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DC DC current mode buck load transient

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pianomania

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I am designign a DC DC current mode buck , but the load transient is too large, what parameter can effect the behavior ?

Error Amp gm ? or bandwidth , or compensation network RC, or zero , pole?
Does any article discuss this topic ?

And if load transient improves , usually the stability becomes worse, what is the best trade-off ?
 

VVV

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Basically the transient response will be affected by the closed-loop system bandwidth. This is equal to the crossover frequency of the open-loop system. So the compensation has to be done in such a way that you have as large bandwidth as possible, while still meeting the criteria for stability:
sufficient phase margin
sufficient gain margin
-1 slope at crossover.

Generally, a good phase margin to strive for is 60deg. It will provide good transient response with reasonable over-undershoot. A good gain margin is 12dB (has to be greater than 7dB). And the -1 slope at crossover is just that: make sure the slope at crossover is -1 or -20dB/ decade.

And again, all this has to happen as as high a crossover frequency as possible. Generally, for "regula" power supplies this is in the few kHz range, for better supplies it runs in the tens of kHz range.

I don't know the actual requirement for you application, but my guess is that if you manage to cross over at close ot 10kHz you should be OK.
 

eem2am

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1. do you mean it oscillates after you switch from say no_load to full_load?

2. Or is it that just the ouput volts sink well down on load change and slowly come back up again?

solve 1 by increasing the filtration of the feedback divider voltage

solve 2 by decreasing the filtration on the feedback divider voltage.

...the compensation networks just act like filters on the divider voltage.....although caps sometimes get used to push the voltage through quicker for detection....like a cap across the upper divider resistor.

you could solve 1 by putting a cap across the lower divider resistor.

you could solve 2 by putting a cap across the upper divider resistor..

(or of course a series RC )
 

ghydda

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Or the inductor simply has too large a value, which prevents output current from changing at a fast pace. In that case, choose a smaller inductance value.
 

iscmos

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here is an article which talks about how to improve load transient, hope it can help u. hit the link:
https://www.powersystems.eetchina.com/ART_8800443716_2400004_TA_ec5a6406.HTM


quote="pianomania"]I am designign a DC DC current mode buck , but the load transient is too large, what parameter can effect the behavior ?

Error Amp gm ? or bandwidth , or compensation network RC, or zero , pole?
Does any article discuss this topic ?

And if load transient improves , usually the stability becomes worse, what is the best trade-off ?[/quote]
 

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