Universal Serial Bus (USB) type-c ™  A compact, reversible data and power connector has been introduced, while supporting USB power delivery (USB PD). USB PDs support power up to 100W (20V, 5A), in addition to general functions, and their support for fast charging of lithium batteries has driven the rapid popularity of USB type-c.

However, like all versions of USB technology, electrostatic discharge (ESD) poses a risk to sensitive silicon devices in USB type-c systems. USB type-c also poses some unique security challenges: high power transmission, compact connectors, and the ability for consumers to easily connect non compliant cables, greatly increasing the probability of overvoltage faults in this technology, making reliable circuit protection crucial.

This article outlines the requirements for USB type-C circuit protection and introduces some examples of protection circuits and components. Then explain how to design and implement protective circuits that protect these systems from ESD and overvoltage fault modes.

For more detailed information on the USB PD itself, please refer to the article in the library, "Design and import USB type-c and use its power transfer function to achieve fast charging".

What are the issues with USB type-c?

Implementing ESD protection for all USB systems is a good design principle. Some standards specify ESD discharge robustness requirements in USB systems (and elsewhere). For example, EN 55024 ("Information technology equipment - Immunity characteristics - Limits and measurement methods") specifies the ability to withstand 4kV contact discharge and 8kV air discharge (Standard B: Instantaneous disturbance and self recovery)

In addition to the ESD risk that affects all USB systems, USB type-c also leads to some unique failure modes. These modes are influenced by two factors: USB PD high power delivery and compact geometry.

If traditional USB systems use USB 2.0 or 3.0 communication protocols, USB PD can be used to achieve high power transmission. However, traditional USB connectors (Figure 1) do not have strict geometric requirements, which to some extent reduces the risk of failure in non USB type-c applications.

Figure 1: The popularity of USB type-c is partly attributed to its compact and reversible connectors. The disadvantage is that the pin spacing is smaller than older USB connectors such as USB type-a. The decrease in the spacing between type-c pins leads to an increase in the probability of short circuits.

The pin spacing of the USB type-c connector is only one quarter of that of the type-a connector. When the pin spacing decreases, twisting the cable or unplugging the connector under high current/voltage conditions can increase the probability of catastrophic short circuits. The accumulation of debris inside the connector may cause similar catastrophic consequences.

In addition, the popularity of type-c has led to a large number of third-party cables and power adapters. Most of them cannot adapt to the high current supported by USB type-c and USB PD standards.

Compared to other versions of USB technology, the mechanical stress, debris, and non compliant cables of compact connectors, as well as high current/voltage, increase the risk of short circuits in USB type-c systems. If a short circuit occurs between the adjacent pins VBUS and CC or SBU of the connector, the downstream circuit may be damaged by a 20V surge voltage (Figure 2).

Figure 2: The cross-sectional view of the USB socket (not all pins shown) illustrates that the CC (connection/configuration) pin and SBU (sideband/audio adapter accessory configuration/additional functions to be defined) pin are adjacent to the bus power supply (VBUS).

The short-circuit risk of PD controllers is particularly high, as the device is directly connected to the CC pin and is designed with a maximum operating voltage of 5V. Due to the fact that during USB PD, the PD controller is responsible for coordinating the maximum current and voltage levels between the charger and the charged device, it is crucial to protect these devices. Abnormal operation of USB PD caused by damage to the PD controller may become a safety hazard. For example, when an unprotected wall adapter experiences a short circuit, it can cause damage to the downstream PD controller (Figure 3).

Figure 3: A faulty wall adapter may short-circuit between VBUS and CC pins, causing downstream PD controllers to face voltage damage of 20V.