A Single Inductor LED Driver Combined with a Cross-Connected Fibonacci-Type Converter and a Buck-Boost Converter

To achieve high voltage gains and high power efficiency, a hybrid light emitting diode (LED) driver with a single inductor is proposed in this paper. Unlike existing LED sink drivers, the proposed driver consists of a cross-connected Fibonacci-type converter and a buck-boost converter. In the proposed driver, the cross-connected Fibonacci-type converter drives the anode terminal of LED strings to realize high voltage gains. On the other hand, the buck-boost converter is connected to the cathode terminal of LED strings to provide output current controllability. Furthermore, the proposed driver achieves a small internal resistance, because these two converter blocks are connected in parallel to an input source. The characteristics of the proposed driver is investigated by theoretical analysis, computer simulations, and breadboard experiments. The obtained results demonstrate that the proposed driver outperforms the conventional LED sink drivers.


Introduction
To illuminate light emitting diode (LED) strings, an LED driver using switching converters is a vital component. Among others, many types of LED drivers employing dc/dc converters have been developed in past studies. The LED drivers can be classified into three types, i.e. i) LED driver using inductors such as boost converter [1] and buck-boost converter [2], ii) inductor-less LED driver such as charge pump [3], and iii) hybrid LED driver combining several dc/dc converters. In particular, the hybrid LED driver with a single inductor attracts much attention, because it can provide not only small off-chip components but also high gain and output current controllability. For example, McRae et al. proposed an LED driver [4] with a buck-boost converter and a Fibonacci converter [5], Eguchi et al. suggested an LED sink driver [6] with a buck-boost converter and an SC doubler [7], and Ranjana et al. designed an LED driver [8] with a buck-boost converter and a Cockcroft-Walton circuit [9,10]. However, the conventional hybrid LED drivers suffer from low power efficiency. In this paper, a hybrid LED driver with a single inductor is proposed for the realization of high voltage gains and high power efficiency. Unlike existing LED drivers, the proposed driver provides high voltage gains and output current controllability by the cross-connected Fibonacci-type converter [11,12] and the buck-boost converter, respectively. Furthermore, by connecting these two converter blocks in parallel with an input source, a small internal resistance is achieved by the proposed topology. To  Fig. 1 describes the circuit configuration of the proposed hybrid LED driver with a single inductor. As it can be seen from Fig. 1, the proposed driver consists of a cross-connected Fibonacci-type converter and a buck-boost converter. In these converter blocks, switches S 1 and S 2 are driven by non-overlapped two-phase pulses Ф 1 and Ф 2 , respectively. Unlike the conventional LED drivers [6,8], the proposed driver does not require series-connection of converter blocks to achieve voltage gains. In the ideal case, the voltages 8×V in and -D/(1 -D)×V in are provided to the anode and cathode terminals of LED strings, respectively, where D denotes a duty cycle of clock pulses. Therefore, the proposed topology realizes smaller internal resistance than the conventional LED drivers with series-connected structure [6,8]. Furthermore, single inductor topology provides a smaller number of off-chip components. The characteristics of the proposed driver will be clarified theoretically in the following section.

Theoretical analysis
By using the four-terminal equivalent model [13], the theoretical analysis of the proposed driver is performed under the conditions that i) time constant is much bigger than the period of clock pulses T, ii) parasitic elements of circuit components are negligibly small, and iii) all switches have the same onresistance. Cross-connected Fibonacci S1 S1 S2 C1,1 S2 S2 S1 C1,2 S1 S1 S2 C1,3 S2 S2 S1 C2,1 S1 S1 S2 C2,2 S2 S2 S1 C2,3 S2 S1 S1 S2 T1 T2 T Φ1 Φ2 Figure 1. Proposed topology combining a buck-boost converter and a cross-connected Fibonacci converter. First, the equivalent circuit of the cross-connected Fibonacci-type converter is analyzed. Since the instantaneous equivalent circuits of the proposed driver is expressed by Fig. 2, the variation of electric
Furthermore, since the cross-connected Fibonacci-type converter has symmetric structure as shown in Fig. 2, the variation of electric charges in the capacitor C i,j , Δq Tk i,j ((i = 1, 2) and (j = 1, 2, 3)), satisfies in a steady state condition. Using (1) and (5), the average input current and output currents, I in and I op , are expressed as Substituting (1)-(5) for (6), we have the relationship between I in and I op as Therefore, the conversion ratio of the cross-connected Fibonacci-type converter is 8.
To obtain the internal resistance R SC of the cross-connected Fibonacci-type converter, energy loss is discussed. The energy loss of the cross-connected Fibonacci-type converter is expressed as In (9), R on denotes the on-resistance of switch S k . Since (9) can be rewritten as the internal resistance of the cross-connected Fibonacci-type converter can be obtained as 64R on . Finally, the four-terminal equivalent model of the cross-connected Fibonacci-type converter can be obatined as because the four-terminal equivalent model can be expressed by K-matrix [13].
In the same way, the four-terminal equivalent model of the buck-boost converter can be obtained as To save space, the detailed analysis of (12) is omitted, because the buck-boost converter is well-known. Combining (11) and (12) because (11) and (12)

Simulation
By comparing the proposed driver with the conventional hybrid LED drivers, i.e. conventional-1 [6] and conventional-2 [8], the effectiveness of the proposed driver is clarified by simulation program with integrated circuit emphasis (SPICE) simulations. Fig. 3 demonstrates the SPICE simulated results, where the circuit parameters were set to V in = 3.7 V, L = 100 μH, C 0 = C i,j = 10 μF, T = 1 μs, D = 0.5, and R on = 0.1 Ω. Due to the difference of driver topologies, the voltage gain of the proposed driver cannot be equal to that of the conventional drivers. As it can be seen from Fig. 3, the proposed driver outperforms the conventional drivers. In the performed simulations, the power efficiency of the proposed driver reaches more than 95% when the output power is 4 W and the voltage gain is about 8.6. It is noteworthy that the proposed driver can improve the power efficiency more than 32 % from the conventional driver [8] when the output power is 4 W.

Experiment
The experimental circuit of the proposed driver was assembled on a breadboard. In the experimental circuit, the following circuit components were used: photo MOS relays AQW217, Darlington sink driver TD 62004APG, Microcontroller PIC12F629, and diodes 11EQS03L. The measured output voltage of the experimental circuit is demonstrated in Fig. 4, where the experiments were performed under the conditions that V in = 3.7 V, L = 100 mH, C 0 = C i,j = 10 μF, f = 800 Hz, and the output load R L = 22 kΩ. As it can be seen from Fig. 4, the experimental circuit generates 29.6 V and 30.7 V for D = 0.5 and D = 0.6, respectively. In other words, the voltage gains of Fig. 4a and 4b are 8 and 8.3, respectively. From these results, the validity of the proposed topology can be confirmed, because high voltage gains and output controllability can were measured experimentally.

Conclusion
In this paper, a novel hybrid LED driver with a single inductor has been proposed. First, theoretical analysis was performed by assuming a four-terminal equivalent model. In the performed theoretical analysis, the output voltage and power efficiency were derived theoretically. Next, to compare power efficiency and voltage gains, SPICE simulations were conducted between the proposed driver and conventional drivers. In the performed simulations, the proposed driver improved about 32 % power efficiency when the output power is 4 W. Finally, the breadboard experiments were conducted concerning the proposed driver. In the performed breadboard tests, the validity of the proposed topology was confirmed, because high voltage gains and output controllability were measured experimentally. A controller design and the IC implementation are left to a future study.