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I recently have had the (dis)pleasure of working on a PSU for an Silicon Graphics Indigo2 Impact workstation. The PSU was manufactured by Zytec, with an SGI part number of 060-8002-001, revision A according to the external sticker. It provides the following rails:

(Total power shall not exceed 385 W)

This power supply differs from the regular Indigo2 supply, as it has an extra 3.5 V supply rail to run the Impact graphics. This rail is broken out via a blue high-current ribbon cable. There is also a small black and yellow twisted sense cable, which must be connected near the power cable on the graphics riser board. If the Impact graphics option is not installed, the termination PCB (mounted on the side of the power supply) must be used to allow the PSU to operate correctly. Without voltage feedback for the Impact rail, it will not work as expected (I have not tested what it does. The rail either will not regulate correctly, or the supply will shut down).

(Note: This is far from complete. It is based off my limited experience repairing one of these supplies, as well as my own research. I will attempt to cite any sources I used.)

This power supply can be divided into two sections (conveniently, the primary and secondary are each on distinct PCBs), as with most line powered switch-mode power supplies, and is of a forward converter topology. Additionally, it has a smaller (self-oscillating flyback?) supply which is used to provide the supply's (unregulated) 18 V standby rail.

The primary side of this power supply was designed around a UC3845 current-mode controller from Texas Instruments. This controller is used to drive the primary-side switching device for this power supply. However, it is being used in an unusual configuration, as it does not perform any kind of regulation. There are two optocouplers (MOC8101) on the primary-side board. One of these is used to control the supply. When the secondary-side board wants to start the supply, a control signal is allowed to float. When this happens, the supply starts oscillating and powers up. The second optocoupler appears to be used for some kind of shutdown signal from the primary board to the secondary, but has not been investigated further. Additionally, while this supply is not power factor corrected, it does have automatic line voltage selection. This functionality is based around an AVS1AC automatic voltage selection switch. This device configures (via a TRIAC) the primary filters as a voltage doubler when used on 120 V, or as a regular rectifier when used on 220 V.

There are several connections between the primary and secondary PCBs in this power supply, split across two connectors and four cables. First, there are the two sets of high-current cables which exit the forward converter transformer directly. These are the square-wave AC outputs, and are (likely, I have not specifically confirmed) used to derive the 5 V and 3.5 V rails of the supply. These connections are soldered directly to the secondary-side board, with a fusible link at the end.

Next, there is a four-pin Molex Mini-Fit Jr. connector. This connector also carries AC square-wave output, at the lower currents required for the +12 V, -12 V, and -5 V rails. It has a connector on the secondary-side PCB end of the cable.

Finally, there is a six-pin control cable. It has a reddish-brown connector on both ends. It carries the standby voltage (+18 V, red wire) and two control signals (yellow wires) from the primary-side to the secondary-side PCB. There are also three grounds (black). One yellow wire provides an isolated power-on signal, and the other is a control signal from the primary-side to the secondary-side of unknown purpose, as previously described.

This board is where most of the complication of this supply lies. As the supply is a forward converter topology, there are additional magnetics on the secondary side, and the supply is entirely secondary-side regulation.

The simplest rail of the supply is the 5 V Standby supply. It is regulated from the 18 V supply via a LM340T-5.0 regulator. The 5 V from the regulator output is provided to the computer, but is also used for operating the control logic on the power supply itself.

The negative voltage rails are the next simplest rails. They are provided using a pair of MC34166 inverting step-down converters. These are self-contained and fairly straightforward to troubleshoot. The circuit appears to be very similar to the typical application found in the datasheet.