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.

Easy gate driver testing - FOD3120V, IGBT, MOSFET

Easy Gate Driver is one of the product designed by me to ease the research and development in power electronic.

It is useful for the people who just started on machine drive, power converter and power inverter.

I have decide it is the best for me to write on blog before migrate it to my website. (testing purpose and getting feedback)

My gate driver allow the changing of optocoupler which enable replacement when it is spoiled, also it has given me to try out various optocouplers in the market.

My two favourite optocouplers so far are Vishay VO3120 and Fairchild FOD3120V. The reason behind is always good performance over price ratio.

Avago Technology had wide selections of gate drive optocouplers but the price always higher than the other manufacturers in same specifications.

Below is the pictures of my gate driver, I named it Easy Gate Driver, meaning it is easy to use.

It is able to drive two independent channels of PWM which make it suitable for synchronous switching DC converter (buck converter and boost converter) or controlling two independent DC converters. Best of all, it can be cascaded to drive multiple phase / multiple level inverter.

Two type of popular power transistor are MOSFET and IGBT, so I will use these two type for testing purposes.

I had chosen three power transistor from element14, the price range is from RM5 to RM10.

First:

Second:

Third:

The optocoupler used in this testing is FOD3120V.

BTW, I really like element14 packaging when they kept the IC in small boxes. The boxes are reusable for keep small electronic components.

Results:
I am using PIC18f4553 to generate PWM. (Dual Mode)

1. No load waveform. (40kHz)

I generated two inverted PWM signal with duty cycle of 50%. 

The output signal voltage is +15V and -5V, the negative bias is added to reduce the switch off time.

2. First load testing. (IGBT, SINGLE, 1.55V, 32A, TO-247AD)

3. Second load testing. (IGBT, 600V, 34A, TO-220)

4. Third load testing. (MOSFET, N-Channel, 600V, 19.3A, TO-220FP-3)

Much to my surprise the rise time and fall time of MOSFET is much higher than IGBT. After checking the datasheet, page 5, the rise time, fall time as follow:

Turn-on delay time td(on)	11ns
Rise time tr	9ns
Turn-off delay time td(off)	71ns
Fall time tf	9ns

Way lower than my testing, but gate resistance used by INFINEON also much slower (3.4 ohm) which higher gate current to charge the gate capacitance.

Conclusion:

Gate resistance has effect on the rise time and fall time, lower gate resistance allow higher current to charge the gate capacitance, thus the rise time is much faster. 

The negative bias is added to allow the gate capacitance to discharge faster thus the fall time also much lower.

NOTE:

I may have make a mistake by mixing up the results of mosfet and igbt (very unlikely),

nevertheless, it is proven, the Easy Gate Driver able to drive MOSFET and IGBT.