# Best way to get a low DC voltage from a high DC voltage for a power supply?

I have a 200V DC power input and I need to, with no external voltages, generate a 12V-24V voltage to run the control electronics off, at about 50mA.

I've considered zener diodes and potential dividers but they waste too much power; the resistor at the top must dissipate 9.4W to provide 0.6W, which is a ridiculous waste and difficult to handle. I've tried to look at how switch mode wall warts do it, but they seem to have some kind of fancy mechanism of powering themselves from the output, which I don't really understand. (Neither do I understand how they initially get a voltage.)

• are you asking how to build something from scratch, or what to buy off the shelf? personally, I wouldn't mess around with home brew at those voltage levels (> 40VDC is generally considered dangerous) Dec 16, 2010 at 16:34
• What you call powering themselves from the output is probably a voltage feedback so that the controller can control the output voltage by varying the duty cycle of the PWM driving the transformer. Dec 16, 2010 at 17:14
• Out of curiosity, what is providing a 200 V input where you also need a 12/24 V, 50 mA source? Dec 16, 2010 at 17:54
• @Nick T, an array of high voltage batteries. Dec 16, 2010 at 18:10
• Electric/hybrid vehicle?
– XTL
Dec 18, 2010 at 12:15

This problem is known as bootstrapping. It's even more of a problem when designing low-voltage boost converters. If you've got a .1V 10A source, sure, you've got 1W of power, and could probably generate close to 200mA at 5V from it, but you need a voltage usable for some electronics. A 5V power supply is conveniently and indefinitely available on your devices' output soon after you solve this problem.

I won't go into design of bootstrapping a boost converter here (because I don't know how...) but I would suggest how you might go about designing a buck converter.

We'll assume that you have a circuit which can generate 12V from 120V when powered with 12V. That's not so hard, there are several designs which could do this. Wikipedia has a simple one, you might look into others in various application notes. If your switch oscillates at a few hundred kilohertz, you should be generating a usable 12V signal in a few milliseconds.

What you need, therefore, is a way to generate a voltage to power your device for just a few milliseconds, and then turn it off. A simple resistor/>12V zener diode system with a transistor just after the resistor will be fine. A PMOS with the gate to your circuit's output should switch this source off soon after your regulator starts working. You'll want to ensure that your load is disconnected when starting up, because this could cause your shunt regulator to go out of regulation.

Break your problem down into two steps:

1. Generate 12V from 200V while powered from an external 12V source.
2. Generate 12V from 200V for a few milliseconds without an external source.

Then combine the two. The first one is arguably more interesting, many textbooks will skip over the second problem. My professor mentioned it as a side note when lecturing.

Other answers have pointed out that you can probably just tap an AC switch-mode regulator after the rectification circuit, but I suspect that you also want to know how it works.

• You don't need to tap the AC switcher after the rectifier, as DC will go through the rectifier unchanged. Dec 16, 2010 at 19:10
• Unchanged except a diode drop or two. Dec 16, 2010 at 23:06
• Right, and a diode drop or two isn't really significant when you are starting with 200V. Dec 16, 2010 at 23:46
• @Brian - I didn't want to differentiate between power supplies with an without transformers, although, now that I think about it, a transformer-based power supply would likely cook if you tried to connect 200V after the rectification. Dec 17, 2010 at 20:37

While this charger is rated for 100 - 240 VAC, the first thing that a switchmode supply does is rectify the input voltage, so 200 VDC will work fine to power it.

(You can of course buy this kind of supply in a variety of output voltages.)

Going with @markranges' suggestion, here's one at Digi-Key that does 85-264 V AC 110-340V DC in and 12 V, 420 mA out.

First, are you able to float your controller at +176V? This way you would not have to drop the voltage. I/O circuitry would have to be fancier.

There are two ways to drop voltages: dissipate power (linear regulation) or switch it and smooth the output (buck). Ideally one would draw only 6mA from the 200V line, but there will be losses in a buck converter. I've never seen a HV DC-DC buck made for low power. You may need to make one on your own, if it's worth it.

I'm not sure which will have larger losses. How much is it worth? :D

• How would you generate the +176V? Wouldn't you still have the issue of making a circuit that "drops" roughly 176V?
– W5VO
Dec 16, 2010 at 16:58
• When you say float do you mean running the control electronics at +200V and GND referenced to 197V~ish? Dec 16, 2010 at 17:07
• @krapht - I believe that is what he is saying (Well, ground referenced to 188V-176V, but pretty close.) Dec 16, 2010 at 18:57
• @W5VO - No, a simple LM317 linear regulator will work fine, and be perfectly happy operating at 200V above earth ground. It would do fine generating 0.6W, and is cheap and simple. That's why 317s are often used as pre-regulators. Heck, you could put 10 of them in a row, let each dissipate a watt of power, and get your 20V that way. Dec 16, 2010 at 18:59
• @reemrevnivek So if I were to use a 7812 instead of a 317, how would that be connected? I'm not really following how you're dodging the 175V+ drop. The current has to go somewhere, that's the LAW!
– W5VO
Dec 17, 2010 at 0:59

What you're looking for, I think, is a High Voltage DC-DC Converter. Something like those found here: http://www.powerstream.com/dcdc.htm

For theory of operation, I recommend reading http://en.wikipedia.org/wiki/Buck_converter and http://en.wikipedia.org/wiki/Switched-mode_power_supply. The short answer to how it gets power is through a feedback circuit.

The switching mode wall worts are pretty simple. The rectify the input voltage (which you don't need to worry about since you have 200VDC already) then turn it into a PWM waveform at fairly high frequency. The transformer can then be rather small and light weight as its operating at the PWM frequency. The turns ratio of the transformer is used to step down to the desired voltage and then filtered or further regulated as you would with any switching regulator. The feedback loop to the PWM controller (which is operating at mains/high voltage) is usually done with an opto-coupler so there is complete isolation between the high voltage and low voltage sides. You may be able to skip that in your case.

What would be ideal is a flyback or a feed forward switching controller. Here are some examples from linear. But your problem is going to be that 200VDC is really high, i've never seen a DC-DC controller that supports more than ~100V input.

However, you should be able to adapt an AC-DC switching controller which are normally designed for use with rectified mains voltages which can be much higher than 200V.

For instance ST makes a bunch of such controllers. I think that you can use this type of topology with 200VDC with minimal modification although i've never tried, you should study the modes of operation further. I'm sure there are many more manufacturers for similar parts as well.

• But how does the controller operate off 200V? Where does it generate its Vcc from? Dec 16, 2010 at 18:11
• the controller operates off the 200V supply. Internally these devices are very simple, it doesn't take many transistors. There is normally a precision internal reference set at some nominal voltage that is used for the comparator. In other words, there is no Vcc, just the supply voltage. The reference can be created in various ways, but is very, very low current so little power is wasted, I'm not sure what they use to create it, maybe a zener, maybe some other construct.
– Mark
Dec 16, 2010 at 22:44
• The reference is frequently our old friend the TL431. electronics.stackexchange.com/questions/7038/… Dec 16, 2010 at 23:48
• @Mark, Most show things like logic gates, comparators, etc. Is the entire circuit designed to run off 200V-300V, or does it generate its supply elsewhere? Dec 17, 2010 at 14:39
• @thomas yea, logic gates are just transistors, as are comparators, as always just use transistors rated for your operational voltage. Its not like they have to switch fast in this application.
– Mark
Dec 17, 2010 at 20:07

Check out Power Integrations Linkswitch TN devices - these will do exactly what you want with good effficiency and low cost, assuming you don't need isolation.

• These look pretty neat. I may use them in my next project. Dec 21, 2010 at 1:49