**4. Methods to improve EMI performance of power converters**

#### **4.1 With** *RC* **snubber**

In the previous section, it has been shown that the ringing in the current (and voltage) waveforms degrades the EMI performance of the circuit. Therefore, damping out the ringing will improve the performance. A simple method to achieve the same is the use of an *RC* snubber. Considering the case of a buck converter, the snubber is connected across the freewheeling diode (snubber is to be connected between switching node and ground), as shown in **Figure 8**. The values of the snubber elements *Rs* and *Cs* are computed as follows:

*Cs*≈3 times the diode junction capacitance. Since the junction capacitance is a varying quantity (with junction voltage), the maximum value corresponding to zero bias value, i.e., *CJ*<sup>0</sup> is considered for calculation. Therefore, *Cs*≈3 480 *pF*. This results in a value of *1.44 nF,* and so a value of *2 nF* is selected.

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

*Rs*≈2*ω*0*Lp* where, *ω*<sup>0</sup> is the angular frequency corresponding to the ringing oscillations to be damped and *Lp* is the parasitic inductance, *1 nH* at present.

The ringing oscillations in the current waveform shown in **Figure 7(a)** have a frequency of ≈500 *MHz*. With these values, *Rs* is computed to be *6.28 Ω,* and therefore, the value of *10 Ω* is selected.

With the snubber elements included in the buck converter circuit as shown in **Figure 8**, the input current is computed and is shown in **Figure 9(a)**. Also shown in the Figure is the current without snubber (shown earlier in **Figure 7(a)**) for comparison.

Clearly, the snubber results in damping out the ringing at the rising edge of the input current waveform. The same inference can also be drawn from the frequency domain characteristics shown in **Figure 9(b)**. The snubber has clearly damped out the resonant peak due to the high-frequency ringing. With snubber included in the circuit, the frequency spectrum plot clearly shows a reduction in the spectral amplitude of the high-frequency components, and hence an improvement in the emission performance.

It is also important to check the power loss in the snubber resistor and its impact on the efficiency of the circuit. The power loss is given by [5], *fswCfV*<sup>2</sup> where, *fsw* is the switching frequency (100 kHz), *V*is the peak voltage which appears across the diode terminals (approximately equal to the input voltage *Vdc*).

The power loss is calculated and found to be *28.8 mW*. The output power is ð Þ <sup>0</sup>*:*4�<sup>12</sup> <sup>2</sup> <sup>5</sup> <sup>¼</sup> <sup>4</sup>*:*<sup>6</sup> *<sup>W</sup>*. Therefore, the power loss in the snubber is <sup>28</sup>*:*2�10�<sup>3</sup> <sup>4</sup>*:*<sup>6</sup> � 100 ¼ 0*:*625% of the power delivered at the output and can be neglected for all practical purposes.

**Figure 8.** *Buck converter with input side* RC *snubber.*

#### **Figure 9.**

*Input currents in buck converters with and without snubber. (a) Temporal variation of input current and (b) comparison of frequency domain characteristics.*

Although the snubber improves the EMI performance of the converter, designing the snubber elements can prove to be difficult since it is difficult to estimate the parasitic inductance accurately and switch capacitance and the maximum voltage at the switching node (due to overshoot). In addition, the snubber damps out or attenuates the high-frequency oscillations, which take place at (or around) a certain frequency. If all high-frequency components (above a certain cutoff or threshold frequency) could be attenuated, further improvement in the EMI performance can be obtained. This can obviously be achieved by using a low pass filter. This is discussed in the following.

### **4.2 With damped** *LC* **filter**

One popular method to reduce emissions from power converter and improve their EMI performance is using filters. Considering the example of the buck converter shown in **Figure 6**, a damped *LC* filter is added on the input side, as shown in **Figure 10**.

The filter elements are the inductance *L <sup>f</sup>* , capacitance *Cf* and the damping resistor *Rf* . The value of the damping resistor is selected from the condition *Rf* << *<sup>R</sup>=D*<sup>2</sup> [6]. Selecting *Rf* <sup>¼</sup> <sup>1</sup> *<sup>Ω</sup>* easily satisfies the criteria. The values of *<sup>L</sup> <sup>f</sup>* and *Cf* are selected to be *100 μH* and *100 pF,* respectively. With these values, the resonant frequency is ≈5 *MHz*. This is sufficiently away from the switching frequency of *100 kHz* and therefore does not slow down the converter's response. However, it provides sufficient attenuation to the high-frequency components and, therefore, is expected to improve EMI performance. To validate the same, the buck converter with the input side filter is simulated, and the input current *Isw* is obtained. This is shown in **Figure 11 (a)**, and the input current without the filter is shown in **Figure 7(a)** for a practical converter. The frequency spectrum of the currents with and without filters is shown in **Figure 11(b)**.

In the time domain current waveform, the effect of the damped *LC* filter in reducing the oscillation is clearly noticeable. The amplitude of the overshoot is also observed to have reduced. The attenuation provided to the high-frequency components can be seen from the frequency response. The reduction in the amplitude at some of the high-frequency components is around *20–30 dB.* This is a significant improvement in the performance of the converter operation from the EMI/EMC point of view. Also, it is worthwhile to note that the damped *LC* filter attenuates both the resonant peaks present in the frequency characteristics. This can be compared with the method employing snubber elements (**Figure 9(b)**), where only one peak has been attenuated.

However, it is important to mention here that the *LC* filter changes the converter dynamics, often leading to a significant degradation in the transient response. Moreover, for converters operated under closed-loop control, the addition of the filter may even make the control system unstable [7]. Therefore, the

**Figure 10.** *Buck converter with input side damped LC filter.*

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

#### **Figure 11.**

*Input currents in buck converters with and without input side filter. (a) Temporal variation of input current and (b) comparison of frequency domain characteristics.*

design of the filter is not a straightforward task, and the converter performance, closed-loop response, and EMI specifications must all be considered in satisfying all the requirements.

So far, the mechanism behind emissions from power converters, along with some of the important factors responsible for degrading the EMI performance, has been discussed. In addition, some popular methods to improve the EMI performance have also been discussed. In the following sections, the measurement of the emissions and analysis of the same through standard receivers will be briefly discussed.

#### **5. Measurements of electromagnetic emissions**

Measurements of emissions are performed to test for EMC compliance of any product. Therefore, the measurements must be carried out so that the results are easy to correlate across different measurement sites or EMC labs. To understand this, assume that the noise current injected by a certain product into the power supply needs to be measured for compliance. This could be conveniently carried out by using a current probe. However, the noise current injected into the supply depends on the equivalent impedance measured between the supply's terminals. This impedance varies over frequency and will be different at different places, time of the day, seasons, etc. As a result, the noise current will also vary, and therefore the measurement results cannot be correlated.

In order to eliminate the above uncertainties, a Line Impedance Stabilization Network (LISN) is employed for the measurement of Conducted Emissions (150 kHz- 30 MHz) [8]. The block diagram of the measurement setup is shown in **Figure 12**.

The LISN is connected between the supply lines and the product, usually referred to as Device under Test (DuT) or Equipment under Test (EuT). LISN has the following two purposes [8]:


Therefore, with **Figure 12**, only the noise current from the DuT flows through the LISN to the supply terminals. Since the impedance is fixed, a linearly related equivalent noise voltage drop is produced at the LISN, which is then fed to a measuring receiver (EMI receiver). The receiver's measured noise is then analyzed, which will be briefly discussed in the following section.

In the case of radiated emissions, the emissions or noise is coupled directly through the air. Hence an antenna is used to capture the emissions, which are then fed to the measuring receiver for analysis. The block diagram is shown in **Figure 13**.

In the measurement of radiated emissions, typically, over the frequency range of 30–200 MHz, the biconical antenna is used, from 200 MHz – 1 GHz, a log-periodic antenna is employed, and beyond 1 GHz, horn type antennas are used. Discussion on antennas is beyond the scope of the present chapter. However, more detailed discussions are available in the literature [8].

The emission or noise measured by the antenna is sometimes passed through a pre-amplifier stage before feeding to the EMI receiver. The noise emitted by the devices or equipment must be within limits set by the standards. In the United States, the Federal Communications Commission (FCC) limits are required to be adhered to whereas, in most of the European countries, the limits specified by the International Special Committee on Radio Interference (CISPR) are followed. In order to test whether the device or equipment emits the noise, lies within the limit, the measured noise (by LISN or Antenna) is analyzed by a measuring receiver or EMI receiver. The standards also stipulate the analysis procedure to be followed by

**Figure 12.** *Block diagram of conducted emission measurement setup.*

**Figure 13.** *Block diagram of radiated emission measurement setup.*

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

such a receiver. The receiver, according to CISPR 16–1-1 standards, is briefly discussed in the following.

### **5.1 EMI receiver according to CISPR 16–1-1**

As specified by the CISPR 16–1-1 standard, a measuring EMI receiver must be employed to obtain the noise produced by the DuT. The receiver is coupled to different devices (LISN for CE or different antennas for RE) depending upon the type of emission to be tested. The input impedance of the receiver must be 50 Ω or as close as possible. Otherwise, impedance mismatch might lead to standing waves which will introduce error in the results. The receiver can be operated in different modes, such as Peak, Quasi-Peak, Average, or RMS. The modes are selected based on the type of signal to be measured [9].

In the following, the peak detector mode is briefly discussed since it can be used for different types of input signals and indicates the worst-case scenario. In other words, if the emission of equipment is above the stipulated limit in the peak detector mode, it will fail all the other modes as well. Discussion and modeling of the receiver in quasi-peak, average and RMS modes can be found in literature [10, 11].

The receiver frequency range is divided into a number of different bands. Each band has its range as well as bandwidth for the Intermediate Filter (IF). **Table 1** specifies the important requirements for the receiver in peak detector mode.


**Table 1.**

*Requirements of CISPR 16–1-1 EMI receiver in peak detector mode.*

**Figure 14.** *EMI receiver output for the input current to the buck converter.*

The input time-domain signal to the EMI receiver is typically transformed to the frequency domain through the FFT algorithm. Thereafter, the windowing operation is performed with the IF filter, determined by the band specified in **Table 1**. An extremely short time constant for charging and a very long time for discharging is employed for the peak detector mode. For the other modes, the time constants are specified in the standard. The output of the peak detector stage is obtained across the frequency range of the EMI receiver to produce the emission result for the DuT [6].

As an example, consider the input currents to the buck converter shown in **Figures 7(a)** and **11(a)**, i.e., without and with the damped input filter, respectively. The noise currents are analyzed with the CISPR 16–1-1 EMI receiver (numerically implemented). The results are presented in **Figure 14**, up to 1 GHz.

The improvement in the emission due to the buck converter with the damped *LC* filter is clearly observed in the EMI receiver output. The output of the EMI receiver can be compared with the limits set by the standards to test if the equipment is compliant with the EMC standards or not. Since the standardized procedure is followed, the results can be correlated across different sites.

### **6. Conclusion**

In this chapter, the emission from power electronic converters has been discussed. Beginning with a discussion on the origin of the emission, which was attributed to the rapid switching transients taking place during the operation of the switches, the effect of the device characteristics as well as circuit parasitic elements were analyzed. At each step, through simulations and computations, the effects were demonstrated to degrade the converter's EMI performance compared to an ideal buck converter performance. Thereafter, a couple of methods employed to improve the EMI performance were discussed and demonstrated.

The importance of stipulating a standard procedure for measuring and analyzing the emissions was briefly discussed. The different coupling devices used in measuring emissions over different frequency ranges were introduced along with the measuring EMI receiver according to CISPR 16–1-1 standard. The results of the buck converter currents were analyzed with a numerically modeled EMI receiver as per the standard, and it was shown that the input filter dramatically improves the EMI performance of the power converter. Finally, since the switching transient is common to all power electronic converters, the discussions hold good for power converters in general.
