Inductor Ripple Current (Buck)
Size the buck-converter inductor from switching frequency and duty — get peak-to-peak ripple and peak current.
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The engineering
In a continuous-conduction buck, the inductor current ramps up while the high-side switch is on and ramps down while the diode (or low-side FET) conducts. The peak-to-peak ripple is set by the volt-second balance: apply the off-state voltage across L for the off time and you get ΔiL = Vout·(1−D)/(L·fsw). The DC average is the load current, so the current triangle rides on top of Iout.
Rule of thumb: designers target ΔiL at 20–40% of full load. Too little ripple wants a bulky inductor; too much pushes the peak current into saturation and hurts efficiency. If ripple exceeds 2·Iout the valley hits zero and you drop into discontinuous conduction — the DCM math above no longer applies.
Sanity check the peak current against the inductor's saturation rating and the FET's current limit, not just the average. A 2 A load with 1 A ripple still peaks at 2.5 A every cycle.
Where this math comes from
Switching regulators grew out of aerospace and computing power needs in the 1960s, but the design equations stayed folklore until academic treatment caught up. The volt-second balance and small-ripple approximation that this card uses became standard teaching through Robert Erickson's work at the University of Colorado.
Erickson's 1997 textbook Fundamentals of Power Electronics (with Dragan Maksimović in later editions) codified the buck-converter ripple analysis into the clean ΔiL expression engineers reach for at the bench, tying inductor sizing directly to frequency and duty cycle.
- 1976Slobodan Ćuk & R. D. MiddlebrookState-space averaging formalizes converter analysis at Caltech.
- 1997Robert W. EricksonFundamentals of Power Electronics standardizes the small-ripple / volt-second approach.
- 2001Erickson & MaksimovićSecond edition extends the buck ripple treatment used industry-wide.
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