Additional effects with higher on-board voltage levels
48V does not simply mean four times 12V, but entry into the world of hybrid vehicles and a huge stride towards electromobility and autonomous driving. Clearance and creepage distances are key for 48V electrical systems. Clearance is the shortest distance in air between two electric conductors. Hazardous arcs can be expected in the event of short circuit, cable breakage or switching under load when operating above roughly 20V. Arcs become very hot and thus pose an immense risk of fire. To manage this risk effectively, special relays and contact systems, characterized by high impact and temperature stability, are applied.
Another solution, which is technically more complex but wise, for preventing arcs is their early detection through measuring and evaluating current and voltage diagrams with the aid of intelligent sensors.
Arcs and the hazards they pose
Besides voltages above 20V, there is another condition for the permanent burning of arcs. Depending on the current and voltage, a minimum output of approximately 100W is needed to ensure an arc burns steadily. Electric circuits in 48V electrical systems are therefore exposed to a high risk of arcs, as this is where consumers with high power requirements are found, e.g. the electric turbocharger, electrically powered steering or dynamic stability control. Suitable protective concepts are thus indispensable. Basically, there are two types of arc:
The simplest type, an arc parallel to the load, occurs due a short circuit of the current-carrying conductor to ground, e.g. due to faulty insulation. Provided the fault current path is sufficiently low resistive, the short circuit or arc current is very high and is added to the load current. This trips the safety fuse, disconnects the electric circuit, and extinguishes the arc. Needless to say, the fuse must be able to effectively extinguish the arc inside it.
The second type, the arc in series to the consumers, is much harder to detect. It occurs when the 'live' circuit is interrupted. Examples of this are the opening of relay contacts or connecting and disconnecting connectors while the current is flowing. Arcs can also occur due to broken wires, loose contacts or a defective ground connection.
Since the arc path is in series with the load, the additional drop in voltage at the arc reduces the load current. This is the reason why conventional safety fuses do not detect series arcs. The breaking capacity of the fuse is not exceeded.
In a conventional electrical system, the safety fuse is the typical protective device that prevents the risk of a cable fire caused by overheating, overcurrents or short circuit. In the future, it will logically be replaced with an electronic solution to ensure every type of arc is detected and extinguished reliably and efficiently.
Electronic fuses offer more options
Besides protecting against arcs, electronic fuses consisting of semiconductor switches offer further benefits. In contrast to safety fuses, electronic fuses can be reset and thus reused. Once triggered, the electronic fuse can be diagnosed and reset via the vehicle bus without the use of tools.
The fuse characteristic of a fuse channel is, to a large degree, freely designable for an electronic fuse. For instance, pure overcurrent breaking capacities can be defined for very high currents, while the I²t characteristic may be used as the breaking limit for medium and small currents. The fuse characteristic can thus be
adapted flexibly to the current-carrying capacity of the connected wire and the dynamic behavior of the load. In contrast, when using safety fuses, it is necessary to stock hardware variants with varying load capacity and tripping characteristics (delayed action).
An electronic fuse combines the safety and the switching function, which is an additional advantage. A safety fuse can only switch off, while a bus-controlled electronic fuse can switch both off and on. This may remove the need for a circuit breaker, a protective device that would additionally be required in a concept featuring a safety fuse.
The next aspect has to do with safety. An electronic fuse with a bus connection can also dump load in a targeted and controlled manner. The electrical system management can thus assume an active role in the distribution of electrical energy, e.g. focusing the available power on a safety-relevant circuit.
Fuse replacement with the iFuse from Bosch
Our partner Bosch is currently working on the development of a highly integrated ASIC, known as iFuse, for 12V, 24V, and 48V applications. Along with a microcontroller and N-Channel Power MOSFETs, the iFuse can be used to design a semiconductor fuse with four channels. To do so, the iFuse integrates various functions of a system basis companion chip, such as power supply, watchdog, and supply voltage monitoring, for the microcontroller. Added to this are the gate drivers for the N-Channel MOSFETs used as switches. Brilliant: The current is measured without the need for external sensors like shunt resistors.
The basic function 'overcurrent shutdown' also occurs autonomously, without the microcontroller being involved. The SPI interface of the µC is used to configure the module, to control the outputs and the watchdog, and to read back the status and diagnosis data.
The iFuse comprises a buck converter for direct connection to the electrical system and downstream linear regulators for the internal power supply and the external supply of the microcontroller. The gate voltages for the switching transistors are generated via an internal boost converter. Both switching regulators have internal FETs, meaning only a few external components are necessary.
The iFuse can be protected against reverse polarity with an external MOSFET. The iFuse operates above a 3.5V power supply - permanently, even straight after switching on and is thus also subject to cold cranking.
In combination with an intelligent load with bus connection, the iFuse enables arc detection. To do so, both the microcontroller via the iFuse and the intelligent load determine the supply voltage and current and compare these two values through the microcontroller in the control unit of the iFuse. If the current determined by the load is less than the current measured by the iFuse, it indicates an arc parallel to the load. If the supply voltage determined by the load is less than the supply voltage measured by the iFuse, an arc in series to the load could be the reason.
Another method to determine an arc is applied by the spectral analysis of the supply voltage or the load current. This task could be assumed by the microcontroller with DSP capability in the control unit of the iFuse. The iFuse provides the required measuring signals which are mapped up to high frequencies. This is usually quite sufficient in conjunction with overcurrent monitoring for loads without built-in 'intelligence' or loads that cannot communicate.
- Four independent fuse channels
- Basic chip system for electronic fuses for 12V ... 48V
- Voltage range up to 70V
- Soft turn-on for large capacitive loads
- Bidirectional current measurement without external shunt resistors
- Intelligent measuring range selection for the highest measuring accuracy at every current level
- Power supply for microcontroller and power MOSFETs
- Specified for a voltage range down to 3.5V
- Watchdog for monitoring the microcontroller
- Internal temperature sensor
- Supply voltage monitoring
- SPI interface to the higher-level microcontroller
- Individual status signals for all four channels
- Compact package, TQFP64epad
- Protective function for overcurrent, stand-alone without µC intervention
- Full protective function up to 3.5V power supply
- Overcurrent detection and turn-off in less than 10µs
- Reverse polarity detection and protection
- Overcurrent detection and turn-off in active and sleep mode
- Individual overtemperature detection of the internal gate drivers
- Supports arc detection without impairing the protective function
The benefits at a glance
- One basic solution for smart fuses suitable for all LV electrical systems
- Less space required due to high integration
- Robust power supply with Automotive Grade
- Current saving modes for energy efficiency
- Easy-to-use system diagnosis
- Fast and accurate current measurement without external components
- Option for bidirectional disconnection as with safety fuses
Find components at www.rutronik24.com.