Inductor-calculation in Buck/Setp-Down

Check the current waveform. It depends on ripple. When current drops to zero each cycle, it becomes discontinuous and the behavior changes. The change is still within reach of an error amplifier, so a variable regulated supply isn't impossible or anything.

Tim

-- Deep Fryer: A very philosophical monk. Website @

Consider dI/dt = V/L. For a given switcher frequency, i.e. a given inductor-charging time, thinking simply, if you need I to be 5x smaller, you'll want L to be 5x larger. This implies sqrt 5 = about 2.2x as many turns, and for 5x lower current you can use 5x smaller wire cross-section area, not to mention core area, so the physical size of the inductor can be smaller.

Think another way - as you go to higher currents, the impedances must go lower. The implies ot only low R but low L. At very very high currents your wires may be thick bars of metal and an inductor may be a huge special pieces of ferrite clamped around the bar (i.e. made with one turn).

Thanks for the replies :)

But what if my designed power supply, calculated to handle a load of 5 amp, suddenly got a lighter load... let's say 100mA, what would then happen??? i know the duty-cycle changes, but what about the output??? would there be more ripple or what???

Overshoot.

When the load becomes so light that the inductor current ramps all the way to zero before the next switching cycle begins, you've entered so-called "discontinuous mode." From a stability point of view, the system has changed so in a poorly-designed (or perhaps just super-cheaply-designed where it was "known" that there'd always be at least a minimum load) power supply the system might go unstable and you'll get an oscillating output, but in most cases you'd expect the power supply would still be stable to really nothing at all "interesting" happens. Further decreases in load will decreate the switch duty cycle, although in general this won't be your worst case of ripple because the load is so very light in the first place. Many controller ICs for switchers have special modes that they enter at light loads that are meant to improve overall efficiency (when the load itself is light, suddenly all the losses in the rest of the switcher become a much bigger factor in the power supply's overall efficiency), such as switching to "burst" mode where they switch for a few cycles at a higher duty cycle and then shut off completely for a bit.

You might want to download a copy of LTspice and play around with the various example circuits. It also sounds as though you might want to pick up a book on switching power supply design

---Joel

Thanks guys

I slowly begins to comprehend but there is still a lot of reading and simulating for me to do (thanks for the info about LTspice, it makes it much easier to understand when you can see the waveforms).

Have a great weekend :) Ole