# Estimation of SMPS' Conducted Emission According to CISPR Standards
## 2 Materials and Methods
### Studied System  
#### 2.2.1 Two-Switch Forward Converter  
The base for this study was the two-switch forward converter [8]. The general concept of the topology is presented in Figure 4. In these kinds of electronic circuits, the high-frequency switching currents effect electromagnetic compatibility, which is a deep concern. Due to the fact that SMPS modules supply other devices in the system, which can work as antennas and emit noise coming from the SMPS, those modules should be designed with an appropriate margin in terms of emissions.  
The exact description and principle of the work of the two-switch forward converter can be found in the literature [8]. For this study, it is essential to define two working cycles presented in Figure 5.  
The first cycle (a) is when the transistors (M1 and M2) are in the conducting state and current flows through them and through the transformer. When the cycle is switched to (b), transistors are in the non-conducting state, and current loops are closed by the two diodes (D2 and D3). From the EMC performance, the C1 capacitor is essential. It should be able to store enough energy so that the AC current will not be drawn from the power supply. Otherwise, the conducted emissions will be high. The inductances of the traces, due to the size of the physical module, are also a concern and should be taken under consideration when the physical layout is performed. The design of SMPS is always based on decisions  
Figure 4: Topology of the two-switch forward converter.  
Figure 5: Two-switch forward converter cycles: (**a**) mosfet transistors are ON; (**b**) mosfet transistors are OFF.  
and risk minimization. The components have physical dimension and pins that connect it to the layout. The shapes of the components and pins create \"keep out\" regions that prevent designers from achieving perfect layout proposed in application notes or technical datasheets of the components. Due to this fact, using the approach proposed by the authors, one can verify if the design of the module meets the design criteria from the conducted emission perspective.  
In this paper, it has been assumed that in order to obtain satisfactory results in terms of conducted emission estimation, it is enough to focus on components related only to the two current loops shown in Figure 5. According to the previous statement, the electrical model of the SMPS has been built in the ANSYS TWIN BUILDER environment, which is presented in Figure 6.  
In order to estimate conducted emission accurately, the electrical models of the components have to represent both static and dynamic behavior. As the time-domain simulator used to solve the conducted emission problem was the Twin Builder from ANSYS, all models used were compatible with this software. It is worth noting that all information required for the model preparation was found in each component's datasheet, except for the Ferrite Bead model of which the parameters were provided by the supplier. It is important to keep in mind that model creation is the key part of the process. The authors took the approach that, once the first electrical simulation has been prepared and verified, then the electrical model is extended by adding the components required for conducted emission estimation. During the second stage, no further changes to the electrical models were introduced. A series resistance of the capacitors and inductors were added to the electrical simulation, and those values are added as separate components.  
Figure 6: Electrical circuit of the two-switch forward converter: (**a**) primary side; (**b**) secondary side.