# 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?

• Technically, the question should be "why aren't they rated with square waves or pulses?" Because you feed flyback transformers with a square wave, not DC and not AC (which refers to a sinusoid). Feb 21, 2021 at 18:06
• Well, transformers are AC devices.
– user16324
Feb 21, 2021 at 18:48
• Give an example of a flyback transformer that has these "confusing" ratings so that we know precisely what the numbers are and someone can properly answer the question specifically and help generally. Feb 21, 2021 at 21:07

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.

• I suggest that transformers intended for switch mode applications should really be rated for peak voltage rather than RMS.
– Frog
Feb 21, 2021 at 18:51
• @Frog I agree, but alas. Feb 21, 2021 at 18:52
• @Frog So then would a VAC transformer rating translate directly to a VDC converter (since VAC ratings are usually RMS), or would the VDC rating of the converter be the peak AC voltage? Feb 21, 2021 at 19:14
• @PaulKujawa You would need to translate RMS to peak. Frog is saying the fact they don't just give you peak is roundabout, Feb 21, 2021 at 19:15
• Exactly. A 1000VRMS transformer is in fact a 1400V peak transformer.
– Frog
Feb 21, 2021 at 19:46

I think you are confused about transformers in general and flyback transformers in particular. Transformers are not fed with DC typically. While you could use half-waved rectified "DC" (and it's not really DC) but full-wave rectified DC would do nothing but generate heat in the primary winding of the transformer. Without AC you get no changing magnetic field and you get nothing coupled into the secondary.

Flyback transformers are usually fed high-frequency AC in order to be able to get an efficient HV output with a relatively small transformer.

**** I edited your question to try to make it more clear. ****

If you are talking specifically about flyback transformer circuits, then I agree. The most common driver takes a DC voltage and switches it to feed the primary of the transformer. Often this is a kHz-range signal.

These are used for two reasons. First the core needed for a 15 kHz flyback is much smaller than one needed for a 60 Hz flyback. Second, it's often desirable to locate the driving frequency above the typical range of human hearing since these things can often be heard and can be quite annoying.

• I think the OP understands that and I suspect that the correct answer would explain how switched DC in the primary becomes AC. "Flyback transformers are usually fed high-frequency AC ..." How? Feb 21, 2021 at 17:51
• I did already understand that, but I can see how my wording could be confusing. I should have specified switched DC. Feb 21, 2021 at 18:35
• Switched DC is not DC. Once you start switching it you have an alternating current which is the same as AC. It's not a sinusoidal AC like you get from mains power but it is alternating. Feb 21, 2021 at 18:38
• We're saying the same thing, just over different durations. All AC is DC on a short enough timescale. :) Feb 21, 2021 at 18:42
• @jwh20 I further edited my question to hopefully make it clearer still. Feb 21, 2021 at 19:04