In the quest to increase converterqqqpower density, switching frequencies are greater than ever, giving increased importance to EMC performance.qqqTraugott Schutz reports.

The analysis and suppression of electromagnetic interference (EMI) generated from high switching frequency converters is more important today given the increase in EMC regulations. The EMI noise in power converters can generally be associated with the layout and design, but design and mechanical constraints can be limiting in many projects.
Switch-mode power supplies (SMPS) generate high frequency noise because of this high frequency switching action, and as with all EMI noise, interference is a complex phenomena, taking the form of both differential and common mode noise.
Many current EMI designs have used trial and error methods, and whilst this is ultimately successful, it is
time-consuming, expensive, and often difficult to optimise performance.
For a cost-effective and time-saving approach to design, the EMI characteristics of a SMPS need to be analysed at an early stage in the project. The most important aspects to be examined first in regards to EMI problems are the high dV/dt and di/dt slew rates in the SMPS, and the circuit layout itself.
We will consider the conducted noise of a 40W ac/dc boost rectifier in Fig. 1, using the key waveforms according to Fig. 2. and the layout geometry of Fig. 3.
This circuit works in the critical conduction mode with variable frequency control (constant on-time) to achieve power correction as shown in Fig. 2.
To quantify the EMI noise caused by the drain node of the MOSFET, and its high dV/dt and di/dt slew rates, parasitic extraction is performed. Critical for the reduction of EMI noise is the layout geometry of the boost circuit.
We find on the palatine partial inductance and capacitance to ground. The partial inductance and capacitance matrix is responsible for the EMI noise, and therefore we have the two different forms of interference; differential-mode (DM) and common-mode (CM) noise.
The CM noise is directly related to stray capacitance (common-mode capacitance). The CM currents are defined as currents which are equal in phase on the hot and return. CM currents return to the source via the system ground. In this circuit, since only the drain node of the MOSFET has high dV/dt slew rate and the MOSFET is mounted flat on the PCB, the major common mode capacitance comes from the capacitance between drain node trace and the ground plane. This can be calculated according the analytical formula or FEA method. Most are of a very high frequency.
DM mode currents are defined as currents which are opposite in phase on the hot line and the return. The parasitic inductance, the high di/dt slew rates in the pulsating switched currents will introduce DM noises. Most are of a lower frequency.
Parasitic elements of passive components may also have important effects on high frequency noise. The winding capacitor of the boost inductor, the equivalent series resistance ESR and equivalent series inductance ESL of the input and output capacitors were determined experimentally using an impedance analyser.
The reduction of this noise will almost certainly require the use of an EMI filter. We analysed a SMPS for a standard PC. We measured the influence to the lines input with different configurations of an EMI filter.
We then measured the influence of no EMI filtering
(Fig. 4), using only capacitors (Fig. 5), and finally only using inductors (Fig. 6).
Without any EMI filter we have noise in the whole frequency range. Up to the MHz range over the limit according the EN norm.
With capacitors we have filtering especially in the high frequency range, but still not enough for the EN norm.
With inductors we have just a small reduction of the noise. The noise is over the limit according the EN norm.
The current compensated inductance is mostly for lower frequency problems and good for CM mode filtering. The capacitors are most for DM filtering if the capacitor is connected line to return. If the capacitor is connected from line to ground it also can be used for CM filtering. The important factor to remember is that there are no hard and fast rules and in many cases the performance of the filter and the positioning of the components are dependent on the application.
To use discrete components within the SMPS is one solution but the trend shows that many customers are turning to EMI filters to solve their problems because of the flexibility it allows. Mechanical constraints sometimes leave a separate EMI filter as the only solution, and along with the speed and ease of the design review process, this makes a powerful argument for EMI filters. SMPS families can be developed very quickly, and offsetting the cost of a filter is the additional functionality such as integrated connectors, switches, fuses and associated approvals.

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Traugott Schutz is with Schaffner EMV AG is based in Luterbach, Switzerland. www.schaffner.com