Troubleshooting SMPS

[ricothetroll]

Hi,

I'm currently troubleshooting an Orange Tiny Terror 500 amplifier. This one has a switched mode power supply, and a class D power amp (500W for less than 5kg !!!!).

When I switched it on, the fuse blew right away (exploded actually).

I found a bad cap (3300uF/16V electro) and the two MOSFETs are shorted, whatever direction I test them (every combination of the 3 pins beeps with my DMM).

The PSU provides two bipolar supplies (+/- 15V and +/- ???V), and supply for the preamp tubes (HV and 6,3V).

I have 3 questions :

- Are the two MOSFETs dead ? I don't know them mosfets too well....

- The MOSFETs are FQA28N50 (Vdss(max) = 500V, Id(max) = 28,4A, Rds(on) = 0,16R @ Vgs = 10V).
I can't find any FQA28N50 in Europe but RS Components sells FDA28N50F. To me the specs look qui similar but again I'm no specialist in SMPS. Do you think those can be an adequate replacement ?

- When those MOSFETs burnt, what other component could have been burnt along with them ? (the most probable...)

Here are pictures of the PSU :
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2295.JPG
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2296.JPG
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2297.JPG
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2298.JPG
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2299.JPG
(the two mosfets are already out of the board and the 3300uF/16V cap replaced by 3x1000uF + 470uF in parallel)

Unfortunately I couldn't find any schematic so I don't even know what kind an SMPS it is (flyback, buck boost....)

Best regards

Eric

 

[ricothetroll]

Another question I forgot to ask :
Can I test the PSU without any load connected ?
Thanx in advance.

 

[JohnRoberts]

Hi,

I'm currently troubleshooting an Orange Tiny Terror 500 amplifier. This one has a switched mode power supply, and a class D power amp (500W for less than 5kg !!!!).

When I switched it on, the fuse blew right away (exploded actually).

I found a bad cap (3300uF/16V electro) and the two MOSFETs are shorted, whatever direction I test them (every combination of the 3 pins beeps with my DMM).

The PSU provides two bipolar supplies (+/- 15V and +/- ???V), and supply for the preamp tubes (HV and 6,3V).

I have 3 questions :

- Are the two MOSFETs dead ? I don't know them mosfets too well....

Yes, semiconductors often fail as a short circuit when over heated. The cap probably failed first, then broke the mosfets.

- The MOSFETs are FQA28N50 (Vdss(max) = 500V, Id(max) = 28,4A, Rds(on) = 0,16R @ Vgs = 10V).
I can't find any FQA28N50 in Europe but RS Components sells FDA28N50F. To me the specs look qui similar but again I'm no specialist in SMPS. Do you think those can be an adequate replacement ?

??? I don't know... The F version has a higher Rds(on) resistance  0.175 ohm max vs 0.155 ohm max, so it will drop more voltage and make more heat when turned hard on and passing current. For modest current PS the extra 0.02 ohm may not matter. But it is different in a way that will affect PS efficiency.

- When those MOSFETs burnt, what other component could have been burnt along with them ? (the most probable...)

??  Maybe the controller IC? I'd look at components in the immediate area of the failed parts.

Here are pictures of the PSU :
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2295.JPG
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2296.JPG
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2297.JPG
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2298.JPG
https://dl.dropboxusercontent.com/u/28610725/Tiny%20Terros%20Bass/DSCF2299.JPG
(the two mosfets are already out of the board and the 3300uF/16V cap replaced by 3x1000uF + 470uF in parallel)

Replacing the i cap with 4 will probably not be quite the same due to lead inductance. Using a single cap would be better.

Unfortunately I couldn't find any schematic so I don't even know what kind an SMPS it is (flyback, buck boost....)

Best regards

Eric

Good luck...

JR

 

[ricothetroll]

Hi John,
Thanx for your reply !
RS sells also "non F" FDA28N50 that have Rds(on) = 0.155R,so I'll take those ones ! Thanx for the tip, I actually didn't know Rds could affect the circuit that much. I'll buy two of those along with the 3300u cap and see how the circuit behaves.
Do you think I can try the PSU without connecting the rest of the circuit (unloaded) ? I'm afraid that it could send come crap and fry some other parts, like the class D amp, that seems to me even harder to debug...
Best regards.
Eric

 

[JohnRoberts]

It will work with reduced load, I do not know about no load,, maybe hang a few resistors across it...

JR

PS: While the Rds on difference does not look like much, the peak current in the switching devices could be a couple times average output...

 

[jwhmca]

100watt light buld as load?

It's something I do for motor controllers all the time...