Hi all,

What do people think should be the maximum number of secondaries on a flyback converter?

This would be for a low power range (sub 20W) with all the secondaries pulling the same amount of power. I am thinking of using six secondary windings (its for a motor drive gate driver power supply).

In the past for various reasons I have used two seperate supplies with 3 secondaries and that has been fine. This time I can overcome the reasons for using two seperate supplies and may be able to use a single supply but I am concerened about cross regulation.

Do you think that the rails can be regulated to within 10% of each other?
Do you know of any suppliers who may have off the shelf carriers/bobbins to take the extra number of windings (looking for a quick turn-around).

Thanks

Strange, I couldn't find it.

Anyway to answer the question, 6 outputs on a flyback is quite common in the motor drive industry. If you have the same fixed load on each, 10% regulation should be quite reasonable.

If you wind three secondaries on one layer, using safety wire, then primary, then the other three secondaries, I think it will work well.

You can regulate the whole thing from another auxiliary and still hold 10% if you build the transformer carefully.

I also would follow Ray's winding advice...
The number of secondaries and cross regulation depends on the coupling between secondaries... Reducing Leakage inductance between secondaries with B-filar, Tri-filar, Penta-filar, which ever applies, is paramount....
Keep in mind the wire coating thickness and type of coating for safety and isolation requirements...If you are dealing with all low-voltage secondaries....the secondary to secondary insulation requirements will not be severe...

It's more of the energy stored during the on time and transferred to the secondaries.So the real limit is how much copper can you cram into the winding window. Sandwiching the secondaries within the primary is the optimum for coupling/regulation for a flyback as well as minimizing the leakage inductance. The bobbin is another limitation. Especially in smaller core geometries, so some designers utilize flying leads while others tend towards toroid cores and fabricate a carrier with press fit terminations to accomodate the windings.

Warning, I don't use flyback often because of the EMI but am currently, so this is the observations from a "scientist" not experienced engineer. Since the Flyback "transformer" is not a transformer in the usual sense but a coupled inductor it seems to me there is no advantage in having the secondaries close to the primary winding. The driver is the changing flux in the core. Therefore I would think that for better regulation between secondaries they should be close together (inter-leaved or right on top of each other) and next to the core. Thus for best regulation the primary might be on the outer layer (at expense of additional primary leakage inductance). Ray said in one of his classes, the secondaries should have the lowest currents closest to the core and highest currents on the outside of the secondary windings, f not trying to do some kind of interleave.

Hi all,

thanks for the comments and advice. For those who are interested the requirements on the transformer are for -40 to +125degC operation and at least 1kV isolation.

Its good to know that it is possible to use a single part. Does anyone know of any off the shelf options that could be easily modified? (I am thinking more of the bobbins etc rather than turns and core selection)

Thanks

I have used Coiltronics Versa-Pac units extensively. I have not looked in detail at the available variations in a while, but 20W in flyback mode may be pushing it since you cannot get as much power from a particular transformer (i.e. multi-turn inductor) in flyback mode. In addition, I recall that they only have 6 windings total. However, Since they are small and cheap, you may be better off using multiple Versa-Pacs with dual controller chips. You can parallel and series the various windings to accomplish a wide range of input-output configurations.

Typically, you would need EE/EVD/EFD(16/25) bobbins. Off the shelf bobbins are max of 10 pins. Since you need minimum of 16 pins, you might want to build a die of your own.
We use multi winding transformer for similar application in EE25 bobbin with 18 pins. We have made die of our own for that bobbin.
As for regulation, you have to be careful if your regulated winding load varies significantly. Even if you make coupling of your other secondaries to regulated winding very tight, you may end up in troubles since your application is gate drive supply.
We have seen that non linear dampers using zeners do help in improving cross-regulation while achieving significant efficiency of converter.

Luke, we have an EE22 bobbin that can fit 9 isolated secondaries. You can contact us at rencousa.com for samples of the bobbin/wound component.