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Author Topic: Power Supply Failures  (Read 21316 times)
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AB2EZ
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« Reply #25 on: January 10, 2014, 12:46:28 PM »

Jim

I am still confused as to how there can be 13.5 ohms of resistance in series with the capacitor... unless the load that it is feeding is drawing only occasional pulses of power from this supply.

I guess that I need a more complete schematic of this power supply (showing the bridge, the 13.5 ohm resistor, the capacitor, and how the load is connected) and a better understanding of the nature of the load it is feeding.

Stu
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« Reply #26 on: January 10, 2014, 01:03:01 PM »

Jim

Since you said that this power supply is one component of a large particle accelerator... is it possible that this supply is basically sitting idle (i.e. delivering no power or very low power to the bigger system) until someone "pushes a button" that causes everything to very briefly turn on to inject a single bunch of electrons into the accelerator. Then, this power supply would sit idle until the next bunch of electrons needs to be injected into the accelerator.

Stu
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AB2EZ
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« Reply #27 on: January 10, 2014, 01:36:04 PM »

If the above is correct, then there could be a large transient voltage pulse produced between both sides of this supply's capacitor and the grounded side of the spark gap (common mode) every time the larger system is pulsed to inject electrons into the accelerator.

Given the path to ground through the MOV's and the spark gap ... this could result in a large pulse of current passing through half of the diodes in the bridge. If the amplitude of the current pulse is large enough... those diodes could be damaged.

If that same current flows through the 13.5 ohm series resistor, it may also cause a voltage drop across the remaining three, reverse biased diodes... that exceeds their PIV rating.

Stu
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« Reply #28 on: January 10, 2014, 02:10:56 PM »

May be a late reply, but the sparkgap surge arrestor   in series with the mov's to ground is  to prevent a to high leakage current from the mov's to ground. A sparkgap has nearly no leakage if not ignited and when its ionised it has something as 0.5 Ohm. The mov's have always a rest current of something as 1 mA each.
The residial  current as in the standards ( UL, Csa, Kema , Gs etc) is less then 0.5 mA. This cannot be met by mov's alone.
Anton
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« Reply #29 on: January 10, 2014, 10:30:17 PM »

May be a late reply, but the sparkgap surge arrestor   in series with the mov's to ground is  to prevent a to high leakage current from the mov's to ground. A sparkgap has nearly no leakage if not ignited and when its ionised it has something as 0.5 Ohm. The mov's have always a rest current of something as 1 mA each.
The residial  current as in the standards ( UL, Csa, Kema , Gs etc) is less then 0.5 mA. This cannot be met by mov's alone.
Anton

  Makes sense Anton. This arrangement has its purpose in compliance issues, but seems to do little to protect that FW bridge, unless it is to keep the isolated bridge package from breaking through beyond its 2500 V RMS rating. Same for the ISO-Topp FET's in the H-Bridge switcher.

I am still confused as to how there can be 13.5 ohms of resistance in series with the capacitor... unless the load that it is feeding is drawing only occasional pulses of power from this supply.

I guess that I need a more complete schematic of this power supply (showing the bridge, the 13.5 ohm resistor, the capacitor, and how the load is connected) and a better understanding of the nature of the load it is feeding.

   Maybe the attached scope plots will better illustrate whet I was trying to express in words. At 2ms per division the +/- sides of the bridge rectifier are lumpy DC ranging from 90-180v, essentially providing dirty power to the high frequency switcher. Both plots are of the same thing, only the scope sweep rate changes.

   I also added a PDF showing one stage of the linear accelerator (linac). Each stage has RF phase shifters to optimize the energy multiplication, and transport efficiency. All the Rf amplifiers slave off a common Rf source that is called the master oscillator. The resonator cans are a bit larger then a beer keg, are VERY high Q...just a few Khz from tuned and the reflected power gets really high. The RF amplifiers have a built in protection circuit to limit the reflected power to no more then 250 watts. Usually when tuned there can be as high as 2750 watts forward, and < 5 watts reflected. The tuning loop is always active since things change fast as parts warm up..even with water cooling.

   The RF stuff runs at various power levels, and not all are used at a time. For some needs they all run (up to 14 of them). They are on CCS, often 24/7. The RF is on whether or not an ion beam is running through the linac. It might take a Forrest of trees to offset the carbon foot print from one of these beasts.

Jim
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* RF based linear accelerator.pdf (105.7 KB - downloaded 164 times.)

* H-Bridge Output 2us per.JPG (128.8 KB, 640x480 - viewed 400 times.)

* H-Bridge Output 2ms per.JPG (117.19 KB, 640x480 - viewed 336 times.)
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AB2EZ
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« Reply #30 on: January 11, 2014, 11:45:44 AM »

Jim

In the second oscilloscope photo... that shows the 2ms per division time scale:

It appears that you are using a pair of single-ended (grounded) probes (2 superimposed traces), rather than a differential probe.

If so, what is the "ground" side of each probe connected to?

I see 3 pulses in the top waveform in 1/60th of a second (16.7ms). I would expect to see 6 pulses in the top waveform in 1/60th of a second. I.e. in each 1/60th of a second (separated by 1/360th of a second)

Phase 1 - Phase 2 reaches its peak positive value
Phase 1 - Phase 3 reaches its peak positive value
Phase 2 - Phase 3 reaches its peak positive value
Phase 2 - Phase 1 reaches its peak positive value
Phase 3 - Phase 1 reaches its peak positive value
Phase 3 - Phase 2 reaches its peak positive value


The first oscilloscope photo (longer time scale) shows the voltage switching between two levels (as if the load were switching between two values). What is the time scale (time/division) of the first photo?

Stu
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WD5JKO
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« Reply #31 on: January 11, 2014, 12:18:08 PM »

Jim

In the second oscilloscope photo... that shows the 2ms per division time scale:

It appears that you are using a pair of single-ended (grounded) probes (2 superimposed traces), rather than a differential probe.

If so, what is the "ground" side of each probe connected to?

The first oscilloscope photo (longer time scale) shows the voltage switching between two levels (as if the load were switching between two values). What is the time scale (time/division) of the first photo?

Stu

Stu,

   The time scale is part of the picture label, i.e 2ms and 2 us per division

The H=Bridge is very similar to this one:
http://omapalvelin.homedns.org/tesla/SSTC/pics-fullbr/bridge.gif

  The + and - power in the image go to the + and - outputs of the 3 phase diode bridge.

The capacitor in series with the transformer is four times 4.7 uf polypropylene in parallel. These get HOT but seldom fail. The transformer is a mule, about 2"X2"X3", and is water cooled. The secondary output is ground referenced...(FW CT rectifier follows) so it isolates the AC Line stuff while making 0-200VDC at around 20 amps maximum.

The capacitors in the image between the + and - rails are the ones that fail. Local to the H-Bridge, clustered between the four FET's are six (I said 4 earlier) .1 uf polypropylene, and then on a daughter PCB there are four .47 uf polypropylene all in parallel across the +/- buss.

The scope probe was ground referenced looking at the transformer primary..the side with the series capacitor, hence seeing +/- 150v (with 3 phase power line ripple on the buss).

Jim
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AB2EZ
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« Reply #32 on: January 11, 2014, 01:04:06 PM »

Jim

okay... with respect to the 2nd oscilloscope trace photo...I think I get it.

The scope ground reference for these measurements (two probes in the 2nd oscilloscope picture) is equivalent to putting a local balanced "Y" load directly across the 3 phase AC source... and defining the point where the 3 legs of the "Y" come together as the measurement ground reference. This measurement ground reference would not be the same as the AC source ground to which the spark gap is connected (which could have all sorts of noise on it, relative to this measurement ground reference). If the oscilloscope has an "a-b" feature, I guess you could subtract the lower trace from the upper trace to display the voltage between "+" and "-", and to cancel out the effects of any extraneous noise on the measurement reference point relative to both "+" and "-".

With respect to the first oscilloscope photo (2us per division), I guess I don't understand how this system works. It is hard for me to see how the diode bridge output voltage (either + or - side) with respect to measurement ground can change that quickly (rise/fall time much less than 2us)

Stu

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AB2EZ
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« Reply #33 on: January 11, 2014, 05:54:26 PM »

Jim

I think I now understand what 1st oscilloscope trace photo is.

There is only one probe producing the one trace that is shown in the 1st photo (whereas, there are two probes producing the two traces shown in the second photo)

The (single) probe's tip is attached to the + side the diode bridge; and the "ground" lead of the probe is attached to the "H bridge" at the point where the source of the top FET on the left side of the "H bridge" attaches to the drain of the FET below it.

It appears that the four switching inputs to the H-bridge are 100kHz (approximately) square waves (i.e. a little longer than 10us to complete one cycle).

Therefore, as the H bridge FETs switch on and off at 100kHz (approximately), the probe's ground connection point is, in effect, switching at 100kHz (approximately) between the voltage on the "-" side of the diode bridge (plus the voltage drop across the bottom FET that is conducting), and the voltage on the "+" side of the diode bridge (minus the voltage across the top FET that is conducting).  

Therefore, what the scope shows is a 100kHz (approximately) square wave whose peak-to-peak amplitude is approximately the voltage between the "+" and "-" terminals of the 3 phase bridge... with the 3 phase ripple superimposed as a fuzz (due to the overlapping of multiple traces that are all triggered to sync with the square wave).

Anyway, getting back to the failures in the 3 phase diode bridge:

It appears that the 13.5 ohm resistor is limiting the average voltage across the capacitor on the output side of the 3 phase bridge... and also setting the shape and amplitude of six charging current pulses per 60Hz cycle that flow through the diodes in the bridge into the capacitor.

However it also appears that the load (power delivered to the "H bridge") is being drawn directly from the +/- terminals of the diode bridge. I.e. the load is not across the capacitor... but, instead, across the series combination of the capacitor and the 13.5 ohm resistor.

Therefore, charging current is flowing into the capacitor via the 13.5 ohm resistor, and discharging current is flowing out of the capacitor via the same 13.5 ohm resistor... and the average power dissipated in the 13.5 ohm resistor (which depends upon the integral of the square of the current) is less than it would be if the "H-bridge" load were directly across the capacitor. 

What is still surprising to me is that the wire wound resistors and the associated heat sink (a pair of 27 ohm 25W resistors in parallel, epoxied to the heat sink) can handle the power that is being dissipated in those resistors... 

Stu

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« Reply #34 on: January 11, 2014, 09:51:40 PM »

Stu,

  Both images are done with one scope probe, and use only one scope channel. Both are done under the same exact conditions within the RF amplifier, i.e RF on at 500 watts RF output. The only difference was the scope sweep rate, 2 ms /div and Line sync to see the 3 phase ripple, and 2 us / div and channel 1 sync to see the ~ 100 KHZ switch rate.

   Let me make a comparison to something else. Imagine a class AB solid state audio amplifier with complimentary NPN and PNP output transistors. There are dual supplies, + and -. The connection between the two transistors (the emitters) will drive a speaker. The +/- power supplies are referenced to a common point (ground). The power supplies have lousy filtering so the ripple at 120 Hz is about 5%. The amplifier has about 20 db NFB.

  So lets drive this amplifier with a 1 Khz sine wave until it is running 50% of whatever the watt rating is into the speaker load. A nice sine wave input becomes an amplified sine wave out. The power supply ripple makes no difference because the amplifier is running well within its dynamic range, and there will be a high power supply rejection ratio (PSRR). The scope is hooked to the output, and to the common point (ground).

  Now lets drive the amplifier until it clips badly as the output slams against the power supply rails. When this happens, the 20DB NFB goes away, and PSRR goes to zero. The clipped portion of the sine wave (1 khz) will now have 120 hz ripple present riding on the flattened peaks. When this is happening, if you adjust the scope sweep to 2 or 5 ms / division, then the scope will mostly show what appears to be two traces of 120 hz ripple...i.e the top trace will show the + supply ripple, and the bottom will show the - supply ripple. if the scope is a digital one, the 1 Khz portion might not even be visible.

  The H-Bridge switcher on each side is behaving like the audio amplifier driven into hard clipping. The ripple is common to each side of the transformer, and the series cap to one side of the primary removes any DC component, and also acts like a high pass filter. There is NO ripple in the 0-200V output that this H-Bridge feeds into.

  Back to the AC 3 phase supply feeding this amplifier. The power distribution to the test building is 480v delta 3 phase. Then it is stepped down to 208v 3 phase 'Y'. Here at the common point in the 'Y', we connect both the neutral, and ground wires at this common point. The electrician ran the 3 phase 208 service to the test room using a 4 wire cable (3 phases and ground...no neutral). Since the ground wire connects back at the transformer the same as a neutral, grounding my scope to the amplifier chassis (power line ground) is a valid way of doing this measurement of the power buss ripple.

  If I wanted to make meaningful measurements of the 100 Khz into the transformer, I'd need two scope probes, sum the channels, and invert one of them to make the measurement. I have two nice 100X compensated probes, so I may try this. I would connect a probe to each side the the transformer primary.

  So what is the rms ripple % of unfiltered 3 phase power? If I follow this thread, I take it to be about 5%.

http://forums.mikeholt.com/showthread.php?t=132994

  These RF amplifiers go back about 20 years. At one intermediate point, the OEM was putting a power diode across that 13.5 ohm resistor (two 27 ohms in parallel). I don't remember which orientation. Whenever I run across one of those diodes, if I need to dismantle that module, I will remove the diode. The 'current' production amplifiers do not use it, nor do the real old ones.

Jim
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AB2EZ
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« Reply #35 on: January 11, 2014, 11:15:10 PM »

Jim

Excellent analogies and reference thread!

I learned a lot "discussing" this with you and the other posters.

I'm sorry that I can't provide much regarding the cause of the 6 diode bridge failures, other than those caused by downstream short circuits, and possibly the temperature rise of the heat sink from the power dissipated in the wire wound resistors.

Best regards
Stu



Stu,

  Both images are done with one scope probe, and use only one scope channel. Both are done under the same exact conditions within the RF amplifier, i.e RF on at 500 watts RF output. The only difference was the scope sweep rate, 2 ms /div and Line sync to see the 3 phase ripple, and 2 us / div and channel 1 sync to see the ~ 100 KHZ switch rate.

   Let me make a comparison to something else. Imagine a class AB solid state audio amplifier with complimentary NPN and PNP output transistors. There are dual supplies, + and -. The connection between the two transistors (the emitters) will drive a speaker. The +/- power supplies are referenced to a common point (ground). The power supplies have lousy filtering so the ripple at 120 Hz is about 5%. The amplifier has about 20 db NFB.

  So lets drive this amplifier with a 1 Khz sine wave until it is running 50% of whatever the watt rating is into the speaker load. A nice sine wave input becomes an amplified sine wave out. The power supply ripple makes no difference because the amplifier is running well within its dynamic range, and there will be a high common mode rejection ratio (CMRR). The scope is hooked to the output, and to the common point (ground).

  Now lets drive the amplifier until it clips badly as the output slams against the power supply rails. When this happens, the 20DB NFB goes away, and CMRR goes to zero. The clipped portion of the sine wave (1 khz) will now have 120 hz ripple present riding on the flattened peaks. When this is happening, if you adjust the scope sweep to 2 or 5 ms / division, then the scope will mostly show what appears to be two traces of 120 hz ripple...i.e the top trace will show the + supply ripple, and the bottom will show the - supply ripple. if the scope is a digital one, the 1 Khz portion might not even be visible.

  The H-Bridge switcher on each side is behaving like the audio amplifier driven into hard clipping. The ripple is common to each side of the transformer, and the series cap to one side of the primary removes any DC component, and also acts like a high pass filter. There is NO ripple in the 0-200V output that this H-Bridge feeds into.

  Back to the AC 3 phase supply feeding this amplifier. The power distribution to the test building is 480v delta 3 phase. Then it is stepped down to 208v 3 phase 'Y'. Here at the common point in the 'Y', we connect both the neutral, and ground wires at this common point. The electrician ran the 3 phase 208 service to the test room using a 4 wire cable (3 phases and ground...no neutral). Since the ground wire connects back at the transformer the same as a neutral, grounding my scope to the amplifier chassis (power line ground) is a valid way of doing this measurement of the power buss ripple.

  If I wanted to make meaningful measurements of the 100 Khz into the transformer, I'd need two scope probes, sum the channels, and invert one of them to make the measurement. I have two nice 100X compensated probes, so I may try this. I would connect a probe to each side the the transformer primary.

  So what is the rms ripple % of unfiltered 3 phase power? If I follow this thread, I take it to be about 5%.

http://forums.mikeholt.com/showthread.php?t=132994

  These RF amplifiers go back about 20 years. At one intermediate point, the OEM was putting a power diode across that 13.5 ohm resistor (two 27 ohms in parallel). I don't remember which orientation. Whenever I run across one of those diodes, if I need to dismantle that module, I will remove the diode. The 'current' production amplifiers do not use it, nor do the real old ones.

Jim
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« Reply #36 on: January 12, 2014, 02:11:47 AM »

Late to this party but 2 cents..

On the single phase setup here, 120V primary peak current at full power is about 54A each half cycle (2.5A peak rectifier current)
(two supplies, one on each side of the 240V to neutral). Not too surprising but substantial, and is the reason why a 240V 50A dryer outlet was put in.

When keying the TX, the simulator says the inrush for one cycle is by simulation 230A but it's probably a lot less due to wiring resistances etc. There was no digital scope to capture the transient with at the time I measured. Anyway it translates to 10.5A through the rectifier for the inrush.

On transients when turning off the HV, which is done by a relay. The set is keyed by control of the power to the primaries of the plate transformers and final screen supply transformer. When the relay is opened there can be a transient over 15KV on the rectifier plate leads. This is why MV rectifiers are used. I am too cautious but don't like large strings of rectifiers and would not put less than a 30KV SS rectifier in place of a 673 rates 15KV. The rectifier that blew on January 18, 2009 (that took a 33A plate transformer relay with it) went because it was bad, not a flashover.

I know it's not the same situation but maybe the information will parallel what you are having and suggest things.
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« Reply #37 on: January 12, 2014, 11:07:41 AM »

Is the diode stack water cooled? Ok. "Loss of water cooling not reported by customer?"
-For whatever reason.  
Chain of custody information, Pump, valve or flow failure, etc.
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« Reply #38 on: January 12, 2014, 12:38:22 PM »

Is the diode stack water cooled? Ok. "Loss of water cooling not reported by customer?"
-For whatever reason.  
Chain of custody information, Pump, valve or flow failure, etc.

  Good question!  Yes that happens. Having 14 parallel, and individually valved 2 gallons / minute flow loops is hard to manage. On occasion the pumping system (a remote chiller) aerates the water such that bubbles build up over time, at a valve, or elbow. These can restrict water flow.

  What should happen is that the amplifier senses an over temp condition (Airpax TO-220 cased sensor), and then shuts down. Sometimes this causes the operator to condemn an amplifier as bad, and a perfectly good amplifier goes through the repair loop. Putting in a new unit will purge out the water lines, so the person believes the old amp was bad, and the new one fixed it. Sometimes the Airpax sensors are bad, and the amplifier does self destruct. The symptom is that the Thin-film RF power parts get hot enough to unsolder themselves, and cause one hell of a flame spray in the process. I have seen several of these bad sensors, and when I find a bad one, that 3 phase bridge rectifier survived.

  One test performed is to run the amp at 1500 watts with the test set water interlock defeated. If an amplifier does not trip off in 2 minutes, then the sensor is bad.

Jim
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« Reply #39 on: January 12, 2014, 01:23:27 PM »

Sounds like the whole thing was designed by physicists  Grin

In some situations loss of data for a moment or two is more critical than meltdown and calling you.
Don't think your ever getting the whole story, heh, heh.

As mentioned, keep the cash cow and don't appear too bright.
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