In modern automotive and industrial applications, reliability is paramount. Products ranging from automotive domain controllers to computer numerical control in industrial applications, whether the end product is simple or complex, can damage a manufacturer's reputation if reliability is not guaranteed. In addition, there is also the cost of warranty repairs and even the cost of recalling the product.
However, electronic circuits will fail all the time, either due to external influences or due to degradation of component performance over time. Therefore, based on good design practices, it is recommended to employ circuit protection devices to ensure that the effects of faults are minimized.
This article describes the limitations of standard circuit protection devices and how eFuses can be used to improve designs.
The most common (and least costly) form of circuit protection is a common fuse. This type of fuse is usually based on a metal wire or a thin strip of metal with a low melting point temperature. The fuse is usually inserted into the power supply line, and when the rated current is exceeded, the heat generated in the fuse will blow the wire/metal band, disconnecting the circuit from the power supply.
This disconnection is usually not instantaneous, and the time it takes for the fuse to "blow" is inversely proportional to the magnitude of the fault current. If the current is only slightly higher than the fuse rating, it may continue to pass through for a period of time, affecting the supply rails, causing circuit failure or damage.
Fuses are not easy to use and once they blow, they need to be replaced (usually by the user). If the user uses the wrong rating (unintentionally or intentionally), it poses a fire risk.
Figure 1: Fuses and PPTC thermistors are commonly used for circuit protection
For circuit protection, other devices such as positive temperature coefficient (PTC) thermistors can also be used. The resistance of these PCB-mounted devices increases as the temperature increases, restricting the flow of current. As long as the overcurrent is not too large (otherwise the PTC thermistor will become open), as the current decreases, the temperature will drop and normal operation will resume.
While fuse replacement is no longer necessary, PTC thermistors are nonlinear and, given their operating principle, are not suitable for applications over a wide temperature range.
eFuse
eFuses (also known as eFuses, a term coined by onsemi when such devices were first introduced) are an alternative that today offers basic circuit protection features as well as many additional features. Typically, they are capable of providing protection against overcurrent (including short circuit), overvoltage, reverse current, and overtemperature.
Although these innovative devices have many applications, they are often used in hot-swap situations, or in scenarios where power failures are frequent. It can also be used in applications where load failures are highly susceptible, or where inrush current needs to be limited.
The enable pin, combined with a high-precision current control mechanism, allows the device to incorporate the functions of a load switch and a fuse, making it a fundamental building block for point-of-load control systems in modern power distribution architectures.
The main advantages of the eFuse are the flexibility and auto-reset function without the need for user intervention. As a smart device, these additional features enable the eFuse to do more in the system than just overvoltage/overcurrent protection, where the eFuse excels.
For example, many eFuses include a power good ("PGOOD") pin that can be used with a system controller to precisely sequence the power rails. Some eFuses also include a three-state pin that can be used to ensure that multiple rails are turned on and off simultaneously.
The ability of eFuses to detect reverse currents (which is not possible with conventional fuses) makes them useful in redundant power supply applications where ORing is required. It is also suitable for situations where a large capacitor is required to maintain charge after the system is shut down, and is typically used to limit the inrush current at start-up.
Figure 2: eFuses offer a variety of features to enhance protection.
In many applications, capacitance (or capacitive loads) can present a challenge, resulting in large inrush currents that can damage components or PCB wiring. eFuses can provide a variety of features to help designers solve this problem, such as self-healing or limiting inrush currents so that the capacitors can be charged in a controlled manner.
As smart devices, eFuses can monitor temperature, voltage, and current and relay data to the system controller. This is especially useful when detecting early warning signs of a fault.
eFuses in automotive applications
As more and more technologies are used in automobiles, the need for circuit protection to ensure reliable operation and prevent damage increases. Circuit protection is especially important in automobiles, where the battery's current capacity is strong enough to destroy delicate electronic components.
eFuses are often used in the power lines of subsystems, such as head-up displays or infotainment systems, to disconnect and shut down these systems in the event of a failure.
In systems that are partially located outside the vehicle, damage to external components can lead to short circuits that can damage the internal circuitry. For example, telematics systems have external LNA and GPS antennas connected via eFuses to protect the interior circuitry.
If the vehicle system is divided into different zones, eFuses can be cascaded within the system to provide overall protection as well as subsystem protection.
Figure 3: Cascaded eFuses are sometimes used in automotive domain control applications.
For example, in an advanced driver assistance system (ADAS) domain controller, a primary fuse can be connected between the power supply and the primary system, while a secondary fuse can be used to protect system peripherals, such as an externally mounted ultrasonic parking assist sensor unit.
Wiring harnesses in modern cars are a complex subsystem. Once the vehicle is assembled, wiring harnesses are expensive to replace and difficult to operate, so protection is crucial. Wiring harnesses are connected to many power-hungry devices (fans, window motors, air conditioners, other actuators). Fuses are often used in front of these systems to protect the wiring harness from excessive currents.
Examples of the latest eFuse technology
The onsemi NIV(S)3071 is a 60 VDC, 65 VTR eFuse that integrates four independent channels into a single 5.0 mm x 6.0 mm package, each capable of supporting up to 2.5A of continuous current (10A total). The RDSon value of only 80 mΩ per channel ensures that energy consumption within the eFuse is kept to a minimum.
All channels have configurable current limits, and other features include output voltage clamping, a digital flag to indicate faults, a configurable current action time, and a fixed 1 ms soft-start.
NIV3071 is capable of operating from a junction temperature (TJ) range of -40ºC to +150ºC and offers 2 kV ESD protection, making it ideal for demanding automotive applications, including 12V and 48V.
The NIV(S)4461 is an electronic fuse that provides overcurrent, undervoltage, and inrush current protection in applications such as industrial automation, telecommunications, computing, and more. The device can support continuous currents up to 360 V.
Notable features of the device include low resistance (typical RDSon = 39 mΩ) and fast run-time period. The device also includes programmable current limit (21-157A), undervoltage, and adjustable slew rate control. User-configurable features include latching and self-healing.
The NIV(S)4461 is available in a DFNN1024 package measuring just 3.0 mm x 3.0 mm. It features an industry-standard pinout and is UL2367 and IEC62368 compliant.
summary
Circuit protection is an essential part of modern design, ensuring circuit and system reliability and minimizing damage in the event of a failure or unexpected situation.
While traditional fuses offer a certain level of protection, modern eFuses offer significantly higher levels of protection with higher accuracy, greater flexibility, and a wider feature set.