High Voltage Linear Regulators

Philldapill · 2009-07-18 05:02

Does anyone know of a high voltage(>50V) linear regulator in a simple package?

I am designing an LED array that will use 120VAC. This array will be a BIG string of 5mm LEDs. This array will be powered by rectified AC, which will be ~170VDC nominal. My LED string will only drop about 135V, and the rest of the voltage will be dropped via a MOSFET acting as a variable resistor. This MOSFET is used to keep the current through the LEDs at exactly 20mA, regardless of the input voltage(~170VDC). My plan is to tap the high voltage at the MOSFET, and use this to power the current sensing circuitry.

Anyway, all that aside, I need a regulator to step down the ~50V to a more moderate 12VDC. I'd like the regulator to be in something like a TO-220 package with minimal components(i.e. capacitors maybe). Switching regulators are probably out of the question as they require too many components.

Mike Green · 2009-07-18 05:09

You really need a switching regulator. The voltage is way too high to use a linear regulator because 95% of the power will have to be dissipated as heat.

Another possibility is to use a Zener and an appropriate power resistor. This way, most of the voltage drop is across the power resistor which is built to handle that kind of abuse. Make sure to fuse the thing.

Philldapill · 2009-07-18 05:15

Thanks, Mike. The Zener was actually my second choice, so it's good to hear it from YOU that it would be ok to work. In fact, now that I think about it, a zener seems to be much more appropriate since it has a very minimal I_q value. I will only be sinking about 1mA from this to drive a low-power OpAmp to take care of the current sensing/MOSFET gate voltage. Thanks!

mctrivia · 2009-07-18 05:15

No way you can drop 169V DC peak to 12V DC in analog it would only be 7% efficient. that is a lot of energy to convert to heat. Can be done with switching regulator fairly easily.

If you place a high voltage cap to keep the voltage up and a 1.7k resister in series your led string will run just as well with no complicated circuit.

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Philldapill · 2009-07-18 06:24

mctrivia, I didn't plan on dropping the 169V via a regulator/zener. The LEDs will drop about 135V, and the other 35V-50V is what the MOSFET and sense resistor will drop. I was thinking about tapping the 35-50V to use for the regulator or zener. In this scenario, the voltage would be dropped down to the 35-50V range by the LEDs, then the current through the LEDs would be split through the regulator, and the rest through the MOSFET. The MOSFET would act as a variable resistor to keep a constant 20mA through the LED string.

mctrivia · 2009-07-18 15:37

http://forums.parallax.com/forums/default.aspx?f=15&m=367566

5V and 3.3V rails can take up to 45V input. Though a purely passive system like I mentioned will work.

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PJAllen · 2009-07-18 16:57

There are ways to·configure·the good ol' LM723 for high voltage.

CounterRotatingProps · 2009-07-18 19:57

Phil,

a couple of thoughts... if you're using the leds in series and the v-drop is a convenient side effect, why not just toss a few more leds in the chain and get closer to the drop you need?

As this is 120vac based, I'm _assuming_ this is going to be used for lighting or lighting effects. If it's hardwired in, make sure you put surge and spike protection ahead of the chain. --- Ask any electrician who installs led lighting, and they'll tell you how these "new fangled LEDS" blow out *way* more than other lights. Even if your power lines are clean, lightning transients does bad things to those little PN junctions. And, like a christmas light chain, one out and ... well you know ...

A related, ranting digression:

I've got a bunch of X-10 lighting and switches in one installation. I am beginning to hate these devices - about 1 out of 20 of the off-the-shelf versions go belly up after a near-by storm. And this install has one of the best and biggest surge/spike protectors upstream. Regardless, in a storm, the PIC controllers inside them either loose their marbles at best, or the whole device is dead. What I've been doing lately is to put a 150v MOV across the hot and neutral. Some legs off the breaker box seem more susceptable than others - despite proper grounding etc - it makes no sense, it's voodoo as far as I can tell. But the MOV's have helped. If we get a mambo storm nearby, I'll pop one or two open and check them. So far, I've found three go bad, while the device is still OK.

You might want to consider something similar. (And if you don't mind a sidewise thread hijacking, I'd be interested to hear from others more experienced in the grey/black arts of powerline conditioning what the *heck* might be going on in my case!)

cheers
-Howard

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mctrivia · 2009-07-18 23:44

Well I am an electrician and have yet had any problems with LEDs. MR16 Halogens and T5 Floresents I have been changing like crazy though. However every building I work in(mostly Home Depot) have 3 phase lightning aresters at the main power feed.

The cheap christmas tree lights have no protection at all except a resister and a diode and they burn for years. The good under counter LED lighting all run on 12V. I have had to replace 1 transformer for an LED string but it did not work from the moment I turned it on.

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Post Edited (mctrivia) : 7/18/2009 11:49:19 PM GMT

Philldapill · 2009-07-19 04:24

The problem with this system, is that I have no idea what the input voltage will be, exactly. It could be as low as 105VAC(rms), or as high as 135VAC(rms). Both of those extremes are fairly unlikely, but anything in between is very well possible. It's good practice to slap yourself once in a while and remember that you're NOT working in an ideal world, and that you're going to have to plan for worst-case scenario.

The reason I can't just add a few more LEDs, is due to the undervoltage case. If I add a few more, the total drop may exceed the supply voltage, and the LEDs won't turn on. If I don't add enough, the supply voltage(~191V) could be way over the total LED drop. I'm having to strike a middle ground by using the MOSFET in a variable resistor mode. The LEDs are only drawing 20mA, so with a fair heatsink, I the MOSFET will be fine under worst case over-voltage.

However, I still need a way to manage that high voltage drop across the MOSFET, in order to power my analog circuitry for the MOSFET gate. The switching regulators look nice, but the input voltage looks like it will MORE than exceed their max input.

kwinn · 2009-07-19 04:25

Philldapill, I have seen and used a 2n3440 with an emitter resistor and zener used to provide a constant current to a display. 20mA at 50V is 1W so if you split the power between the resistor and transistor it should be no problem.

Found the generic schematic. A 12V zener and a 560 ohm emitter resistor should give you close to 20mA.

***** Last night I put the zener diode in upside down. I have uploaded the correct diagram this morning. What can I say... late night is probably not the best time to be doing this sort of thing. My apologies for the mistake.

Post Edited (kwinn) : 7/19/2009 2:38:34 PM GMT

mctrivia · 2009-07-19 04:30

Philldapill you are very right. The CEC dictates line voltage from distribution be 120V+-7% allowable drop from panel to receptacle is 3% so range is 108V-128.4V

240V lines are even werse. Comercial and Industrial use 208V because they are 3 phase. Residential is 240V. Same plug though [noparse]:([/noparse]

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Post Edited (mctrivia) : 7/19/2009 4:37:07 AM GMT

Philldapill · 2009-07-19 07:52

LOL! D'oh! Kwinn, that 4 component solution is both elagent and simple. However, I think the Zener is facing the wrong way. If I understand the circuit, the zener keeps a constant voltage across the base and the emitter resistor. Since the Base-Emitter drop is pretty much constant, then the voltage across the resistor is constant as well. From Ohms Law, the current through the emitter resistor is also constant, providing a constant current source. To adjust the current, simply adjust the emitter resistor. Even if I were to use a 5.1V zener, which I have plenty of, the resistor would drop about 5.1V-0.7V = 4.4V. At 20mA current, that is only 88mW which a small trim pot should be MORE than able to handle.

This is awesome! We were talking about using a switching power supply when 4 components do the job just fine. Man, I can't get over this. I'm literally laughing-out-loud at how simple the solution is. Thank you!

kwinn · 2009-07-19 14:58

You're welcome Phil. Just keep in mind the rest of the power is dissipated by the transistor so it will need a heat sink. And yes, I did put the zener in backwards last night, which I realized this morning and corrected just before your post.

Philldapill · 2009-07-19 16:08

Yeah, I did a quick simulation, and even under the worst case scenario(Vs = 191V), the transistor and resistor combo only dissipate about 1W, with the transistor taking most of the punch(~800mW). This is one of those classic solutions where thinking a little outside the box yields a very simple solution. Thanks again, Kwinn.

Phil Pilgrim (PhiPi) · 2009-07-19 16:47

You can get regulated, switching wall warts for very little money these days. Most will run from 100-250VAC. In addition to the regulation and efficiency, you gain the safety provided by galvanic isolation.

-Phil

CounterRotatingProps · 2009-07-19 22:39

mctrivia said...
Well I am an electrician and have yet had any problems with LEDs. MR16 Halogens and T5 Floresents I have been changing like crazy though. However every building I work in(mostly Home Depot) have 3 phase lightning aresters at the main power feed.

Yeah, I've seen inside those panels - wish I had some of those arresters in my (smaller) installs!

Just mentioned the LED thing to an old electrician friend, who·recently retired after 60 years. He had had 5 or 6 crews going full blast - and the city's electro-mechanical inspectors often called him when they had a code question [noparse]:)[/noparse]·· He stopped offering LED installs because of too many call backs, said that ~ 1 in 3 would go out in a storm (surge prot.·or not).·

We're in Florida, however. - If you live here, you have to like to sweat, like bugs, hurricanes, and lightning strikes [noparse]:)[/noparse])

Well at least the MR16's aren't as anoying as the tubular ones (T3's?) --- I regret installing about 15 outdoor floods that use them. Dang things seem to go out monthly!· I'd like to see an LED version of those! ^^

@Philldapill - looks like we got an embedded·sub-thread going here titled "lightbulb's Lament" :-P

- H

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mctrivia · 2009-07-20 00:31

if it exists you can probably find it here : http://www.superbrightleds.com/led_prods.htm

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High Voltage Linear Regulators

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