I am trying to calculate the required gate current to charge a power mosfet ( or more specifically three in parallel, should just be x3 though). After my research I've come across two conflicting formula's to calculate the amount of current needed to fully charge the gate capacitance of the power mosfets.

Power Mosfet =

__STP80NF55-08__, (Datasheet: http://pdf1.alldatasheet.com/datasheet-pdf/view/24515/STMICROELECTRONICS/STP80NF55-08.html)

__: (Source: http://ww1.microchip.com/downloads/en/AppNotes/00898a.pdf)__

**Formula 1****PGATE = 1/2 CGATE * (VGATE^2) * F**,

where:

QTOTAL = Total Gate Charge Value (most of the

time given in nano-coulombs)

CGATE = Total Gate Capacitance

VGATE = Gate Drive Voltage

**CGATE = QTOTAL / VGATE**-->

Qgate = 115 nC from datasheet.

Vgate = 12V from my circuit.

F=20,000 Hz = PWM switching frequency

Therefore my calculated Cgate = 10 nF

Using this the

**power**= 1/2 ( 10nF ) * (12 ^2) * (20,000) =

**.0138 Watts**

I inferred that I=P/V , therefore the gate current should be .0138/12 =

.00115 =

**1.15 mA**

This is obviously wrong because it definitely needs more than 1.15 mA to charge the the gates.

__(Source: http://www.jimfranklin.info/microchipdatasheets/00786a.pdf)__

**Formula 2:****IG = QG/t(transition)**

IG = Gate Current

Qg= 115 nC from datasheet (Gate charge)

t(transition) = desired transition time

t(rise time) = 85nS from datasheet

t(fall time) = 25nS from datasheet

Now I assumed here we would want to use the rise time because were concerned with turning the mosfet ON, so:

**IG**= (115nC) / (85nS) =

**1.35 Amps**

Now this seems more realistic, maybe a little high, but is completely contradictory to the first method / formula of calculating gate current.

Which method / formula is correct? How can they be so contradictory? The second method doesn't account for the frequency at all though, which is strange.