RE: SSTC Singing Arc Design - Help needed CLASS D IMPLEMENTATION

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Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Dan,

On 12 Sep 2002, at 11:37, Tesla list wrote:

> Original poster: "Mccauley, Daniel H by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <daniel.h.mccauley-at-lmco-dot-com>
> 
> 
> >>>>Okay, i think you convinced me to scrap the TL494. 
> 
> Let me see if I have this correct.  Please correct me if I am wrong or if I
> am missing something.
> 
> 1.  I should use a 555 or 556 timer IC to generate a sawtooth (which is
> present at the capacitor terminal of 555) at
> the RESONANT FREQUENCY of my secondary coil.  So for me, I need a sawtooth
> waveform at 200kHz.
> 
> 2.  I then feed my audio signal into a comparator with the sawtooth signal
> and I should get a PWM waveform out at the frequency set by my sawtooth
> waveform.

Yes

> QUESTION>>>>>>>>>>>  Is there any particular way to bias these 
signals???
> For example if my sawtooth waveform varies from 0 to 5V, should my audio
> signal be centered within that range or doesn't this matter. 

Again, yes. Just use a regulated 5V source (3T regulator) divided by 
a couple of 100k (say) resistors and connect the 2.5V to the audio 
input of the comparator. You then have a roughly 50k input impedance 
for the audio circuit. Capacitor coupling from the audio source 
(which I suggest should produce around 4.5Vpp) is all that's needed.

 Also, how
> should one set the amplitude (threshold) for the sawtooth wave (or is this
> just adjusted to get the best audio quality during running)
> 
> 3.  I then can use the comparator output (say using a pull-up resistor on
> the output at 5V) to get a PWM pulse from 0 to 5V which can then be fed into
> my MOSFET 4421/4422 drivers.

The major problem with simple TL494 ramps is that they are really RC 
generated and hence more of an exponential type waveform.

I have a discrete design for a linear ramp generator which I could 
zip up and send to you as an attachment. It should still be 
reasonably linear at 200kHz. I could also send it to Terry to put on 
his website if there is other interest. I designed it for use in SMPS 
which I often design from the ground up. Once you understand how the 
circuit works it will not be difficult to set the ramp amplitude to a 
desired value. The circuit as it stands generates a (almost) 0V to 
2.5V ramp. I could write up a simple description for the circuit if 
necessary.

Pse let me know. Others might be interested.
 
Regards,
malcolm