Have you noticed the details of MOS transistor driver design?

Have you noticed the details of MOS transistor driver design?

MOSFET driver design generally assumes that MOSFETs are voltage driven and do not require driving current. However, there is a junction capacitor between the G and S stages of MOS, which makes driving MOS less simple.

If ripple and EMI requirements are not considered, the faster the MOS transistor switching speed, the better. The shorter the switching time, the smaller the switching loss. In switching power supplies, the switching loss accounts for a large part of the total loss. Therefore, the quality of the MOS transistor driving circuit directly determines the efficiency of the power supply.

For a MOS transistor, the shorter the time it takes to pull the voltage between GS from 0 to the opening voltage of the transistor, the faster the MOS transistor will open. Similarly, if the GS voltage of the MOS transistor is reduced from the opening voltage to 0V in a shorter time, the speed at which the MOS transistor turns off will also be faster.

From this, we can knMOS transistorow that if we want to raise or lower the GS voltage in a shorter time, we need to provide a larger instantaneous driving current to the MOS gate.

The commonly used method of directly driving MOS with PWM chip output or amplifying MOS with a transistor before driving MOS actually has significant drawbacks in instantaneous driving current.

A better method is to use specialized MOSFET driver chips such as TC4420 to drive MOSFETs. These chips generally have a large instantaneous output current and are also compatible with TTL level inputs.

Attention should be paid to the design of MOS transistor driver circuits:

Because the driving circuit wiring will have parasitic inductance, and the parasitic inductance and MOS transistor junction capacitance will form an LC oscillation circuit,

If the output end of the driver chip is directly connected to the MOS gate, significant oscillations will occur at the rising and falling edges of the PWM wave, leading to rapid heating or even explosion of the MOS transistor;

The general solution is to connect a resistor of about 10 ohms in series with the gate, reduce the Q value of the LC oscillation circuit, and quickly attenuate the oscillation.

Due to the extremely high input impedance of MOS gates, even a small amount of static electricity or interference can cause MOS transistor to mislead. Therefore, it is recommended to connect a 10K resistor in parallel between MOS transistor G and S to reduce the input impedance;

If you are concerned about the coupling of interference on nearby power lines to generate instantaneous high-voltage breakdown of MOS transistor, you can connect an additional 18V TVS transient suppression diode in parallel between the GS. The TVS can be considered a fast response voltage regulator, which can withstand power up to several hundred to several kilowatts in an instant and can be used to absorb instantaneous interference pulses.

Wiring Design of MOS Diode Driver Circuit

The loop area of the MOS transistor driver circuit should be as small as possible, otherwise foreign electromagnetic interference may be introduced.

The bypass capacitor of the driver chip should be as close as possible to the VCC and GND pins of the driver chip, otherwise the inductance of the wiring will greatly affect the instantaneous output current of the chip.