FP38 Datasheet, PDF (4/5 Pages) Power-One – Filter & Ring Core Chokes

Accessories
Filter & Ring Core Chokes FP, L and LP Series
Input Interference Reduction
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 current compensated choke LP183 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
choke.
LP34-3 or
LP183 L/LP20-7
Vi+
Vo+
12013
U
PSR
U_o R_L
C_{ext 1}
Gi–
C_{ext 2}
C_{ext 3}
Go–
Fig. 5
L/LP type chokes and capacitors used as input filter
Typical Application
A voltage drop U_rGo = r_Go • (I_o – I_i) is produced across the
ground loop resistance r_Go. It is superimposed upon the
regulators output voltage U_o and generates the voltage U_R
= U_o – U_{r Go} across the load resistance R_L. Without an input
inductance L_e the current I_i in the input circuit has a rela-
tively high AC component with a basic frequency f_s (regula-
tor's switching frequency of approx. 150 kHz). This alternat-
ing current produces an AC voltage component across r_Go
which is superimposed upon U_RL.
To prevent this phenomenon, an inductance L_e 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 C_e; thus, I_i is a pure direct current. C_e should be
wired as close as possible to the regulator's input terminals
Vi+ and Gi–.
L_e and C_e 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 U_rG in the regulator would be superimposed on the
output voltage.
12015
I_o
Vi+
Vo+
L_e
U
C_e
PSR
U_{r G}
U_o ^RL U_RL
Gi– _rG Go–
r_Go
U_{r Go}
Reduction of Output Ripple
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.
The formula for the remaining output ripple at the load R_L is
calculated as follows:
U_R = u_o • Z_{C ext}/Z_LD
u_o: Output ripple of the regulator
Z_Cex: 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).
Z_LD = 2 π • f_S • L_D
f_S: 150 kHz (regulator switching frequency)
Through the use of a common mode choke LP 183, the
common mode noise at the output can also be further re-
duced.
Consider that the filter not only affects the output ripple but
can also influence the voltage U_R across the load R_L 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/LP20-7 and 20 mV/A for the LP34-3.
The dynamic regulation is dependent on the size of the ca-
pacitor. Generally, the bigger C_ex the smaller is the dy-
namic, however, recovery will be slower.
R_D
L_D Z_LD Z_{C ext}
Vi+
Vo+
12014
U
PSR
U_o
Gi–
Go–
U_R
C_e
R_L
Fig. 7
Low-loss ring core choke with external capacitor (C_ex
approx. 1000 µF) used as output filter
Fig. 6
Reduction of superimposed interference voltages in
grounded power supply systems, caused by ground loops
REV. SEP 29, 2003
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