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QPI-4L Data Sheet Rev. 3.0 Page 3 of 8

Applications Information

The QPI-4 is an active EMI filter providing conducted

common-mode and differential-mode attenuation

from 150 kHz to 30 MHz. The QPI-4 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.

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-4. 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.

The resulting plot in Figure 4 shows the QPI-4 is

effective in reducing the total conducted noise

spectrum measured pre-filter to well below the

EN55022 Class B Quasi-peak detection limit.

Layout and Circuit

Application of the QPI-4 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-4. 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.

Some systems may require the converter positive or

negative input or an output "terminal" to be connected to

PE (Protective Earth) ground for safety or other

considerations. When using the QPI in this situation this

"terminal" must be connected to the converter shield

plane created in the PCB layout under the converter.

Because the PE earth path may pass excessive current

under a fault condition the resistance of this path may be

limited to a low resistance value. To meet the resistance

requirement without degrading filter performance Ry can

be replaced by a 4.7uH inductor rated for the fault current

condition maintaining low power dissipation during a

fault until the protection device clears.

Figure 3 – Conducted EMI profile of a DC-DC Converter

without filter.

Figure 4 – Conducted EMI profile of a DC-DC Converter

with QPI-4.