Reasons of Fuse Blowing in Switching Mode Power Supply Board

[r.raslan]

Hello,
Based on your experience: What could be the reason(s) of blowing the fuses in the switching mode power supply?
thank you!

 

[Ian Rogers]

Main switching transistor not switching... Usually controlled by an opto-coupler fed from the secondary side..

 

[r.raslan]

Ian, thank you, I have some points wishing to be declared:

1) I replace the MOSFET (usually?), does the problem will over?

2) If we say for some reason, the photocoupler ( = feedback) was defective, what is the effect of that on the board? (no output voltage will be produced?, short circuit at the primary side? unregulated output voltage?...etc)

3) I can see a Triack, what its function?

4) Have you ever fixed a SMPW and the problem was the ferrite hight frequency power-transformer?

thank you in advanced!

 

[unclejed613]

in order of commonality 1)switching FET, shorted 2)bridge rectifier shorted 3) switchmode chip shorted, and 4)switching transformer has a shorted turn (not very likely, but does occasionally happen). usually if #2 is true, #1 is also true. if #3 is true, there will usually be a physical indication that this is the case (hole or crack in the chip) and again the FET may also be shorted.

 

[Ian Rogers]

Ian, thank you, I have some points wishing to be declared:

1) I replace the MOSFET (usually?), does the problem will over?

2) If we say for some reason, the photocoupler ( = feedback) was defective, what is the effect of that on the board? (no output voltage will be produced?, short circuit at the primary side? unregulated output voltage?...etc)

3) I can see a Triack, what its function?

4) Have you ever fixed a SMPW and the problem was the ferrite hight frequency power-transformer?

thank you in advanced!

Ans 1) No! IT has blown because of another reason...
Ans 2) The opto / photo-coupler could be defective on both sides... The primary more so though.

Ans 3) Triac's are usually there for "Zero crossing detection"
Ans 4) No!! Waste of time... The SMPS's I use only cost £10... It far cheaper to lob it in the bin....

 

[r.raslan]

thank you all for your time to reply!
There is still one doubt about the Feedback issues,
Please do confirm what will happen if the photocoupler ( = feedback) was broken? Does the system will:
1) have short circuit?
2) have unregulated voltage at the output?
3) No voltage will be produced? (zero volt)?
4) something else? please state then.

 

[Ian Rogers]

I reckon 1 and 3... As 1 will blow the fuse... There will be no output..

 

[r.raslan]

Ok, thank you for the answer.
I have now a practical case as follows:
1) The DC bus is about 300 V DC (which means the Rectifier circuit + big capacitor + AC protection circuits) are all fine.
2) I have tested the following components (by removing them out from the board) :
- Feedback Circuit: Opto-coupler => Test Ok
- Zero-crossing circuit: Triac => Test OK
- Power Transistor => MOSFET Ok
- Controller IC (M51977) => Replaced with new One.

Problem: Still I can not see any output DC voltage.

Some Measurements:
1) The controller IC (M51977) has Vcc = 12 VDC and GND (0 VDC)
2) There is no voltage output at the IC pin # 2 which is Vout (connected to MOSFET through series of resistors)
3) I can measure frequency at the IC Vout pin about 10KHz (I am using DMM that has frequency measurement range)
4) I can not read any frequency nor any voltage at the secondary side of the isolated HF power transformer.


What is your opinion about this problem?

 

[dr pepper]

I just mentioned this on another thread.
I had a similar scenario on a power supply I looked at, the fets were dead, and the gate drive resistors, a couple of diodes and the controller chip, after replacing all the these, the supply still didnt work, the transformer primary was open circuit, the fets shortng put dc on the primary and vapourised the copper before the fuse blew.
Might not be the case with yours however might be worth metering the primary.
!0 Kc sounds low for a switcher 20 to 40 is more like it, sounds like the supply might be protecting itself, or powering up and shutting down in a cycle.