Digital functional block
Figure 1 shows a microcontroller which performs a number of digital functions:
- Digital de-chopper (demodulator) for current measurement
- Resistor temperature compensation for the copper shunts
- DAC shut-down signal for over-current and over-temperature
- End-of-line storage of calibration values for current paths
- Software to calculate exact current values for transfer to an ECU
- Communication interface
- Management of safety features
- Programming of over-temperature and over-current trigger characteristics for up to four channels
When implemented as an IC, it is intended that, wherever possible, digital functions should be implemented as state machines; the device will not include a microcontroller. This approach is simpler and cheaper, it consumes less power, and it better supports the requirement for functional safety.
It should be said that the analysis of the functional safety of this circuit is today in its early stages. The plan for implementing the IC product provides for an ASIL A rating, with provision for a higher safety rating if required by the industry. Since the use of intelligent fuses calls for some external software if the full benefits are to be enjoyed, a full functional safety analysis can only be made in the context of the final application.
One fact, however, should not be forgotten: a melted fuse has no ASIL level at all. In terms of functional safety, then, as well as system cost, fuel efficiency, performance and functionality, the intelligent fuse demonstrated by ams is far superior to the traditional thermal fuse.
The steady growth in the number of electronic, electrical and electro-mechanical functions in cars has given rise to many innovations in the design and operation of automotive power systems. In one domain, however, the car remains stuck in a technological Stone Age: the device of choice for circuit protection is still the fusible cut-out (fuse).
Its use continues in spite of its numerous and serious drawbacks. This is because the traditional fuse has one, very powerful attribute in its favour: its unit cost is very low.
Looked at from a system point of view, however, the implementation of a more intelligent, electronic fuse offers the potential to cut total cost, as well as to reduce the weight of the vehicle substantially. Now ams has introduced a reference design board which shows the industry a design concept for an accurate yet simple intelligent fuse. It gives OEMs the opportunity to evaluate the concept, understand its advantages and cost, and simulate its operation in complex power systems.
This article details the drawbacks of the traditional thermal fuse. It then describes the operation of the new demonstration circuit. It shows how innovative high-side current measurement directly on the board’s copper traces, together with fast analogue over-temperature and over-current shut-down circuits, provide a simple, low-power, low-cost and high-performance alternative to the thermal fuse.
* The ageing of fuses is rarely discussed in the literature, although it is an inherent characteristic of the device. See J. Shi et. Al, Ageing Assessment Condition, Inspection and Lifetime. Journal of Energy and Power Engineering 5 (2011) 892-898
About the authors
Martin Jaiser gained his diploma in electronic engineering from the University of Karlsruhe. He then held several positions in sales and field applications engineering at Analog Devices, Rambus and Elmos. Since 2012, he has been working as a field applications engineer specialising in automotive applications at ams AG, with a particular focus on position sensing, inductive and capacitive sensing, and NFC.
Manfred Brandl is a senior product manager in the mobility sensors division of ams. He began his semiconductor career in 1984 when he joined the then Austria Microsystems as a product engineer. He was then promoted to a role in foundry engineering before joining the automotive business unit in 2000. Since 2003 he has been a product manager for battery management ASSPs. Brandl holds a bachelor’s degree in electrical engineering and a masters degree in mechanical engineering from the Technical University of Graz (Austria). He has been awarded ten patents for inventions in sensors, sensor interfaces, MEMS technology and packaging. He can be reached under the mail address email@example.com
Further information www.ams.com
All illustrations (C) ams.