Analysis on Hiding CharacteristICs of Passive components in PCB

This paper uses SIMple mathematical formulas and electromagnetic theory to explain the hiding behavior and characteristics of passive components** PCB **These are the basic knowLEDge that engineers must have in advance when designing electronic products to pass EMC standards Traditionally, EMC has been regarded as a black magic In fact, electromagnetic compatibility can be understood through mathematical formulas However, even if mathematical analysis methods can be used, these mathematical formulas are still too many for actual EMC circuit design It's too complicated Fortunately, in most practical work, engineers do not need to fully understand the complex mathematical formulas and theoretical basis in the **EMC specification**, as long as they use simple mathematical models, they can understand how to meet EMC requirements**PCB board**

1. Trace of wires and PCB boards

The seEMIngly inconSPIcuous parts, such as wires, traces, fixtures, etc., usually become the transmitters of radio frequency energy (i.e. EM I). Each element has an inductor, which includes the bonding wires of silicon chips and the pins of resistors, capacitors and inductors. Each wire or track contains hidden parasitic capacitance and inductance. These parasitic elements affect the impedance of the wire and are sensitive to frequency. According to the value of LC (determining the self resonant frequency) and the length of PCB track, self resonance may occur between components and PCB track, thus forming an effective radiation antenna. At low frequencies, the conductor basically has only resistance. However, at high frequencies, the conductor has inductive characteristics. Because when it becomes high frequency, it will cause impedance changes, and then change the** EMC design **between the wire or PCB track and the ground. In this case, the ground plane and grounding grid must be used. The main difference between a wire and a **PCB trace **is that the wire is circular and the trace is rectangular. The impedance of conductor or track is composed of resistance R and inductive reactance XL=2 Ï fL. At high frequency, the impedance is defined as Z=R+j XL j2 Ï fL, and there is no capacitive reactance Xc=1/2 Ï fC. When the frequency is higher than 100 kHz, the inductive reactance is greater than the resistance. At this time, the conductor or track is no longer a low resistance connecting conductor, but an inductor. In general, the wire or track working above the audio should be regarded as inductive, not as resistance, but as RF antenna. The length of most antennas is equal to 1/4 or 1/2 wavelength of a specific frequency (Î). Recall that in the EMC specification, the wire or trace is not allowed to work at a frequency lower than a certain frequency, because it will suddenly become an efficient antenna. Inductance and capacitance cause circuit resonance, which is not recorded in its specification. Example: If the trajectory is 10 cm, R=57 m Î), 8 NH/cm, the total inductance is 80 NH. 50 m Î at 100 kHz © Inductive reactance of. At frequencies higher than 100 kHz, the trace is inductive and its resistance value can be ignored. This 10 cm track will form an effective radiation antenna at frequencies above 150 MHz. Because at 150 MHz, its wavelength 206206206Î=2m, so 206206206/20=10 cm=track length; If the frequency is greater than 150 MHz, its wavelength will be SMAller, and its 1/4 or 1/2 value will be close to the length of the track (10 cm), thus gradually forming a perfect antenna.

2. Resistance

Resistors are common components on PCB boards. The information of resistor (carbon composite material, carbon film, mica, winding wire, etc.) limits the influence of frequency response and EMC. Wirewound resistors are not suitable for high frequency applications because the inductance in the wire is too large. Although the carbon film resistor contains inductance, it is sometimes suitable for high-frequency applications because the inductance value of its pin is small. What most people often neglect is the package size and parasitic capacitance of resistors. Parasitic capacitors exist between the two terminals of a resistor. They can destroy normal circuit characteristics at very high frequencies, especially at frequencies as high as GHz. However, for most applications, parasitic capacitance between resistor pins is not more important than pin inductance. When the resistance is tested for ultra-high voltage limit, the change of resistance must be noted. If electrostatic discharge occurs on the resistor, something interesting will happen. If the resistor is a surface mounted component, it is likely that the resistor will be arced. If the resistor has pins, ESD will find a path of high resistance (and high inductance) for the resistor and avoid entering the circuit protected by the resistor. In fact, the real protector is the hidden inductance and capacitance characteristics of the resistor.

3. Capacitor

Capacitors are typically used in power bus bars to provide decoupling, bypass, and maintain constant DC voltage and current. The truly pure capacitor will maintain its capacitance value until it reaches the self resonant frequency. Beyond this self resonant frequency, the capacitor acts like an inductor. This can be explained by the formula: Xc=1/2 Ï Ì fC, where Xc is the capacitive reactance (in Î). For example, for 10mf electrolytic capacitor, the capacitance reactance is 1.6 Ω at 10 kHz); At 100 MHz, it drops to 160m Î). Recall that the short circuit effect will occur at 100 MHz, which is ideal for EMC. However, the power parameters of the electrolytic capacitor: equivalent series inductance and equivalent series resistance will limit the capacitor to operate only below 1 MHz. The use of capacitors is also related to pin inductance and volume structure, which determines the number and size of parasitic inductance. There is parasitic inductance between the bonding lines of the capacitor. When the capacitor exceeds the self resonant frequency, the parasitic inductance will cause the capacitor to behave like an inductor and lose its original function.

4. Inductance

Inductors are used to control electromagnetic interference in PCB. For an inductor, its inductive reactance is proportional to the frequency. This can be explained by the following formula: XL=2 Ï ν FL, where XL is the inductive reactance (in Î). For example, the ideal 10 mH inductance is 628 Ω at 10 kHz); At 100 MHz, it is added to 6.2 M Î). This inductor can be regarded as open circuit at 100 MHz. At 100 MHz, the signal passing through the inductor will cause a degradation of the signal quality (observed in the time domain). Like the capacitor, the electric parameters of the inductor (parasitic capacitance between coils) limit the working frequency of the inductor to less than 1 MHz. The question is, if you can't use an inductor at high frequencies, what should you use? The answer is to use iron powder beads. The iron powder data are iron magnesium or iron nickel alloys, which have high permeability. At high frequency and high impedance, the capacitance between coils in the inductor will be reduced. Iron powder beads are usually only applicable to high-frequency circuits, because at low frequencies, they basically retain the complete characteristics of the inductance (including resistance and reactive components), which causes some losses on the meeting line. At high frequency, it basically has only one resistance component (j Ï L), and the resistance component increases with the frequency. In fact, iron powder beads are high-frequency attenuators of RF energy. In fact, the iron powder ball can be regarded as a resistor in parallel with the inductor. At low frequency, resistance is induced (short circuit) and current flows to inductor; At high frequencies, the high inductive reactance of the inductor forces current through the resistor. In essence, the iron bead is a dissipative device that converts high-frequency energy into heat. In terms of efficiency, it can only be interpreted as resistance, not inductance.

5. Transformer

The transformer is usually present in the power supply. In addition, they can be used to isolate data signals, I/O connections, and power interfaces. Depending on the type and application of the transformer, there may be a mask between the primary and secondary windings. The shield is connected to the grounding reference to prevent capacitive coupling between two groups of coils. Transformers are also widely used to provide common mode insulation. These devices transmit energy through magnetic connection side and secondary coil according to the differential mode signal input through them. In this case, the common mode voltage passing through the side coil will be rejected, so as to achieve the purpose of common mode insulation. However, during the transformer manufacturing process, there is a signal source capacitance between the primary and secondary windings. When the frequency of the circuit increases, the capacitive coupling capacity will also increase, thus damaging the insulation effect of the circuit. If there is enough parasitic capacitance, **high-frequency RF **energy (from fast transient, electrostatic discharge, lightning stroke, etc.) may pass through the transformer, causing the circuit on the other side of the insulation to also receive the transient high voltage or high current. The above explains the hiding characteristics of various passive components in detail, and the following explains why these hiding characteristics will cause electromagnetic interference in **PCB**.

6. On electromagnetic theory

All substances have compositional relationships with other substances. These include:

1) Conductivity: relationship between current and electric field (Ohm's law of matter): J=Ï E.

2) Permeability coefficient: relationship between magnetic flux and magnetic field: B=mH.

3) Dielectric constant: relationship between charge storage and electric field: D=Î µ E.

J=conduction current density, A/m2

Ï = Conductivity of substance

E=electric field strength, V/m

D=electric flux density, coulombs per square meter

Î µ=vacuum dielectric constant, 8.85 pF/m

B=magnetic flux density, Weber/m2 or Tesla

H=magnetic field, A/m

M=permeability of medium, H/m

According to Gauss law, Maxwell equation is also called separation theorem It can be used to explain the electrostatic field E This phenomenon is observed between two boundaries: conductive and non-conductive According to the Gauss law, behavior under boundary conditions products a conductive cage (also called a Faraday cage) that acts as an electrostatic shield In the enclosed area surrounded by Faraday box, electromagnetic waves from the surrounding outside cannot enter the area If there is an electric field in the Faraday box, at its boundary, the charges generated by the electric field are concentrated within the boundary The charge outside the boundary is rejected by the internal electric field on it