resonating LC output of full bridge smps?

[grizedale]

Hi,

I was recently sacked from my job as a SMPS designer at a new start-up Class D amplifier company

Was i sacked because my voltage mode full-bridge SMPS with +/-40V rails would have resonated at the output LC resonant frequency when audio pulses at that frequency were drawn from it?

It was a dual rail output , so there were two LC's.

Each individual LC had a resonant frequency of 630Hz.

Vin was = 90-265VAC...it had a voltage doubler link for USA/Japan mains...(no PFC).
Output power was 330W.
Type 3 compensation.

For the first few months they were lauding me as there new messiah......then all of a sudden they were ranting at me as if i'd just stolen the whole year's profits...then sacked me.

I think that they showed my schematic to a SMPS colsultancy...and the consultancy told them something.....but what they said i dont know.

Do you think it was the danger of the resonating LC output stage of the Full-Bridge?

 

[BradtheRad]

It sounds like you were 95 percent of the way to getting the bugs ironed out. It doesn't make sense to fire a technician who has gotten this far.

Was the product ever a cause of injury to an employee or end user? Resulting in a lawsuit?

Did the product somehow cause loss of revenue to the company? Run over budget?

When somebody does something that doesn't make sense, look for a reason that doesn't make sense.

Did an entity (known or unknown) step in and offer to finish development for less money? (Think 'overseas'.) Did they make extraordinary promises to get the job, promises which may go unkept?

Did someone higher up in the food chain see you as a threat to his or her job?

Did you have a dispute with a higher-up? Express disapproval of a higher-up? It's become uncannily frequent for people to take offense these days.

Were you drawing a lot of overtime? Necessitating extra pay?

Were you offered a salary (or benefits package) to generous for the company to afford? Now or in the future?

Anything to do with expensive medical care?

Etc.

An honest answer may be hard to obtain.

You may have cause to bring suit against the company.

To find out more it would help to have a friend on the inside.

 

[mtwieg]

Did the product meet its claimed specifications? If not, then nobody really has much room to complain.

But in general, yes having an LC filter with a resonant frequency within its operating bandwidth is a bad idea. But that begs the question of if they specifically asked for a higher bandwidth or certain response time on the output. Did they? And what was the damping on the LC, under load?

I would talk to a lawyer if I were you. In general, employees have more rights and protections than they are aware of, and you may very well have a legitimate reason to file suit.

 

[BradtheRad]

A couple more things...

How did you happen to pick 630 Hz as a frequency for your SMPS to operate at?

This may be important, since ultrasonic frequencies are commonly chosen for such products because those high pitches are outside our hearing range.

Was the project assigned to you alone, or was a second department/ entity secretly given the task as well? For the boss to approve/advance whichever he liked and dismiss the other?

 

[FvM]

How did you happen to pick 630 Hz as a frequency for your SMPS to operate at?

It's not the SMPS operation frequency, just an unwanted output filter resonance.

Most buck operation SMPS have a double output filter pole in the kHz range, that is able to oscillate with unsuitable load current steps. As far as I remember your considerations in the forum discussions, you have been emphasizing fast response requirements, but never told a quantitative specification. I heared from this posts, that the project most likely lacks of clear specifications. Possibly there has been an implicite specification like "product should behave as the competition does" or "better as ...".

It's quite normal, that a developer team starting in a new field of application is disregarding state-of-the-art somehow. Sometimes this leads to fresh ideas, more often the design has to be reworked. The problem usually starts when project schedules are thrown over.

Firing developers is more a matter of social relations in the company than pure technical competence. In so far it's difficult to understand it from a distance.

 

[BradtheRad]

An image is taking shape.

First of all, do nothing to prevent them from wanting to hire you back, in case this dismissal is temporary.

Chances are that the CEO (your boss, or your boss's boss) had big ideas about his startup operation. Maybe a few investors too.

I'll bet extreme hopefulness was behind their "lauding you as the new messiah".

Take neither their ranting at you nor their overdone praise seriously.

The boss may not understand all the technical details behind the company's new product. But he can understand that it emits odd noises.

Now the boss has a lot of people he wants to impress. Did he make a show of saying "The product is overdue and doesn't meet specs, so we're starting over with a new engineer, and you, my backers, can yet expect a return on your investment"?

It would have made better sense to tell you any bad news from the SMPS consultant. Then you could continue ironing out the bugs.

To fire you now is like firing the general who lost one battle but won the previous eight.

There's a chance he'll come back and ask you to fix this remaining bug. He might even offer you a 10 percent raise.

In the meantime he'll probably try hiring another engineer. He may or may not succeed.

And in case he doesn't succeed, isn't the boss likely to sack him too?

That's why I recommend you stay on good terms with the people who just sacked you. It may be temporary.

Just try to hold onto some optimism right now.

Anyway someone else 'in the loop' may have noticed your abilities thus far, and make you an offer to come and work for his company.

--------------------

About the 630Hz tones... (in case that was really the cause of your dismissal, and in case you get another shot)...

Can you construct another filter somewhere to trap the buzz? Reduce the buzz?

Pack foam around the components to soften the buzz?

Change the resonant frequency of one of the LC pairs, in case the trouble came from the center frequencies being in sync?

 

[grizedale]

thanks,

-i certainly ran some simulations with the load being pulsed on and off at a frequency equal to the corner frequency of the output power filter....but no oscillations or instability occurred.

-i wouldnt be taking any "action" as it was their absolute right to get rid of me any time they wanted.

 

[FvM]

i certainly ran some simulations with the load being pulsed on and off at a frequency equal to the corner frequency of the output power filter....but no oscillations or instability occurred.

So did you observe different behaviour of the real circuit? Has it been with a class-D amplifier? There may be a problem of the class-D forming a negative load impedance by sinking constant power with varying supply voltage.

it was their absolute right to get rid of me any time they wanted

You mean, the employment has been terminated within a probation period?

 

[grizedale]

im not sure , i was not permanent staff, so they can get rid any time.

 

[mtwieg]

im not sure , i was not permanent staff, so they can get rid any time.

Unless this is explicitly said in your contract, this is probably wrong. Even if it is in the contract, it might be wrong (because such policy isn't usually legal).

Even part time employees as McDonald's (in which pretty much nobody is "permanent") can't be fired purely on the whim of an employer. They need some specific reason related to the behavior or competency of the employee, and they have to back it up, in court if necessary. I'll say it again, employees generally have much more rights than they are aware of (in first world countries anyways).

 

[grizedale]

yes but i like to offer my services to companys...if they dont want my services for whatever reason, then i dont want to be there anyway.

 

[mtwieg]

I can understand that perfectly. But at the same time, if I though I were unjustly fired I probably wouldn't let it go, even if I didn't want the job back, especially if I had lost out on a paycheck or some benefits that I need. But then again, I would probably only work for a unionized company...

Anyways, I wouldn't get too worked up over it.

 

[ALERTLINKS]

Don't you consider your fault of finishing the job and you didn't apply for a job in assebly section. You could be a new messiah once again......