Isolated feedback for off-line switching power supplies with primary-side control

Safety regulations require galvanic isolation between the AC line voltage and the outputs of switch-mode power supplies, making implementation of the feedback-control loop difficult. Four existing schemes for providing feedback information across the isolation barrier are examined. The schemes are identified as opto-couplers, amplitude modulation of the error signal, amplitude modulation of the output voltage, and communication of the drive pulse across the barrier. Each is examined with regard to steady-state and frequency-domain considerations