Automotive Pulses

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Automotive pulses refer to specific transient voltage waveforms that occur in a vehicle's electrical system. These pulses are defined by standards (like ISO 7637, SAE J1113, or LV 124) and used primarily for EMC (Electromagnetic Compatibility) testing of automotive electronic components and systems.

These tests simulate real-world disturbances (like load dumps, switching transients, inductive kicks) to ensure components can survive and operate correctly in harsh automotive environments.

Modern vehicles contain many ECUs, sensors, and actuators. They're exposed to:

• Alternator spikes
• Battery disconnection
• Relay switching
• Ignition transients
• Electrostatic discharge (ESD)

To ensure reliable operation, these pulses are simulated and tested in labs.

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Pulses as per ISO 7637-2

ISO 7637-2 (for 12V and 24V systems)

The most widely used for electrical transients on supply lines.

Pulse	Description	Typical Cause
Pulse 1	Negative spike	Battery disconnect while inductive loads are ON
Pulse 2a/2b	Positive/negative spikes	Switching of inductive loads
Pulse 3a/3b	Fast transients	Arising from relay contact bounce
Pulse 4	Voltage drop	Engine cranking
Pulse 5	Load dump	Battery disconnect while alternator is charging

 

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Pulse Characteristics (ISO 7637-2)

Pulse 1

• Negative Spike
• Cause: Battery disconnection from Inductive Loads (motor, Solenoid)
• Affect: Power supply circuit, Microcontroller
   

 

Parameters	Nominal 12 V system	Nominal 24 V system
Us	−75 V to −150 V	−300 V to −600 V
Ri	10 Ω	50 Ω
td	2 ms	1 ms
tr	1 μs	3 μs
t1	≥0.5 s
t2	200 ms
t3	<100 μs

Pulse 2a

• Positive Spike
• Cause: Sudden interruption of current due to sudden disconnection of large loads
• Affect: Voltage regulators and semiconductors devices
   

 

Parameters	Nominal 12V & 24V system
Us	+37 V to +112 V
Ri	2 Ω
td	0.05 ms
tr	1 μs
t1	0.2 s to 5 s

 

Pulse 2b

• Negative Spike
• Cause: Alternator field decay, Alternator’s field winding de-energises after engine shutdown
• Affect: ECU Reset
  
   

 

Parameters	Nominal 12 V system	Nominal 24 V system
Us	10 V	20 V
Ri	0 Ω to 0.05 Ω
td	0.2 s to 2 s
tr	1 ms
t12	1 ms
t6	1 ms

Pulse 3a

• Fast Transients – Negative
• Cause: Relay chatter (very fast edge), switching processes. Characteristics of these transients are influenced by distributed capacitance and inductance of the wiring harness.
• Affect: Signal Interference and component malfunction
   

 

Parameters	Nominal 12 V system	Nominal 24 V system
Us	−112 V to −220 V	−150 V to −300 V
Ri	50 Ω
td	150 ns ± 45 ns
tr	5 ns ± 1.5 ns
t1	100 μs
t4	10 ms
t5	90 ms

Pulse 3b

• Fast Transients – Positive
• Cause: Relay chatter (very fast edge), switching processes. Characteristics of these transients are influenced by distributed capacitance and inductance of the wiring harness.
• Affect: Sensors and controllers
   

 

Parameters	Nominal 12 V system	Nominal 24 V system
Us	+75 V to +150 V	+150 V to +300 V
Ri	50 Ω
td	150 ns ± 45 ns
tr	5 ns ± 1.5 ns
t1	100 μs
t4	10 ms
t5	90 ms

Pulse 4

• Cranking Pulse - Slow Negative
• Cause: Supply voltage reduction caused by energising starter motor of internal combustion engine.
• Affect: Sensors and controllers
   

 

Parameters	Nominal 12 V system	Nominal 24 V system
Us	− 6V to − 7V	− 12V to − 16V
Ua	− 2.5V to − 6V with |Ua| ≤ |Us|	− 5V to − 12V with |Ua| ≤ |Us|
Ri	0 Ω to 0.02 Ω
t7	15 ms to 40 ms	50 ms to 100 ms
t8	≤ 50ms
t9	0.5 s to 20 s
t10	5ms	10ms
t11	5ms to 100ms	10ms to 100ms

Pulse 5a – Load Dump

• Unsuppressed Alternator Surge- Without Protection
• In the event of a discharged battery being disconnected while the alternator is generating charging current and with other loads remaining on the alternator circuit at this moment.
• Affect: ECU, Sensor, Power circuit
  
   

Parameters	Nominal 12 V system	Nominal 24 V system
Us	65V to 87V	123V to 174V
Ri	0.5 Ω to 4 Ω	1 Ω to 8 Ω
td	40ms to 400ms	100ms to 350ms
tr	10ms

Pulse 5b – Load Dump

• Suppressed Alternator Surge- with protection (like Zener diode, TVS diode)
• In the event of a discharged battery being disconnected while the alternator is generating charging current and with other loads remaining on the alternator circuit at this moment.
• Affect: ECU, Sensor, Power circuit
   

 

Parameters	Nominal 12 V system	Nominal 24 V system
Us	65V to 87V	123V to 174V
Us*	As specified by manufacturer
td	40ms to 400ms	100ms to 350ms

 

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Testing Setup

Usually tested in the lab using an EMC test bench with:

• Pulse generators
• Coupling/decoupling networks
• Oscilloscopes
• Electronic loads or real DUTs

The DUT (Device Under Test) must withstand or operate normally depending on the pulse.

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Other Standards

• SAE J1113 – Similar to ISO 7637, North American usage
• LV 124 / LV 148 – German standards for 12V/48V systems (used by BMW, VW, Daimler, etc.)
• ISO 16750-2 – Broader set including electrical load, jump start, reverse polarity, etc.

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Conducted Emission

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Conducted emissions refer to electromagnetic disturbances that are transmitted through conducting wires or cables connected to an electrical or electronic device. These emissions can propagate along power lines, signal lines (such as data cables), and any other conductive paths that are connected to the device.

Here's a detailed breakdown of conducted emissions:

Sources:

Conducted emissions originate from various sources within electronic equipment:

1. Power Supplies: Switching power supplies and transformers can generate conducted emissions due to switching transients and harmonics.

2. Electronic Circuits: Digital circuits, especially those with fast switching speeds, can produce conducted emissions through power and signal lines.

3. Motors and Drives: Electric motors and motor drives can introduce conducted emissions into power lines due to switching frequencies and power modulation.

4. Cables and Connectors: Poorly shielded or improperly grounded cables can act as antennas, radiating conducted emissions.
    

Frequency Range:

Conducted emissions typically cover a wide frequency range, often from a few kilohertz (kHz) up to several hundred megahertz (MHz), depending on the nature of the emitting device and the frequency of the signals or power being processed.

• Low Frequency Range (LF): Typically below 150 kHz, includes power line harmonics and switching noise from power supplies.

• Radio Frequency Range (RF): Spans from 150 kHz to several hundred MHz, encompassing emissions from digital circuits, clock signals, and other high-frequency components.

Measurement and Testing:

Conducted emissions are typically measured using specialized equipment such as spectrum analyzers and conducted emission measurement receivers. Testing is conducted with the device under test (DUT) connected to a standardized test setup that simulates real-world operating conditions. Measurements are taken across specified frequency ranges to ensure compliance with applicable EMC standards.

Test Setup
 

 

Test Instrument

Below are the some Major Test equipment, required for Conducted Emission Testing

1. EMI Receiver

2. Line Impedance Stabilization Network (LISN)

3. Pulse Limiter

4. RF Cables

Test Parameters

1. Frequency Range: 9kHz to 30MHz (depends on the Product standard)

2. Detector: Quasi-Peak, Average

Test Procedure

1. Setup EUT (Equipment Under Test) at the designated place, 0.8m high on the insulated table.

2. Table must be kept on the Ground Reference plane and at a distance of 0.4m from the Vertical coupling plane.

3. EUT should be energised through LISN with the rated Voltage and current and to be set in the highest configuration to achieve max level of emission from it.

4. RF cable should be connected between LISN and EMI Receiver.

5. Set the desired frequency range, limit lines and other parameters in EMI Receiver to receive the EMI signal from the EUT.

6. Select the peaks closure to limit line and measure the Q-Peak and Average values for final measurement.

7. Repeat step 5 & 6 for every individual line and neutral.

8. Capture the data like emission level in dbµV, Limit at that particular frequency, calculate the margin to mention in the final report. 

9. There are different limit lines as per different product standards.

Most of the standards refers to the below limits lines based on the end use of the product. 

Conducted Emission limits_Class A

Frequency Range (MHz)	Quasi Peak (dbμV)	Average (dbμV)
0.15 - 0.5	79	66
0.5 - 30	73	60

Conducted Emission limits_Class B

Frequency Range (MHz)	Quasi Peak (dbμV)	Average (dbμV)
0.15 - 0.5	66-56	56-46
0.5 - 5.0	56	46
5.0 - 30	60	50

 

Electromagnetic Emissions from the DUT should be less than the limits mentioned above. 

Conducted Emission Spectrum with both the Class A and Class B limits. 

In summary, conducted emission testing ensures that devices meet regulatory EMC limits, contributing to reliable and interference-free operation across electrical and electronic systems.

Common-Mode vs. Differential-Mode Noise

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Electrical Noise

Electrical noise is any unwanted signal superimposed on a desired electrical signal that can distort, interfere, or reduce signal fidelity. In high-speed circuits and EMC analysis, this noise is typically classified as:

• Differential-Mode (DM) Noise

• Common-Mode (CM) Noise

These two have different origins, transmission paths, and countermeasures.

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1. Differential-Mode Noise – “Normal-mode noise”

Differential-mode noise is the voltage difference between two conductors of a signal or power line. It represents the intentional signal path where the noise rides in opposition across the two conductors.

Current Flow:

The noise flows in opposite directions in a loop between the two wires. If one conductor has a +5V spike and the other has -5V, the differential noise is 10V.

Example Use Cases:

• Power rails: +V and GND in DC systems

• Communication: USB, HDMI, Ethernet pairs

• Analog signals: Sensor lines in instrumentation

Typical Sources:

• Switch-mode power supplies (SMPS) – due to rapid switching

• High-speed data lines – signal reflections, ringing

• Magnetic coupling – between traces or cables

• Load changes – from motors, solenoids, or relays

Mitigation Strategies:

• Differential-mode filters: Series inductors with capacitors across lines

• Matched impedance design: Prevents signal reflection and ringing

• Twisted pairs: Cables twisted to cancel out opposing fields

• Short trace lengths: Reduces antenna effect

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2. Common-Mode Noise – “Ground-referenced noise”

Common-mode noise appears equally and in phase on both conductors relative to a common reference point, usually system ground or chassis.

Current Flow:

Both wires carry the noise in the same direction, and the return path flows through the ground plane, earth, or shielding.

Real-World Example:

• Long USB cables pick up RF noise equally on both data lines.

• AC power lines exposed to EMI from nearby radio towers or industrial machines.

Typical Sources:

• Electrostatic coupling: From nearby high-voltage lines

• Radiated emissions: Antenna-like behavior of cables or traces

• Ground potential differences: Between system components (e.g., USB ground loop)

• Parasitic capacitance: Between PCB traces and the chassis

Mitigation Strategies:

• Common-mode chokes: High impedance to CM signals, low to DM signals

• Shielded cables and connectors: With 360° termination to chassis

• Isolated grounds: Breaks in ground loops

• Ferrite beads/clamps: On external cables

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3. Visualizing the Current Paths

 

Noise Type: Differential Mode

Current Direction: Opposite directions on each line, Reference Point: Across the pair, Return Path: One conductor to another

Noise Type: Common Mode

Current Direction: Same direction on both lines, Reference Point: Ground or chassis, Return Path: Through system or earth ground

 

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4. Importance in EMC Testing

Emissions:

• Differential-mode emissions are mostly conducted (via power/signal lines).

• Common-mode emissions often become radiated, as CM currents form large loop areas (acting like antennas).

Compliance Implications:

• Regulatory bodies (e.g., FCC, CISPR, IEC) test both emission types.

• CISPR 22/32 and FCC Part 15 focus heavily on common-mode conducted emissions in lower frequency bands (150 kHz–30 MHz).

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5. Filtering Techniques Comparison

Filter Type: CM Filter

Effective Against:  Common-mode noise, Components Used:  Common-mode choke, Y-capacitors

Filter Type: DM Filter

Effective Against: Differential-mode noise, Components Used:  Series inductors, X-capacitors

Notes:

• Y-capacitors connect from line to ground (handle CM).

• X-capacitors go across the line pair (handle DM).

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6. Example: SMPS Input Filtering

In a switch-mode power supply, both noise types are generated:

• Differential noise arises from the switching node oscillations.

• Common-mode noise results from parasitic capacitance between high-frequency switching nodes and the chassis.

The input filter typically includes:

• X-capacitor across line and neutral (DM)

• Y-capacitors from line/neutral to ground (CM)

• Common-mode choke for both conductors

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7. Application-Specific Impacts

High-Speed Digital Systems:

• Differential signaling (LVDS, HDMI, USB) relies on clean DM paths. Noise degrades data integrity (eye diagrams, jitter).

• CM noise can cause cross-talk between pairs or fail EMC radiated tests.

Automotive:

• CM noise is a major concern due to long wire harnesses acting as antennas.

• Standards like ISO 11452 and CISPR 25 require thorough CM filtering.

Medical Devices:

• Safety and immunity to external EMI are critical—isolation transformers, CM chokes, and filtering are used to ensure patient safety and compliance (e.g., IEC 60601-1-2).

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Conclusion
 

Aspect	Differential-Mode Noise	Common-Mode Noise
Flow Direction	Opposite on conductors	Same on both conductors
Reference	Between lines	Against ground or chassis
Typical Source	Internal circuit switching	External EMI, parasitic coupling
Testing Concern	Conducted emissions	Radiated + conducted emissions
Mitigation	X-caps, twisted pairs, impedance match	CM chokes, Y-caps, shielding

Both types of noise must be addressed for:

• Regulatory compliance

• Signal integrity

• Functional reliability

Designers should always measure both types during EMC testing and implement layered filtering and shielding strategies.