Application information – Diodes ZXGD3105N8 User Manual
Page 12
ZXGD3105N8
Document Number DS35101
Rev. 3 - 2
12 of 14
March 2013
© Diodes Incorporated
A Product Line of
Diodes Incorporated
ZXGD3105N8
Application Information
(cont.)
Layout Guidelines
When laying out the PCB, care must be taken in decoupling the ZXGD3105 closely to V
CC
and ground with 1μF low-ESR, low-ESL X7R type
ceramic bypass capacitor. If the converter’s output voltage is higher than 20V, a 12V zener diode should be connected from the bias pin to GND to
clamp the Gate voltage and protect the synchronous MOSFET. Figure 7 shows the typical connection diagram.
Figure 7 Zener Voltage Clamp Arrangement
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 the 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 (refer Figure 8). Both methods reduce the oscillations by softening the snubber diode’s reverse recovery
characteristic.
Figure 8 Primary Side Snubber Network to Reduce Drain Voltage Oscillations