Alpha Technologies ACS HP Series - Manual (Page 9)

Preliminary
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AIP900-0012-B0-001 Rev. A (01/2012)
1.2 Theory of operation
Please refer to the power circuit block diagram on the following page for a theory of operation of
the ACS HP switched mode AC-DC conversion system. The 187-312 VAC, 50/60 Hz input is fed
through a circuit breaker into a full wave rectifier, which in turn provides a 120 Hz (340V peak)
pulse train to an input filter circuit. The input filter provides a nominal 290 VDC "raw supply", with
approximately 30 VP-P 120 Hz ripple, to the transistor switching circuit. The transistor switching
circuit “chops” the raw supply into a 525VP-P, 100 kHz (nominal) rectangular waveform with
a nominal 66% duty cycle. This high frequency switching waveform is then fed into a ferrite
power transformer, in which the waveform is “stepped down” and isolated. A rectifier circuit then
converts the power transformer output to a DC pulse train with a nominal 160 V peak. Next, a
two-stage output filter averages and smoothes this pulse train downward, providing the nominal
125 VDC output with low noise. A voltage error amplifier circuit senses the output voltage and
compares it with the voltage reference to provide a voltage error signal. Similarly, a current error
amplifier, using a shunt resistor and scaling amplifier, senses the output current and compares
it with the desired maximum output current, in order to provide a current error signal. These
signals are then fed into the pulse width modulator (PWM) via ORing circuitry so that either
voltage or current regulation is achieved. The PWM controls the "ON" time of the switching
transistors, varying the output as commanded by the error amplifiers. The PWM also senses the
switching transistor current on an instantaneous basis to provide cycle-by-cycle protection of the
switching transistors. An auxiliary supply, powered via a small 50/60 Hz transformer, and a DC/
DC converter power the control circuit and front panel circuitry. The PWM receives the ON/OFF
command and clock signal from the front panel circuit and control circuitry.
1.0 Introduction