EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: robzy on August 27, 2024, 02:15:35 am
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How does one choose a MOSFET gate resistor? I'm having trouble finding anything that I'd consider "decent and simple" guidance on it.
In my case its an AO3400 switching a relatively high current (~1A) and I'm considering using a gate driver.
Would you use guidance like TI's SLLA385 https://www.ti.com/lit/ab/slla385a/slla385a.pdf (https://www.ti.com/lit/ab/slla385a/slla385a.pdf) ?
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Sounds like you're over complicating it! 1A is not a high current, that app note is for high power gate drives in SMPS with 10s or 100s of amp MOSFETs at high voltage.
What frequency will it operate at?
Will it be low side or high side switch?
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It would be operating at 100kHz-ish. Although it would be great to design for 1MHz even though the rest of the circuit can't handle it.
It will be a low-side switch. Possibly even driven by a gate driver like the MCP1415.
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Would you use guidance like TI's SLLA385 https://www.ti.com/lit/ab/slla385a/slla385a.pdf (https://www.ti.com/lit/ab/slla385a/slla385a.pdf) ?
I would.
But for a 1 amp circuit @ lowish voltage @ 100khz you could just put in anything from 0-22 ohms.
Apart from improving the gate waveform the only other consideration I know of removing heat from the FET. If you use 0 ohms most of the gate loss will occur in the FET. If you add a small resistor it will absorb most of this.
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The gate resistors on power MOSFETs are there primarily to prevent parasitic VHF oscillations which can be very destructive, that's why they should be placed right next to the gate, small ferrites on the gate lead can also be used.
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Often when the gate resistor approaches 22 ohms or a bit higher a diode is placed across the resistor to assist in draining the gate charge during the turn-off phase. The resistor also helps control EMI by limiting excessively sharp waveform edges and some of the associated ringing, but at a slight loss of efficiency due to dwelling in the linear range a tiny bit longer.
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AO3400 isn't a big transistor. What are you doing with it? What is the load, what voltage/range, what current range? Does it need to be fast, is switching loss a concern? (Probably, with the SOT-23 package, but how much so, is the question.) 100kHz Fsw doesn't tell us much: that's the repeat rate for the current pulses and voltage changes around the device; the actual changes can occur in mere nanoseconds if pushed hard enough -- a 10,000 times smaller time scale!
Conversely, is it a good idea to push it so fast? Is the load current even enough to swing the drain capacitance in that time frame? (Taking an average say Coss = 100pF, and delta V = 20V, 1A can raise it in (100pF)*(20V)/(1A) = 2ns, so that's probably a given.) What's attached to it, input or output -- is EM radiation a concern? This thing switches fast enough, you could interfere with aircraft communications in the right (wrong) situation. That wouldn't be very good!
Is the driver even fast enough? What's the rise/fall time into a representative load (similar Ciss, Qg(tot))? A fast output requires all of a fast transistor, low gate-circuit resistance, and a fast driver.
Besides radiation, slower switching is desirable for load switching applications, like enabling peripherals, switching general loads, coils, etc. A microsecond or slower may be desirable, much slower still (~ms) when inrush current into load capacitance is required (the energy of which is dissipated by the device, often necessitating a much larger e.g. D(2)PAK type, even if not much average current is required).
Tim
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Thanks for the comprehensive answer.
I wish there was a short "beginners guide to applied MOSFETs" which laid out the basics.
For now, I've opted to use BLDC drivers which have been engineered by people smarter than me.