Smart fuse reduces cost, weight of automotive wiring harness: Page 4 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 the IC version of the circuit, the switch will be realised as an internal gate driver and an external MOSFET. The drivers have to operate at high speed, because they need to frequently switch heavy loads, and, in order to avoid excessive switching losses, the MOSFET should spend as little time as possible in the linear region.

Like a conventional thermal fuse, the switch must be on the high side. This means that the driver needs an additional charge pump in order to raise the voltage far enough above the battery voltage to drive the MOSFET.

Accurate current measurement on the high side

A difficult part of the circuit to realise – and the reason why no semiconductor manufacturer before now has implemented this simple but high-performance design – is the high-side current measurement.

Despite being on the high side, this current measurement circuit must offer high accuracy if the circuit is to provide the benefits – such as weight reduction and intelligent power management – described above. Fortunately, ams has developed a technology which can achieve accurate high-side current measurement. What is more, it measures current directly on the board’s copper traces, avoiding the need for an expensive alloy precision shunt resistor.

On the demonstration board, the current measurement components are placed directly on top of the copper traces so that they can measure their temperature as well as the current flow. This eliminates the need for external sensors, while allowing for compensation for the temperature coefficient of the copper. (The dimensions of the copper trace must be specified with reasonable accuracy.)

With this technique for direct measurement on a trace, currents up to around 50A may be measured, using two layers of a four-layer board. Since the circuit is making high-side measurements, there is also a need for level shifters to provide a voltage that the ADC can handle. In the demonstration board, the circuit achieves current-measurement accuracy of ±2% over the operating temperature range. This can be improved if necessary.

The circuit also provides for direct cut-off, bypassing the digital functional block. This ensures that the fuse can switch within a maximum of 20µs when subject to a large current surge. Implementing the comparator and level shifter for this function is another demanding circuit-design challenge.  

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