SMPS Failure Analysis and Repair

[olNick]

SMPS Failure Analysis and Repair

Hi all,

I'm Nick and this is my first post here. I wound up here while searching and learning about smps.
My application is an inverter tig welder that I managed to blow-up, tried half-jacked repair and duly blew it up again.
After studying and more or less understanding smps designs I am in the process of (hopefully) repairing this welder again.
I will relate what happened first and then add my understanding as to why the welder blew up.

Schems are here,

My welder 160 amp https://dl.dropboxusercontent.com/u/85561940/th160.pdf
Component placing FWIW, https://dl.dropboxusercontent.com/u/85561940/thcomp.pdf
Same welder more less but in 200A version https://dl.dropboxusercontent.com/u/85561940/th200.pdf

The 160A schem (my welder shows 4 IGBT'S (HGTG30N60B3, mine came with 2 of them, HGTG40N60B3 from the factory and it is shown this way in some other docs. Also, my board revision (older) has all the stuff that show as NM, Not Mounted

I live off grid and all my heavy loads and tools run off a diesel generator. Said generator is ex-army issue, twin diesel 6kW single phase. It has always run my heavy loads, welder included w/o problem.

1. First "accident"
I was building some metal door frames and using the welder with a smaller cheesy "5.5 kW Honda clone".
I was only tacking the frames and there was not really any problem. I use the small generator as the big one is 100 feet away and I did not want to always go back and forth to start it or shut it down.

Several days later, after all was tacked, I crank up the welder to do the final welding, forgetting I was still connected to the cheesy generator.
Ater about a second of welding the generator starts modulating from full rpm to almost stalling and I see the digital display of the welder blinking.
I turned it off quickly. I tried again with the same blinking. I plugged into real 230 volts and it went bang.

to be continued....

regards,
nick

 

[olNick]

Continued...

I opened the welder and saw that;

Rectifier blew up
Ritard fuse had blown up
Cover and wires blown of the NAIS relay that sends 220 to the control circuit x-former
One of the IBGT's HGTG4060N had exploded
Secondary transformer "looked" like it had leaked, but I called a supplier and he said they look like that but I ordered used one anyway.
They both ohmed out ok, and both looked the same (slight oozing)

2. Second blow up
I replaced all the above parts on the primary side, checked the storage caps were not shorted and figured I'd plug it in and see what happens
4 seconds and boom (should have used a variac/DBT:-(

I'm a bit wiser now and have tried to understand what has happened and maybe fix it this time.

Findings;

Checked my 12v logic side by not giving any power to the bridge and see my display works, all the functions/switches work.
I checked the GDT and while the primary is ok, the secondary is open.
I believe here is/was my problem. "If the gate to emitter circuit is open, the IGBT turns on autonomously and large current flows.." at least if this reference is correct.

http://www.fujielectric.com/product...l/application/box/doc/REH984b/REH984b_04a.pdf

The 4 diodes in the secondary rectifier check out ok..

What do you think? Any other things to check or consider???

to be continued....

regards,

 

[olNick]

Continued...

So, I need;

new GDT (VAC D4097-X055)
outputs
transil diodes across out IGBT's?
maybe it's a good idea to replace filter caps. Currently they are BC (Vishay) http://uk.farnell.com/vishay-bc-components/mal215957471e3/cap-alu-elec-470uf-450v-snap-in/dp/1166656 30x50mm snapin
Can anyone recommend any better capacitors?

My thoughts and questions for a more reliable, cooler machine are;

The board is has provision for 6 IGBT's total. I was thinking to use 4 of them as the schem shows. The heatsink is also driled/tapped for a second pair. Would the existing GDT drive the additional pair of devices bearing in mind the same logic/drive pcb on all models, whether 160, 200A. (which for sure has 4 devices.)
Might as well ask... can I use 3 devices per side since the board drilled for it? In general I have read about various concerns regarding paralleling of outputs.

Tha's about it...

I have other questions for another time...

regards, and thanx for taking the time...

nick

 

[MicrosiM]

I feel sorry for you, but as I can see from the PDF files, this dosent look like an easy machine to fix.

If I am in your place, first thing I would take is the schematics, then from there you can start to trace the circuit.

Replacing parts in place of blown ones is a serious mistake. You must check waves across the IGBTs before PLACING new ones.

You must use a VARIAC

I still dont understand why you plan to replace the filter capacitors?

Can you show some pictures of the actual product? where the damaged areas?

Lot of things to do to fix this machine.

hope that helps for now.

 

[olNick]

I feel sorry for you, but as I can see from the PDF files, this dosent look like an easy machine to fix.

If I am in your place, first thing I would take is the schematics, then from there you can start to trace the circuit

I have done this and see secondary and low level control voltages are ok and the display works

Replacing parts in place of blown ones is a serious mistake. You must check waves across the IGBTs before PLACING new ones.

This I don't get as the existing igbts shorted

You must use a VARIAC

this I realized and said so

I still dont understand why you plan to replace the filter capacitors?

good I thought maybe they might be damaged from the maassive current dump that blew rectifier igbts gdt fuse basically most thjings on primary side

Can you show some pictures of the actual product? where the damaged areas?

well ican try but there is really not much to see as I have removed bad parts

sorry for grammar etc. sending this from tablet
tx for rreply

Lot of things to do to fix this machine.

hope that helps for now

 

[MicrosiM]

You need an oscilloscope to check DRIVE signals at the gate of the IGBTs.

Also you must make sure that the secondary side has no defective parts

 

[olNick]

Ok
First thing I'm trying to do is understand reason for failure..
My thinking is the extreme under voltage/brownout when the generator almost stalled dropped the +-15 volts coming out of the transformer which then gets rewegulated to +- 12 volts for the control to very low voltage and the control/pwm gdt drive etc stopped working and the igbts stayed on and fried or the +- 15v gate drive was too low to switch the igbts off.... or too high whhen generator was overvoltage and exceeded the 20v gate-emmiter limit. and cooked the device.
Once this happened the short sent 300vdc across the gdt secondaries.
After this replacing parts w/o replacing gdt would still bnliw. up due to open g-e circuit

my thoughts, comments welcome...

nick

 

[olNick]

You need an oscilloscope to check DRIVE signals at the gate of the IGBTs.

Also you must make sure that the secondary side has no defective parts

Ok, can I do this w/o igbts mounted? Or at least w/o the e-c high voltage?

 

[MicrosiM]

I don know about the HIGH VOLTAGE section in your machine.


But you can just keep the IGBTs un-mounted (DONT INSTALL THEM)

And while using VARIAC, increase voltage slowly, while monitoring the current draw from the 220VAC line, if increasing quick, then STOP.

Else, you should see nice waves across the E+G of IGBTs, please show pictures of the waves

Regards

 

[olNick]

I don know about the HIGH VOLTAGE section in your machine.


But you can just keep the IGBTs un-mounted (DONT INSTALL THEM)

And while using VARIAC, increase voltage slowly, while monitoring the current draw from the 220VAC line, if increasing quick, then STOP.

Else, you should see nice waves across the E+G of IGBTs, please show pictures of the waves

Regards

Hi,
Thanx for your answers.

There are 2 rectifiers for the incoming ac.

One is a big bridge rectifier (B1) on schem that feeds the main filter caps and generates the ~300 VDC for the IGBT's. This is turned on by the relay K1, I'm guessing with a delay giving the logic circuits a chance to start.

There is a +-15V transformer on board (TV1) on schem that feeds another smaller rectifier (B2) that gives +-15 Vdc (for gate drive) and also gets regulated 10 +-12V for the low voltage circuit. This gets 220VAC as soon as the power switch is turned on.

What I am saying is I can disconnect the B1 so no 300VDC goes to the IGBTs but I have my low voltage circuit working and can check for pulses waveform quality across g-e on device. Granted this is without load...

I cannot use a variac (at least simpley) to bring up the voltage slowly because I will not have enough ac voltage going into B2 -- the control voltage. I believe this under voltage is what cooked things in the first place....

I can use a dim bulb tester instead and limit current....
Also, my scope is an old analog 2 channel scope and I dont have a differential probe...

regards,
nick

 

[MicrosiM]

Remove the bridge that feeds the capacitors to generate the 300VDC


And check waves across E+B of the IGBTs