of 0.8volts.

The Vbe of Q1 needs to be greater than the forward voltage of the clamping diode.

To increase the effective Vbe of Q1 a diode D2 is added.

Diode D3 is added to aide in Q1 turn OFF.

Signal source V2 can remove Q1 base charge via R3 and D3.

Baker Clamp I have only once seen Baker

clamping used in a CRT monitor. The circuit was more complicated

than shown here. I remember there was a 2 mHz

oscillation in the base drive circuit and it took several days to find a fix.

Baker clamping works better with high gain transistors.

Q2 may be added. It is difficult to Baker clamp a

transistor with a hfe is only 2 or 3. Q2 also reduces the current in the

clamp diode(s). D1 & D2 form the clamp diode. C1 makes a +5 volt supply used to

drive Q1 Base.

Baker Clamp A major goal is to reduce the

heat loss in the horizontal switch.

The question is, how to determine when things are working coolest?

Measure the current from the power supply.

Adjust the base drive for least supply current.

Set Ib1 to slightly too much current.

Too much is better than too little.

Snubber Secondary ringing is one of the

worst things that can happen in a base drive circuit.

It is unpredictable, temperature dependent and very subject to slight changes in the transformer.

If there is any change between prototype and production transformers the snubber may need to be changed.

If the transformer manufacturer changes, it is very likely that the leakage inductance will change.

Snubber This waveform shows Base

current ringing in red. At the left Ib1 base current

keeps the transistor ON. The large negative current

(Ib2) happens during storage delay time.

When the transistor opens up large voltage appears across the Collector Emitter shown in blue.

Snubber Any amount of base current

during the flyback pulse results in great heating.

Notice the three small current pulses on the right side.

A small 50mA Base current pulse will cause 500mA of Collector current.

The 500mA of current comes during the center of a 1000 volts flyback pulse.

Safe Operating Area Base back bias will change the

Collector Emitter safe operating area of the power transistor.

In the next graph notice that when the base is negative one or more volts it is safe for 1000 volts VCE.

If the base is grounded the Vce safe voltage is only 550 volts.

It is important to keep the base off bias negative during the time you need the clearance.

Base Breakdown The Base Emitter junction will

breakdown if the voltage is taken too far negative.

The junction will look much like a large zener diode.

Small transistor’s Base junction breakdown at about 6 volts (2N2222A).

This condition will degrade the transistors small signal gain.

Base Breakdown Large power transistors

breakdown at 8 to 15 volts. It is thought that it is not

destructive to breakdown the Base of large transistors.

The Base area is large and can handle several watts of power.

Motorola even recommends inductively driving the base negative to the break down point.

Ib2 Too Large It is difficult to write anything

about power transistors that will be true years from now.

Power transistors have changed over the years.

Some transistors do not like the Ib2 current to exceed Ic current.

Some transistors do not seem to have a problem with it.

Ib2 Too Large Either way, when the turn off

current (Ib2) is greater than the collector current Ic, the Collector Emitter voltage (Vce) will drop.

Notice the two waveforms; one with Ib2 = 0.8 Ic and one with Ib2 = 1.2 Ic.

In a switching power application the bump in Vce has no adverse effect.

In a CRT monitor there may be a vertical bar caused by the sharp change in Vce.

END Notes:

High Voltage Components

HV Components Flyback Transformers Capacitors Bipolar Output Transistors Damper Diodes Bias Diodes

Flyback Transformers Physical Structures

Tire and Pie winding Slot winding Layer winding

Electrical Characteristics Ringing and ‘Ring-less’ types Coupling and effect on impedance Stager tuning Split windings with diodes

Tire and Pie winding One of the original structures

for high voltage transformers. The layers are stacked to

increase the voltage breakdown strength.

Pie winding reduces turn to turn capacitance and increases self resonance.

Each layer is narrow and wound at an angle such that it crisscrosses the layer below and above.

It is wound like a RF choke. Each layer is further from the

core with poorer coupling.

Tire and Pie winding The primary and secondary

windings are separated by layers of tape.

Only the outer winding area was covered with insulating material. Hence the name ‘Tire’.

One long wire makes the entire coil.

The large secondary section forms a simple resonate circuit that is loosely coupled to the primary.

This type of transformer has a low self-resonance frequency.

Tire and Pie winding It also shows poor ringing

characteristics. It has a high ‘Q’ that leads to

‘resonate’ rise. Giving more than the turns ratio voltage output.

This is useful for self oscillating HV supplies. But shows poor self regulation because of its high impedance. Loading lowers the ‘Q’ and output voltage.

To reduce the effective impedance the resonance of the ‘tire’ was tuned to an odd harmonic of the retrace time, flattening the peak of the pulse.

Tire and Pie winding To reduce the ringing the

resonance of the ‘Retrace pulse’ was tuned to an odd harmonic of the trace time, putting the drive pulse out of phase with the ringing frequency.

This type of transformer is not usable in high frequency and multimode displays.

Due to its low resonant frequency and highly tuned frequency dependent nature.

Slot or Bobbin Winding To improve the performance,

the trick is to increase the coupling of the High-Voltage windings to the core.

The slot wound transformer has individual “small” slots that divide up the secondary and allow the wire to be closer to the core.

The plastic Bobbin maintains insulation levels.

Slot or Bobbin Winding This breaks up the winding into

smaller sections, each with a higher resonant frequency.

Each section is loosely coupled to the next and can be tuned slightly off frequency to reduce the ‘Q’ and flatten the ringing.

Stager tuning is an effective way of improving the broadband response of high turns ratio transformers.

This type could be used by Multi-mode and high frequency displays.

High frequency transformers The resonate frequency can be increased still higher

by separating each section with a diode. The circuit changes from several tuned sections in

series to individual tuned sections.

Winding Effects High voltage transformers have

large numbers of turns. Capacitance forms between

each wire inside the transformer.

Intra winding capacitance causes the transformer to form a tuned circuit.

This resonance may be tuned by changing the gap in the core which changes the inductance.

‘Tire’ transformers are tuned to resonate at 3, 5 or 7 times the frequency of the Flyback pulse.

This type of transformer is not well suited for multi-sync use.

Stager tuning Windings can be wound with a

different number of turns in each section giving each section a different resonant frequency.

Stager tuned transformers allowed for multi-frequency monitors.

Slot wound bobbins may have different size slots that can hold a different number of turns.

This was popular in the early 1980’s until the ‘Ring-less’ layer wound Flyback was introduced by Kyushu Mitsubishi in 1994/95.

Multi layer transformers Early in the development of

CRT terminals they operated at 15,750 Hz a Flyback transformer secondary might have 20 to 30 layers of wire.

As horizontal frequencies moved up to the 31,750Hz range A.C. losses and low resonant frequencies became a problem.

The A.C. wire losses increase exponentially with the number of wire layers.

At higher frequencies the windings need to be wider with fewer layers.

Layer wound transformers Dividing a ten layer winding into two five layer windings with a diode

between each will cut the A.C. wire loss in half while dramatically reducing the resonant frequency. (17+17=34 which is about ½ of 67).

A secondary broken into 10 one layer windings with diodes between each will have A.C. wire losses of 10/67 that of a 10 layer winding.

Either way the same number of diodes are required to handle the reverse voltage.

Layers 1 2 3 4 5 6 7 8 9 10A.C. wire loss

1 3 6.3 11 17 24 33 43 54 67

Layer wound transformers Each layer of wire forms a capacitor

with the next layer of wire. Adding layers of tape reduces the capacitance between layers.

While the capacitance can not be eliminated it’s effect can be reduced.

If there is no A.C. voltage across a capacitor no current will flow.

In this example the secondary has been divided into four layers, each wound from right to left.

The bottom layer starts at 0 volts and has a large A.C. signal at the left end.

The signal is rectified by a diode to make D.C. that connects to the start of the next winding.

Layer wound transformers In this way all layers of wire have

D.C. on the right side and an identical A.C. signal at the left side.

There is only D.C. between layers (no A.C. signal between layers). No A.C., no current flow hence, no A.C. capacitance losses.

Wire to wire the capacitance (in a single layer) has relatively little effect. If there is one volt per turn then each cap will have 1 volt A.C. across it.

The intra layer capacitance acts as a diode capacitor multiplier, adding to the strength of this design.

Layer wound transformers Some manufacturers use the same

number of turns on each layer. This causes the layers to resonate almost at the same frequency. This was suppressed with a anti-ringing RL network in series with the cold end of the secondary.

Other manufacturers discovered that winding different numbers of turns would stagger the tuning and cause small intra layer losses that made this transformer truly broad-band and ‘Ring-less”.

This design has all the advantages of tight coupling, high resonance frequency and broad frequency range operating.

Capacitors Materials Losses Crest value ratings Impedance Use

Capacitor Curves Impedance Dissipation

Factor

Capacitor Curves Impedance Dissipation Factor Temp. Coefficient

“S” Capacitor Specifications Typ. Data sheet

Damper Diodes

Data sheets Characteristics T-on T-off V-forward

Damper Diodes

High voltage Bias Diodes 3kv diode characteristics Data sheet

Bipolar Output Transistors Data sheets and how to read IB1 and IB2 Dynamic de-saturation Storage time Fall times Cross over time

Bipolar Transistor Specs.

END Notes:

High Voltage Power Supplies

Generator Topologies Sinusoidal Capacitor Voltage multiplier Fly Back Types Variation on Buck-down

Sinusoidal Free running oscillator Transformer multiplier Capacitor multiplier Voltage feed back Popular in early multi-

mode monitors

Sinusoidal Oscillator A voltage controlled

oscillator is a popular source in free standing high voltage power supplies.

Often the supply is self resonate and changes frequency with load.

These types are more expensive that fly back types.

The free running frequency often beats with the load current and causes screen anomalies.

Capacitor Multiplier Capacitor and Diode.

For 29kv need 8 to 12 5kv rated sections.

Depends on crest value of V1. Stacking circuit. Impedance.

Depends on capacitor value and operating frequency.

Generally low. Used in high beam current systems

like CRT projectors >100w. Low Volume and Expensive.

High Voltage Current Sense

The cold end of the high voltage winding can be returned to ground or other convenient supply.

As beam current increases the voltage drop across R1 increases.

An A to D converter and micro or comparator may monitor current levels.

There are two common levels used.

One for ABL and a higher for X-ray shutdown

Buck-down Regulator The buck-down regulator

used for horizontal size control also makes a good regulator topology for high voltage regulation.

The relative fast response of the “filter” can handle beam current loading provided adequate capacitive filtering is provided at the Anode.

The basic circuit stores energy of the primary of the flyback transformer or on an auxiliary inductor.

PWM

T1


C2S cap

D1

Damper Diode

Q1H. Switch

DYH. Yoke


B+

D2

DIODE

Q2

Buck-down regulator To get the fastest response

to loading the inductance of the storage coil needs to be low.

The operating mode close to discontinuous allows for the greatest line to line change.

Unfortunately this puts a high ripple current and stress on the components.

Higher inductance storage leads to continuous mode with a more level current load.

The response time is slower.

PWM

T1


C2S cap

D1

Damper Diode

Q1H. Switch

DYH. Yoke


B+

D2

DIODE

Q2

Variation on Buck-down A variation on a theme

operates the “storage” inductance in continuous mode.

Providing a base voltage pulse. The regulator input inductor T2

and Q2 operate in the discontinuous mode. Energy may be added line by line as needed.

Very fast response times can be attained.

The Anode capacitance can be greatly reduced.

6000pF @ 30Kv becomes 470pF @ 30Kv

Variation on Buck-down Transformer T2 is added in series

with the flyback transformer T1. Sense the average voltage across

T2 must be zero the addition of T2 will not effect the size of the picture.

A signal V2 is placed across T2 in synchronism with the horizontal scan.

The size and phase of the signal V2 will add or subtract from the flyback pulse as seen by T1 but will not effect the flyback pulse as seen by the deflection yoke.

In this way the high voltage can be regulated without effecting the deflection.

US patent #4,614,899.

Data Ray HV Regulator There are several variations

of this circuit that can be used in single or multi-frequency monitors.

First let us look at a single frequency.

Feed back resistor divider R1 & R2 provide sense voltage for the error amplifier of the Pulse Width Modulator.

Variations in the high voltage will be seen by the PWM causing a change in duty cycle on the gate of Q2.

Data Ray HV Regulator During Trace Q1/D1 are closed. D3 and Q1 pull the voltage on the

bottom end of the primary of T2 to near ground.

The voltage across T2 primary will be zero volts when Q2 is open and B+ volts when Q2 is closed.

While Q2 is closed energy is stored on T2.

During retrace the collector voltage of Q1 will swing upward in a half sine wave.

The bottom end of the primary of T2 will kick upwards charging C1 until it runs out of current.

Data Ray HV Regulator The secondary has a miniature of

the same waveform that is added to the main flyback pulse supplied to the flyback transformer.

As Q2 is held on for more or less time the amount of current charges on to T2 changes and hence the reverse pulse size added to the flyback.

In this way additional supply pulse can be controlled by the PWM driving Q2.

The additional energy needed to regulate the High Voltage comes from T2.

The speed of regulation is very fast as it can go from 0% to 100% on any cycle.

Data Ray HV Regulator For example: During flyback we need a 20,000

volt pulse on the + end of the secondary of T1 under no load conditions. If the flyback transformer has 5% load regulation then under load it’s voltage will drop about 1000 volts.

To get regulation the pulse will have to be boosted up to 21,000 volts under no load.

Through the 20:1 turn ration the primary will need a 1000 volt pulse on the + lead (1050 volts under load).

Data Ray HV Regulator The variables in the horizontal

section (B+, C1 & DY) should be chosen to create a flyback pulse slightly smaller than the 1000 volts needed for high voltage.

If the flyback pulse is 950 volts then the secondary of T2 will need to push downward 50 volts to get 1000 volts across T1.

Under load conditions T2 will need to develop 100 volts to hold the high voltage constant.

Typically T2 has a 10:1 turn ration that allows for 10% changes in high voltage.

F E T Drive I similar to the buck-down

except T2 is wired to the supply (100% duty cycle) and Q2 is a FET that is duty cycle dependent to regulate High Voltage.

The supply is typically operated in the discontinuous mode to maintain response time.

Higher current, High Voltage FETs are needed for this job.

PWM

T1


C2S cap

D1

Damper Diode

Q1H. Switch

DYH. Yoke


B+

D2

DIODE

Q2

Bleeders,Bypass, and Focus

High voltage bleeders

High voltage bypass

Focus voltage

END Notes:

3 _CRT Display Design_A_

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