Applications Information
The QPI-1 is an active EMI filter providing conducted
common-mode and differential-mode attenuation
from 150 kHz to 30 MHz. The QPI-1 attenuates noise at
all frequencies covered by EN55022. Designers should
be aware that to select and qualify an EMI filter they
must test the filter in their equipment under the set up
and conditions specified in the EMI standards that they
are trying to meet.
Filter performance is highly dependent on several
factors and cannot be derived from the attenuation
curves shown in
Figure 2
alone. Performance is a
complex function of filter element impedances and
noise source impedances, which vary in magnitude and
phase over the frequency spectrum. Measurements
should be made in consultation with in-house
compliance testing or an independent EMI test facility.
Figure 4 –
Conducted EMI profile of a DC-DC Converter
with QPI-1.
Test Example
Figures 3 and 4
show the before and after plots of a
DC-DC converter noise profile to demonstrate the
performance of the QPI-1. The plots were taken using
the standard measurement technique and set up defined
in CISPR22. The results show the total noise spectrum
for a standard DC-DC converter under load compared
to the EN55022 Class B Quasi-peak detection limit.
Layout and Circuit
Application of the QPI-1 requires PCB layout practices
similar to those used with passive EMI filters. The circuit
in
Figure 6
and the PCB layout in
Figures 9 & 10
should
be used as a design guide to ensure successful
application of the QPI-1. Refer to the application note
QPI- AN1 for additional PCB layout recommendations.
Note: For optimum performance of the QPI-1 it is
critical to connect the QPI-1 Shield Plane (pins 5 & 6) to
the converter EMI Shield Plane (common point of C1-
C4 "Y" capacitors) thru RY as shown in Figure 6. If the
safety earth-ground connection (chassis or case) is
connected in the power conversion section of the PCB
it should be at the input bus of the QPI-1 Shield Plane,
shown as the optional connection in Figure 6.
Connecting the converter EMI shield plane directly to
chassis ground may degrade the QPI-1 attenuation
performance. The typical value for RY is 10 Ohms. The
recommended part is a 5% tolerance SMT1206 or
similarly rated 0.25 W through-hole resistor.
QPIs will filter multiple DC-DC converters up to the
maximum current rating. The schematic in
Figure 7
shows the required circuit for multiple DC-DC
converters. The Shield Plane should be added under
each converter as a common EMI Shield. Note that only
a single RY is required in a multiple converter
configuration.
Figure 3 –
Conducted EMI profile of a DC-DC Converter
without filter.
The resulting plot in
Figure 4
shows the QPI-1 is
effective in reducing the total conducted noise
spectrum measured pre-filter to well below the
EN55022 Class B Quasi-peak detection limit.
Picor Corporation • www.picorpower.com
QPI-1L Data Sheet Rev. 2.0 Page 3 of 8
VICOR [ VICOR CORPORATION ]
