Power Supply Failures

[WD5JKO]

  OK, this is a post about a recurring failure I am seeing on a piece of commercial gear that I do repairs on. The power supply is part of a 13.56 Mhz 3KW RF amplifier. Since this post is for a commercial part, and part of my livelihood, please moderators feel free to delete if this content is deemed inappropriate.

  There are three power conversions in this design, AC to lumpy DC, lumpy DC to 0-200V DC, and 0-200V DC to 0-3000W RF. Overall the efficiency is about 63% at full output.

  The first conversion is where I see more failures then I think there should be. I am attaching a schematic, and a few pictures of a transient filter the OEM design has.

   The three yellow discs appear to be 20mm diameter varisters, and the little white thing appears to be a sparc gap. If I hi-pot these things, the varisters break over at about 800 volts (1 ma), and the spark gap begins to flicker illumination from within at about 300 volts.

   We are seeing the big FW Bridge die (short), and then the relay on the next turn on becomes a welded mass. Then the customer resets the breaker over and over until it too fries. The power is 3 phase 208 VAC Delta. The FW Bridge has 1200V PRV. I just cannot understand how those MOV's would do much...they don't sacrifice themselves at all...they survive!

   Was thinking of just adding some 300L40 MOV's phase to phase, but that spark gap seems to be there to trap a common mode lightning bolt.

Any thoughts?

Jim
Wd5JKO

[flintstone mop]

This should be a good grey matter exercise for some high power transmitter folks who live here in AMFONE.
 
I'm not a 3 phase power type, but would some imbalance between phases be killing the rectifiers? It must be pretty ugly after the diodes pop and then the relay takes the final hit.

OR how 'bout this......Are the diodes capable of handling the peak voltages and currents?
A call to the manufacturer might help for this recurring failure of the same components. Maybe they have an upgraded replacement diode available.

Fred

[W2VW]

If it were mine I'd add a zero crossing relay and several rectifiers in series and call it a day.

[AB2EZ]

I wonder if the diode bridge is failing because there isn't enough series resistance in each leg to limit the peak current flowing through each diode when it starts to conduct on each cycle.

Stu

[w4bfs]

yup ... needum step start

[N2DTS]

What is the size and rating of the cap?
What is the peak inverse voltage the diodes are supposed to be withstanding?

[WD5JKO]

"OR how 'bout this......Are the diodes capable of handling the peak voltages and currents?
A call to the manufacturer might help for this recurring failure of the same components. Maybe they have an upgraded replacement diode available."

  The AC RMS current per phase is a maximum of 15 amps at 3KW RF out. The Power factor is .9 at 750 watts, and rises to .95 at 3 KW. The thing is PFC (power factor corrected) so the bridge rectifier does not have to charge up a big capacitor..there is very little energy storage in there. The diodes are rated at 30 AMPS RMS, and have a huge transient peak current rating.

"If it were mine I'd add a zero crossing relay and several rectifiers in series and call it a day."

  Not easy to get all three phases at zero crossing at the same time unless the power is off. More diode PRV though might be necessary, or perhaps better transient suppression..the topic of this post. All the power semiconductors are water cooled; everything else cooks in hot stale air. These units consume 5 KW (AC Line) to make 3 KW RF. All this in a box the size of a shoe box.

  The machines these units go into use 14 of these amplifiers. In addition there is a 100KV 100 ma DC supply, and two electromagnets that each consume 5KW. Add in a half dozen roughing pumps, and six turbo pumps, and 4 cryo pumps. Lots of power management....whole thing takes 480V 3 phase at 100 amps / phase. All this to make a multi-mega-electron-volt particle accelerator.

"I wonder if the diode bridge is failing because there isn't enough series resistance in each leg to limit the peak current flowing through each diode when it starts to conduct on each cycle."

  Not much series resistance for sure. Adding just a little would add considerable thermal heat loss and lower efficiency.

  The OEM maker added the varistors and spark gap after many years of not having them. I don't think they do much. Beefing up the transient suppression might mean more failures from blown MOV's.

Jim
Wd5JKO

[DMOD]

According to the schematic, the MOVs don't go directly to ground so they do very little if anything.

As other have alluded to, you either have too much surge current going through the rectifier diodes or a large reverse voltage is spiking them out.

If you have PF inductors ahead of this circuit, you could have large PRV's occurring that are not being quenched.

Phil - AC0OB

[steve_qix]

Wow, it looks like an accident waiting to happen !

There should definitely, absolutely be some sort of step start or other soft start mechanism in place. There's nothing to mitigate the in-rush current - not even a power transformer to add some loss.

A couple of questions - does it always fail on turn on?  If so, inrush current is really looking like the problem.  If it fails while running, there are some major surges coming across the line from someplace.

A step-start would not be a big deal. Another relay would be necessary and of course the series resistors that are shorted out by the step start relay, and whatever you use to establish the time delay.  I use a diode going into an R/C right across the line.  The C charges through the R, and a small relay closes when a critical voltage is reached.  This relay in turn sends power to the step-start relay and closes it.  Done !

Can provide more details if necessary, but assuming the failure occurs on start-up, a step-start will probably fix the problem.

Regards,  Steve

[WD5JKO]

According to the schematic, the MOVs don't go directly to ground so they do very little if anything.

As other have alluded to, you either have too much surge current going through the rectifier diodes or a large reverse voltage is spiking them out.

If you have PF inductors ahead of this circuit, you could have large PRV's occurring that are not being quenched.

   With modern supplies it seems that PFC is more of a cost savings marketed as a feature. They get rid of about 90% of the energy storage causing lumpy DC, and then Taylor the feedback loop to correct for the varying input at some multiple of the power line frequency. Having 3 phase power sure has an advantage since the unfiltered ripple is manageable. So with PFC they replace capacitors with Silicon. With PFC there is less EMI ripple conducted up the AC power line.

  That rectifier diode has an NTE replacement. Like all NTE stuff there is a premium price to be paid. There is also a higher PRV rated part from Powerex..I need to see, but I recall the higher PRV resulted in less current surge ratings. There is very little room to add anything. Modern supplies like this use very high density packaging, and these days everything is approaching 100% surface mount...even at multi-kilowatt levels.

  I am still contemplating using more application specific varistors such as in the picture. For example, If I used V140LA20A's instead, but wired as in the schematic drawn, then I would have 280v RMS rated protection phase to phase. Each has a 340V clamping voltage at 100 amps, so that provides 680V protection phase to phase.

http://www.littelfuse.com/products/varistors/~/media/Electronics_Technical/Datasheets/Varistors/Littelfuse_Varistors_LA_Datasheet.pdf

  The power might be so dirty that varistors as proposed would not survive...

  So why is that gas discharge spark device in there? I see these in some RF Ham transmission line products. Do they survive multiple discharges?

Jim
Wd5JKO

[WD5JKO]

Wow, it looks like an accident waiting to happen !

There should definitely, absolutely be some sort of step start or other soft start mechanism in place. There's nothing to mitigate the in-rush current - not even a power transformer to add some loss.

A couple of questions - does it always fail on turn on?  If so, inrush current is really looking like the problem.  If it fails while running, there are some major surges coming across the line from someplace.

A step-start would not be a big deal. Another relay would be necessary and of course the series resistors that are shorted out by the step start relay, and whatever you use to establish the time delay.  I use a diode going into an R/C right across the line.  The C charges through the R, and a small relay closes when a critical voltage is reached.  This relay in turn sends power to the step-start relay and closes it.  Done !

Can provide more details if necessary, but assuming the failure occurs on start-up, a step-start will probably fix the problem.

Regards,  Steve

  It's hard to say when the failures occur. When I see these units, they are long removed from the machine. I suspect the failures (bridge rectifier) occur from multiple causes. Down stream on the DC buss is a 20A fuse that sometimes opens, but not always. The load is an Iso-Topp H-Bridge with 100A rated Fet's. Sometimes one side of the H-Bridge fuses to a short, which blows the fuse, and sometimes the bridge. When the bridge is blown the NEXT power cycle closes the contactor into a short which welds the contacts. Then the customer resets the breaker into a dead short taking it out too.

  The contactor gets power from an auxiliary 24v supply that verifies proper AC power, and then drops the contactor. So in a sense there is a step start. Without a big capacitor bank to charge up, the turn on transient is uneventful. I monitor current on all three phases; no big surge unless its broken.

  I have a nice collection of dead parts, and some spectacular photos...

Jim
Wd5JKO

[AB2EZ]

The load draws down a fraction of the charge on the capacitor 6x on each cycle ("lumpy" DC at the output of this supply). Therefore, there are 6 current surges (one through each pair of diodes) each cycle, corresponding to the six rechargings of the capacitor from the AC source. The peak current on each of these current surges may be much larger than the rms current on each phase. The surge current rating of the bridge may not apply to surges that pass through each diode twice per cycle.

The lower the AC source resistance, the larger the peak current on each surge will be.

Stu

[KA2DZT]

Could be that the MOVs (if anywhere) should be connected on the other side of the relay.  Problem might be some back surge when the relay is opened.

Fred

[W2VW]

Sorry. I took my 3 phase hat off yesterday when I left work

[w4bfs]

hi Jim .... an interesting problem to be sure .... it's beginning to sound like a problem downstream of the lumpy supply causing a failure cascade that the 20A fuse can't catch .... a turn on surge problem doesn't seem to fit after all; however, the ESR of the capacitor sets the uncharged to going charged current amount not the capacity

it would be interesting to add an overcurrent trip ckt in the 24V contactor supply feed by sensing the negative supply lead with a low value resistor and low coil voltage relay (or suitable optocoupler) .... perhaps this could stop the cascade long enuff to id the culprit

I learned at TI a long time ago that the actual lifetime of high power/current electronics is a function of electromigration of junction materials due to current density and it is finite ....

[flintstone mop]

hi Jim .... an interesting problem to be sure .... it's beginning to sound like a problem downstream of the lumpy supply causing a failure cascade that the 20A fuse can't catch .... a turn on surge problem doesn't seem to fit after all; however, the ESR of the capacitor sets the uncharged to going charged current amount not the capacity

it would be interesting to add an overcurrent trip ckt in the 24V contactor supply feed by sensing the negative supply lead with a low value resistor and low coil voltage relay (or suitable optocoupler) .... perhaps this could stop the cascade long enuff to id the culprit

I learned at TI a long time ago that the actual lifetime of high power/current electronics is a function of electromigration of junction materials due to current density and it is finite ....

Too bad that these units can be removed by the user and now you have no idea what exactly is going on. These units read like they are updated, cheap shortcut designs, a so called improved version, of something more reliable made a number of years ago.
Does the manufacturer have any input as to what the real problem is?
If not, then it's a cash cow for you. Just replace the defective components until the user gets tired of the constant repair bills. How many users of this device?
You are repairing these for many, from a region, or just locally?
Can you give us the make and model of this device?
As you kindly reply to the responses, the more I read that this is a technical merry-go-round........too many things going wrong and no control over what exactly the problem is.
Fred

[WD5JKO]

Could be that the MOVs (if anywhere) should be connected on the other side of the relay.  Problem might be some back surge when the relay is opened.

   That is a possibility, and there might be a way to do this with the space constraints.

hi Jim .... an interesting problem to be sure .... it's beginning to sound like a problem downstream of the lumpy supply causing a failure cascade that the 20A fuse can't catch .... a turn on surge problem doesn't seem to fit after all; however, the ESR of the capacitor sets the uncharged to going charged current amount not the capacity

it would be interesting to add an overcurrent trip ckt in the 24V contactor supply feed by sensing the negative supply lead with a low value resistor and low coil voltage relay (or suitable optocoupler) .... perhaps this could stop the cascade long enuff to id the culprit

I learned at TI a long time ago that the actual lifetime of high power/current electronics is a function of electromigration of junction materials due to current density and it is finite ....

  Thanks. These failures are usually after years of operation, so your "metal migration" concept could fit. That said, certain failures can be immediate initiating a cascade of carnage.

  Looking at the range of three phase FW bridges out there, there are many with very stout ratings.None will fit the pre-drilled, and water cooled heatsink though. Look at the IX YS stuff, this company seems to be leading the industry on many fronts, including FET's for Cla ss E transmitters. We use two of those IX YS Fet's in this box paralleled to make 3 KW RF.

  I am attaching two pictures. It occurred to me though, that some of the bridge failures might be from the isolated mount (2500V RMS rated) punching through. The Varistor / spark gap pictured in post 1 of this thread might help protect with isolation failure, but not do much with diode PRV punch through failures. That said, most of the diode failures have no outward sign of device trauma.

Good input from all, Thanks.

Jim
Wd5JKO

[W2VW]

I was thinking turn off transients may be what is killing the rectifiers.

Some interesting ways of mitigating:

http://www.crydom.com/en/Tech/Whitepapers/3P_HC_whitepaper.pdf

[AB2EZ]

Jim

Put a 0.05 ohm 50W wire wound resistor in series with each of the three branches (to keep things balanced).

With the normal output load attached, measure the voltage waveform across each resistor, using a transformer to couple the voltage across the resistor to a scope.

[Or, even better: use a current probe (rated at 200A or more) if you have one]

You may be surprised by how large (peak amplitude) the current pulses are. On each phase, you should see 2 positive pulses and 2 negative pulses in each 60Hz cycle.

I also agree that the most stressful event in this circuit will occur when the contactor closes, and the capacitor charges up from zero coulombs... but that event will occur only during turn-on... not multiple times per 60Hz cycle.

If the designers skimped on the size of the output capacitor ("lumpy DC" output), they may have reduced the turn-on stress (avoiding the need for a step start circuit) in trade for the stress that occurs multiple times each 60Hz cycle.


Stu

[w7fox]

I would say that the rectifiers are being destroyed by transients.  The voltage rating of 800 volts on the MOVs is too high.  With them connected in Y, they won't conduct until 1600 volts appears from phase to phase.  That is going to wipe out 1200 volt diodes nicely.  I would install a more suitable MOV in delta across the phases.  The peak voltage for 208 is something under 300 volts, so I would pick a MOV around 450 volts or so, I'm not sure what's available.

[DMOD]

It occurred to me though, that some of the bridge failures might be from the isolated mount (2500V RMS rated) punching through. The Varistor / spark gap pictured in post 1 of this thread might help protect with isolation failure, but not do much with diode PRV punch through failures. That said, most of the diode failures have no outward sign of device trauma.

It could also be you have an insufficient heat conducting path from the rectumfiers to the cold plate.

You might also try some Bi-Directional Surge Suppressors such as the NTE 4993,5, two in parallel for each phase leg:

http://www.nteinc.com/specs/4900to4999/pdf/nte4903_99.pdf

I have seen some pretty nasty three-phase power in both 208 and 480VAC lines.

Phil - AC0OB

[AB2EZ]

For each pair of input lines (phases):

If the line to line voltage exceeds the voltage across the capacitor (in either polarity), the bridge will conduct current into the capacitor.

Therefore I think that a line to line voltage surge would result in a forward current surge through the associated pair of forward biased diodes (possibly destroying one or both diodes)... rather than creating a PIV problem.

For line to line voltages, the MOVs will only come into play if the contactor is open.

The MOVs allow all three lines (phases) to share the same spark gap to ground, to protect against a lightning strike that would raise all three lines with respect to ground (I.e. common mode, not line to line)

Stu

I would say that the rectifiers are being destroyed by transients.  The voltage rating of 800 volts on the MOVs is too high.  With them connected in Y, they won't conduct until 1600 volts appears from phase to phase.  That is going to wipe out 1200 volt diodes nicely.  I would install a more suitable MOV in delta across the phases.  The peak voltage for 208 is something under 300 volts, so I would pick a MOV around 450 volts or so, I'm not sure what's available.

[WD5JKO]

What is the size and rating of the cap?

   I was looking at one of these amplifiers yesterday. The cap is 180 uf / 400 V. Now get this, they Epoxy two 27 ohm 10W WW resistors to the water cooled center plate, and the resultant 13.5 ohms is placed in series with that capacitor. Then there is a 200K 1/2 watt across the cap. So there is the impedance to limit peak current from the rectifiers. On occasion I find these resistors shattered, and open...as if the contactor had been chattering..along with the usual cascade of carnage.

I would say that the rectifiers are being destroyed by transients.  The voltage rating of 800 volts on the MOVs is too high.  With them connected in Y, they won't conduct until 1600 volts appears from phase to phase.  That is going to wipe out 1200 volt diodes nicely.  I would install a more suitable MOV in delta across the phases.  The peak voltage for 208 is something under 300 volts, so I would pick a MOV around 450 volts or so, I'm not sure what's available.

   Remember that MOV's are rated for the RMS voltage in a AC circuit, and an energy absorption in Joules.

http://www.littelfuse.com/products/varistors/~/media/Electronics_Technical/Datasheets/Varistors/Littelfuse_Varistors_LA_Datasheet.pdf

The MOVs allow all three lines (phases) to share the same spark gap to ground, to protect against a lightning strike that would raise all three lines with respect to ground (I.e. common mode, not line to line)
Stu

  Good point! I bet there was some sort of compliance test that needed that circuit added to be certified. I used to do Mil Spec 461 testing, and all sorts of things like this had to be done.

Jim
Wd5JKO

[AB2EZ]

Jim

If there really is 13.5 ohms in series with the capacitor... then the load on this supply must be drawing occasional, short duration pulses of power from the supply.

I.e. if there were (as an example) 10A of average current flowing into the load, then the 13.5 ohm resistor would dissipate an average power of 1350 watts!

However, if the load is drawing only an infrequent, short duration pulse of current from the supply, then the capacitor would deliver the corresponding charge (coulombs = the integral over time of the pulse of current being drawn by the load) and the corresponding energy (joules = the integral over time of the pulse of power being drawn by the load).

In between these occasional, short duration pulses of power being drawn by the load... the capacitor would be recharged by the AC source delivering current (and power) through the diode bridge and through the wire wound resistor.

The power flowing into the wire wound resistor would exceed its dissipation rating for the duration of the recharging process... but the wire wound resistor can handle the these short duration dissipation overloads, provided there is enough time between capacitor rechargings for the associated heat to be dissipated by the heat sink.

If all of the above is what is actually happening... then the next thing I would examine is whether the thermal conductivity to the heat sink is insufficient to dissipate the power produced in  the series resistor when the load is drawing its maximum number of short duration pulses of energy per unit time.

Stu

What is the size and rating of the cap?

   I was looking at one of these amplifiers yesterday. The cap is 180 uf / 400 V. Now get this, they Epoxy two 27 ohm 10W WW resistors to the water cooled center plate, and the resultant 13.5 ohms is placed in series with that capacitor. Then there is a 200K 1/2 watt across the cap. So there is the impedance to limit peak current from the rectifiers. On occasion I find these resistors shattered, and open...as if the contactor had been chattering..along with the usual cascade of carnage.

  
 
Jim
Wd5JKO

[WD5JKO]

Jim
If there really is 13.5 ohms in series with the capacitor... then the load on this supply must be drawing occasional, short duration pulses of power from the supply.

   Stu,

   the power buss is bypassed by four .47 uf Poly caps, and then four 0.1uf poly caps. Part of the cascading carnage is these handle a LOT of AC RMS current, and they get HOT. After about 10 years they spill their guts out. Perhaps the resistors have to handle more AFTER the caps start popping...failure is not immediate.

  The power buss feeds a big H-Bridge PWM converter about running 50 Khz. I have been replacing the .47uf's with something more readily available, and with lower ESR. These also get hot, but the overall efficiency of the unit (line in KW with PF correction) versus RF power output increases a few percent, like 62% to 64%..remember there are three power conversions in there, so 64% overall isn't too bad.

Keep in mind these designs came out 20 years ago. Pretty Hi-Tech for that time...Clas s E 3KW RF @ 13.56 Mhz...The size of a shoe box.

Jim
Wd5JKO