Hi everyone,

This is my first post here. I hope it's OK to dive right in and ask...!?

I have a small SMPS here which was blowing fuses. I found two transistors on the board which were all shorted. I checked all other semconductors... diodes and the other transistors and the capacitors too and everything reads OK.

I changed the two faulty transistors and then turned the SMPS on via a variac and through a light-bulb limiter. The symptom I am getting is that the bulb flashes on/off. It 'pulsates' and I haven't brought the variac up to full volts, just in case. I don't want to risk blowing my new transistors.

I seem to always have this scenario. I try fixing a SMPS... I find faulty components, and change them. I spend hours looking and checking all the other components - transistors, diodes... even capacitors with my ESR meter - and all read OK.

But when it comes to powering the unit up - it's almost certain that it will still be 'faulty'.... is there something else I should be checking/doing...?

Am I missing something?

I am starting to get frustrated. I'm great with normal power supplies, but SMPS seem to have the better of me...!

Cheers,
Tom

 

make and model if possible and may be able to dig the schematic up

 

There is usually an IC associated with the transistors.
Quite often this has blown too.

 

Cheers for the replies... the SMPS is from a "dB technologies T4 active three way line array module"

I tried getting schematics but nothing turned up in searches.

Cheers,
Tom

make and model if possible and may be able to dig the schematic up

 

Hi everyone,

This is my first post here. I hope it's OK to dive right in and ask...!?

I have a small SMPS here which was blowing fuses. I found two transistors on the board which were all shorted. I checked all other semconductors... diodes and the other transistors and the capacitors too and everything reads OK.

I changed the two faulty transistors and then turned the SMPS on via a variac and through a light-bulb limiter. The symptom I am getting is that the bulb flashes on/off. It 'pulsates' and I haven't brought the variac up to full volts, just in case. I don't want to risk blowing my new transistors.

I seem to always have this scenario. I try fixing a SMPS... I find faulty components, and change them. I spend hours looking and checking all the other components - transistors, diodes... even capacitors with my ESR meter - and all read OK.

But when it comes to powering the unit up - it's almost certain that it will still be 'faulty'.... is there something else I should be checking/doing...?

Am I missing something?

I am starting to get frustrated. I'm great with normal power supplies, but SMPS seem to have the better of me...!

Cheers,
Tom

When switch transistors fail, it usually punches through the collector-base junction which puts high voltage on the base drive circuitry, usually including an IC and small transistors. They never simply blow the main switch transistors, there is always collateral damage.

 

I dislike having to repair a SMPS. Without a schematic I would most likely shelve it and get a new one.

 

Run it at full volts! The control IC will issue a fault condition if you try to power it through a variac. That could be the entire cause of your new "fault" condition.

And if you run it at full volts and it blows the two transistors again you know you did not fix it right.

Also ditch the light bulb as a load, most modern small SMPS will run fine with no load. If you need to add a load use a power resistor drawing approx 20% of the full load current.

 

Thanks for the help again.

The bulb isn't an actual load for the supply output but in series with the mains input to hopefully take the current of there is still a fault. It has saved newly installed output transistors on amplifiers many times for me!

So, I'm getting the feeling that using variac's and bulb limiters with SMPS's are not the way to go...?

Is there really no way of protecting new components on powering up an SMPS (like there is with the bulb, working on older power supplies) these transistors cost me £8... can't really afford to get in to a "blowing them/ordering more" spiral..!

Hmm.. I'll try and put full volts on it... have to bite the bullet I suppose...!

Cheers,
Tom

Run it at full volts! The control IC will issue a fault condition if you try to power it through a variac. That could be the entire cause of your new "fault" condition.

And if you run it at full volts and it blows the two transistors again you know you did not fix it right.

Also ditch the light bulb as a load, most modern small SMPS will run fine with no load. If you need to add a load use a power resistor drawing approx 20% of the full load current.

 

check or replace the electrolytic caps in the circuit. I fix a lot of smps here, and the caps dry out causing transistors to fail and usually just replacing the caps fixes them.

 

And the last resort is to replace the controller chip.

 

Thanks for the tip... would this be even if those caps read OK on an ESR meter...?

Cheers,
Tom

check or replace the electrolytic caps in the circuit. I fix a lot of smps here, and the caps dry out causing transistors to fail and usually just replacing the caps fixes them.

 

any that have bulges in the tops of the cases, have indications of leakage undeer them, or have a smell of cat urine when the leads are unsoldered are suspect. some might be ok, but if they have ever leaked, they wiont last long.

 

...
So, I'm getting the feeling that using variac's and bulb limiters with SMPS's are not the way to go...?

Is there really no way of protecting new components on powering up an SMPS (like there is with the bulb, working on older power supplies) these transistors cost me £8... can't really afford to get in to a "blowing them/ordering more" spiral..!
...

Using a light bulb (and or variac) on the mains input wire is no good.

There are ways of protecting the power devices, the best way is to replace the mains input fuse with a smaller value. The HV power devices are rated for a few amps, so using a 0.5A fuse etc should blow long before the power devices even if the controller switches them full-on.

You might need an inrush resistor so the main cap inrush current does not take out the fuse, something like a 10 ohm or 20 ohm resistor shouldn't drop the AC volts much during operation but will reduce the cap inrush current when mains is first supplied.

Be warned; the resistor itself can act like a fuse and might smoke up if you have certain faults in the SMPS. I have some old (open circuit) glass fuse bodies with fusible resistors soldered across them, that can be plugged in for testing.

 

Thanks RB and everyone else who replied.
I am learning as I go along... it's great to be able to discuss....

Cheers,
Tom

Using a light bulb (and or variac) on the mains input wire is no good.

There are ways of protecting the power devices, the best way is to replace the mains input fuse with a smaller value. The HV power devices are rated for a few amps, so using a 0.5A fuse etc should blow long before the power devices even if the controller switches them full-on.

You might need an inrush resistor so the main cap inrush current does not take out the fuse, something like a 10 ohm or 20 ohm resistor shouldn't drop the AC volts much during operation but will reduce the cap inrush current when mains is first supplied.

Be warned; the resistor itself can act like a fuse and might smoke up if you have certain faults in the SMPS. I have some old (open circuit) glass fuse bodies with fusible resistors soldered across them, that can be plugged in for testing.

 

Hi again,

Just a quick question about that inrush resistor - does it need to be a high power one or would a common 1/4 or 1/2 watt one do the job...?

Cheers,
Tom

You might need an inrush resistor so the main cap inrush current does not take out the fuse, something like a 10 ohm or 20 ohm resistor shouldn't drop the AC volts much during operation but will reduce the cap inrush current when mains is first supplied.

Be warned; the resistor itself can act like a fuse and might smoke up if you have certain faults in the SMPS. I have some old (open circuit) glass fuse bodies with fusible resistors soldered across them, that can be plugged in for testing.

 

And the last resort is to replace the controller chip.

Not necesarily last resort, it could be cheaper than another set of transistors, so might be a good idea to change it right away if you can get it for a decent price.

 

Hi again,
OK I made up a fuse holder, put in a 0.5amp fuse and added a 20-ohm resistor.
I plugged it in without bulb or variac.

The fuse blew with a big blue flash!

I am going to assume it's still faulty. But since I have checked all semi-conductors and all read fine, also the six caps on there read good with my ESR meter and no resistors read short circuit... so assuming all my checks are accurate, what would be the next move? I feel kinda stuck.

There is a small stand-off circuit board on there with surface mount components. I assume that's the "oscillator chip"... also the only other components are those two transformers.

Can any SMPS experts guide me if possible please?

Cheers,
Tom

 

any that have bulges in the tops of the cases, have indications of leakage undeer them, or have a smell of cat urine when the leads are unsoldered are suspect. some might be ok, but if they have ever leaked, they wiont last long.

Also any electrolytic running hot in a SMPSU is on a downward spiral - check the aluminium can isn't live before feeling the temp!!!

If the heat has shrivelled the cap's plastic sleeve - you don't need an ESR meter to tell you it needs replacing.

Sometimes, after replacing a failed electrolytic, I add a multilayer ceramic chip capacitor on the print side - its only a little, but it helps take the edge off the switching transients. It does sometimes reduce the operating temperature of a hard worked electrolytic.

 

Hi again,
OK I made up a fuse holder, put in a 0.5amp fuse and added a 20-ohm resistor.
I plugged it in without bulb or variac.

The fuse blew with a big blue flash!
...

Did you try a slow blow 0.5A or slightly larger fuse? Maybe try a 1A slow blow type. I suggested 0.5A as you said "a small SMPS".

Some photos of the SMPS might help a lot at this point! You've got access to expert fixers but are just spoon feeding us little bits of information at a time.

Photos and info on what you replaced and why, would be the next step.

 

I think there's no real difference using either an inrush limiting resistor or a lightbulb of enought wattage at the input, except of course that if there's a shortcircuit the lightbulb won't tend to be destroyed. Leave the variac out though.

By the mention of two transformers it seems your smps is a half bridge forward topology, maybe look for a basic diagram on that topology and compare it to your circuit so you can understand what's going on?