Key considerations for USB 3.0 power supply circuit design

In the major update specifications of USB 3.0 by USB-IF, the USB 3.0 interface upgrade not only improves the transmission performance that most industry players are concerned about, but also greatly improves the interface high power supply specification to meet the charging and driving needs of related devices, which also brings new challenges to the protection circuit design of USB interface design, whether on the host side or the device side.

Most consumers hope to significantly shorten the charging time of mobile devices, such as the Apple iPad. Since the device itself is equipped with a high-capacity lithium battery, if it is charged through the low-voltage, low-current USB 2.0 interface, it will inevitably greatly increase the charging time. In addition, when the iPad was first launched on the market, there were concerns that it would charge too slowly or not charge at all through the computer USB 2.0 interface. In order to address this problem, the transformer provided with the iPad uses a transformer with high output current as a random accessory, but on the host side, it still encounters the difficult problem of the USB interface charging speed limit.

PPTC protection components can provide complete over-current protection for the USB interface.

The USB 3.0 interface electrical specifications have been significantly upgraded, which also brings new challenges to related application designs.

The launch of iPad highlights the design trend of increasing battery capacity for future mobile devices to increase battery life. If the old 5V/500mA USB interface is used, the charging experience of the device will be greatly reduced. The 5V/900mA interface electrical specifications of the new generation USB 3.0 interface can at least save twice the charging time of the USB 2.0 interface.

Accelerated charging also brings new design challenges

In the USB 3.0 related specifications, it is clearly required that the device or interface must be designed with current limiting components to enhance the power application protection of the interface. In terms of power supply distribution, the USB 3.0 specification is roughly similar to the USB 2.0 interface. After the SuperSpeed ​​device completes the initial settings with the host, the device can use nearly 900mA of interface current. In terms of power supply voltage, the host port or the port on the HUB will be reduced from the original 4.75V to 4.45V. Even external devices that use the USB interface for power supply must only operate at a 4.00V interface voltage. Other USB 3.0 related specifications, such as instantaneous current limitation, interface current limitation in host standby/sleep mode, etc., except for the high power 900mA configuration of the highest rated current of the interface, other requirements are as strict as the USB 2.0 interface electrical specifications.

According to the electrical specifications of the USB 2.0 interface, a single USB port must supply 5V/500mA power, which is equivalent to 2.5W of power output. In order to respond to the design requirement of shortening the charging time of the device, the 2.0 interface design generally does not allow the host end to charge at the interface specification limit of 2.5W for the sake of charging safety. Most of them will slightly reduce the "actual" output power to charge the device at 2~2.1W. Some devices will even limit the current drawn from the USB bus to a much lower current than the interface limit of 500mA for charging for safety.

Similarly, the 900mA specified by USB 3.0 basically requires battery charging safety considerations in electronic circuit design, and the related charging mechanism design cannot be based on the interface limit, but the benefits of upgrading the device interface (i.e. shortening the charging time) are not significant enough. The design problem that system designers must face when converting the USB interface to 3.0 is the difference in the interface power output power from 2.5W to 4.5W, which is nearly doubled.

In actual design cases, most host-side USB ports are not just one group, but may be 3 to 4 groups. Designers have to manage the power supply circuits of multiple interfaces, power matching and PowerPath design between input voltage source and battery. At the same time, the design must also properly configure circuit protection design to ensure that important ICs are not affected or damaged by transient high voltage. The relevant design solutions must also be able to combine the design trend of extremely small size and the requirements of USB 2.0 retroactive support, which increases the difficulty of developing many related devices.

Improving Power Supply Design with PowerPath ICs

As devices tend to be thinner and smaller, electronic circuits designed for power control usually occupy more PCB area. In addition, the use of multi-layer PCBs is becoming more and more important to reduce the design space of products. However, this also results in less and less available PCB area, making it impossible to introduce power-related protection circuit design.

In actual design cases, most will use a control IC with PowerPath to match the interface design. That is to say, the power control IC itself has corresponding solutions for autonomous management and USB power design. For example, in the charging application state, the host side can give priority to the load for power supply, while increasing the output current to the interface requirements as far as possible within a safe range. This type of power control IC can also perform different PowerPath application modes according to the power supply state of the host device, whether it is 110V AC supply or battery power supply mode.

In charging mode, the general supply status of the host side cannot accelerate the charging efficiency of the connected products at the highest output, because different devices also have their own battery safety characteristics. Most designs must be combined with a battery tracking (Bat-Track) mechanism to confirm the highest possible charging output. Through Bat-Track, the best charging status of the tracking device is obtained and the tolerable charging design of the device is mastered, thereby accelerating the charging process as much as possible.

Key points to consider when considering USB power supply circuits

For USB-powered mobile devices, the application mode carried out through the host end is difficult to avoid. A more complete supporting design must be carried out on the host end to avoid low charging efficiency or high heat loss problems. In terms of electronic circuit design, Over Voltage Protection (OVP) circuits must also be used to prevent transient high voltage from causing accidental damage to the device or the host end.

The industry commonly uses the Polymer Positive Temperature Coefficient (PPTC) design solution for overcurrent protection components to provide good overcurrent protection design for electronic devices equipped with USB 3.0 interfaces. In order to avoid short circuit problems that occur in the connected or unconnected state, or abnormal overcurrent conditions that occur after the device is connected to the host, protection design measures must be taken for power-related pins.

Observing the performance of PPTC components, its electrical characteristics are nonlinear and the resistance components change greatly with temperature. Under normal circumstances, PPTC components can maintain excellent conductivity, but when the components are overheated due to overcurrent, sufficient Joule heat effect is generated on the PPTC components, and the PPTC components form a resistance value change of more than 100 times in a very short time, thereby achieving the design goal of protecting circuits and electronic devices. At present, for the overcurrent protection solutions required by USB 3.0 interface design, the industry will often use PPTC components for circuit design, which can ensure that the product design can meet the electrical specification requirements of the USB 3.0 interface.

Caption: The electrical specifications of the USB 3.0 interface have been significantly upgraded, which also brings new challenges to related application design.