psa242.5 Power-One, psa242.5 Datasheet - Page 4

• Manufacturer Part Number: psa242.5
• Description: Accessories Filter & Ring Core Chokes Fp, L And Lp Series
• Manufacturer: Power-One

REV. SEP 29, 2003
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.
U
Fig. 5
L/LP type chokes and capacitors used as input filter
Typical Application
A voltage drop U
ground loop resistance r
regulators output voltage U
= U
inductance L
tively high AC component with a basic frequency f
tor's switching frequency of approx. 150 kHz). This alternat-
ing current produces an AC voltage component across r
which is superimposed upon U
To prevent this phenomenon, an inductance L
serted into the input circuit. This causes the AC component
of the input current to be supplied entirely from the input
capacitor C
wired as close as possible to the regulator's input terminals
Vi+ and Gi–.
L
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
output voltage.
e
and C
o
– U
C
rG
ext 1
r Go
U
e
in the regulator would be superimposed on the
additionally provide protection against input tran-
LP183
e
across the load resistance R
; thus, I
e
the current I
C
L
rGo
ext 2
e
C
LP34-3 or
L/LP20-7
e
i
= r
is a pure direct current. C
Go
C
Go
ext 3
Gi–
• ( I
Vi+
i
o
. It is superimposed upon the
in the input circuit has a rela-
and generates the voltage U
o
PSR
U
– I
RL
r
G
r G
Vi+
Gi–
.
i
) is produced across the
Vo+
Go–
12015
PSR
r
Go
L
U
. Without an input
Go–
Vo+
o
I
o
U
R
r Go
e
e
L
can be in-
should be
U
s
Filter & Ring Core Chokes FP, L and LP Series
12013
o
(regula-
U
RL
Page 4 of 5
R
L
Go
R
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
calculated as follows:
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
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
namic, however, recovery will be slower.
Fig. 7
Low-loss ring core choke with external capacitor (C
approx. 1000 µ F) used as output filter
Fig. 6
Reduction of superimposed interference voltages in
grounded power supply systems, caused by ground loops
U
U
u
Z
Z
f
S
o
Cex
LD
R
:
:
= 2 π • f
: The impedance of the capacitor at the regu-
= u
Vi+
Gi–
lator's switching frequency (150 kHz) corre-
sponds to the equivalent series resistance
(ESR) of the capacitor (please refer to the
corresponding data sheet).
Output ripple of the regulator
150 kHz (regulator switching frequency)
o
PSR
• Z
C ext
S
Go–
Vo+
• L
/ Z
LD
D
U
o
R
D
R
ex
across the load R
the smaller is the dy-
L
D
Accessories
Z
LD
C
Z
e
C ext
U
R
12014
ex
L
in the
R
L
L
is