Smart fuse reduces cost, weight of automotive wiring harness: Page 5 of 6

September 14, 2017 //By Martin Jaiser, Manfred Brandl, ams
Smart fuse reduces cost, weight of automotive wiring harness
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). Besides of being a very cheap component, there are numerous and serious drawbacks in using simple fuses. But there are alternatives.

In terms of board layout, several current paths are routed together, and covered on the backside with an additional thermal mass (made of copper). This ensures that they are thermally coupled, and so a single temperature measurement may be applied to the entire circuit, with compensation for thermal resistance implemented in the microcontroller (see Figure 2).

The ADC itself has a chopper architecture which produces a zero offset: this enables the circuit to measure current accurately even at low currents. Level shifters drop the analogue voltage over the shunts to the ADC’s voltage domain. Dechopping is implemented in the digital filter of the Sigma-Delta ADC.

A common objection to chopper architectures is that they generate noise. But in this ams design, chopping with ringing cancellation is performed in the analogue domain, and dechopping in the digital (see Figure 3). As a result, noise is negligible, while the offset in the signal path of the current channel is entirely eliminated.


Temperature measurement

In the ams circuit, measurement of the MOSFET’s temperature is performed by external temperature sensors, since most MOSFETs’ internal temperature measurement is not accurate enough. With an additional comparator, this allows for direct over-temperature shut-down, bypassing the digital circuit in the same way as for the over-current shut-down function.

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