Failed function within SMPS giving weak current

[abdulbadii]

TL;DR Summary

faulty component function within SMPS leading to have weak current

What is the very common culprit component and what is its
function within SMPS having weak current, far weaker than its rating while its voltage is always perfect?

 

[hutchphd]

Something with a heat sink. One of the output FETs.

 

[berkeman]

TL;DR Summary: faulty component function within SMPS leading to have weak current

What is the very common culprit component and what is its
function within SMPS having weak current, far weaker than its rating while its voltage is always perfect?

Which power supply is it? Do you have a schematic?

 

[Averagesupernova]

It could be as simple as an over current protection circuit failing. Without knowing more details it's hard to say.

 

[berkeman]

SMPS having weak current, far weaker than its rating while its voltage is always perfect?

Also, I assume the "voltage is always perfect" part does not include when you start to load the output, right? Have you used an oscilloscope to watch what the output does in this early current limit behavior? Many SMPS will use a "burp" mode to try to recover during/after an overcurrent condition. Most linear power supplies will crowbar the output voltage and not attempt to recover until the overcurrent condition is removed. Which do you see on your oscilloscope?

 

[DaveE]

There are way too many possibilities here. We don't even know the SMPS topology you are referring to. Of course something wrong with the control circuit could do that. Also for a very commonly used current-mode controlled SMPS, a problem with the big inductor creating high ripple current.

If you don't know how your PS works, you'll just be guessing. If you think you can fix it by just replacing one thing, you may just be delaying another failure due to an undiagnosed underlying cause. The key troubleshooting step here is to somehow get access oscilloscope measurements of the various currents (transistors, inductors, rectifiers, etc.). This is really hard to do if you aren't in a lab set up for such things.

So, since you're asking us instead of working quietly at your lab bench, I suggest throwing it out and buying one that works as intended.

--- Wait ---That's after you are sure your external measurements are correct and your application is compatible with the PS specifications. We all can fool ourselves with bad data and blame the wrong thing.

PS: If it's an encapsulated brick, no one can fix it, throw it out.

 

[DaveE]

One trick you can try to figure things out is to find the datasheet for the control IC and look at their applications and functional description. These ICs are nearly always used just as the manufacturer intended, adjusted of course for the PS specifications. If the magnetics has an off-the-shelf part number you can do the same with that to get current ratings and inductance or turns ratios, but this is rarely the case, magnetics are almost always custom designs.

 

[abdulbadii]

It's what around us, typical kind and size one that is for laptop, printer, or router etc

input 110-240 V 1.6 A 50-60Hz
output 24 V
2.5 A

read once only max 700 mA, all reading else mostly 400-500 mA

its outstanding Led contradict its performance; its light last long, won't off until around one minute after disconnected from mains power

 

[berkeman]

It's what around us, typical kind and size one that is for laptop, printer, or router etc

input 110-240 V 1.6 A 50-60Hz
output 24 V
2.5 A

read once only max 700 mA, all reading else mostly 400-500 mA

its outstanding Led contradict its performance; its light last long, won't off until around one minute after disconnected from mains power

Woo-wee. So it's a cheap power brick that no longer seems to work. Just buy another one; you are not going to pull apart an ultrasonically welded plastic brick to repair it.

 

[Averagesupernova]

Woo-wee. So it's a cheap power brick that no longer seems to work. Just buy another one; you are not going to pull apart an ultrasonically welded plastic brick to repair it.

If it were disassembled it could be a learning experience. No worries if it gets wrecked. But dealing with anything potted is not fun.

 

[berkeman]

If it were disassembled it could be a learning experience. No worries if it gets wrecked. But dealing with anything potted is not fun.

In my experience power bricks are just ultrasonically welded plastic enclosures, not fully potted. Even so, without a schematic (which we are not getting for a jellybean power brick like this), even I would not try to debug it. (Unless it were my design and it was returned from a customer under RMA.)

 

[Rive]

output 24 V
2.5 A

read once only max 700 mA, all reading else mostly 400-500 mA

Erm.
As long as the 24V is fine, if the current is lower than 2.5A then it's about the load, not about the brick.
Why do you think it's wrong?

 

[DaveE]

Just buy another one; you are not going to pull apart an ultrasonically welded plastic brick to repair it.

Yep. Not worth the effort. You can cut it open, but you'll find it really difficult to work with once you do.
Plus, this, for example, for $20: https://www.digikey.com/en/products/detail/mean-well-usa-inc/IRM-45-24ST/7704685

 

[Tom.G]

@Rive got it right, if the output Voltage is correct but the Current is low then the Power Supply is good; it is just because the item being powered takes only the Current it needs to operate. Any higher current the supply has available can be used to power other stuff at the same time.