Huawei ETS2000 Series Maintenance Manual page 16

Figure1-16
Power transfer circuit
The PWM control chip adopts TL494, including 2 differential amplifiers, 1
external adjustable oscillator, 1 pulse comparer, 1 time overflow controller, and 1
output control circuit.
The differential amplifier can work within the range from AC -0.2V to 0.2V. The
time overflow controller has a fixed offset to assure the stable output in case of the
changed external input. The external oscillating circuit can be set through Rt/Ct
(The working frequency to be set is 100KHZ). The following block diagram
illustrates the functions of TL494:
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