AN10912_15 Datasheet, PDF (27/34 Pages) NXP Semiconductors – SMPS EMC and layout guidlines

NXP Semiconductors
AN10912
SMPS EMC and layout guidlines
ldiff
Rload
C
L
ldiff
Rload
C
L
Rser ldiff
Cpar
C
L Rpar
Rload
Fig 25. HF damping process
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5.8 Shielding
5.8.1 Mount the hot windings at the inner layer
The outer windings of inductors and transformers can act as a shield when connected to a
stable potential. This is capacitive shielding as the area having most dV/dt is covered by
conductive material. Although the dot shown in Figure 26 only indicates current flow and
winding direction, it is also often interpreted as the location of the first winding (see
Figure 26).
this dot indicates both winding direction
and the position of the inner winding
L_p L_s
S_w
Fig 26. Hot windings at inner layer
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5.8.2 Attach a heatsink to the clean potential
Heat-sinks distribute heat away from dissipative elements. In SMPS systems, these
dissipative element are usually the components that carry high currents and voltages,
switches for example. Because of the close proximity of the heat-sinks to these devices,
capacitive coupling always occurs. This results in the complete heat-sink becoming a
capacitive radiator to its surroundings. This is particularly evident where common mode
noise is present. It is therefore recommended that heat-sinks are connected either directly
to a clean potential, such as a clean ground signal, or the use of a decoupling capacitor to
conduct noise toward a clean potential.
5.8.3 Use the component material and heatsink as a shield
Some components, including grounded heat-sinks, can act as shielding for capacitive
conducted noise. One of the constructional methods often encountered in power supplies,
is the use of a heatsink to disperse heat, act as shield and also to provide a mechanical
housing. Because the functionality is combined, it is an economical solution. Another
AN10912
Application note
All information provided in this document is subject to legal disclaimers.
Rev. 1 — 18 February 2011
© NXP B.V. 2011. All rights reserved.
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