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Semiconductor technology for fast protection

semiconductor technology

Authors: emilia mance, Corporate Product Sales Manager Standard Products Rutronik, and Turadj Aliabadi, Senior Marketing Manager for Discrete Semiconductor at Toshiba Electronics Europe

Today, fuses are used less to protect against system failures and more to safeguard against user failures. However, not only the nature of the faults changes, the fuses have also evolved. Fuses are based on a thin section of wire that is designed to "melt" if excessive current is flowing into the application being protected. Thus, they stop the flow of current and safeguard the application. Fuses are sized according to two units: current and time. The current limit defines the upper limit of allowable current flow before the fuse is sacrificed. The time element makes it possible to accommodate current spikes that typically occur beyond the specified limit, such as those that often occur during inrush current when a product is turned on.

The main problem with fuses of this type is the sacrificial part: when a fuse blows, a service technician, for example, will need to check why it blew and replace it. This is obviously time consuming and can cause delays in the process. It can also be challenging and expensive depending on how the fuse is integrated and the accessibility of the equipment. Today, an overload situation is often caused by user error, such as a short circuit when inserting a faulty USB device into a PC or laptop. Rather than using fuses, power supplies for such devices often make use of Polymeric Positive Temperature Coefficient (PPTC) components. This is a class of low impedance resistor. Its resistance increases rapidly due to heating during excessive current flow conditions caused by a fault, thus restricting current flow. Once the fault has been cleared, the fuse cools down and as a consequence returns to its original low resistance. This is how PPTCs offer protection, without the need for a maintenance technician, and work again after “exploding”.

Not the fastest protection feature

It should be noted that neither device is particularly fast in executing its protective function: fuses typically take a second to blow, while PPTCs respond faster, but can take seconds to reach their current constraint. complete. And while the fuses completely disconnect the appliances from the power supply, PPTCs still allow a small amount of current to flow, even after being triggered. Both fuse types are also dependent on ambient temperature, so a “limitation” to the higher operating temperatures must be considered in the design.

The fast fuse

Semiconductor technology has been used to improve or replace a wide variety of components over the past decades. This also applies to fuses. Electronic models, also known as eFuses, are increasingly replacing fuses and PPTC. They provide protection enhancements and have the ability to be reset once the fault has been cleared through a simple logical interface. motherboards (mainboards) of a computer, especially tracks that support SATA hard drives or USB ports, benefit from increased protection.

The eFuses they make use of advanced silicon processes. Low impedance MOSFET switches ensure low current loss when electricity flows. The built-in analog comparators are able to accurately monitor current flow, reacting in less than a microsecond if power needs to be cut off completely. In combination with a processor host, a decision about the cause of the failure and when to restore power can be made via the electronic fuse interface.

Being a product of silicon, the eFuses they provide a number of other useful functions, including overheat monitoring, overvoltage clamp, undervoltage shutdown, and reverse current protection.

eFuses in use

Any application that provides power for the submodules add-onsuch as oscilloscope probes or programmable logic controllers (PLCs), you can take advantage of technology eFuse. Solutions like the series TCKE8xxx de Toshiba they are easily integrated thanks to their compact WSON10B package (3 × 3 × 0,7 mm). The devices supply a short-circuit tripping current of A 5 with an accuracy of ±11 percent. Thanks to the rapid-fire comparator (almost-trip) integrated, the devices remove power under fault conditions within 150 ns. The series also provides the ability to choose models with automatic retry or blocked response. the variants auto-retry they reconnect power automatically as soon as they cool down (temperature around 68 °F or 20 °C), while the latched-response versions close and have to be reset via an EN pin. Compatibility with the standard IEC 62368 simplifies certification of the entire system for customers.

The resistance (RON) of the integrated switch is 28mΩ, while the effect slew rate to control inrush current and undervoltage blocking can be set using external components. Internal temperature monitoring also offers protection and, once 320°F (160°C) is reached, automatically shuts down the output.

La Figure 2 shows that this kind of eFuse can be used to safeguard a charging socket on USB chargers and battery packs. In this case, the TCKE805NL provides the optimum setting, offering latching protection with an overvoltage clamp set at 6,04 V. A 75 kΩ resistor connected to an ILIM pin limits current to 1,5 A, while a 2 nF capacitor provides a turn-on ramp time of 4 ms. 1 µF input and output capacitors located near the VIN and OUT pins reduce overshoot (overshot) and undershoot (undershoot) voltage during sudden changes in current draw. If required, an N-channel FET can also be integrated to protect against reverse currents.

Conclusion

Fuses and PPTCs have been a safety essential for many years. However, the type of protection required today is often against human-caused failures rather than failures of the entire system. The eFuses Configurable applications provide reliable, restartable protection that helps extend the life of applications and decrease the need for service technician support in many cases.

application circuit
Figure 1: Example of an application circuit for an electronic fuse (eFuse) protection solution

The history of the fuse

german physicist George Ohm comes to mind when talking about resistances. Similarly, we remember the British scientist Michael Faraday when selecting capacitors and from American physicist Joseph henry when sizing inductors. But interest in these three basic electrical components was waning when people began to quantify the characteristics of the humble fuse in the XNUMXth century.

usb charging socket
Figure 2. Protection of a USB charging socket using an eFuse TCKE805NL.

The development of a simple fuse, whose name comes from the Latin word “fusion” (cast), goes back primarily to Arthur C. Cockburn. Although part of his experiments was mocked by the newly formed Society of London Telegraph Engineers in your meeting of 1888, went to great lengths to scientifically determine the factors that combine to create a reliable fuse. The result of his work was the following: a fuse must be rated for melt between 150 and 200 percent of the current rating of the circuit being protected. At that time, electric lighting was still in its infancy, and in addition, telegraph workers needed protection from lightning strikes. Thus, the fuse thus became an essential safety component for this fledgling industry.