Gate driver charging current (tested using FOD3120V)

The basic of gate driver is providing sufficient gate voltage and gate current to turn on power transistor. The magnitude of these characteristics decide the turn on & turn off time of power transistor.

Most of the power transistor data sheet only show the gate voltage (below is taken from IR IGBT - irgs4620dpbf datasheet)

It always show the gate voltage (VGE) needed to drive ICE(A), but gate current is equally important to achieve faster switching.

The gate resistance (Rg) need to be perfectly tuned to get best performance (no overshoot and no undershoot)

Below is the picture for undershoot (from my point of view), the nicer the curve, the longer turn on time.

Then, this is my view of good switching on delay time,

I didn't have a picture for overshoot. Just imagine above picture with higher voltage spike inside the red circle.

Now about the gate driver IC (optocoupler), they always show the high output current capability:

FOD3120 – 2.5A

FOD3150 – 1.0A

Do we really need such high gate current?

Yes, we do need it. Nevertheless, only high inrush current is needed to turn on the gate but no needed (or maybe little current) to maintain the gate ON state. Let me show you guys.

Below is the point I probed,

Results (voltage spike is used to represent charging current:)

You can see that current only went through during turn on and turn off.

Let see the zoom version, charging gate:

Zoom version of discharging gate:

I increases the switching frequency to get nicer view of charging current, 

I am using the FOD3120V but I didn't push max current driving capability as it will increase burden to optocoupler and power supplies. Good on board isolated power supply is expensive.  

Conclusion:

Gate voltage is important for driving the power transistor but gate current is equally important to achieve optimum switching. Gate resistance need to be carefully selected to achieve fast voltage ramp without causing overshot.