Solutions for EMC Issues in Automotive System Transmission Lines

Transcription

1 June 23, 2010 Solutions for EMC Issues in Automotive System Transmission Lines FTF-ENT-F0174 Todd Hubing Clemson University and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc.

2 Session Introduction In automotive systems, the wiring harnesses play a key role in coupling electronic noise to and from the components. Shielding and filtering of the wiring harness and connector are not generally viable options; therefore it is important to address cable coupling issues with proper board-level design practices. This presentation reviews the various cable-coupling mechanisms and grounding strategies. It also discusses the options available at the board level for ensuring that automotive components do not put excessive amounts of noise on the harness and that noise on the harness does not affect the proper operation of the component circuitry. This session is intended to help the attendees learn to design electronic components that function properly in harsh electromagnetic environments. Todd Hubing is the Michelin Professor of Vehicle Electronics at the Clemson University International Center for Automotive Research. He has over 25 years experience designing products that meet electromagnetic compatibility requirements. This session is divided into four 40-minute presentations with 10 minutes allocated for questions and one 10-minute break each hour. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 2

3 After completing this session you will be able to: Anticipate possible Electro Magnetic Compatibility (EMC) problems with your product designs Make design decisions affecting the cost and performance of your products Session Objectives Specify grounding and shielding requirements for your products and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 3

4 Agenda 14:00 1. When is a wiring harness a transmission line, and when is it just a bundle of wires? 15:00 16:00 17:00 18:00 2. If it s just a bundle of wires, why do I care how it s configured or routed? 3. When do wiring harnesses look like antennas? 4. How do I design automotive components to prevent harnesses from radiating? 5. How do I protect automotive components from noise or transients on the wiring harness? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 4

5 When is a wiring harness a transmission line? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 5

6 When is a wiring harness a transmission line? Characteristic impedance: Z 0, the ratio of voltage to current in a forward traveling wave. For a pair of wires side-by-side: μ 1 d Z0 = cosh ohms ε 2a wire spacing wire radius For a pair of wires in the same bundle: Z 0 is 100 to 300 ohms. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 6

7 When is a wiring harness a transmission line? Steady state solution is always the wire-pair solution If we don t care about how we get to the steady state, then we don t need to worry about transmission line solutions. In most automotive applications, we don t care! and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 7

8 Signal Termination Eliminating ringing with a series resistor Matched terminations and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 8

9 If it s just a bundle of wires, why do I care how it s configured or routed? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 9

10 Common Impedance Coupling and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 10

11 Where does the return current flow? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 11

12 Where does the return current flow? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 12

13 Identify Current Paths Current takes the path of least impedance! > 100 khz - This is generally the path of least inductance < 10 khz - This is generally the path(s) of least resistance and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 13

14 Identify Current Paths and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 14

15 Identify Current Paths and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 15

16 Identify Current Paths Where does the 10 khz return current flow? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 16

17 Conducted Coupling I 1 aka: Common Impedance Coupling R S1 R S2 + + V S1 V R R S2 RET I RET R L2 V RL2 L1 V RL1 I Requires 2 conductor connections between the source and victim. The only mechanism that couples DC level shifts. Most likely to be dominant at low frequencies, when source and victim share a current return path. Most likely to be dominant when sources are low impedance (high current) circuits. aka also known as and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 17

18 Conducted Coupling Examples Lights dim and radio dies when automobile engine is started. Power bus voltage spikes are heard as audible clicks on an AM radio using the same power source. An electrostatic discharge transient resets a microprocessor causing a system to shutdown. A lightning-induced transient destroys the electronic components in a computer with a wired connection to the internet. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 18

19 Electric Field Coupling V RL2 S2 L2 Crosstalk = 20 log = 20 log VRL1 R 1 S2 R L2 R R + jωc 12 and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 19

20 Electric Field Coupling aka: Capacitive Coupling Requires 0 conductor connections between the source and victim. Coupling proportional to dv/dt. Most likely to be dominant at higher frequencies. Most likely to be dominant when sources are high impedance (high voltage) circuits. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 20

21 Electric Field Coupling Examples Coupling from circuit board heatsinks to cables or enclosures. AM radio interference from overhead power lines. Automotive component noise picked up by the rod antenna in CISPR 25 radiated emissions tests. Microprocessor resets due to indirect electrostatic discharges. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 21

22 Magnetic Field Coupling Crosstalk =20 log VRL2 ωm 12 R L2 = 20 log V R R + R + jωl RL1 when V = 0 L1 L2 S2 22 S2 and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 22

23 Magnetic Field Coupling aka: Inductive Coupling Requires 0 conductor connections between the source and victim. Coupling proportional to di/dt. Most likely to be dominant at higher frequencies. Most likely to be dominant when sources are low impedance (high current) circuits. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 23

24 Magnetic Field Coupling Examples Coupling from power transformers or fluorescent lighting ballasts. Jitter in CRT displays. 60 Hz hum in a handheld AM radio. Hard-drive corruption due to motor or transformer currents. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 24

25 When do wiring harnesses look like antennas? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 25

26 Identifying Antennas What makes an efficient antenna? λ/2 Half-Wave Dipole Electrically Small Loop λ/4 Quarter-Wave Monopole Size Two Halves and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 26

27 Identifying Antennas Good Antenna Parts Poor Antenna Parts <100 MHz >100 MHz <100 MHz >100 MHz Cables Heatsinks Power planes Tall components Seams in shielding enclosures Microstrip or stripline traces Anything that is not big Microstrip or stripline traces Free-space wavelength at 100 MHz is 3 meters and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 27

28 Identifying Antennas Common-Mode vs. Differential Mode Δz E max = I c fδz r s E max = I d f 2 r sδz = I d fδz s r λ and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 28

29 How are common-mode currents induced on cables? Voltage-Driven Mechanism Heatsink ~ VDM Noise voltage V CM ~ Cable Equivalent voltage V CM C V C board r = heatsin k = VDM DM max C C board heatsin k Fboard Fcable E and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 29

30 How are common-mode currents induced on cables? Current-Driven Mechanism Signal current loop induces a voltage between two good antenna parts. - Vcm + At 10 MHz and higher, milliamps of current flowing in a ground plane produces millivolts of voltage across the ground plane. A few millivolts driving a resonant antenna can result in radiated fields exceeding FCC limits. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 30

31 How are common-mode currents induced on cables? Direct-Coupling Mechanism Signals coupled to I/O lines can carry high frequency (HF) power off the board. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 31

32 How are common-mode currents induced on cables? DM-to-CM conversion due to cable and load imbalance and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 32

33 How do I design components to prevent harnesses from radiating? Don t rely on EMC Design Guidelines Be familiar with currents and current paths Learn to recognize good electromagnetic interference (EMI) sources Learn to recognize good antennas Be aware of fundamental EMI radiation mechanisms and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 33

34 Don t Rely on EMC Design Guidelines Board Level Trace routing No trace unrelated to I/O should be located between an I/O connector and the device(s) sending and receiving signals using that connector. All power planes and traces should be routed on the same layer. A trace with a propagation delay more than half the transition time of the signal it carries must have a matched termination. Capacitively-loaded nets must have a total source impedance equal to or greater than one-quarter of the line characteristic impedance or a series resistor must be added to meet this condition. Nets driven at faster than 1V/ns slew rate must have a discrete series resistor at the source. Guard traces should be used to isolate high-speed nets from I/O nets. Guard traces should be connected to the ground plane with vias located less than one-quarter wavelength apart at the highest frequency of interest. All power and ground traces must be at least three times the nominal signal line width. This does not include guard traces. If a ground or power separation is required, the gap must be at least 3 mm wide. Additional decoupling capacitors should be placed on both sides of a power or ground plane gap. Critical nets should be routed in a daisy chain fashion with no stubs or branches. Critical nets should be routed at least 2X from the board edge, where X is the distance between the trace and its return current path. Signals with high-frequency content should not be routed beneath components used for board I/O. Differential pairs radiate much less than single-ended signals even when the traces in the pair are separated by many times their distance above a ground plane. However, imbalance in the pair can result in radiation comparable to an equivalent single-ended signal. The length of high-frequency nets should be minimized. The number of vias in high-frequency nets should be minimized. On a board with power and ground planes, no traces should be used to connect to power or ground. Connections should be made using a via adjacent to the power or ground pad of the component. Gaps or slots in the ground plane should be avoided. They should ONLY be used in situations where it is necessary to control the flow of low-frequency (i.e. less than 100 khz) currents. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 34

35 EE371 Design Problem and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 35

36 EMC Design Guideline Collection and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 36

37 Identify Current Paths Where does the 56 MHz return current flow? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 37

38 Control Transition Times t f t f Control transition times of digital signals! and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 38

39 Control Transition Times CMOS Driver Model CMOS Input Model and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 39

40 Control Transition Times Digital Signal Currents in CMOS Circuits t f t f Control transition times of digital signals: Can use a series resistor or ferrite when load is capacitive. Use appropriate logic for fast signals with matched loads. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 40

41 Control Transition Times Reducing risetime with a series resistor Reducing risetime with a parallel capacitor Good idea Bad idea and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 41

42 Noise Sources and Coupling Mechanisms and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 42

43 Identify Sources Clocks Digital Data Analog signals Power supply switching Arcing Parasitic oscillations Narrow band, consistent Not as narrow as clocks, but clock frequency is usually identifiable. Bandwidth determined by signal source, consistent Appears broadband, but harmonics of switching frequency can be identified, consistent Broadband, intermittent Narrowband, possibly intermittent and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 43

44 Identify Sources Active Devices (Power Pins) For some ICs, the high-frequency currents drawn from the power pins can be much greater than the high-frequency currents in the signals! and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 44

45 Identify Sources Noise on the low-speed I/O For some ICs, significant high-frequency currents appear on low-speed I/O including outputs that never change state during normal operation! and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 45

46 Circuit Board Grounding, Filtering and Shielding and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 46

47 Ground vs. Signal Return AGND Whenever I see more than one of these symbols on the schematic, I know there is [EMC] work for us here. T. Van Doren DGND and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 47

48 Ground vs. Signal Return The purpose of a system ground is to provide a reference voltage and/or a safe path for fault currents. Signal currents flowing on a ground conductor can prevent a ground conductor from serving its intended purpose. Don t confuse ground conductors with signal return conductors. Rules for the routing of ground may conflict with the rules for routing signal or power returns. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 48

49 Ground vs. Signal Return Circuit boards should have high-frequency ground! Why? Conductors referenced to different grounds can be good antennas. Signals referenced to two different grounds will be noisy (i.e., include the noise voltage between the two grounds.) Layouts with more than one ground are more difficult, require more space, and present more opportunities for critical mistakes. Excuses for employing more than one ground are generally based on inaccurate or out-dated information. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 49

50 Ground vs. Signal Return If grounds are divided, it is generally to control the flow of lowfrequency (<100 khz) currents. For example, Isolating battery negative (i.e. chassis ground) from digital ground Isolating digital ground from analog ground in audio circuits. This can be necessary at times to prevent common impedance coupling between circuits with low-frequency high-current signals and other sensitive electronic circuits. HOWEVER, it is still necessary to ensure that there is only 1 highfrequency ground. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 50

51 Ground vs. Signal Return Exercise: Trace the path of the digital and analog return currents. D/A and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 51

52 Ground vs. Signal Return Exercise: Trace the path of the digital and analog return currents. D/A and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 52

53 Ground vs. Signal Return Exercise: Trace the path of the digital and analog return currents. D/A and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 53

54 Ground vs. Signal Return Design Exercise: What is wrong with this design and how would you improve it? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 54

55 Lateral Isolation Chassis GND Rarely appropriate Analog GND Often the source of significant problems Digital GND and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 55

56 Vertical Isolation Digital GND Analog GND Chassis GND Only one plane usually needs to be full size. One or zero vias should connect planes with different labels. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 56

57 Ground vs. Signal Return You don t need to gap a plane to control the flow of high frequency (> 1 MHz) currents. If you provide a low-inductance path for these currents to take, they will confine themselves to this path very well. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 57

58 Ground vs. Signal Return Rules for gapping a ground plane: 1. Don t do it! 2. If you must do it, never ever allow a trace or another plane to cross over the gap. 3. If you must do it, never ever place a gap between two connectors. 4. Remember that the conductors on either side of the gap are at different potentials. 5. See Rule #1! and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 58

59 and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 59

60 Sensitive A/D Isolation ONE VIA Analog GND Digital GND Digital GND If you think you need two vias, then you shouldn t be isolating the analog and digital grounds. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 60

61 Provide a Good HF Chassis Ground at the Connector Cables and enclosures are both good antenna parts. If they are not held to the same potential, they are likely to create a radiation problem. Exceptions: When there is no chassis ground When there are no connectors with cables Note: Sometimes low-frequency isolation between chassis and digital ground is necessary control the flow of low-frequency currents. However, even in these situations it is usually important to provide a good high-frequency connection. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 61

62 Isolating Chassis and Digital Grounds and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 62

63 Better implementation Caps with traces and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 63

64 Filtering Low-Pass Filters and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 64

65 Filtering Design Exercise: Which low-pass filter is most appropriate in each case? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 65

66 Filtering Parasitics and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 66

67 Filtering with Two Capacitors Two capacitors more than twice as good as one. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 67

68 Shielding Electric Field Shielding and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 68

69 Shielding Magnetic Field Shielding (at low frequencies) and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 69

70 Shielding Magnetic Field Shielding (at high frequencies) and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 70

71 Shielding Enclosure Shielding and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 71

72 DC Power Distribution and Decoupling and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 72

73 The Concept of Power Bus Decoupling L P V board V supply L G Power Supply Printed Circuit Board and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 73

74 The Concept of Power Bus Decoupling L P V board V inductance V inductance V supply L G Power Supply Printed Circuit Board and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 74

75 The Concept of Power Bus Decoupling L trace L trace C d C d L d L d C b L trace L trace and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 75

76 Effective Strategies for Choosing and Locating Printed Circuit Board Decoupling Capacitors and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 76

77 Rules for PCB Decoupling? Use small-valued capacitors for high-frequency decoupling. Locate capacitors near the power pins of active devices. Avoid capacitors with a low ESR! Run traces from device to capacitor, then to power planes. Location of decoupling capacitors is not relevant. Use the largest valued capacitors you can find in a given package size. Use 0.01 μf for local decoupling! Use capacitors with a low ESR! Use μf for local decoupling! Locate capacitors near the ground pins of active devices. Never put traces on decoupling capacitors. Local decoupling capacitors should have a range of values from 100 pf to 1 μf! and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 77

78 How much capacitance do you need? Impedance approach L trace L trace Z max ( f ) V = I NOISEMAX DEVICEMAX ( f ) ( f ) Ltrace C b Ltrace Z max 1 2 π 1 f C 2πfL trace 1 = Z 2πf C C 0 min max = 1 2πf Z 0 0 max = 1 (2πf ) 2 L source f0 f 1 and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 78

79 Capacitance Ratio Approach How much capacitance do you need? Recognizing that CMOS loads are capacitances, we are simply using decoupling capacitors to charge load capacitances. Total decoupling capacitance is set to a value that is equal to the total device capacitance times the power bus voltage divided by the maximum power bus noise. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 79

80 How much capacitance do you need? Guidelines approach Let s do it the way that worked for somebody at sometime in the past. include one 0.01 uf local decoupling capacitor for each VCC pin of every active component on the board plus 1 bulk decoupling capacitor with a value equal to 5 times the sum of the local decoupling capacitance.. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 80

81 Printed Circuit Board Decoupling Strategies and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 81

82 Boards with Closely Spaced Power Planes C b C d C d Power Distribution Model ~ (5-500 MHz) Board with power and ground planes and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 82

83 Boards with Closely Spaced Power Planes C = 3.4 nf B L = 5 nh BULK L = 2nH D C BULK = 1 μ F C = 10nF D Bare Board Board with decoupling MHz 1 MHz 10 MHz 100 MHz 1 GHz and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 83

84 For Boards with Closely-Spaced Planes The location of the decoupling capacitors is not critical. The value of the local decoupling capacitors is not critical, but it must be greater than the interplane capacitance. The inductance of the connection is the most important parameter of a local decoupling capacitor. None of the local decoupling capacitors are effective above a couple hundred megahertz. None of the local decoupling capacitors are supplying significant charge in the first few nanoseconds of a transition. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 84

85 and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 85

86 Inductance of Connections to Planes On boards with closely spaced power and ground planes: Generally speaking, 100 decoupling capacitors connected through 1 nh of inductance will be as effective as 500 decoupling capacitors connected through 5 nh of inductance. 5 nh 0.5 nh and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 86

87 Power Bus Decoupling Strategy With closely spaced (<.25 mm) planes Size bulk decoupling to meet board requirements Size local decoupling to meet board requirements Mount local decoupling in most convenient locations Don t put traces on capacitor pads Too much capacitance is ok Too much inductance is not ok References: T. H. Hubing, J. L. Drewniak, T. P. Van Doren, and D. Hockanson, Power Bus Decoupling on Multilayer Printed Circuit Boards, IEEE Transactions on Electromagnetic Compatibility, vol. EMC-37, no. 2, May 1995, pp T. Zeeff and T. Hubing, Reducing power bus impedance at resonance with lossy components, IEEE Transactions on Advanced Packaging, vol. 25, no. 2, May 2002, pp M. Xu, T. Hubing, J. Chen, T. Van Doren, J. Drewniak and R. DuBroff, Power bus decoupling with embedded capacitance in printed circuit board design, IEEE Transactions on Electromagnetic Compatibility, vol. 45, no. 1, Feb. 2003, pp and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 87

88 Boards with Power Planes Spaced >0.5 mm C b C d C d and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 88

89 Boards with Power Planes Spaced >0.5 mm M L TRACE L VIA L VIA L TRACE PORT 1 PORT 2 C BOARD ACTIVE DEVICE DECOUPLING CAPACITOR LOOP A LOOP A and LOOP B SIGNAL PLANE POWER PLANE GROUND PLANE SIGNAL PLANE On boards with a spacing between power and ground planes of ~30 mils (0.75 mm) or more, the inductance of the planes can no longer be neglected. In particular, the mutual inductance between the vias of the active device and the vias of the decoupling capacitor is important. The mutual inductance will tend to cause the majority of the current to be drawn from the nearest decoupling capacitor and not from the planes. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 89

90 Where do I mount the capacitor? Here? V CC GND Here? POWER GND and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 90

91 and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 91

92 For Boards with Widely-Spaced Planes Local decoupling capacitors should be located as close to the active device as possible (near pin attached to most distant plane). The value of the local decoupling capacitors should be 10,000 pf or greater. The inductance of the connection is the most important parameter of a local decoupling capacitor. Local decoupling capacitors can be effective up to 1 GHz or higher if they are connected properly. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 92

93 and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 93

94 Power Bus Decoupling Strategy With Widely Spaced (>.5 mm) Planes: Size bulk decoupling to meet board requirements Size local decoupling to meet device requirements Mount local decoupling near pin connected to furthest plane Don t put traces on capacitor pads Too much capacitance is ok Too much inductance is not ok References: J. Chen, M. Xu, T. Hubing, J. Drewniak, T. Van Doren, and R. DuBroff, Experimental evaluation of power bus decoupling on a 4-layer printed circuit board, Proc. of the 2000 IEEE International Symposium on Electromagnetic Compatibility, Washington D.C., August 2000, pp T. H. Hubing, T. P. Van Doren, F. Sha, J. L. Drewniak, and M. Wilhelm, An Experimental Investigation of 4-Layer Printed Circuit Board Decoupling, Proceedings of the 1995 IEEE International Symposium on Electromagnetic Compatibility, Atlanta, GA, August 1995, pp J. Fan, J. Drewniak, J. Knighten, N. Smith, A. Orlandi, T. Van Doren, T. Hubing and R. DuBroff, Quantifying SMT Decoupling Capacitor Placement in DC Power-Bus Design for Multilayer PCBs, IEEE Transactions on Electromagnetic Compatibility, vol. EMC-43, no. 4, Nov. 2001, pp and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 94

95 Power Bus Decoupling Strategy With No Power Plane: Layout low-inductance power distribution Size bulk decoupling to meet board requirements Size local decoupling to meet device requirements Two caps can be much better than one Avoid resonances by minimizing inductance References: T. Hubing, Printed Circuit Board Power Bus Decoupling, LG Journal of Production Engineering, vol. 3, no. 12, December 2000, pp (Korean language publication). T. Zeeff, T. Hubing, T. Van Doren and D. Pommerenke, Analysis of simple two-capacitor low-pass filters, IEEE Transactions on Electromagnetic Compatibility, vol. 45, no. 4, Nov. 2003, pp and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 95

96 Power Bus Decoupling Strategy Low-impedance Planes or Traces? Choice is based on bandwidth and board complexity Planes are not always the best choice It is possible to achieve good decoupling either way Trace inductance may limit current to active devices Planes Widely Spaced or Closely Spaced? Want local or global decoupling? Want stripline traces? Lower impedances are obtainable with closely spaced planes. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 96

97 Embedded Capacitance Input impedance of a populated 2 x 3 board with a plane separation of about 5 microns: and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 97

98 Decoupling Myth In order to be effective, capacitors must be located within a radius of the active device equal to the distance a wave can travel in the transition time of the circuitry. On boards with closely spaced planes (where this rule is normally applied) none of the capacitors on the board can typically respond within the transition time of the circuitry no matter where they are located. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 98

99 Decoupling Myth Smaller valued capacitors (i.e. 10 pf) respond faster than higher valued capacitors. The ability of a capacitor to supply current quickly is determined by its mounted inductance. The value of the capacitance only affects its ability to respond over longer periods of time. For a given value of inductance, higher valued capacitors are more effective for decoupling. and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 99

100 Printed Circuit Board Layout Guidelines Design rules won t make you a good circuit board designer: Use common sense! Visualize signal current paths Locate antennas and crosstalk paths Be aware of potential EMI sources and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 100

101 Example Design Example: How would you modify this design? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 101

102 Example Design Example: A much better design and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 102

103 Example Design Example: How would you modify this design? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 103

104 Design Review Keep signal loop areas small Don t locate circuitry between connectors Control transition times in digital signals Never cut gaps in a solid return plane and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 104

105 Avoid right angle corners on traces. 20-H rule Multiple decoupling cap values or traces on caps Maximum trace lengths on striplines or microstrip traces Other design rules you may have heard of, but shouldn t be too concerned with and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 105

106 Design Example and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 106

107 Design Example and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 107

108 Design Example 2 and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 108

109 Design Examples and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 109

110 Summary Don t rely on design guidelines! Use common sense Visualize signal current paths Locate antennas and crosstalk paths Be aware of potential EMI sources Ask other engineers to review your designs and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 110

111 How do I protect components from transients on the harness? Design Exercise: Where should the transient protection be grounded? OR Digital GND Chassis GND and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 111

112 Diodes 0.5 volts to ~10 volts Lowest Energy High Capacitance (10 s of pf) Usually fail short Voltage limiting device and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 112

113 Varistors 0.5 volts to 10 s of volts Low Energy Higher Capacitance (10 s of pf) Usually fail short Voltage limiting device and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 113

114 Thyristors 0.5 volts to 10 s of volts Medium Energy Higher Capacitance (10 s of pf) Fail open or short Crowbar device and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 114

115 Gas Discharge Tubes 10 s of volts to 1000 s of volts High Energy Low Capacitance (< 1 pf) Fail open Crowbar device and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 115

116 Session Review 1. When is a wiring harness a transmission line, and when is it just a bundle of wires? 2. If it s just a bundle of wires, why do I care how it s configured or routed? 3. When do wiring harnesses look like antennas? 4. How do I design automotive components to prevent harnesses from radiating? 5. How do I protect automotive components from noise or transients on the wiring harness? and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 116

117 For Further Information Cited Referenfces regarding Power Bus Decoupling Strategy: T. H. Hubing, J. L. Drewniak, T. P. Van Doren, and D. Hockanson, Power Bus Decoupling on Multilayer Printed Circuit Boards, IEEE Transactions on Electromagnetic Compatibility, vol. EMC-37, no. 2, May 1995, pp T. Zeeff and T. Hubing, Reducing power bus impedance at resonance with lossy components, IEEE Transactions on Advanced Packaging, vol. 25, no. 2, May 2002, pp M. Xu, T. Hubing, J. Chen, T. Van Doren, J. Drewniak and R. DuBroff, Power bus decoupling with embedded capacitance in printed circuit board design, IEEE Transactions on Electromagnetic Compatibility, vol. 45, no. 1, Feb. 2003, pp J. Chen, M. Xu, T. Hubing, J. Drewniak, T. Van Doren, and R. DuBroff, Experimental evaluation of power bus decoupling on a 4- layer printed circuit board, Proc. of the 2000 IEEE International Symposium on Electromagnetic Compatibility, Washington D.C., August 2000, pp T. H. Hubing, T. P. Van Doren, F. Sha, J. L. Drewniak, and M. Wilhelm, An Experimental Investigation of 4-Layer Printed Circuit Board Decoupling, Proceedings of the 1995 IEEE International Symposium on Electromagnetic Compatibility, Atlanta, GA, August 1995, pp J. Fan, J. Drewniak, J. Knighten, N. Smith, A. Orlandi, T. Van Doren, T. Hubing and R. DuBroff, Quantifying SMT Decoupling Capacitor Placement in DC Power-Bus Design for Multilayer PCBs, IEEE Transactions on Electromagnetic Compatibility, vol. EMC-43, no. 4, Nov. 2001, pp T. Hubing, Printed Circuit Board Power Bus Decoupling, LG Journal of Production Engineering, vol. 3, no. 12, December 2000, pp (Korean language publication). T. Zeeff, T. Hubing, T. Van Doren and D. Pommerenke, Analysis of simple two-capacitor low-pass filters, IEEE Transactions on Electromagnetic Compatibility, vol. 45, no. 4, Nov. 2003, pp EMC Guideline Collection: and VortiQa are trademarks of Freescale Semiconductor, Inc. All other product or service names are the property of their respective owners Freescale Semiconductor, Inc. 117

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