How to source a 5A of current to charge the gate capacitor of a power MOSFET?

Thread Starter

Younes Thabet

Joined Jan 9, 2019
144
Hello all,
I have been checking the EVAL-ADUM4146 evaluation board for the ADUM4146 gate driver.
In this board, they are using a (2W, +15/-3, +93/-185mA) isolated DC-DC converter module which is the R12P21503D. And in the gate driver datasheet, it says:
  • 11 A short-circuit source current (0 Ω gate resistance)
  • 9 A short-circuit sink current (0 Ω gate resistance)
  • 4.61 A peak current (2 Ω gate resistance)

What I didn't understand is how come the R12P21503D can deliver that 4.61A peak current and sink the 9A current (pic1).
I know the time is very short probably in the nanoseconds but still?

Can someone explain this to me, please?

Also,
1. what's the difference between using DC-DC modules (pic1) and Isolated SMPS with a transformer (pic2) to create voltage rails for the gate driving circuitry, and just using a non-isolated supply while using pulse transformers instead (pic3)?
2. what are the pros and cons of using 2 separate biases for the high and low side of a half-bridge and using a single isolated bias (pic4)?

Thank you.
Regard,
 

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Externet

Joined Nov 29, 2005
2,236
Part1.- Seems you are mixing behaviors. If a mosfet is compared to a very sensitive relay, the gate is like the coil circuit taking almost no current and the contacts are the drain/source circuit capable of high current.
A mosfet gate current is near zero. Is almost insulated, you cannot apply current from a supply to it. You apply voltage and that will make the 'contacts' (as in the relay) to conduct.
 

Thread Starter

Younes Thabet

Joined Jan 9, 2019
144
Part1.- Seems you are mixing behaviors. If a mosfet is compared to a very sensitive relay, the gate is like the coil circuit taking almost no current and the contacts are the drain/source circuit capable of high current.
A mosfet gate current is near zero. Is almost insulated, you cannot apply current from a supply to it. You apply voltage and that will make the 'contacts' (as in the relay) to conduct.
Yeah, I know that MOSFET gates are high impedance when operated in DC voltage but in high frequencies, in order to charge the parasitic capacitances (ex: Cgd) you need a high current for a small period of time!
I might be wrong but that's what google taught me and TBH I am a little confused!
 

Externet

Joined Nov 29, 2005
2,236
If you are talking of high frequencies to be applied to the gate, you make no sense in using your "isolated DC-DC converter module" which is DC, not 'high frequency"
 

Thread Starter

Younes Thabet

Joined Jan 9, 2019
144
If you are talking of high frequencies to be applied to the gate, you make no sense in using your "isolated DC-DC converter module" which is DC, not 'high frequency"
The isolated DC-DC converter module is used to provide voltage rails +15/-3 for the gate voltage. see pic1
 

Thread Starter

Younes Thabet

Joined Jan 9, 2019
144
That is true, but how fast does you application require the MOSFET to turn on and off?
Isolation is unrelated to that.
I intend to use SiC Mosfets so probably 1-3MHz.
This is supposed to be a general-purpose gate driver so not necessarily have very high frequencies but I would like for it to be able to run at up to 5MHz.
 

Thread Starter

Younes Thabet

Joined Jan 9, 2019
144
To answer your question. The high current will come from a capacitor. You do not need a 5A supply for it to supply 5A of current for a fraction of a microsecond.
Is this capacitor part of the power supply or gate driver circuitry? how big is it approximately (value, voltage, package)?
 
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