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Gate driver circuit design

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Xavier Pacheco

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Hi,

I'm trying to calculate the gate resistors for the MOSFET that I selected which has a gate resistance of 1.3 ohms. Well, I attach its electrical characteristics (Parameters.PNG). Also, I add the Vgs vs Total charge curve. I would like to find the suitable configuration for this application (See Gate1.PNG an Gate2.PNG). Some of the gate driver characteristics are also attached [GateDriverParameters.PNG].

The gate driver power supply is 12V.
The sink/source capability of the driver is 5A.

So taking into account the worst scenario where the charging current is equal to the peak current of the driver (5A)

RGon = 12/5 - (0.55 + 1.3) = 0.55 ohms
where 0.55 ohms is the Rds(on) or source resistor of the driver and 1.3 ohms is the mosfet gate resistance.

For the discharge current
ROff = 12/5 -(0.7 + 1.3) = 0.4 ohms
where 0.7 ohms is the sink resistance of the driver.

It's supossed that the charging peak current has to be less than driver peak current (5A). So, how do I know the real charging peak current?
Is this approach right?

In the Gate2.PNG picture, do I need the diode and the discharging resistor?

I'm really stuck at calculating the right gate driver configuration. I will appreciate your help so much.

Additional information:
PWM frequency (gate driver input) = 25 kHz
VDS is about 150-170VDC
The application refers to a Brushed DC motor controller using low side switching as shown in attachment Scheme.PNG

Datasheet of the Mosfet: https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d4625b3ca4ec015b3e42ba4a0744

Gate driver: https://www.mouser.com/datasheet/2/196/Infineon-2EDN752x-2EDN852x-DS--DS-v02_05-EN-1225984.pdf

Useful document about gate resistors: https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d462518ffd8501523ee694b74f18
 

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dknguyen

Well-Known Member
Most Helpful Member
Why don't you just leave space for a gate resistor but install a 0hm resistor (or just short it) and run it like that so the gate driver switches the MOSFET full-blast. Then scope it during operation while increasing the power levels. Scope the all the terminals of the MOSFET to see if there is excessive oscillations, ringing, transients, EMI, etc. As for gate driver heating, monitor the temperature to see if it's heating more than it should. Do all this to see if you actually need a gate resistor.

Similarly, it can be difficult to predict whether you need a diode, discharge resistors, gate clamp diodes, snubbers or other similar things. But if you're worried about ringing and such issues, leave space for those too and scope it in operation to see if they are necessary.

Things that increase the liklehood you may need them:
1. high frequencies (for the gate drive)
2. high frequencies AND high currents
3. long traces or large loops in your gate drive current circulation path
4. A gate driver capable of large peak currents relative to the gate charge on your MOSFET. For this criteria, your driver is 5A and your MOSFET total gate charge is ~150nC so I wouldn't your MOSFET inherently turns on too fast to cause ringing or EMI without a gate drive resistor.

If you can get away with slamming the transistor on and off as fast as possible, you should since it's more efficient that way. Don't use a gate resistor unless you need to reduce gate driver heating or slow the rise/fall time to alleviate ringing, oscillations, transients, or EMI.

Whether you get problem such as excessive EMI, ringing, oscillations, noise, latch up, etc are not things you can do with paper napkin calculations. You need to either simulate it or physically test it to figure out these things, and even simulation can miss these issues since it depends on trace length, parasitics, and layout which you often don't have or can't measure.

Since there exist applications that can't be testing gradually ramping up power/current/frequency and must be tested full-blast and if they fail, will fail immediately and catastropically, I won't say it's not always more trouble than it's worth to calculate/simulate gate drive resistors to try and predict it ahead of time what you need. Or unless you're super trying to optimize things but

But I think often it is a waste of time since there's lots of variables involved and you often don't have nearly all the information you need to do a helpful simulation. Just leave space for the protective gate drive components you think you need and test.

One more thing...your datasheet says MOSFET. Your circuit symbols says IGBT. There is a difference between the two and IGBTs have some gotchas that you may need to provide space/provisions for things like Miller Current/latch up in case you end up needing them (i.e. miller clamp, negative gate drive voltage , etc).

The discharge resistor/diode is probably not needed if you're using a MOSFET. But leave space for it if you're worried or just want to experiment with different rise/fall times. It's the simplest way to deal with Miller Current and Latch-up but that's more of a problem for IGBTs than MOSFETs.
 
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