Application Note #103
HL-8105
Ripple Reduction Technique

Introduction

Switching regulators provide many advantages over linear power supplies, but in some applications the relatively higher output ripple prevents their use. This application note describes a filtering technique that can greatly reduce the output ripple of a switching power supply, and is applicable to step-up (boost), step-down (buck), or flyback (buck-boost) circuits.

By adding L₂ and C₃ to the output of a switch mode power supply, the output ripple is reduced by the attenuation of the filter formed by L₂ and C₃.

Z_L2 = 2πfL₃ L₂ = inductance of L₂ in Henries
f = ripple freq. C₃ = capacitance of C₃ in Farads
ESR_C3 = Equivalent series resistance of C₃ in Ohms

Design Example

At 70KHz a 3uH inductor (HL-8105), has an impedance, Z_L, of 2 x 3.14 x 70,000 x .000003 = 1.318 ohms.
A 1000uF capacitor has an impedance, Z_C, of 1/2 x 3.14 x 70,000 x .001 = .0022ohms.
However, its ESR is probably .05 ohms. (Check manufacturer's data sheet).
Now if the above inductor and capacitor are used to form a filter which is added to a switching supply with output ripple of 1 Vpp the ripple at the output of the filter will be = 1 Vpp.(.052/(1.318 + .0522)) = 38 mVpp.