Working SMPS Secondary Side Rectifier Diode has forward & reverse voltage drop??

[iiooii]

I'm just starting out on learning electronics.
I've come across one of those SMPS cellphone charger (one without optocoupler design)
It stopped working one day.

After some measuring, I found 3 faults, (at least I thought I did)

the secondary side Schottky Rectifier Diode has forward voltage drop of 0.3v and reverse voltage drop of 0.1v (reverse should be OL)

c1 (spec 220uf 50v) measures 4.5nF

c2 (spec 220uf 50v) measure 5.8nf

So I bought a FR305 Diode to replace to schottky diode (I know it won't last, but I'm just interested in learning and fixing the faulty charger)

2 new capacitor of 220uf and 50v to replace the c1 and c2

After removing the old parts and putting in the 3 new parts. the charger is working.

the removed old part measurement:

old schottky diode. no forward/reverse value

both capacitors failed to give any value when measured with multimeter in capacitor mode.

Even though I've fixed the charger. Here's the question :

I'm still able to measure 0.3 v reverse voltage drop on the new diode

I'm still unable to measure the capacitor value of c1 & c2 when it's soldered onto the board (they measure 180uf and 191uf off board)

So essentially the fault I thought I fixed still present, yet the charger has been fixed ....

Can someone point to me what's wrong with my logics?

Simply from the secondary side of SMPS circuit diagram attached, if I measure 0.3v reverse voltage drop across the diode on the secondary side. does that mean I'm measuring the voltage drop between a resistor and a transformer secondary coil to get that 0.3v measurement?

[Majorassburn]

You cannot measure those components "in circuit". Please read up on measuring passive components with DMM's. Best Regards, 

[tautech]

Replace C4 too, a common failure point for SMPS.

[jwet]

Servicing stuff is hard, its good that you've got a schematic.  I was a beginner once.  What you need to do is analyze or imagine what you're really measuring when you put a DMM across something in a circuit.  In some ways, old analog meters gave you more info in may cases- DMM's and diode measuring functions get pretty complicated- not just ohms.

If you measure across the diode in the pos direction, you'll get the forward drop in one polarity, but in the other direction- it gets complicated.  If the diode is good- ie open (off in reverse bias), you'll see the circuit of the output resistor with the caps in parallel and then the series resistance of the output winding (low ohms).  Your meter might interpret this as .3v or a lot of things depending on what it is putting out and how the caps effect the measurement.  Measuring caps with an ohmmeter is  kind of screwy.  They will look like a short at first when they're charging and then will slowly charge up and may look like an open, but its not guaranteed, because they generally leak a bit.

The caps were probably bad initially and if the diode measures bad out of circuit, it was likely bad too.  Likely that the diode failed (shorted) due to an overload, put a bunch of AC on the output caps and they went bad.

Keep learning.

[tautech]

Servicing stuff is hard, its good that you've got a schematic. 

Only as hard as we make it.
With no parts makings, indeed quite difficult however when devices are clearly marked....we all know how to use a search engine and find datasheets don't we ?

I too once thought SMPS as magical voodoo but study of Typical Application circuits in nearly every SMPS controller datasheet and breaking the circuit into its small and simple building blocks has one discover SMPS is not magical at all and really quite simple.

AC supply must be rectified to HV DC from which dropper resistors*¹ provide current for the controller to start then the flyback winding*² takes over for continued operation.

*¹ If these resistors are open or read high the SMPS may not start.
*² Flyback supply is rectified and fed to the VCC supply where both the diode and VCC cap must be in good order for reliable operation.

On the secondary side diode failure can be attributed to overload, probably from exceeding the stated overload cycles its datasheet specifies when the output caps fail due to high ESR and subsequent heating, drying the electrolyte and leading to internal shorts.

Low quality Low ESR caps are common resulting in short SMPS life.....

[iiooii]

Thank you everyone for the detailed reply and knowledge sharing.
@tautech especially for the break down of the SMPS in simple blocks.

[ArdWar]

The 0.3V reading isn't something random. Ignoring the capacitors which tend to throw in circuit test down the drain. If your DMM diode test use 0.3mA test current (which is well within common range of values used) then it'll read 0.3V across 1kΩ resistor (which your R8 appears to be).

[tautech]

Thank you everyone for the detailed reply and knowledge sharing.
@tautech especially for the break down of the SMPS in simple blocks.

Some are simple, some not so much especially when miniature as in wallwarts and such.
Often these compact SMPS use a controller with an inbuilt MOSFET and TNY263-268 are examples.

The Typical application circuits are normally gold and very often followed closely in actual designs.
https://www.power.com/ko/downloads/documents/tinyswitch-ii_family_datasheet.pdf

See Fig15/16 P9

[kevin.gibbs]

When the parts are mounted on the board, other elements affect the measurements. In this case, the transformer winding and resistor R8. Therefore, the parts must be unsoldered when measuring with a multimeter. If I were you, I would also check the other capacitors in the diagram, which are C1, C2, and C4. They could have lost capacitance, too.