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Forum Index : Electronics : SMPS question

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oztules

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Joined: 26/07/2007
Location: Australia
Posts: 1686
Posted: 11:32pm 21 Jul 2008
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The pleasure was all mine Philb It is something I had pondered, but gone no further with. After seeing those waveforms it does look to be as simple as Fanman suggests. You have some serious igbt's there.

Current limit may be as simple as a fuse with those big boys in parallel

I can't help but feel this is the easiest way into the kind of power you are considering.

Looking at the waveform, it looks like a 70% duty cycle wave, so dc voltage would have to be about 1.4 times the output ac rms voltage.

What they seek to do is give an average voltage (spread over the 1/120th of a second that each half cycle is turned on) that will give you the peak voltage of the sine wave you are trying to duplicate (of the normal 110 US, 240v AUS etc), but with the average rms value that that sine wave would have produced.

An example is if we turn on the power for .7071 the time at 1.414 the voltage, we would turn on for a total of .7071x 120th of a second at 156v (for 110vac 60hz) the result averaged out over the 120th second (half wave) would be (.7071 x 156) = 110vac rms not the 156v peak that we started with... which is what we want.
Your AC equiptment was looking for a peak of 156v and an average rms of 110v, and this is a compromise way to deliver it.

If you look on a scope at your grid 110v the peak ac volts should be 1.414 x 110 volts. (about 156v)

If you use just 100% square waves, then 110v dc is enough to give 110vac rms.... but the peak voltage will not be there so resistance heaters, incandescent lights etc would be happy, but lots of other things would not be so pleased.

I suspect that the best way to find out what dc voltage you will need (to match their duty cycle), is to measure the hvdc inside the inverter that you are going to use.

Hope that helps some. Keep us informed of your progress with this. Keep in mind I have not actually done this, but I think this is the theory behind it. (I might give it a whurl just for the hell of it though.... then I will know for sure I guess)


.........oztules


Village idiot...or... just another hack out of his depth
 
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