It is generally believed that MOSFET is voltage driven and does not require driving current. However, there is a junction capacitance between the G and S stages of MOS. This capacitance makes driving MOS not so simple and can easily cause abnormal driving waveforms. The commonly used PWM chip output directly drives MOS or uses a transistor to amplify it and then drives MOS through isolation. In fact, there are great defects in the instantaneous driving current. A better method is to use a dedicated MOSFET driver chip to drive the MOS tube. Siliconlabs has launched the half-bridge MOS driver chip SI823X series isolation driver chip for MOS tube driving.

This article first analyzes several MOS tube drive waveforms:

As shown in Figure 1, if such a round waveform appears, the MOS tube is in danger of exploding. The waveform shows that the MOS tube works in the linear region for a large part of the time, and the loss is extremely huge. Generally, this is because the wiring is too long and the inductance is too large. The gate resistor cannot save you, and you can only redraw the board.

Figure 1 MOS tube abnormal drive waveform 1

As shown in Figure 2, the high-frequency ringing is serious in the driving square wave. The oscillation is serious at the rising and falling edges. In this case, the tube is generally damaged instantly. It is similar to the waveform in Figure 1 above, entering the linear region. The reason is also similar, mainly due to wiring problems.

Figure 2 MOS tube abnormal drive waveform 2

As shown in Figure 3, a fat and round pig wave appears. The rising and falling edges are extremely slow, which is caused by impedance mismatch. The chip driving ability is too poor or the gate resistance is too large. Decisively change the driver chip with a large current and adjust the gate resistance to a smaller value.

Figure 3 MOS tube abnormal drive waveform 3

As shown in Figure 4, there is a square wave with spikes on the rising and falling edges. The high and low levels are distinct, and the level can be called a level at this time because it is flat. 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 appropriately increase the gate resistance.

Figure 4 MOS tube abnormal drive waveform 4

As shown in Figure 5, this is a product with no ringing, no spikes, and no linear loss. This is the most perfect waveform.

Figure 5 MOS tube normal driving waveform

SILICON LABS half-bridge MOS isolation driver chip SI823X has input-output isolation performance of up to 5KVRMS, switching frequency of up to 8MHz, and peak output capacity of up to 4.0A, which can meet the MOS driving requirements of different frequency and power ranges. At the same time, SI823X not only plays the role of isolation, but also has the ability to drive MOS, as well as programmable dead time control, which greatly simplifies the circuit design and reduces the use area of PCB, thereby reducing the probability of driving waveforms in Figures 1 and 2. From the above, it can be seen that the selection of SI823X can simplify the MOS driving circuit while obtaining more stable driving performance.

Source: Shiqiang

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