IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 1, … · IEEE TRANSACTIONS ON POWER...

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 1, JANUARY 2013 289

A Comparative Performance Study of an InterleavedBoost Converter Using Commercial Si and SiC

Diodes for PV ApplicationsCarl Ngai-Man Ho, Senior Member, IEEE, Hannes Breuninger, Member, IEEE, Sami Pettersson, Member, IEEE,

Gerardo Escobar, Senior Member, IEEE, and Francisco Canales, Member, IEEE

Abstract—A performance comparison of an interleaved boostconverter (IBC) using Si and SiC diodes for photovoltaic (PV)energy conversion systems is presented in this paper. The per-formance attributes under investigation include the semiconduc-tor device behavior, thermal requirement, system efficiency, andpower density. The IBC is designed to sustain the dc-link voltage inthe energy conversion system and to provide the maximum powerpoint tracking in the PV system. Due to the absence of reverserecovery current in SiC Schottky diodes, low switching losses aregenerated in diodes and switches. This benefit results in a highersystem efficiency and smaller cooling system design requirement.As a benefit, the volume and weight of the heatsink can be furtherreduced. Furthermore, behaviors of the power semiconductors,which will impact the performance in the system, are discussed inthe paper. The validity of the analysis is confirmed experimentallywith a 2.5-kW IBC prototype with relatively wide power and inputvoltage operating range. The overall performance of the IBC pro-totype using Si and SiC diodes is summarized in a table for easycomparison.

Index Terms—Diode, interleaved boost converter (IBC),MOSFET, photovoltaic (PV), power semiconductor, SiC.

I. INTRODUCTION

PHOTOVOLTAIC (PV) inverters are widely used in res-idential applications as an interface between PV panels

and low-voltage distribution grid. A PV inverter generally con-sists of a dc–ac inverter and a dc–dc converter [1]–[5]. Theinverter is used to feed the dc power into the grid network.Among various inverter topologies, two-level H-bridge invert-ers and three-level inverters are usually adopted in industry dueto simple circuit implementation and high efficiency, respec-tively [1]–[7]. In particular, for a 220-V single-phase grid, theminimum dc-link voltage requirement for the inverter is 350 V.

Manuscript received December 22, 2011; revised March 16, 2012 and April15, 2012; accepted April 16, 2012. Date of current version September 11, 2012.This paper was presented in part at 8th International Conference on PowerElectronics ECCE-Asia, Jeju, Korea, May 30–June 3, 2011. Recommended forpublication by Associate Editor T. Suntio.

C. N.-M. Ho, S. Pettersson, G. Escobar, and F. Canales are with theABB Corporate Research Ltd., CH-5405 Baden-Dattwil, Switzerland (e-mail:[email protected]; [email protected]; [email protected]; [email protected]).

H. Breuninger was with ABB Corporate Research Ltd., CH-5405 Baden-Dattwil, Switzerland. He is now with Brose, 96103 Hallstadt, Germany (e-mail:[email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TPEL.2012.2197830

TABLE ITYPICAL SPECIFICATION OF A PREREGULATOR FOR SINGLE-PHASE

PV INVERTERS

Parameter Value Parameter Value

Input voltage 125V – 650V Max. input voltage 800V

Output voltage 400V Max. rated power 2.5kW

Min. operating frequency 16kHz Max. input current ripple 10% of Iin, max

Max. ambient temperature 50ºC Max. junction temperature 125ºC

This voltage level guarantees that the input voltage of the in-verter is higher than the peak value of the grid voltage. However,PV panels do not provide constant output dc voltage for a longperiod of time. For instance, in single-phase PV inverters forresidential applications, a typical design specification of theoutput voltage range of the PV string is from 125 to 650 V. Itis well known that this variation depends on sun irradiation andpanel surface temperature. Thus, a dc–dc converter, referred aspreregulator, is usually connected between the panels and theinverter to sustain the dc-link voltage. Besides the input voltagerange for maximum power point (MPP) tracking, the absolutemaximum input voltage is specified for designing PV inverters.It is used to avoid the open-circuit voltage of the PV panels fromdamaging the semiconductors in the inverter. Maximum inputcurrent ripple is another important parameter for designing PVinverters. As it is well known that the MPP of PV panels is ata very narrow current range, the input current ripple of the PVinverter has to be reasonably small to increase the MPP trackingaccuracy. These parameters in the inverter design specificationare based on the consideration of the PV panel output charac-teristics. Table I shows a typical design specification of 2.5-kWPV preregulators.

Fig. 1 shows a simplified PV energy conversion system witha sketch of a conventional controller for PV preregulators. Thevoltage source inverter is used to guarantee the output currentquality and manage the dc-link capacitor voltage VDC . The pre-regulator executes the function of MPPT by controlling the PVpanel voltage and current. From topology point of view, twoidentical boost units are used in the circuit. This configurationis named interleaved boost converter (IBC), which is one ofthe most appealing options for PV applications. The reason forthis is that the input current ripple is minimized by forcinga 180◦ phase shift operation between the two switching cells,which results in a ripple cancellation [3]. However, the maindrawback is that semiconductors with high breakdown voltage

0885-8993/$31.00 © 2012 IEEE

290 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 1, JANUARY 2013

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VDC

IPV

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edCurrent balancing u1=1-(e+ed)/Vdc

u2=1-(e-ed)/Vdc

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CiLi

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Fig. 1. Two-stage PV inverter topology using a two-phase IBC with conventional controller.

have to be used due to a possible high input voltage comingfrom the panels. In consequence, the system suffers from the re-verse recovery loss in the high voltage class silicon (Si) diodes(DB) and the commutating loss in the switches (SB). Three-level converter configuration has also been proposed to dealwith the high voltage across the semiconductors [8]. With thistopology, 600-V devices, which present good switching charac-teristics, may be utilized. The switching waveforms are shown inFig. 17(a) of the Appendix. However, the main disadvantage ofthis topology is that the operating current of each semiconduc-tor is double compared to that of the IBC. Thus, the conductionloss is higher than in the IBC according to the rule I2R. A newtype of Schottky diode with silicon carbide (SiC) technologyhas been commercialized in the last decade. The manufacturersclaim that the main advantage of the SiC diodes is zero reverserecovery [9], [10]. This characteristic makes them appropriatefor high switching frequency converters. And thus, two conjec-tures are made for converter design with such devices. First, ahigher system efficiency can be expected due to the lower semi-conductor switching losses with the same switching frequency.Second, a higher system power density can be achieved usingeither smaller inductors with higher switching frequency or asmaller heatsink with fixed switching frequency. Therefore, acomparative study to exhibit the performance improvement in areal system would be of a great interest.

There have been reported in the power electronics literatureseveral comparative studies of the performance between Si diodeand SiC diode at the device level [11]–[13] and at the circuitlevel [14]–[21]. In most of these papers, a low-voltage low-power rated power factor correction (PFC) is the most popularapplication to demonstrate the advantages of SiC diodes. Ingeneral terms, it has been shown that the SiC diodes can bringattractive benefits to the power supply industry. Furthermore,the trend of the use of SiC diodes is toward the high switchingfrequency [18], [19], e.g., 1 MHz, and high-power [20], [21],e.g. 55 kW, applications. The main focuses of those comparisonsare the device behavior and the semiconductor losses. From thedevice point of view, there are large differences in the use ofeither Si or SiC diodes. However, from the system efficiencypoint of view, the semiconductor loss is only a part of the total

loss. Moreover, the testing platforms were not optimized forcomparison, and thus, it is not possible to maximize the benefitsof the devices. As a result, the advantages brought by the SiCdiodes have not been fully demonstrated in the performanceof the evaluated systems. Furthermore, to the best knowledgeof the authors, evaluations have not been presented, so far, inpreregulators for PV applications.

This paper presents a comparative study of the use of com-mercial Si and SiC diodes in an IBC for PV applications. Thatis, two converters are analyzed: a converter using SiC diodes,referred as Si/SiC system, and a converter using Si diodes, re-ferred as Si/Si system. Issues addressed here include 1) thestatic and switching characteristics of two diodes with a Cool-MOS device; 2) the efficiency of the system within the fulloperating range; and 3) the system power density. The validityof the analysis is verified experimentally with a 2.5-kW IBCprototype. Two cooling systems are designed: one for the Si/Sisystem and one for the Si/SiC system. The design of the coolingsystems has been restricted to keep the CoolMOS device at thesame junction temperature in both systems. This is to avoid thechange of conduction loss of the CoolMOSs that may influencethe results of the comparison. The system design procedure isbased on the guideline in [22]. Thus, a platform is set to have afair evaluation of the performance of both the Si/Si and Si/SiCimplementations.

II. SEMICONDUCTOR CHARACTERIZATION

Device characterization is the most straightforward way toevaluate the semiconductor losses in a converter. The lossesare mainly divided into conduction loss and switching loss. Thelosses can be extracted by two types of characterizations, namelystatic and dynamic characterization. Converter designers can op-timize the system by selecting the most suitable semiconductordevices and gate drive circuits based on the extracted loss infor-mation. In this paper, the CoolMOS, IPW90R120C3 by Infineonwas selected as active switch. The first consideration of the de-vice is the breakdown voltage. According to the specification inTable I, the breakdown voltage of the devices cannot be lowerthan the maximum input voltage, 800 V. Moreover, this is due to

HO et al.: COMPARATIVE PERFORMANCE STUDY OF AN INTERLEAVED BOOST CONVERTER 291

TABLE IIPARAMETERS OF THE EVALUATED DEVICES

Parameter SymbolDevices

UnitCoolMOS Si Diode SiC Diode

Manufacturer InfineonST

MicroelectronicsCree

Part No. IPW90R120C3 STTH1210D C2D20120D

Type CoolMOS Ultrafast recovery SiC Schottky

Breakdown Voltage

V 900 1000 1200 V

Rated Current I 23 12 11 A

Max. Junction Temperature

T 150 175 175 ˚C

Thermal Resistance, J-C

R 0.3 1.9 0.48 K/W

Package TO-247 TO-220 TO-247

the fact that the device provides a very low on-state resistance atlow junction temperature. Thus, the conduction loss is relativelylow compared with other Si technology-based active switches,such as insulated gate bipolar transistors. Furthermore, this de-vice can switch at very high di/dt and dv/dt. Hence, it wouldform a very fast switching cell if combined with a SiC diode tominimize the switching losses [10], [23]. A diode benchmarkinghas been done to select a Si-based diode to be comparable tothe SiC diode. The selection was based on two criteria: similarelectrical ratings to the SiC diode and the suitability of the IBCwith the specification in Table I. The details of the benchmark-ing results are given in the Appendix. Eventually, two sets ofswitching cells have been evaluated in the system, both combin-ing the CoolMOS device with either the Si ultrafast diode (Si/Si)or the SiC Schottky diode (Si/SiC). All devices have been testedin both static and dynamic measurements to evaluate the lossesand to design the appropriate heatsinks. Table II shows the keyparameters of the devices under test. These values have beencollected from the corresponding manufacturer datasheets.

A. Static Characterization

The main objective of carrying out the measurements forstatic characteristics is to determine the conduction loss of thespecific devices, which will be used in the power electronicsystems. The Tektronix 371A curve tracer is used to extract theparameters from the semiconductor devices.

Fig. 2 shows the output characteristics of the CoolMOS de-vice where a strong temperature dependence can be observed.Consequently, the heatsink should be designed to limit the max-imum junction temperature for CoolMOS at 75 ◦C to reduce theconduction loss, and thus to guarantee a good performance ofthe overall system. Fig. 3 shows the forward characteristics ofthe tested Si and SiC diodes. Notice that both devices are tem-perature dependent, but the SiC diode has an interesting featurein contrast to the Si diode. The Si diode has a negative tempera-ture coefficient, meaning that if the device is heated up, then theconduction loss of the device reduces. In contrast, the SiC diodehas a positive temperature coefficient at high-current operation,which is higher than 2 A. By comparing both diodes at the samereference point, i.e., 10 A and 75 ◦C, it is observed that the Sidiode has a lower conduction loss than the SiC diode. In fact, the

Saturation Characteristics @ Vgs = 10V

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0 2 4 6 8

Drain-Source Voltage, Vds (V)D

rain

Cur

rent

, Id

(A)

Tc = 25˚CRon=108mΩ



Fig. 2. CoolMOS output characteristics at different temperatures.

Forward Characteristics Si vs SiC diodes

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SiC - 25CSiC - 75CSiC - 125CSi - 25CSi - 75CSi - 125C

Pulse Width = 250µs

Tc = 125˚C

Tc = 25˚CTc = 75˚C

Tc = 125˚C

Tc = 25˚CTc = 75˚C

Fig. 3. Forward characteristics of the Si and SiC diodes at different junctiontemperatures.

turn-on voltages of the Si and SiC diodes are 1.36 and 1.62 V,respectively. However, notice that the turn-on voltage differenceat low-current operating range is not significant.

B. Dynamic Characterization

In practice, the dynamic characteristics of semiconductorscan be extracted by double-pulse tests [24]. Based on the re-sults of this characterization, the switching loss information and


Fig. 4. CoolMOS turn-on switching waveforms.

switching behavior of the semiconductors are determined. Ac-cording to the design specification in Table I, the correspondingdc output voltage during operation of the IBC is 400 V for eachswitching cell. Hence, 400 V has been considered as the dctesting voltage for measurements.

Figs. 4 and 5 show the turn-on and turn-off switching wave-forms of the switching cells. As it can be seen in Fig. 4(b), theCoolMOS turn-off waveforms for both switching cells are verysimilar. This leads to the conclusion that the diode type in thebasic switching cell does not affect the turn-off performanceof the CoolMOS device. In contrast, Fig. 4(a) shows that theperformance of the turn-on waveforms is quite different, whichdepends on the reverse recovery behavior of the diodes. Thereverse recovery current increases the switching loss in the Sidiode and also reflects these characteristics on the drain currentof the CoolMOS device; this is called “commutating loss.” Asa consequence of the high reverse recovery current peak Irr

and the long transient time of the Si diode, the turn-on loss of

Fig. 5. Si and SiC diode reverse recovery switching waveforms.

Switching Losses on Devices,CoolMOS + Si vs SiC Diode,

Conditions @ 400V,10A,75°

0

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600

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0 5 10 15 20Drain Current, iD (A)

Switc

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s)

Switch,on, Ultra Fast diode

Switch,off, Ultra Fast diode

Diode,off, Ultra Fast diode

Switch,on, SiC diode

Switch,off, SiC diode

Diode,off, SiC diode

C ,10V,10Ω,10Ω

Fig. 6. Switching energy loss chart.

the CoolMOS device in the Si/Si switching cell is much largerthan that of the Si/SiC switching cell. In addition, the reverserecovery current peak and the transient duration of the Si diodesstrongly depend on the operating current, the di/dt, and thejunction temperature. SiC diodes have zero reverse recovery inprinciple, but still a small current overshoot can be observeddue to the energy swing between the stray inductance and theparasitic capacitance of the SiC diode, which is only dependenton the di/dt [24].

Fig. 6 shows the information of the losses for different op-erating currents starting from 2 to 18 A. The solid lines anddashed lines represent the losses of the Si/Si and Si/SiC switch-ing cells, respectively. Notice that the turn-off losses for both


TABLE IIILOSS BREAKDOWN OF SEMICONDUCTORS IN IBC AT CRITICAL CONDITION

Condition: V = 125 V, P = 2.5 kW, f = 16 kHzSwitching cell

UnitSi/Si Si/SiC

Switch

Conduction loss P 11.3 11.3 W

Turn-on loss P 8.6 2.9 W

Turn-off loss P 2.4 2.4 W

Total loss P 22.3 16.6 W

Diode

Conduction loss P 4.4 5.1 W

Reverse recovery loss P 5.1 0.6 W

Total loss P 9.5 5.8 W

Total semiconductor loss P 63.6 44.7 W

switching cells are very similar, and that the difference of thediode reverse recovery losses is obviously very large. It can beobserved that the reverse recovery loss of the SiC diode is quiteconstant within the full measured current range, the loss beingaround 40 μJ at the 10-A testing points. On the other hand, the Sidiode suffers seriously from the reverse recovery current prob-lem, the energy loss being 380 μJ at the same testing current. Inother words, there is almost a 90% switching loss reduction inthe diode due to the use of the SiC diode. Besides, the turn-onloss in the switch reduces from 610 to 210 μJ at 10 A by usingthe SiC diode instead of the Si diode. The total switching lossof the Si/SiC switching cell in one switching cycle is one-thirdof the Si diode counterpart. The energy chart obviously showsthe main advantage of the SiC diodes, which is the low switch-ing loss. It is important to mention that the switching behaviorcomparison has been carried out using the same gate resistance,i.e., assuming the same di/dt. However, in practice, the Si/SiCswitching cell can operate faster with a smaller gate resistance,and therefore, the turn-on loss of the switch could be furtherreduced. This possibility is not included in the present compar-ison, as it involves electromagnetic interference issues, whichare out of the scope of this study.

C. Semiconductor Losses in the System

The semiconductor energy loss extraction is used to determinethe semiconductor loss in power electronics systems. Moreover,the cooling system can be designed based on the information.The heatsink is designed to keep the maximum junction tem-perature of the CoolMOS devices at 75 ◦C. For this purpose,the semiconductor losses have to be determined at the criticalthermal conditions, i.e., at 125-V input voltage and 2.5-kW out-put power. The steady-state characteristics of an IBC systemhave already been well documented in [22], [25], and [26]. As areference for the critical condition, the average Iavg , minimumImin , and maximum Imax currents in the switch are 8.27, 10,and 11.73 A, respectively.

Table III summarizes the loss breakdown of the semiconduc-tors in the IBC with Si and SiC diodes at the critical condition.The main improvements by using SiC diodes are on PM on andPD rr , where “PM on” is the turn-on loss of the switches, and“PD rr” is the reverse recovery loss of the diodes. In compar-ison, PM on using SiC diodes is one-third of the one using Sidiodes and PD rr using the SiC diodes is almost one-tenth of the

40

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2522191613107

Loss

, W

Switching Frequency, kHz

Losses in IBC

Si Diode

SiC Diode

Fig. 7. Losses in the IBC with switching frequency variation.

one using Si diodes. As a result, the total semiconductor lossreduction is around 19 W because of SiC diodes.

D. Switching Frequency Evaluation

It is clear, from the losses breakdown table, that the main ad-vantage of the SiC diodes is the semiconductor switching lossreduction. This implies that the optimal switching frequencymay be larger in the SiC-based system. Fig. 7 shows the calcu-lated system losses of the IBC using the two different diodesunder study and using the operating switching frequency as arunning parameter. Optimized inductors are considered at eachfrequency point. The inductor design procedure is based on [22].It shows that the optimal switching frequencies for the Si andSiC diodes are 10 and 11 kHz, respectively. However, to avoidgenerating acoustic noise, the minimum switching frequencyfor the application must be 16 kHz, which is selected as the op-erating frequency in both cases. Accordingly, the inductors arethe same in both cases as well.

III. COOLING SYSTEM EVALUATION

The cooling system requirement for each IBC is designedbased on the semiconductor losses shown in Table III. The junc-tion temperature in both systems is set at 75 ◦C in order to keepthe conduction loss of the CoolMOS devices low. The heatsinksize is different in both cases due to the different semiconductorlosses. Fig. 8 shows the temperature distribution simulation re-sults using the (a) Si and (b) SiC diodes. The criteria to designthe heatsinks are as follows:

1) the junction temperature of the CoolMOS devices andthe ambient temperature are set at 75 ◦C and 50 ◦C,respectively;

2) a SUNON KDE1206PTV2 and an array of two SUNONKDE1204PKV2 fans are used for the Si/Si and the Si/SiC


Fig. 8. Cooling system simulation for CoolMOSs with (a) Si diodes and(b) SiC diodes.

Fan Characteristics

0

0.05

0.1

0.15

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0 5 10 15 20

Airflow (CFM)

Stat

ic P

ress

ure

(Inch

-H2O

)

KDE1206PTV2

KDE1204PKV2 x 2

Fig. 9. Fan characteristics for the two cooling systems.

systems, respectively. Sketches of the disposition of botharrays of fans are shown in Fig. 8(a) and (b). As observedin Fig. 9, the characteristics of both arrays of fans are verysimilar;

3) the material and structure of the heatsinks are basically thesame. By adjusting the height of the fins and the length ofthe heatsink, different thermal resistances for the coolingsystems can be achieved.

A summary of the thermal simulation results is shown inTable IV. Notice that the junction temperatures of the CoolMOSdevices on those two heat sinks are quite similar and close to75 ◦C.

TABLE IVSIMULATING CONDITIONS AND RESULTS OF THE COOLING SYSTEM

Parameter SymbolSi/Si Si/SiC

UnitCoolMOS Si Diode CoolMOS SiC Diode

Power loss P 22.3 9.5 16.6 11.6 W

Thermal resistance, junction to case

R 0.3 1.9 0.3 0.48 K/W

Thermal resistance, case to heatsink

R 0.24 0.45 0.24 0.24 K/W

Ambient temperature T 50 50 50 50 ˚C

Heatsink temperature T 67.3 65.4 67.8 60 ˚C

Junction temperature T 79.7* 98.1* 76.4* 69* ˚C

Thermal resistance, Heatsink

R 0.247 0.41 K/W

*This value is estimated.

Fig. 10. Heatsinks’ size comparison.

Fig. 11. Optimized IBC prototype.

Fig. 10 shows the physical size comparison between the twodesigned heatsinks. The total volume and weight of the heatsinkfor the Si/Si system are 1031 cm3 and 1185 g, respectively, whilethe volume and weight of the heatsink for the Si/SiC system are388 cm3 and 470 g, respectively. As a result, there is a 60%reduction in the dimensions of the Si/SiC heatsink with respectto the Si/Si heatsink.


IV. EXPERIMENTAL VERIFICATIONS

A 2.5-kW hardware platform has been built, as shown in Fig.11. The hardware including the measurement board, the mainpower board, the gate driving circuits, the CoolMOS devices,and the inductors is common for both Si/Si and Si/SiC systemsin the tests. The main differences are the diode type used, thecomponent placement, and the cooling system design, as it wasshown in Fig. 8.

A. Testing System Configuration

The goal of the comparison is to verify that the Si/SiCIBC gives a higher system efficiency than the Si/Si IBC eventhough the former uses a smaller heatsink. To determine the dif-ferences between both approaches, efficiency characterizationstudies have to be performed. Efficiency characterization of PVinverters includes MPPT efficiency and static converter effi-ciency [27], [28]. On the one hand, the MPPT efficiency mainlydepends on the dynamic response of the MPPT controller, i.e.,the ability to track the MPP under variable environmental con-ditions, and negative effects caused by the power fluctuation,the switching ripple, which may produce fluctuations of the op-erating point around the MPP. Normally, the dynamic responseof the semiconductors is considerably much higher than theMPPT control bandwidth. And thus, using the Si or SiC diodesdoes not have an impact on the MPPT efficiency. On the otherhand, the static efficiency refers to the ability of the converterto keep operating at the MPP at constant environmental condi-tions during a given period of time. Clearly, the operating pointcan be anywhere within the specified ranges of the input volt-age and the output power. To characterize the whole system interms of its static efficiency, a PV simulator and a dc electronicload have been connected to the IBC systems under test. Anopen-loop controller has been considered for the IBC, wherethe duty ratio is manually assigned. The measuring points couldbe set easily and accurately by changing the PV curve, the dutycycle, and the output resistance. As the application of the pro-totype is for PV energy conversion, two key standard efficiencymeasurement methods have been adopted, namely, the Euro-pean efficiency and the California Energy Commission (CEC)efficiency [27]–[30]. The definitions of those are as follows:

European efficiency

ηEUR = 0.03 × η5% + 0.06 × η10% + 0.13 × η20% + 0.1

× η30% + 0.48 × η50% + 0.2 × η100% . (1)

CEC efficiency

ηCEC = 0.04 × η10% + 0.05 × η20% + 0.12 × η30% + 0.21

× η50% + 0.53 × η75% + 0.05 × η100% . (2)

The PV simulator, Spitzenberger & Spies PVS7000, has beenused to provide the tested input voltage levels (125, 250, and350 V) for the IBC measurements. To evaluate the IBC per-formance based on the aforementioned standard requirements,21 testing points have been considered in the measurements.Fig. 12 shows three testing points for the PV preregulator effi-ciency characterization. And the energy was dissipated by the

Fig. 12. Operating points of static efficiency measurement.

Fig. 13. Block diagram of the experimental setup for static efficiencymeasurement.

TABLE VMEASURED TEMPERATURES

TA TH,Diode TH,Switch TJ,Diode TJ,Switch Unit

Si/Si 30 43 49 65* 61* ˚C

Si/SiC 27 42 46 48* 55* ˚C

*This value is estimated.

dc electronic load, Chroma 63204. The experiments were car-ried out by open-loop-based control. For this, the duty ratio wasassigned through a computer and a DSP board to the converterto manually reach the MPP. The efficiencies were directly mea-sured using a power analyzer, Yokogawa WT3000. A simplifiedconnecting block diagram is shown in Fig. 13.

B. Experimental Results

Table V shows the measured temperature results when thesystems operate at the critical point, i.e., at 125-V input voltageand 2.5-kW output power. The measured ambient temperaturesare 30 and 27 ◦C for the Si/Si and Si/SiC systems, respectively.The case temperatures of the diodes and the CoolMOS devicesare almost the same in both cases. However, the junction temper-atures are different; the estimated operating temperature of theSi diodes is about 17 ◦C higher than that of the SiC diodes. Thereason for this is that the switching loss of the Si diode is higher,but the thermal resistance of the heatsink for the Si/Si system is


96

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Fig. 14. Experimental efficiency graphs of the IBC prototype using (a) Sidiodes and (b) SiC diodes.

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Fig. 15. Efficiency difference between Fig. 13 (a) and (b).

lower. In fact, the junction temperature of the Si diodes is higherthan that of the SiC diodes, even though the case temperaturesare the same. The junction temperature of the CoolMOS devicesin the Si/Si system is slightly higher than in the Si/SiC system.

Fig. 14 shows the measured efficiency charts under the wholevoltage and power operating condition range. In the graphs, thex-axis is the output power of the system and the y-axis is theinput voltage. Both charts show that the system provides higherefficiencies during operation at high voltage and high power. Inthis operation region, the inductor dc currents and ripple currentsare relatively low, and as a consequence, both the semiconductorlosses and the inductor core losses are reduced. In contrast, theefficiency behavior is deteriorated when the system operatesat the region of low power and low voltage. In this case, theswitching losses due to diode reverse recovery are reduced as thesystem operates in the discontinuous conduction mode region,but the core losses of the inductors dominate in the total lossof the system since the inductors still need to deal with thehigh-amplitude current ripple.

European efficiency

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iency

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(a)

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99.5

100.0

125 150 175 200 225 250 275 300 325 350Input voltage (V)

Effic

iency

(%)

CoolMOS + Si Diode

CoolMOS + SiC Diode

(b)

Fig. 16. Efficiency of the IBC: (a) European, and (b) CEC.

Fig. 15 shows the efficiency difference between the Si/Si andSi/SiC systems. It can be seen that the difference is around 0.4–0.8% within the whole input voltage range and in most parts ofthe operation region except for output powers lower than 250 W.Fig. 16 illustrates the comparisons of both systems considering(a) the European efficiency, and (b) the CEC efficiency. Asobserved, in these two efficiency comparisons, the Si/SiC systemis better than the Si/Si by 0.4–0.8%. This is more evident in theEuropean efficiency.

V. CONCLUSION

A comparative study about the use of Si and SiC diodes inthe implementation of an IBC for PV applications has beenpresented. The study was supported by the measurements ofthe static and switching characteristics of both Si/Si and Si/SiCswitching cells. Furthermore, the optimal operation of the sys-tems considering maximum efficiency was also determined aspart of the study. The optimal frequency was found to be below


TABLE VISUMMARY OF THE COMPARISONS

Parameter Condition Si/Si Si/SiC Unit

Efficiency @2.5kW, 125V 96.7 97.3 %

Heatsink volume 1031 388 cm3

Heatsink weight 1185 470 g

Junction temperature @TA=30˚C 61/69 55/48 ˚C

Inductor volume 338 338 cm

Inductor weight 1142 1142 g

TABLE VIIEVALUATED 600-V DIODES

Manufacturer Fairchild Power Integrations Cree

Part no. F08S60S LQA08TC600 CSD08060

Type Ultra Fast Q-Series SiC Schottky

Breakdown Voltage 600V 600V 600V

Rated Current 8A 8A 8A

the audible frequency, and thus, it was decided to evaluate bothsystems at 16 kHz instead. The systems were designed and eval-uated keeping fair operating conditions such as the same semi-conductor junction temperature and the same passive devices.To complete the study, a 2.5-kW prototype of the IBC withtwo optimized cooling systems was implemented for testing thetwo types of diode technology. Results showed that, in termsof efficiency, volume, and weight, the converter using the SiCdiodes, referred as the Si/SiC system, performed better than theconverter using the Si diodes, referred as the Si/Si system. Thecombined Si/SiC system provided a significantly higher effi-ciency and a higher power density by the simple one-to-onediode replacement plus a system optimization. The evaluationhas been summarized in the form of a table for an easy compar-ison in Table VI.

APPENDIX

TRADEOFF BETWEEN SWITCHING LOSS

AND CONDUCTION LOSS OF DIODES

In low-voltage, low-power, and high-switching-frequency ap-plications, e.g., PFC, fast recovery diodes are required to reducethe switching losses. However, there is a tradeoff for silicon-based diodes, a diode which gives low reverse recovery currentbut generates high forward voltage. Therefore, the diode selec-tion for designing a converter should be based on the overallperformance of the converter. In order to verify the argument,a simple diode characteristic comparison will be given next.Table VII lists three 600-V diodes. They are from differentmanufacturers, but the breakdown voltage and the rated cur-rent are the same according to the corresponding datasheets.F08S60S (ultrafast) and LQA08TC600 (Q-series) are Si-baseddiodes and they are also claimed to have a very short reverserecovery time. CSD08060 is the same type of SiC diode as inTable II, but with 600-V class. Fig. 17(a) shows the measured

(a)

0

2

4

6

8

10

12

14

16

0 0.5 1 1.5 2 2.5 3 3.5

Forw

ard

Cur

rent

(A)

Forward Voltage (V)

Forward Characteristics @125˚CSiC Schottky

Ultra Fast Q-Series

(b)

Fig. 17. Comparisons of 600-V diodes’ (a) reverse recovery current and(b) forward characteristic.

turn-off waveforms of these three diodes. On the one hand,the ultrafast diode needs 60 ns to achieve zero current and itspeak reverse recovery current is 5 A. On the other hand, bothparameters, the reverse recovery current and time, of anotherSi-based diode, Q-series, are also half of the ultrafast diode.And its switching behavior is close to the SiC diode. However,the Q-series diode has higher forward voltage than the othertwo diodes. The measured forward characteristics are shown inFig. 17(b). It can be concluded that there is a tradeoff betweenthe switching loss and the conduction loss for Si-based diodes.The diode selection should depend on the switching frequencyand the current flowing through the diode in a converter design.


TABLE VIIISI-BASED DIODE BENCHMARKING

eulaVnoitidnoCretemaraP

Manufacturer ST Microelectronics International

Rectifier Fairchild

Semiconductor

Model STTH1210D BYT 12PI-

100010ETF12PbF MUR8100E

Breakdown voltage

1000 V 1000 V 1200 V 1000 V

Rated current 12 A 12 A 10 A 8 A

Forwardvoltage

@10A, 150˚C

1.2 V 1.6 V 1.2 V 1.5 V

Peak Irr @8A, 100˚C,

200A/us

14 A 15 A 16 A 13 A

Recovery time

170 ns 650 ns 210 ns 120 ns

RthJC 1.9 ˚C/W 4 ˚C/W 1.5 ˚C/W 2 ˚C/W

Furthermore, the comparison also shows that the SiC diode isthe optimal solution from the performance point of view.

SI-BASED DIODE BENCHMARKING

Semiconductor benchmarking was the first step in this com-parative study. Some commercial diodes have been evaluated,which are available from different manufacturers. The ratingsof all devices are comparable to the SiC diodes, i.e., 1200 V and11 A, as shown in Table II. Besides, they are all reasonable forthe IBC application. Table VIII shows a summary of the bench-marking of the selected diodes. The values of these diodes wereobtained from the corresponding manufacturer datasheets. It canbe seen in Table VIII that the explained tradeoff is valid betweenSTTH1210D and MUR8100E. However, the difference is notdramatically large. It can be expected that the final converterperformance with these devices will be more or less the same.Thus, STTH1210D was selected due to the similar rated currentwith the evaluated SiC diodes.

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Carl Ngai-Man Ho (S’06–M’07–SM’12) receivedthe B.Eng. and M.Eng. double degrees and the Ph.D.degree in electronic engineering from the City Uni-versity of Hong Kong, Kowloon, Hong Kong, in2002 and 2007, respectively. His Ph.D. research wasfocused on the development of dynamic voltageregulation and restoration technology.

From 2002 to 2003, he was a Research Assis-tant at the City University of Hong Kong. From 2003to 2005, he was an Engineer at e.Energy Technol-ogy Ltd., Hong Kong. In May 2007, he joined ABB

Corporate Research, Ltd., Baden-Dattwil, Switzerland, where he is currentlya Principal Scientist and Project Manager of photovoltaic inverter researchprojects. He owns several patents in the area of energy saving and renewableenergy conversion. His research interests include renewable energy conver-sion technologies, power quality, modeling and control of power converters,and characterization of wide bandgap power semiconductor devices and theirapplications.

Hannes Breuninger (M’09) received the Dipl.degree in mechatronics engineering from theKarlsruher Institute of Technology (KIT), Karlsruhe,Germany, in 2011. His Diploma thesis was focusedon control systems for automotive clutches to reducejudder.

From 2009 to 2010, he was an Intern at ABB Cor-porate Research, Ltd., Baden-Dattwil, Switzerland.His tasks implied design, implementation, and per-formance verification of dc–dc converters for photo-voltaic applications. Since 2011, he has been a Test-

ing Engineer with an automobile supplier, Brose, Hallstadt, Germany, and isresponsible for test coordination and validation processes in the DevelopmentDepartment. His studies were focused on power electronic systems and feed-back control systems. He was involved in the design and implementation topicsfor multilevel converters in his student research project.

Sami Pettersson (M’05) received the M.Sc. andD.Sc. degrees in electrical engineering from the Tam-pere University of Technology, Tampere, Finland, in2004 and 2009, respectively, where his research topicwas shunt four-wire active power filter topologies andtheir digital control methods.

He was with the Institute of Power Electronics,Tampere University of Technology, as a ResearchAssistant from 2003 to 2004, and as a Research En-gineer from 2004 to 2008. In December 2008, hejoined the Power Electronics Systems Group, ABB

Corporate Research Ltd., Baden-Dattwil, Switzerland, where he is currently aPrincipal Scientist and Project Manager of a research project related to renew-able energy conversion and industrial motor drive systems. His research interestsinclude modeling, design, and optimization of power converter topologies andsystems, power quality, as well as control of power converters.

Gerardo Escobar (M’00–SM’08) received the B.Sc.degree in electromechanical engineering (specialty inelectronics), the M.Sc. degree in electrical engineer-ing (specialty in automatic control) from the Engi-neering Faculty, National University of Mexico, Mex-ico City, Mexico, in 1991 and 1995, respectively, andthe Ph.D. degree from the Signals and Systems Labo-ratory, Ecole Superieure d’Electricite, Paris, France,in May 1999.

He is currently a Principal Scientist in the PowerElectronics Group, ABB Corporate Research Ltd.,

Baden-Dattwil, Switzerland. His main research interests include nonlinear con-trol design, passivity based control, control of switching power converters, activefilters, inverters, electrical drives, and renewable energy systems.

Francisco Canales (M’95) received the B.S. degreein mechanical and electrical engineering from theUniversidad Veracruzana, Veracruz, Mexico, in 1989,the M.Sc. degree in electronic engineering from theCentro Nacional de Investigacion y Desarrollo Tec-nologico (CENIDET), Cuernavaca, Mexico, in 1994,and the Ph.D. degree in electrical engineering fromthe Virginia Polytechnic Institute and State Univer-sity (Virginia Tech), Blacksburg, in 2003.

He was a Senior Research Assistant at the Centerfor Power Electronics Systems, Virginia Tech, where

he was involved in core research and several industry-sponsored projects. He wasan Associate Professor in the Department of Electronic Engineering, CENIDET.He is currently a Senior Principal Scientist at ABB Corporate Research Ltd.,Baden-Dattwil, Switzerland, where he is involved in research on high-densitytraction converters and industrial applications.

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