The buck converter has been the predominant choice of switch-mode power converter for point-of-load applications. For applications demanding ultra-low voltage conversion ratio (VCR) such as 1/48X, the buck converter suffers severe power loss due to the narrow duty cycle. A multi-stage buck converter has wider duty cycles for sub-converters and achieves higher power efficiency, but it costs at least twice as much as a single-stage buck converter. In this research, we focus on developing a hybrid buck converter that achieves a wide duty cycle but uses only one power inductor and one controller.

A 3-Level hybrid DC-DC power converter is proposed in realizing the minimum ultra-low VCR of 1/48X, by integrating a step-down switched-capacitor (SC) converter with a buck converter. It is designe...[ Read more ]

The buck converter has been the predominant choice of switch-mode power converter for point-of-load applications. For applications demanding ultra-low voltage conversion ratio (VCR) such as 1/48X, the buck converter suffers severe power loss due to the narrow duty cycle. A multi-stage buck converter has wider duty cycles for sub-converters and achieves higher power efficiency, but it costs at least twice as much as a single-stage buck converter. In this research, we focus on developing a hybrid buck converter that achieves a wide duty cycle but uses only one power inductor and one controller.

A 3-Level hybrid DC-DC power converter is proposed in realizing the minimum ultra-low VCR of 1/48X, by integrating a step-down switched-capacitor (SC) converter with a buck converter. It is designed to satisfy point-of-load (PoL) applications with a wide input voltage range, from 12V to 48V, minimum output voltage of 1V, and output current range from 0.25A to 3.0A. The proposed step-down SC converter has a VCR of 1/4X which greatly relieves the VCR of the buck converter to 1/12X for 48V-to-1V conversion, and the duty cycle (D) is then approximately D ≈ 1/12, which is much easier to implement, and the power conversion efficiency (PCE) is shown to be much better than a single-stage buck converter that has very narrow duty cycle of D ≈ 1/48.

Compared to the expensive choice of a multi-stage buck converter and the emerging choice of the double step-down interleaving buck converter, the proposed hybrid converter achieves comparable efficiency but only needs one power inductor and one PWM control loop, thus reducing cost and controller complexity. Furthermore, the VCR of the proposed step-down SC converter can be easily scaled down to (1/N)X by using N switched-capacitor stages. This new approach allows the 3-Level hybrid architecture the flexibility to choose the proper combination of VCR ratios for the SC converter and the buck converter for performance optimization. The proposed converter can achieve 89.2% peak PCE for the 12V-to-1V conversion, and 75.8% peak PCE at 1A to 3A output loading for the 48V-to-1V conversion.

The proposed hybrid converter implements integrated gate drives for the 3-Level stacking power switches, integrated PWM controller for stable steady-state and transient operations, and with a tailormade soft-start system that (1) limits the in-rush current by switching to open-loop PWM control; (2) pre-charges the capacitors of the switched-capacitor converter appropriately, thus protecting the power devices from high-voltage stress during start-up; and (3) switches smoothly from soft-start operation to steady-state PWM control. The proposed hybrid converter was designed and fabricated using a 0.18μm HV BCD process. On-chip power MOSFETs with different power ratings (65V, 45V, 36V, and 16V) are used to improve power conversion efficiency and reduce the overall cost of the converter. Special layout considerations such as placing and isolating power transistors and avoiding latch-up are observed.

To conclude, this work shows a new self-starting 3-Level hybrid converter topology that achieves ultra-low voltage conversion ratio with high power conversion efficient for PoL applications.