What is Buck-Boost Charging?

With Apple unveiling their new MacBook on April 10, 2015, they also opened up a new era in power management for computing devices. The USB-C port in the new MacBook features a true all-in-one port. It is capable of delivering power and bi-directional data at the same time. The technology eliminates a separate charging port, as it integrates the charging functions into the USB-C port.

Intel has released their 6th generation processors, and very soon, a new generation of ultrabook computers, 2-in-1s, tablets, and external devices are expected in the market, ready with the USB-C port. However, with USB-C, fundamental changes are necessary in the existing power delivery architecture. This presents a new challenge to the system designers.

Power Delivery at Present

At present, almost all electronic devices charge through USB-A/B in low power applications. The traditional USB-A/B port offers 5 V DC at up to 2 A current capabilities, but this is insufficient when charging high-power devices. At present, such high-power devices require a separate AC adapter with tens of watts power rating for charging.

For instance, ultrabook computers use different battery stacks ranging from a single-cell battery to 4-cell batteries. Since each Li-ion battery has a typical operating voltage of 2.5 to 4.3 V, from discharge to fully charged status, the ultrabook may have a battery voltage ranging from 2.5 to 17.2 V. Ultrabook computers generally come with a hefty AC adapter with a 20V output.

Therefore, the charger within the ultrabook battery stack has to step down the 20 V DC to make it suitable to charge the battery. This is done through a buck topology. Again, the ultrabook has to provide 5 V on its USB-A/B port for charging an external USB device. To generate this 5 V USB power rail, the ultrabook may have to apply a boost topology if it is using a single-cell battery pack. If it has battery stack of more than one cell, the ultrabook may use a similar buck topology as it does for charging.

Moving to USB-C

USB-C is a standard interface to connect anything to anything. Even though the default is 5 V, the USB-C port is capable of negotiating with a plugged-in device to raise the port voltage to 12 V, 20 V, or any other mutually agreed voltage and mutually agreed current level. Therefore, the maximum power a USB-C port can deliver is 20 V at 5 A, or 100 W. This is more than what most ultrabooks require – about 60 W.

The main consideration involving the use of USB-C technology lies in the absence of input-to-output relationship, which would warrant the use of buck technology when using a 5-20 V adapter voltage to charge a 2.5-17.2 V battery. Likewise, there is no definite output-to-input relationship either, for which a boost topology would be suitable.

This is where the buck-boost approach finds its merit. This operates in buck mode when there is an input-to-output connection and in boost mode when there is an output-to-input connection – the USB-C port being bi-directional. This flexibility allows for a more efficient design using the smallest solution size. It offers the best design solution, achieving all the requirements of a system designer.