Inductor Selection in DC-DC Converters - Page 1
Forum section: electronics / beginners

author: Hiemal
author_rank: Regular Contributor
timestamp: February 02, 2019, 02:28:44 am
content: Pretty basic question.For DC-DC converters, is there any downside to using an inductor of a higher value where the datasheet typically shows smaller value ones?Aka, the good ol' AP3012. Datasheet application circuit shows an inductor of 10 uH.If I sub in a 100 uH or 150 uH inductor, would anything "bad" happen? Would I get better noise filtering on the output?Or, rather, are the size of the inductors more the constraint, since higher value inductors typically are physically larger?Thank you!

author: boB
author_rank: Frequent Contributor
timestamp: February 02, 2019, 02:45:21 am
content: I don't think anything will explode if you use a larger value of inductance.Worse case it might not work as well as it should is all.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 02, 2019, 03:07:20 am
content: Yes.The first symptom is hissing and whining, with generally higher input and output ripple. The second is instability, as increased under/overshoot on transient loads, or outright oscillation.The AP3012 is a peak current mode controller, meaning it is susceptible to chaotic behavior at low ripple ratios (that is, small change in inductor current per cycle, versus the average output current level). Slope compensation helps with this, but only to ripple ratios of maybe 50% or 33%.An average current mode controller won't go chaotic*, but in both cases, a large enough filter time constant will go outside the stable range of the error amplifier's compensation time constant, causing the voltage loop to oscillate. This includes the output filter capacitor as well as the inductor, so don't go overkill on that, either!*Actually a similar thing can happen, but it's limited to period doubling (ripple at Fsw/2), not full-on chaos.If you have an externally compensated controller, you can adjust for the capacitor value, and inductor value to some extent, but you still won't be able to run a peak current mode controller at such a low ripple ratio without it behaving poorly.Most regulators (i.e., integrated switch type) are peak current mode, because it's an excellent combination of performance, reliability (the current limit is very nearly as intended), stability and efficiency. Controllers (external switch) may vary more; or alternately, average current mode control is so easy that you can build your own from a few chips, who needs a proper controller. Average current mode is the most general in terms of suitability with large filter values.Tim

author: boB
author_rank: Frequent Contributor
timestamp: February 02, 2019, 05:12:31 am
content: I didn't notice that it was a peak current mode regulator but I wouldn't be afraid to try it until you have the recommended 10uH inductor.I just wouldn't connect up any Vcc voltage sensitive components to its output to start with just in case it does go chaotic.It still should voltage regulate. I do see that the part has an over voltage protection circuit that stops at 29V it says.Maybe be ready to measure the output voltage and to turn it off if the output voltage should rise higher than wanted.

author: MrAl
author_rank: Super Contributor
timestamp: February 03, 2019, 03:10:00 am
content: Quote from: Hiemal on February 02, 2019, 02:28:44 amHi,It depends on the type of converter, buck, boost for example.A 10uH inductor is going to have roughly 10 times LESS effective series resistance and so going to a 100uH will generally increase that by 10 times, which in turn will limit the duty cycle of the converter or even stop it from reaching the right voltage if the frequency is higher than some threshold limit. This would happen because the inductor must be able to charge up in the time allowed for that given frequency, and if it does not then the output voltage will never reach the required value, or just not under load.

author: spec
author_rank: Frequent Contributor
timestamp: February 03, 2019, 04:17:40 pm
content: Quote from: MrAl on February 03, 2019, 03:10:00 amI think you meant 3.16 MrAl (L=KN2).