MOSFET gate driver requirements

RJJ, 

Can I ask what your end application / topology is? We do have some online programs that are meant to assist with the design process. 

The gate driver strength you need will be dictated by the switching speed / frequency you require. Depending on your application there may be a better forum to assist you with gate driver selection. 

Brett,

This is for a 3 phase motor controller, so I would need 3 half bridges.  I'm planning on using a 5kHz PWM control input.

RJJ

Yeah, pretty low frequency. Just so you know, we actually have some pretty awesome FET motor drivers from the motor control group that have all three phases integrated. They should be able to assist you here. We also have a number of reference designsthat feature our FET and gate drivers paired together.

https://e2e.ti.com/support/applications/motor_drivers/

Can I ask how much current the motor requires, in terms of RMS and peak current under stall conditions?

The motor current is 25A RMS and 30A peak. The motor voltage is 75VDC.

It would be nice to understand the calculations, so I could adjust for different requirements.

Thanks,
RJJ

RJJ,
Ok I just wanted to make sure you really wanted a FET that low resistance. But thats pretty high power so the CDS19536KTT should be fine.

Basically the equations are you turn on will be dictated by gate drive current strength, and Qg of the FET.

FET will start to really turn on at Qg(th) and will continue to turn on until your desired Vgs is achieved according to Figure 4 on the datasheet (also on the front page).

Its a simple t(on) = Qg / Idrive.

Sample for pull down strength.

Brett,

Is this correct?

Ipeak=Qg(th)/(td(on)+tr)

Ipeak=24nC/(13+8)ns=1.14A

Will I damage the gate driver if I exceed the 1A peak drive current?

If I use a 15V supply voltage on the LM5109A with a 20 ohm gate resistor, the worst case gate current would be:

Igate=Vg/Rg=15/20=.75A

Thanks,

RJJ

RJJ, 

There is not a peak current the gate of the FET can handle, but you do need to make sure that the gate voltage of the FET is never exceeded. 

0.75A or 1A is really no concern at all - in fact this is pretty weak turn on. 

Since you have a 15V gate drive, you are still well under 20Vgs max rating of the FET so long as you don't have excessive ringing. 

Brett,

Is that the correct calculation for Ipeak which as I understand is the peak gate current required to charge the gate capacitor of the MOSFET? 

I was more concerned about overdiriving the 1A peak output current of the driver.  Is that not the case?

Thanks,

RJJ

RJJ, 

I think you are thinking about this backwards. The timing values provided on the datasheet are not concrete, but rather they are dictated by the strength of the current they are being driven at. 

The driver is driving a capacitive load which at the start of turn on is zero impendance, such that the only resistance is the Rg of the FET, and the internal resistance of the driver. The voltage across those resistors will dictate the current and therefore the turn on time. 

Brett,

Do I not have to pay attention to the gate current?  What if the peak MOSFET gate current was 2 or more amps?  Would I damage this gate driver that has a peak drive current of 1A or will the turn on time just be longer?

Thanks,

RJJ

RJJ,
That is exactly correct. All you need to worry about is not exceeding the max gate voltage on the FET.

2A (or more is fine)