RE: audio modulated SSTC

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 

Hi,

On Thu, 5 Feb 2004, Tesla list wrote:
 > Original poster: humanb-at-chaoticuniverse-dot-com
 > Can someone show me with a schematic how one would
 > modulate the rails? A simple diagram would help me
 > immensely.

These are by no means final schematics but they should give you some ideas
about the concept. I just drew them up from memory, there might be some
errors there so watch out...
This one is for linear audio modulation, where you'll be vasting a lot of
power as heat in the mosfet/igbt:
   http://www.hut.fi/~jwagner/ele-tmp/audiomod-1.gif
   (hmm I think the R1/R2 vs LM358 supply is a bit screwed up, should
    look different...)
and this second one
   http://www.hut.fi/~jwagner/ele-tmp/audiomod-2.gif
is a current-fed SSTC approach which is a bit more complex as it uses
high freq PWM, and so requires fast one IGBT or MOSFET, an inductor, and
an ultrafast recovery diode. The benefit is that it is _much_ higher
efficiency. The drawback is not only the higher complexity, but also
possible RF mixing products falling into the audio range. e.g., audio PWM
prestage runs at 300kHz, SSTC fullbridge switches at 290kHz (assuming 50%
duty) => 10kHz audio tone "interference" in the output. So you need to
select a sufficiently high audio PWM prestage freq to be on the safe side
- also well above the freq components that the PWM generates while it
runs at >50% duty cycle.

cheers,
  - Jan

--
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  Helsinki University of Technology
  Dept. of Electrical and Communications Engineering
  http://www.hut.fi/~jwagner/ - jwagner-at-cc.hut.fi