**1. Introduction**

In electrical power systems, a number of potential sources capable of producing electromagnetic disturbances or noise are present. These include natural phenomena like lightning or electrical corona and electronic, especially power electronics circuits and devices. These sources either introduce or draw from the supply terminals, non-sinusoidal currents typically characterized by the rapid rise and fall times, or equivalently, high-frequency components (in MHz or GHz range) in their frequency-domain characteristics. Such currents can influence the operation of other circuits or devices present in the electrical systems in two possible ways:

1.By directly reaching the circuits through closed electrical paths

2.By producing electromagnetic fields of sufficient strength, which may couple with the other circuits directly through the air.

The former is known as conducted emission and has a frequency range of interest between 150 kHz to 30 MHz, whereas the latter is referred to as radiated emission, with the frequency range of interest being >30 MHz. With the proliferation of power electronic devices in everyday life, there is a potential increase for devices to interfere with each other. Applications of power electronics in the present times are abundant. This includes everyday household use in laptop and mobile battery chargers, Uninterrupted Power Supply (UPS), induction cooking, etc. Furthermore, in industrial applications such as control of motor drives or railway traction systems, power transmission systems such as Flexible Alternating Current Transmission System (FACTS) and High Voltage Direct Current (HVDC) transmission of electric power, utilization of power electronic systems can be found. In addition, with power electronics finding increased applications in renewable energy systems in recent times, the electromagnetic compatibility of power converter systems is a significant concern.

In view of the above, the present chapter will focus on understanding the origin of the electromagnetic noise from power converters, some methods to reduce emissions, and measurement procedures followed to quantify the noise.

#### **2. Origin of electromagnetic noise from power converters**

Power electronic converters employ semiconductor switching devices, which are operated in a manner required to produce the desired output. For example, consider the DC-DC Buck converter, the basic topology of the same has been shown in **Figure 1**. The buck converter is used to step down the input DC voltage to a lower level at the output. A controlled semiconductor switch, typically a MOSFET, is periodically turned ON and OFF to achieve the purpose.

During every switching action, the currents through certain elements or voltages at certain circuit nodes undergo rapid transition. For example, assuming every element, including the switches shown in **Figure 1**, to be ideal, the input current waveform *Isw* for an ideal buck converter is a periodic pulse train, as shown in **Figure 2(a)**. The current undergoes rapid transitions to zero when the controlled switch is turned OFF and similarly rises rapidly (to the load current) when the switch is turned ON [1].

**Figure 2(a)** shows the current waveform for a switching frequency of *100 kHz*, duty cycle *D* of 0.4, a rise time of *25 ns,* and fall time of *100 ns* of the PWM pulse (control signal to switch). The FFT of the current is shown in **Figure 2(b)**. It can be observed that even though the switching frequency is only *100 kHz,* the current

**Figure 1.** *Buck converter topology.*

*Power Converters Electromagnetic Emissions with Methods to Measure, Compare and Reduce… DOI: http://dx.doi.org/10.5772/intechopen.99711*

**Figure 2.**

*Input current to the ideal Buck converter. (a) Time domain waveform (b) frequency Spectrum of the currents.*

waveform has significant spectral amplitude even in the MHz range (highfrequency components).

Typically, the DC input *Vdc* to the converter is obtained by rectifying the *220 V or 110 V* (50 or 60 *Hz*) AC supply. Therefore, the non-sinusoidal (pulse train) current drawn by the buck converter is reflected on the AC supply side, and the high-frequency components can flow through the electrical paths reaching various other neighboring devices and circuits connected to the supply. This can result in electromagnetic interference, and since the coupling of the electromagnetic noise takes place through closed electrical paths, this forms an example of *conducted emission* (CE), introduced earlier.

In practice, the entire circuitry contains numerous metallic or conducting paths and connections. Although, for example, the input DC supply (and even the load) is connected to the converter through a cable harness, the converter layout could be realized in a Printed Circuit Board (PCB) in which the PCB traces provide the connections. In addition, there are various metallic connectors and wires present in the entire circuit. When the high-frequency currents flow through these conducting elements, some of them (depending on the dimensions and frequency or wavelength) behave like radiating antennas. In such situations, the electromagnetic fields may couple directly through the air between two different circuits. This is the case of *Radiated Emission* (RE) which has earlier been introduced.

The above analysis considering the ideal buck converter topology clearly demonstrates that the switching transients that take place in a power electronic converter can lead to electromagnetic interference through conducted and radiated emissions. Although the buck converter topology has been used as an example for demonstration, it is clear that since the switching transients take place in all power electronic converters, electromagnetic noise is produced by all power electronic converters.

Although the ideal buck converter is a good example to consider for a first analysis, the situation is much more complicated in practical circuits, chiefly due to the presence of parasitic elements. Some of these aspects are discussed in the following section.

#### **3. Effect of non-linearities and parasitic elements**

The power semiconductor switches are inherently non-linear in their operational characteristics. These non-linearities lead to additional system dynamics during the switching transients. This results in worsening of the EMI performance of practical converters compared to analysis carried out for ideal converters.

To demonstrate the above, the buck converter shown in **Figure 1** is simulated again. This time, however, the freewheeling diode is no longer assumed to be ideal. Instead, the well-known quasi-static model (diode models used in LTSpice), presented in **Figure 3**, is employed to obtain the terminal characteristics of the diode [2]. The equations of the model have also been presented below [2]. For more advanced diode models, the reader may refer to recent literature [3, 4].

The model provides the current *id* through the diode and the voltage across its terminals *vd*. To compute*id*, the saturation current *Is*, emission coefficient *N* and the threshold voltage *VT* need to be known. The model also includes a series resistor *Rs* and a non-linear junction capacitor *CJ*. The junction capacitance is a function of the voltage *vd*, and can be computed by knowing the zero-bias junction capacitance *CJ*0, junction grading coefficient *M* and the junction potential *VJ*. For the diode considered, MBRS340, the values as obtained from the LTSpice library are; *Is* ¼ 22*:*6 *μA*, *N* ¼ 1*:*094*, Rs* ¼ *:*042 *Ω, CJ*<sup>0</sup> ¼ 480 *pF, M* ¼ 0*:*61*.VT* is taken to be *26 mV* and *VJ* ¼ 1 *V*, the default value.

The buck converter is simulated with the above diode model, keeping the switching frequency, duty cycle, and the load same as that of the ideal converter analysis. The simulated input current waveform obtained is shown in **Figure 4(a)**.

There is a sharp spike observed in the current waveform at the rising edge in the present case. This is not present in the ideal converter input current shown in **Figure 2(a)**. **Figure 4(b)** shows the rising edge of both the input current and current through the diode are plotted together. The sudden spike in the diode current is required to charge the junction capacitance when the controlled switch (MOSFET) is turned on and the diode is, turned off. This current is drawn directly from the DC supply *Vdc* and therefore reflects as a spike in the input current waveform. The effect of this spike on the emission performance can be clearly understood by comparing the frequency domain characteristics of the input currents in the ideal and practical buck converter cases. For example, the FFTs of the two currents (shown in **Figures 2(a)** and **4(a)**) are shown in **Figure 5**.

From **Figure 5**, it is clear that for the practical converter, the spectral amplitude of the high-frequency components has increased by many times. The difference is even a few tens of *dB* in the higher frequency range. Therefore, it is clearly seen that the nonidealities of the power converter switches can play an extremely important role towards the magnitude of the electromagnetic emissions from the power converter circuit.

The situation in practical converters is further complicated by the parasitic elements which are inadvertently present in the circuits. To demonstrate this, a parasitic inductance of *1 nH* is considered on the converter input side, as shown in **Figure 6**. Such inductances arise due to the connecting cables or wires, PCB traces, contacts, and connectors, etc., and are unavoidable in the practical circuits.

**Figure 3.** *Model of freewheeling diode.*

*Power Converters Electromagnetic Emissions with Methods to Measure, Compare and Reduce… DOI: http://dx.doi.org/10.5772/intechopen.99711*

#### **Figure 4.**

*Simulated currents for the practical Buck converter. (a) Input current and (b) input current and the freewheeling diode current.*

#### **Figure 5.**

*Frequency domain characteristics of the input currents to ideal and practical buck converters.*

#### **Figure 6.** *Buck converter with input side parasitic inductance.*

Keeping everything else unchanged, the above circuit is simulated, and the input current is shown in **Figure 7(a)**. It is observed that the input current contains a damped, high-frequency oscillatory response at the rising (and falling) portions. Such oscillations are known as ringing and are due to the L-C resonance between the parasitic inductance and the diode junction capacitance. The frequency-domain

**Figure 7.** *Buck converter with input side parasitic inductance and diode effects considered. (a) Temporal variation of input current and (b) comparison of frequency domain characteristics.*

characteristics are shown in **Figure 7(b)** along with the earlier two cases for comparison. The spectral amplitude of the current with parasitic inductance (plus diode) is the highest. In addition, resonant peaks are clearly observed in the highfrequency end of the spectrum. Therefore, the parasitic elements increase the highfrequency components of the input current and, correspondingly, the emissions from the power converter. These are, therefore, serious considerations in assessing the EMI performance of the converter.

In practice, it is often necessary to reduce the emission from power converters in order for the design to be EMC compliant. Damping out the ringing shown in **Figure 7(a)** is extremely important in this regard.

The discussion so far has been carried out towards understanding the origin of the electromagnetic noise (emissions) from power converters. In addition, the effects of parasitic elements and switching device features have also been examined. It is quite clear that electromagnetic noise from power converters is unavoidable due to their intrinsic switching operations. However, the design must also be EMC compliant. With this view, the discussion will next be directed towards methods to improve the EMI performance of power converters.
