Automotive Pulse
Understanding Automotive Pulse Testing
A vehicle's 12V power bus is nothing like a laboratory bench supply. During normal operation, the battery voltage ranges from 9V (cold cranking at -40 °C) to 16V (fast charging). During abnormal events, the bus voltage can spike to +40V (load dump with centralized clamping) or dip to 3V (starter motor inrush). These transients happen thousands of times over a vehicle's 15-year life, and every electronic module on the bus must survive every one of them.
ISO 7637-2 Pulse Definitions
| Pulse | Cause | Voltage | Duration | Energy |
|---|---|---|---|---|
| Pulse 1 | Disconnection of inductive load (same wire) | -75 to -150V | 2 ms | Low (0.5 J) |
| Pulse 2a | Disconnection of inductive load (parallel wire) | -50 to -100V | 50 μs | Very low |
| Pulse 2b | Ignition switch-off, alternator field decay | +20 to +40V | 0.2 s | Medium |
| Pulse 3a/3b | Switching process (contact bounce, relay chatter) | -150V / +100V | 150 ns bursts | Very low (ESD-like) |
| Pulse 4 | Starter motor engagement (cranking voltage dip) | Down to 4.5V | 15 to 100 ms | N/A (undervoltage) |
| Pulse 5 | Load dump (battery disconnection under charge) | +40 to +120V | 200 to 400 ms | High (40+ J) |
E = 0.5 × Lalt × Ialt²
Where:
Lalt = Alternator rotor inductance (~10 mH typical)
Ialt = Alternator output current at moment of disconnect (~80A)
E = 0.5 × 0.01 × 6400 = 32 Joules
A centralized Zener clamp at the alternator absorbs most of this energy,
limiting the bus spike to 35 to 40V for 200 ms.
Without clamping: Vpeak can reach 120V.
Frequently Asked Questions
What is a load dump and why is it dangerous?
A load dump occurs when the battery is suddenly disconnected while the alternator charges at full current. The alternator voltage spikes because the stored inductive energy has no battery to absorb it. In a 12V system without clamping, voltage can spike to 80 to 120V for 200 to 400 ms. Modern vehicles use centralized clamping to limit this to 40V, but every ECU must still survive that level per ISO 7637-2 Pulse 5.
What is Pulse 2a and why do RF modules fail it?
Pulse 2a simulates the negative spike when a nearby inductive load switches off (-50 to -100V, 1 to 5 μs rise time). RF modules fail when the negative transient couples through decoupling capacitors to sensitive front-end components with no negative voltage tolerance. Protection requires a fast-response TVS diode on the power input.
How does 48V mild hybrid change pulse requirements?
48V systems introduce LV 148 requirements. Load dump peak increases to ~52V, and cranking dip has two phases: a brief 21V excursion during 48V starter engagement, then potential dip to 18V during assist. Higher bus voltage means protection TVS diodes must handle more energy, and wider voltage swing complicates DC-DC converter design.