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Adjusting SMPS output voltage (discuss.tchncs.de)

submitted 1 year ago by [email protected] to c/[email protected]

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Does anyone recognise these power supplies? They're cheap AliExpress led drivers and I want to change its output voltage to around 22V from 12V. I've read that the way to do this is to adjust the REF voltage on the IC that controls it. It's a KA3845 but I don't understand where that reference voltage is regulated. One voltage is feedback from the output where then other should be a reference.

What would be the best way to approach this? I can't find any schematics on these boards unfortunately.

Thanks.

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[–] [email protected] 7 points 1 year ago

You might be able to increase the voltage 10-20% by changing the feedback, but you will not be able to double it without changing the transformer.

[–] [email protected] 7 points 1 year ago

As others have said, increasing the output by 5/6 is unrealistic. And even it would work, it'd only be for a short while. Best case is that it works and you're home and awake when it catches fire.

Better buy a dedicated powersupply. How expensive something is, is in the eye of the beholder, but it is worth it. Most of the ones on this list can be adjusted 10 or 15% and that should get you there from 24V. And they're actually meant to do it. https://dk.rs-online.com/web/c/stromforsyninger-og-transformere/stromforsyninger/switch-mode-stromforsyninger-smps/?pn=1&rpp=100&sortBy=P_breakPrice1&sortType=ASC&applied-dimensions=4294965650,4290297838,4291523693,4294965001,4293335399,4293589165,4294472088,4294478231,4294273800

[–] [email protected] 6 points 1 year ago* (last edited 1 year ago) (2 children)

Considering there is what appears to be a fixed-stepdown 12 volt AC transformer on the line side of that board, I'd be surprised if you will change anything higher via IC's.

This may be switch-regulated for rectification and line voltage correction but it definitely does not look modifiable.

[–] [email protected] 10 points 1 year ago (1 children)

That's not a mains frequency transformer. Not enough steel. It's a high-frequency all-switchmode supply.

However, that's not to say you can simply adjust the feedback and have it safely deliver near twice the voltage. The secondary side diodes and capacitors probably won't be up to it, or will have a very limited life.

The transformer does have a ratio, and the marking makes it clear that this is a specific part for the 12V model. How much leeway there is will depend on topology. Flyback are generally fairly flexible. Other types less so.

Starting with a 24V model and either adjusting the feedback resistors or adding a few diodes would be near trivial. Many will already have a potentiometer that provides that degree of adjustment. Starting with a 12V model is an uphill battle.

[–] [email protected] 1 points 1 year ago* (last edited 1 year ago) (1 children)

indeed it looks like a Royer oscillator in there (because there are transistors before AND after the transformer).

[–] [email protected] 1 points 1 year ago* (last edited 1 year ago) (1 children)

TO220 package diodes are pretty common in SMPS applications, so I'm not sure that's a guarantee.

EDIT: Not sure what you're getting at here? Royer doesn't seem to need anything other than a rectifier on the secondary, and plenty of topologies use two power transistors on the primary.

The second smaller transformer below the main one does tend to suggest it could be a Royer, at least from my reading. I don't see the extra switches needed for it to be a PFC stage.

There's still a little SO-8 and optocouplers.

[–] [email protected] 1 points 1 year ago

skipped reading the silkscreen … it says Q1 on the lowermost package but the other three are diodes. so could also be a forward converter or something similar.

[–] [email protected] 1 points 1 year ago

Well there certainly is some regulation because attaching a load does not decrease the voltage by much. Increasing the voltage is indeed ambitious for the 12V model but lowering the voltage of the 12V model seems doable.

[–] [email protected] 4 points 1 year ago (1 children)

It's probably not a good idea for actual use, but if you'd like to experiment: looks like the 3-pin devices U4 and U5 on the upper right provide feedback through the optocouplers next to the class Y cap north of the transformer. I bet those are LM431 voltage references (or similar). The passives around them provide filtering, but two of the resistors should form a resistor divider for the Ref pin (lower right pin if the single pin is on top). That divider sets the voltage.

[–] [email protected] 2 points 1 year ago

Thanks. I ordered the 24V model in hopes of adjusting it down to 22V. I will use that to keep a 7s Li-Ion battery at a minimum charge level whereas a solar panel array may increase that voltage higher. It looks like the 24V model's capacitors need to be changed as well since they can't handle the Li-Ion batterys' max charge voltage.

[–] [email protected] 1 points 1 year ago

Replying to myself for informational reason. Modifying voltage was more or less successful. Both optocouplers transmit a reference voltage, so both need to be adjusted simultaneously.

This can be done by changing the value of R19, a 23.2k smd resistor close to the output terminals.

I've attached a 10k pot with a 10k and 6.8k resistor in series and successfully modified the voltage down to 22.5.

The power supply itself is a piece of crap though. It claims to handle 400W but anything over 150W causes the short circuit protection to activate, never mind overheating at 150W very quickly.