Solutions for EMC Issues in Automotive System Transmission Lines

Transcription

1 Solutions for EMC Issues in Automotive System Transmission Lines Todd H. Hubing Michelin Professor of Vehicle Electronics Clemson University A P R TM External Use

2 EMC Requirements and Key Design Considerations Radiated Emissions Radiated Susceptibility Transient Immunity Electrostatic Discharge Bulk Current Injection 1 HF GND Risetime Control Filtered I/O Adequate Decoupling Balance Control 1 HF GND Filtered I/O Adequate Decoupling Balance Control LF Current Path Control Chassis GND on board Filtered I/O Adequate Decoupling LF Current Path Control Chassis GND on board Filtered I/O Adequate Decoupling 1 HF GND Chassis GND on board Filtered I/O Adequate Decoupling Balance Control Designing a product that is guaranteed to meet all of these requirements is relatively easy. Fixing a non-compliant product can be difficult and costly. External Use 1

3 Automotive EMI Sources, Victims and Antennas External Use 2

4 Agenda 10 rules for ensuring you meet your automotive EMC requirements the first time your product is tested. Hour 1 Hour 2 Hour 3 Hour 4 1. Control your transition times 2. Recognize where currents flow 3. Recognize the 4 (not 2) possible coupling mechanisms 4. Identify your antennas 5. Identify your sources 6. Don t rely on EMC design guidelines 7. Don t gap your ground planes 8. There are no shielded enclosures in the automotive world 9. Provide adequate power bus decoupling 10. Provide adequate transient protection External Use 3

5 First Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time, you MUST: External Use 4

6 RLCG Parameters If the length of each section of the RLCG lumped model is small relative to a wavelength (e.g. n <<l/8), the electrical behavior of the model is the same as the electrical behavior of the transmission line. External Use 5

7 Propagation Velocity Determined by the dielectric material Propagation velocity (m/sec) v 1 1 LC Inductance per unit length (H/m) Capacitance per unit length (F/m) This term is independent of the geometry External Use 6

8 Propagation Delay (Electrical Length) RS S1 l Z0 RL t PD v Propagation Delay (sec) The propagation delay is the amount of time required for a signal to propagate from one point to another point (total distance, ) on the transmission line. External Use 7

9 Characteristic Impedance RS S1 l Z0 RL Z 0 R jl L G jc C Characteristic Impedance (ohms) Low-Loss Approximation The characteristic impedance is the ratio of the voltage to the current in a signal traveling in one direction down the transmission line. External Use 8

10 Attenuation RS S1 l Z0 RL V V e 0 x x=0 x=1m R jl G jc j x0 m Ve 0 Attenuation in db/m 20log x1m Ve 0 20 log e 8.7 R 2Z 0 Low-Loss Approximation Attenuation in db/m LC 4.34R Z 0 External Use 9

11 Dispersion RS S1 l Z0 RL x=0 x=1m R 2Z 0 Low-Loss Approximation Attenuation in db/m LC 4.34R Z 0 Notice that attenuation is a function of R, but at high frequencies, R is a function of frequency due to the skin effect. Therefore higher frequencies are attenuated more than lower frequencies. This can change the shape of the signal in the time domain, and this effect is called dispersion. External Use 10

12 When is a cable a transmission line? RS l VS Z0 RL at midpoint in transmission line at midpoint in transmission line Technically, always! But it will be most important to us when the propagation delay is greater than the transition time. External Use 11

13 When must a cable be modeled as a transmission line? RS l VS Z0 RL The answer depends on the application, but generally the following guidelines apply. For digital signals: When t r < 2 * t pd For RF signals: When > l/8 External Use 12

14 An Important Point RS l VS Z0 RL In most applications, anything that must be modeled as a transmission line must have a matched termination. This is usually undesirable from a cost and EMC perspective. Therefore, every effort should usually be taken to ensure that the signal bandwidth is no higher (or transition times are no shorter) than necessary. External Use 13

15 When cables are electrically short They can be modeled using their lumped RLCG parameters. RS l<<λ/8 VS Z0 RL RS VS RL Often, one or none of these parameters is significant relative to the source and load impedances External Use 14

16 When cables are electrically short Be careful not to model short cables or connectors with the full L or C unless you have shown the that other parameter can be neglected. Discontinuities with a characteristic impedance greater than the source and load impedances can be modeled with a lumped inductance. l L eq Z 01 Z 02 Z 01 Z 01 Z 01 Z 01 Z 01 Z 01 < Z 02 << 1 The value of this inductance is less than the value of the lumped parameter L in the RLCG model. Leq L 1 2 R 01 Z 02 External Use 15

17 When cables are electrically short Be careful not to model short cables or connectors with the full L or C unless you have shown the that other parameter can be neglected. Discontinuities with a characteristic impedance less than the source and load impedances can be modeled with a lumped capacitance. l Z 01 Z 02 Z 01 Z 01 Z 01 C eq Z 01 Z 01 Z 02 < Z 01 << 1 The value of this capacitance is less than the value of the lumped parameter C in the RLCG model. Ceq C 1 2 Z 02 R 01 External Use 16

18 When cables are not electrically short To eliminate reflections, transmission lines must have a controlled impedance and must be matched! RS l VS Z0 RL For signals with one source and one load, the match can occur at the source end: R S = Z 0. For signals with one source and more than one load, the match must generally occur at the load end: R L = Z 0. External Use 17

19 When is a wiring harness a transmission line? RS S1 l Z0 RL 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! External Use 18

20 When is a wiring harness a transmission line? RS S1 l Z0 RL If the risetime is much greater than the propagation delay, transmission line can be modeled as lumped element. length = 5 meters propagation delay ~ 30 nsec length = 50 cm propagation delay ~ 3 nsec External Use 19

21 When is a wiring harness a transmission line? 1. Every digital signal transition time should be forced to be >100 ns (unless this would prevent the circuit from working). 2. Signals that must transition faster than 100 ns, should transition in the longest permissible time. 3. Traces or cables that carry signals with transition times > 100 ns should not have matched terminations unless the length of the signal propagation is > 5 meters. 4. Traces or cables that carry signals with transition times > 10 ns should not have matched terminations unless the length of the signal propagation is > 50 cm. External Use 20

22 Control Transition Times CMOS Driver Model CMOS Input Model External Use 21

23 Control Transition Times t f t f Control transition times of digital signals! External Use 22

24 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. External Use 23

25 Control Transition Times Reducing risetime with a series resistor Reducing risetime with a parallel capacitor Good idea Bad idea External Use 24

26 3-METER E-FIELD IN DB(uV/M) Example 1: Microcontroller Output Driver Automotive microcontroller in typical application: Suppose we connected an output of this microcontroller directly up to an impedance-matched antenna Available Information V source = 3.3 V 90.0 Absolute maximum possible emissions! Maximum Radiated Field I max = 20 ma 80.0 FCC Limit C in = 5 pf R series = 0 W CLK Freq = 100 khz Calculated Parameters R source = 165 W T = 10 s t r = 1.82 ns FREQUENCY IN MHZ External Use 25

27 3-METER E-FIELD IN DB(uV/M) Example 1: Microcontroller Output Driver Same output with 20-kW series resistor: Suppose we connected an output of this microcontroller directly up to an impedance-matched antenna Available Information V source = 3.3 V I max = 20 ma Maximum Radiated Field FCC Limit C in = 5 pf 70.0 R series = 20 kw 60.0 CLK Freq = 100 khz 50.0 Calculated Parameters R source = 8165 W T = 10 s t r = ns FREQUENCY IN MHZ External Use 26

28 3-METER E-FIELD IN DB(uV/M) Example 2: Microcontroller Output Driver Same output with 1 MHz output: Suppose we connected an output of this microcontroller directly up to an impedance-matched antenna Available Information V source = 3.3 V I max = 20 ma C in = 5 pf Maximum Radiated Field FCC Limit R series = 0 kw CLK Freq = 1 MHz Calculated Parameters 40.0 R source = 165 W T = 1 s t r = 1.82 ns FREQUENCY IN MHZ External Use 27

29 3-METER E-FIELD IN DB(uV/M) Example 2: Microcontroller Output Driver Same output with 1 MHz output and 8-kW series resistor: Suppose we connected an output of this microcontroller directly up to an impedance-matched antenna Available Information V source = 3.3 V 90.0 Maximum Radiated Field I max = 20 ma 80.0 FCC Limit C in = 5 pf 70.0 R series = 8 kw 60.0 CLK Freq = 1 MHz 50.0 Calculated Parameters 40.0 R source = 8165 W T = 1 s t r = 90 ns FREQUENCY IN MHZ External Use 28

30 Amplitude TM Amplitude Amplitude RLC Circuits 10 Under-damped L R 2 C Time 10 Critically-damped 10 Over-damped L R 2 C 4 2 L R 2 C Time Time External Use 29

31 2nd Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time, you MUST: External Use 30

32 Where does the return current flow? External Use 31

33 Where does the return current flow? External Use 32

34 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 External Use 33

35 Identify Current Paths 6 VDC INPUT Where does the 56 MHz return current flow? +5 volts VOLTAGE REGULATOR ground +5 volts 56 MHZ OSC. +5 volts ground ground External Use 34

36 Identify Current Paths 6 VDC INPUT Where does the 6 VDC return current flow? +5 volts VOLTAGE REGULATOR ground +5 volts 56 MHZ OSC. +5 volts ground ground External Use 35

37 Identify Current Paths External Use 36

38 Identify Current Paths External Use 37

39 Identify Current Paths Where does the 10 khz return current flow? External Use 38

40 3rd Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time, you MUST: External Use 39

41 Common Impedance Coupling I 1 R S1 R S2 + + V S1 V R R S2 RET I RET R L2 V RL2 L1 V RL1 I External Use 40

42 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. External Use 41

43 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. External Use 42

44 Electric Field Coupling aka: Capacitive Coupling C 12 V S1 V S2 C 11 C 22 R L2 + + V R RL2 L1 V - RL1 - R S1 R S2 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. External Use 43

45 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. External Use 44

46 Magnetic Field Coupling aka: Inductive Coupling L 1 M 12 L 2 R S1 R S2 + + R L1 V S1 V S2 R L2 V RL2 V - RL1 - 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. External Use 45

47 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. External Use 46

48 4th Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time, you MUST: External Use 47

49 When do wiring harnesses look like antennas? External Use 48

50 Identifying Antennas What makes an efficient antenna? l/2 Half-Wave Dipole Electrically Small Loop l/4 Quarter-Wave Monopole Size Two Halves External Use 49

51 Identifying Antennas Good Antenna Parts Poor Antenna Parts <100 MHz >100 MHz <100 MHz >100 MHz Cables Heatsinks Power planes Microstrip or stripline traces Microstrip or stripline traces Tall components Seams in shielding enclosures Anything that is not big Free-space wavelength at 100 MHz is 3 meters External Use 50

52 Identifying Antennas Common-Mode vs. Differential Mode z E max I c fz r s E max I d f 2 r sz I d fz s r l External Use 51

53 How are common-mode currents induced on cables? Voltage-Driven Mechanism Heatsink ~ VDM Noise voltage V CM ~ Cable Equivalent voltage V CM C C heat sin k board V DM V DM Cboard r C F F heat sin k board cable E max External Use 52

54 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. External Use 53

55 What do you think of this automotive BCM design? External Use 54

56 How are common-mode currents induced on cables? Direct-Coupling Mechanism Signals coupled to I/O lines can carry HF power off the board. External Use 55

57 How are common-mode currents induced on cables? DM-to-CM conversion due to cable and load imbalance RS S1 Z0 RL External Use 56

58 Common-mode and Differential-mode Current General Definition I h I hi DM I I I CM 1 2 I 1 = 3 Amps I 2 = 5 Amps I I hi 1 DM CM I2 I DM (1 h) I CM AND I DM V Z DM DM External Use 57

59 Driving a Ribbon Cable h = no plane h = small - w/ plane A perfect differential driver driving two adjacent wires in a ribbon cable produces no common-mode current on the ribbon cable. A single-ended driver driving two adjacent wires in a ribbon cable produces a exactly the same amount of common-mode current as a common-mode source with half the signal voltage Don t drive ribbon cable wires with single-ended sources unless you know the common-mode current will not be a problem. External Use 58

60 PCB Driving a Twisted Wire Pair h = 0.5 A perfect differential driver driving a perfect twisted-wire pair produces no common-mode current on the wire pair. A single-ended driver driving a twistedwire pair produces a exactly the same amount of common-mode current as a common-mode source with half the signal voltage Don t drive twisted-wire pairs with single-ended sources unless you know the common-mode current will not be a problem. External Use 59

61 5th Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time, you MUST: External Use 60

62 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 External Use 61

63 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! External Use 62

64 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! External Use 63

65 6th Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time: External Use 64

66 EE371 Design Problem External Use 65

67 EMC Design Guideline Collection External Use 66

68 7th Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time: External Use 67

69 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 External Use 68

70 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. External Use 69

71 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. External Use 70

72 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 high-frequency ground. External Use 71

73 Ground vs. Signal Return Exercise: Trace the path of the digital and analog return currents. D/A PWM Driver Heatsink External Use 72

74 Ground vs. Signal Return Exercise: Trace the path of the digital and analog return currents. D/A PWM Driver Heatsink External Use 73

75 Ground vs. Signal Return Exercise: Trace the path of the digital and analog return currents. D/A PWM Driver Heatsink External Use 74

76 Lateral Isolation Chassis GND Rarely appropriate Analog GND Often the source of significant problems Digital GND External Use 75

77 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. External Use 76

78 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. External Use 77

79 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! External Use 78

80 Provide a good HF chassis ground at 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. External Use 79

81 Question: When do you provide a chassis ground on the board? Answer: Whenever you have a metal enclosure OR a connection to vehicle chassis. Chassis connection to chassis ground Wiring Harness Chassis connection to chassis ground Capacitors connecting chassis ground to the digital return plane Chassis Ground Plane Digital Return Plane External Use 80

82 Chassis Ground Capacitors Caps with traces Better implementation External Use 81

83 Filtering Low-Pass Filters External Use 82

84 Filtering Design Exercise: Which low-pass filter is most appropriate in each case? External Use 83

85 Filtering Parasitics External Use 84

86 Filtering with 2 capacitors Two capacitors are more than twice as good as one. External Use 85

87 8th Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time, you MUST recognize that: External Use 86

88 Shielding (a.) Electric Field Shielding (b.) + V - - V+ (c.) External Use 87

89 Shielding Magnetic Field Shielding (at low frequencies) External Use 88

90 Shielding Magnetic Field Shielding (at high frequencies) External Use 89

91 Shielding Enclosure Shielding External Use 90

92 9th Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time, you MUST: External Use 91

93 The Concept of Power Bus Decoupling L P V board V inductance V inductance V supply L G Power Supply Printed Circuit Board External Use 92

94 The Concept of Power Bus Decoupling L P V board V supply L G Power Supply Printed Circuit Board External Use 93

95 The Concept of Power Bus Decoupling L trace L trace C d C d L d L d C b L trace L trace External Use 94

96 Effective Strategies for Choosing Amount of Decoupling Capacitance Required External Use 95

97 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 F for local decoupling! Use capacitors with a low ESR! Use F 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 F! External Use 96

98 How much capacitance do you need? Impedance approach L trace L trace Z max ( f ) V = I NOISE MAX DEVICE MAX ( f ) ( f ) L trace C b L trace Z max 1 2 p f 1 C 2 p fl trace 1 = Z 2 p f C C 0 min max = 1 2 p f Z 0 = 1 ( 2 p f ) 0 max 2 L source f0 f 1 External Use 97

99 How much capacitance do you need? Capacitance Ratio approach Recognizing that CMOS loads are capacitances, we are simply using decoupling capacitors to charge load capacitances. R + V - C D C L 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. External Use 98

100 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.. External Use 99

101 Effective Strategies for Locating Printed Circuit Board Decoupling Capacitors External Use 100

102 Boards with Closely Spaced Power Planes C b C d C d Power Distribution Model ~ (5-500 MHz) Board with power and ground planes External Use 101

103 Boards with Closely Spaced Power Planes C B = 3.4 nf L BULK = 5 nh L = 2nH D C BULK = 1 F C D = 10nF Bare Board Board with decoupling MHz 1 MHz 10 MHz 100 MHz 1 GHz External Use 102

104 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. External Use 103

105 External Use 104

106 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 External Use 105

107 Power Bus Decoupling Strategy With closely spaced (<.25 mm) planes size bulk decoupling to meet board requirements size high-frequency 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 External Use 106

108 Boards with Power Planes Spaced >0.5 mm C b C d C d External Use 107

109 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. External Use 108

110 Where do I mount the capacitor? Here? V CC GND Here? POWER GND External Use 109

111 External Use 110

112 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. External Use 111

113 External Use 112

114 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 External Use 113

115 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 L 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 External Use 114

116 Power Bus Decoupling Strategy Low-impedance planes or traces? choice 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 obtainable with closely spaced planes. External Use 115

117 Embedded Capacitance Input impedance of a populated 2 x 3 board with a plane separation of about 5 microns External Use 116

118 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. External Use 117

119 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. External Use 118

120 10th Rule of Automotive EMC Design To guarantee that your design will meet its EMC requirements the first time, you MUST: External Use 119

121 Protecting components from transients on the harness Design Exercise: Where should the transient protection be grounded? OR Digital GND Chassis GND External Use 120

122 Transient Protection Options Diodes 0.5 volts to ~10 volts Lowest Energy High Capacitance (10 s of pf) Usually fail short Voltage limiting device Varistors 0.5 volts to 10 s of volts Low Energy Higher Capacitance (10 s of pf) Usually fail short Voltage limiting device Thyristors 0.5 volts to 10 s of volts Low Energy Higher Capacitance (10 s of pf) Usually fail short Voltage limiting device Gas Discharge Tubes 10 s of volts to 1000 s of volts High Energy Low Capacitance (< 1 pf) Fail open Crowbar device External Use 121

123 Current Limiting Devices Fuses 100 ma and up Speed depends on current Low impedance Inexpensive Non-resettable Circuit Breakers Microamps and up Speed independent of current Moderate inductance Expensive Resettable Ground-Fault Interrupters Microamps and up Speed independent of current Moderate inductance Expensive Resettable Positive Temperature Coefficient Devices 100 ma and up Speed depends on current Low to moderate inductance Inexpensive Limit current without opening Resettable External Use 122

124 1. Control your transition times 2. Know how your currents return to their source 3. Recognize the 4 (not 2) possible coupling mechanisms 4. Identify your antennas 5. Identify your sources 6. Don t rely on EMC design guidelines 7. Don t gap your ground planes 8. There are no shielded enclosures in the automotive world 9. Provide adequate power bus decoupling 10. Provide adequate transient protection External Use 123

125 Example Design Example: How would you modify this design? External Use 124

126 Example Design Example: A much better design External Use 125

127 Power Inverter Design Example + Input 80 MHZ OSC. Phase 1 - Input Phase 2 Gate Array Phase 3 Connection to power plane Connection to ground plane External Use 126

128 For More Information External Use 127

129 LearnEMC