No gain/phase analyser for SMPS design is OK?

[treez]

Hello,

I am working for an SMPS company in Chicago.

I am testing the SMPS’s.

Lately I tested a 200W Buck converter with:-

Vin = 120 – 360VDC
V(out) = 50V
I(out) = 0 to 4A
Switching Frequency = 100KHz
Think its current mode but do not have much access to schematic.

The Engineering Manager told me that they do not have a gain/phase analyser, and that they do not use them.

The SMPS Engineer told me that he just does his best to avoid SMPS instability, by putting a very large value of capacitance on the output. –In this way his crossover frequency is so low that he believes he virtually assures himself a good gain and phase margin.

I am told (by the SMPS Engineer) that SMPS’s often go out to the customer, and then end up needing to be returned because of “problems” which they then work on fixing.

The power supplies that I see here often show instability –oscillations of a few KHz on the output, accompanied by a whining of the wound components.

We are going ahead with doing thermal testing (running the SMPS at –10 deg C through to 80 deg C) on the SMPS even though it has oscillations on its output at 25 deg C at certain input line and output load conditions.

Would I be right in saying that the company is seriously handicapping itself by NEVER using gain/phase analysers ?

 

[zappedagain]

I suggest you lease an analyzer. If you have solved the oscillation problems before the lease is up I expect the analyzer will have proven, and paid for, itself.

Sounds like you may be the first one at the company who understands what one can do.

Devil's advocate - can you make the same measurements the hard way with a scope with FFT capability?

John

 

[treez]

I appreciate your getting back zappedagain,

It sounds as if it would be too time consuming to do gain and phase with a scope.

I am certain that i am not the only one who undertands what a gain phase analyser is for at this place.

There are SMPS guys here who've been turning out SMPS's for years and i'm foxed at why they don't use gain phase analysers?

 

[zappedagain]

"too time consuming to do gain and phase with a scope" - do it once and prove if the GP analyzer is worth buying.

Old habits die hard... Why use a #1 screwdriver when a #2 screwdriver only strips the screw 'every once in a while'. Sounds like you may have higher quality standards.

I suggest you ask what engineering uses instead of a GPA. Understanding exactly what the engineer does may help you show him the benefits of a GPA. Or you may realize it that he has a sequence of other tests that he runs through that provide the same information. Hopefully something more quantitative than "he believes he virtually assures himself ...".

If you call a sales rep for a GPA they might be happy to visit with one for a demo. If you can hook it up to a malfunctioning unit and show the issue that may be very valuable.

John

 

[IRstuff]

Has your engineer looked at the oscillations? Clearly, his rule-of-thumb has failed, and if you can get him onboard, it'll be easier to make the case to buy or rent an analyzer.

TTFN

FAQ731-376