In recent years, USB Power Delivery (USB PD) is playing an increasingly important role in the field of consumer electronics. The USB PD 3.1 extends the delivering voltage up to 48V, which requires a buck-boost converter with extreme conversion ratio (textCR= VOUT/VIN) range from 0.1″,10, high-voltage (HW) stress switches, and fast transient response. Into the bargain, the USB Type-C interface with USB PD specification allows power delivery through a single port, which brings about the necessity for bi-directional power converter. To fulfill this task, one of the most common proposals is to implement two power converters for each direction, but this method will be costly to accomplish and will complicate the designs for controllers. Although the Three-level Buck-Boost converter (TLBB) in  can provide buck-boost conversion when V2 > 2V1 (top left of Figure 11.7.1), node Vx will be 2 times V1, which leads to inductor charging in both varphi 1,varphi 2, and fails to transfer power from V2 to V1. To transfer power from V2 to V1 by using the double step-down converter (DSD) , V1 needs to be 4 times larger than V2 due to the limit of CR. In prior DC-DC converters, it is hard to achieve both wide-range CR and bi-directional power conversion. Thus, this paper proposes a symmetric hybrid buck-boost converter (SHBB), which contains two adjustable voltage switched-capacitor (AVSC) cells to satisfy the specifications mentioned above, as well as bi-directional power transmission. Conventional buck-boost (CBB) converter can reach full-range CR, but it encounters low CR=0.1 in the case of 48-to-5V conversion, leading to difficulty in driver and controller designs. The bottom-left of Figure 11.7.1 shows the CR range comparison chart with state-of-the-arts. The DSD converter  can provide extreme CR (24V to 1 V) with relatively wider duty cycles, but its maximum CR is limited to 0.25, which is not qualified for USB PD 3.1. Regarding boost conversion, the hybrid boost converter  and 3-switch boost converter  provide CR larger than 4, but they still fail to meet USB PD 3.1 requirements. The state-of-the-art buck-boost converters ,  can operate in both buck and boost modes but the maximum CR is only 2. Furthermore, since the input and output voltage will vary from 5V to 48V, to tolerate high voltage stress over 96V (2times Vin) in , 48V (Vin) in CBB, and , the usage of 60V HV process is unavoidable, which increases fabrication cost and decreases efficiency.