I'm planning to build a multi-output power supply that allows output voltages between 0 - 30 V and currents below 5 A. The idea is that I will purchase a number of 30V 5.3A power bricks (unknown if they have floating output or negative ground) and create an adjustable isolated buck-boost converter (controlled by one 7805-supplied microcontroller per SMPS) and a linear high efficiency low dropout post-regulator to remove any remaining switching noise. The reason I'm choosing buck-boost topology is that buck converters have some voltage drop so 30V power brick could create only about 28V after the second self-built SMPS and 26V after the linear regulator, so I really need the ability to step voltage both up and down. Also buck-boost allows using 24V input power bricks as input in a pinch with full 0-30V output if they have enough current capacity. Also I'm choosing to build isolated SMPS stages because buck-boost converters convert negative ground to positive ground, which would be weird on the output, and I want the ability to use any 24-30V power brick, grounded (positive or negative) or not, and have the complete freedom to connect outputs in series in any manner, to create for example a dual-sided power supply.

I'm trying to find a suitable toroid core high frequency power transformer for my uses. The specs I need are:

  • Operation way above audible frequencies
  • About 7 amperes of average current handling
  • About 1 mH inductance on both input and output, although this is not an exact requirement -- however, the input and output should have about the same number of turns

I think these specs are fairly odd. Most power transformers have 120-230V in input and a much lower voltage at output (but I want to use safe power bricks instead of building devices that directly work between 120-230V). Some are also intended for 50-60 Hz frequencies only. Also most inductors are just inductors, having only a single winding.

The best I could find is a toroidal common mode power line choke. Specifications here. It seems promising because the 1 mH inductance is measured at 10 kHz (although only at 0.1 mA), and because the DC resistance is very low, and because the rated current is 7 amperes. Furthermore, it has the same number of turns on both windings.

However, this doesn't seem to be an inductor specifically intended for my applications. I'm worried about the following properties:

  • Is the high frequency AC resistance much higher than DC resistance due to skin effect? If the intended use case is for letting 50 - 60 Hz mains AC through and reject high-voltage common mode pulses, the skin effect probably doesn't matter at all. On the other hand, I would use PCB for the switched-mode power supply, and if skin effect is harmful on an inductor, it's probably harmful on the PCB too.
  • Does the toroid core lose energy if used as a transformer between 20 - 100 kHz? (As the intended use case for this choke is in differential mode, and common mode only rejects momentary high-voltage pulses, and it probably doesn't matter if the pulses are fully rejected or only partially rejected and partially converted to heat)
  • Does the inductance change much with frequency?
  • Does the inductance change much with current?

Does it look like that this choke could be used as an isolated two-winding inductor in a buck-boost converter running between 20 - 100 kHz?

If not, how should I find or build a suitable two-winding inductor / 1:1 transformer?

  • I could find a toroid core and wind both windings myself. If so, what are the specs I should look for in a toroid core given my application? What kind of wire should I use? Is skin effect harmful between 20 kHz - 100 kHz in wires capable of handing 7 amperes? If it is, how on Earth are all SMPSs built on printed circuit boards that should suffer from skin effect too?
  • I could find an inductor intended for non-isolated switched mode power supplies and wind the second winding myself. If so, what kind of wire should I use for the second winding?

The circuit is essentially same as the circuit here: https://upload.wikimedia.org/wikipedia/commons/e/e6/Buckboost_conventions.svg

...but with the exception that the inductor is replaced with a 1:1 transformer, and the left side of the circuit is connected to the primary winding, and the right side of the circuit is connected to the secondary winding.

  • \$\begingroup\$ Please get rid of a lot of words by drawing a picture and indicating where your toroid would fit into the scheme of things. At the moment, your question is 99% impregnable. \$\endgroup\$
    – Andy aka
    Jan 3 at 16:21
  • 1
    \$\begingroup\$ A power line choke will not provide the specs you need for an isolated buck boost. Choose a suitable core material and core size, and wind your own. \$\endgroup\$
    – Neil_UK
    Jan 3 at 16:24
  • \$\begingroup\$ No, it will saturate immediately. You need a real transformer for any power transfer. \$\endgroup\$
    – winny
    Jan 3 at 17:34

I can see that it is tempting to use an EMC common mode choke. I would not because the DC current rating of the choke is very low due to core saturation and often the ferrite is more lossey which is good for EMC but not as the main inductor of a SMPS. Also the common mode EMC chokes have significant leakage inductance which is generaly undesirable on hard switched topologies.


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