The TL431 And You
What is a TL431? It is one of many widespread three-terminal programmable references, often referred to as programmable zener diodes due to how they mimic the zener diode's shunt mode voltage regulation.
What is a shunt regulator? It is a regulator that works by sinking however much current as it has to or can across its main terminals to prevent the voltage across them from exceeding a set value, similarly to how surge protectors short out surge energy. In the programmable reference's case, the current it shunts from its cathode to its anode is a function of the voltage difference between the voltage applied to its reference pin relative to its anode and the device's own internal reference, 2.5V in this case. Using a simple voltage divider between the anode and cathode to drive the reference pin makes it possible to program any shunt voltage from the internal reference value (VRef tied to the cathode) up to its maximum voltage rating, provided that total power dissipation also remains within limits. If you read my SL300 repair, you saw my little shunt regulator schematic, which I used to drag the surging 5VSB output down to about 5.2V using a PNP transistor to boost the shunt current.
The TL431 and its countless variants from multiple power management and linear integrated circuit manufacturers are the duct tape of instrumentation electronics. They get used everywhere some sort of input needs to be weighed against a known quantity, and there are many creative ways to use them. In the case of power supplies, they function as comparators to drive the feedback photocouplers due to their flexibility and simplicity: two resistors to set the reference pin ratio, which sets the comparator's threshold, one resistor to limit current through the photocoupler, the *431 and the photocoupler, five parts in total, seven if you include an RC filter to improve transient response. Of course, it has other common uses as well, such as fan speed control and combining (ORing) feedback output from the 3.3V remote sense wire, as was the case in both the SL300 and AR300.
This is the AR300's actual 5VSB feedback circuit. If the 5VSB voltage is below roughly 5.1V, the resistor divider brings the reference pin below 2.5V, the reference stops drawing current and its cathode's voltage rises to rail voltage minus the photocoupler's diode voltage drop. If the output is above 5.1V, the resistor network pulls the reference pin above 2.5V, the cathode current increases, turns on the photocoupler, signaling the primary side to reduce power.
Looking at voltages around the AR300's 431, we see the reference pin at 2.74V and anode at 0V, which means it should be fully turned on. This is further confirmed by the 2.05V cathode-anode (VKA) voltage. The path from cathode to anode within the 431 and direct equivalents has three base-emitter junction voltage drops, and at about 0.65V each, 2V is just about the lowest possible shunt voltage. There is little doubt that the photocoupler and its current-limiting resistor are seeing the balance of the 12V currently present on the 5VSB output.
From the looks of it, the 431 appears to be working perfectly fine. Its state is consistent with a shunt reference trying to pull its cathode down as hard as it can. Time to move on to the next step: the photocoupler's LED.
What does the signal across the photocouper's LED look like in action? I'm glad you asked since I have a screen capture of that--it is that 10VSB picture I said I would get back to later. Now is later.
Here is what happens to the 5VSB feedback photocoupler LEDs as the 5VSB output starts ramping up after the power supply gets plugged in. Until the 5VSB output reaches 5V, voltage across the LED settles at about 1V due to the TL431's minimum operating current of about 100µA. Shortly after the 5VSB supply passes the 5V mark at the vertical cursor, though, voltage across the IR LED increases to 1.5V, indicating a significant current increase through it. You can also see the LED's anode and cathode voltages dropping by roughly 2V when the TL431 turns on. With 5VSB at 12V, the LED's anode at 3.6V and the 75Ω in-between, Ohm's law dictates about 112mA must be flowing through them, which is just over double what the 817 photocoupler's LED is rated for and slightly more than the 100mA the TL431 is specified at.
Despite the high current flowing through the LED, it appears to still be behaving like a typical infrared LED should, so I am inclined to believe it is still perfectly fine after possibly weeks of operating at more than twice its rated power. I did not take measurements while the power supply was connected to the motherboard so I have no idea exactly what the motherboard and 5VSB output were actually exposed to back then, and I had no intention of plugging it back in just to find out. I wonder how much longer this could have continued going on if I had not happened to need to use my Pentium 4 for a while longer. If the photocoupler's LED had failed, there would have been no feedback whatsoever left and things could have become much worse.
The fact that the 5VSB output was indeed 5V (if you overlook switching transients before I added external capacitors or replaced the internal ones) indicates that the primary side of the feedback loop was still at least sort of working, much like the SL300's. After investigation, it looks like it is none the worse for wear.
There's nothing more to see in the relative safety of the secondary side. It's time to start poking around the primary side.