Posted on May 22, 2012

A major feature of the PWM servo amplifier is elimination of a pulse transformer. A 150 kHz pulsewidth modulated signal is applied to U3, with its complementary outputs applied to identical circuits to drive the load. When point A increases, Q2 is on and point B is at ground potential. The Vee for Ul is maintained through Dl, and Ql is held off by D2. When point A decreases, Q2 turns off, point Cis pulled low by C2, which turns Ql on. The time constant for Rl, R3, and C2 can hold Ql on just long enough to allow the voltage at point B to start rising.

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As point B rises, it charges C2 by forward biasing D3, maintaining point C low with respect to Ul, and keeping Ql turned on. With point B at 40 V, D2 is off and point C is held low by Rl and R2, and Vee for Ul is maintained by the charge on Cl. When point A increases again, Q2 again turns on, C2 pushes point C high, and turns Ql off long enough to allow the voltage at point B to start falling. C2 is now discharged by reverse-biased D3, which keeps point C high with respect to Ul, and keeps Ql off. Once point Breaches ground potential, Dl again turns on, recharging Cl, and maintaining Vee to Ul. D2 also turns on and keeps Ql off.

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