Single-Inductor Multiple-Output Switching Converters Wing-Hung Ki and Dongsheng Ma Integrated Power Electronics Laboratory Department of Electrical and Electronic Engineering The Hong Kong University of Science and Technology Clear Water Bay, Hong Kong SAR, China Tel: (852) 2358-8535 Fax: (852) 2358-1485 Email: eemds@ee.ust.hk Abstract A family of singleinductor multiple-output switching converters is presented. They can be classified into same-type, bipolar, and mixed-type converters. Synchronous rectification and control loop design are discussed, and experimental and simulation results of representative converters are presented to verify the functionality of these converters. I. INTRODUCTION Inductors are expensive and bulky elements in switch mode power converters. In an application that requires N output voltages, a straightforward implementation would be using N switching converters, thus requiring N inductors and 2N power devices (transistors and diodes). If the N outputs do not need to be individually controlled, then a transformer with one primary winding and N-1 secondary windings could be used. In such a case, one of the outputs, usually the one with the heaviest load, can be accurately controlled, while the rest would track. Both implementations require bulky magnetics. For a considerable saving in cost, weight and size, it is natural to investigate the possibility of using only one inductor for an N-output converter, and determine if the number of power devices can also be reduced. 11. SIMO BOOST / BOOST CONVERTER Consider two conventional boost converters switching at a frequency of f, (=l/t) and working in discontinuous conduction mode (DCM) with duty ratios DI,T and DlbT. The schematics and waveforms of their inductor currents are shown in Fig. 1. For Converter A, the inductor current ramps up during DI,T, ramps down during D2,T, and stays zero during D3,T = ( 1-DIa-D2,)T, while similar mechanism applies to Converter B. If D1, + DZa < 0.5, and Dlb + D2b < 0.5, the two converters can be organized to work in complementary phases @, and b, such that they can be combined in parallel as shown in Fig. 2 [l]. The timing V. V* Fig. 1. Two boost converters and their DCM inductor currents. scheme is a form of time multiplexing (TM). The diodes D, and Db in Fig. 1 are replaced by switches Sa and sb. Otherwise, during D2,T and D2bT, the inductor current cannot differentiate between D, and Db, and will charge up both outputs at the same time, and the outputs cannot be controlled independently. The power switches Sa and Sb close in @, and +, respectively are to divert power to the respective outputs V,, and Vob. Hence, a single-inductor dual-output (SIDO) boosthoost converter is obtained. Note also that only three switches are needed. Time multiplexing can be extended to N outputs, and each output should occupy a time slot of T/N for charging and discharging the inductor, to form a single-inductor multiple-output (SIMO) converter. The idea of SIMO converters has been disclosed in a patent [2], with two timing schemes that are different from the above (Fig. 3). In both schemes, the converter operates at This research is in part supported by the Hong Kong Research Grant Council CERG HKUST 6217/98E and DAG 00/01.EG30. Fig. 2. The SIDO boosthoost converter and its inductor current. 0-7803-7067-8/01/$10.00 02001 IEEE 226
the boundary of CCM (continuous conduction mode) and DCM. In the first scheme (Fig. 3a), each period is divided into three phases, not necessarily of equal duration. In phase 1, the inductor is charged from 0 to I,,. In phase 2, the inductor discharges into the first converter until IL = Ip2, and in phase 3, the inductor drains IL into the second converter until JL = 0. It is clear that information from both converters is needed to determine I,,, and any change in one phase necessarily affects the other two phases that makes the control of the two outputs interdependent. The same argument applies to the second control scheme (Fig. 3b). Hence, the design of the controller would be very complicated, and no product is selling in the market yet. Actually, the scheme would work if the converter is allowed to enter DCM, but not at the boundary of CCM and DCM. A (a) Control scheme 1 output instead, then the output voltage becomes positive. To do so requires a total of 4 switches (Fig. 4). It is easy to employ volt-second balance in confirming that the conversion ratio for the positive flyback converter is M = V,N, = D/( 1 -D) [ 31. Now, for a boost or a flyback converter, charging of the inductor depends solely on the source (generator) voltage V,. To divert energy to the output, one switch per output is needed. Hence, for an N-output boosthoost or flyback- /flyback- converter, N+l switches are needed (Fig. 2 and Fig. 5b). The figures only show dual-output converters, but it is easy to extend to the N-output case. Note that one switch is used for charging the inductor, while N switches are used to divert energy to the N outputs. For a buck converter, the two switches reside on the input side, and no switch resides on the output side. To construct an N-output bucklbuck converter, the two switches at the input side are needed, and each output requires one switch for separation (Fig. 5a). Hence, a total of N+2 switches are needed for the N-output buckhuck converter. Similarly, N+3 switches are needed for the N-output flyback+/flyback+ converter (Fig. 5c). (b) Control scheme 2 Fig. 3. Two control schemes in [2]. 111. CLASSIFICATION OF SIMO CONVERTERS A. SIMO same-type converters Fig. 5a. The SIDO buckhuck converter. Conventional classifications of second-order converters are the buck, the boost, and the buck-boost converter. Many researchers reserve the name flyback for a buck-boost converter that has an isolation transformer. In this paper, we label the buck-boost converter as the flyback converter. -, / VO, The flyback converter attains a negative output voltage because the inductor drains current from the output during discharge. If the discharging current is pumped into the PT ;DIT f * * * t7 Fig. 4. The flyback converter with positive output. Fig. 5b. The SIDO flyback-/flyback- converter. WaT. Fig. 5c. The SIDO flyback+/flyback+ converter. 221
B. SIMO Bipolar Converters In many applications such as LCD and CCDs, both positive and negative power supplies are needed. Since a buck or a boost converter gives a positive output, while a flyback converter gives a negative output, it is natural to construct buck/flyback- (Fig. 6a), boostlflyback- (Fig. 6b) and flyback+/flyback- converters (Fig. 6c) [4]. The boosthlyback- converter requires only N+2 switches. The bucwflyback- and flyback+/flyback- converters need three switches to charge the inductor for the positive and negative outputs differently. Hence, an N-output buck/ flyback- or flyback+/flyback- converter requires N+3 switches. For buck and boost converters, switching arrangement can be made to turn their conversion ratios negative, yet the numbers of switches required are prohibitive and impractical. It is worth mentioning that the boost/flybackconverter has been turned into a commercial product [5], because the output switches can be replaced by diodes, and the control circuitry can be much simplified C. SIMO Mixed-Type Converters To exhaust all possible combinations of SIMO converters, it is natural to extend the SIMO scheme to include converters with the same polarity but having mixed-type outputs. This brings us three more converters: buckhoost (Fig. 7a): buck/flyback+ (Fig. 7b) and boost/flyback+ (Fig. 7c) converters. By identifying four different converters, the buck, the boost, the flyback+ and the flyback- converters, a total of ten SIMO converters can be constructed. They can be group according to the number of switches needed (Figs. 2, 5, 6 and 7), and are listed in Table I. TABLE I. SIMO CONVERTERS WITH N OUTPUTS boosdflyback- I unipolar (positive) I N+3 flyback+/flyback+ I unipolar(positivc) 1 N-3 bucwflyback- bipolar flyback+/flyback- I bipolar N+3 N+3 Fig. 6a. The SIDO buawflyback- converter. Fig. 7a. The SIDO buckhoost converter. Fig. 6b. The SIDO boost/flyback- converter. DaT Fig. 6c. The SIDO flyback+/flyback- converter. Fig. 7b. The SIDO buck/flyback+ converter. 228
Fig. 7c. The SIDO boost/flyback+ converter IV. IMPLEMENTATIONS OF SIDO CONVERTERS The proposed converters share similar features in implementation if TM control in Section I1 is employed. Here, the boosthoost converter in Section I1 and the bipolar boost/flyback- converter are chosen to illustrate the designs with and without synchronous rectification. Fig. 8 shows a SIDO boosthoost converter with synchronous rectification. Switches Sa and Sb are implemented by power transistors instead of diodes because a 0.7V of diode drop degrades the efficiency of the converter, especially for a low-voltage battery-operated application. The over-current sensing circuit limits the maximum currents of the inductor and power devices and prevents excessive current from damaging the converter. If the inductor current starts to ramp up at t = 0, then it begins to go negative at t = (DI+D2)T. Bidirectional switches (MOSFETs) cannot block the reverse current automatically as diodes do. Therefore, zero-current sensing is needed to turn off the switches during DST. In the control loop, a multiplexer is used to sample the output voltages of the error amplifiers of the two sub-converters in the respective phase. The two PMOS power transistors M,, and Mpb should be driven using the highest output voltage of the converter instead of V, to avoid substrate leakage. Fig. 9 shows the schematic of the SlDO boost/flybackconverter. The switches SI and S2 in Fig.% are NMOS power transistors, while Sa and Sb are implemented with diodes. Diodes could be used because the two outputs are connected at different nodes of the inductor, which simplifies the controller, since two control signals are eliminated, and only two signals for duty ratios are needed to control MI and MZ. V. EXPERIMENTAL AND SIMULATION RESULTS Fig. 8. SIMO boosthoost converter with synchronous rectification. Converters with many of the proposed topologies have been designed. In particular, an integrated SIDO boost/ boost converter has been fabricated (Fig. 10) [6]. Table I1 summarizes the performance of the integrated boosthoost converter for system-on-chip (SOC) applications mentioned above. Fig. 11 shows the measured inductor current. Fig. 12 shows the voltage at node V, in the steady state. Fig. 13 shows two output voltages with reference to the inductor current. Each filtering capacitor has an ESR (equivalent series resistance) of 1 O O d and the parasitic resistance of the inductor is 125mR. Fig. 9. SIMO boodflyback- converter. i Fig. 10. Chip photo of SIDO boosthoost converter. 229
TABLE 11. PERFORMANCE OF INTEGRATED SIMO BOOSTiBOOST CONVERTER currents of the outputs and output voltages in the steady state, and Fig. 16 shows the voltages at the two nodes ofvxb and VXf of the inductor. Each filtering capacitor also has an ESR of SO&. TABLE 111. PERFORMANCE OF SIMO BOOST/FLYBACK- CONVERTER A SIDO boostlflyback- converter is designed for LCD applications and its performance is listed in Table 111. Similarly, Fig. 14 and Fig. 15 show the inductor current, Fig. 1 1. Inductor current of SIDO boosthoost converter. D 1.00 V Fig. 15. Output voltages of boosthlyback- converter. Fig. 12 Voltage at the node of V,. Fig. 13. Two output voltages with reference to inductor current. 230
VI. CONCLUSIONS A new family of single-inductor multiple-output (SIMO) converters is introduced, among which ten possible secondorder SIMO converter topologies are proposed. Time multiplexing is employed in switching energy to individual outputs. Synchronous rectification is used to replace diodes with transistors in enhancing efficiency. Design examples are discussed, with extensive experimental and simulation results presented in verifying the validity of the concept and the design. REFERENCES programming, IEEE/ACM ASP-DAC LSI Universiry Design Contest, pp. 19-20, Jan. 200 1. [2] T. Li, Single inductor multiple output boost regulator, US Patent 5,757,174, June 13,2000. [3] R. W. Erickson, Fundamental of Power Electronics, Boston: Kluwer Academic Publisher, 1999. [4] D. Ma, W-H Ki, P. Mok and C-Y. Tsui, Single-inductor multiple-output switching converters with bipolar outputs, IEEE Int. Symp. on Ckts. and Syst., May 200 1, in press. [5] MAX685, Dual-Output (Positive and Negative) DCDC Converter for CCD and LCD, www.maxim-ic.com. [6] D. Ma, W-H Ki, C-Y. Tsui and P. Mok, A 1.8V singleinductor dual-output switching converter for power reduction techniques, IEEE VLSI Symp. On Ckts., June 2001, in press. [I] D. Ma, W-H Ki, C-Y. Tsui and P. Mok, A single-inductor dual-output boost converter for variable voltage 23 1