APPLICATION NOTE #103

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 thier 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 switching power supply, the output ripple is reduced by the attenuation of the filter formed by L2 and C3.

Ripple (Vout) = Ripple (VC2)(ZC3/( ZL2 + ZC3))
Where: ZC3 = 1/SC3 + ESRC3 = 1/2

fC3 + ESRC3
ZL2 = 2

fL3
L2 = inductance of L2 in Henries
f = ripple feq.
C3 = capacitance of C3 in Farads
ESRC3 = Equivalent series resistance of C3 in Ohms

DESIGN EXAMPLE:

At 70 KHz a 3uH inductor (HL-8105), has an impedance,
ZL, of 2 x 3.14 x 70,000 x .000003 = 1.318 ohms.

A 1000 uF capacitor has an impedance,
ZC, of 1/2 x 3.14 x 70,000 x .001 = .0022 ohms
However, its ESR is probably .05 ohms. (Check manufacturers 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.

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