For most USB applications, the USB port is used to charge personal electronic devices. With the demand for more processing power comes the need for more power in a highly efficient and thermally cool solution. In order to meet the ever-growing trend of increased maximum power for USB applications, the system end equipment must be designed to accommodate the newly specified USB Type-C 1.2 (15 W) and USB PD 3.0 (100 W) power requirements detailed in Table 1-1 and illustrated in Figure 1-1.

The USB Type-C application is then further segmented into different power and data roles as shown in Figure 1-2. This application note focuses on a no data/source only USB Type-C end equipment with the use of a high-efficiency switching buck converter as the intermediary DC/DC conversion for a well-regulated output voltage rail.

Popular designs for source-only AC/DC adapter takes the input AC voltage through an EMI filter and bridge rectifier. The EMI filter is used to ensure noise is not introduced to the input and the bridge rectifier provides a DC voltage to the quasi-resonant flyback (QRF) power stage. The QRF converter then provides a large step down secondary output voltage of ~21V while meeting isolated safety requirement. Since the secondary output of the flyback is loosely regulated using an auxiliary winding to provide voltage information reflected back to the primary side, an intermediary and well-regulated buck converter takes the flyback output voltage and provides another step down voltage to power the downstream personal electronic devices. The system is well-isolated from the input AC source because of the bridge and diode rectification and the output voltage is well-regulated because of the step down buck converter. Typical output voltages in USB Type-C applications are: 3.3 V, 5 V, 9 V, 15 V, and 20 V. Figure 2-1 shows a typical system block diagram for a dual-port AC/DC adapter.

The buck converter is selected to meet the following design requirements for USB Type-C applications listed below:

• Small form factor and thermally efficient solution
• Dynamic output voltage scalability
• High output current capability with good thermal performance to achieve up to 5A to meet USB PD 3.0 specifications