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Old 22nd July 2004, 04:33 PM   #1
Tensop is offline Tensop  Lebanon
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Default MOSFET gate resistors..

Hey guys,

From what ive been taught; resistors on the gates of mosfets are designed to slow down the switchon/off time of mosfets. To reduce ringing in affiliated circuitry, and its safe to assume that it balances the load on mosfets also? hopefuly im right with this.. now, on to the question...

How do i know what is the right value resistor to use?

ive seen on high speed, low voltage, high accuracy circuitry(computer motherboard SMPS's(5058) driving CPUs for example) use very low value resistors between the output of the SMPS controller and the mosfet, typically around 4-10 ohms.

However, ive noticed in another area of design; car audio SMPS controllers such as the 3525 where its higher voltage(1.5 volts at 60 amps compared to 12 Volts at 50 amps) where the gate resistor value is usualy 50-100 ohms.

Is this due to the latter having to deal with a more reactive load?

is there any easy way to determine which value mosfet gate resistor i should use? and how does it change when using multiple mosfets in parallel?

i've noticed(for example) on the output stage of an audio amplifier(BJT) where in the case of it having 2 BJT's per phase it would be .22 Ohm, and in the case of 4 BJT's per phase it would be .47 Ohm - But this is done purely for Load balancing right..? And would mosfets follow the same principal?
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Old 22nd July 2004, 04:54 PM   #2
sam9 is offline sam9  United States
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There is a white paper / app note on the Exicon website that mentions gate resistors. It doesn't answer your question but it's explanation may give you some help.

Basically the explanation is that the gate capacitance and gate resistance in the MOSFET device itself forms a low pass filter that rolls off HF. However, additional resistance is needed to lower the pole to a point where HF oscillation is supressed. Unfortunately precise values were not given. (N-type and P-type need different values BTW.)

Anyway, its a starting point

www.Exicon.com look at the bottom part of the page for the link. It's a .pdf.
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Old 22nd July 2004, 06:15 PM   #3
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Just do it like Nelson Pass and use 100-221 Ohm gate resistors. It will work just fine.
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Old 23rd July 2004, 01:53 AM   #4
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If this is for a switch mode poweer supply the lower the value the better. Sometime you can get buy with out a resistor, don't do that and for other part you can't. Anyway I would start with a 20 to 40 ohm resistor and work up. Meause the rise time on the drain to get a feel how the resistor affects the switching times.


It's normal in most of these audio designs to see resistors from 170 to 220 ohm.
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Old 23rd July 2004, 09:25 AM   #5
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Hi,

In my experience the value needed to stop oscilation increases with higher Drain-Source voltages. On a IRF540 i needed 150 Ohm minimum, with 30Volt D-S voltage maximum.

greetz,
Thijs
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