bogdan2011
Member
I'd like to open this topic on transformer cores and calculating them. I've had some issues with my SMPS projects that I guess are related to the transformer core, mainly too much primary current, big inductive spikes on the primary winding, audible noises coming from the transformer or not being able to sustain the load (the secondary voltage would collapse even with a small load). I'd also like to isolate transformer issues from the issues with the output inductor.
So I've built myself a small test rig that can be powered from a bench power supply with current limiting. Most of the transformer cores I tested behave badly, drawing a lot of current and making audible noises. Some of them are salvaged from electronics but I also have store bought ETD29/39 cores.
Some waveforms are pictured below. The one that looks better is from a green toroid, the only one that behaves well. I've also had the same issues with buck/boost converters that use toroid inductors from computer power supplies - the only core that behaved well was the green one. Frequencies are around 50-100kHz.
So what am I doing wrong? I use the formula below for calculating the primary turns, with a Bmax value of 1500.
Also, can a transformer be tested with no output circuitry (just the two windings), no feedback or even without a secondary winding, just the primary?
So I've built myself a small test rig that can be powered from a bench power supply with current limiting. Most of the transformer cores I tested behave badly, drawing a lot of current and making audible noises. Some of them are salvaged from electronics but I also have store bought ETD29/39 cores.
Some waveforms are pictured below. The one that looks better is from a green toroid, the only one that behaves well. I've also had the same issues with buck/boost converters that use toroid inductors from computer power supplies - the only core that behaved well was the green one. Frequencies are around 50-100kHz.
So what am I doing wrong? I use the formula below for calculating the primary turns, with a Bmax value of 1500.
Also, can a transformer be tested with no output circuitry (just the two windings), no feedback or even without a secondary winding, just the primary?