# calculating capacitance of transistor gates

• 6th July 2019, 00:55
Zak28
calculating capacitance of transistor gates
Is it as simple as C= V/Q for this part maximum gate capacitance at 15v would be 119e-9/15 yeilding 7.93nF?

https://i.ibb.co/7X79xqL/img.png

https://www.st.com/content/ccc/resou...DM00052344.pdf
• 6th July 2019, 01:34
KlausST
Re: calculating capacitance of transistor gates
Hi,

I'm not sure what your question is...

The output capacitance is acting as miller capacitance, thus ...
* Although the voltage at the gate changes only about 15V
* the voltage at the output capacitance may change 415V.
This causes high Q.

Klaus
• 6th July 2019, 02:26
Zak28
Re: calculating capacitance of transistor gates
Quote:

Hi,

I'm not sure what your question is...

The output capacitance is acting as miller capacitance, thus ...
* Although the voltage at the gate changes only about 15V
* the voltage at the output capacitance may change 415V.
This causes high Q.

Klaus

Hence max gate charge was determined at 415Vds was to obtain Vgs capacitance to obtain peak current which apparently is 7.93nF for 15Vgs at specified Ids.
• 6th July 2019, 11:32
FvM
Re: calculating capacitance of transistor gates
I don't understand the purpose of deriving a capacitance value from total gate charge, because there's no physical capacitor representing the charge. The real behavior is better characterized by Figure 8, gate charge vs gate-source voltage.

Total gate charge can be used to easily calculate the per pulse gate driver energy and average gate driver supply current.
• 6th July 2019, 11:33
CataM
Re: calculating capacitance of transistor gates
The effective input capacitance is calculated as in post #1. You have capacitance curves in the datasheet.
• 6th July 2019, 18:19
dick_freebird
Re: calculating capacitance of transistor gates
Total gate charge is useful to calculate switching losses
but treating it as a fixed capacitance will not be accurate
when it comes to transient switching behavior. Be sure
you want the right things.
• 6th July 2019, 21:12
Zak28
Re: calculating capacitance of transistor gates
This was to determine peak Igs current for gate driver which is 7.93nF for mentioned part.
• 6th July 2019, 23:11
CataM
Re: calculating capacitance of transistor gates
Peak current into gate of transistor is simply V_driver/Rgate. No need for any capacitance value.
• 7th July 2019, 01:12
Zak28
Re: calculating capacitance of transistor gates
Is Rgate the series resistor between gate driver output and transistor gate?
• 7th July 2019, 16:59
CataM
Re: calculating capacitance of transistor gates
It includes the gate driver's resistance and transistor's internal resistance as well.