# Why gate drivers have series resistors on gates?

Although gate drivers are often used to interface low-voltage logic with high gate thresholds, it is my understanding that their primary function is to ensure fast gate switching by supplying high current to deal with gate capacitance.

Over the past few days I've been through dozens of the datasheets, and most of them show series resistors in the typical application examples. It seems to me this defeats the whole point of using gate driver. The only explanation I have is that the engineers are balancing between EMI (requires slow slope) and heat (requires fast slope).

Furthermore, many IGBT-capable chips provide separate paths for ON and OFF, with resistor in the ON path and direct connection (Miller clamp) for the OFF path. If EMI can be ignored to reliably switch the IGBT off, why not connect both directly and be done with it? Especially with IGBT switching times already 10x slower than those of FETs.

• Note that some drivers use two paralleled "series resistor with diode" ... Dec 15, 2023 at 9:06
• @Antonio51 Yes, that too. Often seen on drivers with single outputs to separate ON and OFF paths Dec 15, 2023 at 10:26

The gate resistor can help damp oscillation due to the inductive elements in the gate drive circuit interacting with the gate capacitance.

Compared with the driver impedance, it gives a linear impact under the control of the circuit designer.

With a high current gate drive, and no gate resistor, you will commonly get ringing on the gate, often with disastrous results.

it is my understanding that their primary function is to ensure fast gate switching by supplying high current to deal with gate capacitance.

For that particular purpose a push-pull totem pole driver which comprises two transistors and 1 or 2 resistors can be used instead. It's generally not space-eater, and also it's customisable i.e. if one transistor does not serve well then can be replaced with another one. So you don't have to go for a gate driver IC just for high peak current output.

The primary function of a gate driver can be different for different applications, such as -

• High-side switching (internal bootstrap driver or switched-capacitor booster)
• Half-bridge or push-pull converters (internal fixed or adjustable dead time generation)
• Synchronous rectification (Monitoring drain-source voltage and generating proper gate drive signal)
• Reverse polarity protection and inrush limiting

most of them show series resistors in the typical application examples.

The only explanation I have is that the engineers are balancing between EMI (requires slow slope) and heat (requires fast slope).

Gate stopper resistor can have different purposes such as reducing the peak gate drive currents to make life easier for the drivers (a good example is logic-level-gate MOSFETs driven by MCUs which have GPIOs rated for a few milliamps only), or to reduce EMI issues coming from fast switching, or to prevent oscillations especially when there are multiple MOSFETs in parallel.

Sometimes you want different speeds for turn on and off i.e. fast turn on vs relatively slower/acceptable turn off, or vice versa. In those cases, resistors along with diodes or transistors serve really good. Few examples are boost PFC regulators, flyback converters, or half-bridge LLC converters.

• "So you don't have to go for a gate driver IC just for high peak current output." Some of the ICs are just that - two BJTs in one package, e.g. ZXGD3006. Its datasheet too has a diode and a resistor. Dec 15, 2023 at 10:37

Yes, the gate driver can be used to drive gate of different transistors in different scenarios.

If you manufacture a one single gate driver, it only makes sense to make it as fast as possible and make it push and pull as much current as possible.

The adjustment of the driver IC and FETs to suit your application can then be done simply with a resistor. It allows the designer to make the switching only as fast as required for the application.

For example a FET can have a maximum current limit for driving the gate, and too fast switching requires more energy and it easily causes EMI problems.

It is far more convenient to adjust a resistor than trying to match which driver to use with which FETs when you need to do adjustments.

• The drivers manufactured for a range of uses and the simplest way to adjust them for particular application is using gate resistor. Yes, I think while very simplistic, this answer is most on the point of the original question. Dec 15, 2023 at 10:52

In your circuit, you choose a balance between switching speed and EMI, and the simplest way to do this is with a cheap resistor.

Gate drivers are designed with a low output impedance to allow fast switching. A resistor can slow down a gate driver's output signal, but there is no easy way to speed it up.

• The resistor can also help damp oscillation due to the inductive elements in the drive circuit interacting with the gate capacitance. Dec 15, 2023 at 9:11
• @colintd It's your point that first came to my mind. Not that of CL's. Dec 15, 2023 at 9:39