Electronic – Questions about SMPS Toroid winding

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I'm making a simple SMPS to create multiple isolated outputs to audio effects circuits.

I need to make a toroid pulse transformer which has one primary and
at least 10 secondaries.
These secondaries will be used in pairs to make 5 positive and negative outputs.

I've never wound my own transformer before so I have some basic questions :

  1. Can I wind each 'pair' together. That is physically push two wires through the core together for each turn? Then use those pairs to create my positive and negative outputs? Will that have any effect on noise or galvanic isolation etc?

  2. Can I safely wind my secondaries with thinner wire? The intention is that the primary will pass 3 amps, and each secondary will only be expected to supply 100mA or so. Is it safe to do that?

  3. Will the transformer still work if the iron powder compound is not exactly right. So in an ideal world I'd be using a T130-3 (switching at 100KHz) But what if it's a T130-6 etc? Will it work, but just much less efficiently?

  4. what would happen if I wound secondaries, on top of other secondaries? Would that work? So if I ran out of space on the toroid, could I just wind more secondaries on top of other ones? I appreciate that there'd be some difference in output voltage, but would those coils still work?

  5. Sort of related to question 1. Would my negative secondary coils need to be wound in the opposite direction? And also, should my secondaries be wound in the same direction as the primary?

PS :

  • T130 core size : OD-1.3" ID-0.7" height:0.25" (roughly)
  • Type 3 iron powder (grey clear) : Carbonyl HP Permiability 35 80Khz-500Khz
  • Type 6 iron powder (white clear) : Carbonyl SF Permiability 8.5 (10Mhz up)

Best Answer

  1. Yes - it's called bifilar winding.
  2. Yes. Work out copper loss (I^2*R) for each winding, choose wire such that the loss on the primary is about the same as all the secondary losses added together (and as small as possible, i.e. use most of the space). Copper loss is simply the power loss due to current passing through the wire's resistance. Work out the length of wire, look up the resistance per metre of your chosen wire diameter in a table (online) and plug in your current. If losses don't match, choose more appropriate wire. Doesn't have to be precise - see the difference between adjacent wire sizes.
  3. Probably not, but it depends on your definition of "work". You need to work out the flux density in the core, and compare it with where each iron saturates. Add datasheets for both cores to the question if you're in any doubt what to do. This is basic to transformer design, easy to get wronf if you don't understand.
  4. It'll work, but the coupling between secondaries will be affected - e.g. noise on one sec may be coupled more strongly to another.
  5. You can wind everything in the same direction - just pick the correct end of each winding when wiring it up. (Easiest done if you have an oscillator and a scope so you can monitor the phase of each winding).
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