[jan-jaap]

Part 2, where we focus on how the SGI PowerOne PSU is controlled using the 12-pin connector on the input module.

The SGI version of the PowerOne SPM5 series is different from a standard model in two ways:

1. The meaning of the inhibit signal on the 12-pin connector. The standard SPM5 series enable outputs by default when input power is applied (they are 'ON'). The SGI model doesn't. If you connect the inhibit signal to ground, the SGI PSU enables outputs. You may have noticed a blue jumper wire on the 12-pin connector in some of my photos: that's how I switch on the PSU outside a system. This behavior is configured with positions 4 and 6 of SW2 in the control circuit and documented in PowerOne "High Power Modular Series Applications Note #H7 For Product Series: HPF3, HPF5, & SPF3".
   
2. There's an extra circuit board installed, between the 12-pin connector and the PSU control circuit. It's a simple circuit, I traced it and it seems to release the inhibit signal if the overtemp signal goes active. In other words: it switches off the system if the PSU overheats, rather than just signaling this condition to an external system controller. 
   

This is the PSU control circuit:



I didn't fully reverse engineer it, but you can recognize major components:

• It's powered from a secondary winding of the auxiliary bias supply I discussed before. So it's galvanically isolated from the mains. It has to: it connects to the 12-pin connector on the input module.
  
• You can spot an 7805 regulator.
  
• You can recognize some voltage comparators and an Airpax overtemp switch.
  
• The SW2 controls behavior including the inhibit function, but is largely undocumented.
  
• The (white) MOC8104 optocoupler allows the control circuit to control the auxiliary bias inverter.
  

I was interested in the function of that optocoupler, so I did some measurements. Remember: the aux bias supply is always on, even if the PSU outputs are inhibited. In this state, the voltage drop over the LED side of the optocoupler is 1.17V (DC). The voltage drop over the C-E is 0.2V. In other words: LED is on, phototransistor in conducting. If I switch on the power supply, the voltage drop over the LED rises to 2.67V, and the drop over the phototransistor to 13.7V. This voltage drop over the LED is only possible if no current is flowing through it. The phototransistor is no longer conducting.

What this does to the aux bias inverter (the circuit built around the UC3844N and the BUK456) is kind of interesting.

This is what the PWM output of the UC3844 looks like when the PSU outputs are off (inhibited):



And with the PSU switched on, it looks like:



Somehow, when the PSU is in standby, every 3rd pulse from the UC3844 is missing, and the amplitude is much less as well. The voltage generated (and used) by the aux bias supply goes up when the PSU is switched 'ON'!. The UC3844 is not like a digital circuit that requires a fixed supply voltage. When the PSU is switched 'ON', the voltage goes up which also makes the PSU fan spin faster. I knew this, I just never realized how this works...

The application note hints at this as well:

2. Inhibit Signal
What is the purpose of this signal?
By activating this signal, the end user can inhibit all output modules simultaneously. This signal interfaces with the pulsewidth modulation circuitry of all modules and turns the pulsewidth modulation IC either ON or OFF.

The pulsewidth modulation IC discussed here is probably on the output modules, because obviously the aux bias supply inverter is always 'ON'.