Study of the Multilevel Converters in DC-DC Applications

Transcription

1 th Annual IEEE Power Electronics Specialists Coilference Aachen. Germany, 2004 Study of the Multilevel Converters in DC-DC Applications Fan Zhang'. ', Fang Z. Peng'.', and Zhaoming Qian' College of Electrical Engineering, Zhejiang University' Department of Electrical and Computer Engineering, Michigan Stat,: UniversitJ 2120 Engineering Building, East Lansing, MI Phone: zhangf@msu.edu Abstract- Multilevel converters have been demonstrated to have many advantages such as low harmonic, low voltage stress, and high power capability. However, most of the researches are focused on high power AC-DC and DC-AC applications. This paper presents several multilevel DC-DC converters, which can be used in automotive applications as well as in high power applications. Based on the diode-clamp, flying-capacitor, and cascaded multilevel inverters, their correlative DC-DC converters are derived. A 1 kw prototype of four-level flying-capacitor DC-DC converter used in automotive systems was built as an example to demonstrate the advantages. 1. INTRODUCTION Multilevel converters have been widely used in power conversion for high power applications. The advantages of the multilevel converter include less harmonic distortion, low EMI, and low voltage stress on devices. By providing more than two levels, multilevel converter can achieve smoother and less distorted AC-DC, DC-AC, and DC-DC power conversion. Many literatures have discussed the multilevel technology on AC-DC or DC-AC converters, most of them are used in utility and large motor drive applications [I]. Some literatures discussed about multilevel DC-DC converters and the voltage choppers [2-4]. In these DC-DC applications, multilevel structures can reduce or even eliminate the magnetic components, which help to reduce the size and cost of the converter. However, multilevel DC-DC converter will meet more balance problems than that in AC-DC or DC-AC converter. Because of the asymmetric DC output voltage, fewer redundant switching states will available in the DC-DC multilevel converter. Also, the difference of characteristics for each individual component, either on semiconductors or on passive components, will cause the voltage unbalanced. In most cases, a dynamic balance control strategy is necessary to balance the capacitor voltages, which requires enough redundant switching states. But unfortunately, not all multilevel shctures have enough switching states, which means some of them can't be used in DC-DC applications. This paper discusses several multilevel DC-DC topologies both in non-isolated and isolated applications. Based on the theoretical analysis, different structures for multilevel DC- DC converter are compared and a four-level flying-capacitor DC-DC prototype used in 42 V automotive systems will be analyzed as one example. Compared with the traditional converter, this converter uses the same number of devices but with no or minimal inductance requirement. The efficiency can achieve '98% and it is easy to integrate because of its compact sice and lightweight. Simulation and experimental results from this IkW prototype verify the analysis and demonstrate the advantages of the flyingcapacitor multilevel DC-DC converter. 11. THEORI?TICAL ANALYSIS There are three major rnultilevel topologies: diode-clamp, flying-capacitor, and caxaded. They all have correlative topologies in DC-DC applications. Because of the asymmetric DC output, some multilevel converters will have serious balance problem at a unique structure. In this section, these multilevel DC-DC converters both in nonisolated and isolated applications will be discussed. For convenience, the (converters discussed in this paper are all step-down converters, although some of them can transfer the energy bi-directional. Assume the input voltage is 3 Vdc and the output,roltage is I Vdc, which means the voltage transfer ratio is 3: I. A. Non-isolated DC-DC converter 1) Diode-clamp DC-DC rnultilevel inverter Fig. I shows a four-lebel diode-clamp multilevel inverter. This structure usually is used in DC-AC applications. By using dynamic balance control, it won't have balance problem. When it is used for DC-DC applications, its redundant switching states will he reduced. Sometimes it will be not enough to kee,p voltage balanced. To get V,,,, = I at output side, V,, must be higher than VbO. Furthermore, to get the minimal voltage ripple and smallest size filter at the output side, the optimized control strategy should be satisfied with V,, - Vbo = IV,. From this point, only some unique combinations of switching states can be used in the control strategy. Table 1 shows all thf: switching states of phase-a and phase-b. There are three combinations of the switching states that can get I Vdc.at the output. They are marked as blue, yellow, and green areas from top to bottom. Each of them has 1/3 duty cycle in one period, and this can keep the converter voltages balanced. If same grounding is required for the output and input sides, in table 1, simply keep Vbo equals to zero, which means only the green ar,:a can be used. There's no way to keep the voltages balanced. The two upper capacitors will keep charging and the lower capacitor will be discharged to zero /04/$20.M) IEEE. I702

2 rh Annual IEEE Power Elecrronics Specialisrs Conference Aachen, Germany, I Fig. 1. Four-level diode-clamp DC-DC converter. Table I Combination of switching states (Diode-clamp) Fig. 2. Four-level flying-capacitors DC-DC converter. Table 11 Combination of switching states (flying-capacitors) L@gq F l v,, = I!& 2) Flying capacitor multilevel DC-DC converter Fig. 2 shows a four-level flying capacitor multilevel DC- DC converter. Same as the analysis in the diode-clamp multilevel converter, we can get all the switching states in table 11. It s obviously that the flying-capacitor type multilevel converter has more redundant switching states. The yellow area actually has nine combinations of switching states to get IV, at output side. The green area has three combinations of the switching states. More redundancies make it easy to select the proper switching states and keep the voltages balanced. If same grounding is required for the output and input sides, we can simply keep V,, equals to zero, which means only the green area can be used in the control. These three combinations of switching states(&,, S, So, = 100, 010, 001) in this topology are enough to keep the voltages balanced. So we get another four-level flying-capacitor DC- DC converter with same grounding, which is shown in Fig. 3. This topology was presented in [5]. Some charge pump circuits have the similar structure with this flying-capacitors converter. As shown in Fig. 4, the three-level DC-DC converter is exactly the well-known voltage doubler. This four-level flying-capacitors DC-DC converter also can be seen as a voltage tripler. More detail analysis will be discussed in the next section of this paper as an example. 3) Cascaded multilevel DC-DC converter The cascaded converter use two or more same structure in Fig. 3. Four level flying-capacitor DC-DC converter with same grounding. series to provide multilevel voltages. The advantages include modularized structure, self-powered capability, and low voltage stress on the device. A generalized multilevel converter is shown in Fig. 5, which is built with six P2 cells [6]. The control strategy is same as that for the flyingcapacitor type converter which is shown in table

3 2w4 35Ih Annual IEEE Power Elecrronics Speciulisrs Cor$erencz Auchen. Gerniuny, 2004 Fig. 4. Three-level flying-capacitors DC-DC converter and traditional voltage doubler. B. Isolated multilevel DC-DC converter The isolated DC-DC multilevel converters can get easily from the correlative non-isolated converters. The isolated diode-clamp multilevel D13-DC converter is shown in Fig. 7. The correlative flying-capacitors multilevel DC-DC converter is shown in Fig. 8. Fig. 9 shows an example for cascaded DC-DC conveker. It has series structure in high voltage side and parallel structure in low voltage side. The advantage is observable especially at high power applications. It can distribute the total power with two or more transformers, and use same number of active devices as that used in Fig. 7 and Fig.8. Among these three types of multilevel converter, the diode-clamp multilevel DC-DC converter has fewer redundant switching states, and has limitation at some applications. The flying-:apacitor multilevel converter has more redundant switching states and is easy to use in different applications. The only disadvantage is that more bulk capacitors are used. The cascaded converter has many advantages, such as modularized structure and low voltage stress, etc. It is the most suitable multilevel converter for DC-DC applications. Fig. 5. Generalized multilevel DC-DC converter. 0 I& ~~ S., S., Sd, 1% YO?,, Fig. 7. Diode-clamp multiled DC-DC converter (isolated). Fig. 6. Simplified circuit from the generalized multilevel DC-DC converter Similarly, when same grounding required, we simply use three switching states (Se,, So,, S.3 = 100, 010, 001) shown in the green area. The detail analysis can be found in [7]. We notice that the point b is always connected to the output point a, so V,, also can be seen as the output voltage. Simplified the converter shown in Fig. 5 and we get a new topology, which is shown in Fig. 6. The control in this circuit is very simple. 50% duty cycle is applied to all the switches, and Sol is complemented to So.,. In several cycles, the voltage across each capacitor will be charged balanced. Compared to the converter shown in Fig. 3, it uses same number of active devices but has fewer capacitors. Filter " I Fig. 8. Flying-capacitors multilevel DC-DC converter (isolated) 1704

4 rh Annul IEEE Power Elecrronics Specialisrs Conference Aachen, Gemany, 2004 sal _5-1 I 0 0 I v'k S, I n I o j j s, 0 0 r F T y IYh j j Fig. 11. Switching sequence of the four-level DC-DC converter. j Fig. 9. Cascaded converter with series structures at high voltage side and parallel structures at low voltage side DETAIL ANALYSIS FOR FLYING-CAPACITORS MULTILEVEL DC-DC CONVERTER In this section, a four-level flying-capacitors DC-DC converter will be discussed in detail. We know that the three-level flying-capacitors DC-DC converter is exactly a voltage doubler. The four-level DC-DC converter also can be seen as a voltage tripler. This is a suitable converter for automotive 42 V114 V dual-voltage systems, which is shown in Fig. IO. The voltage at low-voltage-side is 113 of that at high-voltage-side, which is locked to the high-voltage-side battery, just behaves like a true battery. The converter has three switching states (S,,, Sa>, So, = 100,010, 001) that generate an output voltage of IVdc on the low voltage side. Fig. I I shows the switching sequence. The converter operates at a fixed duty ratio and fixed switching frequency without pulse width modulation. The voltage across each capacitor is balanced automatically. Compared to the traditional Buck converter with large inductor, this converter has many advantages. First of all, V, is close to I vdc, so the inductor at low voltage side can be minimal or even eliminated. Secondly, low dddt in switching instant, which contributes to the EM1 reduction. Also, there is no voltage spikes due to the capacitor clamp in this multilevel circuit. I Vh 1V. EXPERIMENTS AND PROTOTYPE FOR 42 V AUTOMOTIVE SYSTEMS Based on the theoretical analysis in section 111, a 1 kw prototype used in 42 V/14 V automotive systems was built to demonstrate the advantages. The prototype is shown in Fig. 12. There's no inductor inside and only three pins: +42 V, +I4 V, and GND. The prototype was hand-wired on Vector boards with discrete DIP chips. The size can he further reduced with PCB and SMD components. Fig. 13 shows the efficiency of this prototype, which was measured using Yokogawa's power analyzers PZ4000. The efficiency curve shows 98% was achieved at 300 W output and more than 95% at full load. Fig. 12. Photograph of the 42 V/ 14 V prototype % Baliery voltage I Fig. 10. Four-level DC-DC converter used in 42 automotive systems. 92 I IWO I200 outpvt Power (W) Fig. 13. The measured efficiency. I705

5 th Annual IEEE Power Electronics Specialists Corlference Aachen, Germany V. CONCLUSION This paper presents several multilevel DC-DC converters that can be used in non-isolated or isolated applications. Based on the theory analysis, the comparison for the three types multilevel DC-DC converter (diode-clamp, flyingcapacitor, and cascaded) is presented. The diode-clamp type converter has fewer redundant switching states and has limitation in DC-DC applications. The flying-capacitor can be easy to balance at different applications but more capacitors is used. The cascaded converter has many advantages, such as modularized structure and low voltage stress. It is the most suitable type converter for DC-DC applications. As an example, a 1 kw prototype of four-level flyingcapacitor DC-DC converter was built to demonstrate the advantages. It can be used in 42 Vi14 V dual voltage automolive systems. The advantages include high efficiency, magnetic-less circuit, and low EMI. RE:?ERENCES [I]. J. S. Lai and F. Z. Pcng, Multilcvcl converters - A ncw breed of powcr CO~YC~CCS. leee Trans. Ind. Application, vol. 32, pp , MayIJunc T. A. Meynard and H. Fcrch. Multilcvcl convcrsian: High voltagc chappcr and voltagc-sourcc invcrtcps.)( in Prcc. IEEE PESC 92, 1992,pp [3]. K. D. T. Nga and R. Wcbstcr, Stcady-Statc Analysis and Dcsign ofa Switchcd-Capacitor DC-D Z Convcrtcr. IEEFJPESC, [41. Francois and B.; Hauticr, J.P. Multilcvcl SlNClUR-S for four-lcvel DCIDC voltagc convcnians: Enhanccd analysis and control design issues". IEEEIPESC, 2002, pp [5]. Fang Z. Peng. Fan Zhang. and Zhaoming Qian. A Novel Compact DC-DC Convcrtcr for 42 \ Systems", IEEWPESC, [6]. F. Z. Pcng, A Gcncraliz,d Multilcvcl lnvcrtcr Topologv with Sclf Voltage Balancing, IEEE Transactions on Industry Applications, Vol.37.No.2,pp. 6I1-61f,March/April [7]. Fang 2. Pcng, Fan Zhang. and Zhaoming Qian, A magnetic-less DC- DC COnVCrtCr for dual voltage automotive systcms, IEEE Transactions on Industry Applications, Vo1.39, No.2, pp MarcldApril