Today's consumers have quickly become dependent on mobile devices that use the USB-C or USB-Type C communication interface standard -- from smartphones and tablets to wearables and laptops. The USB port also doubles as a quick charging port for most of these devices. As a result, designing strong protection against electrostatic discharge (ESD) and overheating conditions has never been more important.

The USB-Implementers Forum (USB-IF) has made four major revisions to the standard 1. It was first standardized in 1996 and has been evolving at higher rates and allowing greater power carrying capacity. The USB standard started from version 1.0, and has been developed to version 2.0, version 3.x, and currently updated to version 4, USB4. Table 1 lists the versions from 2.0 to USB4 and shows how the capabilities of each version have increased dramatically.

 To handle higher data transfer rates and higher power transfer, the USB Type-C cable and connector standard has been updated to version 2.12 and the USB-PD (Power transfer) standard has been updated to version 3.1. Figure 1 shows the Type-C connector that implements the enhanced USB feature set. The PD version allows charging and powering devices via a USB port. The maximum power capacity has increased from 2.5W (5V @0.5A) to 100W (20V @ 5A) and is now in the 240W (48V @ 5A) power range. The higher power capacity will open up new power and charging applications for USB-C, such as gaming laptops, expandable docks, 4K monitors and all-in-ones.

 Figure 1: USB Type-A and Type-C connectors. The Type-C connector has 24 pins compared to the 4 pins of the Type-A connector. The distance between signal contacts of the Type-c connector is 0.5 mm. (Credit: Littelfuse)

Challenges to product reliability

While evolving standards have improved data transfer rates and increased charging power, they do not directly prescribe specific ways to protect USB ports from external hazards. This article will introduce methods to eliminate the possibility of electrostatic discharge and overheating conditions leading to failure. These technologies are critical to making products more reliable and powerful.

 Protects the USB port from electrostatic discharge

Electronic circuits exposed to the external environment through cables and connectors, such as USB ports, are potential targets for electrostatic discharge (ESD). ESD shock can occur through direct human contact or through air (if an energy source arcing an electronic circuit). ESD shock up to 30 kV or more, fast rise time, and can melt silicon and wire, current up to 30 A. ESD has so much power that it can cause components to fail completely.

In addition, ESD shock can cause more subtle damage. ESD current can cause soft failures, including state changes in logic devices, latches, or unpredictable behavior. This can lead to data flow corruption. Data needs to be retransmitted, which slows down the data transfer rate. If a latch failure occurs, the system will need to be restarted. ESD can also lead to potential defects in which components are still functional but have degraded and may fail prematurely. The product must have strong anti-static capability to obtain high reliability. They must also comply with international standards such as IEC 61000-4-2 before they can be sold in all regions of the world. Figure 2 shows the electrostatic discharge simulation test waveforms specified in IEC 61000-4-2. Products must be CE certified.

Figure 2: ESD test waveform specified in IEC 61000-4-2. (Credit: Littelfuse)

A variety of products are available to protect communication ports from ESD damage. Figure 3 shows the recommended protection elements that can be used for USB interface lines with power supply capacity up to 100 W and extended power supply range up to 240 W. The recommended element is the transient voltage suppressor (TVS) diode. Table 2 describes the component technologies and their respective characteristics and advantages.

 Figure 3: USB interface block diagram shows recommended ESD protection components (see Table 2). (Littelfuse company)

 Table 2: Recommended USB protection technologies (source: Littelfuse)

For USB 2.0 lines, consider using the SP3530 unidirectional TVS diode or equivalent. The TVS diode can safely absorb 22kV ESD shocks, almost three times the 8kV level required by IEC 61000-4-2, without attenuation. Typically, a low capacitance of 0.3 pF minimizes interference with signal conversion. The component is packaged with 0201 surface mount, designed to save PC board space. The SuperSpeed line requires a component with the lowest possible capacitance so as not to degrade high-speed data transmission performance. For example, the SP3213 bi-directional TVS diode, two anode-to-anode connected diodes provide protection against ESD shocks up to 12 kV. These diodes typically have a leakage current of only 20 nA to minimize circuit power consumption and are packaged in compact µDFN-2 surface-mount packages. For sideband use (SBU) and configuration channel (CC) lines, consider the SP1006 unidirectional TVS diode. The element can safely absorb 30 kV ESD shock in µ DF-2 packages. The SP1006 is a very reliable TVS diode that complies with the AEC-Q101 standard and is suitable for automotive applications with USB communications. Vbus lines require TVS diodes to withstand higher power levels than signal line protection devices. SPHV series 200 W TVS diode can protect Vbus lines with a capacity of 100 W. The SPHV diode can withstand 30 kV ESD shock and is aEC-Q101 certified in surface mount package. An example solution for extended power range interfaces is the SMBJ diode. It has a higher peak power rating of 600 W than SPHV diodes and can absorb ESD shocks up to 30 kV. Like other TVS diodes recommended for USB ports, the SMBJ diode is a surface mount element. Each different TVS diode has the functionality necessary to protect a particular set of lines from ESD without interfering with the line's functionality. Incorporating these diodes into the circuit protects against immediate failures, soft failures, and potential premature failures.

Prevents USB Type-C plugs and sockets from overheating

The HIGH density of the USB Type-C connector makes it more susceptible to fouling by dirt and dust, which can cause resistive failures between the power supply and the ground. Coupled with the higher power on Vbus lines, USB connectors are at greater risk of overheating, which can damage connectors, cables, and connected port electronics. An increase in temperature could melt the connector or, in the worst case, start a fire.

The solution to prevent overheating is a digital temperature indicator designed to comply with the USB Type-C cable and connector specification. When the temperature indicator detects a temperature of 100° C or higher, its resistance increases by at least five (5) tenfold. The example component technology cited in this article is Littelfuse's unique setP digital temperature indicator. Its characteristic curve is shown in FIG. 4.

Figure 4: Resistance-temperature curve for a temperature indicator using Littelfuse SETP as an example. (Credit: Littelfuse)

As shown in Figure 3, the temperature indicator is placed in the configuration channel line. It is not placed in the Vbus line, so it does not reduce any voltage or power, nor does it reduce the power supply capacity on the Vbus line. If the component detects a temperature of 100°C, its resistance increases significantly. The USB protocol interprets high resistance as an open circuit connection between the source connection, the Vbus and receiver connection, the load, and the Vbus line being disabled. When the conditions causing overheating are corrected and the sensor temperature drops below the 100°C threshold, its resistance is reset to a low temperature value of about 10 ω and the Vbus is powered back on. For best results, temperature indicators should be built into USB plugs and/or sockets to monitor the connector temperature at the source of the failure. Unlike positive temperature coefficient devices or miniature circuit breakers that must be in a Vbus line, digital temperature indicators do not consume power and reduce power transmission capacity. In addition, these other components are limited to 100 W and lower power, which will prevent them from being used in EXTENDED power range USB Type-C applications.

The temperature sensor should be small in size to enable detection at the source of the fault. It should also be able to change its resistance state in as little as one (1) second to prevent damage to cables and electronic components. Figure 5 shows how the temperature indicator maintains a safe connector surface temperature during an overheating failure.

Lower rise in connector surface temperature when temperature indicator (A Littelfuse setP) is used for overheat protection. (Credit: Littelfuse)

conclusion

Without proper protection, ESD or debris in USB Type-C connectors can cause field failures in valuable consumer electronics that users rely on daily. Electronics engineers can protect their latest designs by using TVS diodes to protect USB lines from ESD interference and digital temperature indicators to prevent connectors from overheating. As mobile devices become smaller and more complex and the need for faster charging continues to increase, designers face the additional challenge of finding smaller surface-mount protection elements to fit into the limited space and minimize PCB space to implement the necessary protection.

Anticipating these important design considerations can help prevent end-user problems. It also helps improve product performance, prolong product life and improve consumer satisfaction.