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Question on testing/characterizing DCDC regulators (design process)
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E.Buer:
Hi,
I am interested to hear how you go about testing/characterizing DC/DC regulators that you design/use in a design, and which tests are necessary.

* Besides simulation, do you measure the stability of the regulator prototype? If so, how?
* Do you measure at different loads?
* Do you use small signal analysis, noise injecting etc. to obtain a loop response?
* Do you measure the efficiency?Is it normal to just wing it, and assume that the regulator works like the simulation, given that the output voltage is correct?

If you could talk me through the design and development process, that would be helpful!
Thank you!
David Hess:

--- Quote from: E.Buer on June 08, 2019, 02:29:53 pm ---Besides simulation, do you measure the stability of the regulator prototype? If so, how?
--- End quote ---

Yes, at a minimum I do load and line stability tests.  The load stability test may make it unnecessary to do a separate feedback loop stability test.


--- Quote ---Do you measure at different loads?
--- End quote ---

Yes, and different input and output voltages if applicable.  The loop response will change significantly if the regulator crosses the continuous conduction mode boundary and this will need to be taken into account in one way or another.  In discontinuous conduction mode, the loop response changes with load.


--- Quote ---Do you use small signal analysis, noise injecting etc. to obtain a loop response?
--- End quote ---

As mentioned above, the load stability test may be sufficient in place of measuring the loop response separately.  In addition, I do not have a dedicated network analyser; I just have never had enough need for one.


--- Quote ---Do you measure the efficiency?
--- End quote ---

Only if there is a specific reason to.  I would for optimization during the design process.


--- Quote ---Is it normal to just wing it, and assume that the regulator works like the simulation, given that the output voltage is correct?
--- End quote ---

Oh, I never do that.  And my simulation may only be paper calculations.  I usually only do simulation to verify or refine the design calculations and measurements.
MagicSmoker:
This is really too broad a question with a different answer for each combination of parameters such as how big a budget (time and money) you have, how fast the transient response needs to be, whether the converter topology is prone to issues to begin with (e.g. - boost or flyback-derived converters in CCM), how wide the input voltage range is, how wide the output voltage and load current range is, how rapidly can/will load current change (dI/dt), is the converter transformer-isolated, how expensive the product is, how serious are the consequences of failure, what kind of MTBF is needed, etc.

Over the years I've gotten fairly good at anticipating when a very casual design and validation approach can be used and when I actually need to employ more rigorous testing schedules. I used to be a proud member of the "hates SPICE" crowd but LTSpice made a believer out of me and now I use it for at least the first-pass workup of every new design, if not full AC/transient analysis of the loop. Regardless of how much simulation you do, though, you still have to verify the circuit operates correctly in the real world. Again, whether that entails full-blown frequency response analysis with a tracking generator and signal injection transformer or just toggling a bank of resistor loads on and off and looking for ringing on the scope depends on the parameters mentioned above.

For example, non-isolated buck converters - especially if slope-compensated current-mode control is used - are so easy to get working and so forgiving of their loop compensation components that you would arguably look incompetent if you had to resort to real-world frequency analysis. In contrast, a transformer isolated CCM flyback is practically metastable by design, so unless you slug the hell out of the control loop it will almost certainly have combinations of line and load (especially load dI/dt) that are outright unstable.
David Hess:
Often with the designs I work with, I end up doing the SPICE simulation after measuring the loop response because there are too many unknowns which I have to derive from actual measurements.
ocset:
The Ridley engineering group make a gain phase analyser  ( AP300) which is good for measuring the gain and phase margin of an SMPS…
It can be used very quickly in most cases.

http://www.ridleyengineering.com/analyzer.html

The following 4 videos go through use of the AP300 frequency analyser, which you can use to measure gain and phase margin of the SMPS feedback loop.




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