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.
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.