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no image Sunday, November 26, 2017 by Deepak P

Transformer sizing for AC-DC Power converter

While designing a transformer for AC-DC power converter (SCR full wave half controlled thyristor-diode bridge rectifier) with DC output power of 240W @24VDC, we picked up a transformer with rating of 600VA, 230/40VAC to account for output voltage regulation, losses in rectifier & choke of a DC LC filter.

But gradually loading the power converter to 10A drops my output voltage to as low as 21V and even my regulator couldn't help it

Is my transformer not sufficiently rated?

My circuit schematic is as below

Schematic

Transformer name plate details: Rating: 600VA Primary voltage / Current: 230V AC / 2.6A Secondary voltage / current: 40V AC / 15A Frequency - 1-Phase Insulation class-F

Comments

  • by  Rick Curl
    Hi Deepak-

    If the transformer actually does meet specification, it should be more than big enough. I think you should find out where the problem is happening. It could be that the DC resistance of the choke is too high. It could be that the filter capacitor is much too small.  it could be a problem in the regulator circuit. 

    The first thing I would do is to load the supply to 10 amps and see if anything gets suspiciously hot.

    If you have an oscilloscope available, load the supply to 10 amps and then start checking for excessive voltage drop, starting with the transformer secondary, then the rectifiers, SCR's, and inductor. 

    If you don't have an oscilloscope you can at least use a multimeter to check the voltage across the transformer secondary while the supply is loaded.  If it looks ok, check the voltage drop across the inductor.

    Please let us know what you find.  I'd be interested to know what the problem turns out to be.

    -Rick

     

  • by  David Ashton
    Deepak...a couple more comments.  Are you sure your SCRs are conducting for the full half wave that each handles?  As Rick says, an oscilloscope would show this.  If you don't have a scope, try changing the SCRs to diodes and then put a load on and see what happens.   I'd say on projects like this a scope is pretty well essential.


    It would also show if your choke or capacitor are causing the problem.  A rough rule of thumb is 2000 uF per amp, so you should be looking at 20000 uF, though the fact that you're using a choke may reduce this (that figure is for straight rectifier-capacitor circuits).  Again, you could try removing the choke (just short circuit it) and test again and see if that changes anything.

    If you don't have a scope, try and get one, even a cheapie 10 MHz (even a 1 MHz) scope would be fine at mains frequencies.  

    Best of luck / David

  • by  PeterTraneus Anderson

    I would not short out the inductor, as the surge current into the capacitor could blow the rectifiers. Like David, I suggest replacing the SCRs with diodes and seeing what you get. For an inductor-input filter with enough inductance so the rectifiers are always conducting (assuming plain rectifiers, not SCRs), the output DC voltage is the average AC input voltage minus rectifier and inductor and transformer drops. The average of AC is about 90% of the RMS, so you should get 36 VDC out under load, minus 2 V for two rectifiers in series, minus the IR drop in the inductor. If you have a spare identical transformer, you could use its secondary as the inductor.

    I am glad to see you include the reverse diode across the rectifier output. This is necessary because the inductor-input filter is designed assuming the instantaneous current through the inductor is always positive.

    To check if the capacitor is big enough, use your voltmeter to measure both DC and AC at the output. The AC should be small compared to the DC. You might have to put a capacitor in series with the AC meter to get an AC-only reading.

    A cheap USB oscilloscope dongle should work. Oscilloscope software using the sound card in your computer should also work, though you may want to replace the input series coupling capacitors with larger ones to get better low-frequency response.

    See O. H. Schade, "Analysis of Rectifier Operation", Proceedings of the IRE, Volume 31 issue 7, July 1943, pages 341 through 361, for detailed discussions of rectifier and filter functions. Treat your silicon rectifiers as mercury-vapor rectifiers with one tenth the voltage drop. ieeexplore.ieee.org has this paper, though they charge for it.

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