Today, I share four common MOS transistor gate driving circuits. Have they all been used?1. IC direct drive typeThe direct driving of this power IC is the most common and simplest driving mode.

Fig. 1 IC directly drives MOS gate

Using this method, we should pay attention to several parameters and their effects.

First, check the power IC manual for the maximum peak driving current, because different IC chips have different driving capabilities.

Secondly, check the parasitic capacitance of MOSFET, such as C1, C2 and C3 in the figure. If the capacitance is large, the energy required to turn on the MOS tube is also large.

If the power IC does not have enough peak drive current, the transistor will turn on at a slower speed.If the driving capability is insufficient, high-frequency oscillation may occur at the rising edge, and even reducing Rg in Figure 1 will not solve the problem! However, IC driving ability, parasitic capacitance of MOSFET, switching speed of MOSFET and other factors will also affect the choice of driving resistance, so Rg cannot be reduced indefinitely.

2. totem pole circuit enhances driving.The function of the driving circuit is to increase the current supply capacity and quickly complete the charging process of the gate capacitor input. This topology increases the turn-on time, but reduces the turn-off time, and the switch tube can be turned on quickly, avoiding the high-frequency oscillation of the rising edge.

Figure 2 totem pole circuit enhanced drive

3. Drive the circuit to speed up the turn-off of the MOS tube.

At the moment of turning off, the driving circuit can provide a path with the lowest impedance as possible, so that the capacitor between the gate and the source of the MOSFET can be discharged quickly, and the switching tube can be turned off quickly.

In order to ensure the fast discharge of the capacitor C2 between the gate and the source, an Rg2 and a diode D1 are connected in parallel to Rg1.D1 usually adopts fast recovery diode, which shortens the turn-off time and reduces the turn-off loss; The function of Rg2 is to prevent the power IC from burning out due to excessive current when it is turned off.

Fig. 3 accelerating MOS transistor shutdown circuit

The totem pole circuit can also be turned off quickly. When the driving capability of the power IC is sufficient, the circuit in Figure 2 can be improved to the form shown in the following figure.

Fig. 4 improved accelerating MOS transistor shutdown circuitIt is very common to use triode to discharge the electricity of GS capacitor. If the emitter of Q1 has no resistance, the capacitor between the gate and the source of PNP transistor will be short-circuited when it is turned on, which can realize the discharge in the shortest time and minimize the cross loss when it is turned off.Fig. 4. Because of the existence of triode, the capacitor current between the gate and the source will not be discharged directly through the power IC, which improves the reliability of the circuit.

4. The transformer drive circuit accelerates the turn-off of MOS tube.In order to meet the requirements of driving high-side MOS transistors, as shown in Figure 5, transformer drivers are usually used, sometimes for safety isolation.The purpose of R1 is to restrain the parasitic inductance on PCB from forming LC oscillation with C1, and its design purpose is to isolate DC and AC, while preventing the magnetic core from saturation.

Figure 5 High-side MOSFET driver circuit

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