Re: DC to AC converter. - can this be done?

Joerg wrote:
mook Johnson wrote:

I have a buddy that is struggling with a design. Heres what he's trying to do.
He recieves a 45 - 200Hz 500mV p-p sine wave through a signal transformer and would like to buffer it and drive and high current version of the signal to an output. Output load cna be anywhere from open circuit to .25 ohms.

So far he had a 6:1 transformer primary being driven by NPN-PNP totempole and an OPamp to elimiante the xover distortion (aka simple audio amp). It is desired that the secondary be allowed to have about 2Vp-p of 60-120Hz common mode voltage on it from external sources so it can't be tied to ground but it doesn not need to truelly be isolated.

What he is attempting to do is to AC couple off both sides of the output coil and bring that differential signal back into to the primary side to a second opamp. The feedback signal will be compared by this opamp to the reference signal and any load dependant droop will be corrected out. with all the elememts in the loop (opamp 1, opamp 2+ totem, 6:1 transformer, etc, stability is a bit shaky. He even ran into the dreaded motorboating problem.

Basically its a DC to AC linear inverter problem but with an isolated output and stiff voltage requirements on the output voltage.

I have some ideas about using a isolated power supply to generate the rails for a power opamp and us it to buffer the signal transformer. This would generate quite a bit it heat even with the +/-5V rails most power opamps require. The low load resistance is whats challenging.

How would you analog types go about attacking this problem?


Just bought running shoes from Alpine Design and this model was called "Analog". Seriously. So I guess I qualify ;-)

If the size of the transformer is a concern one could take a different tack: Build a switch mode power supply, basically taken from a TI/Unitrode app note or similar. Make sure its regulation bandwidth is well in excess of 200Hz, usually not a big deal.

Now transfer the power at over 100kHz, rectify at the output, easy on the output cap value, load that pretty good so the cap will bleed down fast enough for a 200Hz upper limit and provide the usual TL431+optocoupler scheme for the feedback.

You would modulate in via the FB input of you regulator. It's not really trivial to build a switcher that goes from truly zero to full scale but it's been done before. SEPIC is what I usually do but that's not isolated. Flyback or push-pull may be your ticket because IIUC the output ground side needs to be floating to inject that other 2Vpp signal.

There is a series of really tiny lab bench supplies from Lascar Electronics. Don't know if they can be hacked to modulate the output but AFAIK they are fully isolated and quite cheap:
http://www.lascarelectronics.com/

It sounds like the OP needs to source AC to his load; this rules out a 'regular' switcher. One could consider a switcher with synchronous rectification, although things would get complex that way.

I wonder if one could use a class D amplifier chip intended for subwoofers? You'd have to apply external feedback, but that should be straight forward. You may even have to power it from an isolated supply, but you'd still have something that was built of individually straightforward blocks.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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Relevant Pages

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    ... So far he had a 6:1 transformer primary being driven by NPN-PNP totempole and an OPamp to elimiante the xover distortion. ... What he is attempting to do is to AC couple off both sides of the output coil and bring that differential signal back into to the primary side to a second opamp. ... The feedback signal will be compared by this opamp to the reference signal and any load dependant droop will be corrected out. ... I have some ideas about using a isolated power supply to generate the rails for a power opamp and us it to buffer the signal transformer. ...
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