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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I have tried to hack a 300W ATX psu following uzzors guide ( ), and I feel like I'm stranded at a strange problem.
I have connected a 3.3ohm resistor on 5V and a 15ohm resistor on 12V to run it with a load.
I have located the voltage reference resistors for ground, 5V and 12V and put a potentiometer in series with them.
I have disabled the over voltage transistor by keeping pin 4 on the KA7500B grounded.
When I turn the potentiometer the PT starts being more noisy, screaming and the heatsinks gets warmer faster, but im still left with +5V and +12V on the rails. So I'm no longer sure what is missing or if I did something wrong. :/
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Sounds strange. Check pin 13 too, it should remain at V+ the entire time. It's for output control and if low will turn off the outputs. Other than that I'm stumped. Have you checked the rectifier and capacitor ratings? It seems as though the power is increasing, but being diverted somewhere else...
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I got +5V at pin 1,2 and 13, there is no change in pin 1 voltage when adjusting the potentiometer, so maybe there isnt a feedback or its still not loaded enough?
Its a PSU visible identical to the one you used in the article, so ratings are properly the same, but as I cant even get it above 12V I figured something else must be wrong.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
The voltage should change regardless of the load, and I think you've loaded it enough to kee pit stable. What bothers me is that the pin 1 voltage remains constant although the voltage divider ratio changes. Pin 16 is also a non-inverting input, maybe it was used instead?
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Its status quo trying the mod on pin 16's resistors, I'm on the edge of building a SMPS from the ground instead, as its almost as simple as modding an existing :)
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Just to add, many of the cheap PC PSU's will explode even before reaching their maximum current ratings.
E.g. a cheap "300W" PSU with the 12V rail rated at 10 amps might actually supply only 6A before reaching a meltdown state, so on the 12V rail you will get only 70W out of a "300W" PSU...
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
You don't want to be messing with the feedback resistors, as this alters the loop gain of the power supply. Doing this is likely to make the control loop unstable - Symptoms of instability are a squealing noise, excessive heating, wild oscillations in output voltage and bits exploding.
The loop gain of the power supply is something that the original designer took great efforts to control over a wide frequency range using a whole load of complicated maths and theory. It's a difficult excercise to get this right at the design stage so you don't want to mess with it unless you have a very good understanding of control theory.
In order to change the output voltage you want to alter the set-point (or the reference voltage) that is fed to the voltage error-amplifier (Vea), NOT the loop gain. In other words, leave the feedback resistors that are connected to the EA's inverting input alone. The resistors that you want to be changing are the ones that present the reference voltage to the non-inverting input of the Error Amplifier. By changing the reference voltage to the EA, the EA will servo the feedback voltage to equal the new reference, and the output will change accordingly. There _should_ be no instability problems because the loop gain hasn't been messed with.
DISCLAIMER: Attempting to alter the output voltage of a SMPSU from its original design voltage can fail for a whole load of reasons, such as:
1. Saturation of the transformer 2. Exceeding PIV of rectifiers 3. Exceeding reflected voltage in flyback converter 4. Exceeding smoothing capacitor rated voltage 5. Exceeding rated output current or power rating 6. Change of control loop dynamics with duty ratio for some converters 7. Insufficient or excessive bias voltage for the controller (auxilliary winding that supplies the PWM controller can now be under- or over-voltage.) 8. Maximum or minimum duty ratio reached 9. Excessive snubber dissipation
As for whether it is easier to mod an existing design or start from scratch, It depends on your skill level. It's probably easier to mod an existing design if the voltage change is only +/- 25%. For anything greater you'd probably have to do a lot of calculations and change so much that you might as well start with a blank piece of paper. If you've got the right skills you can evaluate both routes to get to the new spec that you want.
It is possible to do things like double the output voltage of a SMPSU (and half the rated output current) by only changing about 5 or 6 components. And if it is done right the control side is tricked into thinking nothing has changed. Such drastic changes from the original spec require a lot of thought and planning to be successful.
-Richie.
EDIT: looking at the schematic here:
I'd change resistors R19 & R24 to alter the output voltage by altering the reference voltage into pin 2 of the Voltage EA. You want to change the ratio of R19 to R24 as these act as a potential divider. However you want to keep the parallel combination of R19 & R24 to be roughly the same as it was before, as this acts with R18 and C1 to form part of the control loop's critical compensation circuitry. Leave R20, R21, R25 & R26 alone as changing these will directly alter the loop gain.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I tried with 47k potmeters in series with R19 and R24 equilants in my PSU and restored the feedback.
Exactly same results as before, no matter the values / ratio there is no change in output voltage.
My skill level is low regarding SMPS and SSTC designs so I thought this would be an ok start to reverse engineer something that works and try to alter it abit to gain knowledge of its ways.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
It should be working, there must be something else in place limiting the voltage. Have you tried decreasing the output voltage, maybe there's still some protection circuits in place.
For future reference Richie sent me this explanation of stability problems which can/will occur when modding, and how they can be reduced.
When you modify the feedback resistors in a SMPSU you are altering the set-point (i.e. the voltage that the control loop aims to keep the output regulated at) and also the loop gain (basically how drastically the control loop reacts to changes in the output voltage.)
You should really change the reference voltage being fed into the Voltage Error-amplifier (and leave the feedback resistors alone.) This results in only the set-point changing, and no change to the loop gain. Therefore the regulation and stability of the PSU are not affected.
I'm guessing that the SMPSU that you modded was to increase the output voltage only? It is possible to get reasonably acceptable results by decreasing the amount of feedback in an attempt to raise the output voltage from the supply. Decreasing the amount of feedback tricks the supply into thinking the output voltage is lower than it really is, and it increases the duty-ratio to compensate. The reason you can get away with this is that decreasing the feedback also decreases the loop gain. This acts to make the control loop more sluggish and more stable. So in summary, you can increase the output voltage slightly by decreasing the amount of voltage feedback to the error-amplifier, without much risk of instability.
Trying to decrease the output voltage by increasing feedback is where the problems start. Increasing the feedback does initially trick the PSU into thinking the output is over-voltage and decreases its duty-ratio accordingly. However it also increases the loop gain and eventually the PSU will over-compensate for any disturbances at the output. The result is that it will usually break into oscillation before the output has reached half of the previous output voltage. This is because the control loop is normally designed with this 2:1 safety margin built in to allow for all of the component tolerances in the supply itself and the feedback path.
If you didn't encounter any stability problems in modding your SMPSU, then i'm guessing you only tried to raise the output voltage, and didn't try to make it variable down to zero? The later is where you really do need to alter the reference to get any success.
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PSU hack troubles
