There are two basic topologies for the connection of the re-

circulating “Y” capacitors, referred to as “open-frame” and “base-

plate”. Figure 4 illustrates how a converters total conducted noise

profile can vary with one topology versus another. The EMI

generated by the “base-plate” configuration is much greater than that

generated by the “open-frame” in this example. Selecting the right

topology will greatly reduce the amount of EMI signal that needs to

be filtered.

Figure 2 shows the base-plate topology of re-circulating “Y”

capacitors. Here, CY1 to CY4 are connected to each power node of

the dc-dc converter, and then are connected together on a copper

shield plane created under the converter. The addition of the copper

shield plane helps in the containment of the radiated EMI,

converting it back to conducted EMI and shunting it back to

its source.

In Figure 5, the open-frame topology is shown where the “Y”

capacitors (CY1 and CY2) re-circulate the EMI signals between the

positive input and output, and the negative input and output nodes

of the power conversion stage. Safety and system requirements may

dictate that some converters must have the output return (-) directly

connected to earth ground. This connection can impact performance

of some filter solutions, where the QPI-21 works well in this

configuration.

Rev 1.5

vicorpower.com

Page 5 of 15

08/2015

800 927.9474

QPI-21

Figure 4 — An unfiltered converter’s response to “open-frame”

(light blue) and “base-plate” (purple) EMI configurations

CY1

CY2

Shield

BUS+

QPI+

BUS-

QPI-

IN+

IN-

OUT+

OUT-

BUS+

BUS-

Chassis/Shield

CB1

CIN

LOAD

+

+

PC

PR

SENSE+

SC

SENSE-

XY+

XY-

Optional Chassis Connection

Figure 5 — Typical “open-frame" application with an optional connection of the chassis to OUT- of the converter