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I built my own step-up transformer to be driven using a ZVS using a core from a flyback transformer in order to charge some 450 V capacitors. I wanted to see how well my transformer operated with a load on the secondary side and that's when I discovered an issue. With a 6.8 \$\Omega\$ resistor across the secondary I measured this on the oscilloscope. enter image description here

It looked fine to me so I would expect around 2.4 V peaks on the primary since my turn ratio is about 10. But I measured this for the voltage on the primary.

enter image description here

So my step-up transformer is now a step-down transformer?? I initially thought the secondary was saturating due to the relatively large current causing the secondary inductance to drop below the primary inductance thus making it a step-down transformer. So I tested a 1.2 \$k\Omega\$ across the secondary to reduce the current but it was still acting as a step down transformer this time even more (4 V peak on the secondary side). I really don't know why this is happening.

Schematic:

enter image description here

Update:

I tested larger values for the secondary load as suggested by Antonio51. I started with 1M\$\Omega\$ just to be safe and it all worked fine, nominal secondary voltage off 440V peak. I then went down to 100k\$\Omega\$; also worked fine. 6.6k\$\Omega\$ also worked so I decided to test my luck and tried 180\$\Omega\$. This caused the secondary voltage to drop to around 330V peaks while the primary voltage did not drop in accordance. So I think it's safe to say it works for loads above 1k\$\Omega\$ but why it doesn't for smaller loads is beyond me.

Also, just to relieve any confusion, that resistor I tested in my orginal post which I thought was 1.2k\$\Omega\$ was actually 1.2\$\Omega\$.

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  • \$\begingroup\$ What was the output without the load? How many primary and secondary turns? What gauge wire? \$\endgroup\$
    – Andy aka
    Dec 17, 2022 at 8:35
  • \$\begingroup\$ Output with no load is 440V peak. 6 primary turns with 12 AWG. Secondary is 105 turns with 20 AWG magnet wire. \$\endgroup\$ Dec 17, 2022 at 9:00
  • \$\begingroup\$ It doesn't add up. The primary voltage is 24 volts p-p and should, on the secondary produce 420 volts p-p yet you say 440 volts peak (aka 880 volts p-p). \$\endgroup\$
    – Andy aka
    Dec 17, 2022 at 9:12
  • \$\begingroup\$ You may be looking at the wrong image? When loaded the secondary is 24V peak (48V p-p) not the primary. When not loaded, the primary is 42V peak (84 p-p) thus creating 420 peak (840 p-p) (in my case 440V peak becuase of extra windings) \$\endgroup\$ Dec 17, 2022 at 9:16
  • \$\begingroup\$ I'm not looking at any image. I'm reading your comment and doing math on that (knowing that it is a ZVS circuit) \$\endgroup\$
    – Andy aka
    Dec 17, 2022 at 9:18

1 Answer 1

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Here is the resulting behavior for a "charging" capacitor (100 k for "safety" discharge).

Note that the peak current is somewhat "big", quasi 50 A.

Note that you can reduce the peak current with L2 & L3 bigger and coupling it, but the charge of the capacitor is slower.

enter image description here

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  • \$\begingroup\$ Thank you this is helpful. I think I now know why the voltage drops so much with load. My orginal theory was that the secondary was saturating and the secondary inductance drops below the 15uH primary thus making the transformer step-down instead. But I tested what I thought was a 1.2k\$\Omega\$ resistor but it was actually 1.2\$\Omega\$ and thus saturating even more than the 6.8\$\Omega\$ causing even lower voltage on the secondary. But with larger resistors the current is lower and the secondary doesn't saturate as much or at all which explains why it worked fine with larger loads. \$\endgroup\$ Dec 18, 2022 at 10:39

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