Rev 1.5

vicorpower.com

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08/2015

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QPI-21

Applications Information

EMI Sources

Many of the components in today’s power conversion modules are

sources of high-frequency EMI noise generation. Diodes, high-

frequency switching devices, transformers and inductors, and circuit

layouts passing high dv/dt or di/dt signals are all potential sources

of EMI.

EMI is propagated either by radiated or conductive means. Radiated

EMI can be sourced from these components as well as by circuit

loops that act like antennas and broadcast the noise signals to

neighboring circuit paths. This also means that these loops can act as

receivers of a broadcasted signal. This radiated EMI noise can be

reduced by proper circuit layout and by shielding potential sources

of EMI transmission.

There are two basic forms of conducted EMI that typically need to be

filtered; namely common-mode (CM) and differential-mode (DM)

EMI. Differential-mode resides in the normal power loop of a power

source and its load; where the signal travels from the source to the

load and then returns to the source. Common-mode is a signal that

travels through both leads of the source and is returned to earth via

parasitic pathways, either capacitively or inductively coupled.

Figure 6 to Figure 9 are the resulting EMI plots, after filtering by the

QPI-21, of the total noise, both common and differential mode, of a

DOSA (Distributed-power Open Standards Alliance) compliant

converter. These converters are mounted on a QPI-21 evaluation

board and tested under various loads. The red and blue traces

represent the positive and negative branches of total noise, as

measured using an industry standard LISN setup, as is shown in

Figures 11 to 13.

Differential-mode EMI is typically larger in magnitude than

common-mode, since common-mode is produced by the physical

imbalances in the differential loop path. Reducing differential EMI

will cause a reduction in common-mode EMI.

Passive EMI Filtering

The basic premise of filtering EMI is to insert a high-impedance, at

the EMI’s base frequency, in both the differential and common-mode

paths as it returns to the power source. Passive filters use common-

mode chokes and “Y” capacitors to filter out common-mode EMI.

These chokes are designed to present a high-impedance at the EMI

frequency in series with the return path, and a low impedance path

to the earth signal via the “Y” caps. This network will force the EMI

signals to re-circulate within a confined area and not to propagate to

the outside world. Often two common-mode networks are required

to filter EMI within the frequency span required to pass the

EN55022 class B limits.

The other component of the passive filter is the differential LC

network. Again, the inductor is chosen such that it will present a

high-impedance in the differential EMI loop at the EMI’s base

frequency. The differential capacitor will then shunt the EMI back to

its source. The QPI-21 was specifically designed to work with

conventional switching frequency converters like Vicor’s Brick

products; Micro, Mini and Maxi modules; as well as converters from

various vendors.

Active EMI Filtering

The QPI-21 active EMI filter uses the same basic principles for

filtering as the passive approach, but its active common-mode filter

can perform as well as a passive filter, when filtering lower

frequencies, in much less board area and power dissipation.

Typically, the lower the frequency the greater the needed inductance

would be to properly filter the EMI signal. This means either a larger

core or a greater number of turns on a smaller core. A larger core

requires more board space, where a smaller core with more turns has

a greater amount of unwanted parasitics that can affect the filters

ability to attenuate EMI signals.

Figure 3 is a simplified schematic of the QPI-21’s active and passive

circuitry used for EMI filtering. The QPI-21’s active filter uses a small

high-frequency common-mode transformer to filter the higher

frequencies and adds a sensing element to

it so that the lower frequency common mode signal can be sensed

and a correction signal can be generated and inserted into the shield

connection. By this means, the QPI-21 is capable of providing EMI

filtering of converters in far less space than standard passive filters

and can provide filtering over the entire EN55022 class B range.

EMI Management

The more effectively EMI is managed at the source, namely the

power converter, the less EMI attenuation will be required to meet a

given standard. The addition of “Y” capacitors to the input and

output power nodes of the converter will help to limit the amount of

common mode EMI that will propagate to the input source.

CM w/Aux Winding

LDM

CDM

Y Caps

Active CM Filter

Figure 3 — Simplified Active EMI filter circuit