Help improving eff of a 24v -> 350v flyback converter

Mike · 2006-01-19T03:09:00+00:00

I have built a 24v input flyback converter. The output is adjustablefrom 45 to 350v. It works prettty well except that the effiency isonly about 68% at 220vdc out & a 100mA load.Is 75%-80% efficiency a realisic goal for such a converter?The temperature of the secondary winding of the transformer veryquickly rises 35 deg C under the 100mA load. The transformer coredoesn't seem to be the source temperature rise. The secondary heats uprapidly and then the core starts heating up. The transistor warms uponly slightly.Does it sound like the transformer is the major cause for the powerloss?The output voltage has about 25-100mv of noise riding on it. I say25-100mv because it varies as I put my hand close to the core or thefeedback resistor and as I raise the board above the surface of thebench. There are 2 components to the noise , A spike at the switchingfrequency(75KHz) along with some ringing and a much lower frequencycomplex sinusoidal-ish wave.What level of noise should I expect to be able to achieve?I have an LC filter on the output after the sense resistor.The transformer core is an unknown type of gapped E core. It wassalvaged from a 6mHy 10A filter inductor that I was told was made forhigh powered speaker core 1.6" square, 0.25" thick and0.5" high. The center posts are 0.5" dia with a .11" gap. I wound theprimary first then wound the secondary over the top of the pri and ithas 1:15 turns ratio (26T of #22 and 390T of #29). The DCR of thesecondary is 6.4ohms. I can't find my notes now, but the priinductance measured either 60 or 80uHy.I read one app note from International rectifier that said to alwayswind the pri first to minimize the length of wire in the primarywinding and have seen other app notes that say to interleave thewindings, I.E, wind half the sec then the pri then the other half thesec.I was wondering if it would buy me any efficiency to interleave thewindings when I rewind the transformer?Also I want to wind the sec with double #29 instead of just a...

Jan 19, 2006 20 Replies

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NonDigital 20 years ago

Use Litz wire in the inductor

Recover energy from the flyback pulse
Switch the FETs as hard as possible to achieve fast switching

get the fastest diode you can find
use a free Spice program to model the converter and get it optimized there first.

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Help improving eff of a 24v -> 350v flyback converter

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