Author Topic: What do you using to calculate response and feedback for regulators?  (Read 253 times)

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Offline Miyuki

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Hi folks.
Wanna ask what you are using to calculate feedback compensations and check the stability of regulators? Do you use some fancy tools, a simple spreadsheet, or just a piece of paper?
Just wondering as a lazy person how to ease my life. Especially when I want a second stage output filter. 

For example, now I have a high current boost converter (with about 50A input current) and the output filter is a PI CLC filter with 30uF 80uH 30uF
I'm curious if you have some simple tool where you can just put your numbers in and it recommends compensation and shows frequency response and margins

 

Offline T3sl4co1l

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Re: What do you using to calculate response and feedback for regulators?
« Reply #1 on: April 11, 2021, 05:05:37 pm »
In general?  No, you need to know internals of the regulator and do Bode plots and figure it out that way.

In particular?  There are often tools available.  Most manufacturers have their simulation tools, and many have formulas or spreadsheets that get in the right ballpark.

I would never consider design equations authoritative and accurate; they are subject to too many approximations.  And may not even be applicable over the operating range (the system is nonlinear).  They can be good starting points however.

Also, regulators with internal compensation, you have no choice; just use the recommended values, including the inductor as it is part of the internal settings too.

Offhand, 30uF sounds awfully small for a boost with 50A input, just from how much ripple the poor thing(s) have to handle.  That also suggests a pole much higher than most controls would be comfortable with.

And, 80uH sounds enormous at, whatever the output current is (10 or 20A?).

Finally, the ratio of the two is rather high (sqrt(80uH / 15uF) -- note the caps act in series with respect to the inductor), which implies rather poor output impedance (a step load of ~10A will yield a step change in voltage of ~23V or more!).

Tim
« Last Edit: April 11, 2021, 05:08:09 pm by T3sl4co1l »
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Offline Miyuki

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Re: What do you using to calculate response and feedback for regulators?
« Reply #2 on: April 11, 2021, 05:22:32 pm »
Output is a constant load, but the input side is highly variable (high range but slow change rate) that is the reason for weird design
And they are high ripple film capacitors, it is within the rating.

I normally would put it to SPICE and calculate plots there. And adjust compensation. I am curious if is a more automatic or simpler way. 
 

Offline David Hess

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Re: What do you using to calculate response and feedback for regulators?
« Reply #3 on: April 14, 2021, 06:24:19 pm »
In general?  No, you need to know internals of the regulator and do Bode plots and figure it out that way.

And that is why I often start by measuring the response, and then using the bode plot and known circuit values to work out the unknown ones, like the internals of the regulator or pass element.  Then I can calculate the ideal component values for the response that I want, but the results are usually not much better than the empirical results.

So my tools of choice are a function generator and oscilloscope to measure the impulse and network response.  If I had a suitable network analyzer I would use that as well.  And then pencil and semi-log paper.
 


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