Re: DC to AC converter. - can this be done?
- From: Tim Wescott <tim@xxxxxxxxxxxxxxxx>
- Date: Mon, 18 Dec 2006 20:33:11 -0800
mook Johnson wrote:
I have a buddy that is struggling with a design. Heres what he's trying to do.The 45Hz makes it challenging, too, if you're trying to put it through a transformer that doesn't break your back.
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?
One way to approach this would be to use transformers that pass well beyond the desired frequencies, then condition the frequency response of the circuit with the feedback components. This would help with the instability, but it would make transformer selection difficult.
Ultimately what you want is a circuit which has a high common-mode impedance and a low differential mode impedance. Your transformer idea is just one way to make this happen. Another way would be to build a pair of current drivers, with feed back coming from a differential amplifier. This would ease your stability difficulties (and shrink your circuit), yet it would still have enough pitfalls to make it really fun.
You could mix & match, also, by using a diff-amp feedback but a transformer in the forward path. This would ease the need for such an absurd transformer that I suggested in the all-transformer solution.
No matter what you do you'll have to deal with stability issues. This all sounds very doable, but not trivial.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
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