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Abstract – This paper proposes a new DC-DC converter. The

DC-DC Multilevel Boost Converter, based on one inductor, oneswitch, 2N-1 diodes and 2N-1 capacitor, for N levels plus thereference (total N+1 levels), is a boost converter able to controland maintain the same voltage in all the Nx output levels, andable to control the input current.

This converter is based on the multilevel converters principle,and it is proposed to be used as DC-link in applications whereseveral controlled voltage levels are needed with self balancingand unidirectional current flow, such as photovoltaic (PV) or fuelcell generation systems with multilevel inverters. Used to feed amultilevel inverter, the proposed topology achieves a self voltagebalancing; experimental results prove the principle of theproposition.

Index Terms – DC-DC converter, multilevel converter, boostconverter, voltage multiplier.

I. INTRODUCTION

ultilevel converters have attracted interest in powerconversion [1][8]; they already are a very important

alternative in high power applications [1][2]. It has beenshown that they are useful in virtually all power conversionprocesses such as ac-dc, dc-ac, dc-dc and ac-dc-ac [3].

Some of the advantages of multilevel converters againsttraditional topologies are: (i) low harmonic distortion, (ii) lowvoltage stress, (iii) low EMI noise, (iv) low switchingfrequency, (v) high efficiency, (vi) ability to operate withoutmagnetic components [2][3][8]. All these advantages makemultilevel converters one of the most important topics inpower electronics, and industrial application research, and insome applications they can get modular topologies [2].

After 30 years of research, a lot of structures and variationshave been developed. The three major topologies are: (i) diodeclamped, also called neutral point, (ii) capacitor clamped, alsocalled flying capacitor, (iii) and cascaded multi-cell. Fromthose ones, several hybrid topologies have appeared [1][3]. Ageneralized topology is proposed in [3], showing that all basicstructures and new multilevel topologies can be derived fromthe generalized topology.

The main multilevel converters’ applications are focused inhigh power motor drives, Fig.1a, static VARs compensation,and other utility applications [1][2][8]; they are also suitablefor FACTS devices[4]. They can also been applied to DC-DCconversion in low power, specially for automotiveapplications, Fig.1b [6][10], and renewable energy systems[4][5[7].

Authors are with CINVESTAV – Unidad Guadalajara- MEXICO

In DC-DC conversion, it has been shown in [2] that DC-DCconverters can be derived from the three basic topologies,initially proposed for dc-ac and ac-dc conversion, and from thegeneralized topology; they can eliminate magnetic componentsin the converter, although they exhibit limitations in thecapacitor’s voltage balancing, making this one the maindrawback.

Fig. 1. Examples on multilevel converters applications: (a) high power, traindrive system in Japan [12], (b) Low power, DC-DC converter for automotive

system [10].

II. RENEWABLE SYSTEMS APPLICATION

Renewable energy systems offer clean power andindependence from fossil fuel, since the product of thechemical reaction in fuel cells is H2O when H2 is used as fuel;therefore, the fuel cells are environmentally cleaner thantraditional generators [5[9][12].

Nowadays, the main energy generation is based onconcentrated high power plants; in the future, a diverse anddisperse generation will become a major energy source [4][7].

According to these scenarios, the increase in world energydemand will be supplied by renewable energy technologies,which will provide 30% - 50% of world energy by 2050 [9].

This scenario makes multilevel topologies ideal forrenewable energy generation systems. Some applications havebeen developed [4][5], the major ones related with multilevelconverters based renewable systems who use the renewablesource in the optimum operating point and the DC-Linkvoltage balance. Renewable systems are often made by a dc-dcconverter that can track the maximum power operating point,and an inverter to deliver the active power into the utility.

Novel DC-DC Multilevel Boost Converter

Julio C. Rosas-Caro, Juan M. Ramírez, Pedro Martín García-Vite.Power System Department Guadalajara Campus of CINVESTAV, Guadalajara City Mexico.

M

978-1-4244-1668-4/08/$25.00 ©2008 IEEE

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It is known that for applications on active power transfer,such as motor drives, or renewable applications, conventionalmultilevel topologies require either isolated dc power sources,or a complicated voltage balancing circuit and control tosupport and maintain each voltage level [3], and they areneither operable nor complete for active power conversionbecause they depend on outside circuits for voltage balancing[3].

The multilevel configuration makes it possible to utilize lowvoltage MOSFETs, which have extremely low on-resistanceand are low cost, because of large production volume forswitching power supplies used in communications andcomputer industries [6]. These low cost-size switchesproportionate the possibility of integrating inverters in a smallspace and make renewable systems inverters compact andcheap.

Fig. 2. Five level diode clamped inverter for renewable energy generationsystem.

The challenge is to link the DC renewable energy sourcewith a DC-AC multilevel inverter. Such links should bebalanced, and is highly desirable to be self-balancing to avoidcomplex control strategy, Fig.2. It also requires a high boostratio, which is a challenge for transformer-less DC-DCconverters [11], although for utility connected renewableapplications the boost ratio can be larger than five.

For multilevel inverters based renewable applications, it ishighly desirable to design a DC-DC converter to overcomesuch challenges, and connect a N-level multilevel inverter tothe utility with a high boost ratio, self-balanced voltage, andunidirectional current. The DC-DC converters proposed in thispaper overcome such difficulties.

This paper proposes a new DC-DC converter, namedMultilevel Boost Converter (MBC), based on one inductor,one switch, 2N-1 diodes and 2N-1 capacitor for N levels. It isa boost converter PWM controlled and able to maintain thesame voltage in all N output levels and able to control theinput current.

This converter is based on the multilevel convertersprinciple; and it is proposed to be used as DC-link inapplications where several voltage levels are needed with self-balancing and unidirectional current flow, such as photovoltaic

(PV) or fuel cell generation systems with multilevel inverters.Used to feed a multilevel inverter, the proposed topologiesachieve a self-balanced voltage. Experimental results in a lowpower prototype are exhibited.

III. BOOST CONVERTER

This section analyzes the conventional boost converter andthe maximum boost ratio considering the inductor losses. Theboost ratio is limited by the inductor’s equivalent seriesresistance (ESR) [13]. Due to this limitation, the maximumboost ratio can be calculated through the output power(voltage and current) and the inductor’s ESR, which can beestimated and measured during and after the inductor’s design.This is especially important when the desired boost factor isbigger than five; however, this information is not available inmost of commercial literature. Here the ESR effect in the boostratio is analyzed for the conventional boost converter and forthe proposed topology.

Fig. 3. Conventional boost converter.

Fig. 3 illustrates the boost converter. The duty cycle, D, isdefined as the time relationship that the switch is on relative tothe total switching period. Assuming that the current in allinductors and the voltage in all capacitors keep constant, as isusual in the switching converters’ steady state analysis, insteady state the DC voltage in the inductor

LV can be

expressed as (1) and is equal to zero:

0))(1()( =−−−= CininL VVDVDV (1)

From (1) at steady state it can be verified that thecapacitor’s voltage becomes:

DV

V

in

C

−=

1

1 (2)

The DC inductor’s current can be obtained by the input andoutput power:

O

CCoutCLin R

VVIVIV ==

O

C

in

CL R

V

V

VI = (3)

Substituting (2) into (3), the inductor current can be expressedas:

O

CL RD

VI

)1( −= (4)

Eq. (2) is obtained from the boost converter’s ideal model,while the inductor’s losses can be added through an ESR (

LR ),

Fig 4. An analog equation to that in (1) can be obtained for themodel with ESR as (5).

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Fig. 4. Boost converter with the inductor’s ESR.

0))(1()( =−−−+−= LLCinLLinL RIVVDRIVDV (5)

From (5) it is possible to get (6):

CLLin

LLCinLLin

VDDDRIDDV

RIDVDVDRDIDV

)1()1()1(

0)1()1()1(

−=+−−+−+=−−−−−+−

LLCin RIVDV +−= )1( (6)

Remembering (4) and substituting it into (6), the boostfactor considering the inductor’s losses can be approximatedby (7).

−+−=

−+−=

O

LCin

LO

CCin

RD

RDVV

RRD

VVDV

)1()1(

)1()1(

O

Lin

C

RD

RDV

V

)1()1(

1

−+−

= (7)

If the inductor losses are ignored ( 0=LR ), then (7)

becomes (2). Eq. (7) Gives us an idea to the effect in the boostfactor, and explains why the real boost factor gets a maximumbefore D=1 and then becomes 0, Fig. 5. From (7) it can also beseen that the boost ratio is limited for the relation between theinductor’s ESR and the output resistance. By varying (7), theactual boost factor against the duty cycle can be obtained. InFig. 5 the ideal case is drawn (RL/RO = 0.00) besides sometypical values RL/RO.

Fig. 5. Voltage gain versus duty cycle for different values of ESR/Ro.

It is noteworthy from Fig. 5 that the boost factor is quasi-linear from [1, 2] when the duty cycle is from [0, 0.5], butafter that, the boost factor becomes non-linear. The necessaryboost factor for renewable generation systems is from [2, 6-8].This behavior complicates the boost converter control inrenewable energy generation systems.

IV. DC-DC MULTILEVEL BOOST CONVERTER

Fig. 6 depicts the proposed topology. It is a Nx DC-DCconverter based on one switch, 2N-1 diodes and 2N-1capacitors. One advantage of the topology is that the numberof levels can be extended adding capacitors and diodes.

The lowest part of the converter, Fig. 7, is the conventionalDC-DC boost converter, then the voltage gain also holds forthe boost converter in (2). The difference between themultilevel boost converter and the conventional one is that inthe multilevel boost converter, the output is Vc times N, whereN+1 is the converter’s number of levels, Fig. 6.

Fig. 6. DC-DC Multilevel Boost Converter for N+1 levels.

Fig. 7. Lowest part of the proposed topology.

The multilevel principle of this converter will be explainedthrough a 4x (5 levels) dc-dc boost converter, Fig. 8. Toexplain the multilevel principle, assume that the switch (S) isswitching with a duty cycle (D) of 0.5.

Fig. 8. Switch-on state.

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During the switch-on state, the inductor is connected to Vinvoltage, Fig. 8a. If C6’s voltage is smaller than C7’s voltagethen C7 clamps C6’s voltage through D6 and switch (S), Fig.8b. At the same time, if the voltage across C4+C6 is smallerthan the voltage across C5+C7, then C5 and C7 clamp thevoltage across C4 and C6 through D4 and S, Fig. 8c.Similarly, C3, C5, and C7 clamp the voltage across C2, C4,and C6, Fig. 8d.

When the switch turn off, the inductor current closes D7,and switches all diodes. During the switch-off state, theinductor current closes D7 charging C7, Fig. 9a. When D7closes, C6 and the voltage in Vin plus the inductor’s voltageclamp the voltage across C5 and C7 through D5, Fig. 9b.Similarly, the voltage across the inductor plus Vin, C4, and C6clamp the voltage across C3, C5, and C7 through D3. Finally,the voltage across C1, C3, C5, and C7 is clamped by C2, C4,C6, Vin, and the inductor’s voltage, Fig. 9c.

Fig. 9. Switch-off state.

It can be seen that D1, D3, D5, and D7 switch in asynchronously way, complemented with D2, D4, D6, and S,Figs. 8-9.

V. LOSSES TAKEN INTO ACCOUNT

In this section the boost ratio will be analyzed, which is veryimportant because the inductor losses limit the theoreticallymaximum boost ratio, and the analysis gives importantinformation to designers. As it has been mentioned, eq. (2)holds in (7) for the first level, but the total output voltage is Ntimes Vc. Thus, the new voltage gain can be expressed by:

DV

V

in

C

−=

1

1 thenD

N

V

V

in

out

−=

1(8)

The input DC current can be expressed in terms of theoutput current and input-output voltage as:

O

C

O

CC

O

outoutoutoutLin R

VN

R

NVNV

R

VVIVIV

22

====

O

C

O

C

in

CL RD

VN

R

VN

V

VI

)1(

22

−== (9)

From (9) it can be seen that the input current can becontrolled with D in the PWM which is important in someapplications such as renewable energy based distributed

generation systems where sometimes is necessary to track themaximum power point.

From Fig. 7 the following expressions can be derived and itcan be seen that (5) and (6) holds for the first level in the newtopology. Remembering the relation between the capacitorsvoltage vs. the output voltage and (9) the eq. (10) can bederived with the next procedure in which the inductors powerlosses are considered.

LLCin

CLLin

LLCinLLin

LLCinLLinL

RIVDV

VDDDRIDDV

RIDVDVDRDIDV

RIVVDRIVDV

+−=−=+−−+−+

=−−−−−+−=−−−+−=

)1(

)1()1()1(

0)1()1()1(

0))(1()(

LO

outoutin R

RD

NV

N

VDV

)1()1(

−+−= (10)

From (10), the boost ratio for the novel topology may beexpressed as (11).

O

Lout

in

RD

NR

N

DV

V

)1(

)1(1

−+−

= (11)

It is important to note that (11) is actually a general equationthat includes the conventional boost converter if 1=N in theideal model when 0=LR , or in the model considering losses

whenLR is considered.

Fig. 10. Voltage gain versus duty cycle for different values of ESR/Ro in thenovel multilevel boost converter (N=4).

Analogously to Fig. 5, Fig. 10 shows the boost factor forN=4 (4x, 5 levels); Vout is divided in C1, C3, C5, and C7,Figs. 8-9.

From Fig. 10, the theoretical boost ratio limitation causedby the inductor losses is still a limitation but, the quasi-linearoperative region is larger and includes higher boost factors.Thus, the multilevel boost converter can operate in the highboost ratio region, this region is also around D=0.5 which isthe better point to operate the multilevel strategy.

Likewise, the maximum boost ratio is farther than D=1,which is an operative point difficult to implement with non-ideal switches.

All these characteristics made the multilevel boost convertera real option to implement in renewable energy generation

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system transformer-less and based on a multilevel inverter. Forinstance, the five-level diode clamped multilevel inverter witha large linear operation region and a high boost factor withduty cycles near 0.5 instead of duty cycles near 1.

VI. CENTRAL SOURCE

A variation of the proposed topology is discussed in thissection. One of the disadvantages of the proposed topology,Fig. 6, is that the current in semiconductors is higher in thelower levels. This is a disadvantage of most of the DC-DCmultilevel converters. For instance, in Fig. 1b the lower devicedissipates more power than the higher one, and this fact comefrom the energy conservation principle: for the same powerhigher voltage means lower current and vice versa.

Fig. 11. Topology variation with medium source.

Fig. 11 displays the topology variation with the input sourcevoltage at the medium position. Similarly to Fig. 6, thetopology can be easily extended to any number of levels, butthe current at the medium semiconductors is less than thecurrent in the bottom semiconductors. Fig 12 present the 4xvariant.

Fig. 12. 4x (5 levels) multilevel boost converter variant.

It is noteworthy that the multilevel operation holds and thecapacitor’s voltage are balanced, regardless on the load andconfiguration, non-medium, or a medium source. Thus, theDC-DC Multilevel Boost Converter is an important alternativeto feed multilevel inverters.

VII. EXPERIMENTAL RESULTS

A low power prototype is built to show experimentally theoperating principle of this novel converter. A Freescale eightbits microcontroller is employed to provide the PWM signal inan open loop structure. Fig. 13 shows the prototype schematicof a 3x (4 levels) multilevel boost converter. Fig. 14-15 exhibitthe test bench; the inductor has a value of of 25mH and allcapacitors are 12000 F; the inductor is added externally tothe PBC prototype.

Fig. 13. Prototype schematic: L = 25mH, C= 12mF.

Fig. 14. (a) Isolated gate drive power supply (b) diodes (c) IGBT (d) gatedrive power input (e) output voltage (f) input voltage.

Fig. 15. (a) Isolated gate drive power supply (b) diodes (c) IGBT (d) gatedrive power input (e) output voltage (f) input voltage.

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Several tests are carried out. Figs. 16-17 show the resultantwaveforms; results are as expected.

Fig. 16. (a) Switch voltage (b) lower capacitor voltage (c) input voltage.

Fig. 17. (a) Switch voltage (b) total output voltage (c) input voltage.

VIII. CONCLUSION

The proposed Multilevel Boost Converter (MBC) is basedon one inductor, one switch, 2N-1 diodes and 2N-1 capacitorfor Nx (N+1 levels). It is a boost converter PWM controlled,able to maintain the same voltage in all Nx output levels andable to control the input current.

This converter is based on the multilevel convertersprinciple, where all devices block one voltage level whichallows developing high voltage converters with low voltagedevices. It is proposed to be employed as DC-link inapplications where several voltage levels are required withself-balancing, unidirectional current flow, and PWM control,such as photovoltaic (PV) or fuel cell generation systems withmultilevel inverters. Utilized to feed a multilevel inverter, theproposed topology achieves a self-balanced voltage.Experimental results show the applicability of the noveldevice.

ACKNOWLEDGMENT

Authors thank to CONACyT under grant 54067. Specialthanks are given to Prof. Fang Peng and people from powerelectronics and motor drives lab of Michigan State Universityfor their suggestions.

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