APC Unit 1

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Prepared By Mr.M.RAVICHANDRAN AP / EEE Page 1

SELVAM COLLEGE OF TECHNOLOGY, NAMAKKAL 637 003DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

Analysis of Power ConverterUNITI Single Phase AC - DC Converter

Introduction;

It has been said that people do not use electricity, but rather they use communication, light,

mechanical work, entertainment, and all the tangible benefits of both energy and electronics. In this sense,

electrical engineering as a discipline is much involved in energy conversion and information. In the general

world of electronics engineering, the circuits engineers design and use are intended to convert information.

This is true of both analog and digital circuit design. In radio frequency applications, energy and information

are sometimes on more equal footing, but the main function of any circuit is information transfer. What about

the conversion and control of electrical energy itself? Energy is a critical need in every human endeavor. The

capabilities and flexibility of modern electronics must be brought to bear to meet the challenges of reliable,

efficient energy. It is essential to consider how electronic circuits and systems can be applied to the

challenges of energy conversion and management. This is the framework of power electronics, a discipline

defined in terms of electrical energy conversion, applications, and electronic devices. More specifically,

DEFINITION: Power electronics involves the study of electronic circuits intended to control the flow of

electrical energy. These circuits handle power flow at levels much higher than the individual device ratings.

1. Static Characteristics of Power Diode

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Forward recovery time, tFRis the time required for thediode voltage to drop to a particular value after

the forward current starts to flow.

Reverse recovery time trr is the time interval between the application of reverse voltage and the

reverse cur-rent dropped to a particular value as shown in Fig.

Parameter ta is the interval between the zero crossing of the diode current to when it becomes IRR.

On the other hand, tb is the time interval from the maximum reverse recovery current to

approximately 0.25 of IRR. The ratio of the two parameters ta and tb is known as the softness factor

(SF).

Diodes with abrupt recovery characteristics are used for high frequency switching. In practice, a

design engineer frequently needs to calculate the reverse recovery time.

This is in order to evaluate the pos-sibility of high frequency switching. As a thumb rule, the lower

tRR the faster the diode can be switched.

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2. Static Characteristics of SCR

Aplot of the anode current (iA) as a function of anodecathode voltage (vAK) is shown in Fig.

The forward-blocking mode is shown as the low-current portion of the graph (solid curve around

operating point 1).

With zero gate current and posi-tivev AK, the forward characteristic in the off- or blocking-state is

determined by the center junction J2, which is reverse biased. At operating point 1 very little

current flows (Ico only) through the device.

However, if the applied voltage exceeds the forward-blocking voltage, the thyristor switches to its

on-or conducting-state (shown as operating point 2) because ofcarrier multiplication.

The effect of gate cur-rent is to lower the blocking voltage at which switching takes place. The

thyristor moves rapidly along the negatively-sloped portion of the curve until it reaches a stable

operating point determined by the external circuit (point 2).

The portion of the graph indicating forward-conduction shows the large values ofiAthat may be

conducted at relatively low values of vAK, similar to a power diode.

As the thyristor moves from forward-blocking to forward-conduction, the external circuit must allow

sufficient anode current to flow to keep the device latched.

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The minimum anode current that will cause the device to remain in forward-conduction as it switches

from forward-blocking is called the latching current IL.

If the thyristor is already in forward-conduction and the anode current is reduced, the device can

move its operating mode from forward-conduction back to forward-blocking.

The minimum value of anode current nec-essary to keep the device in forward-conduction after it has

been operating at a high anode current value is called the holding currentIH.

The holding current value is lower than the latching current value as indicated in Fig. The reverse

thyristor characteristic, quadrant III of Fig. is determined by the outer two junctions (J1 and J3),

which are reverse biased in this operating mode (appliedvAKis neg-ative).

Symmetric thyristors are designed so that J1will reach reverse breakdown due to carrier

multiplication at an applied reverse potential near the forward breakdown value (operating point 3

in Fig.

The forward- and reverse-blocking junc-tions are usually fabricated at the same time with a very long

diffusion process (1050h) at high temperatures (>1200C). This process produces symmetric

blocking properties.

Wafer edge termination processing causes the forward-blocking capability to be reduced to about

90% of the reverse-blocking capability. Edge termination is discussed below.

Asymmetric devices are made to optimize forward-conduction and turn-off properties, and as such

reach reverse breakdown at a lower voltage than that applied in the forward direction.This is accomplished by designing the asymmetric thyristor with a much thinnern-base than is used in

symmetric structures. The thin n-base leads to improved properties such as lower for-ward drop and

shorter switching times.

Asymmetric devices are generally used in applications when only forward volt-ages (positive,vAK)

are to be applied (including many inverter designs).

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whereV0 =voltage intercept, models the voltage across the cathode and anode forward biased

junctions andR0=on state resistance.

When average and RMS values of on-state current (ITAV, ITRMS) are known, then the on-state

power dissipation PON can be determined usingV0andR0. That is,

Off State Characteristics

Unlike the standard thyristor, the GTO does not include cathode emitter shorts to prevent

non-gated turn-on effects due todv/dtinduced forward biased leakage current.

In the off-state of the GTO, steps should, therefore, be taken to prevent such potentially

dangerous triggering. This can be accomplished by either connecting the recommended value

of resistance between the gate and the cathode (RGK) or by main-taining a small reverse bias

on the gate contact (VRG=2V).

This will prevent the cathode emitter becoming forward biased and therefore sustain the GTO

thyristor in the off state.

The peak off-state voltage is a function of resistanceRGK. This is shown in Fig. Under

ordinary operating con-ditions, GTOs are biased with a negative gate voltage of around15V

supplied from the gate drive unit during the off-state interval.

Nevertheless, provision ofRGKmay be desir-able design practice in the event of the gate-

drive failure for any reason (RGK

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Single phase fully controlled half wave rectifier with R load

This is the simplest and probably the most widely used rectifier circuit albeit at relatively small

power levels.

The output voltage and current of this rectifier are strongly influenced by the type of the load. In

this section, operation of this rectifier with resistive, inductive and capacitive loads will be

discussed.

Fig shows the circuit diagram and the waveforms of a single phase

uncontrolled half wave rectifier. If the switch S is closed at at t = 0, the diode

D becomes forward biased in the the interval 0 < t . If the diode is

assumed to be ideal then

With a resistive load ripple factor of i0 will also be same.

Because of such high ripple content in the output voltage and current this

rectifier is seldom used with a pure resistive load.

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The ripple factor of output current can be reduced to same extent by

connecting an inductor in series with the load resistance as shown in Fig 2.3 (a).

As in the previous case, the diode D is forward biased when the switch S is

turned on. at t= 0.

However, due to the load inductance i0 increases more slowly. Eventually at t

= ,v0 becomes zero again. However, i0 is still positive at this point. Therefore,

D continues to conduct beyond t = while the negative supply voltage is

supported by the inductor till its current becomes zero at t= .

Beyond this point, D becomes reverse biased. Both v0 and i0 remains zero till the

beginning of the next cycle where upon the same process repeat

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Single phase fully controlled half wave rectifier with RL load

Fig. shows the circuit diagram of a single phase fully controlled halfwave rectifier

supplying a purely resistive load.

At t = 0 when the input supply voltage becomes positive the thyristor T becomes

forward biased. However, unlike a diode, it does not turn ON till a gate pulse is

applied at t= .

During the period 0 < t ,the thyristor blocks the supply voltage and the load voltage

remains zero as shown in fig 10.1(b). Consequently, no load current flows during this

interval.

As soon as a gate pulse is applied to the thyristor at t= it turns ON. The voltage across

the thyristor collapses to almost zero and the full supply voltage appears across the load.

From this point onwards the load voltage follows the supply voltage.

The load being purely resistive the load current io is proportional to the load voltage. At

t = as the supply voltage passes through the negative going zero crossing the load

voltage and hence the load current becomes zero and tries to reverse direction.

In the process the thyristor undergoes reverse recovery and starts blocking the negative

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supply voltage. Therefore, the load voltage and the load current remains clamped at zero

till the thyristor is fired again at t= 2+ .

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Single phase fully controlled bridge converter RLE Load

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Fig shows the circuit diagram of a single phase fully controlled bridge converter.

It is one of the most popular converter circuits and is widely used in the speed

control of separately excited dc machines. Indeed, the RLE load shown in this figure may represent the electrical

equivalent circuit of a separately excited dc motor. The single phase fully controlled

bridge converter is obtained by replacing all the diode of the corresponding

uncontrolled converter by thyristors.

Thyristors T1 and T2 are fired together while T3 and T4 are fired 180 after T1 and

T2. From the circuit diagram of Fig 10.3(a) it is clear that for any load current to

flow at least one thyristor from the top group (T1, T3) and one thyristor from the

bottom group (T2, T4) must conduct.

It can also be argued that neither T1T3 nor T2T4 can conduct simultaneously For

example whenever T3 and T4 are in the forward blocking state and a gate pulse is

applied to them, they turn ON and at the same time a negative voltage is applied

across T1 and T2 commutating them immediately.

Similar argument holds for T1 and T2. For the same reason T1T4 or T2T3 can not

conduct simultaneously.

Therefore, the only possible conduction modes when the current i0 can flow are

T1T2 and T3T4. Of course it is possible that at a given moment none of the

thyristors conduct.

This situation will typically occur when the load current becomes zero in between

the firings of T1T2 and T3T4. Once the load current becomes zero all thyristors

remain off.

In this mode the load current remains zero. Consequently the converter is said to be

operating in the discontinuous conduction mode. Fig shows the voltage across

different devices and the dc output voltage during each of these conduction modes.

It is to be noted that whenever T1 and T2 conducts, the voltage across T3 and T4

becomesvi. Therefore T3 and T4 can be fired only when vi is negative i.e, over the

negative half cycle of the input supply voltage.

Similarly T1 and T2 can be fired only over the positive half cycle of the input

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supply. The voltage across the devices when none of the thyristors conduct

depends on the off state impedance of each device.

The values listed in Fig assume identical devices. Under normal operating

condition of the converter the load current may or may not remain zero over

some interval of the input voltage cycle.

If i0 is always greater than zero then the converter is said to be operating in the

continuous conduction mode. In this mode of operation of the converter T1T2 and

T3T4 conducts for alternate half cycle of the input supply.

Continuous Conduction Mode:

As has been explained earlier in the continuous conduction mode of operation i0never

becomes zero, therefore, either T1

T2

or T3

T4

conducts. Fig. shows the waveforms of

different variables in the steady state. The firing angle of the converter is . The angle

is given by

It is assumed that at t = 0-

T3T

4was conducting. As T

1T

2are fired at t = they turn on

commutating T3

T4

immediately. T3

T4

are again fired at t = + . Till this point T1

T2

conducts. The period of conduction of different thyristors are pictorially depicted in the

second waveform (also called the conduction diagram)

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The dc link voltage waveform shown next follows from this conduction diagram and the

conduction table shown in Fig (b). It is observed that the emf source E is greater than the dc

link voltage till t = . Therefore, the load current i0continues to fall till this point.

However, as T1T

2are fired at this point v

0becomes greater than E and i

0starts increasing

through R-L and E. At t = v0

again equals E. Depending upon the load circuit

parameters io

reaches its maximum at around this point and starts falling afterwards.

Continuous conduction mode will be possible only if i0

remains greater than zero till T3T

4are

fired at t = + where upon the same process repeats.

The resulting i0

waveform is shown below v0. The input ac current waveform i

iis obtained

from i0by noting that whenever T

1T

2conducts i

i= i

0and i

i= - i

0whenever T

3T

4conducts. The

last waveform shows the typical voltage waveform across the thyristor T1.

It is to be noted that when the thyristor turns off at t = + a negative voltage is applied

across it for a duration of . The thyristor must turn off during this interval for successful

operation of the converter.

It is noted that the dc voltage waveform is periodic over half the input cycle. Therefore, it

can be expressed in a Fourier series as follows.

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Fourier series expression of v0is important because it provides a simple method of estimating

individual and total RMS harmonic current injected into the load as follows:

The impedance offered by the load at nth harmonic frequency is given by

it can be argued that in an inductive circuit IonRMS

0 as fast as 1/n2

. So in practice it will be

sufficient to consider only first few harmonics to obtain a reasonably accurate estimate of

IOHRMS

form equation This method will be useful, for example, while calculating the required

current derating of a dc motor to be used with such a converter.

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However to obtain the current rating of the device to be used it is necessary to find out a

closed form expression of i0. This will also help to establish the condition under which the

converter will operate in the continuous conduction mode.

To begin with we observe that the voltage waveform and hence the current waveform is

periodic over an interval . Therefore, finding out an expression for i0

over any interval of

length will be sufficient. We choose the interval t + .

Now at steady state 00t=t=+i=i since i0is periodic over the chosen interval. Using this

boundary condition we obtain

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Dis Continuous Conduction Mode:

So far we have assumed that the converter operates in continuous conduction mode without

paying attention to the load condition required for it. In figure 10.4 the voltage across the R

and L component of the load is negative in the region - t + .

Therefore i0continues to decrease till a new pair of thyristor is fired at t = + . Now if the

value of R, L and E are such that i0becomes zero before t = + the conduction becomes

discontinuous.

Obviously then, at the boundary between continuous and discontinuous conduction the

minimum value of i0

which occurs at t = and t = + will be zero. Pu tting this

condition in we obtain the condition for continuous conduction as.

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Fig shows waveforms of different variables on the boundary between continuous and

discontinuous conduction modes and in the discontinuous conduction mode.

It should be stressed that on the boundary between continuous and discontinuous conductionmodes the load current is still continuous.

Therefore, all the analysis of continuous conduction mode applies to this case as well. However

in the discontinuous conduction mode i0

remains zero for certain interval. During this interval

none of the thyristors conduct.

These intervals are shown by hatched lines in the conduction diagram of Fig 10.6(b). In this

conduction mode i0

starts rising from zero as T1T

2are fired at t = . The load current continues

to increase till t = . After this, the output voltage v0falls below the emf E and i

0decreases

till t = when it becomes zero.

Since the thyristors cannot conduct current in the reverse direction i0

remains at zero till t = +

when T3and T

4are fired. During the period t + none of the thyristors conduct.

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During this period v0

attains the value E. Performance of the rectifier such as VOAV

, VORMS

,

IOAV

, IORMS

etc can be found in terms of , and . For example

It is observed that the performance of the converter is strongly affected by the value of . The

value of in terms of the load parameters (i.e, , and Z) and can be found as follows.

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Inverter mode of operation:

The expression for average dc voltage from a single phase fully controlled converter in

continuous conduction mode was

For < /2, Vd

> 0. Since the thyristors conducts current only in one direction I0

> 0 always.

Therefore power flowing to the dc side P = V0I0

> 0 for < /2. However for > /2, V0

< 0.

Hence P < 0. This may be interpreted as the load side giving power back to the ac side and the

converter in this case operate as a line commutated current source inverter.

So it may be tempting to conclude that the same converter circuit may be operated as an

inverter by just increasing beyond /2. This might have been true had it been possible to

maintain continuous conduction for < /2 without making any modification to the converter

or load connection.

To supply power, the load EMF source can be utilized. However the connection of this source

in Fig 10.3 is such that it can only absorb power but can not supply it. In fact, if an attempt is

made to supply power to the ac side (by making > /2) the energy stored in the load inductor

will be exhausted and the current will become discontinuous as shown in Fig.

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Therefore for sustained inverter mode of operation the connection of E must be reversed as

shown in Fig (b). Fig (a) and (b) below shows the waveforms of the inverter operating in

continuous conduction mode and discontinuous conduction mode respectively. Analysis of

the converter remains unaltered from the rectifier mode of operation provided is defined as

shown below

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Prepared By Mr M RAVICHANDRAN AP / EEE Page 24

Prepared By Approved By

M.Ravichandran Prof.P.Manimekalai

AP / EEE HOD / EEE

APC Unit 1

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