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Table of Contents
SGI Indigo2 Impact Power Supply Repair Notes
Introduction
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:
| Voltage (V) | Current (A) | Power (W) |
|---|---|---|
| +5.0 | 40.0 | 200 |
| +3.5 | 36.0 | 126 |
| +3.5 | 12.0 | 42 |
| +12.0 | 4.25 | 51 |
| -12.0 | 0.5 | 6 |
| -5.0 | 0.8 | 4 |
| +5V Standby | 0.1 | 0.5 |
(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).
Basic Theory of Operation
(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.
Primary Side
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.