Rectifier Bridge of Boost PFC is dead...how?

[Faringdon]

Hi,
[EDIT...This post is now solved, and was a simple , intermittent, dry joint, please do not bother reading, touching the dry component pads actually intermittently connected it up------END OF EDIT}
Do you know why my  GBJ2510 diode bridges  died? Two of these diode bridges were used  together in the bridge of a Boost PFC. (as attached).

[EDIT...please see post #4, which just might be what the problem is. END_OF_EDIT]

This boost PFC has only ever been powered up with a 3.15A Mains fuse in it. (Load has never yet been above 7W). This PCB has  only ever been powered up  with mains through a 47R inrush resistor. (later shorted out by relay)
I have now removed the  two GBJ2510’s from the PCB, and their heatsinks.
Mains here is some 225VAC from my cheap DMM.
The Boost PFC  has previously powered a 7W load at the correct 390V.
Previously, when I did the manual inrush into the PFC output caps, I did notice that the output caps only went up to some 300VDC…..but I confess to suspecting it may have been due to the scope being out of cal, and to be honest , at the time, the PFC FETs blew up, which distracted me away from the 300V reading.
I then fixed the Boost PFC (It seemed that I possibly had  accidentally  desoldered the current sense connection).
With the Boost PFC fixed, I then adjusted the control card signal  resistors for the intended, ,future  increased load of  250W…….Then again I powered up, (with just 7W load) through the 47R inrush resistor, but noticed only some 70VDC on the PFC output caps. The drop in voltage was occurring in the bridge diodes (GBJ2510). I had the full correct mains VAC just upstream of the bridge.
I removed the two  GBJ2510’s, and find that they  both still DMM as a diode bridge. However, when you go onto resistance range of 20k, and probe with red to anode and black to cathode, they show some 15k of resistance (with the probes the other way they show open circuit). All the bridge diodes do this, on both GBJ2510’s. Even stranger, is that when you switch to 200k resistance range, they then show as some 90k. (and again open circuit with the probes swapped round).
Do you know how this could  have happened? These are 1kV  rated rectifier bridges.
There is a MOV at the mains input.
MOV  (V275LA10P)
https://www.farnell.com/datasheets/2608835.pdf

GBJ2510 diode bridge
https://www.mouser.co.uk/datasheet/2/115/ds21221-2940656.pdf

The diff two mode chokes are very low value at 46uH (just 13 turns round three stacked MS-090125-2 sendust torroids). The common mode choke is just 2 x 6 turns round a MS-090125-2 torroid. (This SMPS is prototype only, to get to the intended 2kW power level.
I am now just wondering if a big mains transient did indeed take out the diode bridges, and maybe that’s what killed the FETs too. (it’s a dual Boost PFC).

[jonpaul]

Test diodes with curve traces a DVM reading is useless

MOVs are NOT good clamps for line transients.

Check mains power quality eg inductive loads, or (UK) ring bus faults

j

[bdunham7]

Measuring diodes using the resistance ranges requires understanding exactly how the resistance range functions (test current and voltage).  A 90K reading on the 200K range just means that the voltage drop across the diode is 0.45 x the full range test voltage at the DMM's test current--if it even uses a fixed test current.  If you get that reading with a resistor, you can say that it is a 90K resistor.  With non-ohmic devices, not so much.

The results you have are probably completely normal and your diode bridges are likely fine.  If they're not fine, you're going to have to use other methods to figure that out.

[jonpaul]

Farrington:

We used to use a DV HV insulation tester to check diode bridge breakdown, eg 0-30kV 1..10..100 uA FS Danbridge.

The TEK 576, 577 curve traces are better below 1600 V


Finally A boost PFC my require a minimum load to run.

At zero or very light loads the worst case of the controller or switch device may be exceeded.

Just a thought,

j

[Faringdon]

The results you have are probably completely normal and your diode bridges are likely fine.

Thanks BDunham7, and Actually, having thought this through whilst at Mcdonalds, i think youre probabaly right......i likely had a semi-dry joint with the rectifiers.......they looked soldered, but they were big PTHs and my soldering iron is cheap....Possibly , Somehow i managed to solder myself a high value resistance, which varied each time i powered up.

I may even have tightened the heatsink screws with the bridges soldered in, and put pressure on the solder joints.....i'm not certain yet, but i am now on this trail....oops My sincere apologies to all answerees, and readers, ill put in an edit to the top post now.

[jonpaul]

F: We use Use only genuine Metcal Smartheat irons like my SP-200 desing of 1991,

Especially  on heave connections, ground planes and power PCBs.

Insist on ONLY Eutectic 63/37 solder LEADED NOT LEAD FREE

Lesson: CHEAP (Chinese) Irons waste time and money. GET A DECENT SOLDERING SETUP


Jon

[Faringdon]

Thanks, ive just resoldered the diode bridges......and again all i am getting is 70V on the Boost PFC caps, with 40k resistive  loading...and Boost PFC disabled...still with Boost PFC disabled, i shorted out the 47R inrush resistor, and the 1A mains input fuse blew straight away.
Cout is 270uF.
EDIT...I am claiming mug-of-the-year award for myself...it was the wire links to the inrush resistor...they werent actually soldered...just touching..and intermittent open ..END OF EDIT
My apologies

[uer166]

Thanks, ive just resoldered the diode bridges......and again all i am getting is 70V on the Boost PFC caps, with 40k resistive  loading...and Boost PFC disabled...still with Boost PFC disabled, i shorted out the 47R inrush resistor, and the 1A mains input fuse blew straight away.
Cout is 270uF.
EDIT...I am claiming mug-of-the-year award for myself...it was the wire links to the inrush resistor...they werent actually soldered...just touching..and intermittent open ..END OF EDIT
My apologies

Do you own any test equipment at all? Scope? Diff probe? I'm not sure how far you can get with this by trying random things until something works. A start-up trace of the DC link would be the first thing you should have done, and immediately saw that the precharge is broken.