Selecting Critical Components: MOSFET Gate Drive Adjustment Circuit

This article discusses components to adjust the switching of an external MOSFET of the power supply IC BD7682FJ, and the method of adjustment.

MOSFET Gate Drive Adjustment Circuit: R16, R17, R18, D17

In order to optimize the switching operation of an external MOSFET Q1, a circuit to adjust the gate driving signal from the OUT pin of the BD7682FJ is configured using R16, R17, R18, and D17 (see the circuit diagram). This circuit affects the MOSFET losses and noise, and so must be optimized while checking the MOSFET switching waveform and losses.

The speed when switching on is adjusted using R16 and R17, which are inserted in series into the gate driving signal line.

The speed when switching off is adjusted by the combination of the diode D17 and R16, used to draw out charge.

By decreasing the resistance values, switching (rise and fall times) is made faster.

In this circuit example, we set R16=10Ω/0.25W, R17=150Ω, and D17=Schottky barrier diode RB160L-60 (60V/1A).

Switching losses in a quasi-resonant converter essentially do not occur at switch-on, and the losses during switching-off are dominant.

In order to reduce the switching loss at switch-off, R16 is made smaller to increase the switch-off speed. However, a sharp change in current results, and the switching noise is increased.

There is a tradeoff between switching loss and noise. Hence the temperature increase (=loss) and the noise of the MOSFET while incorporated into the product are measured, to check whether the temperature rise and noise level are within allowable ranges. Starting with the above-described component values, adjustments are made as necessary.

Moreover, current pulses flow in R16, and so the ability of the resistor to withstand pulses must be confirmed.

R18 is a resistor that pulls down the MOSFET gate. It should be set in the vicinity of 10 kΩ to 100 kΩ.