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Circuit protection will never be the end of electronics development

2020-05-19
Circuit protection is like insurance; at best, it can be seen as an afterthought, and even when installed in place, it is often not enough. While underinvestment in insurance can threaten the stable operation of a business, inadequate circuit protection can lead to more serious consequences such as loss of life.

We illustrate the importance of circuit protection in the case of swissair flight 111, which departed from John f. Kennedy international airport in New York on September 2, 1998. The flight was operated by the 7-year-old McDonnell Douglas md-11, which recently upgraded its in-flight entertainment (IFE) system. Smoke from 52 minutes after taking off, the cockpit suddenly and the crew immediately declared a state of emergency response, and tried to alternate to halifax, the airport, but due to the cockpit ceiling electric control cable caused the fire burned out of control and crashed in 8 km of the sea from the nova scotia coast, killing all 215 passengers and 14 crew members.

The crash investigation found that the materials used in one section of the new IFE were the main cause of the crash, and that the materials, which were supposed to be fireproof, burned up and spread to critical control lines. Although it is impossible to say for sure, it is assumed that the electric arc between the IFE wires was the cause of the fire. Although these wires are fitted with circuit breakers, they do not trip because of arcing. This is a true case of 229 deaths caused by inadequate circuit protection. Such circuits are now equipped with arc fault detection protections to trip when an arc is sensed (not including the arc produced by normal operations such as pressing a switch).

Usb-pd brings more danger

Although the Swiss MD - 11 is caused by electrical failure rather than electronic failure, but now more and more circuits are enough to produce arc (and may endanger the fire of life) of voltage and current, such as the upgrade of the USB power supply (USB - PD), it can support up to 20 v and 5 a (maximum power of 100 w) of high voltage and current. Compared to the 5V voltage and 3A current (15W) of USB type-c, the upgrade of usb-pd is a big improvement, but it also greatly increases the possibility of danger.

In addition to the risks associated with high voltage and current, usb-pd can cause other problems when used with USB type-c connectors and cables. This is because the pin spacing of the USB type-c connector is only 0.5mm, one fifth of that of the type-a and type-b connectors, thus increasing the risk of A short circuit due to the slight distortion of the connector during insertion or removal. Impurities that build up inside the connector can have a similar effect. In addition, the popularity of USB type-c has also led to the significant development of cables, although many cables are still unable to carry 100W of power, but they are not identified. However, these signs do not guarantee security; If the consumer wants to use an unspecified cable, it can also be plugged into a usb-pd socket as easily as a qualified cable.

Arcs are not the only danger when usb-pd is used at high voltages and currents. Because the main bus power pin is very close to the other pins of the connector, a short circuit can easily expose the downstream electronics to a power surge such as a 20V short circuit voltage that can cause a fault. For example, the inductance of a one-metre-long USB cable can "oscillate", causing the peak voltage to be much higher than the 20V short-circuit voltage (sometimes twice as high). For some applications, the failure of downstream equipment that is affected by overvoltage may cause safety problems, as those devices that are commonly used to control the maximum operating current and voltage of cables are the most vulnerable to damage.

Full circuit protection

Usb-pd can produce arcs or damage components when running at the highest rated current and voltage, so it cannot be said that the protection circuit is completely useless. In applications where usb-pd maximum power mode is frequently used, for example when charging a portable computer battery, full circuit protection must be provided.

Transient voltage suppression (TVS) diodes installed between the pin and ground of a USB type-c socket are relatively simple and inexpensive circuit protection. In the case of a transient short circuit, the TVS diode "pinches" the peak voltage to a level that the connected part can withstand. While TVS diodes provide good transient protection, they are not ideal for continuous overvoltage events. To solve these problems, an additional circuit, similar to overvoltage protection, paired with an n-channel MOSFET is required. During a continuous overvoltage event, the guard triggers the nMOSFET to disconnect the load from the input, thereby preventing overloading of the connected downstream device. But TVS diodes, guards, and nmosfets still can't withstand all overvoltage situations; Occasionally, short circuits around USB cables occur. In this case, the inductance of the socket is very low, making the voltage rise faster than the response speed of the protection device and nMOSFET, so more clamping devices can be used to extend the voltage rise time, so that the protection device has enough time to cut off.

Comprehensive protection virtually increases the cost and complexity of usb-pd applications, but this can be avoided by selecting the right components. Manufacturers are now starting to offer integrated devices that integrate TVS diodes, protection and clamps into a single package (the nMOSFET is usually kept as a discrete chip), saving money and space while simplifying usb-pd protection design.

conclusion

Circuit protection will never be the end of electronics development. However, solution development engineers need to have the knowledge to take appropriate protective measures to prevent material damage and prevent people from injury or even death. Circuit protection is like insurance; at best, it can be seen as an afterthought, and even when installed in place, it is often not enough. While underinvestment in insurance can threaten the stable operation of a business, inadequate circuit protection can lead to more serious consequences such as loss of life.


We illustrate the importance of circuit protection in the case of swissair flight 111, which departed from John f. Kennedy international airport in New York on September 2, 1998. The flight was operated by the 7-year-old McDonnell Douglas md-11, which recently upgraded its in-flight entertainment (IFE) system. Smoke from 52 minutes after taking off, the cockpit suddenly and the crew immediately declared a state of emergency response, and tried to alternate to halifax, the airport, but due to the cockpit ceiling electric control cable caused the fire burned out of control and crashed in 8 km of the sea from the nova scotia coast, killing all 215 passengers and 14 crew members.

The crash investigation found that the materials used in one section of the new IFE were the main cause of the crash, and that the materials, which were supposed to be fireproof, burned up and spread to critical control lines. Although it is impossible to say for sure, it is assumed that the electric arc between the IFE wires was the cause of the fire. Although these wires are fitted with circuit breakers, they do not trip because of arcing. This is a true case of 229 deaths caused by inadequate circuit protection. Such circuits are now equipped with arc fault detection protections to trip when an arc is sensed (not including the arc produced by normal operations such as pressing a switch).

Usb-pd brings more danger

Although the Swiss MD - 11 is caused by electrical failure rather than electronic failure, but now more and more circuits are enough to produce arc (and may endanger the fire of life) of voltage and current, such as the upgrade of the USB power supply (USB - PD), it can support up to 20 v and 5 a (maximum power of 100 w) of high voltage and current. Compared to the 5V voltage and 3A current (15W) of USB type-c, the upgrade of usb-pd is a big improvement, but it also greatly increases the possibility of danger.

In addition to the risks associated with high voltage and current, usb-pd can cause other problems when used with USB type-c connectors and cables. This is because the pin spacing of the USB type-c connector is only 0.5mm, one fifth of that of the type-a and type-b connectors, thus increasing the risk of A short circuit due to the slight distortion of the connector during insertion or removal. Impurities that build up inside the connector can have a similar effect. In addition, the popularity of USB type-c has also led to the significant development of cables, although many cables are still unable to carry 100W of power, but they are not identified. However, these signs do not guarantee security; If the consumer wants to use an unspecified cable, it can also be plugged into a usb-pd socket as easily as a qualified cable.

Arcs are not the only danger when usb-pd is used at high voltages and currents. Because the main bus power pin is very close to the other pins of the connector, a short circuit can easily expose the downstream electronics to a power surge such as a 20V short circuit voltage that can cause a fault. For example, the inductance of a one-metre-long USB cable can "oscillate", causing the peak voltage to be much higher than the 20V short-circuit voltage (sometimes twice as high). For some applications, the failure of downstream equipment that is affected by overvoltage may cause safety problems, as those devices that are commonly used to control the maximum operating current and voltage of cables are the most vulnerable to damage.

Full circuit protection

Usb-pd can produce arcs or damage components when running at the highest rated current and voltage, so it cannot be said that the protection circuit is completely useless. In applications where usb-pd maximum power mode is frequently used, for example when charging a portable computer battery, full circuit protection must be provided.

Transient voltage suppression (TVS) diodes installed between the pin and ground of a USB type-c socket are relatively simple and inexpensive circuit protection. In the case of a transient short circuit, the TVS diode "pinches" the peak voltage to a level that the connected part can withstand. While TVS diodes provide good transient protection, they are not ideal for continuous overvoltage events. To solve these problems, an additional circuit, similar to overvoltage protection, paired with an n-channel MOSFET is required. During a continuous overvoltage event, the guard triggers the nMOSFET to disconnect the load from the input, thereby preventing overloading of the connected downstream device. But TVS diodes, guards, and nmosfets still can't withstand all overvoltage situations; Occasionally, short circuits around USB cables occur. In this case, the inductance of the socket is very low, making the voltage rise faster than the response speed of the protection device and nMOSFET, so more clamping devices can be used to extend the voltage rise time, so that the protection device has enough time to cut off.

Comprehensive protection virtually increases the cost and complexity of usb-pd applications, but this can be avoided by selecting the right components. Manufacturers are now starting to offer integrated devices that integrate TVS diodes, protection and clamps into a single package (the nMOSFET is usually kept as a discrete chip), saving money and space while simplifying usb-pd protection design.