Gate resistors 6 amp mosfet driver

Thread Starter

MGPERF

Joined Jun 8, 2021
70
Hello,

I am using a TC4425MJA 6 amp mosfet driver and am wondering what resistors to use. I am thinking 2.2 ohm in order to make it so I still have the full 6 amps at 15 volts. However is this too low? Would this almost be the same as having no resistor?
 

Papabravo

Joined Feb 24, 2006
18,964
The size of the gate resistor has nothing to do with the amount of drain current. On the other hand, it would be a poor choice run a device up to and including it absolute maximum limits. Is there some reason you are heading in this direction?
FYI, the main purpose of the gate resistor is damping.
 

Thread Starter

MGPERF

Joined Jun 8, 2021
70
Thanks for your response I calculated the gate current as 2.2 ohms and 15v so a max of 6.8 amps or just above the max output of the driver. Forgive me if I'm wrong but say if I put a 10 ohm resistor in I'd only be getting 1.5 amps. Is my thinking correct or is there another more important factor? I am using the full 6 amps because I want to use mosfets with less resistance and therefore a higher gate charge. Hoping it will cut down on heatsinking requirements.
 

Papabravo

Joined Feb 24, 2006
18,964
Thanks for your response I calculated the gate current as 2.2 ohms and 15v so a max of 6.8 amps or just above the max output of the driver. Forgive me if I'm wrong but say if I put a 10 ohm resistor in I'd only be getting 1.5 amps. Is my thinking correct or is there another more important factor? I am using the full 6 amps because I want to use mosfets with less resistance and therefore a higher gate charge. Hoping it will cut down on heatsinking requirements.
It doesn't sound like you know what you are doing. Gate current is not measured in ohms, and there is no DC current into or out of the gate; a gate looks like a big capacitor. You should go back and review the basics of MOSFET operation.
Drain current depends on gate voltage. In order to minimize switching losses you need AC gate current to charge and discharge the gate capacitance and pass through the Miller plateau quickly.
 

Thread Starter

MGPERF

Joined Jun 8, 2021
70
Papabravo, yes I understand the gates are voltage controlled however I am under the impression that you still need a lot of gate current to switch mosfets quickly (gate charge 86 Nc). If I put a 10 ohm resistor in for R15 would it limit the amount of current out of the driver to 1.5 amps?

1657067640464.png
 

sagor

Joined Mar 10, 2019
713
Current to the gate is in the order of uA or even nA, very low (gate to Source leakage). Initial current to the gate may depend on gate capacitance, but that is very low as well. The gate is a voltage control, and gate current has little to do with turning the MOSFET on, only the voltage (in basic terms). The gate does not draw any real current. Current is from Drain to Source
 

Papabravo

Joined Feb 24, 2006
18,964
Papabravo, yes I understand the gates are voltage controlled however I am under the impression that you still need a lot of gate current to switch mosfets quickly (gate charge 86 Nc). If I put a 10 ohm resistor in for R15 would it limit the amount of current out of the driver to 1.5 amps?

View attachment 270835
I don't think it works that way, but you go ahead and do whatever you want. In the following simulation I have an ideal voltage source of 15V going through a 2.2 Ω gate resistor ant the current NEVER reaches anything like 6.8 Amperes. The Maximum Drain current is controlled by the 12V source and the 2Ω drain resistor. The current into and out of the gate is an AC current and it happens pretty fast.
1657071432005.png
 

ronsimpson

Joined Oct 7, 2019
2,242
There is no reason you have to use 6A for the gate current. The gate current is just for the time it takes to charge up/discharge the gate capacitance. ( also the G-D capacitance )
 

Sensacell

Joined Jun 19, 2012
3,031
Current to the gate is in the order of uA or even nA, very low (gate to Source leakage). Initial current to the gate may depend on gate capacitance, but that is very low as well. The gate is a voltage control, and gate current has little to do with turning the MOSFET on, only the voltage (in basic terms). The gate does not draw any real current. Current is from Drain to Source
But that all goes out the window as soon as you try to switch the MOSFET quickly, you then need to supply large gate currents both to charge and discharge the gate capacitance.
That's what gate drivers are for.
 

Papabravo

Joined Feb 24, 2006
18,964
But that all goes out the window as soon as you try to switch the MOSFET quickly, you then need to supply large gate currents both to charge and discharge the gate capacitance.
That's what gate drivers are for.
All that is true, but you don't get the gate current by dividing the voltage of the source driving the gate by the gate resistor. That amount of current will never flow.
 

Ian0

Joined Aug 7, 2020
5,816
The TC4425 already has an output resistance of about 3Ω, so the peak output current will be drive voltage divided by 5.2Ω not 2.2Ω.
 
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