ZXGD3105N8

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November 2011

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ZXGD3105N8

Layout guidelines

ESR, low-ESL X7R type ceramic bypass capacitor. If the converter’s output voltage is higher than 15V, a series

voltage regulator between the converter’s output voltage and the Vcc pin, can be used to get a stable Vcc voltage.

GND is the ground reference for the internal high voltage amplifier as well as the current return for the gate driver. So

the ground return loop should be as short as possible. Sufficient PCB copper area should be allocated to the Vcc and

GND pin for heat dissipation especially for high switching frequency application.

Any stray inductance involved by the load current 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 controller Drain pin

and MOSFET’s terminal should be kept less than 10mm. MOSFET packages with 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 reverse recovery of the snubber diode. These

high frequency oscillations are reflected across the transformer to the drain terminal of the synchronous MOSFET.

The synchronous controller 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 or an R-C network across the diode. Both methods reduce the oscillations by softening the

snubber diode’s reverse recovery characteristic.

Figure 4 - Primary side snubber network to reduce drain voltage oscillations