原创:Jacob Beningo@DigiKey得捷电子
With the ubiquity of electronic devices in homes, offices, and industries, the need for high-speed, compact, low-cost, resettable, and adjustable circuit protection devices is becoming increasingly important to ensure user safety and maximum equipment uptime. Traditional fusing methods have inaccurate fusing currents, slow response times, and often inconvenient fuse replacement.
While it's possible to design a proper protection scheme from scratch, it's not easy to meet the demanding latency and accuracy requirements in a resettable device. In addition, the same solution is now expected to include features such as adjustable overcurrent protection, adjustable inrush current slew rate, overvoltage clamping, reverse current blocking, and thermal protection. This design requires a large number of discrete components and several ICs, which not only takes up a large area on the PC board, increases cost, but also delays time to market. The increasing difficulty is to meet high reliability requirements and meet the requirements of international safety standards such as IEC/UL62368-1 and UL2367.
To do this, designers can turn to electronic fuse (eFuse) ICs to provide nanosecond (ns) short-circuit protection, which is a million times faster than traditional fuses or PPTC devices.
An introduction to eFuse and how it works
This article explains why faster, more robust, more compact, more reliable, and more economical circuit protection is needed before introducing the eFuse and how it works. It then describes several eFuses from Toshiba Electronic Devices and Storage Corporation and explains how these devices meet the circuit protection needs of designers in terms of affordability, compactness, and ruggedness.
Circuit protection requirements
Overcurrent conditions, short circuits, overloads, and overvoltages are some of the basic circuit protection needs of electronic systems. In the overcurrent state, there is an excess of current flowing through the conductor. This can lead to high levels of heat, fire, or risk of damage to the equipment. Overcurrent faults can be caused by short circuits, overloads, design failures, component failures, and arc or ground faults. In order to protect circuit and device users, overcurrent protection requires transient action.
In the presence of an overload, excessive currents are not immediately dangerous, but the consequences of long-term overloads are just as unsafe as high overcurrents. Overload protection is achieved with various time delays depending on the degree of overload. As the overload increases, the latency decreases. Overload protection can be achieved with a delay or slow fuse.
Overvoltage conditions can lead to unstable system operation and can also lead to excessive heat generation and increase the risk of fire. Overvoltages also pose an immediate danger to the system user or operator. Like overcurrent, overvoltage protection requires quick action to cut off the power supply.
To ensure safe and stable operation, some applications also benefit from additional protection functions in addition to the basic protection functions, including adjustable levels of overvoltage and overcurrent protection, start-up inrush current control, thermal protection, and reverse current blocking. A variety of different circuit protection devices are available to meet the needs of different combinations of these protection functions.
How eFuses work
Compared to traditional fuses and PPTC devices, the eFuse IC enables a wider range of protection functions and a higher level of control (Figure 1). In addition to high-speed short-circuit protection, the eFuse offers accurate overvoltage clamping, adjustable overcurrent protection, and adjustable voltage and current slew rate control to minimize inrush current and thermal shutdown. The different versions also include a built-in reverse current blocking function.
Figure 1: The eFuse can replace traditional fuses or PPTC devices with more protection features and a higher level of control. (Image source: Toshiba)
One of the key factors in eFuse's performance is the internal power MOSFETs, which typically have "on" resistance in the milliohm (mΩ) range and can handle high output currents (Figure 2). During normal operation, the very low on-resistance of the power MOSFET ensures that the voltage at the VOUT terminal is nearly identical to that at the VIN terminal. When a short circuit is detected, the MOSFET disconnects very quickly, and when the system returns to normal, the MOSFET is used to control the inrush current.
Figure 2: The low on-resistance power MOSFET (top center) is key to the eFuse's ability to achieve fast action and controlled start-up. (Image source: Toshiba)
In addition to power MOSFETs, the active nature of eFuses contributes to numerous performance benefits (Table 1). Conventional fuses and PPTC are passive components with very low accuracy of trip current. They all rely on Joule heating and are time-consuming, which increases their reaction time. The eFuse, on the other hand, constantly monitors the current and activates short-circuit protection once it reaches 1.6 times the adjustable current limit. Once activated, eFuse's ultra-high-speed short-circuit protection technology reduces current to near zero in just 150 to 320 nanoseconds, while fuses and PPTC have a reaction time of 1 second or more. This fast response time reduces system stress, which enhances robustness. Since the eFuse eFuse cannot be destroyed by a short circuit, it can be used multiple times.
Form source: Toshiba
Table 1: Compared to fuses and PPTC (Convergence Switch) devices, eFuse ICs offer faster protection, higher accuracy, and more comprehensive protection features.
Compared to traditional fuses that are single-use devices, eFuse helps reduce maintenance costs and shorten recovery and repair times. eFuse has two types of failback: auto-recovery and lock-out protection: the former automatically resumes normal operation after the fault condition is eliminated, and the latter recovers when an external signal is applied after the fault is eliminated. The eFuse also offers overvoltage and thermal protection, which is not possible with conventional fuses or PPTC.
Select eFuses
Choosing the right eFuse typically starts with the power rails of your application. For 5 to 12 volt power rails, the eFuse is a good choice. Rated for input voltages up to 18 V, currents of 5 A, IEC 62368-1 certified to UL2367, the series is available in a WSON10B package measuring 3.0 mm x 3.0 mm x 0.7 mm tall and measuring 0.5 mm pitch (Figure 3).
Figure 3: Toshiba's eFuses are available in a 3 mm x 3 mm, 0.7 mm tall WSON10B surface-mount package. (Image source: Toshiba)
For designers, the TCKE8xx family offers increased flexibility including overcurrent value adjustment by external resistor settings, slew rate control by external capacitor settings, over- and under-voltage protection, thermal shutdown, and a control pin for select external reverse current blocking FETs.
Designers can also choose between three different overvoltage clamps: a 6.04 V clamp for 5 V systems (e.g., ), a 15.1 V clamp for 12 V systems (inclusive), and a clampless voltage (e.g., Figure 4). Depending on the model, the overvoltage protection is divided into two modes: automatic retry and clamping, and the clamping level is set with an accuracy of 7%. Undervoltage lockout can be set by an external resistor. Thermal shutdown protects the IC from overtemperature by disconnecting the eFuse when it exceeds 160°C. Models with auto-recovery thermal protection restart when the temperature drops by 20°C.
Image credit: Toshiba
Figure 4: The TCKE8xx family of eFuses includes a TCKE805 with a clamping voltage of 6.04 V for 5 V systems, a TCKE812 with a clamping voltage of 15.1 V for 12 V systems, and TCKE800 without a clamping voltage.
To ensure stable operation, these eFuses have the option for designers to set current and voltage slopes at startup (Figure 5). When power is applied, a huge inrush current flows into the output capacitor and trips the eFuse, resulting in unstable operation. External capacitors on the dV/dT pins of the eFuse can be used to set the start-up ramp for voltage and current to prevent no-tripping.
Figure 5: Designers can set start-up ramps for voltage and current to ensure stable operation of the eFuse. (Image source: Toshiba)
Depending on the application requirements, designers can add an external N-channel power MOS to block the reverse current, a transient voltage suppression (TVS) diode to protect the input transient voltage, and a Schottky barrier diode (SBD) to protect against negative voltage spikes at the eFuse output (Figure 6). Reverse current blocking is useful in applications such as hot-swappable disk drives and battery chargers. The external MOSFET is controlled via the EFET pins.
A TVS diode is required in a system where there will be an instantaneous voltage on the power bus that exceeds the maximum rating of the eFuse. In some applications, negative voltage spikes can occur at the output of the eFuse, and optional SBDs can protect the ICs and other devices on the load side, as well as the eFuse itself. Toshiba recommends as an external MOSFET, as a TVS diode, and as an SBD.
Image credit: Toshiba
Figure 6: A typical application for the TCKE8xx family of eFuses shows an optional TVS for input transient voltage protection, an SBD for negative voltage spike protection on the output pin, and an external MOSFET to block the reverse current.
eFuse with built-in reverse current blocking MOSFET
For applications that require the smallest possible solution with reverse current blocking, designers can use an eFuse with two internal MOSFETs (Figure 7). The second internal MOSFET has no performance loss, and the combined on-resistance of the two MOSFETs is only 53 mΩ, which is about the same as when using an external blocking MOSFET.
Image credit: Toshiba
Figure 7: TCKE712BNL, the RF eFuse includes two MOSFETs (top center) for reverse current blocking without the need for an external MOSFET.
In contrast to the fixed voltage design of the TCKE8xx series, the input voltage range of the RF TCKE712BNL is 4.4 to 13.2 V. To support this possible input voltage range, the device has an overvoltage protection (OVP) pin that enables designers to set the level of overvoltage protection to suit specific system needs. In addition, a FLAG pin has been added to the TCKE712BNL to provide an open-drain signal output indicating a fault condition.
epilogue
Ensuring the circuitry and user protection functions of electronic systems is critical, especially in the current situation of equipment proliferation and increased potential for failure. At the same time, designers must minimize cost and packaging while also providing maximum protection flexibility to meet appropriate protection standards.
eFuse offers ultra-fast action speeds, excellent accuracy, reliability, and reusability. These devices are high-performance and highly flexible, making them an alternative to traditional fuses and PPTC devices, but they also have a variety of built-in features that greatly simplify the design of circuits and user protection.