# [SOLVED]calculating capacitance of transistor gates

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#### KlausST

##### Super Moderator
Staff member
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

Zak28

### Zak28

points: 2

#### Zak28

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.

#### FvM

##### Super Moderator
Staff member
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.

Zak28

### Zak28

points: 2

#### CataM

The effective input capacitance is calculated as in post #1. You have capacitance curves in the datasheet.

Zak28

### Zak28

points: 2

#### dick_freebird

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.

Zak28

### Zak28

points: 2

#### Zak28

This was to determine peak Igs current for gate driver which is 7.93nF for mentioned part.

#### CataM

Peak current into gate of transistor is simply V_driver/Rgate. No need for any capacitance value.

Zak28

### Zak28

points: 2

#### Zak28

Is Rgate the series resistor between gate driver output and transistor gate?

#### CataM

It includes the gate driver's resistance and transistor's internal resistance as well.

Zak28

points: 2