MOS tube drive circuit design, how to make the MOS tube turn on and off quickly?

It is generally believed that MOSFET (MOS tube) is driven by voltage and does not require driving current. However, there is a junction capacitance between the G pole and the S pole of the MOS tube. This capacitance will make driving the MOS not so simple.

Regarding the design of the MOS tube drive circuit, this article talks about how to make the MOS tube turn on and off quickly.

It is generally believed that MOSFET (MOS tube) is driven by voltage and does not require driving current. However, there is a junction capacitance between the G pole and the S pole of the MOS tube. This capacitance will make driving the MOS not so simple.

The three capacitors in the figure below are the junction capacitance of the MOS tube, and the inductance is the parasitic inductance of the circuit trace:

MOS tube drive circuit design, how to make the MOS tube turn on and off quickly?

If the ripple, EMI and inrush current requirements are not considered, the faster the switching speed of the MOS tube, the better. Because the shorter the switching time, the smaller the switching loss, and the switching loss accounts for a large part of the total loss in the switching power supply, so the quality of the MOS tube drive circuit directly determines the efficiency of the power supply.

How to quickly turn on and turn off the MOS tube?

For a MOS tube, the shorter the time it takes to pull the voltage between GS from 0 to the turn-on voltage of the tube, the faster the turn-on speed of the MOS tube will be. Similarly, if the GS voltage of the MOS tube is reduced from the turn-on voltage to 0V, the shorter the time, the faster the turn-off speed of the MOS tube is.

From this we can know that if you want to pull the GS voltage up or down in a shorter time, you must give the MOS transistor gate a larger instantaneous drive current.

The commonly used PWM chip output directly drives the MOS or uses a transistor to amplify and then drives the MOS. In fact, the instantaneous driving current has a big flaw.

A better method is to use a dedicated MOSFET driver chip such as TC4420 to drive the MOS tube. This type of chip generally has a large instantaneous output current and is also compatible with TTL level input. The internal structure of the MOSFET driver chip is as follows:

MOS tube drive circuit design, how to make the MOS tube turn on and off quickly?

Points to note in MOS drive circuit design:

Because the driving circuit wiring will have parasitic inductance, and the parasitic inductance and the junction capacitance of the MOS tube will form an LC oscillation circuit. If the output terminal of the driving chip is directly connected to the gate of the MOS tube, it will be affected by the rising and falling edges of the PWM wave. The MOS tube generates a lot of oscillations, causing the MOS tube to heat up sharply or even explode. The general solution is to connect a resistance of about 10 ohms in series with the grid to reduce the Q value of the LC oscillation circuit, so that the oscillation is quickly attenuated.

Because of the high input impedance of the MOS transistor gate, a little static electricity or interference may cause the MOS transistor to misconduct. Therefore, it is recommended to connect a 10K resistor in parallel between the G pole and the S pole of the MOS transistor to reduce the input impedance.

If you are worried about the coupling of interference on the nearby power line to produce instantaneous high voltage breakdown MOS tube, you can connect a TVS transient suppression diode of about 18V in parallel between GS.

TVS can be regarded as a voltage regulator tube with a fast response speed, which can withstand power up to several hundreds to thousands of watts in an instant, which can be used to absorb instant interference pulses.

In summary, MOS tube drive circuit reference:

MOS tube drive circuit design, how to make the MOS tube turn on and off quickly?

MOS tube drive circuit design, how to make the MOS tube turn on and off quickly?

Wiring design of MOS tube drive circuit:

The loop area of ​​the MOS tube drive circuit should be as small as possible, otherwise external 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 trace will greatly affect the instantaneous output current of the chip.

Common MOS tube drive waveforms:

If there is such a round wave, just wait for a nuclear explosion. A large part of the time the tube is working in the linear region, the loss is extremely huge.

Generally, in this case, the wiring is too long and the inductance is too large, and the gate resistance can’t save you. You can only redraw the board.

High frequency ringing severely disfigured square wave:

Vibration is severe on the rising and falling edges. In this case, the tube generally dies instantly, which is similar to the previous case and enters the linear region.

The reason is similar, mainly due to wiring problems. Fat and round fat pig wave.

The rising and falling edges are extremely slow, which is caused by impedance mismatch.

The chip’s drive capability is too poor or the gate resistance is too large.

Resolutely change the high-current driver chip, and adjust the gate resistance to a small adjustment.

The swollen face was filled with sine waves. The triangle wave was born in Fang Bo’s family:

The driving circuit impedance is too large, this is the tube must kill the wave, the solution is the same as above.

The popular face shape, the square wave that everyone loves:

The high and low levels are distinct, and the level can be called level at this time because it is level. The edge is steep, the switching speed is fast, the loss is small, and there is a slight oscillation, which is acceptable. The tube cannot enter the linear region. If you have obsessive-compulsive disorder, you can adjust the gate resistance appropriately.

The handsome boy wave with square and square, the three-no product with no ringing and no peak wireless loss, this is the perfect waveform of Z.

The Links:   LM48014F LMG5320XUFC IGBTS

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