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Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Steve, you might want to look into something called "one cycle control" used in switched-mode power supplies. It is a current-mode control technique where you control the pulse width of the power switches to slew the inductor current where you want it to be in just one switching cycle. I only looked briefly at it but it sounded much like a delta-sigma DAC to me. In power electronics International Rectifier use it to implement active PFC by controlling input current to follow a reference sinewave.
Also a point relating to Dr. Slack's post about delta-sigma modulators: A very high switching frequency for a Class D audio power amplifier in the quiescent state isn't necessarily as damaging as you might first think, because little actual load current flows around the quiescent point. Most of the losses will likely be due to charging and discharging Cds of MOSFETs at high frequency, particularly if the supply voltage is high too.
Provided the switching frequency drops when there is real power throughput near the modulation peaks and troughs then switching losses won't be too bad. Besides a high average switching frequency is good for small signals because this is where you want the most resolution and your ear is most sensitive to distortion and noise.
I did my MEng degree thesis on Class D audio amplification in 1994, and built a 1kW prototype. I'm ashamed to say it used a bog standard H-bridge and bi-polar PWM and the best THD figure was about 5%!!! It got me my degree though! These days I think mobile audio devices with Class D amps use things like Tripath that are based on delta-sigma modulation anyway. The difficult part as I understand is closing the feedback loop around the output filter in order to minimise the effects of load impedance variations on the frequency response.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
If I understand right, the output filter in a Class-D amp would be one of the integrators. (OK, it's actually a 2nd order filter,
I'm not exactly clear what you're planning, but if it's to close a feedback loop round the whole of the class D output, then every pole in your filter, every 90 deg phase shift, counts as an order. It's well known that classical control systems can be stabilised in the face of multiple integrators with a suitable phase lead network, and I don't see why that shouldn't work for delta sigma as well. But the last thing you want to go rail to rail conditionally unstable is a beefy amp driving your favourite speaker.
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Best method of high fidelity audio isolation?
It got me my degree though! These days I think mobile audio devices with Class D amps use things like Tripath that are based on delta-sigma modulation anyway. The difficult part as I understand is closing the feedback loop around the output filter in order to minimise the effects of load impedance variations on the frequency response.