Switchmode PSU conversion - Page 1
Forum section: electronics / beginners

author: Flunze
author_rank: Regular Contributor
timestamp: September 11, 2012, 08:37:24 pm
content: I got a very "simple" project. Everybody knows, that PSU's are very expensive, if it comes to higher voltages at good current with a resulting power of over 500 Watts.I would like to simply reconstruct the circuit of a few Switchmode PSU's just laying around and collecting dust. My plan is, to put the Primary sides in parallel and the Secondary correctly in row. That is very, very, simple and will definitely work. But as I want to have 900-1000 Watts as Output, I have to use more, than just the standart Switching transistors in this supplies, cause they definitely would burn down.There are 2 Ways, to increase the power handling capability: But them in parallel, if I got the same types many times and simply add some Current limiting Resistors there, to not kill them. The other way is, to use my very own Power transistors.I wonder, if the "IRFP 460 Power MOSFET" (500V at 20 Amps) would do this Job? I could also put some in series, if realy neccesary... The problem is, I realy, realy don't know, if the Switching speed of 18ns On/Off and Off/On would be fast enough for that kind of operation. What about that?Thanks for letting me know.Edit:Hmmm... I would like to create my own PCB with that, but unfortunately I don't know, how to create the Primary side Circuit. There are many Schematics, but so confusing - looks almost like a Computer.Is there any very simple Design for that?

author: Psi
author_rank: Super Contributor
timestamp: September 11, 2012, 11:50:04 pm
content: I'm unsure if you want to create a switchmode or linear supply?For 1000W a linear supply would be extremely bulky and not worth even trying to make, in my opinion.What kind of output voltage range do you want, this is important info for deciding on the best approach.

author: Flunze
author_rank: Regular Contributor
timestamp: September 12, 2012, 05:07:23 am
content: I will need about 90Volts. Do you think, this can not be stable? Than I would be happy with 95V and 90V under load. The Capaciturs will not hold more than 100V. So this is critical. What do you think, will be the Voltage Drop under a maximum Load of 10Amps - 900W?By the way, I will use +90V GND and -90V, so it will actually only be 450 Watts, so 5Amps under max. Load per output.This looks more like a realistic an d possible design, right?

author: ptricks
author_rank: Frequent Contributor
timestamp: September 12, 2012, 01:07:53 pm
content: Voltage drop can be whatever you decide you want it to be and that you can afford.+90 & -90 is 180volts x 5A = 900 watts.Regulation becomes more critical as the voltage increases, if your supply was 12VDC then 5% is only .60 volts so within 11.4-12.0VDC.Increase that to 90VDC and the 5% is now 4.5 volts or 85.5vdc , it all depends on what accuracy you need.I really don't suggest you build a SMPS with such high outputs if you don't understand how the whole circuit works. SMPS can do the job you want but they are not the easiest thing to design and altering even one component can make the whole thing fail.Before attempting what you want , try building a small low current SMPS from beginning to end of your own design.Edit:Something kept bugging me about the capacitors will not hold more than 100V statement because I don't think I understood what you meant. If you are saying that the capacitors that will be powered by the supply are rated at 100V and you want to supply 90-95VDC to them, you are asking for trouble. The rating on capacitors is nowhere near perfect, there is always a margin of error and even then you should pick a higher voltage based on what the peak voltages will be. I wouldn't put anything over 80VDC on a 100VDC cap.

author: Flunze
author_rank: Regular Contributor
timestamp: September 12, 2012, 06:45:00 pm
content: I want to create this PSU with the good old TL494. It needs something between about 7V and 40V. I found some voltage Regulators, very cheap, that can convert up to 450Vdc to 10 Volts. But it will only give out 30 milliamps. The TL494 does not require more than 10mA for working at 40V. As I am working with just 10V, it will be less than that, as the datasheet says. Well, the thing is, that it needs to power N-Channel MOSFETs. At a frequency between 100kHz and 150kHz this can be a problem, cause the Mosfets (IRFP460) all have a small capacity... I could not realy find something usefull in the Datasheets.How much current will it require, to trigger the IRFP460 at 150kHz at 10V? Will it exceed 30mA?If yes, is it possible, to put some of the regulator in parallel with some resistors, so it will not burn out them?

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 13, 2012, 09:56:01 am
content: i suspect the problem is not the regulator or pwm or the mosfet chip, but the high frequency smps transformer and its snubber.

author: Flunze
author_rank: Regular Contributor
timestamp: September 13, 2012, 08:03:47 pm
content: What about that? Generally, could that work? I guess, it is very simple, to understand the logic of what I am planning, right?EDIT:How about a much simpler Design, that could also be working very, very good?

author: Flunze
author_rank: Regular Contributor
timestamp: September 14, 2012, 07:09:59 am
content: You did not say anything about the other Idea. What about that?