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Modifying a server power supply into an adjustable bench supply
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Recyclojunk64:
I am trying to modify a switchmode server power supply for adjustable output, from 0 to 42V, by tapping into the feedback loop and adjusting the reference voltage with a 10 turn pot.

I have previously done this with a few ATX supplies using TL494 controller chips, and it usually worked quite well. Except that none of them have survived the torture i put them through (see profile pic). So this time I hope to have a somewhat more robust and powerfull supply that will handle any electronic circuit I throw at it. In the future I also plan on adding a linear tracking output stage to it for better regulation and contstant-current/current-limiting, but I won't worry about that for now.

BTW, the 42V output is rated at 66A, which is about 2.8kW, so I think it should be suitable for most of the stuff I'm going to use it for. Plus 12V, 3.3V and 5V rails adding up to about 300W (from a separate transformer). It also uses a 15A C19 input connector (at 240V).


Now on to modifying it. Firstly I dissabled the over/undervoltage protection. The controller chips is on the primary side, using an opamp and optocoupler (in linear mode) for voltage feedback. The problem though is that the circuit is only adjusting the on-time of the transistors. This is a half-bridge topology, and when I measured the duty cycle I found that I could modify it from 48% down to 33% by decreasing the reference voltage on the opamp, but I soon realised that I was also increasing the frequency from 76kHz to nearly 500kHz! Not so good for the drive transistors. The controller is a CXA8038P http://www.datasheet4u.com/pdf/CXA8038P-pdf/500903 which has two pins for timing resistors, one for On Time and the other for Dead Time control, with the optocoupler connected to the On Time pin. And two external totem poles for driving the main transistors.

So I'm asking for advice on what the best thing to do here might be. Perhaps someone knows of a simmilar smps controller chip that has fixed frequency but variable duty cycle that I could deadbug into the circuit somehow. As I'd like to keep the overcurrent and other protections. Or an easy method of adjusting both the On and Off time at the same time maybe? Or maybe I should use a low side controller and use two optocouplers to get the drive signals to the mains side totem poles? And also anything I might want too look out for, I have some idea of what I'm doing but i'm no expert.

More pics to come
bktemp:
LLC resonant power supplies have a quite limited output voltage range. Adjusting the output voltage over a wide range is impossible.
Using PWM does not work, because the transformers are specially designed for resonant operation (large leakage inductance).
If you want an adjustable output voltage using this SMPS, you need to add a buck converter at its output.
Recyclojunk64:
Thanks for the reply. Not really what I was hoping to hear, I think if I was to have a buck converter on the output that will probably be just as large as the power supply itself for 3kW. In which case I might as well make it an isolated buck converter and run it straight from rectified mains.

But is there any chance I might be able to convert this supply to a different topology? Is there another topology that is driven with a half bridge and does ZVS, that I could adjust the duty cycle to adjust the output? Or maybe something that doesn't use ZVS but will still output over 2KW using only the three half-bridges of 20N50F mosfets, perhaps I could replace them with faster ones so that they dissipate less heat when switching? And either replace the transformers or modify them to have less leakage inductance if necessary? I have lots of different cores and litz wire. I would quite like to make it adjustable as it's pretty much useless to me if I can't adjust it over a wide range.
bktemp:
One big adavantage of LLC resonant converters is the absence of an inductor at the output, because it is integrated into the transformer.
So if you change the topology, you most likely need to add an inductor after the output rectifier.
Siwastaja:

--- Quote ---I think if I was to have a buck converter on the output that will probably be just as large as the power supply itself for 3kW
--- End quote ---

Nope - a non-isolating, simple, 42V input buck converter is likely to be 10x smaller, 10x cheaper, and a lot simpler to design than a mains-powered isolated supply - and it's easy to achieve good efficiency.

You can also add proper current limit to the buck stage to make it a real lab supply.
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