Can I ask why FET or IGBT need gate drivers, though they are designed to use voltage amount on the gate?

Thread Starter

earlycastle

Joined Apr 18, 2021
5
I've always curious about that.

BJT triggered by base current so, I understand they need current amplifying on base.

But, FETs or IGBTs are triggered by gate voltage(as I know) then, WHY, WHY they need gate drivers?
 

crutschow

Joined Mar 14, 2008
27,697
WHY, WHY they need gate drivers?
Why why indeed.
You only have half the story.

It's true that FETs are voltage operated devices and take no DC gate current in the ON or OFF state.
So gate drivers aren't usually necessarily for low frequency switching or slow ON/OFF operation.

But look at high power MOSFET data sheets and note their high gate capacitance.
So to achieve rapid rise and fall time and keep the MOSFET from lingering in the active region during switching to minimize dissipation at the typical high switching frequency of devices such as SMPS, you need to supply a lot of gate transient current (often more than an amp) to rapidly charge and discharge that capacitance.
That's provided by a gate driver.

Make sense?
 

andrewmm

Joined Feb 25, 2011
1,751
I've always curious about that.

BJT triggered by base current so, I understand they need current amplifying on base.

But, FETs or IGBTs are triggered by gate voltage(as I know) then, WHY, WHY they need gate drivers?
One other reason that gate drivers can be used, is that the control logic might only be putting out say 0 to 3v3 , where as the IGBT might need a much higher voltage to drive it,
 

BobTPH

Joined Jun 5, 2013
3,652
And a third....

When an N channel MOSFET is used as a high side switch, it needs a voltage that exceeds the voltage on the drain. Often that is the highest voltage available. A high-side gate driver uses a bootstrap capacitor to provide that higher voltage.

Bob
 

Thread Starter

earlycastle

Joined Apr 18, 2021
5
Thanks for all the reply of you.
And I guess I could understand about that.

So, as I understand, FETs or IGBTs triggerd voltage, YES, But, "The ENOUGH VOLTAGE ON ENOUGH SHORT PERIOD for rising or fall time to trigger gate" was the essential matter for it.

I think I've been locked up in too slow world. I had to think about the electron movement speed.

And, yes, bootstrap. mcu voltage can't fulfill the maximum gate voltage(above the drain voltage), that's the one of the main reason.

Thank you.
 

andrewmm

Joined Feb 25, 2011
1,751
Thanks for all the reply of you.
And I guess I could understand about that.

So, as I understand, FETs or IGBTs triggerd voltage, YES, But, "The ENOUGH VOLTAGE ON ENOUGH SHORT PERIOD for rising or fall time to trigger gate" was the essential matter for it.

I think I've been locked up in too slow world. I had to think about the electron movement speed.

And, yes, bootstrap. mcu voltage can't fulfill the maximum gate voltage(above the drain voltage), that's the one of the main reason.

Thank you.
Well done you for getting back.

Thank you,
 
Why why indeed.
You only have half the story.

It's true that FETs are voltage operated devices and take no DC gate current in the ON or OFF state.
So gate drivers aren't usually necessarily for low frequency switching or slow ON/OFF operation.

But look at high power MOSFET data sheets and note their high gate capacitance.
So to achieve rapid rise and fall time and keep the MOSFET from lingering in the active region during switching to minimize dissipation at the typical high switching frequency of devices such as SMPS, you need to supply a lot of gate transient current (often more than an amp) to rapidly charge and discharge that capacitance.
That's provided by a gate driver.

Make sense?
Thank you for such a clear and comprehensive explanation. it's good that on this forum I can find answers to all my questions
 
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