Electronics > Repair
Fixing an industrial Smps with a blown mosfet.
Fflint:
I bought this 1900W 48V switch mode power supply made by ASCOM for a Cnc project few years ago. When it arrived I tested it and it used to work fine. Fast forward few years of storage in dry, but sometimes cold conditions (up to - 20, in a hermetic box) when I plugged it into my 6a fused test mains socket it blew up two out of 8 n channel mosfets it has. Unfortunately the 6a B rated fuse wasn't fast enough. When I opened the chassis I also noticed a ferrite core of what looks like an EMI suppression choke cracked in half and fell onto the pcb.
The core is E shape and uses two plastic gaps so I assumed extra tenth of a mm of glue is acceptable. I glued it together. I also replaced two blown mosfets.
I tested all mosfets and two bi-diode packs right next to them. I measured the esr of electrolytes on the high voltage side in circuit, is that useful or do I have to pull them out? The measurement came as 1100uF and 0.03ohm esr (3large caps in parallel).
This time I tried powering up the SMPS with an old style incandescent halogen light bulb in series (400W). The psu clicks to power on, the halogen light flashes, the psu switches off and the cycle repeats.
This is a 1900W psu, do you think I should try powering it up with a larger load in series? I have a 2000W heater I could use in lieu of the light bulb. However, I don't want to blow up more of those mosfets. Although not that expensive, they are quite hard to find. Or should I continue looking for more faulty components before using the bigger load in series?
The SMPS has a large control board that detects the mains voltage and switches it on for the ac mains rectifier. Then after the rectifier we have the previously mentioned choke. Unfortunately tracing signals in the back is difficult due to large capacitors obscuring the view from the top so I only found out what I could probe.
I'm having difficulty understanding how 8 mosfets and 2 bi-diode packs are all connected. I understand simple single and double mosfet SMPS schematics, but with more mosfets I'm a bit lost. Could someone point me in a direction of some example schematics how such multi mosfet PSUs are designed?
For example two mosfets are in parallel. They seem to have their sources directly connected to the minus HV SC rail. They are controlled one driver IC. The drains "go somewhere non obvious". Then I found the plus HV DC goes through those 2 bi-diode packs and then there are high voltage electrolytes in parallel. Probing around I did find the HV DC plus past the capacitors on one of the remaining mosfets, but as I mentioned the tracks on top are obscured so tracing is very difficult seeing only one side of the board.
There are some driver ICs there. Could someone suggest some method of checking them, please?
Discrete diodes and resistors look fine. I tested diodes I could get to and they are OK. I might have missed two underneath a transformer radiator.
Most SMPS repair guides I find deal with small, simple SMPSes that have a single mosfet or a pair. Is there some guide that deals with large ones, perhaps?
If there is no guide, and no one has any idea how to troubleshoot other than start desoldering components and testing them one by one, could someone with experience of using a light bulb in series comment if it is possible the SMPS could be OK and still flash a 400W halogen bulb while powering on?
Quick back of the hand calculation. It has 1100uF of caps. Assuming 340V it can store 0.374C of charge (~64J). Such charge could be charged at 340V with 1.1amps at 1ms. That's 374W for 1ms. Is it possible to see a 1ms flash? I have no idea. I'm kind of tempted to try the 2kw heater in series instead.
What do you think?
TheMG:
The light bulb trick (or heater resistor or whatever) in series I find really doesn't work well for SMPS testing, since most have an under-voltage lockout. Even if it does it probably wouldn't save the transistors anyways as the charge in the bulk capacitors is usually enough to fry them (although perhaps less catastrophically).
My approach usually with large SMPS that have blown primary switching transistors, is to take them all out, that way I can fire up the unit and check the gate drive waveforms for proper operation without risking blowing up anything. Usually if the driver IC is sending proper gate switching waveforms, the feedback network is working, and nothing is shorted on the secondary side, it should be safe to populate new switching transistors and fire it up.
The large "choke" after the rectifier is likely the inductor for the PFC boost converter. It's job is to step up the rectified mains voltage to somewhere around 380V typically, while also approximating a sinusoidal current waveform to achieve close to unity power factor, especially important with high power SMPS.
jonpaul:
Bonjour, been designing SMPS since 1970s, debugging a blown 2 KW supply is not trivial. Also you need the proper safety and lab equipment.
BEWARE: 2 KW off line SMPS has severe DANGER OF SHOCK, DEATH OR DAMAGE TO EQUIPMENT
Just a few quick tips I am jammed here today.......
1/ 240V /120V 1 phase or 3 ph input mains
2/ You should use an isolation transformer (2-5 KVA) and a variac NOT a series lamp.
3/ Assume any blown FET has also blown the drivers.
4/ Try to get or trace a schematic.
5/ 2KW is most likely a full wave bridge or possibly double forward.
6/ Run driver on separate LV lab supply with NO HV bus to check drivers.
Bon Chance and BE SAFE!
Jon
Fflint:
Thank you both for really good tips.
Jon, I do have a variac and an isolation transformer, but I wasn't sure how to use them to protect it from frying more mosfets. I assumed if there was a short I needed current limiting. That's why the light bulb is used. Is there a better way?
You mentioned running the drivers off a separate low voltage psu to test. I'll definitely do it.
I have the usual test equipment such as an oscilloscope(probes allegedly good up to 600V), bench psus up to 60V etc.
Further answers and comments online below.
--- Quote from: jonpaul on November 25, 2021, 08:42:52 am ---Bonjour, been designing SMPS since 1970s, debugging a blown 2 KW supply is not trivial. Also you need the proper safety and lab equipment.
BEWARE: 2 KW off line SMPS has severe DANGER OF SHOCK, DEATH OR DAMAGE TO EQUIPMENT
Just a few quick tips I am jammed here today.......
--- End quote ---
They are very useful, thank you. Regarding safety I have a proper bench I'm testing it on. So it sits there in a stable way (no risk I'll grab it instinctively if it trips etc). I have a switchable power extension before the isolation transformer. I use it to power on/power off the AC. No live wires lying around being connected together by hand... Other than that, my floor is made of wood and I do have an RCD in the power panel so I'm hoping that would help. Also I always discharge the HV caps before working on it after powering it on. Unfortunately that relies on remembering to do it, which is not great, but I don't know of any better solution.
--- Quote ---
1/ 240V /120V 1 phase or 3 ph input mains
--- End quote ---
1 phase 240v. The manufacturer claims the smps will work from 180v to 240v. Here is a manufacturer page if you're interested to see the exact model. http://web.tiscali.it/gnnsites/rdz_SMPS48V1900.htm (only in Italian)
--- Quote ---
2/ You should use an isolation transformer (2-5 KVA) and a variac NOT a series lamp.
--- End quote ---
I mentioned it above. I wasn't sure low voltage would be enough to protect mosfets in case of a short so I wanted some way of current limiting. If possible, please let me know if there is a better way while still having current limit capability
--- Quote ---
3/ Assume any blown FET has also blown the drivers.
--- End quote ---
OK. I'll do that. Two blown mosfets used the same driver. I'll get a replacement.
Edit: not so fast. I thought the mosfet were hard to find.. This driver IC is very hard to find. Only one distributor located half a world away has it. I guess I'll have make sure 100% it is faulty before I spend half of the value of the Smps on shipping a part for it.
--- Quote ---
4/ Try to get or trace a schematic.
--- End quote ---
There is no schematic available online. I searched in the usual way. So I'm trying to trace it myself. If I have to I might remove those big caps to get to the top of the board.
--- Quote ---
5/ 2KW is most likely a full wave bridge or possibly double forward.
6/ Run driver on separate LV lab supply with NO HV bus to check drivers.
Bon Chance and BE SAFE!
Jon
--- End quote ---
Will do :-) Thanks
Fflint:
I've pulled the driver chip out and I tested it on its own by applying power and pulling inputs up or down. It is fine. I also pulled out a bunch of other components surrounding the driver chip. All tested fine (3 caps and a 2 diodes) . I've reassembled everything and I tried using the heater for current limiting. The heater is selectable 1kw,2kw or 3.5kw. With 1kw the smps clicks repeatedly same as with the 400W light bulb. With 2kw my 6amp circuit breaker triggers. I haven't tried on a bigger breaker circuit. Unfortunately I don't remember if when this smps was originally tested it was on a 6 or 16amp circuit. Is it possible it can be fine and still trigger the 6amp fuse on startup?
I think that's unlikely.
Before I start pulling all mosfets out, is there something else to try? I attach few photos.
Connection wise there are 6 mosfets, not 8. 3 are on the HV minus and 3 are on HV plus. After more testing I found out only one out of 3 in the minus circuit was blow (2 were replaced, but one tests fine on the bench). The minus mosfets are on the right, plus on the left.
The transformer and low voltage rectifier are fine.
The first two photos show how logic boards are connected. The smps is missing a number of clamps holding mosfets to the radiator and some screws. This is just for testing.
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