Hi I just watched a video tutorial explaining how switch mode regulators work. I want to confirm I've understood correctly. The inductor used in a switch mode power supply is there to provide power to the load when the internal pass transistor is switched off right? Am struggling to find the exact inductor used for the minty boast usb charger, it wouldn't matter if I was to use one with a higher inductance right? It uses 10uH inductor btw

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It's the output capacitor that provides power to the load, while the inductor is made to squirt pulses of current onto the cap. In a boost regulator (makes a higher voltage than the input power), the inductor only squirts current onto the output cap for part of each switching cycle. During the other part, the inductor is being energized so that it can provide the next squirt. In a buck regulator (makes a lower voltage than the input power), the switching is done on the input to the inductor. It can always be supplying output current if that is how the supply is designed.

No, you should not change the inductance without understanding all the tradeoffs that went into deciding on that particular value. There are various considerations, like the peak current relative to what the switch and the inductor itself can handle, ripple current on the output and input caps, total current delivery capability, and more. Another parameter of inductors is their saturation current. That you can safely make bigger, but not smaller without carefully understanding the design and being sure what the peak current thru the inductor is.

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    \$\begingroup\$ +1 for "squirt pulses" and "squirt current". So is that why guns on Star Trek that squirt pulses are called Phasers? Euler would be so proud. \$\endgroup\$ – JonnyBoats Feb 20 '12 at 17:05

A higher-valued inductor will hold more energy with a given amount of current flowing through it than would a smaller-valued inductor, but the rate at which current flows through the inductor will increase or decrease more slowly than with a smaller one. The rate at which energy flows into an inductor is proportional to the applied voltage and current being pushed through it. If a large inductor doesn't have much current flowing through it (e.g. because the switcher has a very light load), there is a limit to how quickly the switcher can "ramp up" to start delivering more current. Additionally, if a large inductor does have a lot of current flowing through it, that current represents energy that's going to have to go somewhere, even if the load current is suddenly reduced by 99.9%.

Many smaller switchers overcome these issues by using relatively small inductors, and operating in what's called "discontinuous mode". The basic idea is that if an inductor is small enough, one can start each cycle with zero current flowing through the inductor, and still put enough energy into the inductor to handle the load's demand for the next cycle. Using a larger inductor would either reduce the amount of power the supply could handle, or else would require it to run at a lower switching frequency.

Other switchers use what's called "continuous mode". These units expect that changes to inductor current will occur slowly, spread out over several cycles. On such units, using a larger inductor may improve performance under stable load conditions, but decrease the unit's ability to respond quickly to changing load conditions.


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