# How to convert 700V AC to 5V DC 500mA?

I have a device that requires approximately 500 mA at 5 VDC, and I am required to get this power from two legs of a three phase power supply. The voltage between these legs can be between 110 and 700 Vrms. This device and its power supply will be enclosed in a small, hot area which will be exposed to fairly large amounts of vibration. There is no need for isolation, and the transformers I have found are too large anyways.

I had previously used this design: http://www.ti.com/lit/an/slua721/slua721.pdf as a starting point for a supply that operated up to 525 Vrms. With some small modifications and a simple DC/DC buck supply at the output I was able to get the required 500 mA at 5V DC, but now of course the bosses say we need to push the input range up a bit. I went back to try and redesign this to work with 700 Vrms, and I have run into some problems.

For one, my rectified DC Voltage is going to be up to 1000 V. Since this design uses BJTs, this seems to limit my choice in transistors to those which can only withstand 200 mA max or are too large for my application.

I am thinking that an IBGT would probably be a better choice, but I am unsure of the implications of replacing the BJT in this circuit:

Can anyone tell me if it would be possible to replace Q1 with an IGBT to handle higher voltages?

If there are better suggestions on how to get from 700 Vrms to 5 VDC I am open to suggestions.

[Edit 1:] This device will be located in the same physical location as the high voltage load that is being power by the three phase supply. It is essentially inside of the casing of a three phase motor. There is no realistic possibility of a human coming close to this device while it is powered. I have isolated DC/DC supplies coming off of the 5 VDC output for powering the microcontroller and sensors. Given this, is isolation really necessary?

[Edit 2:] I have not been able to located any potential transformers that might be small enough for this application. Could someone point me in the right direction?

• I'd be inclined to add a transformer to step down the voltages to more reasonable levels. If it wasn't for the 7:1 input voltage ratio I would have suggested picking a transformer to give you an appropriate output for an international 90 - 240 V switched mode PSU. A 10:1 transformer brings you down to 10 to 70 V. You might be able to design a buck SMPS for that input range. – Transistor Jun 6 '16 at 20:30
• I'd start think of non electrically coupled systems, like a light source and a solar panel, frankly. – Ian Bland Jun 6 '16 at 20:31
• "110 and 700 Vrms" and "inside of the casing of a three phase motor" strongly suggest that you are working with the output of a variable frequency drive (VFD). You need to know the exact nature of the power supply before you do anything else. If this is VFD power, you could be facing 2000 volt high frequency voltage spikes. Also a VFD output would be less than 10 Hz at 110 V if 110 V is actually the minimum. If it is the minimum at which you need to supply power, it still may not be the minimum that the transformer will see. – Charles Cowie Jun 6 '16 at 22:30
• This may or may not be driven by a VFD. High voltage transients will be handled with a MOSFET based overvoltage protection circuit. – user3196752 Jun 6 '16 at 22:48
• are you arc-welding or something? 500 Amps?? holy crap. – robert bristow-johnson Jun 7 '16 at 2:41

Is the supply power at a consistent AC frequency? If so, use a small transformer to take the worst of the bite out of the high voltage. You need a transformer certified for near the actual voltages you'll encounter, that is also commercially available at sane prices, such as the 575V popular in Canada.

You need like 3 VA ( not 3 KVA, just 3 VA) so any transformer will do.

So for instance if you can find a 575V to 48V transformer, that will re-range your voltage to 9-60VAC give or take.

Sorry, didn't realize I essentially duplicated Transistor's comment, but I'm saying don't brew your own transformer, buy one that is UL/CSA/CE listed for at least near the high voltage.

And you really, really want the isolation. Seriously. And that's why to use listed parts on the HV side.

You should get a low-power potential transformer (PT) that will step down your 700VAC to ~120VAC. It will need to have very high isolation ratings, 10s of kV, usually. This will enable you to safely generate an isolated supply using normal 5V offline power converters. DO NOT attempt to directly do non-isolated power conversion from your AC supply. With highly variable input voltage, however, it may be necessary to rectify the PT output and use a high-input-voltage buck converter to produce the required 5V.

Possible small PTs could be found from Hammond Manufacturing (240 to 600 VAC nominal inputs), FASE (100 to 1500 VAC nominal inputs), and Langer-Messtechnik (100 to 1000 VAC nominal inputs). If there is a variable-frequency input, though, these transformers may not be rated, even for non-precision use, with such an input.

• That is a more precise device than is needed, but at the end of the day it's about dollars, and the hard part is finding a transformer small enough, and this is perfect. So it may well be the least expensive option. Good find! – Harper - Reinstate Monica Jun 6 '16 at 20:52
• @Harper they are also rated for high voltage input, high isolation, and come with safety ratings. They are also fairly frequently used to drive instrumentation at medium voltage (2.4k to ~15k VAC) installations. – user2943160 Jun 7 '16 at 2:48

Thank you everyone for the responses. I have contacted TI, and found a solution that should work for my needs. I will be using a rectified flyback topology similar to this:

The UCC28704 will operate up to Tj = 150C. My biggest challenge now will be finding the transformer. This topology allows for a much smaller transformer, but I'm not sure I will be able to find something off-the-shelf that will work. I'll update this answer as I proceed with this design.

You could start with a circuit that handles the global range of consumer circuit voltages (such as the one shown below for 85VAC thru 265VAC). And then throw in some series dropping components (resistors, capacitors, whatever). And use some voltage-sensitive relay circuits to "un-short" the series elements as the incoming voltage rises.