So, back on the bench trying to fix ebay junk

This time a Digimess SM3040 power supply. Something of a beast of a PSU supplying up to 30V/40A, it's a switcher (well, a 1.2kW linear would be a bit of a handful), without PFC as far as I can see.
I forgot to take a photo of the front but here's a shot of the closely related SM5020 I found on the web.

It's the well known Mastech chassis again - there must be literally hundreds of different makes and models based on these designs.
The fault was "blows mains fuse", actually that wasn't quite true as it took out the breaker when I tried it without touching the fuse at all - not surprisingly I found that two of the main switching IRG4PC50UD's are dead short and I was equally unsurprised to find this soft start thermistor has
literally blown its lid.

Something a bit odd is going on around the leg of this IGBT as well - not sure whether it has got hot enough to melt the solder or whether this is left over from manufacture.

Overall construction is reasonable, primary side components are on one half, then heatsink and magnetics in the middle (where they can be cooled by the fan)



However there aren't any air gaps on the PCB to help isolate the high voltage stuff, probably not that much of a problem given that the main board isn't especially crowded and the overall design provides fairly good separation. I haven't checked that adequate creepage has been allowed everywhere though and I'm a little suspicious about the various daughter boards, eg the aux power supply board has the hight voltage DC on part of it, no slots on the PCB and then feeds low voltage DC into the control circuits.

The high current paths are all heavily re-enforced so it should stand up to the max load of 40A with no problems - except, curiously, for the positive output (see photo below). Although this is connected by an 8 AWG cable (good for 75A or so) it isn't soldered all that well to the board, nor is this trace endowed with extra copper. The heavy 8AWG cables run to the terminals on the back, then lighter 12AWG cables run to the front binding posts. This really is another weakness of the design - whilst it's clear that you are supposed to connect high current loads to the rear binding posts there nothing limiting current from the front post as far as I can see - and more than enough capacity to melt the banana sockets


The meter current shunt looks the part though


Finally the schematic - this is for the SM5020 but is basically the same except for a few component substitutions and, in practice, a few of the components on the board differ from the schematic as well (the diagram is a bit large - click on the "thumbnail" below for the full Monty, all the photos above are linked to full resolution versions as well).

I confess to having only a tenuous grasp of SMPS functions so I do have a couple of questions for the experts. The design is based on a TL494 PWM - I've seen quite a few similar to this - with an auxiliary supply built around a TOP223. After the basic SMPS there appears to be a linear regulator stage built around a 75NF75 FET. So far so good.
One thing I don't understand is the function of the thyristor SCR001, especially as it is paralleled by quite a low value resistor. In fact this is an SG210 inrush suppressor in the unit I have although the schematic has 27ohms for the 5020 and 20 ohms for the 3040 (at least I think it's 20ohms, it says 10ohms x 2 but doesn't say whether series or parallel). This is supposed to have a cold resistance of 7ohms and a resistance at rated current of 0.2ohms.
Another thing I can't figure out is the regulation - I assume the SMPS is supposed to track a little above the output voltage then the final regulation is a linear regulator (the 75NF75 and regulator built around U401) but I can't see how the voltage feedback is done - there's a path back to the TL494 via R302 but I don't see how any tracking is achieved.
I've not got this up and running yet as I'm waiting for replacement parts - I've opted for IRG4PC40UD's as they are 1/3 the price of the 50 and I can't see that a 55A part is actually called for. In any case the schematic says IRF460LC which is only rated at 21A. Similarly I'm assuming that the thyristor, should it need to be replaced does not need to be a 55A part either - does that sound reasonable?