Chapter 3 G rounding Grounding Electromagnetic Compatibility Compatibility Engineering by Henry W Ott.

Transcription

1 Chapter 3 Grounding Electromagnetic Compatibility Engineering by Henry W. Ott

2 Introduction Grounding is one of the primary ways of minimizing unwanted noise and of producing a safe system. A good ground system must be designed. A well-designed ground system can often provide protection against unwanted interference and emission, without any additional per- unit cost to the product. The word ground can mean many different things to many different people. Two categories of grounds: (1) safety grounds, (2) signal grounds. Item (2) is better termed as signal returns --- signal returns and power returns. In most cases, safety grounds do not carry current, except during a fault. 2

3 Introduction In addition: (1) chassis grounds, (2) earth grounds. Safety grounds are usually connected to the earth or some conducting body that serves in place of the earth. Signal grounds may or may not be connected to the earth. Basic objective of grounding: First make it safe and then make it work properly without compromising the safety. 3

4 AC Power Distribution and Safety Grounds National Electric Code (NEC) in the United States. Changes to the code are made every 3 years. The basic purpose of power system grounding: protect personnel, animals, structure, and buildings from harm because of electrical shock or fire. In facility wiring, i this is usually accomplished by: 1. Insuring the operation of a protective device in the event of a fault. 2. Minimizing i i i the V between conductive enclosure and other metal parts. 3. Providing lightning protection. Service Entrance Branch Circuits Noise Control 4

5 AC Power Distribution and Safety Grounds Earth Grounds Isolated Grounds Separately Derived Systems Grounding Myth 1. The earth is a low-impedance path for ground current. 2. The earth is an equipotential. 3. The impedance of a conductor is determined by its resistance. 4. To operate with low noise, a circuit or system must be connected to an earth ground. 5. To reduce noise, an electronic system should be connected to a separate quiet ground by use of a separate, isolated ground rod. 6. An earth ground is unidirectional, with current only flowing into the ground. 5

6 AC Power Distribution and Safety Grounds 7. An isolated receptacle is not grounded. 8. A system designer can name ground conductors by the type of the current that they should carry, and the electrons will comply and only flow in the appropriately p designated conductors. 6

7 Better definition for a signal ground: a low-impedance path for current to return to the source. Three basic objectives of signal grounding: 1. Not to interrupt the ground return path. 2. Return the current through the smallest loop possible. 3. Be aware of possible common impedance coupling in the ground. The most important characteristic of a ground conductor: Z = R + jωl g g g In designing a ground, it is important to ask: How does the ground current tfl flow? Ground voltage V = I Z g g g 7

8 To reduce Vg ---- Z g I g 8

9 The proper signal ground system is determined by many things, such as the type of circuitry, f of operation, the size of the system, self-contained or distributed, safety, ESD,. The grounding always involves compromise. Grounding problems have more than one acceptable solution. Grounding is hierarchical. Three categories of signal grounds: 1. Single-point grounds. 2. Multipoint grounds. 3. Hbid Hybrid grounds. In general, it is desirable that the topology of the power distribution system follows that of the ground. 9

10 Single-Point Ground Systems Single-point grounds are most effectively used at low frequencies, dc up to 20 khz. With single-point grounding, we control the ground topology to direct the ground current to flow where we want it to flow, which decreases in the sensitive portion of the ground. I g 10

11 A V I I I Z = ( ) 1 VC = ( I1+ I2 + I3) Z1+ ( I2 + I3) Z2 + I3Z3 This circuit is least desirable single-point grounding system, but it is commonly used because of its simplicity. The configuration should not be used between circuits that operate at widely different e current levels. e When the system is used, the most critical circuit should be the one nearest the primary ground point. 11

12 VA = I1Z1 VC = I3Z3 This system can be mechanically cumbersome. Most practical single-point systems are actually a combination of the series and parallel connections. Group ground leads selectively, so that circuits of widely varying power and noise levels do not share the same ground return wire. 12

13 At high frequencies, the singlepoint ground system is undesirable because the inductance of the ground conductors increase the ground impedance. Multipoint Ground Systems Multipoint grounds are used at high frequency (above 100 khz) and in digital circuitry. L g Z = R + jωl g g g V = I Z g g g 13

14 In order to minimize L g, ground planes or grids can be used. A good low-inductance ground is necessary on any PCB that contains high-frequency or digital logic circuits. The ground plane provides a lowinductance return for signal currents and allows for the possibility of using constant impedance transmission lines for signal interconnections. 14

15 Common Impedance Coupling I2 V V Z I I L1 = S1 ( ) G 1+ 2 IZ 1 G : intracircuit noise voltage IZ 2 G : intercircuit noise voltage I 1 Common impedance coupling becomes a problem when two or more circuits share a common ground + one or more of the following: 1. A high-impedance ground. 2. A large ground current. 3. A very sensitive, low-noise margin circuit, connected to the ground. Normally, at frequencies < 100 khz, a single-point ground system may be preferable; when > 100 khz, a multipoint i t ground system is best. 15

16 Hybrid Grounds 16

17 When many equipment enclosures must be grounded to the power system ground, but it is desirable to have a single-point signal ground for the circuitry. 17

18 Chassis Grounds Chassis ground is any conductor that is connected to the equipment s metal enclosure. The key to minimizing noise and interference is to determine where and how to connect the signal ground to the chassis. It is important to establish a low-impedance connection between the chassis and the circuit ground in the I/O area of the board. Establishing a low-impedance connection between the circuit ground and the chassis in the I/O area is also advantageous with respect to radio frequency immunity. 18

19 Equipment/System Grounding Introduction Electronic circuits for many systems are mounted in large equipment racks or cabinets. A typical system will consist of one or more of these equipment enclosures. Equipment grounding objectives include electrical safety, lightning protection, EMC control, and signal integrity. Three types of systems will be considered: 1. Isolated systems. 2. Clustered systems. 3. Distributed systems. Isolated Systems An isolated system is one in which all functions are contained within a single enclosure with no external signal connections to other grounded systems. 19

20 Equipment/System Grounding Examples: vending machines, TV sets, component stereo system, desktop computer,. This system is simplest of all systems and the easiest to ground properly. The enclosure ground can be provided by 1. the ac power ground (green wire) when powered by single-phase ac. 2. a separate ground conductor run with the power cable when powered by three-phase ac. Internal signals should be grounded as appropriate for the type of circuitry and frequency of operation. 20