SMPS inverter voltage feedback methods

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LOL :)

my little PFC is almost up and running. I have to seriously hobble the loop gain to force PFC action though - at present its trying to regulate out the 100Hz ripple :(

Still, it pretty much fired up first time, so I am happy. And I got a nice sine-looking input current. shame about the bodged up (kludged to you americans) tranformer though - I got my guys in SF to send me some PQ26/20 core sets, and thats exactly what I got. A bobbin, a bobbin, my kingdom for a bobbin. So I went and designed a monstrosity in an ETD34, which was nearby. If I had a reasonably large 60u koolmu toroid I would have used that, but mine are all 125u, so the core losses are too high, and more turns gives me too much L. Oh well, hexafilar windings are fun.

As an aside, anyone know where i can get one of those cute little boxes Jim Williams talks about (AN47 et al) with a male BNC at one side, a female at the other? yes I know I can make such a thing with a diecast box, but it only needs to contain 2 diodes, so I wanted a little pre-fab box.....

Cheers Terry

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In a pinch I made my own bobbin out of a Ferrero Rocher box. Of course, prior to that all those bonbons in there had to be consumed ...

I just use the little blue Pomona boxes but some of those T-connectors can be hacked by wacking off one of the female ports with an angle grinder. Then you end up with an angled piece but if that's not a concern why not try it? Soldering to the outside (if you have to) is a bear though.

Right now I am looking for something similar but SMA. All it has to do is contain anti-parallel diodes to ground, as a limiter.

--
Regards, Joerg

http://www.analogconsultants.com/

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LOL :)

duh. I just made one. Thanks Joerg!

egg zackerly!

Cheers Terry

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Yep, there can be no flux in the K2 transformer since the two primary coils have mirror image currents and are wound the same direction, heres their waveforms:

cheers, Jamie

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Ok thanks, will try a square wave drive :)

cheers, Jamie

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Hi,

Removed the L1-L3 and added ideal switches to operate the two transistors as push pull, output of L4 is still just AM modulated peaks, think I will have to use a peak detector circuit, like one of these puppies:

I think maybe this is what was the idea from the start but I am a bit thick headed :D

cheers, Jamie

On Jan 31, 8:30 pm, Jamie Morken wrote: [....]

Also consider: +12V ! !VCC\\ LM339 ---/\\/\\---+12V IN ----!+ \\ ! ! >---+----+ --!- / ! ! ! !VSS/ ! === ! ! ! ! ! -12V ! GND ------------

For the accuracy of this transformer chopped voltage measurement, what are some techniques to improve accuracy, or does it require per unit calibration because of the variances in components?

I would like to have 12bits of voltage measurement resolution, over the whole AC input range.

Also is there a way to decrease the AC voltage divider so that there is higher amplitude feeding into the push pull transformer? As MooseFet said the EB junction breakdowns are the limit to this, I tried using back to back fets to allow for AC blocking here, but didn't work to well. The benefit of higher amplitude is more current in the push pull transformer and higher signal coming out.

cheers, Jamie

The leakage inductance of the transformer is your enemy. So long as it is small and the transformer is well manufactured, the errors in the transformer will repeat nearly exactly.

You want to be sure to keep the chopping frequency far away from any other frequency or harmonic in the design.

The performance of the chopper transistors needs a bit of optimizing to make sure that the transistor parameters don't come into the issue. You need to provide more than enough base drive for the lowest HFE transistor. You can change to JFET or MOSFET switching if the numbers come out better for you.

JFETs: Don't forward bias the gate enough to cause conduction.

MOSFETS: Beware of the capacitive coupling of the gate drive.

Two ways to deal with this:

a. Switch to a known voltage once in a while. Such as a reference on the isolated side. For 12 bit accuracy that won't exactly be a cheap one or has to be calibrated.

b. Use an FM scheme to transfer the information. That eliminates the influence of the leakage inductance.

--
Regards, Joerg

http://www.analogconsultants.com/

What would an FM method look like for this application?

cheers, Jamie

Two methods come to mind:

One would be a simple voltage to frequency conversion. Unfortunately the old standby LM331 only goes to 100kHz and you want more BW so you'd have to roll your own. It also doesn't come in SMT which can be a drag. Now this gets transferred across a toroid and on the other side you translate the frequency back into a voltage which can often be done using the same chip. Or a micro just counts the pulses against its timer which is pretty easy. For some reason I always run out of timers :-(

Method two would be a PLL where the voltage you want to log modulates the loop. On the other side you place a detector. Remember AFC? That used a detector scheme that measures the DC a station created at the end of the IF chain. The DC was an indicator how far your radio was out of tune and its oscillator was regulated to a point where it's right on. So you need the first part of the AGC, sans loop.

[...]

--
Regards, Joerg

http://www.analogconsultants.com/

Thanks, I have an FPGA on the secondary side so counting pulses would be a perfect application for that, the tricky part is getting the primary side voltage to frequency, it might require its own clean power supply for that section I think, like if using the PLL to generate the frequency.

cheers, Jamie

Well, yeah, but that's just a matter of a TLV431 or TL431. Under 10c a pop.

--
Regards, Joerg

http://www.analogconsultants.com/

For using a PLL, would the 74HCT4046 work for this 200kHz bandwidth or is there another PLL that would be better to use? I've never used a PLL directly so am not sure the best way to implement the voltage to frequency generation for feeding the transformer the FM signal.

cheers, Jamie

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Yes, the 4046 would be able to do that. However, in order to obtain a reasonable conversion factor precision and linearity you may have to servo it. That gets old pretty quickly. IMHO a pulse modulation scheme with DC restore on the system side is more economical.

--
Regards, Joerg

http://www.analogconsultants.com/

Which form of pulse modulation would be good to implement to convert a

120VAC signal for this and then send it over the isolation transformer or perhaps opto?

That link shows 5 different pulse modulation methods, PAM, PWM, PCM, PPM, PDM :)

cheers, Jamie

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This one is easiest:

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Regards, Joerg

http://www.analogconsultants.com/

Back to this one then I think? :)

cheers, Jamie

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