Calculating Required gate current for power mosfets

Thread Starter

surfline

Joined Aug 12, 2009
36
Hi,

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)

Formula 1: (Source: http://ww1.microchip.com/downloads/en/AppNotes/00898a.pdf)

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.

Formula 2: (Source: http://www.jimfranklin.info/microchipdatasheets/00786a.pdf)

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.
 

beenthere

Joined Apr 20, 2004
15,819
Neither of your calculations are absolutely correct. The FET is a field effect device, and so conduction is controlled by the electric field on the gate. The time to turn the device on and off is controlled by how fast that charge is placed on/removed from the gate terminal.

The Ebook chapter - http://www.allaboutcircuits.com/vol_1/chpt_13/2.html - gives you the means of calculating the time to move that charge.
 

Thread Starter

surfline

Joined Aug 12, 2009
36
What about how to calculate the amount of current needed to charge it. I am trying to calculate this in order to find the right driver IC to drive these power mosfets, so I need to know the right amount of output current.
 
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Papabravo

Joined Feb 24, 2006
19,551
There is no "right" amount of current. Any amount of current that you choose would charge the gate capacitance to a fixed value of voltage IF AND ONLY IF the gate represented a constant impedance load. Sadly it does not behave like a constant impedance load. There is a voltage plateau where the voltage stays constant for a period of time before resuming its rise to the final value.

One way to see this, for a specific device, is to use a SPICE simulation. It accurately models this behavior. Of course there is no substitute for building a breadboard to confirm the results of the simulation.
 
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