Volume 22 Issue 5 - June 8, 2012 PDF
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Transformerless DC-DC Converters with High Step-up Voltage Gain
Lung-Sheng Yang, Tsorng-Juu Liang* and Jiann-Fuh Chen
Department of Electrical Engineering, College of Electrical Engineering and Computer Science, National Cheng Kung University
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Abstract -- Conventional DC-DC boost converters are unable to provide high step-up voltage gain due to the effect of power switches, rectifier diodes, and the equivalent series resistance of inductors and capacitors. A topology is presented to achieve high step-up voltage gain, as shown in Fig .1. However, this converter has two issues: 1) Three power devices exist in the current-flow path during the switch-on period, and two power devices exist in the current-flow path during the switch-off period, and 2) the voltage stress on the active switch is equal to the output voltage. This paper proposes transformerless DC-DC converters to achieve high step-up voltage gain without an extremely high duty ratio, as shown in Fig. 2. In the proposed converter, two inductors with the same level of inductance are charged in parallel during the switch-on period, and are discharged in series during the switch-off period. The structure of the proposed converter is very simple. Only one power stage is used. Compared with the converter in Fig. 1, the proposed converter has the following merits: 1) Two power devices exist in the current-flow path during the switch-on period, and one power device exists in the current-flow path during the switch-off period; 2) the voltage stresses on the active switches are less than the output voltage; and 3) under the same operating conditions, including input voltage, output voltage, and output power, the current stress on the active switch during the switch-on period is equal to the half of the current stress on the active switch of the converter in Fig. 1. Finally, to verify the performance of the proposed converter, a 40-W prototype circuit is built in the laboratory for use in an automobile headlamp application. The circuit specifications and components are selected as Vin = 12 V, Vo = 60 – 100 V, fs = 100 kHz, Po = 40 W, L1 = L2 = 100 μH, and Co = 68 μF.

Under the conditions Vin = 12 V, Vo = 100 V, and Po = 40 W, some experimental results are shown in Fig. 3. Fig. 3(a) shows that iL1 is equal to iL2. Moreover, the input current is equal to twice the level of the inductor current during the switch-on period, and is equal to the inductor current during the switch-off period. Fig. 3(b) shows the current waveforms iS1 and iS2.  As can be seen in Fig. 3(c), the voltage stresses on S1 and S2 are equal to (Vo+Vin)/2. Fig. 3(d) shows the voltage and current of the output diode. Fig. 4 shows the dynamic response under the output power variation between 5-W light-load and 40-W full-load. One can see that the output voltage is well regulated. Fig. 5 shows the ideal and experimental voltage gains under Vin = 12 V, Vo = 60 – 100 V, and Po = 40 W. Fig. 6 shows the experimental efficiency of the proposed converter and the converter in Fig. 1 under Vin = 12 V, Vo = 60 – 100 V, and Po = 40 W. It is seen that the proposed converter has higher efficiency than the converter in Fig. 1.
Fig. 1. Transformerless DC-DC high step-up converter. (Reference as follows)

B. Axelrod, Y. Berkovich, and A. Ioinovici, “Switched-capacitor/switched-inductor structures for getting transformerless hybrid DC-DC PWM converters,” IEEE Trans. Circuits Syst. I, Reg. papers, vol. 55, no. 2, pp. 687-696, Mar. 2008.

Fig. 2. Proposed high step-up DC-DC converter.

Fig. 3. Some experimental waveforms of the proposed converter.
(a) iin,iL1 and iL2;(b) iS1 and iS2;(c) vS1 and vS2;(d) vDo and iDo

Fig. 4. Dynamic response of the proposed converter.
(a) Po is charged from 5 to 40 W. (b) Po is charged from 40 to 5 W.

Fig. 5. Ideal and experimental voltage gain of the proposed converter under Vin = 12 V, Vo = 60 – 100 V, and Po = 40 W.

Fig. 6. Experimental efficiency of the proposed converter and the converter in Fig. 1 under Vin = 12 V, Vo = 60 – 100 V, and Po = 40 W.
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