MPhil Thesis Presentation
Analysis and Design of Three-State Controlled Transition Mode for a Buck-Boost Converter with Efficiency and Dynamic Response Enhancement

by Mr Xiao Hao HU

 :  29 Jul 2013 (Mon)
 :  10:30am
Venue  :  Room 3584, 3/F (Lifts 27-28), HKUST

Examination Committee
Prof Wing Hung KI, ECE/HKUST (Chairman)
Prof Philip K T MOK, ECE/HKUST (Thesis Supervisor)
Integrated non-inverting Buck-Boost converter is drawing more and more attention because it can achieve both voltage step-down and step-up. Conventionally, a Buck-Boost converter is operated in Buck Mode, Boost Mode or Transition Mode when the input voltage is larger than, smaller than or close to the output voltage, respectively. However, Transition Mode has a lower efficiency compared with Buck Mode and Boost Mode mainly due to its larger average inductor current and inductor current ripple.

In order to minimize the power loss of Transition Mode, an additional state, where both the input and output are connected to the inductor, has been introduced in prior art. This leads to a Three-State Transition Mode which has a lower average inductor current and thus a higher efficiency compared to conventional Transition Mode. This technique has been used in some applications to enhance efficiency and achieve smooth mode transition. However, detailed analyses on how the duration of the additional state may affect the efficiency are not provided. The dependences of other converter parameters such as average inductor current, inductor current ripple and output voltage ripple on the additional state are not revealed. Moreover, it still remains unknown how the small-signal dynamic behavior of the power converter will be modified by incorporating a third state.

In this thesis, detailed analyses on Three-State Transition Mode are given, which show the conduction loss reduction due to the reduced average inductor current and inductor current ripple. In addition, with this state, output voltage ripple is also reduced. Furthermore, dynamic behavior of Three-State Transition Mode is also analyzed, which indicates a shift of complex poles and right-half-plane (RHP) zero to higher frequency with this additional state. This implies an improvement in dynamic response.

A prototype circuit of a Buck-Boost converter operating in Three-State Transition Mode has been designed and implemented using AMS 0.35μm CMOS technology to verify the analyses. A novel Ramp and CLK Generator is proposed and utilized in the control circuitry to achieve regulation for Three-State Transition Mode.

Measurement results show that 30% efficiency improvement can be achieved at the maximum loading current of 500mA. Gain-Phase Analyzer measurement verifies that the location of complex poles of Three-State Transition Mode is moved to twice as high as conventional Transition Mode. The location of RHP zero is also higher with the additional state. Besides, converter using Three-State Transition Mode can be stabilized with a smaller compensation capacitor.

*** ALL ARE WELCOME !! ***