Semi-technical: Can I run a PicoPSU off a SMPS?

[2timer]

I suppose that technically, this would be like having the PicoPSU in "series" to the SMPS.

The point of this is to use the PicoPSU to provide power, and use the SMPS to boost the 12V amp output from 14.5A to 25A, allowing me to use a full 150W graphics card, all within a 3.5L space.

The SMPS model is a Mean Well EPP-300-12. It was selected mainly for the price and rating, but it has 2 screw terminal posts on the output side.

Question: would the following wiring configuration work, or do I need additional wires running somewhere?

 

[PottedMeat]

i dont see a problem

Power (with Airflow) 300 Watts
Power (convection) 200 Watts

i would put a fan on the psu though.

 

[2timer]

i dont see a problem



i would put a fan on the psu though.

Thanks.

This SMPS was chosen because it's the cheapest 300W unit I can find with a warranty. So I didn't select it for any reason except price and rating. It has a 2 pin fan header (+12v and COM), so I'm going to definitely put a fan on it, the spec sheet "recommends" airflow across the 3" side, but I don't think it would matter if I had an 80 mm fan blowing from the top?

 

[mindless1]

Only you can measure temperature at load in the case. If it gets hot you have to derate the current capability of the unit meaning it is only a 300W PSU with their recommended cooling in "most" uses. They already did that for you and rate as 200W free air, so it should be fine powering a 150W video card.

BUT you cannot do it the way you have pictured. You need to link the grounds from both together so there aren't ground loops/currents in the system but must keep the 12V rails separate because they are both regulated based on feedback.

So, you need to verify that your video card doesn't have continuity (multimeter measurement) between the power socket's 12V pins and the PCI-E slot's 12V pins which are pins 1, 2, and 3 on both sides (see a PCI-e connector pinout diagram). If no continuity you can power the video card with the 300W PSU, but if there is then you would need to cut the traces or grind away the contacts and put in jumper wires from the connector to the subcircuit using power from the slot. Let the Pico power what it can, deciding how much current is available for CPU, HDDs if present, etc then what it can't do would also be powered by the 300W PSU but you never want the 12V rails of each to have continuity, a conductive path to each other.

You also should tie the PS-On signal from both PSU together so they have synchronous starting. Hopefully that will work, or else you may be left finding another way like the output of one triggering a relay that pulls the PS-On line from the other PSU down to ground to turn it on.

At 5" x 3" and $82, that PSU isn't all that much smaller yet more expensive than just pulling the PCB out of some standard ATX PSU and using it like an open frame PSU. I mean something like a Corsair builder series 430W, then you wouldn't need the Pico at all. You could get the modular (wiring harness has connectors to PSU) version or cut off unwanted wires at the PCB and shorten the rest. I'm not clear on the actual space you have available when you described a 3.5L area.

 

[darthsidious]

It probably will work, but keep in mind : High efficiency power supplies are among the most challenging loads for a power converter to drive from a stability perspective.

Most power converters regulate their output voltage by measuring the output voltage, and increasing/decreasing the output current when the output voltage goes down/up to bring it back to the expected level. this assumes loads whose current draw goes up as their input voltage goes up.

A power converter is kind of the opposite. Assume the converter has a constant output load (your graphics card for example). The input power is also constant then. As power is V*I, this implies when the input voltage to your power converter goes up, it's current draw goes down! This is the opposite of a resistive load, which is what most power supplies are designed for, and makes it hard to regulate. If your PicoPSU behaves like this, it will be hard for the SMPS to drive it.

In real life, there are enough other losses (and complex control systems), that this will likely not be an issue. But just something to keep in mind.