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How to calculate gate current of MOSFET gate or MOSFET gate driver

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kushal nandanwar

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I am using IRFB4110PbF MOSFET , how can I calculate current required by MOSFET gate at 5kHz frequency.
 

find Qg parameter in fet datasheet

current needed for gate drive is f(sw) * Qg
ensure your gate drive voltage is same as the datasheet conditions, otherwise it wont be the right formula that you see above.


Remember that Q=CV, where v is the gate voltage that you are driving up to, and Q is the charge needed to get it there, and c is the capacitance seen by the gate drive, which is a combo of gs cap and dg cap...especially with the miller effect
 

Hi,

Your Mosfet is from International Rectifier.
They have very good documents / application notes.

Read through them.

To your question:
Your question is about average current? Or peak drive current?

For average current you need V_DS, V_GS, frequency, c_iss, c_oss, and switching current.

Klaus
 

average current and peak drive current both.
VDS is 24V
VGS is 12V
frequency 5Khz
 

Hi,

Google for "gate drive current application note site:www.irf.com" and find what you are looking for.

Klaus
 

treez said:
find Qg parameter in fet datasheet

current needed for gate drive is f(sw) * Qg
ensure your gate drive voltage is same as the datasheet conditions, otherwise it wont be the right formula that you see above.


Remember that Q=CV, where v is the gate voltage that you are driving up to, and Q is the charge needed to get it there, and c is the capacitance seen by the gate drive, which is a combo of gs cap and dg cap...especially with the miller effect
Click to expand...

Means if Qg = 14nC
and f(sw) = 500Khz

then driver current 0.007A
 

Hi,

Qg in datasheet is given for testconditions. They will not meet the conditions of your application.

But for a raw estimation of average current it may be OK.

Klaus
 

KlausST said:
Hi,

Qg in datasheet is given for testconditions. They will not meet the conditions of your application.

But for a raw estimation of average current it may be OK.

Klaus
Click to expand...

if
0.007A is average current , then Why we need 3A to 10A current driver in buck convert to drive MOSFET.
 
that is instantaneous current not average current, the fet needs a zap of current to bring it on quickly
 

Hi,

Let's say you want to switch the FET within 30 ns, then you need to provide the charge of 14nC in 30ns.
This means you need need about half an ampere. ==> I = Q / t,
But this current will not be constant during this 30ns, so there will be an even higher peak current. Expect at least 1A.

Klaus
 
Please confirm whether I have understood this correctly. I am selecting isolated DC-DC supplies for power to the gate drivers for FGA 25N120.

The total gate charge is given as 200nC. So for a switching frequency of even 20kHz, the time period is 50usec so on an average, the power supply must be able to supply an average current of at least 200nC/50usec = 4mA. Is it really that small? Obviously I am the capacitor overkill kind of guy so I will give each driver a 470uF cap right next to it which will take care of all surges etc.

Only question is whether the 4mA is sufficient to replenish the charge every cycle. Also will the DC-DC converter be able to provide the surge inrush current that is drawn by the 470uF cap the first time it charges.

- - - Updated - - -

So is something like this OK? It is rated at 66mA output at 15V and 100uF max output capacitance. So I'll have an output capacitance slightly less than 100uF. But is this converter OK as an isolated gate drive power supply?

https://in.element14.com/xp-power/itb0515s/dc-dc-converter-15v-0-0667a-sip/dp/2475857
 

Hi,

4mA seems OK.

Obviously I am the capacitor overkill kind of guy so I will give each driver a 470uF cap right next to it which will take care of all surges etc.
Click to expand...
Typically you use bootstrap circuits with capacitors 100nF. .. 1uF. They have the same job, with the same gate currents...

My hint: instead of using a bulk electrolytic capacitor use a much smaller ceramics capacitor... and often the voltage is more clean)
A standard line frequency electrolytic capacitor is about useless for this application.

The DCDC converter has a coupling capacity of 50pF. But with a high dU/dt of the output signal you may expect current peaks. To avoid this I recommend a common mode choke at the output followed by a ceramics capacitor.

Klaus
 

What kind , value and rating of common mode choke?
 

Hi,

rated DC current is known.

Value depends on voltage (VDS), frequency and max. expected current.

With I and U you can calculate XL.
then with frequency you can calculate L.


Klaus
 

I'm confused. What does Vds have to do with it? Do you mean Vgs?

And current is gate current right?
 

Hi,

You gave no schematic, so i can only guess.

With a halfbridge design the upper highside FET gate supply voltage (often bootstrap) shifts with the output voltage = power power supply voltage. I guessed this is VDS (when FET is high impedance).
To be true i don´t know, but you should know.

Klaus
 

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