Classic voltage regulator circuit at high voltage - Page 1
Forum section: electronics / projects

author: 001
author_rank: Super Contributor
timestamp: December 25, 2017, 12:57:32 pm
content: Hi!Any ideas to impoove/mod this classic circuit for say 200V 0.1A? 400V 0.1A?What about minimal rated voltage for transistors?Is it ok to use high voltage CRT transistors (hfe=10) as voltage amplifier?What is the best: low zener voltage and high circuit gain or vice versa?What drop voltage at pass transistor is enough?Thanx!

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2017, 03:19:42 pm
content: The problem with using bipolar transistors for the pass element, is at high collector-emitter differentials, the maximum collector current is severely limited by secondary breakdown. A 5A 400V BJT, rated to 50W will only be able to continuously dissipate a few watts at most, at the full voltage rating. Connecting more than one transistor in series (cascode) can help to alleviate the problem somewhat, but it starts to become impractical, at such high voltages.High voltage BJTs are only good as switches and nothing else. Replace the pass element with an N-MOSFET and you stand a chance of being able to dissipate a reasonable amount of power, at a high voltage.

author: David Hess
author_rank: Super Contributor
timestamp: December 25, 2017, 03:32:30 pm
content: The rated voltages for the transistors will need to be sufficient. The pass transistor needs to be able to support the needed power dissipation and its safe operating area should also be considered under adverse conditions or if it will be required to sustain high collector-emitter voltages. If the reference voltage is increased, then the error amplifier transistor should be protected against reverse base-emitter breakdown. I might add:1. Current limiting or foldback current limiting to protect the output transistor.2. Temperature compensation of the -2mV/C Vbe of the error amplifier transistor. This might be done by selecting the correct reference and the original design might have used a 5.6 volt zener diode for this reason.3. Constant current sources to replace the bias resistors for better line regulation.

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2017, 03:39:09 pm
content: Quote from: 001 on December 25, 2017, 03:26:43 pmIn old CRT monitors, the BJT acts as a saturated switch and is either on or off. The time when the voltage across it is high, whilst current is flowing is very short. Most of the time the transistor has either: a low voltage across it and some current flowing through it, or a high voltage across it and no current flowing.QuoteI think you've confused current gain and transconductance. Current gain is simply the collector (or drain) current divided by the base (or gate) current. Transconductance is the change in base-emitter (or gate) voltage vs the change in collector (or drain) current.MOSFETs have a lower transconductance than BJTs, but a much higher current gain, since at DC, the gate current is near zero.Although the safe operating area of the IRFP460 isn't specified for DC, it's highly likely it will be fine for 0.1A at 400V.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 25, 2017, 03:54:36 pm
content: Asked and answered.https://www.eevblog.com/forum/projects/looking-for-yours-opinions!-hv-stabilized-power-supply/On to my block list, you!

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2017, 04:02:44 pm
content: Quote from: Alex Eisenhut on December 25, 2017, 03:54:36 pmLink fixed.Although I see your point. I decided to give him the benefit of the doubt. That thread is very long and it must be easy to miss the information you're looking for.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 25, 2017, 09:11:50 pm
content: Quote from: Hero999 on December 25, 2017, 04:02:44 pmHuh, thanks, who knows why the link was broken in the first place. He's gonna just repeat the same thing over and over, modify this, add that, remove this, ask the same question, ignore the answer, move on to the next component he finds randomly.We've already done everything for him except cut the steak in his plate and feed it to him.

author: David Hess
author_rank: Super Contributor
timestamp: December 25, 2017, 09:54:07 pm
content: Quote from: 001 on December 25, 2017, 08:14:36 pmYes, changing the pass transistor type will not affect the regulation.QuoteIt is better but still lacks current limiting.

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2017, 02:59:28 am
content: Quote from: 001 on December 25, 2017, 11:11:05 pmIt is possible to do but it causes all kinds of difficulties. Usually instead of high voltage Darlington pairs or even single high voltage transistors, two transistor cascode amplifiers are used with a fast low voltage transistor driving the emitter/source of a slow high voltage transistor. If you have to go that far, it might be better to implement a floating regulator design with a pass transistor cascode.

author: oldway
author_rank: Super Contributor
timestamp: December 26, 2017, 02:27:09 pm
content: Quote from: Alex Eisenhut on December 25, 2017, 03:54:36 pmHe is still looking for the holy grail ....a solid state power supply 350 or 400V reliable and safe made with 2 or 3 cheap transistors (or recovered in an old TV because he does not want to spend anything)It has been said dozens of times that miracles do not exist. (at least in electronics) but he insists .... we are probably all liars or incompetents .....The truth is that he made a big mistake: he sold a high-voltage power supply that worked perfectly for the simple reason that it was an old tube technology.He refuses to acknowledge his mistake and wants, almost demands, to find a solution to his problem.No, it is not impossible to achieve to design a high voltage solid state power supply, but it requires a level of technology that is not within the reach of a beginner like 001.Nobody here is willing to waste time and money developing such a project for free, we have already said this several times .... I think he confuses a forum with a free design office. ..We are all volunteer collaborators, not slaves at his disposal.His insistance begins to become unpleasant, if not misplaced.

author: Zero999
author_rank: Super Contributor
timestamp: December 26, 2017, 04:16:56 pm
content: I think there's a fundamental lack of understanding, on the part of the original poster, as to why a BJT is no good as a pass element.Look at the data sheet for the BUL743. It's rated to 500V, a continuous current of 12A and power dissipation of 100W.http://www.st.com/content/ccc/resource/technical/document/datasheet/2c/aa/3e/26/25/2d/41/8d/CD00219617.pdf/files/CD00219617.pdf/jcr:content/translations/en.CD00219617.pdfNow, look at the DC part of the safe operating area graph. It shows what voltage/current the transistor can continuously handle, before without being damaged. Note that it can only pass 12A, when the collector-emitter voltage is below about 8V. The the current rating then declines proportionally, with the voltage up until around 30V, at just over 3A. Below around 30V, the voltage and current are limited by the power dissipation of the transistor, which is given at 100W. Above 30V the maximum current declines very sharply, down to 90mA at 70V (a power dissipation of just 6.3W, then more shallowly until the full voltage rating of 500V where it's just 20mA, a power rating of 10W.You can look at other BJT data sheets with a similar voltage and current ratings and you'll find they're all similar. It's the device physics which as the limiting factor.MOSFETs also have a safe operating area, but it tends to be less restrictive, than that of BJTs. Always check the data sheet.

author: oldway
author_rank: Super Contributor
timestamp: December 26, 2017, 05:45:55 pm
content: It does not seem to be a simple fundamental lack of understanding as 001 wrote:Quotehttps://www.eevblog.com/forum/projects/looking-for-yours-opinions!-hv-stabilized-power-supply/Safe operating area of BJT is "holywar or rocket sience"......

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2017, 07:57:58 pm
content: QuoteI could.QuoteIt requires careful consideration of secondary breakdown in bipolars and thermal instability in MOSFETs, thermal performance, and safety features like safe operating area protection and over-voltage protection.QuoteThere are switching topologies which could be used. For low noise and relatively low complexity, a higher current but lower voltage linear regulator could drive an inverter. Now the power pass transistor has to handle 4 times the current but only 1/4th of the voltage relaxing the secondary breakdown limitations considerably. With a 10:1 ratio, a pair of cheap 2N3055s could be used. Jim Williams designed some high voltage power supplies that worked this way.I did some searching and there are some suitable $4 power MOSFETs which could do it in a single device although it may be advantageous to use multiple devices in parallel to lower the case-to-ambient thermal resistance. On the other hand, there are some much cheaper power bipolars which could do it when 2 to 4 are placed in series although this makes the circuit more complicated. But if multiple MOSFETs were used to lower the case-to-ambient thermal resistance anyway, why not? With a maximum potential power dissipation of 400 volts x 200 milliamps = 80 watts, there will be plenty of space for more multiple transistors. The thing to watch out for when selecting power MOSFETs are their FBSOA (forward bias safe operating) curves; if they are not specifically rated for linear operation and do not show their instability region at low currents and high voltages, then they are not suitable.But unless there is a requirement to supply high current at low voltages, foldback current limiting can be used to protect the pass transistor. With a cheap bipolar transistor, this means being able to use a single device to put out 200 milliamps from 400 to 300 volts and then decreasing current at lower voltages with 50 milliamps at 0 volts.

author: oldway
author_rank: Super Contributor
timestamp: December 26, 2017, 08:54:43 pm
content: Hello David,I think we're going nowhere.We are starting again a long 12-pages discussion that has already been made .... Is it worth it?Do'nt you understand that 001 does not care about all these technical considerations, he wants a ready-made diagram that can be assembled in maximum one hour with cheap and easy to find components.

author: floobydust
author_rank: Super Contributor
timestamp: December 26, 2017, 09:55:12 pm
content: Quote from: oldway on December 26, 2017, 08:54:43 pm+1I also advise against starting another circle-jerk on this topic. It's just wastes the community's energy.Some people want to figure out maths and enjoy the learning. Other people want only the answer to the math exam question.

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2017, 01:53:47 pm
content: Quote from: 001 on December 25, 2017, 12:57:32 pmEditing and deleting posts, after people have replied (unless to fix minor errors) is bad forum etiquette. Please refrain from doing it in the future.What was wrong with the ideas suggested in the other thread?Please ask specific questions about what you do not understand, rather than making general statements.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 27, 2017, 11:54:20 pm
content: Hahah, nice. I mean, he's calling it like it is... no?The meta-lesson is this: you get what you pay for. If you're not willing or able to put in the effort yourself, and you can't afford to purchase professional advice, and you can't afford to purchase a ready-made device, well -- what's left?Life is hard, people make things, and people argue.Put in your own labor, somehow or another, whether it's learning how to follow the given advice, or affording to get better service otherwise.Tim