MOSFET gate current calculation

I am calculating required gate current to turn on my MOSFET-BUK9M42-60E.
I am driving this MOSFET directly from microcontroller. It is logic level MOSFET.
I have these data from datasheet of MOSFET.

Vgs(th)=2.1V(max)
Vgs(supply from microcontroller)= 5v
Vds(battery voltage)=15v
Qg(total)=8.3 nC

Load details:
Switching frequency of load=1000Hz
Operating duty cycle of load=0 to 100%(consider 2% duty cycle for worse case analysis)

Can anyone help me to calculate gate current with these data?
I am confused whether to take Vgs(th) or Vgs(supply from microcontroller) to choose Qg. Because Qg depends on gate-source voltage.

Hi,

First:
Do you want to know peak current or average current?

* peak current determines switching speed. Peak current often is not determined by the switching device, but the driver circuit.
* average current in most cases is not much informative, mabe to calculate VCC current (only useful for very low power applications)

If there is low peak driving current, then V_DS rise and fall times will be higher causing increaed overall power dissipation.

Qg: simplly have a look at Figure13. It clearly says "V_GS" and not "V_GS_th". And you see that it depends on V_DS (miller effect)

Average current is: switching_frequency * total_gate_charge. independent of duty cycle.

*****
I assume you need to look for basic informations:
Every MOSFET and IGBT manufacturer and every driver_IC manufacturer should provide application notes on how to design and calculate driving circuits.
Read through them. In a forum we can´t give more detailed and more complete informations.

Klaus

Put the circuit diagram, how you want to handle this.
Switching frequency - 1 Khz, Time period = 1ms. 2% Duty cycle - ON time = 20 uS.
Assume 10% of ON time as Rise time and 10% as Fall time(Best case) - Time required to charge MOSFET gate 2uS. To provide 8.3nC charge in time 2uS, uC should able to provide ~4.2mA current pulse for every 2uS (Only during charging, discharging depends on circuit implementation).
Note: these are very conservative assumptions which i took for a rough calculation.

To provide 8.3nC charge in time 2uS, uC should able to provide ~4.2mA current pulse for every 2uS (Only during charging, discharging depends on circuit implementation).

Click to expand...

Need a series resistor (e.g. 50 to 100 ohm) to limit the uC port current within maximum ratings.

Need a series resistor (e.g. 50 to 100 ohm) to limit the uC port current within maximum ratings.
--Yes, i agree

Hi,

I need average current.

Yes, i can understand in forum, we can't give all the information.

But how I am calculating is--
Ig(avg)=Qg/Tsw(on)

Tsw(on)= on time of MOSFET(for that we need to consider duty cycle)
I am taking 2% duty cycle.

Tsw(on)= switching frequency*0.02

And one more information is, I got to know by diagram that I am not using any gate resistor.

Am I heading in right direction?

Hi,

From post#2:

Average current is: switching_frequency * total_gate_charge. independent of duty cycle.

Click to expand...

This is the average current drawn from the power supply.

Klaus

Your formula is the current that is needed for one single charge of gate with a given switching time.

None of both formulas includes "duty cycle".

Added:
Sadly this application note is currently not available at TI.com...
Thus an alternative source:
https://web.mit.edu/6.131/www/document/gate_drive.pdf

Doesn't make sense. Average current is simply Qg*fsw, it doesn't depend on the duty cycle or gate voltage rise time. But it's not clear why you are asking for average current, it's only relevant in terms of overall power consumption.

IO maximum current rating has to be considered additionally, it depends on voltage rise time and circuit impedance. As already mentioned, you'll usually need a series resistor to keep the microcontroller ratings.

Best guide to the exact gate current needed is the one by Balogh, as attached.
The Qg quoted in the FET datasheet will be for a certain Vds(off), which may not be what is your circumstance....so
read the pages of balogh...its kind of at the beginning.

- - - Updated - - -

Balog guide to gate drive
http://www.radio-sensors.se/download/gate-driver2.pdf