alexan_e
Administrator
Hi
I'm using the attached schematic in a design I made which consists of 2 H bridges to drive a bipolar motor (2ohm , 1Amp).
I'm using a 50KHz switching frequency with a chopper to drive the motor with constant current.
The schematic is actually 4 times the circuit I'm showing here and I need it to be able to switch the output fast.
I know that there is a gate capacitance that I have to charge so that the mosfet becomes active so I have used a driver with a 50ohm gate resistor which shows a low on delay.
My problem is switching off thew mosfet, I tried many things and I saw that to switch the mosfet off quickly I have to discharge the mosfet gate capacitance quickly and the only way I was able to do that was by using a 100 ohm pull up resistor to the gate (graph1).
When I tried with a 1K resistor the the off delay is so big that the square wave is very distorted (graph2) , and if I change to 10k I don't get a square wave at all (in the output)
IS there a better way to switch fast a mosfet because this 100 ohm resistor also pulls current that delays the on time.
In the graphs green trace (left axis) is input voltage to the base of the transistor
and the red trace (right axis) is the voltage at the drain of the mosfet (output)
Could you please tell me if there is a better way to drive the mosfets using discrete components?
Can i use some other way to discharge the gate fast?
Alex
I'm using the attached schematic in a design I made which consists of 2 H bridges to drive a bipolar motor (2ohm , 1Amp).
I'm using a 50KHz switching frequency with a chopper to drive the motor with constant current.
The schematic is actually 4 times the circuit I'm showing here and I need it to be able to switch the output fast.
I know that there is a gate capacitance that I have to charge so that the mosfet becomes active so I have used a driver with a 50ohm gate resistor which shows a low on delay.
My problem is switching off thew mosfet, I tried many things and I saw that to switch the mosfet off quickly I have to discharge the mosfet gate capacitance quickly and the only way I was able to do that was by using a 100 ohm pull up resistor to the gate (graph1).
When I tried with a 1K resistor the the off delay is so big that the square wave is very distorted (graph2) , and if I change to 10k I don't get a square wave at all (in the output)
IS there a better way to switch fast a mosfet because this 100 ohm resistor also pulls current that delays the on time.
In the graphs green trace (left axis) is input voltage to the base of the transistor
and the red trace (right axis) is the voltage at the drain of the mosfet (output)
Could you please tell me if there is a better way to drive the mosfets using discrete components?
Can i use some other way to discharge the gate fast?
Alex