A variety of USB cables and connectors have been used over the years for recharging mobile devices like smartphones, tablets, and laptops and transferring data to and from them. Today’s consumers want cables that can charge a variety of devices at the appropriate power levels, as well as support higher data transfer speeds. This has led many manufacturers to embrace the USB Type-C standard, published in August 2014.
Power levels of up to 100W and small pin-to-pin spacing make the risk of dangerous overheating quite high for USB Type-C devices and the cables used to charge them. Environmental factors can also lead to equipment overheating. Dust, dirt, lint, water, and other liquids are just a few examples of environmental conditions that can lead to failure. Mechanical failures such as bent pins or worn cables can also lead to overcurrent events. Other mechanical failures may lead to worn-out devices; over time, these worn-out devices can also become the source of heat, eventually leading to failure or even fire.
USB Type-C hand-shaking uses a configuration process over the Configuration Channel, usually referred to as the CC pin. The configuration process is used to confirm multiple items, including attachment or detachment of the cable, plug orientation, and agreement of power to be delivered over the cable.
Until recently, the methods for protecting against overheating damage were less than optimal. However, Littelfuse has developed a new series of digital temperature indicators, PolySwitch setP (Figure 1), that are designed to be placed inside the plug, onto the CC line. The setP device senses heat, which causes its resistance to rise and thereby increase the voltage present on the CC line. The CC line’s increased voltage causes the system to assume there is no connection, so the system turns VBUS power off. Watch this short video to see how setP works.
Centering the setP device on the printed circuit board (PCB) as close to the connector mounting pins as possible will provide the best protection performance. In accordance with the electrical creepage and clearance guidelines within IPC-221A, Section 6.3.7, the setP device should be no closer than 0.25 mm to components without conformal coating and no closer than 0.13 mm for components with conformal coating. Littelfuse advises locating a setP device no further than 3 mm from the pins of the connector. Testing to determine how temperature changes for various setP placement locations on the PCB indicates that for every 1 mm further from the connector, the temperature decreases ∼5oC.
For example, while the temperature at location A (Figure 2) reaches 100oC, the temperature at location B would be 85oC. The thermal response during a fault situation will vary depending on the materials selected and other factors related to the overall design. Testing can help determine the optimal placement of the setP device for each design.
To learn more about how to enhance the safety of USB-C connectors and cables, download a free copy of the “setP™ Digital Temperature Indicators for USB Type C Cables Design & Installation Guide ” available from Littelfuse.