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.
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.
・The gate driving signal is adjusted to optimize the loss and noise of the switching transistor.
・If the switching rise and fall times are made faster, losses are reduced, but the switching noise increases.