22-09-2019, 12:27 PM
(22-09-2019, 04:48 AM)Nick Wrote: I suspect one optocoupler is to take the PS25A power good signal back to the primary and the second one is from the error amp for the +5VSB. The TL594 should be on the primary side. I suspect this is not that an unusual design although HP do odd stuff with connectors etc.
Yes, I did say that one is obviously for the standby supply - 12V in this case - but I'm not sure why a PG signal would go back from the secondary to the primary. That's all taken care of by the PFC section. That section runs all the time - even when the unit is in standby. If it didn't, then the SB rail would disappear. In standby, the PFC chip can run in pulse-skip mode to minimise the energy it uses. Of course, PG is generated for the motheboard, but the primary side doesn't need to know about that.
It's possible it's being used to shut down the supply in the event of an overload or over-voltage situation. I'm not seeing a conventional crowbar across the output - and those are a bit iffy given the high currents encountered in modern supplies - so these days OVP tends to implemented in a less brutal fashion. The TO220 in the middle of the PCB looked like it might have been an SCR (and D261 is conveniently placed to suggest it might be a zener associated with it), but I've got other ideas about what it might be...
But then, the shutdown could equally be achieved on the secondary side. So until I get my hands on one of these, that really is just a guess.
As for the TL594 being on the secondary side, that's actually very common and makes a lot of sense.
First, for a bridge converter, you need a drive transformer for the upper switch. So if going to the effort of specifying a transformer, you might as well drive both switches with it, even if the lower one could be done without. If nothing else, it's a zero-effort way to ensure both switches get the same waveform from the same source impedance - any waveform shaping required can be done once on the primary side of the transformer, rather than having to be done twice, perhaps in two different ways.
Having put the transformer there, you now have freedom of choice about where you put the controller IC.
If you put it on the primary side, you need voltage feedback from the secondary. That's just an optocoupler, but it's an optocoupler you wouldn't need otherwise.
Next, you have to power the controller IC. This means an additional winding on the main switching transformer. Commonly done, of course, but still extra cost.
That done, what about start up? You need to power the controller IC for the first few cycles before the supply establishes operation ("the engine fires"). Historically, this is usually done with a resistor dropping down the 300-something volts down to whatever the chip requires. This resistor wastes heat, so needs to be a large value (few hundred k), and the capacitor across the IC rails (the "car battery") needs to be in good condition so it can "turn the engine over" once it has been charged. Of course, high value resistors (car battery chargers) with lots of volts across them fail high, and capacitors (car batteries) fail. One or the other of these two components account for nearly all start-up faults in switched-mode power supplies...
With the controller on the secondary side, you clearly still have a start-up problem. One way to resolve that is to make the bridge on the primary side self-oscillating. Once the thing gets going, the controller IC takes charge. Neat. That's what the cheap Chinese PSU discussed in the other thread does.
Alternatively, you can put a completely separate supply in there. Luckily, we need a standby supply anyway, so it might as well power the TL594 while it's at it. So, that's a win-win. Lots of good reasons to put the TL594 on the secondary side, and lots of good reasons to not put in on the primary side.
What about the start-up circuit for the standby circuit?
While we don't know the part used in this example, most of these 7-pin ICs incorporate the start-up circuitry within the package. Usually it's a MOS-FET connected to the drain of the main switching MOS-FET. At power-up, the switching MOS-FET isn't being driven, so the unregulated DC input is present at its drain. The start-up MOS-FET passes a small current through to the control circuit and gets it going. When running, the start-up MOS-FET is turned off. No high-value resistors needed, and no need to send the raw DC into the IC on a separate pin. It's pretty smart IMHO...
Incidentally, those who remember the Thorn TX10 will be quite familiar with all this. This chassis was isolated - relatively unusual for the time - and had all the control on the secondary side. A pulse transformer carried the base drive, and a conventional 50Hz mains transformer powered the secondary side. Of course, the set had a standby mode, and most sets at the time that could be switched to standby used a 50Hz transformer for that purpose. Back then, we weren't so worried about standby power consumption.
All fascinating stuff. Well, I think so...
