How can I limit Buck Converter output? - Page 1
Forum section: electronics / beginners

author: Greorge
author_rank: Regular Contributor
timestamp: February 18, 2014, 11:35:34 pm
content: I read Wikipedia web page on Buck Converters:http://en.wikipedia.org/wiki/Buck_converterBig part of it is way over my head.Looks that I have to make sure, that device taking power from Buck Converter cannot exceed maximum output.Heat sink can help to go higher, but higher limit cannot be exceeded for risk of catastrophic failure.To prevent the problem in a first place, how can I prevent Buck Converter getting anywhere near the maximum output?Can I install some kind of Amperage limiter, that no matter what I connect to Buck Converter it will not go above set level?

author: sleemanj
author_rank: Super Contributor
timestamp: February 18, 2014, 11:47:00 pm
content: Quote from: Greorge on February 18, 2014, 11:35:34 pmSimplest answer:

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2014, 12:04:06 am
content: Simple: you use a control method which actively controls inductor current.Anything else should be considered highly suspect!Tim

author: Corporate666
author_rank: Unknown
timestamp: February 19, 2014, 04:10:03 am
content: Buck converter chip is going to have a feedback/sense function, so by putting a sense resistor of known (low) value in the output path, you can set voltage based in relation to the sense resistor voltage and now it's a constant current converter.This is how most of the LED drivers I've seen work.

author: Greorge
author_rank: Regular Contributor
timestamp: February 19, 2014, 06:57:22 am
content: Does it mean that I can put resistor on (+) or (-) outlet, to stop Buck Converter trying to supply current that would damage it?

author: Corporate666
author_rank: Unknown
timestamp: February 19, 2014, 09:10:31 am
content: Quote from: Greorge on February 19, 2014, 06:57:22 amSomething (could be a timer, microcontroller, or dedicated chip) is controlling the switching of the inductor to effect the buck regulator... and that something will have a feedback circuit so it can hit it's target voltage.If you were to put a resistor on the output of your regulator and connect the other end of that resistor to ground, you would develop a voltage across that resistor that would be relative to the current flowing through it (V=IR). You could use that voltage in your feedback loop, so instead of your buck regulator outputting a constant voltage, it would not output a constant current.You can also put two resistors in series from the output of your regulator to ground and take your voltage reading from between them - which will then be a voltage divider and prevent a (possibly) too high voltage at your output from overloading your feedback circuit max voltage limits (i.e. if you are bucking 100V down to 20V and controlling it with a 3.3V micro). All you really need to do is figure out what the voltage is for the feedback loop, size your two resistors appropriately, and connect the feedback to the center of the two - very simple - and you have a constant current buck regulator.

author: Greorge
author_rank: Regular Contributor
timestamp: June 15, 2014, 05:09:53 am
content: I don't have problem with voltage.I would not like buck device to burn out because I try to take too much current out of it.To make it extreme case:I would like buck converter to produce not more than 2A even if outlets are shorted.(It is not what I want to do, but I just want Buck converter not go above its safe working limit)

author: theatrus
author_rank: Frequent Contributor
timestamp: June 15, 2014, 05:54:18 am
content: Are you using an existing buck controller? Or are you designing one from scratch?

author: Kremmen
author_rank: Super Contributor
timestamp: June 15, 2014, 06:02:41 am
content: You do understand that the buck acts as a voltage source, producing a given voltage and as a consequence of the connected load, a given current (Ohm's law &c). You can specify either the voltage or the current; the connected load will then cause the other one to fall to a specific value.So, your normal buck converter will have a voltage feedback mechanism to control the output voltage. If you also wish to control (or limit) the current then you need another feedback circuit for that. First you need to sense the current and this is normally done by inducing a voltage loss over a small valued resistor, proportional to the current. The voltage is then suitably processed in the (buck) circuit to limit the load current to the maximum allowed by dropping the output voltage.Based on the above, your easiest solution is to pick a controller that already includes the current sense/limit input and do the circuit based on that.

author: Greorge
author_rank: Regular Contributor
timestamp: June 15, 2014, 06:10:50 am
content: Got existing one.Having Buck Converter with both V and A regulation on board is the way to go.Will have a look on E-Bay for some that can regulate both.(Why didn't I think about it before?)Thank you very much for help