Sixteen 100v isolated power supplies

high voltageisolationpowerpower supply

I need to create sixteen isolated power supplies on a single PCB.

Input voltage 9v DC.
Output voltage 100v DC.
Output current 10mA each.
No precise regulation needed.
Efficiency not terribly important.

I was thinking that maybe I could just wind my own transformer with one primary coil, and 16 secondary coils. Drive the primary coil with an H-Bridge, then use bridge rectifiers and capacitors to smooth the outputs.

Does this sound sensible, or is there a better way to do it?
Do I need a sinusoidal input waveform, or will a square wave do?
Does it matter how many turns I use? Is more turns better? (As long as the ratio is correct)
Does the frequency matter?

My cost and space budget are small, but negotiable. So I'd rather not just buy a whole load of off-the-shelf supplies.

Added: Schematic

100v isolated supply

Best Answer

The tone of your question implies that you have little-to-no experience with switching power supply design.

You are going to have an incredibly difficult time if you want to make a transformer with a single primary and sixteen secondaries. The construction of a transformer is often more critical than the hard electrical/magnetic parameters (turns ratio and core material) due to their being so many degrees of freedom (leakage inductances, coupling ratios, copper loss in the windings, interwinding capacitances, etc.).

If the secondaries have to be isolated from the primary, but can be common to each other, you can go with a single secondary winding rated for all the power you need, and use point-of-load converters (bucks or synchronous bucks) to regulate each rail and provide overload protection (to keep one rail from bringing down the entire bus). You can get complete synchronous buck stages in 2mm square packages (a few external parts and you're done.)

If all 16 rails have to be isolated from each other, I'd recommend not using more than four secondaries per transformer (obviously you need four converters). You could go with a flyback converter design, which simplifies the secondaries (no filter inductors needed) and allows for output > input with galvanic isolation. There are many integrated flyback controllers on the market that contain the MOSFET and control circuitry, just wire up some feedback through an opto and away you go.

You (of course) need a properly-designed transformer, so "yes" the turns do matter as well as the actual number of turns used. The number of turns impacts the inductance, peak current and peak flux density of the transformer. A proper transformer design optimizes the number of turns to minimize core and copper losses, and requires a thorough design procedure. There is no 'magic' number, and more is not always better. For a flyback converter, there are more/different constraints, since the transformer has to be designed to store a certain amount of energy.

Your space budget is small. Forget about sinusoidal waveforms. Forget about low frequency operation. You need high-frequency conversion to minimize the space, which (in its simplest form) involves square waves. Of course, there are efficiency tradeoffs with higher frequency operation. (Space doesn't come free.)

Related Solutions

Electronic – Why is it important for mains power supplies to be isolated

Because the mains supply is very unpredictable, and can do all sorts of things outside its nominal specification, which might damage components or at least break the nominal design assumptions. A non-isolated design also has all its voltages referenced to one of the mains conductors, which might or might not have a useful/safe relationship to other potentials in your environment (like earth/ground, for example).

If the only stuff on the low-voltage side is inaccessible electronics, then non-isolated supplies are fine - they tend to much be cheaper/simpler than isolated supplies, and lots of household equipment uses them. Even things like televisions used to work like this, if you go right back to before the time when they had external video/audio connections. The antenna connection was the only external socket, and that was capacitor-isolated.

If a human being or 3rd party piece of equipment needs to interconnect with the low-voltage side of your design, then an isolated supply both gives you a clear barrier across which dangerous voltages won't pass, even in the case of component failure, and it means your circuit is now 'floating' relative to the mains. In turn, that means you can arrange for all the electronics to operate near ground potential, with all your interconnected equipment having at least roughly the same voltage reference to work from.

Electronic – DC/DC isolated supplies in series for higher voltage

The proper way to do this is with two diodes per brick. One diode has its anode connected to the brick's + output. The other diode has its anode connected to your brick's - output. Both diodes' cathodes are connected together and used as the + output. The - output is connected to the brick's - out.

The PSpice-like connection for two bricks would be:

V1 1 0 48V
D1 1 2 D1N4002
D2 0 2 D1N4002
V2 3 2 12V
D3 3 4 D1N4002
D4 2 4 D1N4002
R1 4 0 1k

Vicor has a good application note on how to do this. The nice thing about this topology is that you can sequence or disable bricks however you want. I have used this in an application with 4 95 V Vicor bricks to get an output of 90, 180, 270, or 360 V, depending on how many bricks were enabled. Remember, one diode per brick will always be forward-biased, and each diode will need to be rated for the maximum output voltage of the whole array.

Finally, make sure you know how much current your load will draw at each possible voltage. All bricks must be able to provide the full load current (due to Kirchoff's Current Law).

Best Answer

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Electronic – Why is it important for mains power supplies to be isolated

Electronic – DC/DC isolated supplies in series for higher voltage

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