A3986 Rgate berechnen

[Alex (Gast)]

Guten Tag, ich kann nicht verstehen, wie man die Widerstande für MOSFET 
berechnet. Ich habe schon im Internet recherchieren. Folgente 
Information gefunden:

Schaltung:http: 
www.allegromicro.com/en/Products/Part_Numbers/3986/3986.pdf


 Pull up on resistance is about 40 ohm, Pull down On resistance is about 
19 ohm @ 25 mA. I think you are being a little bit optimistic here. This 
little fellow does not have the driving capacity for short turn on and 
off times needed to control FETs like IRF540 at high currents, for 
example the total gate charge for a gate voltage of 15 volt is about 
70nC.

Cgate= Qtotal / Vgate = 4.6666 nF= 4666.6 pf

using Qtotal = Icharge * T charge you can calculate the current 
necessary to get the Switching time desired, lets say you want a Tcharge 
of 80nS then you need an Icharge= 70 nC / 80 nS = 875 mA this is an 
average current, the charging current is not constant but this is the 
calculation necessary to find the right driver for a Mosfet.

Try using the A3986 parameters to calculate the Tcharge using the 
following formula:

Tcharge = ((Rdriver + Rgate) * Ctotal)*TC

Where:
Rdriver = RDS-ON of the output driver stage
Rgate = any external gate resistance between
the driver and the gate of the MOSFET or
IGBT
Ctotal = the total gate charge value divided by
the gate voltage
TC = number of time constants= 3 (for 95% of the charging voltage after 
time Tcharge)

Qtotal = 70 nC
Vgate = 10V
Tcharge = 80 nsec.
TC = 3
Rgate = 0 ohms
Rdriver = (Tcharge/TC*Ctotal) - Rgate
Rdriver = (80 nsec./ 3 * 7 nF) - 0 ohms
Rdriver = 3.8 ohms (versus your Pull -up on resistance of 40 ohm)

Was heisst Tcharge? Oder wie gross soll sie werden? Wozu ist die ganze 
Geschichte mit Rgate?