Gate resistor for power MOSFET - Page 1
Forum section: electronics / projects

author: Neukyhm
author_rank: Contributor
timestamp: February 15, 2018, 03:10:18 pm
content: Hi everyone, I'm designing the attached circuit. A train pulse turns on and off a IRFP250N, 50% duty cycle at 113kHz. The train pulse comes from a IR2110 that allows up to 2A output, so at 12volts, R_gate can be as low as 6 ohm for fast switching.Is this value OK? I don't want my MOSFET to burn because slow switching considering the voltage it will switch, 150V (vs 200V theorical maximum) and no more than 4A at drain.I have also seen some circuits that use an antiparallel UF4007 with R_gate, I'm not sure why.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 15, 2018, 06:33:43 pm
content: There is nothing wrong with your circuit but I assume the load is resistive as shown.The normal reason for a gate resistor is to slow down turn-on a bit to reduce EMI and ringing, values up to ~22R are common.The diode is to decrease turn-off speed by effectivly shorting out the resistor, however a schottky is needed (like a BAT54) to do any good, a normal fast diode (UF4007) isn't much good due to the higher Vf.What are you using the high side of the IR2110 for and have you ensured adiquite high side gate power from the boost circuit ?Also note the 1K pulldown is not normally required and imposes a static gate current requirement, definety bad news in the high side!

author: Dave
author_rank: Super Contributor
timestamp: February 16, 2018, 05:39:09 am
content: The diode is there for inductive loads. The current is zero when you're switching the transistor on, so you can turn it on nice and slowly (less EMI, like fourtytwo42 mentioned), but the current is at its peak when you're turning it off, so you want to do it quickly to minimize the heat dissipation in the transistor.You would usually put a resistor in series with the bypass diode, so the gate resistance is merely reduced, not eliminated. You want some resistance there to prevent ringing.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 16, 2018, 04:58:36 pm
content: It all depends what you want to acheive, don't forget to count the driver source impedance as part of Rg, that I why you don't always want a discreet resistor! If you have an inductive load and freewheeling diodes then there reverse recovery energy is an important part of the overall equation. Use of a scope to measure real results or modelling in Spice helps a great deal in determining final valuesRegarding the pulldown resistor again it depends upon the mosfet threshold voltage and the driver charecteristics, the IR2110 has UVLO circuits that prevent its outputs exceeding most mosfet thresholds during supply transitions but many drivers do not contain uvlo circuits.

author: pzil
author_rank: Newbie
timestamp: February 17, 2018, 02:56:21 pm
content: If you have an inductive load ?1K Pull-down resistor?Best adjusted to 10K ?and the diode to confirm whether to series resistance?You better confirm by oscillograph measuring Vgs and Vds position of the waveform to confirm?

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 17, 2018, 03:31:31 pm
content: You are allowed to use 200V rated MOSFET on 150VDC bus only if you have a grey beard.Leo

author: ocset
author_rank: Super Contributor
timestamp: February 18, 2018, 06:24:51 pm
content: I think the higher 10k or so was recomended because in the high side if you are using a bootstrap driver then you dont want to be sourcing much current out of the probably small bootstrap capacitor....but you seem to say you dont have a hi side fet anywhere , even though you are using a driver which has a high side bootstrapper...so therefore i guess you can keep your 1k if you like.As long as you can provide for the small current that will flow through it.....I like low value GS resistors i must admit..becuase of that fact that you can get esd damage to fets, and GS resistors help snub this out....eg when the circuit is powered off and someone touches it etc.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: February 18, 2018, 06:41:34 pm
content: Quote from: treez on February 18, 2018, 06:24:51 pmYep, if you zap the gate with standard HBM ESD gun the 1.5k gate-source resistor will bring that 2kV discharge to much safer 1kV level (assuming Rgs has zero inductance.)Leo