Author Topic: Disabled power supply blows up expensive board... how?  (Read 2877 times)

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Offline dmg

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Re: Disabled power supply blows up expensive board... how?
« Reply #25 on: January 11, 2018, 09:53:00 PM »
Here's an update. During this time we tried simulating the board with a dummy load, consisting of an electrolytic capacitor with similar specs to those of the tantalum that blew up (electrolytics can tolerate much more without blowing up with toxic smoke) and found some interesting things.

If you proceed exactly as we proceeded when the incident happened (power supply channel turned off), nothing happens. The output stays at 0V, no transient, no nothing. No matter how you connect the supply to the dummy load, nothing happens. If the capacitor is pre-charged it just discharges with a nice exponential curve (so the output has confirmed bleeding resistors)

If you then turn on the supply with the dummy load already connected, the output just rises nicely to 1.8V, no overshoots or weird stuff.

BUT, if you turn on the power supply channel with the dummy load disconnected and you then hot-plug the dummy load, in some cases there's a several 10's of volts transient (>40V), almost 3 seconds long, and then the output stays at 1.8V. That scenario would have explained our issue... if it wasn't for the fact that the PSU supply channel was disabled. We triple-checked that it was indeed disabled before plugging anything. Both my colleague and I are 100% sure that it was off.

We'll keep investigating... (the manufacturer hasn't answered yet, we'll try new communication channels).
 

Offline oldway

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Re: Disabled power supply blows up expensive board... how?
« Reply #26 on: January 12, 2018, 02:24:42 AM »
I have downloaded the service manual from the TTI CPX 200, I don't know if the shematics are the same as CPX 200D
https://elektrotanya.com/tti_cpx200_sm.pdf/download.html

1) it is a SMPS, not a linear power supply....not a good idea to try spare some hundreds of $ when you are testing $ 10.000.00 boards. Also no crowbar protection....Why did you not buy a Keyside power supply ?

2) Output is internally never disconnected from power converter, this is not safe for the circuit connected to the output.

3) Output circuit has very high value capacitors (because it is a SMPS !)...On the schematic, I see 2.200 + 470 + 470 = 3.140µF....That's crazy !!!!!

There are three 1.200R 2W resistors in parallel, to discharge the output capacitors....

But, what probably happened: for some reason, during adjust of current limit, these condensators has been charged to the maximum output voltage.
They don't discharge instantanely, you did not measure the output voltage before connecting the power supply to the board, the output condensators where not still fully discharged, there was still a voltage greater than 1.8V and you killed your board....
« Last Edit: January 12, 2018, 02:27:35 AM by oldway »
 

Offline dmg

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Re: Disabled power supply blows up expensive board... how?
« Reply #27 on: January 12, 2018, 04:50:57 AM »
I have downloaded the service manual from the TTI CPX 200, I don't know if the shematics are the same as CPX 200D
https://elektrotanya.com/tti_cpx200_sm.pdf/download.html

1) it is a SMPS, not a linear power supply....not a good idea to try spare some hundreds of $ when you are testing $ 10.000.00 boards. Also no crowbar protection....Why did you not buy a Keyside power supply ?

2) Output is internally never disconnected from power converter, this is not safe for the circuit connected to the output.

3) Output circuit has very high value capacitors (because it is a SMPS !)...On the schematic, I see 2.200 + 470 + 470 = 3.140µF....That's crazy !!!!!

There are three 1.200R 2W resistors in parallel, to discharge the output capacitors....

But, what probably happened: for some reason, during adjust of current limit, these condensators has been charged to the maximum output voltage.
They don't discharge instantanely, you did not measure the output voltage before connecting the power supply to the board, the output condensators where not still fully discharged, there was still a voltage greater than 1.8V and you killed your board....

Ours is the CPX200D, different specs than the CPX200, but it looks like it's SMPS too. Didn't see that coming actually... my bet was that it was some sort of dual stage supply, with SMPS pre regulation and linear postregulation. Damn it, I should have RTFM better before choosing it.

The "input never disconnected from the power converter" stuff must be true for this model too, I don't hear any relay clicking when I disconnect the output.

As for the why we didn't get a Keysight, well, we do already have Agilent/HP old-ish supplies that work a charm and we already use them to power expensive stuff, in fact the first supply I used to power this board without problems is an Agilent. The thing is, none of our supplies had remote sense capability and we needed it for this board. TTi is a very reputable PSU brand and we decided to go for this one, first because it's hard to find supplies with remote sensing in other manufacturers and secondly because of the PowerFlex stuff.

We are going to repair the damaged board by replacing the expensive IC that got burned and we've also designed a "modchip" board that would be soldered to the expensive board. It contains a linear, ultra low noise regulator, reverse polarity protection, overvoltage protection and all that stuff. That way we can power the expensive board from say 2.5V and have proper local regulation at 1.8V.
 

Offline _Wim_

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Re: Disabled power supply blows up expensive board... how?
« Reply #28 on: January 12, 2018, 05:12:07 AM »
Thanks for the update. Always good to know the rootcause so we can avoid making the same mistake!  :-+
 

Offline prasimix

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Re: Disabled power supply blows up expensive board... how?
« Reply #29 on: January 12, 2018, 09:20:36 AM »
The "input never disconnected from the power converter" stuff must be true for this model too, I don't hear any relay clicking when I disconnect the output.

It's not necessary to have a relay, it could be mosfet switch, too.
Anyway, what you described is very strange, it's hard to imagine that design is so bad that max. voltage was present for 3 seconds on the output due to possibly control loop(s) failure. It looks more that you had a really bad luck with your unit, and hopefully TTi will let you know soon what could be a problem.
 

Offline Neomys Sapiens

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Re: Disabled power supply blows up expensive board... how?
« Reply #30 on: January 12, 2018, 10:46:38 AM »
I do not think that we already know the root cause. Several shortcomings of the PS have been found, but the effect has not been definitely linked to one of them under the conditions described.
The assumption with the chargeup during current programming would be possible. Now it would be interesting if the OP could give us a timescale for the actions which he was describing so exactly. This might verify or invalidate the hypothesis.
 

Offline oldway

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Re: Disabled power supply blows up expensive board... how?
« Reply #31 on: January 12, 2018, 06:37:16 PM »
I do not think that we already know the root cause. Several shortcomings of the PS have been found, but the effect has not been definitely linked to one of them under the conditions described.
The assumption with the chargeup during current programming would be possible. Now it would be interesting if the OP could give us a timescale for the actions which he was describing so exactly. This might verify or invalidate the hypothesis.
In addition to the defects and limitations of the design of this power supply, there are many other elements that confirm the explanation that I gave:

1) it was a new power supply, nobody was used to its use: a wrong maneuver could easily have been committed.

"We have just bought a new TTI CPX200D power supply for our lab, and it arrived today. We needed a power supply with remote sense capability to better power an RF board. The supply arrived this morning"

2) the settings were made without load. This make possible the internal 3.140µF to be charged at higher voltage than 1.8V.
"The RF board gets its power from a connector,"

3) The fact that the tantalum capacitor has started to burn proves that a voltage well above 1.8V has been applied to the input of the RF module and that, with enough energy to burn the capacitor .... This is not ESD or leakage current.

"Just when the power connector was starting to mate with its socket in the RF board, a tantalium capacitor connected across the power rail just at the RF board power connector blew up and started burning, and the board is now ruined. "

4) The output voltage of the power supply has not been checked with a multimeter before connecting the RF module. They relied only on the fact that the power was with its output "disabled"

"so we set the current limit to 50mA) and proceeded to connect the power connector to the RF board, all of this with the supply output DISABLED."
 

Offline dmg

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Re: Disabled power supply blows up expensive board... how?
« Reply #32 on: January 12, 2018, 09:45:23 PM »
I do not think that we already know the root cause. Several shortcomings of the PS have been found, but the effect has not been definitely linked to one of them under the conditions described.
The assumption with the chargeup during current programming would be possible. Now it would be interesting if the OP could give us a timescale for the actions which he was describing so exactly. This might verify or invalidate the hypothesis.
In addition to the defects and limitations of the design of this power supply, there are many other elements that confirm the explanation that I gave:

1) it was a new power supply, nobody was used to its use: a wrong maneuver could easily have been committed.

"We have just bought a new TTI CPX200D power supply for our lab, and it arrived today. We needed a power supply with remote sense capability to better power an RF board. The supply arrived this morning"

2) the settings were made without load. This make possible the internal 3.140µF to be charged at higher voltage than 1.8V.
"The RF board gets its power from a connector,"

3) The fact that the tantalum capacitor has started to burn proves that a voltage well above 1.8V has been applied to the input of the RF module and that, with enough energy to burn the capacitor .... This is not ESD or leakage current.

"Just when the power connector was starting to mate with its socket in the RF board, a tantalium capacitor connected across the power rail just at the RF board power connector blew up and started burning, and the board is now ruined. "

4) The output voltage of the power supply has not been checked with a multimeter before connecting the RF module. They relied only on the fact that the power was with its output "disabled"

"so we set the current limit to 50mA) and proceeded to connect the power connector to the RF board, all of this with the supply output DISABLED."

About 1: Yes, it was new. I myself was familiar with TTi supplies from a proyect I worked on a while ago. We spent part of the morning playing with it after it arrived and we did some tests to check its functionality.

About 2: maybe I didn't explain myself well. What I meant is that the RF board has a power connector that mates with a connectorized wire harness also provided with the evaluation kit. That wire harness is what we connected to the power supply after twisting the wires. We did the settings without load, yes, but we turned the channel on and off after setting it. We also had a monitoring multimeter connected in parallel with the supply output, and we checked that it was outputting 1.8V when on, 0V when off.

About 3: yes, it was a high energy event. The tantalum cap was obliterated. Surely enough with tantalum caps you only need enough energy to heat the cap to ignition temperature and then it carries on by itself, but still... The cap was a 16V rated one.

About 4: yes, it was checked before, a multimeter was always present. We disabled the ouput channel before mating the power connector precisely to avoid hot-plug and the issues it carries. Unfortunately our multimeter can't log and it wasn't set to max hold or anything, so we don't know what peak voltage was applied to the board.


We don't yet know the root cause. We have found an scenario where the supply goes to 40V if you hot-plug a capacitive load with remote sense and the capacitor is discharged (if it's precharged to 1.8V or so it doesn't trigger the high voltage transient). But that scenario requires hot-plugging the load with the output channel enabled. It was disabled in our case, we're sure about that.

Now we need to see if it's possible to somehow trigger the transient with the supply channel disabled.

 

Offline oldway

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Re: Disabled power supply blows up expensive board... how?
« Reply #33 on: January 13, 2018, 06:33:05 AM »
Quote
Now we need to see if it's possible to somehow trigger the transient with the supply channel disabled.
Yes, it is.... :scared:

If you look at the schematics, you will see that the output on/off switch controlled two separate functions : the "disabled" indication and the function itself.
There is absolutely no guarantee that when the disable function is displayed, the control is actually disabled.

It is quite a crap design, perhaps good for hobby but for sure not safe enough to use with $ 10.000 boards.
 

Offline ogden

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Re: Disabled power supply blows up expensive board... how?
« Reply #34 on: January 13, 2018, 07:06:08 AM »
About 4: yes, it was checked before, a multimeter was always present. We disabled the ouput channel before mating the power connector precisely to avoid hot-plug and the issues it carries. Unfortunately our multimeter can't log and it wasn't set to max hold or anything, so we don't know what peak voltage was applied to the board.

You still have that supply, right? Test supply, check that scenario with other kind of load and scope as logger. If supply is to blame, then who knows.. maybe you can get 10k$ from TTI or their insurer :)
 

Offline Kevin.D

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Re: Disabled power supply blows up expensive board... how?
« Reply #35 on: January 13, 2018, 12:00:29 PM »
Looking at the CPX200 (10A) (schematic from link above ) and a CPX400a (20A) schematic and they are radically different.
The CPX 200 a dual switched flyback (No separate post regulator).
the cpx 400a is a dual switched forward converter + magamp for pre-regulation followed by linear output stage.
I also use a TTI supply the older Pl quad series with 'hard' switch disable and remote sensing, pure linear no pre-regulation. Been a very reliable supply so far.

A couple of possibilities to consider.
These supplies (and many others) use resistors (PTC in the cpx200) to guard against remote sense accidents. But as this previous post suggests (IMHO) I think resistors are inferior to diodes as guards as they offer no protection to loads in the 2 scenario's noted there . http://www.eevblog.com/forum/projects/korad-ka3003d-redesign-and-upgrade/msg1017611/#msg1017611

A 'guard diode' would always protect the load since it clamps the main output wire voltage to the remote sense wire voltage with a max difference of Vf ( i.e  it protects the Load from excessive voltage in preference to protecting the sense wire from excessive current ).

The two scenario's :-

1 / If main wire has become disconnected (or similarly sense wires are connected first before main wires) then the main output terminal voltage 'Vmain' can rise up to to Vmax, this can happen because any current flowing through the guard resistors to the load causes a voltage drop 'Vrguard', so now Vmain = Vsense + Vrguard, any large output caps also get charged to Vmain. These will then get discharged into the load when the main wires are finally connected.
This may be occurring when your 'hot-plugging' which is connecting both the Vsense wires to the load a few mS before the main wire makes contact to the load. In those few mS Cout could charge to near Vmax. 
 
The time constant of the CPX200's output caps and three parrallel output resistors is  3000u*400 = 1.2s thus for them discharge from 40V to ~ 2V = 3.6s , is this the 3 Seconds over voltage your referring to ?.

Another scenario that 'guard resistors' offer no protection against  :-
2/  Mixing up polarity of sense wires . Again Vout will go up to Vmax for same reasons as 1 above and will remain there (if supply has an OVP trip that senses over voltage at the main wire (but not at the Vsense node) then this should activate and shut supply down) .

Is it possible to accidentally re-enable a bench supply .?
Well yes if it's one that disables it's output simply by setting Vset to 0V (a 'soft' disable) then consider it still in a normal 'on' state, it's just that it's output is now regulated to 0V . So as with any 'on' supply if we confuse it by applying a voltage to the Vsense terminal it will react. So a brief negative voltage applied to it's Vsense terminals (as would happen in a reverse polarity Vsense connection or Vsense connected to a negatively charged cap ) would cause the output voltage to rise sharply an attempt to maintain it's Vsense terminals at 0V. 
But if that schematic linked in this thread is the correct one for your CXP200D I can't see how the output could be accidentally re-enabled by the method above since it looks like the output disable operates by shutting down the PWM oscillator that drives the flyback (shutdown by mcu).
 
A quick test :- with supply output disabled and in remote sense mode , charge a largish test cap  (~-1V) and connect it to ONLY the Vsense leads (such that +Vsense and -Vsense will momentarily be reverse polarity ) whilst monitoring the main Vout terminals with scope.).

regards
« Last Edit: January 13, 2018, 12:07:04 PM by Kevin.D »
 
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Offline wraper

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Re: Disabled power supply blows up expensive board... how?
« Reply #36 on: January 13, 2018, 12:25:19 PM »
Both my colleague and I are 100% sure that it was off.
Human memory is very unreliable, especially in accurately remembering a shocking/unexpected moments. You may verify something 3x and then accidentally press enable button while connecting wires.
 

Offline Mr. Scram

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Re: Disabled power supply blows up expensive board... how?
« Reply #37 on: January 13, 2018, 01:18:09 PM »

Human memory is very unreliable, especially in accurately remembering a shocking/unexpected moments. You may verify something 3x and then accidentally press enable button while connecting wires.
The best part is our ability to lie to ourselves how reliable our memory is. Our brain is constantly trying to construct a coherent experience. Reality and truth don't seem to be a priority, only the appearance of it.
 
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Offline oldway

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Re: Disabled power supply blows up expensive board... how?
« Reply #38 on: January 13, 2018, 04:25:31 PM »
For me, it is clear that this power supply is intended for hobby use and is not at all suitable for a professional job with very expensive boards.

There are many errors or limitations in the project of this power that make it not safe for the circuits it feeds.

The big mistake in this accident is the choice of the brand and the model of the power supply ....
 

Online ez24

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Re: Disabled power supply blows up expensive board... how?
« Reply #39 on: January 13, 2018, 04:47:35 PM »
Watching - I like detective stories
 

Offline KE5FX

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Re: Disabled power supply blows up expensive board... how?
« Reply #40 on: January 13, 2018, 06:16:17 PM »
The big mistake in this accident is the choice of the brand and the model of the power supply ....

As far as I'm concerned, the root cause is the manufacturer shipping a $10,000.00 demo board with no onboard regulation.  No excuse for that. 

It's in the nature of power supplies to misbehave destructively.  The only safe assumption is that they will fail in the most unsafe way(s) possible.  Power supplies are devilishly hard to get right, but they aren't sexy enough to attract the attention of talented, experienced EEs. 

And you don't have to buy dodgy Chinese hacks to put your hardware at risk.  People love the old HP/Harrison supplies, for example, but what happens when the wirewound pots they used develop a dirty spot?  They fail by spiking to max voltage while you're adjusting them.  Then there's the HP 6284A that blew up who-knows-how-many parts until someone (me) noticed that it was spiking on turn-off because of a race condition between the reference and output rails. 

Even the IC manufacturers aren't immune.  Who thought it was a good idea to build 78xx regulators without reverse shunt diodes....?  Again, when they fail, they fail high.  I could rant for days about this stuff...
 

Online Damianos

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Re: Disabled power supply blows up expensive board... how?
« Reply #41 on: January 13, 2018, 06:29:50 PM »
Watching - I like detective stories
Enjoy it! But it seems more like a boring comedy...
Maybe the culprit is the material of the power supply feet! Are they of plastic or rubber? We have to analyze this!

From the initial description, it seems that was a user fault. They had to test a sensitive device, so they used a power supply with remote sense. But, instead of having fixed connections for this, they decided to connect it "on fly"! The consequences are known...
 

Offline dmg

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Re: Disabled power supply blows up expensive board... how?
« Reply #42 on: January 14, 2018, 12:53:16 AM »
Both my colleague and I are 100% sure that it was off.
Human memory is very unreliable, especially in accurately remembering a shocking/unexpected moments. You may verify something 3x and then accidentally press enable button while connecting wires.

The sequence of events was as follows:

-Take power connector from the table, proceed to plug in
-Before connector is mated, BOOM.jpg
-I instinctively disconnect the wire and back off, my colleague backs off too.
-I go check the supply to turn it off, realize it is already off
-Without touching anything, we take picutres of everything
-Pictures show that the channel enable LED was off, and current meter shows set current (50mA) instead of 0, which is what it would show if the channel was ON

That's why I say we're pretty sure it was off. In any case, this was destined to happen. One time we would actually forget to turn the supply off or something like that and it would have blown off anyway.
We learned several lessons with this, the most important one is to setup a video camera and record everything the next time we try to power an expensive board xD. I'll keep updating.

Watching - I like detective stories
Enjoy it! But it seems more like a boring comedy...
Maybe the culprit is the material of the power supply feet! Are they of plastic or rubber? We have to analyze this!

From the initial description, it seems that was a user fault. They had to test a sensitive device, so they used a power supply with remote sense. But, instead of having fixed connections for this, they decided to connect it "on fly"! The consequences are known...

We connected it on the fly to avoid any startup surprise with the supply. I thought it was more reasonable to first power-up the supply, set it up properly and then, after it's properly setup, connect the board and finally enable the output channel.

I thought it was much safer to plug in the board with the supply in a known state and with its dials locked than to press the AC switch button with the board plugged in. Moreso in this case, where there's nothing between a 1000€ IC on a 10000€ board and the supply itself. We used the remote sense not because we liked the idea (it's idiotic not to have local regulation on-board) but because it was the manufacturer's recommended way of doing things. I didn't like the idea from the very beginning, also considering that the IC can draw current in pulses.

Now that the board is ruined, if we get a replacement or if we fix this board we'll hack it off either way and install a "modchip" that I designed with proper local regulation and protection.

« Last Edit: January 14, 2018, 01:09:08 AM by dmg »
 

Offline wraper

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Re: Disabled power supply blows up expensive board... how?
« Reply #43 on: January 14, 2018, 02:01:50 AM »
The sequence of events was as follows:

-Take power connector from the table, proceed to plug in
-Before connector is mated, BOOM.jpg
-I instinctively disconnect the wire and back off, my colleague backs off too.
-I go check the supply to turn it off, realize it is already off
-Without touching anything, we take picutres of everything
-Pictures show that the channel enable LED was off, and current meter shows set current (50mA) instead of 0, which is what it would show if the channel was ON

That's why I say we're pretty sure it was off. In any case, this was destined to happen. One time we would actually forget to turn the supply off or something like that and it would have blown off anyway.
You could easily forget that you first disabled PSU before disconnecting the wire as a first measure.  It's really hard to remember accurately when something like this happens. The only way to be sure would be only if you were taking video during the incident.
 

Offline KE5FX

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Re: Disabled power supply blows up expensive board... how?
« Reply #44 on: January 14, 2018, 08:56:34 AM »

You could easily forget that you first disabled PSU before disconnecting the wire as a first measure.  It's really hard to remember accurately when something like this happens. The only way to be sure would be only if you were taking video during the incident.

That seems like quite a stretch.  If something blows up just as you're plugging it in, your attention isn't going to wander back to the power supply button.  You're going to yank the connector out.

This is how I deal with $1000 eval boards with goofy power requirements and no supply conditioning:



The box just contains a couple of 3-terminal regulators.  Nothing fancy, but they can save the chip from most mistakes made by the board designer, the PSU, or me.  :-BROKE
 

Offline wraper

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Re: Disabled power supply blows up expensive board... how?
« Reply #45 on: January 14, 2018, 09:04:16 AM »
If something blows up just as you're plugging it in, your attention isn't going to wander back to the power supply button.
When you plug the wire and 10k board starts burning, the first thing you do is start panicking and loose attention to what you do.
 

Offline KE5FX

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Re: Disabled power supply blows up expensive board... how?
« Reply #46 on: January 14, 2018, 09:07:07 AM »
If something blows up just as you're plugging it in, your attention isn't going to wander back to the power supply button.
When you plug the wire and 10k board starts burning, the first thing you do is start panicking and loose attention to what you do.

Even if your hands are on the plug at the time?  Sorry, not buying it.
 

Offline KrudyZ

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Re: Disabled power supply blows up expensive board... how?
« Reply #47 on: January 14, 2018, 10:11:58 AM »
I think we already established that this is a pretty crappy supply.
For one, it can only regulate up, not down. If it is disable it will still rely on the internal bleed resistors to lower the output voltage.
WRT the PWM being disabled by the MCU: Even if the MCU could do this, we don't know if it's actually implemented that way.
Maybe they really just set the VSET voltage to zero and hope for the best.
If you look at the voltage amp in the schematics, you will notice it's a pure integrator. If the Vsense lines are disconnected from the output lines then there is no limit.
You should measure the resistance between The pos output and the pos sense and the neg output and the neg sense to see if the PTCs (why are they using PTCs?) are indeed there.
If there is an open, then your output voltage could be anywhere no matter what it is set to.
You could also test this directly by measuring the output voltage and then introduce a voltage difference on the sense terminal.

As others have said, I would definitely get rid of this supply. It's wrong in so many ways...
 

Offline wraper

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Re: Disabled power supply blows up expensive board... how?
« Reply #48 on: January 14, 2018, 10:36:21 AM »
I think we already established that this is a pretty crappy supply.
For one, it can only regulate up, not down. If it is disable it will still rely on the internal bleed resistors to lower the output voltage.
WRT the PWM being disabled by the MCU: Even if the MCU could do this, we don't know if it's actually implemented that way.
Maybe they really just set the VSET voltage to zero and hope for the best.
If you look at the voltage amp in the schematics, you will notice it's a pure integrator. If the Vsense lines are disconnected from the output lines then there is no limit.
You should measure the resistance between The pos output and the pos sense and the neg output and the neg sense to see if the PTCs (why are they using PTCs?) are indeed there.
If there is an open, then your output voltage could be anywhere no matter what it is set to.
You could also test this directly by measuring the output voltage and then introduce a voltage difference on the sense terminal.

As others have said, I would definitely get rid of this supply. It's wrong in so many ways...
It's already established that you did not check the schematic of CPX200D, not even CPX200 which is completely different supply, otherwise you won't write what's in bold.
 

Offline wraper

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Re: Disabled power supply blows up expensive board... how?
« Reply #49 on: January 14, 2018, 10:41:52 AM »
As of the safest way using TTI PSUs with remote sense. Disable output, set sense switch to local, connect all wires, set switch to remote, lastly press OUTPUT to enable.
« Last Edit: January 14, 2018, 10:44:39 AM by wraper »
 


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