Switching Mode PSU

[Kochcha]

Hi,

I am trouble shooting on a switching mode psu.The fault is high ripples when connected to a load.It is providing 13.5Vdc/5A output...I have change all the capacitors on the unit and still the ripples are high about 2volts p-p riding on the dc line...Any suggestion

 

[itsmoked]

2 Volts!?!?!

You mean:
Or 0.2V?
or 0.02V?
or 0.002V?

How are you measuring this?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

Are those ripples line frequency or switching frequency?

 

[Kochcha]

Hi Keith,

It is 2volts peak to peak (Spikes)...I used a oscillscope to measure.

Rgds
Kochcha

 

[itsmoked]

Okay..
This was THE problem in the first place? The ripple?

And you replaced the caps and IT CHANGED NOTHING?

There are several possibilities. If you would tell us a LOT more we could probably be more helpful.

Switchers must have ripple in their outputs or they cannot properly control the cycle to cycle wave forms correctly. Of course large ripple is wrong too. You could get large ripple if the switcher was running at way too low a frequency. Or if the sense resistor that controls the current limit has changed value.

Can you hook it to a much smaller load and see what happens? Can your load be wrong or faulty and be sinking too much current?

If the caps are ultra low ESR caps and one failed open this could greatly increase the ripple. If you then replaced the all the low ESR caps with standard caps you would have the same problem again. You might need 5 times as many regular caps to get the ESR down to the design value.

What frequency is your ripple?

What frequency is your switcher supposed to be using?

What is the current your load is ACTUALLY drawing as measured with a TRUE RMS reading voltmeter?

What is the measured output voltage?

Can the supply be run with no load? (Some switchers CANNOT take this.) If your's can, what happens when you run with the load disconnected?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[OperaHouse]

This has to be asked, is it real? Sometimes leads can pick up a lot of noise around a switcher. 2V of switching frequency on a cap would be creating some heat.

 

[Skogsgurra]

One question I always ask in cases like this is: Where is your ground clip?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Kochcha]

Yes,i did replace all the caps with standard ones.How do you differentiate ESR cap from the standard ones?.......

What frequency is your ripple? 50Hz

What frequency is your switcher supposed to be using? 50Hz

What is the current your load is ACTUALLY drawing as measured with a TRUE RMS reading voltmeter?...5Amps.

What is the measured output voltage...13.5Vdc

Rgds
Kochcha

 

[itsmoked]

Ah ha!

All switchers use much much higher frequencies then your line frequency. That is the fundamental basis of their existence. If they didn't, they'd be 10 times more expensive and the size of bread boxes.

So for you to tell us that the ripple frequency IS your line frequency, means you are measuring something related to your line power or grounding or 'scope clip placement' or a bad line filter or line filter ground. It's not your switcher having a problem.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jimkirk]

You also say that the frequency your switcher is supposed to be using is 50 Hz. Usually switchers in this power range switch at more like 50 kHz to 500 kHz. There is a whole 'nother animal that does phase conduction control at the mains frequency, sort of a "light dimmer" like of thing, often used as a pre-regulator for a linear supply. Is that what you've got, or was that a typo?

 

[OperaHouse]

At 5A, you likely wouldn't use half wave rectification. So that would be 100Hz. This is either a typo or a very confused student.