I am trying do design a HF power supply/inverter for school. I stumbled across a problem.
When i feed a IRF740 a sine wave on the gate, the MOSFET gives a modified sine wave on the drain. Why is that? Can this be filtered?
Some screenshots of simulation:
The diodes and transformer are ideal.
This sounds normal. Look carefully at how the FETs are biased. Almost certainly they don't stay in the linear region over the full range of input signal. Since you are getting blips, look closely at what is happening when the gate voltages are low. Probably the FETs are already mostly off with a gate voltage above the minimum, so lowering it doesn't cause much change on the output.
You said this for high frequency. Depending on how high "high" is, you actually want the FETs to turn off for part of each cycle, more like a class C amplifier than a class B. You then reduce the harmonics with a L-C tank on the output.
The gate source threshold voltage for an IRF740 is typically 3V. At this voltage, there will be about 250 micro amps flowing down the drain. By 4.5 volts this will have typically have risen to 700mA: -
With 5V on the gate you might expect to see nearly 3A flowing and at 5.5V on the gate you might expect a drain current of over 6A.
It's very non-linear and your driving sinewave for a lot of its cycle will be below the gate threshold hence you just see the tops of the sinewave in your picture: -
You've made it tad worse by using a diode in series with the gate drive voltage. This isn't needed because MOSFET gates don't care if they get reverse biased (absolute maximum rating for gate-source is +/- 20V).
What I'd suggest doing is driving the two gates from a transformer with a cetre-tap and bias that centre-tap at maybe 2 to 3V and feed a single sinewave into the primary - then look at your simulation results to see the improvement.
