Feature: Power electronics
Figure 1: Differential (left) and common mode (right) filters
Figure 2: A simulation model shows insertion loss
speed drives or designing filters for switched-mode power supplies (SMPSs) such as DC-DC converters. As always, any discussion of noise must begin with a clear
separation between differential mode and common mode. We will start with differential mode. Figure 3 shows the differential-mode impedance
characteristics of a DC-DC converter. Over a defined frequency range, the output impedance of the converter is extremely low. This is exactly what we would expect: a high output impedance would imply higher losses (I²R), so SMPSs are typically designed to present a low output impedance. This is usually achieved through appropriate output capacitor selection and careful layout. Beyond a certain frequency, however, the impedance inevitably increases as capacitor ESL begins to dominate. The input impedance of an SMPS, on the other hand, is a
much more complicated story. It depends on several factors, including: • For AC-powered mains products, whether a PFC circuit is used. PFC forces the input current to be in phase with the mains voltage. To achieve this, the control loop bandwidth must be relatively low, which results in increased input impedance, starting from frequencies as low as tens of Hz. • The front-end EMI filter design. Within certain frequency
ranges, the input impedance may be dominated by high- impedance components, particularly inductors.
• The control algorithm, along with several other secondary effects. As a result, the differential-mode input impedance of
an SMPS is complex and strongly frequency dependent. In practice, it is often relatively high in the frequency range where conducted EMC tests begin (9kHz or 150kHz). For reference, we showed the input impedance of a three-phase voltage-source inverter in Figure 3b; note that in this case the impedance is expressed in dBΩ. So far, the discussion has focused mainly on differential
mode. For common mode, the situation is conceptually simpler, but practically more challenging. Ideally, we would like to achieve a very high common-mode impedance, because for a given common-mode voltage this would result in a smaller common-mode current. In real systems, however, this is extremely difficult to achieve due to unavoidable parasitic capacitances between the power converter and earth/chassis. When testing products like an SMPS or a motor drive,
line impedance stabilising networks (LISNs) are used. From a differential-mode perspective, the simplified diagram in Figure 4a shows the effective load impedance. In this case,
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