While design protocols for the ubiquitous and now well-established USB Type-C connectors may be not necessarily remain in the forefront of design engineers’ minds, the issue of ESD and temperature remains a relevant and important ongoing consideration. For electronics design engineers in India, as elsewhere, electrostatic discharge (ESD) and excessive temperature can result in semiconductor and electronics failures, and understanding how and why they can occur can be extremely useful to developers of chips.
Integrated circuits which are subjected to ESD stress, for example, can result in selective semiconductor structure melting due to high currents, and can commonly result in shorts, opens, leakage or resistive shorts, among other failures. Both ESD and EOS (electrical overstress) can lead to burnt metallization, oxide or dielectric breakdown, or junction damage. Similarly, fast wired charging can, for instance, result in safety issues. Without proper and careful circuit design, including temperature monitoring, cables and connectors can accumulate contaminants, causing them to heat up quickly and potentially destroy the cable or, worse still, the mobile device being charged.
With so many USB Type-C connectors in existence today, minimizing ESD and/or overheating failures in the field is a priority for electronics designers. A number of methods exist to achieve this. Mobile device engineers can, for example, protect their designs by using TVS diodes to safeguard USB lines, and digital temperature indicators to protect the USB Type-C connectors.
Many consumers now rely on mobile devices – from laptops and wearables, to smartphones and tablets – that incorporate the USB-C, or USB-Type C, communication interface standard. The USB port also doubles as the fast-charging port for most of these devices. and so designing robust protection against electrostatic discharge (ESD) and overheating conditions is vital.
The USB-Implementers Forum (USB-IF) was first standardized in 1996 and has been evolving with higher speeds and allowing more powercarrying capacity. The USB standard is now in its 4th revision or USB4.
Table 1 lists the versions from 2.0 to USB4 and shows how the maximum throughput of each version has substantially increased.
To handle higher data transmission rates and higher power delivery, the USB Type-C cable and connector standard has been updated to revision 2.12 and the USB-PD (power delivery) standard has been updated to revision 3.1. Figure 1 shows the Type-C connector which can implement the enhanced USB feature sets. The PD revisions allow devices to be charged and powered through the USB interface. The maximum power capacity has increased from 2.5W (5V @ 0.5A) through 100W (20V @ 5A) to, currently, an extended power range of
240W (48V @ 5A). The higher power capacity will open new powering and charging applications for USB-C, such as gaming notebooks, docking stations, 4K monitors, and all-in-one computers.
Although the evolving standards have enhanced data transmission rates and amplified charging power, the standards do not specify methods for protecting the USB interface from external hazards. This article addresses methods to eliminate the possibility of failure from ESD and overheating conditions, techniques that are essential for ensuring a more consistent and robust product.
Ensuring ESD protection for USB ports
Any electronic circuitry including USB ports that are exposed to the environment through cables and connectors are potential targets for electrostatic discharge (ESD) strikes via direct contact from a person or through the air if a source of energy arcs to an electronic circuit. ESD strikes can be up to 30kV or more with fast rise times and can melt silicon and conductor traces with currents up to 30A, which can lead to total failure of components.
They also have the ability to also cause more subtle damage. Current due to ESD can cause soft failures, including a state change in a logic device, latch-up, or unpredictable behavior, possibly leading to a corruption of a data stream, meaning data will need to be re-sent, slowing down the data transmission rate. In the case of a latch-up failure, the system will need a reboot. ESD can also cause a latent defect in which a component still functions but is degraded and can fail prematurely. For high reliability, products need to be robust to ESD
and also comply with globally recognized international standards such as IEC 61000-4-23. Figure 2 shows an ESD-simulated test waveform specified by IEC 61000-4-2 which a product must be capable of withstanding for CE certification.
There are a wide range of products available to protect communication ports from ESD damage. Figure 3 shows recommended protection components for lines on USB interfaces with up to 100W power delivery capacity and extended power delivery range up to 240W. The recommended components are transient voltage suppressor (TVS) diodes. Table 2 describes the component technologies and their respective features and benefits.
Table 2: Recommended USB protection technologies (Source: Littelfuse,Inc.)For the USB 2.0 lines, an SP3530 unidirectional TVS diode or equivalent may be considered. This TVS diode can safely absorb, without degradation, a 22kV ESD strike, almost 3 times the 8kV level required by IEC 61000-4-2. Typically, a low capacitance of 0.3pF minimizes interference with signal transitions. This component is available in a 0201 surface-mount package designed to save PC board space. The so-called SuperSpeed (or USB 3.0) lines require a component with the lowest possible capacitance so as not to degrade the highspeed data transmissions. For example, the SP3213 bidirectional TVS diodes, two diodes connected anode-to-anode, provide a minimum of protection for ESD strikes up to 12kV. These diodes draw typically 20nA of leakage current to minimize circuit power consumption and are in a compact μDFN-2 surface-mount package.
For the sideband use (SBU) and configuration channel (CC) lines, the SP1006 unidirectional TVS diode may be used. This component can safely absorb a 30kV ESD strike in a μDFN-2 package. The SP1006 is a very rugged TVS diode and is AEC-Q101-qualified for use in automotive applications of USB communication.
Vbus lines require TVS diodes that can withstand a higher level of power than the signal line protection devices. The SPHV series of 200W TVS diodes protects a Vbus line with 100W capacity. The SPHV diode withstands 30kV from ESD strikes and is AEC-Q101-qualified in a surface-mount package.
For the Extended Power Range interface, an example solution is the SMBJ diode. It has a higher peak power rating, 600W, than the SPHV diodes and can absorb ESD strikes as high as 30kV. Like the other recommended TVS diodes for USB ports, the SMBJ diodes are surface-mount components.
Each of the different TVS diodes serve a function necessary to protect a specific set of lines from ESD and do not interfere with the functionality of the line. Incorporating these diodes into the circuit will prevent immediate failures, soft failures, and latent, premature failures.
How to protect USB Type-C plugs and receptacles from overheating
The high density of the USB Type-C connector means there is more opportunity for contamination from dirt and dust to cause resistive faults between power and ground. Combined with higher power on the Vbus line, the USB connector has a greater risk of overheating, possibly damaging the connector, cable, and the attached port electronics, with the temperature rise having the potential to melt a connector that could result in a fire.
The solution for preventing overheating is a digital temperature indicator designed to be compliant with the USB Type-C cable and connector specifications. The temperature indicator increases its resistance by at least
five decades when it detects a temperature of 100°C or greater. The example component technology referenced in this article is the unique setP digital temperature indicator from Littelfuse. Its characteristic curve is shown in Figure 4.
Figure 3, reveals the temperature indicator is placed in the configuration channel line. It is not placed in the Vbus line so that it does not drop any voltage or power and does not reduce the capacity of the power delivery on the Vbus line. If the component detects the temperature reaching 100°C, its resistance increases substantially. The USB protocol interprets the high resistance as an open connection between the source connection, Vbus, and the sink connection, the load, and the Vbus line is de-activated.
When the condition causing the overheating is corrected and the temperature of the sensor falls below the 100°C threshold, its resistance resets to its low-temperature value of around 10Ω and Vbus is reenergized. For best results, the temperature indicator should be built into a USB plug and/or receptacle so that it can monitor the connector temperature at the source of the fault.
Unlike a positive temperature coefficient device or a mini-circuit breaker, which must be in the Vbus line, a digital temperature indicator does not consume power and reduce power delivery capacity. In addition, these other components are limited to 100W and lower power preventing their use in the extended power range USB Type-C application.
A temperature sensor needs to be be small in size enabling detection at the source of faults. It also needs to change its resistive state in one (1) second to prevent damage to the cable and electronic components. Figure 5 shows how a temperature indicator maintains a safe connector surface temperature during an overtemperature fault.
Without proper protection, ESD or debris in USB Type-C connectors can cause field failures in the valuable electronics consumers rely on. Electronics engineers are able to protect their latest designs using TVS diodes to protect the USB lines from ESD, and digital temperature indicators to protect connectors from overheating. As mobile devices become smaller and more complex and the demand for faster charging continues to increase, designers need to access smaller surface-mount protection components to address the limited space while minimizing the PCB real estate required to provide the necessary protection methods. Consideration of these key design considerations helps prevent problems for the end- users and also contributes to more robust product performance, longer product lifetime, and greater consumer satisfaction.