A car’s 12 V system is only “12 V” in the same way the ocean is “flat.” In reality, it’s constantly being kicked around by motors switching on and off, relays snapping open and closed, and an alternator that’s always chasing whatever load the vehicle throws at it. Any electronics tied into that rail have to ride through those electrical jolts without letting the damage make it past the front door.
One of the tougher events in that environment is load dump, which represents a sudden and sustained overvoltage condition that can stress every component connected to the supply rail.
It happens when the battery becomes disconnected while the alternator is supplying charging current. With the battery no longer absorbing that energy, the voltage on the electrical system rises until the alternator or another protection device limits it.
For an ECU packed with processors, sensors and power-management electronics, what happens to that voltage before it reaches the circuitry matters.
Transient voltage suppressor (TVS) diodes are one way to handle the problem. But selecting a TVS isn’t only about choosing a device capable of surviving a large pulse. Its clamping behavior determines the voltage the rest of the circuit still has to withstand.
A new TVS family from Littelfuse takes a different approach to that part of the problem.
What Happens During a Load Dump?
Load dump is different from the short voltage spikes that also occur on automotive power lines. It is a relatively long, high-energy transient.
The suppressed version was historically identified as Pulse 5b under ISO 7637-2. The load-dump requirements were later moved to ISO 16750-2, where they are covered as Test A for systems without centralized suppression and Test B for systems with centralized load-dump suppression.
In a 12 V system, Test B represents a vehicle in which the alternator itself limits the load-dump voltage.
The ECU still needs protection from the remaining transient.
A TVS diode placed across the supply normally stays out of the way. Once the voltage exceeds its breakdown region, the diode begins conducting and diverts transient current away from the protected circuitry.
The important number for everything downstream is the clamping voltage.
That isn’t necessarily the same as the TVS diode’s breakdown voltage. As transient current rises, the voltage across a conventional TVS also rises. The downstream components therefore have to tolerate the voltage that remains while the TVS is conducting, not simply the nominal voltage of the vehicle’s electrical system.
This creates a design tradeoff. A TVS needs enough standoff voltage to remain inactive during normal vehicle operation, while its clamping voltage has to stay low enough to protect the circuitry behind it.
Flattening the TVS Response
Littelfuse is addressing that tradeoff with its new TPSMD-FL FlatSuppressX TVS diode series, developed for load-dump protection on 12 V automotive power rails.
Instead of the I-V response of a conventional TVS, the devices use a foldback characteristic. After the TVS enters its breakdown region, the design limits how far the clamping voltage rises as transient current increases.
In practical terms, Littelfuse is trying to create more separation between normal operating voltage and the maximum voltage that reaches the protected electronics during the transient.
That matters because a higher clamp voltage can force engineers to select downstream components with higher voltage ratings or add another protection stage. Keeping the clamp voltage lower could allow the TVS to handle more of the protection itself.
It doesn’t eliminate the need to evaluate the complete power-input design. The allowable clamp voltage still depends on the ratings of the circuitry downstream and the characteristics of the transient. But it gives engineers another way to manage the protection margin.
A 3 kW Device in an SMC Package
The TPSMD-FL series is rated for 3,000 W of peak pulse power and comes in a DO-214AB, or SMC, surface-mount package.
The series includes devices with working standoff voltages from 20 V to 28 V. Littelfuse specifies the family for ISO 16750-2 Test B load-dump protection, formerly identified as ISO 7637-2 Pulse 5b.
The parts are also AEC-Q101 qualified and rated for IEC 61000-4-2 electrostatic-discharge protection of up to ±30 kV for both contact and air discharge.
Those characteristics put the device in front-end protection applications including body control modules, powertrain electronics, infotainment systems and zonal control units.
The zonal example is particularly interesting. As automakers move some vehicle functions into zonal controllers, those modules bring together electronics that previously might have been spread across several ECUs. Their power inputs still have to contend with the electrical environment of the vehicle.
Reducing the number of components required for transient protection could therefore help with more than BOM cost. It could also save PCB area in modules where power conversion, communications and processing hardware are competing for space.
The Clamp Voltage Is Part of the System Design
TVS diodes are easy to describe as components that absorb voltage spikes. Designing with one requires a more careful look at what happens during the transient.
The device has to survive the energy of the pulse. The protected electronics have to survive the remaining clamped voltage. And the protection circuit has to do both repeatedly under the conditions defined for the vehicle.
That is why the TPSMD-FL’s foldback characteristic is more interesting than its 3 kW rating alone.
Rather than simply increasing how much transient power the TVS can handle, Littelfuse is trying to control the voltage the rest of the circuit sees while that transient is occurring.
For automotive engineers working on increasingly electronics-heavy vehicles, that could make the front end of the power supply a little simpler.
