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Old 23rd June 2007, 02:49 PM   #1
DaBit is offline DaBit  Netherlands
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Default One big transformer or several small ones. Which is better?

I am once more about to convert a 220VAC appliance to 12V; in this case an electronic 150W HID lamp ballast. Since the device also accepts DC this is an easy task: use a push-pull stage to generate 200-350VDC from the 12VDC input, and feed that to the ballasts' input.

But, I want to fit the 12V->200..350V converter into the void areas of the ballast so the occupied volume of the ballast won't increase.

Now, I do have some space to fit the electronics, except for the transformer. But what I can do is to use a couple of EFD20 cores, parallel the primaries, and put the secondaries in series.

Thus, instead of one 1:22 transformer I could use three 1:7.5 transformers. With the primaries parallelled and secondaries in series this acts like a 1:22.5 transformer.

I have built it as a simple unregulated push-pull using a couple of Wurth WE-Flex transformers, and it works. Drain voltage on the FET's looks nice too, even without snubbering and maximum input voltage there is little oscillation and spikes. Efficiency is above 90% with 110W output. Transformers become 23C warmer than the environment, the MOSFET's (FDP65N06) hardly need heatsinking.

I only see advantages in doing so (provided that the cores are free ). Three cores instead of one provides better heat dissipation and lower leakage inductance.


What is your opinion on this?
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Old 24th June 2007, 06:08 PM   #2
Eva is offline Eva  Spain
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Go on with it, series secondaries are fine and you are already getting good results
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