Abstract. The EMC properties of a cubic metallic shield are highly affected by its resonances. At the resonant frequencies, the shielding effectiveness (SE) collapses, which results in high field strengths inside the cavity. This can cause failure or even breakdown of electronic devices inside the shield. The resonant behaviour is mainly determined by the quality or Q factor of the shield. In this paper, the effects of the aperture size on the Q factor and the SE of an electrically large, cubic shield are analysed. At first, a method is developed in order to determine the Q factor based on the resonance behaviour of the shield in time domain. Only the first resonance of the shield is considered therefore. The results are evaluated for different aperture diameters and compared with theory for the Q factor. The dominant coupling mechanism of electromagnetic energy into the shield is thus identified.Then the effect of aperture size on the SE is analysed. The excitation of resonances is very probable if the interfering signal is an ultrawideband (UWB) pulse, which constitutes a typical intentional electromagnetic interference (IEMI) scenario. Therefore, the relation between aperture size and SE is analysed using the theory of the transient SE for a broadband signal with a constant spectral density distribution. The results show, that a "worst case" aperture size exists, where the SE has its minimum.
Abstract. Power semiconductors are used in a growing number of applications.
Furthermore, faster switching transients and therefore higher switching
frequencies can be realised. This leads to higher radiated interferences at
higher frequencies. In power electronic applications, often a cooling concept
for the semiconductors is required to comply with the allowed temperature
range. Typically, heat sinks are made of aluminium and their conductive
behaviour can cause EMC problems. Via capacitive coupling, voltage transients
are transferred from the power semiconductor to the heat sink, so that the
heat sink behaves as an antenna and radiated interferences occur.
Furthermore, a common mode current will appear if the heat sink is grounded.
In this paper, a promising approach to reduce radiated interferences from the
heat sink and solve the common mode issue by using an electrically isolating
ceramic heat sink is presented. The influence of an aluminium nitride ceramic
heat sink on cooling performance and EMC behaviour has been investigated. For
this purpose, two geometrical identically heat sinks made from aluminium and
aluminium nitride have been compared regarding thermal performance and
radiated interferences characterised by S-parameters.
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