Introduction to EMC

1.1 Problems and definitions

All operational electrical or electronic devices produce interference, which may affect the operation of the device itself and/or that of nearby electrical or electronic equipment. Electromagnetic compatibility (EMC) is a domain which is concerned with the coupling of devices and aims to use all possible means to guarantee the “harmonious” operation of a set of nearby, or coupled, equipment. EMC may be compared to a set of rules for “peaceful coexistence”, and is based on a set of standards that must be respected. EMC includes both a scientific aspect, which consists of studying the way in which a device interferes with (or pollutes) its environment, via different types of connections to the “victims”, and a standardization aspect, concerning the specification of acceptable thresholds for interference emission, and of sensitivity thresholds at victim level.

The nature of interacting equipment is highly variable. In some cases, elements are truly separate (for example, a television and a telephone); however, they may also form part of the same device (for example, the power supply and motherboard of a personal computer (PC)). Generally speaking, interference may propagate along electric wires (or PCB tracks): this is known as conducted interference. It may also propagate through empty space (i.e. air or a vacuum) in the case of radiated interference.

“Low-frequency” interference is essentially propagated toward victims by conduction, while higher frequencies are mostly propagated by radiation, as the use of filters allows us to prevent their propagation by conduction. This method is relatively cheap (or natural, given the inductive behavior of connection wires and, for example, the capacitive character of PCB tracks with ground and power supply planes). However, further study is required, as components (inductances and capacitors) are not always able to operate at the frequencies in question.

Conducted and radiated interference will be covered in detail in Chapters 2 and 3; in the case of conducted interference, particular attention will be given to the spectral breakdown of interference (notably for applications connected to the 50 Hz) network:

− electrostatic interference (static electricity, a type of interference often ignored in power electronics);

− very low-frequency interference (flicker, < 10 Hz);

− “low-frequency” harmonic interference, of the order of a few multiples of 50 Hz;

− “medium-frequency” interference, linked to the switching frequency (and to its first multiples, for example, from 10 to 100 kHz for industrial speed variation drives);

− high-frequency (HF) interference, linked to the switching time in the switches (> 1 MHz);

− environmental interference (cosmic or solar radiation, lightning).

Static interference and very low-frequency interference are specific elements, not directly linked to the switching mechanism; flicker, for example, is linked to variable use (on a human time scale) of electrical energy. “Low-frequency” interference is limited to current switching converters (diodes, thyristors and triacs), and thus also belongs to a specific category.

We will, therefore, focus on the two types of interferences which are most widespread in transistor-based converters (choppers, inverters and switch-mode power supplies) used in forced switching, i.e. “medium and high-frequency” interferences.

Environmental interference and the associated protection equipment will be covered in two specific chapters: one in Chapter 2, in relation to conducted interference, and the other in Chapter 3, concerning radiated interference.

The remainder of this chapter is devoted to sources of interference encountered in power electronics: both “natural” interference (lightning and electrostatic discharge) and artificial interference, created by switching, which is at the heart of the EMC problem for electronic power converters.

1.2 “Natural” interference

1.2.1 Static electricity

When two different materials are rubbed together, static electricity may be produced. This is particularly true in relation to the human body and certain fabrics; as the body behaves in a capacitive manner, discharge may occur on contact with electronic circuits. Fragile and/or poorly protected components may be damaged by this phenomenon, so preventive measures should be taken.

The human body has a surface equivalent to that of a sphere with a diameter of 1 m. Considering the capacitance of a spherical capacitor (with two concentric frames of radius r1 and r2, where r1 < r2), direct application of the Gauss formula gives us an expression of the capacitance C as follows:

=4πε0εr1r1−1r2

In this case, regarding the intrinsic capacitance of the human body, the external frame of radius r2 must be considered to extend to infinity. This gives us the following expression:

∞=4πε0εrr1

As we live in air, r=1, giving a capacitance of 56 pF for r1 = 0.5 m.

This capacitance is clearly affected by proximity to the ground, which adds around 100 pF, and additional capacitances may be added, linked to walls or furniture located close to the body (varying from approximately 50 to 100 pF). This gives an overall parallel association of capacitances of around 200 pF. Note, moreover, that this capacitance is not the only element in the equivalent model of the body when charging or discharging: skin contact is resistive, with a value varying from 500 Ω to 10 kΩ for different individuals and according to the contact surface (the end of a finger or the palm of a hand); this value is also affected by the humidity of the skin. Thus, the human body can be assimilated to a series R, C circuit (an inductance may even be included, with a value of less than 100 nH).

Electrostatic charge can easily reach very high values without the individual in question being aware of it, as voltages under 3.5 kV cannot be felt. Table 1.1 shows two examples of charges produced by walking on two different materials and for two different levels of air humidity.

1.2.2 Lightning

Lightning (see Figure 1.1) is a common natural phenomenon, with an estimated 32 million bolts worldwide each year. Figure 1.2 (a) shows a global map with a color scale representing the number of lightning bolts per square kilometer per year. Note that the majority of lightning occurs close to the equator, and that strikes at sea are considerably rarer (but not unknown). Note, however, that lightning does not simply concern discharge from a cloud to the ground: 60 % of lightning bolts form inside a cloud or between two clouds.

France alone receives 1 million lightning bolts per year on an average. The map in terms...