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Old 27th September 2006, 12:22 PM   #11
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when N-channel discusses the voltage which the transistors have to stand off in an off-line smps with 240V primary you should also consider that the insulation of the wire forming the transformer primary windings has to be highly rated as well.
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Old 27th September 2006, 01:09 PM   #12
Eva is offline Eva  Spain
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What values of inductance and capacitance are you both employing in the output filter? Separate or coupled inductors? And which diode model?
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Old 27th September 2006, 02:00 PM   #13
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Jack,

TNX for the comps. I may know a thing or two, but from what I have read from EVA's posts, it appears she probably knows more than all of us combined.

a2005r2003:

I would recommend Coupled inductors for the output as EVA recommends, but you need to keep in mind that the BIG yellow toroid immediately after the output rectifiers is an energy-storing coil, and not a filtering coil. Its function in an AT/ATX PSU is much the same as in a srtaightforward buck converter: It integrates (averages) the output voltage(s) over time. Together with the filter cap, it forms an LC filter that will affect your supply's response to chaing line voltage and output load. This coil is best wound bi-filar, with one winding for each output. This will provide excellent cross-regulation between the equal, but opposite outputs (+/-35V), and help to keep things symmetrical about the zero-volt line. The filter inductors after the first caps can be wound on individual cores, as they do not perform cross-regulation. Then you are free to place additional caps after the filter inductors, effectively resulting in an L-C-L-C output filter.

Choosing the L-C values is critical, because as previously mentioned, it affects the PSU's response to changing conditions, but also affects things on the primary side, because the inductance in the energy-storing coil is "reflected" back to the primary side. This can have adverse affects on the switching conditions of your main switching transistors, as EVA also mentions.

Did you, by chance, change anything in the driver transformer? I hope not, as this type of drive (proportional-base drive) is specifically suited to the transistors and inductance(s) in the original PSU. It is NOT suited for driving MOSFETs, nor is MOSFET drive suited for driving bi-polar NPNs.

The driver transformer's design is critical, as its core must saturate much more quickly than the main transformer. Indeed, part of the primary current path is wound on this core. This helps the supply start up before the PWM control chip kicks in. In the start-up mode, the primary side essentially runs like a free oscillator, but only for a second or two, as the PWM action of the TL494 has not yet taken control. Because the PWM chip is on the secondary side, there is no start-up circuit for it like there would otherwise be if it were on the primary side, but that is another issue. As the secondary side voltage begins to build up (usually within the first few cycles) voltage is applied to the PWM and things go from there.

The reason the values of the inductance of both the driver- and main- transforemer are critical is that they determine the free-running frequency in start-up mode. Changing the secondary windings of the main power transformer definitely will afftect things, and changes will need to be made elsewhere in the whole circuit to bring things "back into balance". Start by checking out the feedback compensation at the PWM's error amp input. Following Marty Brown's chapter on Feedback compensation, you can choose compensation components based on your new configuration. This would be a good start. Anyway,. I'm beginning to babble here, so I will stop for now and catch up with my thoughts. As EVA said, you're lucky your main transistors merely got hot, and didn't let the magic smoke out (voilently)!

Hope these provide some insight.

Steve (N-Channel)
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Old 27th September 2006, 06:51 PM   #14
blmn is offline blmn  Brazil
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If the power supply has a feedback loop connected to the secondary that you changed, it must be changed too.

Regards,
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Old 27th September 2006, 10:43 PM   #15
Eva is offline Eva  Spain
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The main transformer will be OK as soon as the primary windings remain exactly the same, wire insulation is not damaged, and the core is not damaged during disassembly. Any air gaps in the ferrite due to a cracked core or due to bad assembling will cause trouble (of an overheating or exploding flavour).

The turn ratio has to be carefully chosen so that the PSU is capable of producing the required output voltage with the minimum desired mains input voltage.

For a 160V AC minimum input and +-35V output a quick calculation tells us to use...

176*1.4142~=250V (mains peak value)
250V/2=125V (it's a half bridge)
125*.8=100V (accounting for max. duty cycle and rectifier losses)
n=100/35=2.86 (turn ratio)
40/2.86=14 (turns in each secondary)

So with 15 turns you did the right choice.

Calculating an output filter is not so simple, though.
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Old 28th September 2006, 04:08 PM   #16
m-tech is offline m-tech  Serbia
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Eva,

Can you tell us, how to calculate capacitor value ?
Inductor is not problem ( http://schmidt-walter.fbe.fh-darmsta..._e/smps_e.html ), but output capacitor ...

Thanks.
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Old 2nd October 2006, 03:31 PM   #17
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m-tech,

Yes. Check out Brown's chapter on Power Factor Correction. Very good and thorough, however, since his example uses Critical-conduction Mode (CCM) and not fixed-frequency, his example is only good for about 300W Max.

Steve
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Old 17th October 2006, 11:36 AM   #18
chas1 is offline chas1  Dominican Republic
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Default Output cap

M-tech

If you have Pressman's "Switching Power Supply Design" refer
to pg #448 if not see below:

Quote:
Using a constant of 65 x 10E-6 (This assumes that over a large range of aluminum electrolytic capacitor magnitudes and voltage ratings, that ResrCo is constant and equal to 65 x10E-6) End Quote:

Co = di/Vor where di is twice the minimum output current and Vor is your required output ripple voltage.

His example is based on 1 amp min output current and .05 Vor


Co = 65 x 10E-6 X (2 X 1 amp) / .05 volts = 2600 ufd

chas1
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Old 9th June 2011, 03:54 PM   #19
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After Several Years :
I Can Modify PC PSU and It is Very Important than U Use Original Driver Section( don`t modify it) and use GOOD Feedback
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Old 11th August 2011, 06:19 AM   #20
Junm is offline Junm  Philippines
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Quote:
Originally Posted by a2005r2003 View Post
After Several Years :
I Can Modify PC PSU and It is Very Important than U Use Original Driver Section( don`t modify it) and use GOOD Feedback
Hi, I got lot of PC ps from old dell p3 servers and sun and have pretty good specs...can u help me out how to modify it for higher voltage output like 24 to 35 volts. or if possible convert the +/- 12vdc output to at least +/-24v with equal amperage?
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