EE-321 N

Lecture-9

Thyristor Gate Characteristics...contd

Design Consideration

Design Considerations

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Contd...

• The design of the firing circuit can be carried out with the help of the above figure.

• The first triggering ckt feeds power to the gate-cathode ckt of the SCR and satisfies the following equation

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s g g sE V I R

Contd...

• Where,

• The value of Rs should be such that the current Es/Rs does not cause any damage to the source or to the SCR. Otherwise an external resistance may be added in the circuit

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Es = Source voltage

Vg = G-K voltage

Ig = Gate current

Rs = Source internal resistance

Contd... • A resistance R1 is also connected across G-K

terminals which provides an easy path to the flow of leakage current between SCR terminals.

• If Ig(min) & Vg(min) are the min. gate current and voltage to turn ON an SCR, then current through R1 is Vg(min) / R1 & the ckt eqn. now becomes

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(min)

(min) (min)

1

g

s g g s

VE V I R

R

Contd...

• Intersection of the load line of the triggering source and the gate characteristics would give the operating (triggering) point

• In order to minimize the turn ON time and avoid unreliable turn ON, the triggering pt. should within the safe gate drive area and as close as possible to the power dissipation curve (curve 3 in gate characteristics)

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Different Types of Triggering Signals

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AC Signal

Short Duration Pulse

Long Duration Pulse (as good as DC)

HF train/sequence of pulses

1. DC Signal

• A DC voltage of proper magnitude and polarity is applied between the G & K of SCR such that G becomes +ve w. r. t. K.

• When the applied voltage is sufficient to produce the required gate current (IL), the device starts conduction

• However, continuous application of DC signal causes gate power dissipation & also there is no isolation between the power & control circuit

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2. AC Signal

• Most commonly used for SCRs used in AC applications

• Provides proper isolation between the control & power circuit

• Convenient control of firing angle is obtained by changing the phase angle of the control signal

• However, gate drive is present only for half cycle and presence of S/D transformer adds to the cost

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3. Pulse Signal

• Most popular method of triggering the SCR

• Gate drive consists of a single pulse or a sequence of HF pulses appearing periodically

• Requires pulse T/F for isolation

• The main advantage is that there is no need of applying the signal continuously which reduces the gate power dissipation drastically

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Contd...

• However, the average gate power dissipation Pgav must be < the peak instantaneous gate power dissipation Pgm

• If δ is the duty ratio/cycle, then

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gav gmP P

Basic Requirements

Whatever be the method adopted for triggering, the triggering signal has to satisfy the following basic requirements:

Signal should be of adequate amplitude & sufficiently short rise time.

It should be of adequate duration.

It should occur at a time when the main circuit conditions are favorable for conduction

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Problem 1

An SCR has Vg-Ig characteristic given by the equation

In a certain application, the gate voltage consists of rectangular pulses of 12 V and of duration 50 µs with duty cycle 0.2. Determine:

(a) The value of external series resistance to limit the power dissipation to 5 W.

(b) Average power dissipation

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1.5 8g gV I

Problem 2

For an SCR, the G-K characteristic is given by a straight line with gradient 16 V/amp passing through the origin, the max turn ON time is 4 µs & the min gate current to turn ON is 500 mA. If the gate voltage is 15 V, determine: (a) Resistance to be connected in series with the

SCR gate (b) Gate power dissipation, given that pulse width

is equal to the turn ON time and the avg gate power dissipation is 0.3 W.

(c) Max triggering frequency that will be possible with pulse firing

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Solution on Board

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