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AuxPSU is an obsolete circuit. It is no longer needed because a new high-power Computer PSU is used.

AuxPSU - is an auxiliary power supply unit for 12V heavy demanding devices of the embedded PC. Its purpose is to source pure 200 to 300 Watts of energy via the 12V rail.

The need in a separate PSU raises, when installing even mid-level dual-core CPU (around 50-70W, 110W in case of AMD Fusion design) and mid-level GPU (around 150W).

This design is possible, because 4 pin CPU connector on the ATX motherboard is always a separate power rail. Powering up a GPU from a separate power rail may be more tricky. The 6-pin (75W) or 8-pin (150W) PCI-Express power connector does usually have separate 12V rail, but it is not guaranteed by a standard.

In each case, all the power rails must have common ground (Black wire). Balancing power with diodes (addition of currents) is not an acceptable solution, because it forces each of the PSUs to work under the extreme load before its output voltage goes below the output voltage of the concurring PSU. Even the usage of an "ideal" diode schematic (when diode is replaced by a specially controlled N-channel transistor, and there is no pure voltage drop) does not solve problem entirely. In such case, the power supply of a special design, which allows a parallel (even redundant) connection must be used.


Schematic capture

AUXPSU schematic capture. Version 2.1. This board is designed to provide the high power output (300W) via the 12V rail to supply computer's CPU and GPU.

User first plugs XS1 to the Power Splitter output (4 Pin Molex 22.2V output), marked "Computer2". When external power is applied to circuit, the Power Block is opened (switched off), and no current is flowing through it.

User also plugs Computer ATX PSU 4-pin Molex connector to the on-board socket XS2.

Computer PSU is initially turned off, and Switch block is powered down. In this state, the Power Block is also in its initial "open" state.

When external signal starts ATX PSU (turn-on switch on motherboard), the PSU begins to supply it's power rails, including 12V and 5V, and the appropriate voltage rises on XS2 pins. The 12V level turns on the Switch (Q1), and so the Power Block is also turned "ON" (all P-channel transistors Q1,Q2,Q3 are switched to "closed" state).

In this moment the direct connection (with very low RDS(ON) of Q1/3 of the line) from Vs to Vpwr is established.

Vicor Mini DC-DC converters DD1,DD2,DD3 are connected in parallel with fail-safe scheme (decoupling capacitors C_PR1-C_PR3).

Paralleled current output is then dispatched via the XS5 (GPU 8-pin), XS6 (CPU 4-pin) and XS7 (Peripherals 6-pin) connectors.

PCB Layout

Two-sided PCB is shown below:

PCB of the AUXPSU. This version is not copper-poured (but the final version is).

Device photo

This is a copper poured version. There are minor flaws in the current design. Also, only one of three DC-DC converters is mounted, so the output power of the 12V rail is approximately 100W.

A photo of the assembled AUXPSU

Test results

See also

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