In switching circuits and driver circuits, a resistor is often connected in series with the gate of a MOSFET. To be honest, many engineers don't like this practice because it affects efficiency, but without it, the circuit is prone to oscillation, which is a love-hate relationship. So, what is the purpose of this resistor?

Firstly, it limits the drive current to prevent excessive current, which could damage the driving chip due to insufficient drive capability.

As we discussed before, when a MOSFET turns on, it charges various capacitors. At that moment, the capacitors act as short circuits, and the current is very high. The driving chip cannot handle such a large current, so the resistor plays a role in limiting the current and protecting the chip.

However, the resistance value should not be too large. If it is too large, the conduction and turn-off times of the MOSFET will increase, leading to increased switching losses.

Secondly, as mentioned earlier, it can eliminate oscillation signals at the gate of the MOSFET.

There is parasitic inductance in the wiring from the driver chip to the MOSFET, and the internal motor lead of the MOSFET also has inductance. In addition, there is parasitic capacitance between the poles of the MOSFET. Therefore, the MOSFET switching circuit is equivalent to an LC low-pass filter circuit.

The ratio of its output to its input is the gain.

When the gain is greater than 1, the noise is amplified.

Near its resonant frequency, the LC circuit will have a higher gain. If there is no resistor introduced, the gain will be infinite.

The driving signal is a step signal with a rich frequency component, which must have signals near the resonant frequency point, making it easy to resonate and oscillate.

The presence of the resistor is to provide damping, absorb such oscillation signals, and increase the resistance value of the series resistor to reduce the gain at the resonant frequency point. Generally, a resistance value of around 10 ohms is sufficient, with the formula Rg > 2(L/Cgs)^0.5.

So, should the resistor be placed at the driver end or the MOSFET end?

In some low-power MOSFET circuits, both the driver end and the MOSFET end can be used. However, for high-power MOSFET switching circuits, because the MOSFET is prone to interference during turn-off, which may cause misoperation, the resistor is generally placed at the MOSFET end.

Here's a brief explanation:

In many cases, in switch-mode power supplies and driver circuits, most gate series resistors are placed at the MOSFET end.

At this point, we need to consider impedance matching and treat the signal as a high-speed signal.

High-speed signal: The rising edge of the signal is less than 6 times the transmission delay.

However, in general, the gate wiring length does not exceed 10 cm, and the rise time of the driving circuit is not very small, so there is no need to consider it as a high-speed signal, and it is not necessary to place it near the source-end IC.

So, what are the benefits of placing it near the MOSFET?

For the gate of the MOSFET, the received signal is not significantly different in the setting shown in the figure.

This is because the inductance and resistance are always in series. If they are considered as a whole, the impedance is R+jwL, which is completely the same for the circuit.

Above is all the content for today. Thank you for reading! VBsemi will continue to update~