I want to drive a MOSFET with a 50Hz, 50% duty cycle clock pulse. The voltage to be switched is as high as 300V DC. The section inside the dashed lines will stay electrically isolated from the rest of the circuit. I am planning to use an optocoupler for isolation (I'm open for any other suggestion).
Best solution I can find is using a voltage divider network (as seen in the second schematics) and control it with the optocoupler. But I don't think that this is an optimal solution, because there are two major problems.
- I have to use a bulky power resistor for R1.
- Switching losses. Gate capacitance of a typical MOSFET is about 5nF. Time constant for a 5nF capacitor charging over a 20k\$\Omega\$ resistor is 100\$\mu\$s which is 0.5% of the switching period. The maximum current through RL will be 10A, and the Rds,on resistance of the MOSFET will be 50m\$\Omega\$ at most. So the switching loss will approximately be
\$(10A)^2 \times 50m\Omega \times 0.5\% = 25mW\$.
I can increase the values of R1 and R2, but this time the switching loss will increase. There is a trade off. Either way there will be a heat source in the circuit, and some energy will be lost.
What is the efficient way of driving a MOSFET like this?
EDIT: What about this circuit?
With enough dead-time between on states of the two optocouplers, the required rated power of the R1 resistor may stay below 1W. Do you see any problem in this new circuit model?
EDIT2: I decided to put external darlington transistors for supplying more current to the MOSFET gate.
The gate voltage will drop down to 1.4V at low state, at which most MOSFETs are completely turned off. Is this new circuit feasible?