Electronic – How do VAC ratings of flyback transformers translate to VDC ratings of flyback converters


I am trying to design a 40-50W flyback converter but I am struggling to select an appropriately rated transformer. While most flyback transformers are rated in AC volts, plenty of flyback converters use DC volts (just switched on and off.)

How, then, does one translate the VAC rating of a flyback transformer to the VDC rating of a flyback converter?

Best Answer

One interpretation is that rating is not there to give you any indication of how the transformer should be used in a flyback application, but only to tell you what peak voltages it was designed to work with. The likely reason for this is there are too may conditions, caveats, and gotchas with different flyback applications to give you a simple number rating. But a maximum working voltage is easier to give.

Since you don't use a transformer by running DC through it (which is what a DC rating would imply), you would have to use sinusoidal AC voltage to obtain this maximum working voltage. Square waves and pulses, which is what you actually use with a flyback transformer, are not true DC.

You get something similar with ferrite beads where the current rating tells you when the ferrite bead will over heat, but you never use it at that since the ferrite bead saturates at much, much, much lower currents defeating the purpose of the bead if you use it anywhere near the current rating. But it's an simpler number to give.