**2.2 Measures used to eliminate EMI**

The technical measures used to reduce EMI in any EMC chain are called interference suppressors. Interference suppressors include filters, surge protection devices (SPDs), shielding, and grounding. Through these measures, the electromagnetic coupling between the source and the victim is eliminated. **Figure 4** shows the relationship between the EMI source and the victim via coupling.

SPDs eliminate the destructive effect of current pulses and high-energy voltage pulses. EMC grounding diverts interfering signals from the victim, creates a reference ground plane on PCBs, grounds the cable shield and metal enclosures for safety purposes. The shielding attenuates EMI propagated by radiation.

<sup>2</sup> The LISN is a low pass filter located between the AC or DC power source and the DUT to create a known impedance and provide a port for measuring HF noise. The main function of the LISN is to provide an accurate impedance to the EUT power port. It also isolates unwanted radio-frequency signals from a power source.

*EMI Pre-Compliance Measurements Reveal Sources of Interference DOI: http://dx.doi.org/10.5772/intechopen.99754*

In EMC, the couplings are classified as follows:


Knowing the principle of how coupling works using equivalent models, a procedure can make to reduce coupling [11]. **Figure 5** shows simplified equivalent circuits for galvanic and inductive coupling between the source and the victim. The magnitude of the interfering voltage on the series impedance according to **Figure 5a** can calculate as follows

$$
\mu\_{\rm EMI}(t) = R \cdot i(t) + L \cdot \frac{\rm di}{\rm dt} \tag{1}
$$

where *R* is the resistance of the conductor, *L* is the self-inductance of the conductor, and *i*(*t*) is the disturbing current. According to Eq. (1), several measures can perform to eliminate the interfering voltage. For example, reduce the voltage drop across resistor *R* and inductance *L*. The resistance *R* can reduce by shortening the length of the conductors, reducing the number of bends, or increasing the crosssection of the conductors. These resistivity reduction measures follow from Eq. (2)

$$R = \rho \cdot \frac{l}{S} \tag{2}$$

where *l* is the length of the conductor, *S* is the conductor cross-section, and ϱ is the specific resistivity of the conductor. The voltage drop across the reactance component is done by reducing the inductance *L* or reducing the time changes (d*i*(*t*)*/*d*t*)

**Figure 5.**

*Equivalent circuit: Galvanic coupling (a) and inductive coupling (b); R – Resistance, L – Self-inductance, M12 – Mutual inductance between wires, i – Disturbing current, uEMI – Disturbing voltage, H – Magnetic field strength.*

of the interfering current. The magnitude of the interfering voltage across the inductive coupling can calculate as follows

$$
\mu\_{\rm EMl}(t) = M\_{12} \frac{\rm di}{\rm dt} \tag{3}
$$

where *i(t)* is the disturbing current and *M*12 is the mutual inductance between wires. The mutual inductance between the conductors is bound to the magnetic flux as follows

$$\mathbf{O}\_{12} = B\_1 \cdot \mathbf{S}\_2 \tag{4}$$

where *S*2 is the area bounding the victim's circumference, and *B*1 is the magnetic flux density generated by the EMI source. Based on (Eqs. (3) and (4)), the following measures can use to reduce the inductive coupling: reducing the mutual inductance by shortening the length of the parallel conductors, increasing the mutual distance between the circuits, orthogonal distribution of the circuits, and others. Similarly, an analysis of equivalent circuits for capacitive coupling and electromagnetic coupling can perform.

## **3. Precompliance measurements examples**

In the following section, we will show three examples of pre-compliance measurements. The first case will concern the detection of EMI sources in a prototype of an LED street light. The second case will matter the detection of EMI sources from the LED information board. Finally, the third case points to unexpected changes in EMI radiation during the development of audio equipment.

#### **3.1 Prototype of a LED street light**

The best choice to achieve the highest system efficiency is a switched-mode power supply. Correcting the harmonic distortion of the input line current is one of the main goals in designing the street light power supply. For the LED street light EN 61000-3-2, Class C applies at full load [12, 13]. EN 55015 is a product family standard (largely based on CISPR 15). Key EMC standards include EN 61000-3-2 for limits on harmonic current emissions, EN 61000-3-3 for limits on voltage changes, voltage fluctuations, flicker, and EN 61547 for immunity requirements.

The pre-compliance measurements realized on the prototype of LED street light focus on the power line conducted emissions and harmonic current emissions. The power line conducted emissions measured on the first 60 W LED driver shows **Figure 6**. It is clear from **Figure 6** that the level of emission of the LED driver is below the limits specified in the standard EN 55015. Therefore, a peak detector in a frequency range from 9 kHz to 30 MHz is used.

**Table 1** lists harmonic currents generated by the 60 W LED driver. The power factor (PF) is 0.444, and the corresponding limit for the 3rd harmonics is 13.32%. The third column of **Table 1** lists the multiples of limits. The corresponding average total harmonic distortion (THD) of the current generated by the 60 W LED driver is equal to 181.33% the maximal THD is 181.73%.

#### **Figure 6.**

*Limits and measured EMI level of the conducted emissions on the first 60 W LED driver between the phase conductor and earth.*


#### **Table 1.**

*Measured harmonic current emissions of the 60 W LED driver – Class C, power factor PF = 0.444.*

It is clear from **Table 1** that the harmonic currents generated by the 60 W LED driver far exceed the limits set by the standard. This fact is confirmed by the time course of the terminal voltage and the current flowing into the first 60 W LED driver shown in **Figure 7**. Measured values of the odd harmonics extremely exceed the specified limits, thus resulting in the high value of THD of the supply current and low power factor.

**Figure 7** shows the time course of the terminal voltage and current flowing into the 60 W LED driver and the THD of the current through the 60 W LED driver and corresponding PF measured for the first 2500 minutes of measurement. It is clear that the current time course in **Figure 7** is periodic but does not have a sinusoidal shape. The deformation of the current waveform is due to harmonic currents emission from the LED driver.

For comparison, **Table 2** lists the harmonic currents generated by the 50 W LED driver. Measured levels of the conducted emissions on the 50 W LED driver

#### **Figure 7.**

*Time course of the terminal voltage and current flowing into the 60 W LED driver during one period (left), and corresponding THD and PF measured in phase conductor during 2500 minutes (right).*


#### **Table 2.**

*Measured harmonic current emissions of the 50 W LED driver – Class C, power factor PF = 0.991.*

#### **Figure 8.**

*Time course of the terminal voltage and current flowing into the 50 W LED driver during one period (left), and corresponding THD and PF measured in phase conductor during 600 seconds (right).*

between the phase conductor and earth need not indicate as they are below the specified limits. It is clear from **Table 2** that the 50 W driver meets the requirements for harmonic current emissions with a sufficient margin. The corresponding average THD of the current generated by the 50 W LED driver is equal to 10.73%. The maximal THD is 10.77%.

**Figure 8** shows the time course of the terminal voltage and current flowing into the 50 W LED driver and the trend over time of the THD of the current through the 50 W LED driver and corresponding PF measured for the first 600 seconds of measurement.

### *EMI Pre-Compliance Measurements Reveal Sources of Interference DOI: http://dx.doi.org/10.5772/intechopen.99754*

By comparing the measured data, it can conclude that all LED drivers comply with the standard EN 55015 with a sufficient margin and are not sources of EMI. However, as for the harmonic current emissions, the measurements have shown the lack of some drivers. The reason lies in permissible limits according to the standard EN 61000-3-2 were exceeded. Further, the THD of the supply current is high, and the measured PF is low. Therefore, harmonic current emissions are due to either improper circuit design of the LED driver or wrong LED driver design concerning the rated load of the LED street light.
