A better design method for fault detection of USB-C cables

Wireless device users have long been able to charge quickly. At the same time, they are reluctant to worry about the potential dangers associated with the higher power levels of fast charging. Cables with USB connectors are often used in these charging applications.
Traditionally, polymer positive temperature coefficient (PPTC) devices or miniature circuit breakers have been designed into the connector to provide overheat protection for the cable. With the development of the USB-Type-C (USB-C) and USB-powered (USB-PD) specifications and the higher power levels they support (up to 100 W), these solutions must be revisited.
Chargers with different versions of USB cables and connectors have been used to charge various mobile devices and wearable devices (such as mobile phones, tablets, and laptops) for many years. Many consumers have multiple types of cables for charging and data transmission, and these cables are obtained from all the devices they have purchased over the years. Today, consumers expect cables to be able to charge various devices at an appropriate power level and support higher data transmission speeds. This has led many manufacturers to accept the USB-C standard, which was first released in August 2014.
Basic knowledge of charging
In any charging system, three devices must work together: the device being charged, the cable and the charger. The USB-C cable is equipped with one or more symmetrical (hence, reversible) 24-pin connectors. One end of the USB-C charger has an AC plug (used to plug into a wall outlet), and the other end has a cable with a USB-C connector (used to plug in the device to be charged) or the USB-C output port allows USB-C to be inserted Cable (Figure 1).


Figure 1 USB-C output port
From a protection point of view, the USB-C cable must be able to carry the appropriate voltage and current. For chargers with isolated or fixed cables, the cable must be able to handle the voltage output of the charger. Cables with type C plugs on both ends must be able to handle 21V and at least 3A. Cables containing special electronic marking ICs can carry 5A current. Any equipment placed in the power path, especially protection devices, must also be able to withstand these voltage and current levels.
The root cause of charging damage
There are two factors that make the USB-C cable vulnerable to damage. One factor is the high power that these cables must carry when using connectors with very tight pin spacing, which increases the risk of failure that can lead to thermal events. The second factor is the vulnerability of the connector to contamination and the associated risk of uncontrolled overheating. When dust, metal particles, hair or other debris are stuck on the USB-C cable connector or the connector pins are deformed, it may cause a resistance failure from the power cord to the ground. These resistive faults only increase a very small current, which will cause a dangerous temperature rise. Caused damage to cables and equipment (Figure 2)


Figure 2 The resistance failure of the USB-C connector may damage the cable and the device being charged.
The traditional method of protecting USB cables from overheating involves finding PPTC devices or miniature circuit breakers (also known as thermal circuit breakers) on the VBUS power line. The selected device will be placed on the printed circuit board inside the connector to sense the temperature rise caused by the resistive fault. However, using this method, when trying to protect up to 100W of power, may bring some challenges to the power supply design engineer.
PPTC and miniature circuit breakers both cause power loss, making it more difficult for power supply manufacturers to meet mandatory efficiency requirements. It is also difficult to install any kind of device within the enclosed range of the connector. For example, a PPTC suitable for protecting a 60W charger usually has a footprint of 1210 (3.2×2.5 mm), while a small circuit breaker is often larger. PPTC and miniature circuit breakers used to protect 100 W need to be larger. , The miniature circuit breaker has a relatively weak mechanical structure, and its bimetal material may be deformed during the cable assembly process, which will prevent the miniature circuit breaker from providing protection during heating failures.
A new method of protection
One way to solve these shortcomings is to place a different type of protection device in the communication channel (CC) of the USB-C plug instead of on the VBUS line. The device design to protect the USB-C cable from high temperature conditions is the compact PolySwitch setP digital temperature indicator (Figure 3). setP senses the temperature rise, and then "warns" the system to shut down the power.


Figure 3 PolySwitch setP digital temperature indicator helps protect the USB Type-C plug from overheating. They are designed to comply with USB Type-C specifications and can even help protect the level of USB power supply.
As mentioned earlier, when the connector fails due to contamination, it will cause the temperature of the cable and the mobile device connected to it to rise to dangerous levels. However, when the setP digital temperature indicator is placed on the CC line, when it detects a temperature higher than 100°C, it will switch from a low resistance state to a high resistance state (Figure 4). This increased resistance causes the voltage on the CC line to increase beyond the value defined by the USB Implementers Forum (USB-IF) to determine whether the source and sink are separated.


Figure 4 The resistance and temperature curve of setP digital temperature indicator
The charging system assumes that the cable has been disconnected. Because the voltage is higher than the specified value mentioned earlier, the charging system turns off the power supply through the VBUS line. This prevents the connectors, cables and the device being charged from overheating. Once the user disconnects the cable and removes the debris from the connector, the cable can resume normal operation.


Figure 5 Miniature circuit breaker, SMD and setP device size comparison
With a compact 0805 (2.0×1.2 mm) footprint, setP devices are at least 30% smaller than most other solutions for this application (Figure 5). Their rugged physical structure enables them to reliably withstand modern cable assembly and molding operations. They are very suitable for USB Type-C charging cables, USB powered charging cables and chargers with USB Type-C cables.
USB-C cables provide mobile device owners with a fast and simple way to charge the device and transfer data. Now, with the development of new methods of overheating protection, they can be confident that the process will also be safer.
Sidebar: Charging playback


The 　　 mobile phone (compared to the "car phone" that obtains electricity from the vehicle's electrical system while the engine is running) was introduced more than thirty years ago. Early adopters quickly discovered that charging them was a time-consuming and sometimes risky process. One of them is the Motorola DynaTAC 8000X, the size of a brick, powered by nickel-cadmium batteries. The initial price is close to 4,000 US dollars, the weight is 1.75 pounds, 8000 × needs 10 hours of continuous charging to provide 30 minutes of talk time and 6 hours of standby time.
The other is a bulky desktop charger designed to charge 8000X in one hour. It quickly became notorious for overheating the battery, causing accelerated damage to the battery (limiting the number of charge and discharge cycles), and sometimes short-circuiting the battery And need to replace the battery.
Today, compared with the 1980s, charging hardware designers and consumers have much higher expectations for mobile phones and other wireless devices, but the danger of overheating may still be related to the charging process.