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How to convert a HP HSTNS-PL18 psu to 13.8V ?

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magic:
Another possibility is some sort of overvoltage protection at the primary, although I think that would more likely shut down the PSU completely until it's power cycled.
Any pics or schematics? What sort of controller is there?

rjmusto:
Hi and thanks for the reply.

I have not been able to find any schematics for these supplies.  Lots of YouTube videos showing how to mod them for 13.8V, but not seen anybody then run them hard.....  I first thought I just had a faulty one, but when the second did the same, from a differewnt sipplier, different batch, I figured it was a design issue.

I'd be much happier if I could find a schematic, don't like doing these 'blind faith' mods.

Here's a photo though. The yellow jumper wire is the mod.

Ralph

magic:
If you look up datasheets of the ICs, they will contain typical application schematics.
UC3818 appears to be a PFC controller.
UC3895 is a PWM.
Not sure what the third one is.

I'm suprised to see the PWM on the right side of the isolation gap, does that mean that the PFC is isolated and produces output-ground-referced HV DC which is then stepped down by the PWM? :-//

Anyway, it's not entirely a straightforward circuit so modding it will take a few hours of reverse engineering and searching through datasheets and application notes to make sense of the circuit.

rjmusto:
My gut feeling on this was perhaps the same as yours, that some envelope protection was kicking in. Since these supplies are meant to be set to 12v, it's reasonable to assume that overvoltage protection could be set to 14v or 15v and so 13.8v is getting kind of close. The environment they are intended for demands tight controls, so seems a fair assumption.

There are 3 other trimmers on the control board. I marked their set positions and then tried tweaking each one in turn to see if I got a change. One of them did. So I can now set the output to 13.8v and go up to over 25A with no issue.  Since I could pull over 20A previously with lower voltages, I don't think it is a current sense issue, but a voltage sense one. And this new setting has upped the trip point. Ok, it's a guess, but makes sense (no pun intended).

It seems odd that, given the popularity of these supplies in the hobby market, that this behaviour has not been mentioned elsewhere.

Anyhow, I'll soak test these two and see how it goes...

eblc1388:

--- Quote from: rjmusto on October 16, 2021, 11:13:45 am ---I'd be much happier if I could find a schematic, don't like doing these 'blind faith' mods.

--- End quote ---

In the following link and post#1, there is a pdf schematic file of a similar supply PD11, which uses the same PWM UCC3895D controller on the LV side. You can look at it and get some idea of how the voltage and current feedback circuits are connected.

https://www.eevblog.com/forum/projects/hp-hstns-pl11-over-voltage-protection-voltage-increase/msg3136804/#msg3136804

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