Uploaded January 2026 | Updated September 2026, 2 weeks ago
This one is using BC550's to drive the BD139's and runs at 70khz. It was interesting trying to work out the cause of seeing this one start drooping in output power at about 4-5W loads around the 14V output region. Every single waveform was rock solid including the AC output on the secondary. I started thinking there was damage to the diodes going on hurting at least one of them but I couldn't ever really see it.. After deciding to just replace the UF4007's to get less drop anyway the same thermal issue was still occurring somewhere. At that point I started blowing air on the transformer and saw the cooling helped. Tried to shield the air from the switches just in case. So it seems this transformer was very gently telling me to cut it out without any damage to the circuit.
If you're trying to build a very efficient converter for your own reasons then I'm probably not a person to be referring to here. If you just want something that won't burn up and seems to provide you an adequate isolated power rail for whatever, driving switches or isolating ground loops etc.. These last 3 builds have given me the impression it's not so difficult so long as you sacrifice efficiency. Give yourself some headroom on the switches (without giving too much), set a little dead time if you want, and let some linear regulator on the other side handle the rest. Using a well-wound transformer or just one manufactured for the purpose seems to save all the headache. An equivalent circuit that does 24V and 18V out at approx 3-4W no problem can probably be made much smaller
This one is using BC550's to drive the BD139's and runs at 70khz. It was interesting trying to work out the cause of seeing this one start drooping in output power at about 4-5W loads around the 14V output region. Every single waveform was rock solid including the AC output on the secondary. I started thinking there was damage to the diodes going on hurting at least one of them but I couldn't ever really see it.. After deciding to just replace the UF4007's to get less drop anyway the same thermal issue was still occurring somewhere. At that point I started blowing air on the transformer and saw the cooling helped. Tried to shield the air from the switches just in case. So it seems this transformer was very gently telling me to cut it out without any damage to the circuit.
If you're trying to build a very efficient converter for your own reasons then I'm probably not a person to be referring to here. If you just want something that won't burn up and seems to provide you an adequate isolated power rail for whatever, driving switches or isolating ground loops etc.. These last 3 builds have given me the impression it's not so difficult so long as you sacrifice efficiency. Give yourself some headroom on the switches (without giving too much), set a little dead time if you want, and let some linear regulator on the other side handle the rest. Using a well-wound transformer or just one manufactured for the purpose seems to save all the headache. An equivalent circuit that does 24V and 18V out at approx 3-4W no problem can probably be made much smaller
