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.