Analysis and application of switched-capacitor transformers by formulation

Abstract

The operating characteristics of the switched capacitor transformer are formulated and analyzed by means of the state-space averaging method for the three types: 1/2-step-down type, 2-step-up type, and polarity inverting type. The following conclusions are obtained: (1) The static characteristic is equivalent to that of a battery with the induced voltage of the input times the transformer ratio and the output resistance represented by the duty ratio and the on-resistances of the switches. (2) Using switches having small on-resistances and selecting a high duty ratio and the frequency five to ten times the natural frequency of the circuit, the output resistance can be made small and a high transforming efficiency of 80 to 90 percent can be realized. (3) The dynamic characteristic is a first order for the step-down type and a second order for the step-up and polarization inverting type in which no oscillation results with a feedback. In comparison with a switching converter using a reactor, the present structure is superior in stability. These conclusions were confirmed experimentally. Next, as an application of the switched-capacitor transformer, a dc-dc converter with a 12-V input and a 5-V output was constructed. As a method for regulating the output, an on-resistance control was used instead of a conventional PWM control and a frequency control. The test results are excellent as: (1) the ripple of the output voltage is reduced to one-tenth that for the PWM control, (2) the maximum output power is 50 W (10 A) and the efficiency is 80 percent for the specified input and output, and (3) the load response is excellent.