Picor Corporation • www.picorpower.com

QPI-10-CB1 Data Sheet Rev. 1.1 Page 4 of 7

EMI Bypass Configurations

EMI bypass capacitors (“Y” capacitors) are an essential

element in a switch-mode DC-DC filter application as

these capacitors provide a return path for common-

mode noise currents to their source; so careful attention

to bypass capacitor implementation is essential for a

successful EMI filter design. This carrier board is

preconfigured in a “base-plate” EMI topology, which

uses six “Y” capacitors (only four are actually used in a

BCM configuration), as illustrated in Figure 7 below.

This is the preferred topology for VI Chip applications.

The carrier board can be manually reconfigured to an

“open frame” topology, which uses four “Y” caps in a

PRM/VTM combination (two for a BCM), as shown in

Figure 8. The open-frame approach may attenuate

certain load dependent noise better than the base-plate

method. The carrier board provides an ideal test vehicle

for making a comparison between the two “Y”

capacitor configurations. To reconfigure to “open

frame” carefully remove capacitors CY5 through CY10

and place four of those six capacitors in the positions

marked as CY1 through CY4.

EMI Performance and Test Set Up

The EMI plots in Figures 9 through 14 are the total noise

measurements, on both the positive and negative lines

of the QPI-10LZ with various VI Chip configurations,

BUS+

QPI+

BUS-

QPI-

QPI

VIN+

VOUT+

VIN-

VOUT-

PRM

VIN+

VOUT+

VIN-

VOUT-

VTM

CY5

CY6

CY9

CY7

CY10

CY8

C1

47 uF

LISN

LISN

PCB Plane Under Converter

~1.25 m

Shielded Box

Shield Plane (Earth Ground)

PCB Board

BUS

SUPPLY

SW

Figure 7 – Basic EMI measurement setup for “base-plate” configuration.

BUS+

QPI+

BUS-

QPI-

QPI

VIN+

VOUT+

VIN-

VOUT-

PRM

VIN+

VOUT+

VIN-

VOUT-

VTM

CY3

CY1

CY4

CY2

C1

47 uF

LISN

LISN

PCB Plane

Under Converter

~1.25 m

Shielded Box

Shield Plane (Earth Ground)

PCB Board

BUS

SUPPLY

SW

Figure 8 – Basic EMI measurement setup for “open-frame” configuration.

using the basic base-plate standard “Y” cap

configuration. Figure 7 shows the basic EMI

measurement set up that was used to achieve these

results. Figure 8 shows the alternate set up method

when converting to the open-frame approach.

In Figure 7 (base-plate method), capacitors CY5 through

CY10 represent the recirculation capacitors that are

connected to each of the four input and output

terminals, then are commoned to a shield plane that

has been created underneath the converter. Since the

PRM/ VTM pair is similar to a conventional converter,

which is split into to halves, two additional “Y” caps

(CY7 and CY9) were added to the PRM’s output (the

input to the VTM), referenced to the shield plane.

In Figure 8 (open-frame method), four “Y” capacitors

are used (CY1 through CY4) rather than the two “Y”

caps that a conventional converter would require, once

again because of the topology split created by the pair.

In a BCM application there is no topology split so the

set up would require two fewer capacitors for either

configuration. The open-frame method would only

need one pair of input an output caps, referenced to

the shield plane on either side of the BCM. And for

open-frame method a pair of “Y” caps across the

positive input to positive output, and negative input

and negative output would be sufficient.