Design Ideas: March 2, 1995

Placing the averaging filter inductor in the output return leg of an isolated forward-converter power supply can significantly reduce the common-mode noise the supply generates. This technique can eliminate some of the high-frequency spikes present on the output voltage. The inductor has a blocking effect on the high-frequency currents that the primary switching transistor generates. These currents flow through the transformer's stray capacitance to system ground. This configuration allows you to connect the cases of the output rectifier diodes directly to the dc output voltage and eliminates the high dV/dt normally present on the tabs. This connection prevents common-mode current flow through the isolation-barrier capacitance between the diodes' cases and the grounded heat sink.

However, this approach prevents the generation of a secondary-side bias voltage, normally accomplished by peak charging a capacitor with the voltage pulse at the input of the averaging inductor. The circuit in Fig 1 solves this problem. An inverting charge-pump arrangement generates a positive dc bias voltage from the negative voltage pulse present on the ac side of the inductor. C₁ charges through R₁ and D₁ during the on time of the forward converter and discharges into C₂ during the flyback period. The generated bias voltage is approximately equal to V_PEAK(WINDING)+V_OUT.