High Step-up Boost Converter Integrated with Voltage-Doubler Ki-Bum Park, Gun-Woo Moon, and Myung-Joong Youn Department of Electrical Engineering, KAIST, Daejeon, Republic of Korea E-mail: [email protected] Abstract -- The voltage-doubler provides an additional step- up gain on top of that of the boost converter, while distributing voltage stresses on devices as well. The interface between the boost converter and the voltage-doubler is accomplished by a transformer and a balancing capacitor, which also constitute a resonant tank. Since this resonant operation shapes the current sinusoidal, a switch turn-off loss and a reverse recovery on diode can be reduced. Therefore, the proposed converter is promising for high step-up applications with high efficiency Fig. 1. Boost converter with auxiliary step-up circuit. Index Terms— Boost converter and voltage-doubler. I. INTRODUCTION For a battery powered system, electric vehicles, fuel cell system, and photovoltaic systems, where low voltage sources need to be converted to a high voltage of output, non-isolated high step-up conversion techniques find increasing necessities. A classical boost converter is generally used for its simple structure and continuous input current. However, it is hard to satisfy both high voltage conversion ratio and high efficiency at the same time with a plain boost converter. In high output voltage applications, moreover, high voltage stress on switch Fig. 2. Proposed boost converter integrated with voltage-doubler. and diode degrades the performance of devices, causing a severe hard switching loss, a conduction loss, and a reverse recovery problem [1]-[8]. To relieve abovementioned limitations in high step-up applications, various types of step-up techniques can be applied [4]-[14]. A coupled-inductor boost converter is a favorable candidate for its simple structure, however an input current ripple is large and an auxiliary circuit is required to suppress the switch voltage spike [5]-[9]. A voltage multiplier cell or a switch-capacitor circuit can be useful to raise a step- up gain in collaboration with classical topologies [10]-[12]. As the output voltage is increased, however, the number of stage is increased, requiring more capacitors and diodes. Besides, a current snubber is required to reduce the reverse recovery on diode. To raise step-up gain of a boost converter further in non- isolated applications, the alternative structure, which combines a boost converter with an auxiliary circuit in series, can be considered as shown in Fig. 1 [15],[16]. Proper selection of an auxiliary module can give many advantages such as high step-up capability, design flexibility, and distributed voltage stress. In this paper, as an auxiliary step- up circuit, the voltage-doubler is integrated with a boost converter as shown in Fig. 2 [17]. The voltage stresses on the Fig. 3. Key waveforms in BR region. diodes Do2 and Do3 in the volage-doubler are clamped to its 978-1-4244-5287-3/10/$26.00 ©2010 IEEE 810 (a) (b) (c) (d) (e) (f) Fig. 4. Topological states in BR region. (a) Mode 1 [ t0 ~ t1 ]. (b) Mode 2 [ t1 ~ t2 ]. (c) Mode 3 [ t2 ~ t3 ]. (d) Mode 4 [ t3 ~ t4 ]. (e) Mode 5 [ t4 ~ t5 ]. (f) Mode 6 [ t5 ~ t6 ]. output, Vo2 + Vo3, therefore it is inherently suitable for high in a similar way to conventional resonant converters. The voltage application. detailed operation is presented as follows. The interface between the boost converter and the voltage- TLC= 2p (1) doubler is accomplished by the transformer, which also R lkg R contribute to a step-up gain by the turn ratio n. Since a square A. BR region ( TR/2 < DTS ) voltage waveform, i.e., AC voltage, is applied across the switch Q, the transformer can be inserted in parallel with Q. The key waveform and the topological states in BR region are shown in Fig. 3 and 4, respectively. Then, CR is inserted into the primary side of the transformer to make up for a flux-balance of the transformer. Thereby, the Mode 1 [ t0 ~ t1 ] : Q is on-state and VS is applied to the voltage-doubler is coupled with the boost converter by boost inductor LB. The boost inductor current ILb flows sharing the common switch. That is, with a switching action through Q and is increased linearly. At the same time, the of Q, both the boost converter and the voltage-doubler are voltage-doubler is operated with a common switching action operated. Moreover, the leakage inductance of the of Q. The powering path from CR to the lower output Vo2 of transformer Llkg and CR constitute a resonant tank. Since the the voltage-doubler is formed through the transformer, Q, and resonant operation between Llkg and CR shapes the current Do2, as presented by the dotted line. Llkg and CR constitute a sinusoidal during Q on-state, a switch turn-off loss and a resonant tank and derive a powering current with a sinusoidal reverse recovery on diode can be reduced. As a result, the shape. The resonant capacitor voltage VCr is decreased. The proposed converter is promising for high step-up isolated switch current IQ comprises ILb and the resonant current of the applications with high efficiency. voltage-doubler as well. Since Do2 is turned-off with very slow slope of IDo2, a reverse recovery would be minimized. II. OPERATION PRINCIPLES Do3 is blocked by Vo2 + Vo3. The proposed converter combines the boost converter and Mode 2 [ t1 ~ t2 ] : Since TR/2 is shorter than switch on- the voltage-doubler, with a common switching function of Q, interval DTS, the resonant operation is finished at t1 before Q employing a pulse-width modulation (PWM). Since the is turn-off. Therefore, only ILb flows through Q and the voltage-douber utilize the resonant operation between Llkg and resonant current does not increases a turn-off current. That is, CR, its operation can be divided into two regions according to it does not affect turn-off loss at all. Since no current flows the relationship between the resonant period TR in (1) and through CR, VCr keeps its value. duty cycle D. That is, the above-resonant (AR) region [ TR/2 Mode 3 [ t2 ~ t3 ] : Q is turned-off at t2 and ILb flow > DTS ] and the below-resonant (BR) region [ TR/2 < DTS ], through Do1. Meanwhile, the voltage-doubler is starting to be conducted in the opposite direction. The resonant operation 811 n o i t a r n r u t r e m r o f s n a r T Fig. 7. Transformer turn ratio n according to a variation of M. voltage-doubler. The ILb charges CR, increasing VCr linearly. Do1 is blocked. Mode 5 [ t4 ~ t5 ] : At t4, VCr is increased enough to conduct Do1 again. In this mode, as contrary to mode 3, the powering path from Vo3 to Vo1, is formed through Do3, the transformer and Do1. Therefore, IDo1 is increased and IDo3 is Fig. 5. Key waveforms in AR region. decreased by the resonant operation between Llkg and CR. Here, a reverser recovery of Do3 can be reduced because of the slow slope of IDo3. Mode 6 [ t5 ~ t6 ] : IDo3 reaches zero at t5 and the entire of ILb flow through Do1. Since no current flows through CR, VCr keeps its value. As D is increased, mode 5 and mode 6 fade gradually and disappear finally. B. AR region ( T /2 > DT ) R S (a) The operation in AR region is similar to that of BR region except that mode 2 of BR region, where ILb flows solely through Q, is ignored since TR/2 is longer than DTS. The key waveform and the topological states in AR region are shown in Fig. 5 and 6, respectively. Since some topological states are the same to those of BR region, only different topological states, the intervals t0 ~ t1 and t2 ~ t3, are presented in Fig. 6. The topological states of the intervals t1 ~ t2, t3 ~ t4, and t4 ~ t5 in AR region are corresponding to Figs. 4(a), 4(c), and 4(d), (b) respectively. Fig. 6. Topological states in AR region. (a) t0 ~ t1. (b) t2 ~ t3. In AR region, a switch current at the turn-off instant of t2 comprises I and I as well, therefore a turn-off loss can be Lb lkg increased compared with that in BR region where only ILb path from the output of the boost converter, Vo1, to the upper flows through at the switch turn-off instant. output of the voltage-doubler, Vo3, is formed through Do1, the transformer and Do3, as presented by the dotted line. III. ANALYSIS AND CHARACTERISTICS Therefore, by the resonant operation between L and C , I lkg R Do3 A. Input-Output Voltage Gain is increased. Since the resonant current flows through Do1 in For the sake of analysis, assuming the ripple of V is the opposite direction to ILb, IDo1 is decreased accordingly. Cr ignored and using a flux-balance on the boost inductor and Do2 is blocked by Vo2 + Vo3. the transformer, several voltage equations are obtained as Mode 4 [ t3 ~ t4 ] : IDo1 reaches zero at t3, therefore the follows. entire of ILb flows through the transformer and Do3 of the 812 Fig. 9. Transformer turn ratio n according to a variation of M. (a) o 20 I y b 18 M = 12 n = 3.5 d e 16 voltage stress on the voltage-doubler is n times higher z i l M = 10 a 14 compared with that on the boost converter. Since the voltage- m r o 12 M = 8.3 doubler provides n/(1+n) of the output voltage, the n t n 10 M = 7 transformer handles n/(1+n) of the total power accordingly.