Demystifying USB-C boost-buck battery charging

Apple Inc. launched a new era of power management for computing devices on April 10, 2015. The new MacBook features a USB-C™ port - a true all-in-one port that can transfer data and power in both directions simultaneously. This technology eliminates the MagSafe charging port on the MacBook by integrating charging functionality into the USB-C port.

Now, with the release of the 6th generation Intel Core processors, a new generation of Ultrabooks, tablets, 2-in-1s, and external devices are ready to catch the USB-C charging express. However, adopting USB-C charging requires a fundamental change in the existing power delivery architecture and presents new challenges to system design engineers.

This article analyzes traditional PC power architectures and describes how they will change with the widespread adoption of USB-C power delivery using the reversible USB Type-C™ cable connector. We will explore different battery charging methods and explain how the USB-C boost-buck charging topology can provide the flexibility, high efficiency, and small solution size that design engineers demand.

Current methods of powering computing devices

Charging electronic devices via USB-A/B ports is widely used in low-power applications such as smartphones and tablets. Conventional USB-A/B ports are capable of providing a 5V supply voltage with a maximum current of 2A. However, this power level is insufficient to charge high-power devices, which are typically charged using an AC (alternating current) power adapter rated at tens of watts.

Figure 1. Current power transmission architecture

The typical power architecture of current computing devices is shown in Figure 1. It includes an AC power adapter that converts AC voltage to

Convert to DC (Direct Current) voltage and use 20V to charge the main electronic device, in this case, an Ultrabook computer. Ultrabooks can use different battery packs ranging from 1 cell to 4 cells. The typical operating voltage range of each lithium-ion cell is 2.5V - 4.3V from discharged to full. Therefore, the battery voltage range of Ultrabooks is 2.5V - 17.2V.

In the case of a 20V DC power adapter, the battery charger uses what is called a buck topology to step the 20V DC down to the battery charging voltage. The 5V USB-A/B port of an Ultrabook is capable of charging USB peripherals such as a smartphone or tablet. To generate the 5V voltage in the USB-A/B port, the Ultrabook uses a similar buck topology to generate the 5V USB power rail from its internal 2, 3, or 4-cell battery pack, or a boost topology in the case of a 1-cell battery pack.

Moving to USB-C

USB-C is changing the way electronic devices are charged. It is a standard interface that can connect any device. In addition to data transmission, USB-C also supports bidirectional power transmission at a higher current. At the default 5V voltage, the USB-C port is able to "negotiate" with the plugged-in device and increase the port voltage to 12V, 20V, or another mutually agreed voltage at the current level agreed upon by both parties. The maximum power that a USB-C port can provide is 100W (20V×5A), which is more than enough to charge a computer, not to mention that most 15-inch ultrabooks only need about 60W of power. It is not difficult to understand why electronic device manufacturers are equipping their next-generation products with USB-C ports, among which Apple was the first to adopt this technology on the new MacBook launched last year.

Figure 2. New power delivery architecture based on USB Type-C connector plug

By moving to USB-C charging, traditional power architectures also need to change as mobile system manufacturers move to USB-C ports. Figure 2 shows how a USB-C port can connect to any device. A USB-C power adapter with a voltage range of 5V-20V can charge a main electronic device such as an Ultrabook using a 1, 2, 3, or 4-cell battery pack. These main electronic devices can also charge external electronic devices such as tablets, smartphones, power banks, and more.

Different USB-C battery charging options

A unique challenge of the new power delivery architecture is how to charge a 2.5V-17.2V battery using a 5V-20V power adapter, because there is no clear "input to output" and "output to input" relationship, requiring a buck topology in the former case and a boost topology in the latter case.

Figure 3. Pre-boost solution.

Figure 3 shows a solution based on the pre-boost concept. This solution boosts the voltage of the USB power adapter to a level greater than the maximum USB adapter voltage, such as 25V, and then uses a buck charger to charge the battery. This solution requires an additional boost converter, which increases the cost and size of the solution and reduces the overall efficiency due to the additional power loss in the pre-boost stage.

Figure 4. Buck charger or boost charger solution

Figure 4 shows a solution based on the buck charger or boost charger concept. This solution obtains voltage from the USB power adapter and uses a buck charger or a boost charger depending on the input/output voltage relationship. Although this solution eliminates the additional power loss of the pre-boost solution, it still requires an additional boost charger, which also increases the solution size and cost.

Figure 5. Boost-buck charger solution.

Figure 5 shows a boost-buck topology solution. The boost-buck topology can operate in buck mode when there is an "input to output" relationship, in boost mode when there is an "output to input" relationship, or in boost-buck mode when there is a bidirectional "input ≈ output" relationship. This flexibility allows for a better design with the smallest solution size and the best overall efficiency. This solution meets all the requirements of the system design engineer.

Figure 6. Boost-buck charger topology

The first USB-C boost-buck battery charging solution on the market is the Intersil ISL9237. Figure 6 shows the topology of the ISL9237 boost-buck charger. The device consists of four switching FETs, an inductor, and the battery connection FET (BEFT). The four switching FETs are divided into two groups: the forward buck port and the forward boost port. By operating either port, the topology can operate in forward buck mode or forward boost mode to charge the battery. It can also operate in reverse buck mode to power external electronic devices through the USB port, such as tablets, smartphones, or emerging portable power banks that can charge any device.

The ISL9237 offers a rich feature set and is an SMBus charger that can communicate with an SMBus host. It complies with the USB 3.1 specification and the latest Intel IMVP8 PROCHOT# and PSYS requirements, and has protection features for battery voltage drop, adapter overcurrent, battery overcurrent, and overheating. The ISL9237 provides two-level adapter current limiting, fully supporting programming of current limit and duration to take advantage of the adapter's surge current tolerance. It also supports external mobile power supplies and any travel power adapters, including those that have not announced their current handling capabilities.

in conclusion

Thanks to the ISL9237 USB-C buck-boost battery charger, mobile PC systems and other portable devices can now take advantage of the best bidirectional power delivery technology based on the reversible USB Type-C connector. USB-C simplifies the way computers and external electronic devices transfer power and data in both directions. The USB-C ecosystem is ushering in a new future where any device can transfer data and power with just a single, thin cable.

About the Author

Jia Wei is the Marketing and Application Director of Intersil's Mobile Power Products Division. Mr. Wei manages a team of 30 engineers and is the promoter and key decision maker of Intersil's computing device power business. He has published more than 20 articles to date and holds seven U.S. patents. Mr. Wei holds a Master of Science in Electrical Engineering (MSEE) and a Ph.D. from the Center for Power Electronics Systems at Virginia Polytechnic Institute and State University.