With the pervasiveness of Electronic devices in the home, office, and industry, the need for circuit protection that is compact, low cost, high speed, resettable, and adjustable is increasingly important to ensure user safety and maximum device uptime. Conventional fusing approaches suffer from imprecise breaking currents and slow response times and typically are encumbered with the inconvenience of having to replace the fuse.
While it’s possible to design a suitable protection solution from scratch, it’s not easy to achieve the demanding latency and precision requirements in a resettable device. In addition, that same solution is now also expected to feature adjustable overcurrent protection, adjustable inrush-current slew rate, overvoltage clamping, reverse-current blocking, and thermal protection. Such a design requires numerous discrete components and several ICs that together occupy a significant area on the pc board, raise costs, and delay time to market. Increasing the difficulty is the need for high levels of reliability and the requirement to meet international safety standards such as IEC/UL62368-1 and UL2367.
To address these requirements, designers can instead turn to electronic fuse (eFuse) ICs to provide nanosecond (ns) short-circuit protection, which is about one million times faster than conventional fuses or PPTC devices.
This article describes why faster, more robust, compact, reliable, and more cost-effective circuit protection is required, before introducing eFuses and how they work. It then introduces several eFuse options from Toshiba Electronic Devices and Storage Corporation and shows how they support designers’ needs for cost-effective, compact, and robust protection.
Circuit protection needs
Overcurrent conditions, short circuits, overloads, and overvoltages are some of the basic circuit protection needs of electronic systems. During an overcurrent condition, an excessive current is flowing through a conductor. This can lead to high levels of heat generation and the risk of fire or damage to the equipment. Overcurrent conditions can be caused by short circuits, excessive loads, design faults, component failures, and arc or ground faults. To protect circuits and device users, overcurrent protection needs to operate instantly.
Overload conditions exist when the excessive current is not immediately dangerous, but the long-term consequences can be just as unsafe as a high overcurrent condition. Overload protection is implemented with various time delays based on the level of the overload. As the overload condition rises, the delay decreases. Overload protection can be implemented with time-delay or slow-blow fuses.
Overvoltage conditions can result in unstable system operation and can also lead to the generation of excessive heat and increased potential for fire. Overvoltages can also present immediate danger to system users or operators. As with overcurrent, overvoltage protection needs to operate quickly to cut off the source.
Some applications benefit from additional protective functions beyond just the basics to ensure safe and stable operation including adjustable levels of overvoltage and overcurrent protection, start-up inrush current control, thermal protection, and reverse current blocking. Various circuit protection devices can satisfy different combinations of these circuit protection needs.
How eFuses work
eFuse ICs provide more extensive protection functions and higher levels of control compared with conventional fuses and PPTC devices (Figure 1). In addition to high-speed short-circuit protection, eFuses provide precise overvoltage clamping, adjustable overcurrent protection, adjustable voltage, and current slew rate control to minimize inrush currents and thermal shutdown. Versions also include built-in reverse current blocking.
Figure 1: An eFuse can replace conventional fuses or PPTC devices and provide additional protection functions and higher levels of control. (Image source: Toshiba)
One of the keys to eFuse performance is the internal power MOSFET with an “ON” resistance that is typically in the milliohm (mΩ) range and that can handle high output currents (Figure 2). During normal operation, the very low ON resistance of the power MOSFET ensures that the voltage at VOUT is almost identical to the voltage at VIN. When a short circuit is detected, the MOSFET switches off very quickly, and when the system returns to normal, the MOSFET is used to control the inrush current.
Figure 2: A low ON resistance power MOSFET (top center) is key to providing the fast action and controlled startup capabilities of eFuses. (Image source: Toshiba)
In addition to the power MOSFET, the active nature of eFuses contributes to their numerous performance advantages (Table 1). Conventional fuses and PPTCs are passive devices with a low accuracy with respect to the tripping current. They rely on Joule heating that takes time to develop, increasing their reaction times. An eFuse, on the other hand, is constantly monitoring the current, and once it reaches 1.6 times the adjustable current limit level, short-circuit protection is initiated. Once initiated, the ultra-high-speed short-circuit protection technique in eFuses reduces the current to near zero in only 150 to 320 ns, compared with the 1 second or longer reaction times of fuses and PPTCs. This fast reaction time reduces system stresses, enhancing robustness. Since an eFuse is not destroyed by a short circuit, it can be used multiple times.
Table 1: eFuse ICs provide faster protective speed, higher levels of precision, and a more complete suite of protection functions compared with fuses and PPTC (poly switch) devices. (Table source: Toshiba)
Compared with conventional fuses, which are single-use devices, eFuses contribute to reduced maintenance costs and less recovery and repair time. Two types of recovery from fault conditions are available with eFuses: Auto recovery will return to normal operation once the fault condition is removed, and; Latched protection that recovers when an external signal is applied after the fault is eliminated. Overvoltage and thermal protection are also provided with eFuses but are not possible using conventional fuses or PPTCs.
Selection of eFuses
The selection of the appropriate eFuse typically starts with the application’s power rails. For 5 to 12-volt power rails, the TCKE8xx Series eFuses are a good option. They are rated for up to 18 volts input and 5 amperes (A), are IEC 62368-1 certified, meet the requirements of UL2367, and come in a WSON10B package that measures 3.0 mm x 3.0 mm x 0.7 mm high, with a 0.5 mm pitch (Figure 5).
Figure 3: Toshiba eFuses are packaged in a 3 mm x 3 mm, 0.7 mm high WSON10B surface mount package. (Image source: Toshiba)
The TCKE8xx series offer designers flexibility, including an adjustable overcurrent limit set by an external resistor, an adjustable slew rate control set by an external capacitor, overvoltage and under-voltage protection, thermal shutdown, and a control pin for an optional external reverse-current blocking FET.
Designers also have a choice of three different overvoltage clamping levels; 6.04 volts for 5 volt systems (for example, the TCKE805NL,RF), 15.1 volts for 12 volt systems (including the TCKE812NL,RF), and without clamping (such as the TCKE800NL,RF) (Figure 4). Overvoltage protection is available as auto-retry and clamping, depending on the model, and clamping levels are set with a precision of 7%. The under-voltage lockout is programmable using an external resistor. Thermal shutdown protects the IC from an overtemperature condition by shutting the eFuse off when its temperature exceeds 160 degrees Celsius (°C). Models with auto-recovery thermal protection restart when the temperature drops by 20°C.
Figure 4: The TCKE8xx series eFuses are available with clamping voltages of 6.04 volts for 5 volt systems (TCKE805), 15.1 volts for 12 volt systems (TCKE812), and without clamping (TCKE800). (Image source: Toshiba)
To ensure stable operation, these eFuses include the option for designers to set the current and voltage ramp rate upon startup (Figure 5). When power is turned on, a large inrush current can flow into the output capacitor and trip the eFuse, resulting in unstable operation. An external capacitor on the dV/dT pin of the eFuse sets the startup ramp rate for the voltage and current, preventing nuisance tripping.
Figure 5: Designers can set the startup ramp rate of the voltage and current to ensure stable operation of the eFuse. (Image source: Toshiba)
Depending upon the application requirements, designers can add an external N-channel power MOSFET for reverse current blocking, a transient voltage suppression (TVS) diode for protection from input transient voltages, and a Schottky barrier diode (SBD) for protection from negative voltage spikes on the output of the eFuse (Figure 6). Reverse current blocking can be useful in applications such as hot-swap disk drives and battery chargers. The external MOSFET is controlled by the EFET pin.
The addition of a TVS diode is needed in systems that experience transient voltages on the power bus that exceed the maximum rating of the eFuse. In some applications, a negative voltage spike can appear on the output of the eFuse, and the optional SBD protects ICs and other devices on the load side, as well as the eFuse. Toshiba recommends the SSM6K513NU,LF as the external MOSFET, the DF2S23P2CTC,L3F as the TVS diode, and the CUHS20S30,H3F as the SBD.
Figure 6: Typical application for TCKE8xx series eFuses showing the optional TVS for input transient voltage protection, the SBD for protection from negative voltage spikes on the output pin, and an external MOSFET for reverse current blocking. (Image source: Toshiba)
eFuse with built-in reverse current blocking MOSFET
For applications that need the smallest possible solution and reverse current blocking, designers can turn to the TCKE712BNL,RF eFuse that includes two internal MOSFETs (Figure 7). There is no performance penalty associated with the second internal MOSFET; the combined ON resistances of both MOSFETs is only 53 mΩ, about the same as when using an external blocking MOSFET.
Figure 7: The TCKE712BNL,RF eFuse includes two MOSFETs (top center) to enable reverse current blocking without the need for an external MOSFET. (Image source: Toshiba)
Compared with the fixed voltage designs of the TCKE8xx Series, the TCKE712BNL,RF has an input voltage range from 4.4 to 13.2 volts. To support this range of possible input voltages, it has an overvoltage protection (OVP) pin that enables designers to set the overvoltage protection level to accommodate specific system needs. Additionally, the TCKE712BNL has an added FLAG pin that provides an open drain signal output indicating the presence of a fault condition.
Ensuring circuit and user protection in electronic systems is critical, particularly as devices proliferate and the potential for failure increases. At the same time, designers must keep costs and footprint to a minimum, while also achieving maximum protection flexibility, and meeting appropriate protection standards.
With ultra-fast operation, precision, reliability, and reusability, eFuses not only provide designers a high-performance, flexible, alternative to conventional fuses and PPTC devices, but they also come with a wide range of built-in features that greatly simplify the task of circuit and user protection design.
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