Semiconductor switches for electronic fuses
Ralf Hickl, Rutronik
Fuses protect the components in electrical circuits. If an overload or short-circuit occurs, the fuse triggers and opens the circuit. This prevents damage and keeps the cost of repairs down. Ideally, the shut-off characteristic is designed in such a way that the current is cut off before other components, such as the cables, semi-conductors or passive components on the circuit are damaged.
It follows therefore that a correctly-dimensioned fuse is the weakest link on the circuit. The thermal characteristic of the protected object is crucial; for example the electric cable, the wiring harness or the semiconductor switch in the connected control unit. Instead of safety fuses or electro-magnetically triggered mechanical contacts, electronic fuses contain semiconductor switches along with their control logic including protective and diagnostic functions.
The characteristics of fuses
Safety fuses are single-use products. They conduct, protect, and separate in both directions of current. This bi-directionality is an advantage in circuits with loads that - depending on the operating status - are motor or generator actioned. This includes, for example, traction inverters, which are also able to brake regeneratively.
On the other hand, electronic fuses can be triggered and reset any number of times. Due to the body diode, designs with a single MOSFET as the switch disconnect in only one direction of current. So, for bidirectional disconnection, two anti-serial MOSFETS need to be switched, which means a higher cost. Unlike safety fuses, electronic equivalents have their own power requirement. This needs to be taken into account for battery-driven devices in particular.
Conventional safety fuses are characterised by stipulating the rated current and trigger behavior (sluggish, agile, super-agile etc.) In the case of electronic fuses it is possible to set flexible cut-out criteria. This includes: Pt spec (maximum load integral or melting integral), over-current, over-voltage or under-voltage at the fuse input, over-load (power) and over-temperature - ambient or in the fuse.
The fuse manufacturer can also set the trigger behavior for an electronic fuse. So for example, it is possible first to limit the current and then cut out, or cut out immediately and wait for a reset. Because an electronic fuse already contains a switch, it can also be used for this function. So any further switch is superfluous and other functions such as a soft start can take place by controlling the switch with pulse width modulation (PWM).
Safety fuses are typically a single-use product. Once they have fulfilled their function they are unusable and must be replaced. With an electronic fuse, a soft start can be made using a configurable edge steepness on the semi-conductor switch or by controlling it with a PWM. Once triggered, it can switch back on automatically, and cyclically with a previously defined repeat number of reset attempts. Alternatively it is possible to reset only on request. Here a network connection is advantageous.
Particularly in systems that must meet a Safety Integrity Level (ASIL), an essential product characteristic is a diagnostics capability. Here too, electronic fuses provide advantages because it is possible to integrate the diagnostics directly. Moreover, the constant current metering can create and analyze a load profile. The result can be used as an early warning as well as a means to take preventative measures such as preventative maintenance and replace worn parts.
Other characteristics of electronic fuses
In the event of an over-load the electronic fuse scores highly with additional features. In the event of a short-circuit the cut-out is faster which means that the voltage dip on the rest of the on-board power system lasts for a shorter time. This is important for the electronic circuits installed there, which, after a protracted power dip and subsequent power-up will need a reset and reboot, before they are operational again. On critical assistance systems which could fail if a reboot were to occur during a journey, this could be extremely dangerous - the key phrase is "secure power supply".
An electronic fuse can be designed so that the current is limited to a defined maximum before it cuts out. Even where there is a short-circuit, the onboard power system voltage does not dip so much that other control units go to reset. In addition, the leakage inductance energy charge is restricted. This keeps voltage spikes low when oscillation occurs.
With no spark gap there is no arcing on switching/cut-out. This makes an electronic fuse more suitable for use in explosion protected areas.
Some components were developed for very specific applications, a smart high side driver for example still requires additional logic before it can be used as a fuse in practical use. The STEF01 by STMicroelectronics is well regarded everywhere. Its overcurrent (OC), overvoltage (OV), undervoltage (UVLO), and the slew rate are configurable by means of an external passive component (fully programmable). An external FET can be installed to implement a reverse-current blocking circuit.
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