Single-Inductor Multiple-Output Switching Converters with Bipolar Outputs Dongsheng Ma, Wing-HungKi, Philip K. T.Mok and Chi-Ying Tsui Department of Electrical and Electronic Engineering The Hong Kong University of Science and Technolog?, Clear Water Bay, Hong Kong SAR, China Fax: (852) 2358-1 485 E-mail:
[email protected] ABSTRACT A family of single-inductor multiple-outpiit switching converters that provide both positive and negative output voltages is presented. With time multiplexing control, both step-up and step-down bipolar outputs could be achieved. Technical considerations on control loop design and implementing synchronous rectification for single chip converter are discussed. Simulation results are presented to show the validity of the proposed converters.
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Figure 1. Four second-order DC-DC converters
1. INTRODUCTION For battery-operated portable applications, efficiency and size are critical in evaluating the performance of a system. To optimize power and speed, multiple supply voltages are needed to power up different functional blocks [l]. For some applications, these voltages should have both positive and negative polarities [2]. LCD or CCD subsystems embedded in PDAs, mobile phones, and digital canieras are some of the examples. Conventional implementation of M positive and N negative output voltages would require M+N inductors or transformer windings and at least 2(M+N) power switches. Weight and volume would present a packaging problem, while cost would be high.
In this paper, topologies of switching converters that use only one inductor [3][4] to simultaneouslyprovide both positive and negative output voltages are proposed. Architecture and control schemes of the converters will be introduced in Section 2, implementation issues will be discussed in Section 3, and simulation results will be presented in Section 4 in confirming the functionality of these converters.
2. SIMO BIPOLAR CONVERTERS 2.1 The Boost-Flyback Converter
converters suggests that a configurable converter (Fig.2) can be obtained if S, and Sb are added to the boost converter (or S3 and S, to the flyback converter). For this configurable converter, if SI is always closed and Sb always opened, a boost converter is obtained. If S , is always closed and S, always opened, a flyback converter is obtained. In fact, a more economical use of this configurable converter could be achieved. Let us return to the boost and flyback converters in Fig. 1. If both converters are working in the discontinuous conduction mode (DCM), a possible scheme of their inductor currents could be as shown in Fig.3. The boost converter (Converter A) has a positive output, and the flyback converter (Converter B) has a negative output. For consistency, in this paper, we always assign Converter A to have a positive output voltage V,,, and Converter B to have a negative output voltage -Vob.
For Converter A, during DI,T, the inductor current IL ramps up and the inductor is charged with a voltage of VL = V,, During D,,T, IL ramps down with VL= V,-V,,, and during D3,T (= (1D,,-D,)T), IL stays zero. Apply volt-second balance to the inductor, the conversion ratio is given by M, = V,N, = V,JV, = I/( I-D), and gives a positive output voltage V,,. Similarly, for Converter B, Mb = VJV, = -VodVg = -D/( I-D). Hence, the output voltage is negative. Obviously, if Dla+D2,< 0.5 and
The three basic second-order converters are the buck, boost and buck-boost (flyback) converters (Fig.1). For a positive generator source voltage V,, both the buck and the boost converters give a positive output voltage V,, while the flyback converter gives a negative output voltage -Vo. To improve flexibility in design, the non-inverting flyback converter is also introduced [SI (Fig.1). A closer look at the boost and flyback
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This research is in part supported by the Hong Kong Research Grant Council CERG HKUST6217/98E.
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Figure 6. Timing diagram of buck-flyback converter two switches in series, which are the same for both SIDO converters. Figure 4. Timing diagram of boost-flyback converter Dlb+DZb< 0.5, the two inductor currents can be assigned to occupy different parts of the switching cycle without affecting each other. If the above timing scheme is applied to the configurable converter with each subconverter working in complementary clock phases @, and @, a single-inductor dual-output (SIDO) bipolar converter is achieved [2]. When = 1, it operates as a boost converter, and when (+, = 1, it operates as a flyback converter (Fig.4). Thus, time multiplexing (TM) is employed in diverting the inductor current and energy to V,, and -V,b in the respective phases. If the two outputs are achieved by two converters (the conventional implementation), two inductors and four switches are needed. For the SIDO implementation, only one inductor and four switches are needed. If all the switches are of the same size, the SIDO implementation has lower efftciency, but the saving of one external inductor would mean a smaller overall layout of the converter, which would be preferred in many applications.
2.3 The Flyback-Flyback Converter
To add flexibility in design, the positive output could be achieved by the non-inverter flyback converter and the timing diagram are shown in Fig.7. Topologically, the flyback-flyback converter is the same as the buck-flyback converter, but with a different switching matrix, which can be inferred from a careful study of Fig.7. Although inverting buck and boost converters can be constructed, they need too many switches to be practical. Hence, by combining the three second order non-inverting converters with the inverting flyback converter, we have exhausted all possible second order bipolar topologies.
2.2 The Buck-Flyback Converter The boost converter always provides a voltage that is higher than the source voltage V,. For applications that a voltage lower than V, is needed, the boost sub-converter in the boostflyback converter could be replaced by a buck counterpart, and a SIDO buck-flyback converter is thus obtained (Fig.5). The timing diagram is shown in Fig.6. This converter needs one inductor and five switches, but the efficiency is not necessarily worse than the boost-flyback converter. In each charging or discharging instant of the inductor, the current passes through
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Figure 8. M+N outputs flyback-flyback converter
2.4 SIMO Converters 6
It is obvious that the proposed SIDO bipolar converters can be extended to give M positive outputs and N negative outputs. One example of this single-inductor multiple-output (SIMO) converter is the flyback-flyback converter shown in Fig.8. The timing diagram is not provided, but would not be difficult to produce with a good understanding of the TM control. This converter consists of one inductor and M+N+3 switches. Compared with the conventional approach, where M+N inductors and 4M+2N switches have to be used, the SIMO converter saves many components. It is straightforward to extend the case to SIMO buck-flyback and boost-flyback converters. Table 1 summarizes topology comparison between the proposed converters and their conventional counterparts. Table 1. Comparison on Different Converter Topologies Numberof Numberof Type of converter inductors switches 1 M+N+2 Boost-flyback converter M+N 2M + 2N Conventional implementation Buck-flvback converter I 1 I M+N+3 I 2M t 2N Conventional implementation I MtN M+N+3 Flyback-flyback converter I I Conventional implementation I M+N 1 4M+2N
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3. IMPLEMENTATION OF CONVERTERS The three proposed converters share similar features and the SIDO flyback-flyback and boost-flyback converters are chosen for illustration. Fig.9 shows the flyback-flyback converter with
a closed loop controller that employs PWM time-multiplexing control. The switches S I and S2 are realized by PMOS power transistors M, and M2, and S3 is realized by an NMOS power transistor M3, while S, and Sb are replaced by power diodes. Using diodes simplifies the control, since two control signals and the associated driving circuits are eliminated, and only three signals for duty ratios are multiplexed to control MI, M2 and M3. If the converter is to be implemented on a single integrated circuit, then special attention is needed in connecting the wells and the substrate, and in driving M1 and M2. For the PMOS MI and M2, the gate drive should range from -Vob to V, and the potential of its well should be V,, such that they can be shut off without leaking. Meanwhile, the gate drive of the NMOS transistor M3 should range from zero to V,. . Fig. 10 shows the schematic of the boost-flyback converter with closed-loop control. In this converter, power transistors M, and Mb could also be replaced by diodes [2]. Yet, the diode drop of 0.7V degrades the efficiency of the converter, especially for low-voltage applications. To boost the efficiency, synchronous rectification is employed. Since M, and Mb serve as bidirectional switches, zero current sensing circuits are needed to prevent inductor current from going negative during D3,T and D3bT, by turning off the corresponding transistors. Similarly, the connection of wells and substrate should ensure that transistors could be fully tumed on and off. For the PMOS transistor Ma, the gate drive should range from OV to V,, and the well should be connected to Vo,. For Mbr the gate drive should range from -v,b to V,,, and either a PMOS or an NMOS could be used.
4. SIMULATION RESULTS Table 2. Design specifications of the two converters
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Fig. 11 shows the current waveforms of the flyback-flyback converter. The upper trace shows the inductor current, while the second and third traces show the diodes currents of Converter A and Converter B, respectively. Fig.12 shows the waveforms of the two output voltages and their ripple voltages. Fig.13 shows the voltages at the nodes of V, and V, labelled in Fig.9, which are helpful to evaluate the performance of the converter. Similarly, Fig. 14- 16 show the related waveforms of the boost-flyback converter, i.e., current, output voltages and node voltages Vxb and V,6
5. CONCLUSIONS In this paper, a family of SIMO bipolar converters is proposed. Architectures, control schemes and design issues are discussed in detail. Simulation results show that the converters can successfully provide well-regulated multiple bipolar outputs G t h a single inductor. Compared with conventional converters, the numbers of inductors and power devices are reduced significantly, which is very desirable for portable applications.
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