# PC power supply as DC/DC converter



## jhuebner (Apr 30, 2010)

*PC power supply as DC/DC converter 500V*

Hi,

has anyone done this and can point out the matching thread? The search function didn't help...

For my 500V battery pack I am planning to use a PC power supply for DC/DC converter. Same arguments as always: commodity item, cheap, proven.

The input elcaps can be swapped for some higher voltage, lower capacity film caps, the input 400V bipolar transistors (2SC2625) for some higher voltage ones (2SC 4237)

Cool?

EDIT: actually found some threads now. But no one ever modified the power supply for higher input voltage.


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## nulluser (Mar 4, 2012)

Just out of curiosity, why 500 volts? 

Also keep in mind that 500 volts DC is no joke. Brushing against 72V or even perhaps 120V is not too crazy, but 500VDC is ruthless.


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## jhuebner (Apr 30, 2010)

nulluser said:


> Just out of curiosity, why 500 volts?
> 
> Also keep in mind that 500 volts DC is no joke. Brushing against 72V or even perhaps 120V is not too crazy, but 500VDC is ruthless.


I'm running a standard industrial acim, wants 360Vac @ 3600rpm.
I know its dangerous, I hope the accident rate is low enough for me to finish the car.


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## evnz (Jul 24, 2010)

No idea on that voltage but gav(kiwiev)did it on his car there will be a set on youtube on what he did


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## Tesseract (Sep 27, 2008)

jhuebner said:


> ...
> For my 500V battery pack I am planning to use a PC power supply for DC/DC converter. Same arguments as always: commodity item, cheap, proven....


Identify the topology first, look at the primary, switch and secondary rectifier waveforms at high line (probably 275VAC) and no load second. Now ask yourself if the transistors are going to be capable of operating with a 20% shorter on time and/or the primary is not going to saturate when you add another 100-120V on top of the input.

In other words, I'd plan on having to rewind the transformer, and then the practicality of this venture rapidly turns questionable. Proper transformer design and construction is the very deepest end of the SMPS pool.


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## jhuebner (Apr 30, 2010)

I am a few hours into the project now and its looking good so far. Here's what I did first:

- Replace the 400V input bridge transistors with 800V version 2SC 4237
- Replace the 2x200V input caps with 2x400V film caps

Then I removed stuff thats no longer needed:
- precharge PTC
- rectifier
- passive PFC inductor
- All power outlets except one
- PS-On connected to ground (immediate startup)

Next I looked at the auxillary power supply. It uses a KA1H0165R power switch that is controlled from the secondary side with an opto coupler. It is rated at 650V maximum Drain-Source voltage. When I applied 220Vdc the spikes reached about 400V. So at 500V they'll probably reach 900V.
To start out I disconnected the aux supply to see what happens. Little surprise: the PSU doesn't start up. Next I connected it back on through a ampmeter. The meter displayed 6mA current draw. Next thing I noticed was that the aux supply is only needed for startup, after that the secondary side can supply itself over the main transformer.
Now I ended up connecting the aux supply to the midpoint of the input caps where it only sees about half the voltage. The soft start time is now fairly long (about 10s) but thats ok for the application.

Finally I cranked up the voltage to [email protected] (unregulated). It sags to 13.5V @ 10A.

Concerning the duty cycle problem I added some load on the 5V rail.

The input voltage ripple is only [email protected]

Tomorrow I'll test with a 600V DC supply and add pictures.

Here are some mildly informative pics:
















_Input current @ 11A output current. What you see is a 40kHz AC current of 0.3A with a DC offset of 0.57A_


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## jhuebner (Apr 30, 2010)

So today I did the 500V test. I started at 300V and slowly turned up towards 500V, letting it rest in between. At 475V something went up in smoke and the PSU shut down.

Seems I'd forgotten a component along the path: a 385V varistor which is supposed to do what it did: short the input on over voltage. Removed it and the PSU starts again. Will do the next test probably next tuesday.

I'm thinking of replacing the varistor with a higher voltage one to keep the protection.


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## subcooledheatpump (Mar 5, 2012)

I've used a PC powersupply as a DC/DC before. A few differences, I am running 600 volts DC for an industrial induction motor, but I used two powersupplies in series, so each got 300 volts. It actually worked well except for the output voltage wasn't high enough. I didn't want to bother with modifying the output voltage so I abandoned it and simply charge an accessory battery when the vehicle is charging. If you still want to mess with it, try two in series


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## jhuebner (Apr 30, 2010)

Yeah I considered that but then wasn't sure whether it'd work. How did you parallel the outputs? What if one device needs more current than the other, doesn't that disbalance the voltage of the two?


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## subcooledheatpump (Mar 5, 2012)

I just twisted all matching colors togther on the output side. Seemed to share current pretty well. That could be a problem though, I'm sure there is some variance between the supply voltages.


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## steven4601 (Nov 11, 2010)

Likely a forward type topology, changing the components you mentioned should aid in obtaining your goal. The ringing might get worse and you may also need higher volage ouput diodes. 

Safety:
However, you need to humiseal the entire circuit board. Top and bottom... Clearances for home power supplies are usually not more than what is required. So your making a sparkgap in the shape of an ATX supply 

Add proper DC rated fuses for the input. It would be less than fun to strand in a fireball on the highway or Alps due to a modded 20 dollar/euro computer supply 

Ive seen a few dc arc fires start, (rectified 3x63A 400Vac) they take no prisoners .


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## jhuebner (Apr 30, 2010)

subcooledheatpump said:


> I just twisted all matching colors togther on the output side. Seemed to share current pretty well. That could be a problem though, I'm sure there is some variance between the supply voltages.


I guess when under load the current will flow OUT of both supplies whereas with little load the current might flow out of one and into the other. Still something to be tried if my current plan doesn't work out.



steven4601 said:


> Likely a forward type topology, changing the components you mentioned should aid in obtaining your goal. The ringing might get worse and you may also need higher volage ouput diodes.
> 
> Safety:
> However, you need to humiseal the entire circuit board. Top and bottom... Clearances for home power supplies are usually not more than what is required. So your making a sparkgap in the shape of an ATX supply
> ...


With humiseal you mean coating the PCB with some kind of plastic?
Just came across the fuse problem as well, the one thats built in is not designed to interrupt 500Vdc.

Apart from that I could finally complete the high voltage test. Results:
- Output voltage stable up to 560V input (tested about 15 min.)
- No weird noises or smells
- No unusual current draw, 170mA in current for 5,[email protected] output current -> 81% efficiency
- Device doesn't start up above 300V. Must be started at 300V and then voltage can be risen to whatever needed.

Last thursday I tested with my battery pack of 360V and a 12V lead battery connected to the output. Results:
- As noticed before device doesn't start up at 360V (fan spins shortly)

Back home I tested again with 230V:
- Device still doesn't start up. Reason: one input bridge transistor dead
- Replaced it, works again
- Device doesn't start up when 12V battery is connected to output. Can be fixed be connecting the battery through a relay.

So I have at least three problems:
- Doesn't start up above 300V, might even break
- Output relay must be added
- Possible spark gap problem

The first one might stop the show as I don't have a simple solution for it.


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