Radiated EMI is traditionally derived and analyzed using electromagnetic field theory, but for engineering applications there are a limited number of complex formulations relevant to understanding and solving EMI problems.
Another way to mitigate EMI problems is to create a clear circuit model. Figure 1 shows how radiated EMI propagates through a dipole antenna consisting of input and output power cables. In this case, the CM noise source of the converter is the main cause of EMI.
Figure 1: Mechanisms and models of radiated EMI generation
According to Thevenin’s theorem, a converter can be modeled as a voltage source and its series impedance. At the same time, the antenna can be transformed into three impedances (RL, Rr, and XA), which represent its own power dissipation, radiated energy, and near-field energy stored by the antenna, respectively. Below, we will discuss further on the transducer and the antenna, respectively.
As shown in Figure 2, the weaker the power supply of the converter, the less energy is radiated. Ideally, there is no impedance between the input and output of a non-isolated converter because ground impedance is ignored. In this case, the equivalent source (VCM) is zero and no EMI radiation is generated. In practice, however, the PCB traces between ground create inductance and create a voltage drop between the input (P1) and the output (P3), resulting in radiated EMI.
Figure 2: Ideal and practical buck-boost converter circuit models
Based on the above principles, let’s create a radiated EMI model. Similar to the conducted EMI model, the active switch is replaced with a voltage source and a current source based on the substitution theorem. As shown in Figure 3, both voltage and current sources generate EMI noise, where a is the voltage source (VSW) and b is the current source (ID).
Figure 3: Radiated EMI noise sources for a buck-boost converter
The transfer function from each source to the equivalent CM source can be derived from the model in Figure 3. In this experiment, the voltage source and current source are measured by an oscilloscope, the magnitude of each impedance is measured by an impedance analyzer, and the equivalent source is predicted by calculation. The predictions shown in Figure 4 match the measurements from an equivalent CM source, validating the model.
Figure 4: Comparison of Predicted and Measured Values from Equivalent CM Sources
Fixed antenna transmission gain is measured based on the position and length of the antenna, and these values are also fixed during EMI testing. Using the equivalent CM source and impedance obtained above, the next step is to predict the actual EMI noise. Figure 5 depicts the process of predicting buck-boost converter EMI noise. Figure 5a shows the prediction process of radiated EMI, and Figure 5b shows the comparison of the predicted and measured results. The comparison results confirm that the predicted radiated EMI noise is almost in agreement with the measured value.
Figure 5: Radiated EMI prediction process and comparison of predicted and actual radiated EMI noise
An effective way to suppress radiated EMI is to reduce the ground impedance. For non-isolated converters, there are two commonly used suppression techniques. The first is to minimize the distance between the input and output to reduce the ground trace impedance; the second is to add a small bypass capacitor between the input and output nodes to reduce the impedance between the input and output (see Image 6).
Figure 6: EMI reduction by modifying the PCB layout
Figure 7 demonstrates the effectiveness of these techniques. Among them, Figure 7a compares the original EMI performance with the modified PCB layout between the input and output nodes, and Figure 7b shows the EMI measurement results after adding crossover capacitors. Both techniques are effective in reducing EMI noise.
Figure 7: EMI reduction by improving PCB layout and adding crossover capacitors
This article reviews methods for controlling radiated EMI noise for transducers and antennas, using the physical meaning of their circuit models. Modeling and analysis methods are critical to EMI problems, helping automotive electronics engineers meet EMI requirements and improve safety standards in automotive electronics.