Gast
#1139518
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?