Bose Lifestyle PS 28 Troubleshooting Manual page 7

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.
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