T
treez
Guest
Hello,
I have difficulty getting work in electronics now because at a previous company where I worked as an Electronic Engineer in 2012, I revealed a design mistake in the company’s 250W Full Bridge converter. The designers felt that the mistake was merely a “tolerance issue”, and that I should have just kept quiet about it.
After I revealed the design error, the Engineering Manager was extremely angry with my boss (the Chief Engineer). The Chief Engineer was then very unhappy with me because he felt that the problem was simply a “tolerance issue” that would never have ‘reared its ugly head’. He felt that I should not have gone forward and revealed the error.
This Chief Engineer speaks regularly with recruitment consultants and of course provides my reference for work at this company (I don’t work there any more). As you’d expect ,my reference from this Chief Engineer is always absolutely terrible. I thus now have difficulty getting work in Electronics.
What should I do?
The Full Bridge converter in question is shown in two pdf’s below. I had to make it in two parts because otherwise you would have been able to see the company name in the schematic title window.
Spec of Full Bridge Converter:
P(out) = 250W
V(in) = 48V
V(out) = 26V
F(sw) = 300KHz.
Isolated secondary
Isolated feedback by optocoupler/TL431
Current mode.
Details of design Fault.
The design fault concerned the feedback loop.
The converter operated fine in the lab into an electronic load when the optocoupler , “ISO1” was CNY17-X by either Fairchild or Vishay. However, when this optocoupler was the Fairchild one, then the converter became unstable when operated with its actual intended load (the intended load had quite a large amount of input capacitance, obviously unlike the electronic load)
The instability precipitated itself by making the power supply output voltage so full of oscillation, that the load could not operate at all properly, and the whole equipment that was supplied by this SMPS mal-functioned.
(by the way, I may have got Vishay and Fairchild the wrong way round in the above explanation, and I cant remember which specific version of “CNY17-X” the optocoupler was).
Anyway, before the fault was revealed, I was asked to do tests on this Full Bridge Converter. (In the lab, using the electronic load)
I noticed on the schematic that the feedback loop appeared to be very unusual (you can see this yourself on the schematic).
Unusual-ness of the feedback loop:
In the secondary, following after the split secondary diodes, there is 40uF of ceramic capacitance (C47, C48, C49 & C50)…as you can see, the node with this capacitance is called “MAIN_PSU”.
Going downstream, there is then a 680nH inductor (L4).
Then there is a 68uF electrolytic capacitor, C54. This electrolytic capacitor has a 220 milliOhm resistor (R37) in series with it.
The feedback node is the “MAIN_PSU” node, and this can be seen feeding into the “REF” pin of the TL431 (U8)
…I am sure you would agree with me that this feedback loop architecture is very strange. Taking the TL431 feedback from the “MAIN_PSU” node is very strange indeed. Also, the resistor in series with the main output electrolytic capacitor, C54 is very unusual and necessary.
I would say that L4 should be massively less than 680nH, and preferably not used at all. I would also say that the 220milliOhm resistor, R37, in series with the output capacitor , C54, is totally unnecessary, and serves only to increase the possibility of instability.
The TL431 feedback should certainly not be taken from upstream of L4. If anything, the “MAIN_PSU” node should only be used as a kind of “fast lane” feedback via R47 & C63.
Anyway, despite the feedback loop’s unusual-ness, the converter was seen to be stable when running into an electronic load in the lab.
However, as mentioned, when the optocoupler “ISO1” was changed for a same-part-number optocoupler by Fairchild instead of Vishay, and the load was changed to be the actual load, then the Full Bridge converter became unstable. The actual load had a significant amount of input capacitance, which obviously the electronic load did not have.
Chief Engineers response
Initially, I approached the Chief Engineer with my concerns about the feedback loop. He told me that the converter was seen to be stable in the lab when run into the electronic load, and that I should therefore just keep quiet about the feedback loop, because there was no instability seen at that stage.
In these early days, we did not have access to the actual load which this Full Bridge SMPS would be powering, since the actual load was itself being developed, and not only that, it was a top secret device (due to it incorporating the company’s IP) , and so the company were keeping it under lock and key. Thus we just used the lab’s electronic loads.
Anyway, I was still not comfortable with the feedback loop. However, my feedback loop calculation came up with a phase margin of 45 degrees, so I could not formally prove to the Chief Engineer that the Phase margin was inadequate. Of course, I had no access to the load, and so didn't realise that the L4/Load_capacitance resonance frequency of 8700Hz was just under the crossover frequency of 13,300Hz
Anyway, I decided to send the schematic (in confidence) to a fellow who I presumed to be a friend. This friend was an Applications Engineer at a Semiconductor IC company. I Requested if this Applications Engineer could assist myself with repeating a feedback loop calculation for this Full Bridge SMPS.
Anyway, this so called friend did not give me help with the feedback loop calculation, but instead, he immediately contacted my Engineering Manager where I worked, and said that he had seen the schematic, (also said that I had sent it to him) and told the Engineering Manager that the feedback loop was a great cause of concern to him.
This applications engineer proposed to my Engineering Manager that the Full Bridge SMPS be operated into the actual intended load (instead of an electronic load). He also proposed that the optocoupler “ISO1” be changed for a same-part-number optocoupler but made by Fairchild. (our PCB assembly house actually had the option of using either the Vishay or the Fairchild optocoupler part, but were just by chance using the Vishay part until the Applications Engineer came into the picture)
My Engineering Manager did this, (changed the optocoupler “ISO1” for the Fairchild version of it) and the Full Bridge SMPS went unstable.
Several hundred units (of the overall equipment that the SMPS powered) had been delivered to initial product test sites, and indeed, some of these were showing the instability and not working. (the ones that weren’t working had the Fairchild version of the optocoupler in them)
However, the units had not yet been delivered to actual customers, so despite hundreds of thousands of pounds being wasted, it was not the disaster that it could have been if the fault had not been detected till later on.
Anyway, the result of this was that the feedback loop was completely changed to make it more standard, and it then worked fine.
However, my Chief Engineer got severely reprimanded by the Engineering Manager for having let an unstable SMPS go into initial production and site testing.
The Chief Engineer came to find out that it was me that had contacted the Applications Engineer about this issue.
This Chief Engineer was furious with me for “going behind his back” and discussing my concerns with others. The Chief Engineer felt that the Fairchild version of the optocoupler was only introduced by the Engineering Manager due to my having spoken about the feedback loop. This Chief Engineer thought that if this optocoupler had remained as the Vishay version, then the instability would never have precipitated, and everything would have been fine.
The Chief Engineer was very, very unhappy with me indeed. This Chief Engineer had lots of “buddies” in the company, and encouraged all of them to express disgust at me whenever possible. –I found that his ”buddies” would swear at me under their breath as I walked past, etc etc, and I often found that equipment such as soldering irons etc, would mysteriously disappear off my bench when I returned from coffee break etc.
Also, after I left the company, this Chief Engineer (who is regularly in contact with recruitment consultants) has made sure that my reference is as bad as possible.
What should I do about this? This Chief Engineer’s opinion is widely respected by recruitment consultants and hiring Managers in other electronics companies.
I have difficulty getting work in electronics now because at a previous company where I worked as an Electronic Engineer in 2012, I revealed a design mistake in the company’s 250W Full Bridge converter. The designers felt that the mistake was merely a “tolerance issue”, and that I should have just kept quiet about it.
After I revealed the design error, the Engineering Manager was extremely angry with my boss (the Chief Engineer). The Chief Engineer was then very unhappy with me because he felt that the problem was simply a “tolerance issue” that would never have ‘reared its ugly head’. He felt that I should not have gone forward and revealed the error.
This Chief Engineer speaks regularly with recruitment consultants and of course provides my reference for work at this company (I don’t work there any more). As you’d expect ,my reference from this Chief Engineer is always absolutely terrible. I thus now have difficulty getting work in Electronics.
What should I do?
The Full Bridge converter in question is shown in two pdf’s below. I had to make it in two parts because otherwise you would have been able to see the company name in the schematic title window.
Spec of Full Bridge Converter:
P(out) = 250W
V(in) = 48V
V(out) = 26V
F(sw) = 300KHz.
Isolated secondary
Isolated feedback by optocoupler/TL431
Current mode.
Details of design Fault.
The design fault concerned the feedback loop.
The converter operated fine in the lab into an electronic load when the optocoupler , “ISO1” was CNY17-X by either Fairchild or Vishay. However, when this optocoupler was the Fairchild one, then the converter became unstable when operated with its actual intended load (the intended load had quite a large amount of input capacitance, obviously unlike the electronic load)
The instability precipitated itself by making the power supply output voltage so full of oscillation, that the load could not operate at all properly, and the whole equipment that was supplied by this SMPS mal-functioned.
(by the way, I may have got Vishay and Fairchild the wrong way round in the above explanation, and I cant remember which specific version of “CNY17-X” the optocoupler was).
Anyway, before the fault was revealed, I was asked to do tests on this Full Bridge Converter. (In the lab, using the electronic load)
I noticed on the schematic that the feedback loop appeared to be very unusual (you can see this yourself on the schematic).
Unusual-ness of the feedback loop:
In the secondary, following after the split secondary diodes, there is 40uF of ceramic capacitance (C47, C48, C49 & C50)…as you can see, the node with this capacitance is called “MAIN_PSU”.
Going downstream, there is then a 680nH inductor (L4).
Then there is a 68uF electrolytic capacitor, C54. This electrolytic capacitor has a 220 milliOhm resistor (R37) in series with it.
The feedback node is the “MAIN_PSU” node, and this can be seen feeding into the “REF” pin of the TL431 (U8)
…I am sure you would agree with me that this feedback loop architecture is very strange. Taking the TL431 feedback from the “MAIN_PSU” node is very strange indeed. Also, the resistor in series with the main output electrolytic capacitor, C54 is very unusual and necessary.
I would say that L4 should be massively less than 680nH, and preferably not used at all. I would also say that the 220milliOhm resistor, R37, in series with the output capacitor , C54, is totally unnecessary, and serves only to increase the possibility of instability.
The TL431 feedback should certainly not be taken from upstream of L4. If anything, the “MAIN_PSU” node should only be used as a kind of “fast lane” feedback via R47 & C63.
Anyway, despite the feedback loop’s unusual-ness, the converter was seen to be stable when running into an electronic load in the lab.
However, as mentioned, when the optocoupler “ISO1” was changed for a same-part-number optocoupler by Fairchild instead of Vishay, and the load was changed to be the actual load, then the Full Bridge converter became unstable. The actual load had a significant amount of input capacitance, which obviously the electronic load did not have.
Chief Engineers response
Initially, I approached the Chief Engineer with my concerns about the feedback loop. He told me that the converter was seen to be stable in the lab when run into the electronic load, and that I should therefore just keep quiet about the feedback loop, because there was no instability seen at that stage.
In these early days, we did not have access to the actual load which this Full Bridge SMPS would be powering, since the actual load was itself being developed, and not only that, it was a top secret device (due to it incorporating the company’s IP) , and so the company were keeping it under lock and key. Thus we just used the lab’s electronic loads.
Anyway, I was still not comfortable with the feedback loop. However, my feedback loop calculation came up with a phase margin of 45 degrees, so I could not formally prove to the Chief Engineer that the Phase margin was inadequate. Of course, I had no access to the load, and so didn't realise that the L4/Load_capacitance resonance frequency of 8700Hz was just under the crossover frequency of 13,300Hz
Anyway, I decided to send the schematic (in confidence) to a fellow who I presumed to be a friend. This friend was an Applications Engineer at a Semiconductor IC company. I Requested if this Applications Engineer could assist myself with repeating a feedback loop calculation for this Full Bridge SMPS.
Anyway, this so called friend did not give me help with the feedback loop calculation, but instead, he immediately contacted my Engineering Manager where I worked, and said that he had seen the schematic, (also said that I had sent it to him) and told the Engineering Manager that the feedback loop was a great cause of concern to him.
This applications engineer proposed to my Engineering Manager that the Full Bridge SMPS be operated into the actual intended load (instead of an electronic load). He also proposed that the optocoupler “ISO1” be changed for a same-part-number optocoupler but made by Fairchild. (our PCB assembly house actually had the option of using either the Vishay or the Fairchild optocoupler part, but were just by chance using the Vishay part until the Applications Engineer came into the picture)
My Engineering Manager did this, (changed the optocoupler “ISO1” for the Fairchild version of it) and the Full Bridge SMPS went unstable.
Several hundred units (of the overall equipment that the SMPS powered) had been delivered to initial product test sites, and indeed, some of these were showing the instability and not working. (the ones that weren’t working had the Fairchild version of the optocoupler in them)
However, the units had not yet been delivered to actual customers, so despite hundreds of thousands of pounds being wasted, it was not the disaster that it could have been if the fault had not been detected till later on.
Anyway, the result of this was that the feedback loop was completely changed to make it more standard, and it then worked fine.
However, my Chief Engineer got severely reprimanded by the Engineering Manager for having let an unstable SMPS go into initial production and site testing.
The Chief Engineer came to find out that it was me that had contacted the Applications Engineer about this issue.
This Chief Engineer was furious with me for “going behind his back” and discussing my concerns with others. The Chief Engineer felt that the Fairchild version of the optocoupler was only introduced by the Engineering Manager due to my having spoken about the feedback loop. This Chief Engineer thought that if this optocoupler had remained as the Vishay version, then the instability would never have precipitated, and everything would have been fine.
The Chief Engineer was very, very unhappy with me indeed. This Chief Engineer had lots of “buddies” in the company, and encouraged all of them to express disgust at me whenever possible. –I found that his ”buddies” would swear at me under their breath as I walked past, etc etc, and I often found that equipment such as soldering irons etc, would mysteriously disappear off my bench when I returned from coffee break etc.
Also, after I left the company, this Chief Engineer (who is regularly in contact with recruitment consultants) has made sure that my reference is as bad as possible.
What should I do about this? This Chief Engineer’s opinion is widely respected by recruitment consultants and hiring Managers in other electronics companies.
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