ZXGD3104EV2

Issue 1 – October, 2011

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© Diodes Incorporated 2011

3

Conditioning the power supply to maximize efficiency

Any stray inductance in the load current path may cause distortion of the drain-to-source voltage

waveform, leading to premature turn-off of the synchronous MOSFET. In order to avoid this issue,

drain voltage sensing should be done as physically close to the drain terminals as possible. The PCB

track length between the Drain pin and the MOSFET’s terminal should be kept to less than 10mm.

MOSFET packages with a low internal wire-bond inductance are preferred for high switching

frequency power conversion, to minimize body-diode conduction.

After the primary MOSFET turns off, its drain voltage oscillates due to the reverse recovery of the

snubber diode. These high frequency oscillations are reflected into the transformer secondary winding

and the drain terminal of the synchronous MOSFET. The synchronous IC senses the drain voltage

ringing, causing its gate output voltage to oscillate. The synchronous MOSFET cannot be fully

enhanced until the drain voltage stabilizes.

In order to prevent this issue, the oscillations on the primary MOSFET can be damped with either a

series resistor Rd to the snubber diode Dsnub, or an R-C network across Dsnub as shown in Figure

3. Both methods reduce the oscillations by softening the snubber diode’s reverse recovery

characteristic.

Figure 3: Techniques to prevent/reduce gate voltage oscillations