I have built a 24v input flyback converter. The output is adjustable from 45 to 350v. It works prettty well except that the effiency is only 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 very quickly rises 35 deg C under the 100mA load. The transformer core doesn't seem to be the source temperature rise. The secondary heats up rapidly and then the core starts heating up. The transistor warms up only slightly. Does it sound like the transformer is the major cause for the power loss?
The output voltage has about 25-100mv of noise riding on it. I say
25-100mv because it varies as I put my hand close to the core or the feedback resistor and as I raise the board above the surface of the bench. There are 2 components to the noise , A spike at the switching frequency(75KHz) along with some ringing and a much lower frequency complex 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 was salvaged from a 6mHy 10A filter inductor that I was told was made for high powered speaker crossovers.The core 1.6" square, 0.25" thick and
0.5" high. The center posts are 0.5" dia with a .11" gap. I wound the primary first then wound the secondary over the top of the pri and it has 1:15 turns ratio (26T of #22 and 390T of #29). The DCR of the secondary is 6.4ohms. I can't find my notes now, but the pri inductance measured either 60 or 80uHy. I read one app note from International rectifier that said to always wind the pri first to minimize the length of wire in the primary winding and have seen other app notes that say to interleave the windings, I.E, wind half the sec then the pri then the other half the sec. I was wondering if it would buy me any efficiency to interleave the windings when I rewind the transformer? Also I want to wind the sec with double #29 instead of just a single piece of wire, but I don't think there is enough room to fit it in. I am also wondering what the effect on efficiency would be if I dropped a turn on the pri and reduced the turns ratio to 1:12 in order for the new sec winding to fit and cut back on the amount of copper in the new winding? TIA for any help or ideas on improving efficiency. I have posted a pdf schematic that includes a photo of the core in abse with the subject "350v flyback" if that would help any.Mike
Sorry about the long winded post, but I wanted to provide as much info as I can.