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I'm designing a regulator/converter that can generate -120/-108/+60Vdc for a current of 180mA each from a supply of 12Vdc or 24Vdc (5A like it's nothing, 15A tops) and I was choosing a MC34063 but the absolute maximum rating for Vout is 40Vdc:

  • Is it sane to overspec for 300%? In the datasheet there is a circuit for use with external transistors, but I really know little of supply design.
  • Is there another ubiquitous IC for doing this? (I googled it and all that I can find are car inverters)

EDIT: the three separate rails can be independently regulated

EDIT: for the record there is a circuit called the Villard cascade that works for very low currents (like the CRT screens), but it was too dangerous to prototype (by me).


Final Update I found a full LED replacement for the screen I was planning to supply and for the old ones we salvaged supplies from older equipment.

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frankly if your not familiar with power supply design you shouldn't be starting with generating dangerous voltages. Additionally i assume your listed outputs are DC given that the part you listed is a DC-DC converter.

+60V DC will penetrate your skin (not to speak of -108 or -120). It takes <100mA of current through your heart to send it into fibrillation. Which you could easily do with +60V, dry skin and getting +vdd on one hand and GND on the other, wet skin is far worse.

All that aside what you need is a DC to DC boost converter controller with invert for the negative voltages and external switching transistors that can handle the voltage.

You should build a few low voltage designs using this topology so you fully understand it before you attempt to create what your asking for.

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  • \$\begingroup\$ thank you, as I know the basics of supply design I can try generating +12/-24/-22Vdc (20% of the final values) and see all the voltages and currents for myself, and THEN maybe do it full scale. Also, I have a coworker that is a power E.E., I only do low "power" electronics, say less than 12Vdc. BTW: I'm a security nutjob too, I have a special rubber pad for my boards, plastic tools, etc. \$\endgroup\$ – J.P.Wack Oct 27 '10 at 10:56
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First question. No. It is not a good idea to exceed the rating of the converter's output switch, because it may break down. The switch is like a diode, it can only handle so many volts across it in the off state without being damaged. The converter's switch may become uncontrollable at high voltages due to the breakdown state. In fact, you would be pushing it for 40V output, because it's more than likely your output will have ripple on it.

Second question. I think the best solution for this would either be three boost/boost-buck converters, or you could use an SMPS controller chip (like would be used in a computer power supply) with a custom wound transformer. The problem with solution #2 is it depends on the loading of individual rails - the voltage varies if you load one rail more than another, but it doesn't matter too much for low current levels. It depends on what you want: the most expensive and reliable solution or the cheapest and slightly less reliable.

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  • \$\begingroup\$ sorry for not being specific, the three rails can be independent \$\endgroup\$ – J.P.Wack Oct 26 '10 at 18:18
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This isn't exactly a hobbyist project by any stretch of the imagination, but:

If you're into transformers (or need isolation) you could design a flyback converter. The -120V rail would be your regulated output, and the -108V and +60V rails would be slave windings (loosely regulated simply based on the turns ratio). There are many integrated devices on the market which contain a HV MOSFET and simple controller for flybacks.

If you don't care about isolation, you could design a boost for the +60 and something exotic like a Cuk for the -108/-120V rails.

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