> I've been looking for information on high power class D audio amplifiers
> and now I'm looking at the feasiblity of using a PIC as the controller.
>
> Preliminary requirements are:
> - class D 200-250Khz switching freq.
> - single channel
> - 200W RMS into a 4ohm load
> - 12V supply voltage
> - freq. response 100-10KHz
> - THD 5%
>
> I would like to do this without using a switching power supply to get
> the positive rail, but instead using a full-bridge to drive a step-up
> transformer that is connected to the speaker.  The output would be low
> pass filtered and compared to the incoming audio signal to generate an
> error signal to adjust the duty cycle to allow the output to track the
> input.
>
> Driving the full bridge. Can the PIC port pins drive MOSFETs or IGBTs
> without expensive drive circuits or significant power loss at 200KHz and
> high currents? What about implementing a ZVS (zero voltage switching)
> resonant design to reduce the drive requirements for switching the FETs
> by turning them off and on when the current is minimum?
>
> EMI.  By varying the switching frequency in code from say 200Khz to
> 230Khz, would I see much reduction is EMI?  Or is this not a drastic
> enough spread to see a noticable reduction in emissions?
>
> Suggestions and comments are greatly appreciated.
>
>
The ability of the PIC to drive the MOSFETs can be worked out.
Look up the specs on the mosfet, what is its gate input capacitance?
Xc=1/ (2pi x freq x c).... if this is less than 200 or so ohms.. no
dice, I am picking the PIC hasn't a prayer of directly driving  power
MOSFETs at 200kHz.
Changing the frequency from 200kHz to 230kHz will not help EMI.
200kHz is a bloody fast switching speed for a micro, thats only 5uS
per cycle to do the computing.
Gate capacitance is the same no matter what the voltage across or
current through the channel.
There are some class D amplifying chips out there like Texas Inst
TPA005D02 etc
_____________________________

Lance Allen
Technical Officer
Uni of Auckland
Psych Dept
New Zealand
_____________________________