Unit - II Switching Characteristics of Devices · Unit - II Switching Characteristics of Devices 10...

(AUTONOMOUS)Shamshabad, Hyderabad - 501218

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

Unit - II

Switching Characteristics of Devices

110 January 2018



Contents:

Diode as a switch

Piece wise linear characteristics of a diode

Diode switching times

Transistor-switching times and transistor in saturation

Design of transistor as a switch

Temperature variation of saturation parameters

210 January 2018



Diode as a Switch:

310 January 2018

The essential electrical characteristic of a pn-junction is that it constitutes a diode

which permits the easy flow of current in one direction but restrains the flow in

opposite direction.

When the positive terminal of the battery is connected to the p-type and the negative

terminal to the n-type of the pn-unction diode, then the bias is said to be forward

bias.

When the positive terminal of the battery is connected to the n-type and the negative

terminal to the p-type of the pn-junction diode, then bias is said to reverse bias.



Diode as a Switch:

410 January 2018

If forward voltage is applied Vf>V0, then the diode conducts. Hence the forward bias

diode acts as closed switch.

If Reverse voltage is applied VR



V- I Characteristics of Diode :

510 January 2018



Diode current equation:

Diode Current equation is given by,

Where, I = diode current

Io = diode reverse saturation current at room temperature.

V = external voltage applied to the diode

η = a constant, 1 for Ge and 2 for Si

VT = kT/q = T/11600,volt equivalent of temperature, i.e, thermal voltage,

610 January 2018

0 1

T

V

VI I e



Diode models:

710 January 2018

S. No. Type Model Characteristics

1.Piece-wise linear model with finite Rf

Vγ ≠0 rf ≠ 0 rr = ∞

2.Piece-wise linear model with Rf=0

Vγ ≠0 rf =0 rr = ∞

3. Ideal model

Vγ = 0 rf =0 rr = ∞

f f fV V I R

fV V

fV 0



Piece-wise linear characteristics:

810 January 2018

One technique for obtaining an equivalent circuit for a diode is to approximate the

characteristics of the device by straight-line segments. The resulting equivalent circuit

is naturally called the piecewise-linear equivalent circuit.

With Rf=0:

Forward bias Reverse bias V- I Characteristics

With finite Rf:

Forward bias Reverse bias V- I Characteristics



Diode switching times:

910 January 2018

Diode conducts when forward bias applied and block when reverse bias applied.

If reverse bias is applied when the diode is conducting vice versa, we expect

conduction to stop or start instantaneously.

But practically, it will not happen. A certain time will taken by the diode to change its

mode. This is because of the junction capacitances.

The behavior of the diode during this time is called Switching Characteristics.



Forward recovery time tfr:

1010 January 2018

When forward bias is applied, the time taken by the diode current reverse and attain

a steady state value. It is very small in practical (few nano seconds).

It does not create any problem in switching operation.

Reverse recovery time trr:

The time taken by the diode to conduct when switched from forward to reverse bias.

When switching from the conducting to the blocking state, a diode has stored charge

that must first be discharged before the diode blocks reverse current. This discharge

takes a finite amount of time known as the Reverse Recovery Time, or trr

It is very crucial important in high speed switching and hence need to be consider

carefully. It is lies between few nano seconds to few milli seconds.


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING 1110 January 2018

Consider the simple diode circuit and an input waveform

Till time t1: The forward voltage is applied, holes from p type will diffuse into n type

and electrons from n type will diffuse to p type. The value RL is large enough such

that drop across RL is large compared to drop across diode. IF=VF/RL.

In these region, majority carriers becomes minority carriers.

At t=t1: The reverse bias is applied, it acts as forward voltage for the large no of

minority charge carriers progressing though the junction With the results, IR=IF.



Certain time is needed for these minority charge carriers to become majority

carriers. This time interval is termed as Storage time (ts).

At t=t2: After this transition has passed off, the reverse current decreases

exponentially for a time tt and equal to IS (reverse saturation current). This time

interval is called Transition time (tt).

The reverse recovery time is sum of storage time and transition time.

rr s tt t t



For quick switching from ON to OFF, the reverse recovery time should be as small as

possible.

The reverse recovery time is depends on time constant RC, where C is the transition

capacitance.

Transition capacitance plays important role in switching characteristics.

Practically, the range of reverse recovery time is from few nano seconds to one micro

second.

The total switching time trr has the limit on the maximum operating frequency ie

rr

max

rr

T 10t

1 1f

T 10t



Transistor as a Switch:

1510 January 2018

Common emitter transistor circuit arranged to function as a switch

Cut off Region (OFF Switch): When the emitter –

base voltage is zero or reverse biased, the base

current is zero. Collector current almost zero.

Hence the transistor is in OFF state. VCC=VCE

Saturation Region (ON Switch): When the

transistor base is mode positive with respect to the

emitter, a base current IB flows, the collector

current IC is equal to β times of IB. Hence the

transistor is in ON state. VCE=0.



Design of Transistor as a Switch:

1610 January 2018

The transistor acts as switch is driven between cut-off and saturation regions.

For Vi=V(0): When transistor is kept at cut-off, so

the output is VCC or V(1).

For Vi=V(1): When transistor is kept at cut-off, so

the output is VCE(sat) or V(0).

To improve the transient response of the inverter,

the capacitor C is used across R1.



Design:

When Vi=V(0), the transistor is in cut-off . The open circuit base voltage is

If V(0)=0V, then

When Vi=V(1), the transistor is in saturation

From the fig,

1710 January 2018

2 1B BB

1 2 1 2

R RV V(0) V

R R R R

1B BB

1 2

RV V

R R

CC CE(sat) CC B(min)

C fe(min)

V V II , I

R h

1 2B R RI I I



Design:

Current through R1 is,

Current through R2 is,

Base current IB must be equal to

Where,

VBE(cutoff)=0V for Silicon transistor VCE(sat)=0.3V for Silicon transistor

VBE(cutoff)=-0.1V for Germanium transistor VCE(sat)=0.1V for Germanium

VBE(sat)=0.7V for Silicon transistor

VBE(sat)=0.3V for Germanium transistor

1810 January 2018

BE(sat)

1

1

V(1) VI

R

BE(sat) BB

2

2

V ( V )I

R

CC CE(sat )

B

fe(min) C

V V1I

h R



Transistor switching times:

1910 January 2018

The speed with which a transistor can be switched ON and OFF is a crucial factor in

high speed switching operations.

Let the input current i.e base current IB be a pulse as shown. The variation of output

current IC with time would be shown in fig.

Between 0 to t1, the transistor does not conduct instantly. It does not conduct start.

Instead there is delay.

The time that elapses during this delay, together with the time required for the

current to rise to 10% of its maximum value, is called delay time td.




2010 January 2018

Even after the collector current has begun to flow, it does not attain its maximum

value instantaneously.

There is one more time delay termed as rise time.

The time taken by the collector current to rise from 10% to 90% of its maximum

value is called rise time tr.

The total turn ON time is the sum of delay time and rise time.

ON d rTurn ON time t t t




2110 January 2018

Between t1 to t2: The transistor does not stop instantaneously.

Because the transistor is driven to saturation when it is ON, and in this state, the

collector to base bias also forward bias just like emitter bias junction.

When IB abruptly ceases, the charge carriers crossing the collector base junction get

trapped i.e stored in the depletion region.




2210 January 2018

There is time lag between instant IB=0 and instant IC starts to decrease i.e termed

storage time ts.

It is defined as the time interval between the instant IB=0 and instant IC has fallen to

90% of its maximum value is called Storage time ts.

The time required for IC to fall from 90% to 10% is called fall time tf.

Sometimes, the time taken by collector current to fall from 10% of its maximum

value to the level of ICBO is also considered while evaluating TOFF. This time interval

called decay time.

OFF S fTurn off time t t t



Saturation parameters of transistor:

2410 January 2018

If the transistor switch is to be used in a fast switching circuit, RL must be small.

When the transistor is in saturation, the transistor current is given by as (VCC/RL). It

already seen that RL must be small in order to minimize the time of charging of the

capacitor. Hence the collector supply also needs to be small.

The voltage swing at the transistor switch is given by [VCC - VCE(sat)].

Saturation resistance: When a transistor is in saturation, a quantity usually referred

to CE saturation resistance is defined as [VCE(sat)/IC]. It is generally denoted as RCS or

RCES or RCE(sat).



Temperature variation of saturation parameters:

2510 January 2018

VBE(sat): If IB and IC are constant and the temperature changes, voltage VBE tends to

change. The typical temperature sensitivity range is -1.5 to -2mV/0C, in respect of

both silicon and germanium transistors.

When the transistor is in saturation, both junctions are forward biased. It can be

visualized that the two diodes are connected in series back to back. Hence whenever

there is a change of temperature, the changes of voltages in the two junctions are

cancelled to each other with the result that VCE(sat) remains unaltered. This is

applicable for only small and moderate transistor.

The current gain hfe(β) is also temperature dependent. At small and moderate

currents, hfe is increases with increase of temperature.

Unit - II Switching Characteristics of Devices · Unit - II Switching Characteristics of Devices 10...

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