Battery charging for new USB-C applications

USB Type-C charging is becoming popular amongst consumers because it’s convenient to use the same USB Type-C cord for many household items. USB Type-C is even expanding to support higher-power applications such as power tools and e-bikes. As shown in Figure 1, while USB Type-C used to support up to 100 W of power with Standard Power Range (SPR), it can now support as much as 240 W with Extended Power Range (EPR).

Behind the goal to be able to get charge from different types of USB Type-C source including SPR and EPR comes the challenge of designing an application that can support the entire voltage range of USB Type-C PD EPR. Imagine a leaf blower designed with five-cell lithium-ion (Li-ion) batteries and a USB Type-C input. Because the batteries are Li-ion, the full charge voltage of each battery cell is 4.2 V, which means that the voltage of the battery pack (V_BATT) at full charge is 21 V. The owner of the leaf blower could use many USB wall adapters, but for this example, assume that they have a 45- or 140-W USB Type-C wall adapter, which means that the input voltage (V_IN) to the battery charger could be 15 V or 28 V. The question now is how to fulfill these requirements:

• Charge the battery when V_IN > V_BATT or V_IN < V_BATT.
• Charge the battery as quickly as possible to minimize charging time.
• Keep the leaf blower cool during charging to help prevent overheating.
• Use one charger IC across different platforms with different cell counts.

The component that addresses each of these requirements is a buck-boost battery charge controller with external switching field-effect transistors (FETs). The buck-boost portion of the charger helps address the first requirement to have V_IN > V_BATT or V_IN < V_BATT. Using four FETs and an inductor in the configuration shown in Figure 2, the battery charger can operate in buck mode when V_IN > V_BATT shown in Figure 3 and in boost mode when V_IN < V_BATT shown in Figure 4.

The external FETs help address the second requirement. Selecting external FETs improves thermal performance and helps reduce charge times with a higher charge-current capability. In addition, external FETs provide a larger area to dissipate heat during charging. For these reasons, an external metal-oxide-semiconductor FET buck-boost battery charger enables applications that can handle the new USB PD EPR levels and help provide a quick charging experience.