Power supply Loop evaluation

I am using a linear Technologies power supply chip 8616. I have used LTpowe rcad and ltspice to try to get a handle on the closed-loop response of this power supply. I don't think I believe the LT powercad results for the clos ed-loop performance. How would you conduct a real experiment with the real power supply chip to ascertain the closed-loop performance? What I am doing is injecting a second current Source into the load resistor and trying to see how quickly the power supply output voltage can respond to the current low changes at the output. It allows the power supply to change current req uirements both with a square wave and sine waves from say .5 amps to 1 and

1/4 amps. I get nice results with feedback capacitor values that LT power c ad shows is practically unstable
Reply to
bulegoge
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Switching the load, or squirting in some current from a function generator, is a great way to evaluate loop stability.

One thing that makes sims different from reality is output capacitor ESR. A little ESR in an aluminum or tantalum cap can really stabilize a switcher loop. An ideal L followed by an ideal C is a terrible thing to close a loop around.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

+1, a little ESR goes a long way m
Reply to
makolber

Does the ESR make as much of a difference in a current control Loop? From what I have read voltage feedback loop is much more touchy to the output capacitor where is a current Loop is not as touchy?

Reply to
bulegoge

Depends on the topology. I've found that LT Spice does a pretty accurate job if you add some ESR to the output caps in the sim. It's best to measure the actual cap ESRs.

Any time you take the feedback from across the output cap, ESR adds a zero to the loop. Zeroes are good.

I like the ESR behavior of tantalum caps, but they have to be derated maybe 3:1 on voltage, or they may explode.

Polymer aluminum caps are great parts; esr's may actually be a bit low for best loop stability, but better than zero. I have been known to add a 30 milliohm resistor, or a PCB trace, in series with a polymer or ceramic cap to better define the ESR.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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Reply to
John Larkin

Remember, Bode only works for linear time-invariant circuits. If your loop is non-linear (biased however it is in the simulation), the results are crap. If the Bode plots (either simulated or measured) differ from the step response, the step response is correct.

Reply to
krw

Only to the extent that the non-linearities matter. _Everything_ is nonlinear, after all. Search on "describing function analysis".

Used with care, you can do more with Bode than might seem apparent -- but at some point, they do become crap.

Yes. Except you can't go assume it's the step response of a linear circuit and analyze from there.

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Tim Wescott 
Wescott Design Services 
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Tim Wescott

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