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PS18/28/35 Troubleshooting Guide

Theory of Operation

1.4 High-frequency Resonant Converter

This section describes the theory of operation for a high-frequency half-bridge resonant DC-DC

converter, the heart of the power supply. Figure 5 shows its detailed circuit, where the high-fre-

quency transformer is represented by a leakage inductor Llk, a magnetizing inductor Lm, and an

ideal step-down transformer.

Figure 5. High-frequency half-bridge resonant DC-DC converter

With gate signals from the controller IC, the two power MOSFETs Q101 and Q102 are turned on

and off alternately to invert the DC input voltage to a high-frequency square wave voltage. The

high-frequency square wave voltage is applied to a series resonant circuit made of a resonant

capacitor C115 and leakage inductor Llk of the transformer. The resonant circuit shapes the

current waveform to a quasi sinusoidal waveform. Figure 6 shows oscillograms of the voltage

across MOSFET Q101 and current through capacitor C115. Such a smooth current waveform

lowers electromagnetic interference (EMI) emissions from the power supply.

Figure 6. MOSFET Q101 voltage and C115 current waveforms

The transformer transfers high-frequency power from the primary winding to two secondary

windings. The rectifier diodes D201A, D201B, D203, D204 rectify the high-frequency power and

the two capacitors C201 and C202 filter out high-frequency contents. A low pass L-C filter is

added to each rail to further reduce the high-frequency noise. Two DC voltages, one positive and

one negative, are obtained at the output.