The turn-off speed of MOSFET depends on the gate drive circuit. Increasing the switching speed may reduce switching losses. Today, we will share four more practical turn-off circuits.

Simple shutdown circuit

The simplest technique is to use an anti-parallel diode. Here, the RGATE resistor adjusts the MOSFET turn-on speed, and during the turn-off process, the anti-parallel diode shunts the resistor.

As the gate-to-source voltage approaches 0V, the effect of the diode becomes less, but this circuit has a drawback in that the gate turn-off current flows through the output impedance of the driver.

 PNP shutdown circuit

This is a common circuit. Due to the existence of Qoff, the gate and source form a local short circuit at the MOSFET end during the turn-off period. The RGATE resistor limits the turn-on speed, the diode provides a path for the turn-on current, and protects the transistor from reverse breakdown at the beginning of the turn-on.

 The advantage of this circuit is that the high peak discharge current of the MOSFET input capacitor is limited by the two transistors, so that the turn-off current does not return to the driver, avoiding false turn-on. At the same time, the power loss of the driver is reduced by two times.

NPN Shutdown Circuit

Similar to the previous circuit, it can effectively control the gate discharge current locally.

 The difference is that the NPN transistor allows the gate to be closer to GND and can also provide a bias mechanism to keep the MOSFET off during power-up.

However, there is a disadvantage here, because QOFF is an inverting stage and requires an inverted PWM signal provided by QINV. The inverter consumes current in the driver when the MOSFET is turned on, reducing the circuit efficiency.

NMOS shutdown circuit

 Compared with the previous circuit, this circuit has fewer components . It uses a dual driver to provide an inverted PWM signal for a small Nmos, which can provide a very fast switching speed and completely discharge the MOSFET gate to 0V.

The resistor here not only sets the switching speed, but also prevents the formation of breakdown current between the two outputs of the driver when the driving signal timing is imperfect.