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Old 29th December 2006, 03:31 PM   #1
simingx is offline simingx  Singapore
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Default Built a 250W SMPS, possible to modify?

I built this power supply and it's been working fine at 250W output power (build log is here).

I would like to scale it up to supply a pair of UcD400's (about +/-60V @ 8A). If I understand correctly, I need to:

1) Use a bigger core at a higher frequency (Currently it's an ETD39 @ 50kHz).

2) Obviously, change the turns ratio on the transformer. Referring to this site, for a minimum of 270V and maximum of 340V input and 100kHz, the transformer should be a ETD59 core with 29 (15+15) primary windings and 27 (14+14) secondary windings for a total of 120V output volts?

3) To save some trouble, I might not use the semi-synchronous rectification described in the original article. Instead, I will probably implement a standard fullwave bridge rectifier with a center tapped winding.

But then, adding a few more windings for the synchronous rectifiers probably isn't too difficult either...

4) The FETs will have to be upgraded also, I see the maximum current through each FET is something like 9 amps.
Will the existing drive circuitry be able to properly drive bigger FETs?

Will any other (subtle) modifications (besides the obvious changing of OVP and regulation voltage dividers) be required?
Can the existing snubbers still be used successfully?

Thanks!
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Old 30th December 2006, 08:41 AM   #2
luka is offline luka  Slovenia
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1.) Maybe you need bigger core, but not as big as ETD59. It depends on how much wire will you try to put in, as for power, ETD44 can put over 1kw at freq. 50~100kHz.

2.)At input of 270v freq 50kHz, turn ratio would be PRI = 1.4xSEC
I put 90v for sec votage since this page calculates for unregulated supply.This way you can have it regulated.

3.)Best and simple

4.)9A x Vin/2 = 1.4kW+!!. Try to get IRF450, since they are the best for their price, at least for me
If you are using original gate trafo that was made to drive NPN tran., it will NOT work with fets.

Snubbers will still be useful if you use the same freq., I think
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Old 30th December 2006, 02:03 PM   #3
simingx is offline simingx  Singapore
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Thanks for the guide..

Quote:
4.)9A x Vin/2 = 1.4kW+!!. Try to get IRF450, since they are the best for their price, at least for me
If you are using original gate trafo that was made to drive NPN tran., it will NOT work with fets.
I have a pair of IRFP264's lying around (250V / 38A / Rds = 0.075ohm), these should do as the half-bridge voltage is only 170V (with 340V DC side voltage)?

I am using a re-wound gate driver transformer with 26 turns on primary (to the SG3525) and 16+16 turns on the secondary (to the FETs). This should give me slightly more than 10 volts to drive the MOSFET gates...
I will probably be rewinding another gate transformer using an RM core... these are much less hassle than trying to strip the windings from the old transformers :P
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Old 30th December 2006, 02:19 PM   #4
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You will have full DC bus voltage over the transistor when the other one is conducting. So you need FETs with a higher voltage rating.
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Old 31st December 2006, 02:02 PM   #5
simingx is offline simingx  Singapore
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Quote:
Originally posted by luka

Snubbers will still be useful if you use the same freq., I think
I'm probably going to go up to 100kHz... the snubbers on the primary side are 100 ohms + 2.2nF and those on the secondary side are 10 ohm + 10nF. Will these still be usable?


Quote:
Originally posted by megajocke
You will have full DC bus voltage over the transistor when the other one is conducting. So you need FETs with a higher voltage rating.
So I might have to get IRFP460's or something...
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Old 31st December 2006, 02:32 PM   #6
luka is offline luka  Slovenia
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hi

I don't know much about snubbers, becaude I don't use them so I can't tell you for sure.

Yes, or some like IRFP460.
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Old 31st December 2006, 02:47 PM   #7
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I have seen this project and would like to build it. If you're going to go for 120V (+/-60V) @ 8A (close to 1kW), I would give the Full-Bridge topology some serious consideration. Yes, the half-bridge is good for 200-1000W, and, yes, it is simpler to implement, but you will have less drain currents through the MOSFETs and lower primary-side current in the main Xfmr with the fullbridge. Not to mention, capacity to expand the power levels.

Steve
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Old 31st December 2006, 02:54 PM   #8
simingx is offline simingx  Singapore
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N-channel,

Go for it! Winding new transformers isn't difficult and it's guaranteed that they will be at least the correct physical size (some of those AT psu's have such a small main transformer that I'm quite suspicious of them!).

Just one point, I would use (at least) 200V rated polypropylene capacitors in the snubbers (C5 and C8). This point isn't really clear in the text.

For the fullbridge design, am I correct in saying that I just need to put 4 windings on the driver transformer instead of just 2?
Then each pair of the driver windings will go to opposite legs of the transistors?
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Old 31st December 2006, 03:04 PM   #9
luka is offline luka  Slovenia
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hi

Yes you are corrent, but you must pay attencion to how you connect them to fets.You don't want to open 2 mosfets that are connected between +supply and ground, or do you ?
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Old 31st December 2006, 03:12 PM   #10
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simingx,

Don't tempt me! I need another unfinished project like a hole in the head! Seriously though, you are correct in your understanding that four separate windings will be needed for the secondary side of the driver Xfmr. I agree with you on the 200V rating for the snubbers. Probably not necessary, but better to be safe.

As for transformer core size from all those AT & ATX boxes, here is a dirtly little secret every manufacturer of all these standard (and fancy 500W & up) boxes don't tell ya': for a 200-250W rated PSU, the core is good for only about 60-70W continuous and 200-250W on a time-limited basis. This notion is predicated on the idea that, for powering loads with a wide dynamic range (such as disk drive motors and gain-card amplifiers) the power supply will see full power levels for only a second or two, and not continuously.

If you crack open some older AT boxes, I mean like first-generation ones, you will find that the cores are at least twice the size as those found in contemporary boxes. Couple o' reasons for this: 1) Efficiencies got better, eliminating the need for oversized components, and 2) smaller parts were cheaper, and, for the most part, could do the job as well.

Steve
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