Mosfet gate resistors, not so simple.

[nigelwright7557]

Ever wondered where the mosfet gate resistor value comes from ?
http://www.tij.co.jp/jp/lit/an/slla385/slla385.pdf

 

[vzaichenko]

Hi Nigel,

A good article, however, this kind of calculation is valid for switching mode applications (3.75 MHz switching is used as an example in the article).

In case the MOSFET is used in the linear mode, as, say, an output device, source inductance becomes less important (low enough), so the value of the gate resistor may be calculated, assuming Rg and Ciss form a low-pass filter, setting its roll-off frequency (Fr) to 0.5-1.0 MHz.

Fr = 1 / (2 * PI * Rg * Ciss)
thus
Rg = 1 / (2 * PI * Ciss * Fr)

For, say, IRFP240 with Ciss = 1200 pF, setting Fr = 600 KHz:
Rg = 220R

This value can be adjusted based on the tests in practical conditions.

Cheers,
Valery

 

[AKSA]

Yes, very helpful and gives some science to the choosing.......
Many thanks to Nigel and Valery.

Hugh

 

[Dave S]

Ciss varies with changes in VDS.
Ciss comprises Cgs + Cgd. Cgs is bootstrapped in a source follower stage.

So it's mainly the (highly variable) miller capacitance that needs to be considered.

 

[vzaichenko]

Ciss varies with changes in VDS.
Ciss comprises Cgs + Cgd. Cgs is bootstrapped in a source follower stage.

So it's mainly the (highly variable) miller capacitance that needs to be considered.

Hi Dave,

You're right, however, the datasheet Ciss value at 25V or so gives an estimation good enough for initial Rg value calculation.

Then it may be tuned based on the simulations and live tests.

Cheers,
Valery

 

[CBS240]

As pointed out it is mostly the variable Cgd that comprises Ciss in source follower configuration. Under load conditions, RF oscillation burst will take place at max Cdg (min Vds) if the gate resistor is to small to dampen the internal reactive components.

Placing an external zero between the gate and drain, a gate Zobel, as close to the device package as possible will also dampen the resonance, allowing a smaller gate resistor and higher Ft operation whilst maintaining stability.

 

[jameshillj]

Surprisingly, the type of resistor can make a significant difference here too - foil, film, carbon, etc - don't know why.

 

[sajti]

Cgd is the most important, if You use the MOSFET as source follower. Cgs reduced, because of the gain is close to 1. I would say, that the series resistor must be as small as possible to avoid the oscillation. I used 100ohms generally, but very close to the gate.

Sajti