EE-321 N

Lecture-15

Ramp Control Scheme

Introduction

• Similar to the cosine scheme, there is another one called ramp comparator approach

• As the name suggests, this scheme involves comparison of a ramp signal against a dc control signal

• This scheme is suitable for Biphase Midpoint converter and can be extended to three phase converters also

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A Biphase midpoint converter

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Block Diagram & Waveforms

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Vr

2

Comparator 1

Synchronized I/P Supply

M/S 1 (For T1)

Operation

• The t/f secondary terminal provides synchronized ac voltage which is converted to square wave by using a ZCD

• The negative portion of the square wave is suppressed by using a diode

• Square wave is used to generate synchronized ramp signal

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Operation...contd

• As soon as the control signal becomes equal to the ramp signal, output from comparator is obtained

• Comparator o/p is also a square wave whose leading edge lags by an angle α, w. r. t. the commencement of the ramp

• Comparator o/p wave is sufficient to trigger the thyristor but pulse width is variable

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Operation...contd

• But if a fixed duration pulse is required then a monostable chip is used which provides a small duration pulse signal or a pulse train.

• Again the relation between the firing angle and the control voltage is linear given by

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rc c

r r

V Vk

V V