Hi-Rel
Grade
EMI Filter FGDS-2A-50V
4- EMI Filter Electrical Schematics (continued)
4-4 R*C* Network Discussion
The RC damping network is a network used for stability purposes in negative input impedance systems such as DC/ DC
converters.
R and C is a network whose function is to lower the filter’s output impedance.
The value of the capacitor C strongly depends on the application’s conditions (input voltage range and total power drawn
from the source) as well as the standards that the equipment has to meet MIL-STD-461C, or D/ E, ... this because
measurements method (LISN) differs from one standard to another affecting the C value.
DC/ DC converters are negative input impedance systems whereas a filter composed of passive elements displays a positive
output impedance to the converter. To ensure the stability of the whole system “LISN + input Filter + DC/ DC converters”, the
filter output impedance must be kept below the converter’s input impedance, which is given by the following formula :
Vin2
____
Vin2 x h
_________
Zin =
=
Pin
Po
where :
Vin is the converter input voltage,
Pin is the converter input power,
Po is the converter output power
h is the efficiency of the converter.
As it can be seen from the preceding equation, the worst case for system’s stability is at Vin
which should be considered for the filter design.
, so this is the condition
min
As the filter is made of low ESR inductors and ceramic capacitors, it has an important Q which causes a sharp increase of
the filter’s output impedance at the resonant frequency and leads to a violation of the stability criteria, causing the system
to break into oscillations.
Consequently, the values of C and R have to be adjusted to dampen sufficiently the filter’s resonance and make its
output impedance lower than the converter’s input impedance (a 10dB margin between both values is recommended).
The following formula can be used to determine the capacitor value :
( LLISN + 4 .10-6 ) x Po x k
6
_________________________________
C >
Vin2 x (R + RL)
Where :
L
is the LISN inductor value
LISN
Po is the filter output power
R is the capacitor esr (equivalent serial resistor) of C
R is the input bus equivalent resistor. It is the sum of the line + LISN network + input connector resistors
L
k is the stability margin factor.
This factor is given by the following formula : Stability margin (dB) = 20 log (k)
Vin is the converter input voltage
4
Gaia Converter FC05-050.03/ 08 Revision B
©
For locations, phone, fax, E-Mail see back cover
ETC [ ETC ]
