Optimization of current-limiting solutions for USB 3.0 - Embedded.com

Optimization of current-limiting solutions for USB 3.0

Abstract
In addition to transfer speed enhancement in USB 3.0, the requirement for power supply is also increased to meet various peripheral demands. The article introduces the Polymeric Positive Temperature Coefficient (PPTC) device, which is an over-current protection device often used in the industry, and compares it with low-voltage solid-state switch for USB 3.0 applications. It also looks at power delivery in USB 3.0, and differences compared to USB 2.0 are explained, along with how PPTC devices can be used for over-current protection.

USB 3.0 versus USB 2.0
USB 3.0 has a need for speed and a need for power: As the USB 3.0 specification was finalized last November, not only did the transfer speed increase to a blazing 5.0 gigabits/second (Gbps), but the power budget was also boosted from 500 mA to 900 mA. The increase in speed and power brings new challenges to both the system and component level, from integrating new data-transfer protocols and new power-management schemes, to successfully transferring data between host and device.

Among those upgrades in USB 3.0, one critical thing remains unchanged: the safety requirement. System builders now need to allocate enough supply current for these ports, and at the same time carefully incorporate the current-limiting function.

Recent I/O port designs incorporate power lines to directly power peripheral devices, or to power the EEPROM in the load-sink device for data reading. These powered pins need to be protected, to prevent possible short-circuit scenarios while disconnected, or to prevent malfunctions in over-current situations on the connected device. PPTC devices are well-suited to provide such protection, and they have also become the mainstream in electronic designs to meet various specification requirement and safety standards, such as UL 60950.

Introduction and over-current protection
Polymeric positive temperature coefficient devices, commonly known as PPTC devices, can be treated as a non-linear, temperature-dependant resistor. Under normal operation, PPTC devices are highly conductive so the circuit can operate normally. However, if an over-current event occurs, faulty current will generate enough heat on the PPTC device to exceed its switching temperature, causing a resistance jump of four to six orders of magnitude (104 to 106 times) from the norminal value. Therefore, the over-current condition can be eliminated, and the objective of protecting the circuit device is achieved.

The USB 3.0 specification requires a current-limiting device for power protection, and PPTC devices or low-voltage solid-state switches are often used for this limiting. Most of the time, design engineers prefer to use PPTC devices to prevent the circuit from damage, for various reasons. Due to the increase of power budget in USB 3.0, current-limiting devices must pass through more power while maintaining a voltage-drop limit. Table 1 shows the comparative advantages of PPTC versus low-voltage solid-state switches in USB 3.0 applications, for factors such as price, resistance, maximum fault power, and ESD sensitivity.


Table 1: PPTC and solid-state switch comparison in 1-amp applications.
(Click on image to enlarge)

Power delivery difference from compared to USB 2.0
Compared to USB 2.0, USB 3.0 creates new power-management schemes to achieve overall platform power efficiency, by defining new link states and mechanisms. However, in terms of power distribution, USB 3.0 remains similar to USB 2.0, with an increased power budget and more generous voltage-drop requirement.

“SuperSpeed” devices can now draw up to 900 mA after initiation with the host. The supplied voltage requirement at the connector of hub or root ports has been decreased from 4.75 V to 4.45 V, and bus-powered devices should be capable of operating with input voltage as low as 4.00 V. Other requirements. such as inrush current limiting and suspend current limitation. stay as stringent as before, with updated power allocation of 150 mA or 900 mA for high-power devices.

USB 3.0 over-current protection
Although USB 3.0 lowers the supplied voltage requirement from hosts, Polytronics still recommends designers use products in which guarantee PPTC voltage drop of less than 0.1 V under full load, to ensure greater compatibility with USB 2.0 peripherals and give greater voltage margins to other on-board components and traces. In addition, these current-limiting devices should hold full load current for high-power devices above 50°C, to avoid unwanted trip conditions due to thermal derating, especially on the rear USB ports of desktop PCs.

Related articles

  • “PPTC devices help protect electric motors, transformers and industrial controllers,” click here
  • “Maintain USB signal integrity when adding ESD protection,” click here
  • “Which USB is Right for Your Application? (Part 2),” click here

About the authors
Chun-Pai Hsieh holds a master of electrical engineering degree from Cornell University, with concentration in embedded systems. He is presently the senior engineer at Polytronics Technology Corporation in Hsinchu, Taiwan. He is responsible for technical service and development, and he often interacts with engineers and customers in the industry to develop new applications. He is also in charge of marketing communication in the company.
Ching-Han Yu received the master degree in Electronic Engineering from Chung Yuan Christian University in 2005. He is presently quality manager at Polytronics Technology Corporation in Hsinchu, Taiwan. He is responsible for new circuit-protector development and its application. He works closely with PC and battery customers to develop new-generation circuit protectors to meet their needs. In his 11 years at Polytronics Technology Corporation, he held positions such as research and development manager, and product manager.

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