【reason】
关于MOS管驱动电路设计，本文谈一谈如何让MOS管快速开启和关闭。
It is generally believed that MOSFET (MOS tube) is voltage driven and does not require driving current. However, there is a junction capacitance between the G and S poles 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 inductor is the parasitic inductance of the circuit wiring.
If 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. In switching power supplies, switching losses account for a large part of the total losses. Therefore, the quality of the MOS tube drive circuit directly determines the efficiency of the power supply.
How to quickly open and close the MOS tube?
【principle】
For a MOS tube, if 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 MOS tube will turn on. Similarly, if the time for reducing the GS voltage of the MOS tube from the turn-on voltage to 0V is shorter, the faster the MOS tube turns off.
From this we can know that if we want to pull the GS voltage up or down in a shorter time, we must give the MOS tube gate a larger instantaneous drive current.
The commonly used method of directly driving the MOS with the output of the PWM chip or using a transistor to amplify the MOS and then drive the MOS has great shortcomings in terms of instantaneous driving current.
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 driver
circuit Things that need attention in the design :
Because the drive 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 driver chip is directly connected to the gate of the MOS tube, the PWM The rising and falling edges of the wave will produce a large oscillation, causing the MOS tube to heat up rapidly or even explode. The general solution is to connect a resistor of about 10 ohms in series to the gate to reduce the Q value of the LC oscillation circuit and quickly attenuate the oscillation.
Due to the high input impedance characteristics of the MOS tube gate, a little static electricity or interference may cause the MOS tube to mislead, so it is recommended to connect a 10K resistor in parallel between the G and S poles of the MOS tube to reduce the input impedance.
If you are worried that interference from nearby power lines will be coupled to produce an instantaneous high voltage breakdown of the MOS tube, you can connect a TVS transient suppression diode of about 18V in parallel between the GS.
TVS can be thought of as a voltage regulator tube with a fast response speed. It can withstand instantaneous power of hundreds to even kilowatts and can be used to absorb instantaneous interference pulses.
In summary, MOS tube drive circuit reference:
Wiring design of MOS tube drive circuit:
The loop area of the MOS tube drive circuit must 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 wiring will greatly affect the instantaneous output current of the chip.
Common MOS tube drive waveforms:
If such a round waveform appears, just wait for a nuclear explosion. The tube works in the linear region for a large part of the time, and the loss is extremely huge.
Generally, in this case, the wiring is too long and the inductance is too large. Even the gate resistor cannot save you, so you can only redraw the board.
Disfigured square wave with severe high-frequency ringing:
serious oscillation on the rising and falling edges. In this case, the tube usually dies instantly, which is similar to the previous situation and enters the linear zone.
The reasons are similar, mainly due to wiring issues. Fat and round fat pig boobs.
The rising and falling edges are extremely slow, which is caused by impedance mismatch.
The chip driving capability is too poor or the gate resistance is too large.
Decisively replace the driver chip with a high current and adjust the gate resistance to a small value and it will be OK.
Their triangular wave is a square wave that is filled with sine waves:
the impedance of the driving circuit is too high, which is a sure-fire wave for the tube. The solution is the same as above.
The popular face shape, the square wave that everyone loves:
the high and low levels are clear, and the level can be called level at this time, because it is flat. The edge is steep, the switching speed is fast, the loss is very small, and there is a slight oscillation, which is acceptable. The tube cannot enter the linear zone. If you have obsessive-compulsive disorder, you can increase the gate resistance appropriately.
A square and handsome wave, a three-free product with no ringing, no peaks, and no linear loss. This is the most perfect waveform.
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