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EE-321 N
Lecture-15
Ramp Control Scheme
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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)
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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