This will be the 0V reference for the main transformer when added later.The midpoint between C3 & C4 looks as if it should be connected to something (perhaps just a 0v reference).
The schematic I have drawn matches the schematic I found on the net for the SHIDO ATX 250 watt power supply which I have also attached.I can't see where the standing collector current into the output transistor of IC3 comes from (perhaps there's a wrong connection around D501 or ZD1).
The resistor values on the base of Q2 seem to lead to the wrong currents (perhaps wrong values or wrong connections?).
Currently I don't have access to a storage scope, I only have an older 60Mhz analog scope.It would be worth applying a lower voltage to DCin & DCout for testing that Q1 & Q2 are switching correctly. If you can watch their collectors on a storage 'scope you can check the relative timing and perhaps a clue to the fault?
I have now replaced both IC3 and IC4 and I'm still getting the same results (No output volts)Did u check primary section diodes, IC3 and IC4?
As stated before, I did not make the transformer, it is the one from the orginal working power supply, The schematic works in LTspice, all be it that I have guessed the inductance values for each part of the transformer. So I don't think that this is the issue, also the circuit worked for a few seconds before smoking the transistors the first time it was powered up.Make sure you have the phasing of the L3a/b correct
PC power supplies seem to generally require a load on
either the +5V or +3.3V output, to defeat some open
circuit detect function. Which one, depends on the
generation of PC / supply. Try a 10 ohm resistor to
ground on each output in turn and see if any of them
do the trick. Then you can work on finding out if a
higher value will keep you lit and waste less power
I've cobbled a couple this way for 5V lab bricks.
I will retrace the original PCB circuit and check If I have the transformer pins connected correctly.I agree with E-design and betwitxt that if one of the windings L3a or L3b was cross-wired (out of phase), then it is very unlikely to oscillate.
The fan and led both worked for about 20-30 seconds before the transistors went up in smoke that first time. Would this not have been longer than what the initial switch-on pulse could provide? If so then this may indicate that the transformer is connected correctly, I'll still check it to confirm.Referring to the brief period when your circuit powered a fan and an LED, I wonder if that energy could have been transferred during just one, initial, switch-on pulse, and that there was not a period of full oscillations?
D03 is a 1N5819 which is a 40V, 1A Schottky diode.Finally, your schematic shows D03 as a zenner, though this will not be the cause the problem either - just an observation.
I will re-check the entire pcb, although the circuit is 99.9% identical to the SHIDO schematic other than a couple of component values, So far the only difference are the orignial circuit use 4.7 ohm for R502 and a 220 ohm for R507, I have tried these as well and the circuit still doesn't work.If a simple reversal of either of the winding terminals still doesn't fix the circuit, I wonder if you might benefit from re-checking against the original PCB tracks around the bases of the 2 transistors Q1 and Q2 and the collector of the opto IC3?
I will try this after re-checking the intire circuit against the original.It should be possible to check that the circuit is at least oscillating by lowering the 680k bias resistor to say 68k and test the circuit on a lower voltage lab supply (~30VDC) with current limit.
I think I might try this as well after the using the LAB supply method.If you can't do that put a small wattage (around 5-10W) bulb rated at mains voltage in series with the DC supply to the switcher.
Thank you for your suggestions and explanation as to how the circuit should work.Operation:
At the moment of applied DC initial turn on of Q2 is via R501 (680k). With Q2 turning on we get regenerative feedback via winding L3b. In other words as the dot end of L3a is pulled down through Q2, the non-dot end of L3b will move more positive providing more drive to Q2 base through D502, C502 and R504. This will cause Q2 to turn on quickly. Because of this current, the voltage over R502 will increase. When this voltage is enough (around 0.6 to 0.7V), Q1 will start to turn on and divert base current away from Q2. With base current being robbed from Q2, it will start to turn off. This will cause the collector voltage of Q2 to go up. Due to classic flyback action all winding voltages reverse and L3b will aid in turning off Q2 through C502 and R504 at this time. The maximum current flow is limited by R502 and turn on b-e voltage of Q1 (Imax = Vbe/R502). The feedback circuit through IC3 will help turn on Q1 and hence turn off Q2 sooner to establish voltage regulation action on the +5VSB line.
I understood, that you are referring to post #1 (single ended, self-oscillating) and #5 (push-pull, PWM controller driven)? There aren't much similarities in my opinion.although the circuit is 99.9% identical to the SHIDO schematic
I understood, that you are referring to post #1 (single ended, self-oscillating) and #5 (push-pull, PWM controller driven)? There aren't much similarities in my opinion.
I was aware the pinouts where different and had updated my footprints to allow for the difference. The mistake I made was when I read the datasheet I misunderstood which way up the pins where viewed from. I read it as c-b-e instead of the correct e-b-c, hence why the 180 degree rotatiion of the device should fix the problem for this PCB. I'll have to update my footprint again before building the next board.The C945 reads e-c-b and the 3904 e-b-c so make sure you get it connected correctly.
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