Specifications for a flyback transformer

[electrotwelve]

I'm building an SMPS that takes in 110/220V AC and outputs +/- 3.3V DC. The chosen topology is flyback and as is the case of flyback transformers, I need to get one custom made. I have the Np:Ns and Naux:Ns turns ratios and I was hoping anyone here would help me know and state what other attributes I need to specify to the manufacturer.

[electrotwelve]

Primary inductance and output current would be a good start. You might actually be able to find one either at digikey or coilcraft.

Thanks! That helps. I did find a few on Digikey but they all output 5V DC and I didn't want another regulator chip that converts the 5V to 3.3V. Didn't find any that output 3.3V DC.

[T3sl4co1l]

Well, "rated voltage", for a flyback transformer, is just a suggestion...

You can run full rated current, at proportionally lower voltage, by reducing the frequency not-quite-proportionally.  Reason being, the on-time remains the same, but the off-time increases proportionally.

If you don't even need full rated current, or a wide line input voltage range, you don't even need to worry about that!

Tim

[Monadnock]

Unless I'm doing something really unusual I use Power Integrations' PI Expert software to ballpark the design. It will create a spec. You don't have to use one of their controllers, but you do need to be operating at the same frequency as one of their chips.

[nctnico]

I'd try and use a readily made transformer for Coilcraft. I had a special transformer wound over a decade ago but it took a few tries for the coil-winders to get it right. The biggest problem was leakage inductance but I have to admit that at 120W the design was on the edge what you can get from a flyback mains power supply.

[electrotwelve]

You can put windings in parallel or series. For just one example.

http://www.digikey.ca/product-detail/en/wurth-electronics-midcom/750813002/732-3192-ND/2650425

Series tie pin 7 to 5 this is now your common pin 8 is your plus 3.3V and pin6 is your minus 3.3V. You have to half the current though, instead of 1A you have 500mA on each rail. I'm sure there is more there or at coilcraft. You don't need one that says 3.3V otput but it will take some experience to determine whether or not you can adapt one. Main concern is peak primary current.

Thanks for the link. Digikey does ship to my country but the shipping charges are really high. Which is why I'm getting one made here locally. Perhaps with the same specs as this one?

[electrotwelve]

Unless I'm doing something really unusual I use Power Integrations' PI Expert software to ballpark the design. It will create a spec. You don't have to use one of their controllers, but you do need to be operating at the same frequency as one of their chips.

Interesting you should mention PI. I was going through their catalog and found some very feasible solutions for non-isolated supplies. After all, the end equipment that is being powered (which includes a microcontroller) does not have any human interface connection. Which is why I'm wondering if at all an isolated supply is really needed in the first place. They would be far more cheaper but not sure about their reliability.

[electrotwelve]

I'd try and use a readily made transformer for Coilcraft.

I searched their website and catalog but failed to find a suitable one They are quite expensive too in my part of the world.