Categorized by the type of core on which inductors are wound:

Transcription

1 Inductors Categorized by the type of core on which inductors are wound: air core and magnetic core. The magnetic core inductors can be subdivided depending on whether the core is open or closed. Equivalent circuit for an inductor Parallel resonance will occur at some frequency. JHLin, EMC; Passive Components 14

2 Inductors Air core or open magnetic core inductors are most likely to cause interference. The magnetic core is more susceptible to magnetic fields than the air core inductor. As a matter of fact, open magnetic core inductors (rod cores) are often used as receive antennas for small AM radios. It is often necessary to shield inductors to confine H and E fields. JHLin, EMC; Passive Components 15

3 Transformers Actual transformers Faraday shield: a grounded conductor placed between the two windings. As a general rule, the shield should be connected to a point that is the other end of the noise source. JHLin, EMC; Passive Components 16

4 Transformers Two unshielded transformers as electrostatic shielding JHLin, EMC; Passive Components 17

5 Resistors Fixed resistors can be grouped into three basic classes: (1) wirewound, (2) film type, and (3) composition. Equivalent circuit for a resistor In a typical composition resistor, the shunt C is in the order of pf. Except for wirewound resistors or very low value resistors of other types, L can normally be neglected during circuit analysis. 18

6 Resistors The shunt C can be important when high-value resistors are used. Noise in Resistors 1. All resistors, regardless of their construction, generate a noise voltage. JHLin, EMC; Passive Components 19

7 Resistors This voltage results from thermal noise and other sources, such as shot and contact noise. Thermal noise can never be eliminated. Wirewound resistors are the quietest. The composition resistor has the most noise. Also has contact noise. JHLin, EMC; Passive Components 20

8 Resistors The noise produced by film-type resistors, is much less than that produced by composition resistors, but it is more than that produced by wirewound resistors. Another important factor that affects the noise in a resistor is its power rating: the resistor with the higher power rating normally has the lower noise. Variable resistors generate all the inherent noise of fixed resistors, but in addition they generate noise from wiper contact. This additional noise is proportional to current through the resistor and the value of its resistance. JHLin, EMC; Passive Components 21

9 Conductors Conductor characteristics are very important to the noise and high-f performance of electronic circuit. In many cases, they are actually the most important component in the circuit. The inductance is more important than the resistance for a conductor. Inductance of Round Conductors 1. Eqns. (5-1) (5-2) for external inductance. 2. Internal inductance ~ 1.27 nh/in, independent of wire size. 3. Eqn. (5-3) for loop inductance h L 5.071ln nh/in 0.8w t Inductance of Rectangular Conductors for h w. 1. The loop inductance of a rectangular PCB trace JHLin, EMC; Passive Components 22

10 Conductors 2. A flat rectangular conductor has less inductance than a round conductor with the same cross-sectional area. Resistance of Round Conductors 1. Selection of conductor size is generally determined by the maximum allowable dc voltage drop in the conductor. 2. Eqns. (5-6) (5-9). 3. The ac resistance of a conductor is directly proportional to the at high frequency. f Resistance of Rectangular Conductors 1. A rectangular conductor has less ac resistance than a round conductor of the same cross-sectional area. 2. Eqns. (5-10) (5-13). JHLin, EMC; Passive Components 23

11 Conductors 3. High-f current in a rectangular conductor is contained within a thickness of one skin depth of the surface. 4. If the conductor is close to another current-carrying conductor, the ac resistance will be greater than predicted by those equations. Transmission Line section to be skipped. JHLin, EMC; Passive Components 24

12 Ferrites A class of nonconductive ceramics that consists of oxides of iron, cobalt, nickel, zinc, magnesium, and some rare earth metals. The variety of ferrites available is large. Ferrites have one major advantage over ferromagnetic materials: high electrical resistivity that results in low eddy-current losses up into the GHz frequency range. Many different configurations JHLin, EMC; Passive Components 25

13 Ferrites Ferrites provide an inexpensive way of coupling high-f resistance into a circuit without introducing power loss at dc or affecting any low-f signals present. Ferrite beads are small. Ferrites are most effective in providing attenuation of unwanted signals above 10 MHz. (in some applications, they can be effective as low 1 MHz) Both L and R are dependent f. 26

14 Ferrites Impedance data for a typical ferrite core. JHLin, EMC; Passive Components 27

15 Ferrites When used in noise suppression, ferrites are usually used in the frequency range where their impedance is primarily resistive. The most common ferrite geometry used in noise-suppression applications is the cylindrical core or bead. The greater the length of the cylinder, the higher the impedance. JHLin, EMC; Passive Components 28

16 Ferrites To be effective, the ferrite must add an impedance greater than the sum of the source and load impedance, at the frequency of interest. Most ferrites have impedances of a few hundred ohms or less. L-filter to keep high-f oscillator noise from the load JHLin, EMC; Passive Components 29

17 Ferrites Damp out ringing on long interconnection between fast logic gates Suppress parasitic oscillation in horizontal output circuit JHLin, EMC; Passive Components 30

18 Ferrites Eliminate the interference of lowlevel circuits by the high-f communication noise. JHLin, EMC; Passive Components 31

19 Ferrites JHLin, EMC; Passive Components 32

20 Ferrites The ferrite impedance will decrease with increasing current. JHLin, EMC; Passive Components 33

21 Ferrites Ferrite core used as common-mode chokes on a USB cable to suppress radiated emission JHLin, EMC; Passive Components 34