A Comparative Study of Three-Level DC-DC Converters

Mehdi Narimani, Gerry Moschopoulos and Dunisha Wijeratne

Department of Electrical and Computer Engineering Western University, Canada London, ON, Canada

Abstract— The main focus of this paper is a comparative study of several three-level isolated dc-dc converters. The main feature of three-level converters is that the voltage stress across their switches is half the DC bus voltage so that they can have fewer switching losses than comparable two level dc-dc converters. In this paper, the efficiency performance of a three-level dc-dc converter is compared to ZVS three-level and ZVZCS three-level dc-dc converters. The basic concepts of each topology are reviewed, and a comparison involving the number of passive elements and active switches used, the stresses of the semiconductors devices, and efficiency measurements based on experimental results is made. Conclusions based on this comparison are presented as well.

I. INTRODUCTION The two-level, zero voltage switching (ZVS) PWM, full-

bridge (FB) dc-dc converter is widely used in industry as a high switching frequency converter for higher power applications. This converter can operate with ZVS as energy stored in the transformer leakage and magnetizing inductances can be used to discharge switch output capacitances before switches are turned on. The load range that the converter can operate with ZVS is proportional to the amount of current that is allowed to flow in the primary side of the converter as more primary current results in more available energy to discharge switch output capacitances. This, however, also means that the increased primary current creates more conduction losses and thus a trade-off between switching loss reduction and increased conduction losses must be considered [1]-[2].

There are applications where the peak voltage stress across a two-level ZVS-PWM FB converter can be high. For example, in applications where there is a front-end three-phase ac-dc converter that operates with power factor correction (PFC), the output voltage of this converter can be high, which raises the voltage rating of the switches in any downstream dc-dc converter; this creates a need for more expensive, less available devices and also increases switching losses. As a result, so-called three-level dc-dc converters have been developed with the converter proposed in [3] being one of the first converters of this type to be proposed. In three-level isolated dc-dc converters, the

voltage stress of the power switches is only half of the input dc voltage, which allows for the selection of more suitable MOSFET devices for isolated dc-dc converters with high input voltage applications.

A number of topologies have been proposed recently [4]-[16] to improve the efficiency of three-level dc-dc converters. These topologies can operate with improved efficiency through the use of various soft-switching techniques for the power switch devices. Although many such topologies have been proposed, there have been few comparative studies that compare the strengths and weakness of any one topology with respect to others. The main focus of this paper is to examine various representative topologies including the basic fundamental three-level dc-dc topology, a three-level dc-dc converter with ZVS for two switches but with reduced circulating primary current and simple passive auxiliary circuit, a ZVS three-level dc-dc converter with a passive auxiliary circuit and extended ZVS operating range and a zero-voltage zero-current switching (ZVZCS) three-level converter. These topologies have been selected as they represent a variety of soft-switching mechanisms.

This paper briefly explains the basic concepts of each converter, reviews its strengths and weaknesses, and presents experimental results obtained from a prototype converter. The paper ends by making comparisons between the converters in terms of cost, complexity, performance and efficiency and making conclusions about the properties and characteristics of all the examined converters.

II. CIRCUIT DESCRIPTIONS The basic operation of each three-level dc-dc converter

considered in the paper is explained in this section;

A. Conventional Three-Level DC-DC Converter The conventional three-level dc-dc converter is shown in

Fig.1 [3]. Typical gating signals for the four converter switches are shown in Fig. 2. Equivalent circuit diagrams that show the modes of operation for a half-switching cycle for the three-level converter in Fig. 1 are shown in Fig. 3. Switch pairs S1 and S2 or S3 and S4 are turned on simultaneously; when either S1 or S4 is turned off, the

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converter enters a freewheeling mode of operation. The discharging of switch output capacitances is initiated by the turning off of either S2 or S3. The converter has the following modes of operation:

Mode 1: During this mode, switches S1 and S2 are on, half the dc bus voltage, Vin/2, is impressed across the transformer primary, and energy is transferred to the output.

Mode 2: During this mode, switch S1 is turned off and the voltage of CS1 increases from zero to Vin/2 while the voltage of (CS3+CS4) decreases from Vin to Vin/2. Switches S3 and S4 are each exposed to a voltage of Vin/4.

Mode 3: During this mode, the converter is in a freewheeling mode of operation as current freewheels through S2 and D1. It should be noted that the voltage across S1 during this mode is Vin/2.

Mode 4: The converter enters this mode when S2 is turned

Figure1. Three-level dc-dc converter

Figure 2. Typical waveforms of a three-level converter

Mode 1

Mode 2

Mode 3

Mode 4

Figure 3. Modes of operation for a three-level dc-dc converter

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off. The transformer primary current discharges the output capacitances of switches S3 and S4 so that current flows through the body diodes of these devices. The mode ends when S3 and S4 are turned on and the converter enters Mode 1 but with S3 and S4 on instead of S1 and S2.

It should be noted that if in Mode 4, there is enough energy in the leakage inductance Llk, this energy can completely discharge body capacitors of bottom switches and thus these switches can be turned on with ZVS. This condition can happen at heavy load conditions where the primary current is high. However, at light load conditions when leakage inductance does not have enough energy to discharge body capacitors, ZVS cannot happen that concludes switching losses at light load conditions. In order to improve the switching losses and consequently the efficiency of the conventional three-level DC-DC converter, there are some approaches which are explained in the next sections.

B. Three-Level DC-DC Converter with ZVS for Two Switches In the converter shown in Fig.4, a flying capacitor has

been added between top and bottom switches to make a path for the discharging of the output capacitance of S1 and S4 so that these switches can turn on with ZVS. The converter has the following modes of operation and equivalent circuit diagrams that show the modes of operation for a half-switching cycle for the three-level converter in Fig. 4 are shown in Fig. 6.

Mode 1: During this mode, switches S1 and S2 are ON and energy from dc bus capacitor C1 is transferred to the output load. Mode 2: In this mode, S1 is OFF and S2 remains ON. Capacitor Cs1 charges and capacitor Cs4 discharges through Cf until Cs4, the output capacitance of S4, clamps to zero. This mode ends when S4 turns on with ZVS. Mode 3: In this mode, S1 is OFF, the primary current of the main transformer circulates through diode D1 and S2 and the load inductor current freewheels in the secondary of the transformer. Mode 4: In this mode, S1 and S2 are OFF and the current in the transformer primary charges capacitor C2 through the body diode of S3 and switch S4. This mode ends when switches S3 and S4 are switched on and a symmetrical period begins. In this mode, the load inductor current continues to transfer energy from input to the output.

The main advantage of this converter is that the converter just uses a flying capacitor to guarantee ZVS for at least two top and bottom switches and thus the converter needs less circulating current to make ZVS for the other switches. Typical waveforms of the converter are shown in Fig. 5. The converter employs standard phase-shift PWM (PWM) technique which is found in commercially available ICs.

C. ZVS Three-Level DC-DC Converter Fig.7 shows a ZVS three-level converter [6]. In this

converter, an auxiliary inductor and two diodes have been added to the transformer primary side of the conventional converter. The auxiliary inductor Lr stores enough energy to discharge body capacitors of the power switches. The modes of operation of this converter are similar to those of the conventional converter, except that the load range for ZVS operation can be increased due to the energy that can be stored in Lr. The analysis and design of the ZVS three-level dc-dc converter can be found in [6].

Figure 4. A three-level dc-dc converter with ZVS for two switches

Figure 5. Typical waveforms of a three-level dc –dc converter with ZVS for

two switches

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D. ZVZCS Three-Level DC-DC Converter This converter, shown in Fig. 8, allows ZVS for

switches Q1 and Q4 (leading switches) and ZCS for switches Q2 and Q3 (lagging switches). The flying capacitor provides a path for Q1 and Q4 to completely discharge their output capacitances. Series diodes and the resonance between the series capacitor and the transformer leakage inductance help Q2 and Q3 operate with ZCS [8]. The main feature of this converter is that is operates with less circulating current when it is in a freewheeling mode of operation. The analysis and design of the ZVS three-level dc-dc converter can be found in [8].

III. EXPERIMENTAL RESULTS AND CONVERTER COMPARISONS In order to compare experimentally the behaviour of

the converters, four prototypes were built with the input data presented in Table I. The design procedures of the converters studied in this paper are presented in [3], [6]-[8] and will be discussed in final paper. The components used for the implementation of the converters are presented in Table II.

Figure 7. A ZVS three-level converter and typical waveforms [6]

Mode 1

Mode 2

Mode 3

Mode 4

Figure 6. Modes of operation for a three-level dc-dc converter

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TABLE I: CONVERTER PARAMETERS

Input voltage - Vi (V) 400

Output voltage - Vo (V) 48

Output power - Po (kW) 700

Switching frequency - fs (kHz) 100

Table II presents a summarised comparison among the

four converters examined. Considering the number of semiconductors devices employed, it is clear that the original three-level converter uses less power diodes and passive components than other converters. From the point of view of switch stresses, all the converters present the same results, with the same peak voltage across the switches (half of the input voltage).

Efficiency measurements for each converter are shown in Fig. 9. The main sources of losses are diodes and MOSFETs conduction losses, magnetic and snubber losses. The switching losses at full load are practically zero due to zero-voltage turn-on and capacitive turn-off snubbing. As can be seen from Fig.9, the original three-level converter has the minimum efficiency at light loads and heavy loads in compare to other converters. It is because it has no soft switching for the power switches. The three-level converter with ZVS for two switches and the ZVZCS three-level converter have almost the same efficiency curve, but the number of components for two-switch ZVS is less than ZVZCS converter. As a result, the two-switch ZVS converter is less expensive than the ZVZCS converter. Fig.9

Fig. 8. A ZVZCS three-level converter and typical waveforms [8]

TABLE II. CONVERTER COMPONENTS

COMPONENT CONVETERS

Three-Level Three-Level ZVS (two Switches) Three-Level ZVS Three-Level ZVZCS

S1-S4 4 * IRFP460 - 500V, 20A 2 * IRFP460 - 500V, 20A 2 * G4PH50U (IGBT)

Clamp diodes 2 * MUR 860 2 * MUR 860 4 * MUR 860 2 * MUR 860

Diode Series - - - 2 * MUR 860

Flying Capacitors - 1 * 220 µF 1 * 220 µF 1 * 220 µF

Auxilaury Inductor - - - 1 * 15 µH

Bus Capaciotrs 2 * 220 uF

Rectifire diodes DSSK60-02A

Figure 8. Efficiency of the different type of the three-level converters

0 100 200 300 400 500 600 70087

88

89

90

91

92

93

94

95

96

97

98

Output Power ( W )

Eff

icie

ncy

( %

)

TLTL ZVS (2-SW)TL ZCZVSTL ZVS

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shows that full ZVS converter (ZVS for all switches) has the best efficiency numbers at light loads and heavy loads. It is because it can provide a wide range of ZVS for all the power switches and it has no power diodes in series with power switches as ZVZCS converter has. Based on the study that has been done, it can be concluded that ZVS three-level converter has the best performance in terms of efficiency, but it is also the most expensive one.

IV. CONCLUSION The efficiency performance of four different three-level

isolated dc-dc converters was compared in this paper. The paper compared a three-level dc-dc converter to two ZVS three-level converters and a ZVZCS three-level dc-dc converter. The basic concepts of each topology were reviewed, and a comparison involving the number of passive elements and active switches used, the stresses of the semiconductors devices, and efficiency measurements based on experimental results was made. Conclusions based on this comparison were presented as well.

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