I'm trying to ride a full bridge inverter using SPWM to control the MOSFETs, the signal SPWM that I use to control the MOSFETs comes from an NI elvis, but it only provides 5V and I know the fact that the VGS from the MOSFETs must be higher than the VDS and I'm trying to convert at least 20VDC (I want 50VDC but that's another problem with the bridge), so im need a minimum voltage VGS of 20V. I know there are drivers as the IR2110/12 use to control the mosfets but at least for me it doesn't work, so i'm using the optocoupler 4n35 for amplify the input signal of SPWM,it works well while i'm not exceeding the value of 10Vcc, it amplifies the input signal well, but when i'm using more than 10Vcc in the optocoupler the output signal cross for zero and it goes to negatives values and it doesn't work for control the MOSFETs.
Can someone give me some alternative for my problem o the 4N35 cannot be used for my aplication?
These are the output signals and the circuit that i'm using.
Best Answer
Driving a MOSFET directly from an optocoupler seems a poor idea, especially for PWM operation, because you won't get the switching speed you need.
Dependings on the MOSFET, the voltage and current, the grid intensity needed during switching can range from hundreds of mA to several amperes. This is required to shorten the switching time and thus limit commutation losses.
With your design, during tur-off, the current from the grid is limited to:
$$ \frac{20V}{10k} = 2mA $$
This is orders of magnitude too low.
Also consider that, even during turn-on, optocouplers tends to be slowed down if the load resistance (10k here) is too high. I'm afraid the value of the resistor does not fit your needs.
I definitely think that using a MOSFET driver is a good idea. If IR2110 - like drivers do not work for your project, maybe you could consider components like MCP1406/07 ? They can be seen as a powerfull digital buffer/inverter, that can drive a MOSFET.