How to calculate switching energy affected by gate resistor?

switching energy?

Hi!

If I switch voltage to Mosfet or IGBT gate, it charges gate capacitance and takes certain amount of energy. How does it affect If there is gate resistor which restricts current flow. I know it decreases the switching energy but what is the equation to calculate it?

Re: switching energy?

it does not decrease the energy that is needed to put into the transistor gate if the time is long enough to charge the gate to the driving voltage. The resistor does increase the switching time.

Look at the Q-V curve from the data sheet. Integrate the energy E=Q*V over the voltage swing. (Remember that the units of volt are joules/coulomb.)

Re: switching energy?

The gate resisitor has no effect on the energy required to switch the MOSFET on and off. Only the MOSFET's switching speed will change and that will affect the MOSFET's switching losses, but that is entirely different from the gate drive energy.

To calculate the required gate drive power, look at the gate charge curve for your MOSFET. Read off the chart the gate charge at the maximum Vgs that you will be driving the MOSFET with. If you are driving it with 5V, see what the gate charge is at 5V; for 10V drive, go further up on the curve and find the corresponding gate charge for that drive voltage.

So now you have: Pg=Qg*Vg/Tsw=Qg*Vg*fsw

This is the power required for the gate drive, REGARDLESS of the value of the gate drive resistor. Simply because you need to supply it to charge the gate to the same voltage every time. And then you discharge the gate, losing that energy.
It is true that more of it is dissipated in the resistor and less in the driver transistors, if the resistor is larger. But the total power required to drive the gate is the same.

Re: switching energy?

I think you can't decrease switching energy by series a resistor

between driver and the gate. because the gate's input capacitance

is also charged to full voltage. you only slow down the mosfet

and lead to great mosfet switching power loss.

best regards

teepee said:

Hi!

If I switch voltage to Mosfet or IGBT gate, it charges gate capacitance and takes certain amount of energy. How does it affect If there is gate resistor which restricts current flow. I know it decreases the switching energy but what is the equation to calculate it?

Click to expand...

but..

Thanks for replies.

I think the Figure 77 in HCPL-316J datasheet put me into confusion. There is a plot "Switching Energy vs. Gate Resistance". I think that plot shows how much energy is generated in IC output buffer transistors with different gate resistor values.

Re: switching energy?

That would make sense.
The total energy (power) is the same, but how much of it is dissipated in the drivers depends on the external gate resistance.