Development of an AC-DC converter using differential type switched-capacitor transformers

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A new switched-capacitor (SC) transformer is presented whose output voltage is nearly equal to the difference between the two input voltages. By using the proposed SC transformers, an AC-DC converter is realized. The features of this circuit are as follows. (1) It can be made in small size and light weight and the generation of the magnetic field is extremely few, since magnetic parts such as coils and transformers are not used. (2) The numbers of capacitors and switches do not increase, even if a large step-down ratio is required.The first test circuit can convert the commercial power line (100 V / 60 Hz) into a DC voltage (50 V) with more than 80 % power conversion efficiency. Due to the phase control, the power conversion efficiency can be kept higher than 80 %, even if the input voltage is changed from 55 V to 120 V.During this research period, it can be seen that the output voltage ripple becomes large, since all the clocks are synchronized with the input voltage. In the second test circuit, the output side clocks are driven by a 100 kHz frequency without synchronizing with the input voltage to decrease the output voltage ripple. Since the number of the charge-transfer capacitors in the second circuit is increased by 5, the circuit configuration becomes complex and malfunction may occur by noise if analog circuits are used. Therefore, a timing pulse generator (Time 98) using a personal computer is used to obtain the clocks. The second test circuit can convert the commercial power line (100 V / 60 Hz) into a DC voltage (20 V / 0.5 A) and the measured maximum power conversion efficiency is 89.5 %. The regulated output voltage is obtained even if the input voltage is changed from 20 V to 120 V.