Low pass filter help - pure sine wave inverter

[George]

I've been designing a pure sine wave inverter (linear motor drive) - but come a little unstuck with filtering the wave form, at the moment I think I may need a redesign but I'm hoping someone here can help point me in the right direction with my current build.

I need a sine wave to drive a coil with a magnet inside, back and forth (quietly hence sine) I've made the PIC output a pseudo full wave rectified signal with PWM, then another output toggles every half cycle, these signals go to an AND gate and inverting buffer which gives me a PWM output alternating on two pins. the theory was to build a half bridge and have the top transistor pwm its half wave, then the bottom do the same. So I built it up with a center tap transformer providing top, bottom and 0V lines, look it up on a scope and thats what it does - yay! If I filter it with a resistor and capacitor it looks just about like a bought one.

However things act a little crazy when I hook my big coil up to it. If for example it's PWMing at 50% the high side FET turns on and the voltage goes to +100V for 25us (20kHz) then as it turns off, the field collapses and drives the voltage back past 0V to -100V where the diode on the low side catches it and sends it back to supply, this -100V lingers for another 25us till the high side is turned back on. However the net effect is it cancels out the magnetic field, making the magnet produce very small fields at 50% PWM.

I'm currently thinking about putting a big inductor in series then a cap after it, however my thinking is the same thing will effectively happen - ie. the filtering inductor will also spike back to -100V but then it will take significant time to charge rendering 50% duty cycle much lower than 50% current output.

The current design will be good if it works, it is very easy for me to change the amplitude on the fly. Perhaps I look on ebay for a flux capacitor?

If it will help I could grab some scope screen captures to better illustrate whats going on.

Thanks for taking the time to read my sob story and as always many thanks in advance for suggestions

[rsocor01]

Hi,

Your coil is introducing inductance to the circuit (you probably already know this). You can compensate the load impedance of the circuit by connecting a load capacitor to it. You might need to experiment with different values until you get it right. It would be better if you post the schematics so everybody can see exactly what is going on.

Robert