Electrical
Accessory Products
Filter & Ring Core Chokes
Reduction of Output Ripple
Input Interference Reduction
Even though switching regulators have an inherently low
output ripple, certain sensitive applications need even fur-
ther reduction. In such cases, the low-loss ring core chokes
designed to reduce disturbances at the input can also be
used for reducing the ripple on the output voltage. The
chokes in combination with an external capacitor can
achieve even better results than the Filter Blocks with re-
gard to the ripple and dynamic regulation.
Using L- or LP-series chokes together with an additional
external capacitor a similar attenuation can be achieved as
with filter blocks. The capacitor between the choke and the
converter input is necessary in order to avoid possible oscil-
lations caused by the negative input impedance of the regu-
lator. This phenomenon could cause the input voltage to
leave the specified regulator input range. The relatively
high ripple current flowing through the capacitor must be
considered for the design. Refer also to: Technical Informa-
tion: Installation & Application.
The formula for the remaining output ripple at the load RL is
calculated as follows:
The current compensated choke LP 183 has a high perme-
ability ring core with two identical separate windings. The
normal operating current will only see the small stray in-
ductance between the windings. However common mode
interference will be blocked by the full inductance of the
UR = uo • ZCex/ZLD
uo:
Output ripple of the regulator
ZCex
:
The impedance of the capacitor at the regu-
lator's switching frequency (150 kHz) corre-
sponds to the equivalent series resistance
(ESR) of the capacitor (please refer to the
corresponding data sheet).
choke.
LP 34-3 or
LP 183 L/LP 20-7
12013
Vo+
Go–
Vi+
Gi–
ZLD = 2 π • fS • LD
U
fS:
150 kHz (regulator switching frequency)
Uo
RL
PSR
Through the use of a common mode choke LP 183, the
common mode noise at the output can also be further re-
duced.
Cext 1
Cext 3
Cext 2
Fig. 5
Consider that the filter not only affects the output ripple but
can also influence the voltage UR across the load RL in the
event of load changes. The static regulation increases with
the ohmic resistance of the choke, i.e. 6 mV/A for the choke
L/LP 20-7 and 20 mV/A for the LP 34-3.
L/LP type chokes and capacitors used as input filter
Typical Application
A voltage drop UrGo = rGo • (Io – Ii) is produced across the
ground loop resistance rGo. It is superimposed upon the
regulators output voltage Uo and generates the voltage UR
= Uo – Ur Go across the load resistance RL. Without an input
inductance Le the current Ii in the input circuit has a rela-
tively high AC component with a basic frequency fs (regula-
tor's switching frequency of approx. 150 kHz). This alternat-
ing current produces an AC voltage component across rGo
The dynamic regulation is dependent on the size of the ca-
pacitor. Generally, the bigger Cex the smaller is the dy-
namic, however, recovery will be slower.
LD ZLD ZC ext
RD
12014
Vo+
Go–
Vi+
which is superimposed upon URL
.
U
UR
PSR
Uo
RL
Ce
To prevent this phenomenon, an inductance Le can be in-
serted into the input circuit. This causes the AC component
of the input current to be supplied entirely from the input
capacitor Ce; thus, Ii is a pure direct current. Ce should be
wired as close as possible to the regulator's input terminals
Vi+ and Gi–.
Gi–
Fig. 7
Low-loss ring core choke with external capacitor (Cex
approx. 1000 µF) used as output filter
Le and Ce additionally provide protection against input tran-
sients and reduce radio interference voltages.
External connection of Gi– and Go– or connection via a
common ground is not recommended. The internal voltage
drop UrG in the regulator would be superimposed on the
output voltage.
12015
Io
Vo+
Vi+
Le
PSR
RL
U
URL
Uo
Ce
UrG
Gi–
Go–
rG
rGo
UrGo
Fig. 6
Reduction of superimposed interference voltages in
grounded power supply systems, caused by ground loops
Edition 6/01.03
4/5
