It can be concluded that a resistance value that is too large or too small will have a certain adverse effect on the MOSFET drive circuit. How to determine the appropriate resistance value generally depends on the current capacity and rated voltage of the MOSFET, as well as the switching frequency, to select the value of Rg. Abstract generated by the author through intelligent technology Useful

We often hear about adding a resistor in front of the MOSFET gate. So, why add this resistor, and furthermore, why add a 100Ω resistor?

Adding a resistor in front of the MOSFET gate?

The MOSFET is a voltage-controlled device. Generally, the conduction of the MOSFET only requires the gate voltage to exceed its threshold voltage, and there is no need for gate current.

So fundamentally, there is no need to connect any resistor in series with the gate of the MOSFET.

There is another situation, which is the parasitic capacitance at the gate of the MOSFET. Generally, to speed up the conduction and cutoff of the MOSFET, and reduce the losses during conduction and cutoff, the equivalent resistance on the gate should be as small as possible, preferably 0.

But we often see a resistor connected in series with the gate in circuits involving MOSFETs. Like this:

So why connect this resistor?

In the switching state, the usual explanation is to prevent the MOSFET from producing oscillating waveforms during switching, as this would increase the MOSFET switching losses. Moreover, if the oscillation is too large, it can cause the MOSFET to breakdown.

Furthermore, why is the resistor 100Ω?

I saw a simulation experiment online, where a resistor R3 was connected in series with the gate of the MOSFET in the MOSFET circuit, and simulations were performed with resistances of 1 ohm, 10 ohms, and 50 ohms:

• When R3 is 1 ohm, there is a high-frequency oscillation signal on the output voltage Vds.

• When R3 is 10 ohms, the high-frequency oscillation signal on the output voltage Vds is significantly attenuated.

• When R3 is 50 ohms, the rising edge of the output voltage Vds becomes slow. A step is generated on the gate voltage due to the Miller capacitance effect between the drain and gate. This significantly increases the power consumption of the corresponding MOSFET.

In short, if the value is too small, it will cause output ringing; if it is too large, it will increase the MOSFET's switching transition time, thereby increasing its power consumption.

After reading this, you might still not be clear about the role of the 100Ω resistor. Let's delve deeper into the switch oscillation we mentioned earlier. This is a MOSFET drive circuit diagram:

There are various inductances distributed in the power MOSFET drive circuit, such as L in the diagram. They, together with the MOSFET's Cgd and Cge, form a resonant circuit: they resonate with the high-frequency harmonic components in the switch drive signal, causing fluctuations in the output voltage of the power tube.

The gate series resistor Rg of the MOSFET will increase the loss in the MOSFET drive loop and then reduce the Q value of the resonant circuit, causing the inductance and capacitance resonance phenomenon to decay rapidly.

Here we can understand that the resistor connected in series with the gate of the MOSFET is determined based on the specific MOSFET and the distribution of stray inductances in the circuit.

This is related to what we mentioned above about the resistance value affecting. Let's elaborate below:

As shown in the above figure, when the value of Rg is relatively small, the overshoot of the drive voltage is relatively high, and there is more oscillation. The larger the inductance L, the more obvious this effect is, which will affect the performance of the MOSFET and other devices.

In addition, the peak value of the drive current is also relatively large. However, under normal circumstances, the driving current output capability of the IC is limited.

When the resistance value is too large, when the actual driving current reaches the maximum value of the IC output, the IC output is equivalent to a constant current source, which will linearly charge the Cgs. The rise rate of the driving voltage waveform will slow down.

If the driving waveform rises slowly, it will have an adverse effect if there is a large current passing through the MOSFET.

It can be concluded that a resistance value that is too large or too small will have a certain adverse effect on the MOSFET drive circuit. How to determine the appropriate resistance value generally depends on the current capacity and rated voltage of the MOSFET, as well as the switching frequency, to select the value of Rg. Remember the sentence above?

The resistor connected in series with the gate of the MOSFET is determined based on the specific MOSFET and the distribution of stray inductances in the circuit.

Therefore, it is not a fixed value, but 100Ω is a common value because it is a "range value" often obtained in daily use of driver circuit applications. However, the specific application needs to be determined by experimental testing.

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