A Comparative Performance Study of an Interleaved Boost Converter using Commercialized Si and SiC Diodes for PV Applications

Transcription

1 [WeD4-] 8th International Conference on Power Electronics - ECCE Asia May 3-June 3, 11, The Shilla Jeju, Korea A Comparative Performance Study of an Interleaved Boost Converter using Commercialized Si and SiC Diodes for PV Applications C.N.M. Ho, H. Breuninger *, S. Pettersson, G. Escobar, and F. Canales ABB Switzerland Ltd., Corporate Research, Switzerland * Former employee at ABB Switzerland Ltd., Corporate Research, Switzerland Abstract-- A performance comparison of an interleaved boost converter (IBC) using Si and SiC diodes for PV energy conversion systems is presented in this paper. Performance attributes under investigation include the device behavior, thermal requirement, system efficiency and power density. The interleaved boost converter is designed for sustaining the dc link voltage in the energy conversion system. Due to the absence of reverse recovery current in SiC Schottky diodes, low switching loss is generated in the diodes and the switches. This benefit causes higher system efficiency and lower cooling system design requirement. As a benefit, the volume and weight of the heatsink can be further reduced. Furthermore, behaviors of semiconductors and steady-state characteristics of IBC are discussed in the paper. The validity of the analyses is confirmed experimentally by using a.5 kw IBC prototype with wide power and input voltage operating range. Index Terms-- Power semiconductor, SiC, diode, MOSFET, interleaved boost converter, PV. I. INTRODUCTION A PV converter generally consists of a dc-ac inverter and a dc-dc converter [1]-[5]. The inverter is used to feed the dc power into the grid network. Among various inverter topologies, two-level H-bridge inverters and three-level inverters are usually adopted in industry due to simple circuit implementation and high efficiency, respectively [1]-[6]. In general, for the case of V single phase grids, the minimum dc link voltage of inverter is 35V. That voltage level can guarantee that the input voltage of the inverter is higher than the peak value of grid voltage. However, PV panels do not provide constant output dc voltage, in fact, the typical output voltage range is from 15V to 65V, which depends on sun irradiation and panel surface temperature. Thus, a dcdc converter is connected usually in between the panels and the inverter to sustain the dc link voltage according to the typical specification as shown in Table I. Fig. 1 shows a simplified PV energy conversion system. Notice that two identical boost units are used in the circuit. This configuration is named interleaved boost converter (IBC), which is one of the most promising options for PV applications. The reason for this is that, the conduction loss in active and passive components can be significantly reduced by current sharing; the input current ripple can be minimized by 18 phase shift in two switching cells; and it is a simple circuit structure. However, the main drawback is that semiconductors with high breakdown voltage have to be used due to a possible high input voltage coming from the panels. In consequence, the system suffers from the reverse recovery loss in the high voltage class Si diodes (DB) and the commutation loss in the switches (SB). A new type of schottky diode, Silicon Carbide (SiC) schottky diode, has been commercialized in the last decade. The manufacturers claim that the advantages of the SiC diodes are zero reverse recovery and high maximum junction temperature [7]-[8]. It is appropriate for high switching frequency converters. Therefore, higher system efficiency and higher system power density can be achieved with the use of such devices. However, the real performance improvement in a system has to be exhibited by a comparative study. TABLE I SPECIFICATION OF A BOOST CONVERTER FOR SINGLE-PHASE PV INVERTERS PRE-REGULATION. Parameter Value Parameter Value Input Voltage Output Voltage Operating Frequency Max. Ambient Temperature Fig. 1 15V 65V 4V 16kHz 5ºC Max. Input voltage Max. Rated Power Input Current Ripple Max. Junction Temperature Two-stage topology for PV inverter using IBC. 8V.5kW 1% of I In, max 15ºC /11/$6. 11 IEEE

2 Parameter TABLE II PARAMETERS OF THE EVALUATED DIVICES Devices Symbol CoolMOS Si Diode SiC Diode Unit Manufacturer Infineon ST Microelectronics Cree Part No. IPW9R1C3 STTH11D CD1D Type CoolMOS Ultrafast recovery SiC Schottky Breakdown Voltage V BD V Rated Current I D A Max. Junction Temp. T J,max C Thermal Resistance, J-C R thjc K/W Package TO-47 TO- TO-47 There are already in the literature of power electronics several comparative studies of performance between Si diode and SiC diode at device-level [9]-[13] and circuitlevel [14]-[]. In most of the referenced articles, a low voltage and low power rated PFC is the most popular application for demonstrating the advantages of SiC diodes. It has been shown that the SiC diodes can bring benefits in the power supply industry. Furthermore, the trend of the use of SiC diodes is toward the high switching frequency [18]-[19] and high power []-[] applications. Those comparisons have focused on the device behavior or loss in the semiconductors. From the device point of view, there are large differences in those two diodes. But from the system efficiency point of view, the semiconductor loss is only a part of the total loss. Moreover, the testing platforms were not optimized for comparison. The devices could not maximize their self benefit. Thus, the advantages brought by SiC diodes have not really been demonstrated. Moreover, in the best knowledge of the authors, the result is not presented in pre-regulators for PV applications. This paper presents a comparative study of the use of commercialized Si diodes and SiC diodes in an interleaved boost converter for PV applications. Issues addressed here include 1) the static and switching characteristics of two diodes with a CoolMOS device, ) the efficiency of the system in full operating range and 3) the system power density. The validity of the analyses is verified experimentally with a.5 kw interleaved boost converter. Two cooling systems are designed for the two types of diodes in order to keep the CoolMOS device at the same junction temperature. Thus, a fair platform is set to evaluate the system performance for both the Si and SiC diodes. Drain Current, Id (A) Fig. Forward Current, I F (A) Saturation Vgs = 1V Tc = 5 C Ron=18m Drain-Source Voltage, Vds (V) Tc = 75 C Ron=156m Tc = 15 C Ron=5m CoolMOS output characteristics in different temperatures. Pulse Width = 5 s D t C l < Forward Characteristics Si vs SiC diodes Tc = 15 C Tc = 75 C Tc = 5 C Tc = 15 C Tc = 75 C Tc = 5 C SiC - 5C SiC - 75C SiC - 15C Si - 5C Si - 75C Si - 15C Fig. 3 Forward Voltage, VF (V) Forward characteristics of the Si and SiC diodes at different Junction Temperatures.

3 II. SEMICONDUCTOR CHARACTERIZATION The semiconductor loss is mainly divided into conduction loss and switching loss. The losses can be extracted by two types of characterizations, static characterization and dynamic characterization. The loss information helps designers to optimize the system by selecting the most suitable semiconductor devices and gate drive circuits. In the paper, CoolMOS, IPW9R1C3, by Infineon was selected as active switches. It is because the device provides very low onstate resistance at the low junction temperature, thus, the conduction loss is relative low compared with other Si technology based active switches, such as IGBTs. Moreover, it can switch at very high di/dt and dv/dt during turn-off transients, it would form a very fast switching cell with a SiC diode to minimize the switching losses [3]-[4]. Two sets of switching cell have been tested in the system in the combination of the CoolMOS device and the Si Ultrafast diode (full-si) or the SiC Schottky diode (hybrid). All devices have been tested in both static and dynamic measurements to evaluate the losses and to design appropriate heat sinks. Table II shows the key parameters of the devices under test. The values are collected from the corresponding manufacturer data sheets. A. Static Characterization The main objective of carrying out the measurements for static characteristics is to determine the conduction loss of the specific devices, which will be used in the power electronic systems. The Tektronix 371A Curve Tracer is used to extract the parameters from the semiconductor devices. Fig. shows the output characteristics of CoolMOS device where strong temperature dependent can be observed. Consequently the heat sink should be designed to limit the maximum junction temperature for CoolMOS at 75 C to reduce the conduction loss to guarantee a good performance for the overall system. Fig. 3 shows the forward characteristics of the tested Si and SiC diodes. Both devices are temperature dependent, but the SiC diode has an interesting contrast to the Si diode. The Si diode has a negative temperature coefficient, meaning that if the device is heating up, the conduction loss of the device reduces. On the other hand, the SiC diode has a positive temperature coefficient in high current operation, higher than A. By comparing both diodes at the reference point, 1 A and 75 C, the Si diode has a lower conduction loss than the SiC diode. The turn-on voltage of the Si and SiC diodes are 1.36 V and 1.6 V, respectively. However, by the observation, the turn-on voltage difference of low current operating range is not significant. Fig. 4 Fig. 5 CoolMOS turn-on switching waveforms CoolMOS turn-off switching waveforms Fig. 6 Si and SiC diode reverse recovery switching waveforms.

4 TABLE III LOSSES BREAKDOWN OF SEMICONDUCTORS IN IBC Switching Cells Parameter Switch Diode Loss Switch Diode Freq D I avg P M_con P M_on P M_off P M_t P D_con P D_rr P D_t khz A W W W W W W W W CoolMOS Si CoolMOS SiC Switching Loss, E ( Ws) Fig Switching Losses on Devices, CoolMOS + Si vs SiC Diode, 4V,1A,75 C,1V,1,1 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 Drain Current, id (A) Switching energy loss chart B. Dynamic Characterization Practically, the dynamic characteristics of semiconductors can be extracted by double pulse tests [5]. By the use of the results in the characterization, the switching loss information and switching behavior of semiconductors can be determined. According to the design specification in Table I, the corresponding dc voltage of the IBC is 4 V for each switching cell. Thus, the dc testing voltage was set at 4 V as well for the measurements. Fig. 4 - Fig. 6 show the turn-on and turn-off switching waveforms of the switching cells. It can be seen that the CoolMOS turn-off waveforms for both switching cells are very similar in Fig. 5. This leads to the conclusion that the type of diode in a basic switching cell does not affect the turn-off performance of the CoolMOS device. On the other hand, the performance of the turn-on waveforms is quite different, which depends on the reverse recovery behavior of the diodes, as shown in Fig. 6. The reverse recovery current increases the switching loss in the Si diode and also reflects on the drain current of the CoolMOS device; this is called commutation loss. Because of the high reverse recovery current peak (I rr ) and the long transient time of the Si diode, the turnon loss on the CoolMOS device in full-si switching cell is much larger than that of the hybrid switching cell. In addition, the reverse recovery current peak and duration of Si diodes are function of the operating current, di/dt and the junction temperature. SiC diodes have zero reverse recovery in principle, but a small current overshoot can be regarded due to the energy swing between the stray inductor and the parasitic capacitor of the SiC diode, which is only dependent on the di/dt [5]. Fig. 7 shows the loss information for operating currents from A to 18 A. The solid lines and dash lines are the losses of the full-si and hybrid switching cells, respectively. Among the losses, the turn-off losses, pink lines, for both switching cells are the same. The difference of the diode reverse recovery losses is obviously very large. It can be observed that the reverse recovery loss of the SiC diode is quite stable in the full measured current range, the loss being around 4 J at 1 A testing points. On the other hand, the Si diode suffers seriously from the reverse recovery current problem, the energy loss being 38 J at the same testing current. In other words, there is almost a 9% switching loss reduction due to the diode by the use of SiC diode. Besides, the turn-on loss in the switch reduces from 61 J to 1 J at 1 A by using the SiC diode instead of the Si diode. The total switching loss of the hybrid switching cell in one switching cycle is one-third of using Si diode. The energy chart shows the main advantage of the SiC diodes, which is low switching loss. In addition, the switching behavior comparison is based on the same gate resistance, i.e. the same di/dt, but practically, the hybrid switching cell can operate faster with smaller gate resistance. Therefore, the turn-on loss may be further reduced. But it is not included in this comparison and must be further evaluated due to EMI issues. C. Semiconductor losses in the system The semiconductor energy loss extraction is used to determine the semiconductor loss in power electronics systems. Moreover, the cooling system can be designed based on that information. In order to design the heat sink to keep the junction temperature of the CoolMOS devices at 75 C as maximum, the semiconductor loss has to be determined at critical conditions, 15 V input voltage and.5 kw output power. The system steady state characteristics have already well documented in [6]-[8]. Fig. 8 shows the key waveforms for the IBC. As a reference for the critical condition, the average, minimum and maximum currents are 8.7 A, 1 A and A, respectively.

5 Fig. 9 Cooling system simulation for CoolMOSs with Si diodes CoolMOS IPW9R1C + CD1D Fig. 8 Typical waveform of IBC at rated power Table III summarized the loss break down of the semiconductors in the IBC with Si and SiC diodes at the critical condition. The main improvements by the use of SiC diodes are in the loss of P M_on and P D_rr, where P M_on is the turn-on loss of the switches, and P D_rr is the reverse recovery loss of the diodes. In the comparisons, P M_on of using the SiC diodes is one-third of using the Si diodes. And P D_rr of using the SiC diodes is almost one-tenth of using the Si diodes. Therefore, there is around a 19 W total semiconductor loss reduction by using SiC diodes. III. COOLING SYSTEM EVALUATION The cooling systems are designed based on the semiconductor losses in Table III. The junction temperature is set at 75 C in order to keep the conduction loss of the CoolMOS devices low. The heat sink size is different in those two cases due to the different semiconductor losses. Fig. 9 and Fig. 1 show the thermal distributing simulation results for using the Si and SiC diodes. The criteria of the heat sink designs are as follows, The junction temperature of the CoolMOS devices and the ambient temperature are 75 C and 5 C, respectively. One SUNON KDE16PTV and two SUNON KDE14PKV fans are used in Fig. 9 and Fig. 1, respectively. Both fan characteristics are shown in Fig. 11. As can be seen, the characteristics of two sets of fan are very similar. The material and structure of the heat sinks are generally the same. By adjusting the height of the fins and length of the heat sink, different thermal resistances for the cooling systems can be achieved. Fig. 1 Cooling system simulation for CoolMOSs with SiC diodes Fan Characteristics. Static Pressure (Inch-HO) KDE16PTV KDE14PKV x Airflow (CFM) Fig. 11 Fan characteristics for the two cooling system Fig. 1 Heatsinks comparison.

6 TABLE IV SIMULATING CONDITIONS AND RESULTS OF THE COOLING SYSTEM Parameter CoolMOS Si Diode CoolMOS SiC Si Switching Cell SiC Switching Cell Unit Diode P Loss W R JC K/W R CH K/W T A C T H C T J 79.7*.1* 76.4* 69* C R HS K/W *The value is estimated. both cases. However, the junction temperatures are different, the Si diodes are operating about 17 C higher temperature than the SiC diodes by estimation. It is because the switching loss in the Si diode is higher, but the thermal resistance of the heatsink for the Si diodes is lower. The junction temperature of the Si diodes is higher than that of SiC diodes, even when the case temperatures are the same. The junction temperature of the CoolMOS devices in the system using Si diodes is slightly higher than when the SiC diodes are used Efficiency Graph - CoolMOS + Si Diode - Experimental Results 1 3 Input Voltage (V) Output Power (W) 9 Fig. 13 Photograph of the optimized IBC prototype. A summary of the thermal simulation result is shown in Table IV. The junction temperatures of CoolMOS devices on those two heat sinks are very similar and close to 75 C. Fig. 1 shows the physical size comparison between the two designed heat sinks. The total volume and weight of the heat sink for Si diodes are 131 cm 3 and 1185 g, respectively, and the volume and weight of the heat sink for SiC diodes are 388 cm 3 and 47 g, respectively. There is a 6% reduction from these two aspects by the use of SiC diodes. IV. EXPERIMENTAL VERIFICATIONS A.5 kw hardware platform has been built. As shown in Fig. 13, which shows the hardware prototype with SiC diodes, the measurement board, the main power board and the inductors are common for testing. The cooling system is different and based on the design shown in Fig. 9 and Fig. 1. Besides, the switching board consists of the gate drivers and the semiconductors. The gate driving circuits and the CoolMOS devices are the same in the two systems. The differences are the type of diode and the component placement, which is shown in Fig. 9 and Fig. 1. Table V shows the measured temperature results when the systems were operating at the critical point, 15 V input voltage and.5 kw output power. The measured ambient temperatures were 3 C and 7 C for Si and SiC systems, respectively. The case temperatures of the diodes and the CoolMOS devices are almost the same in Fig. 14 Input Voltage (V) Fig. 15 Input Voltage (V) Experimental efficiency graphs of the IBC prototype using Si diodes Efficiency Graph - CoolMOS + SiC Diode - Experimental Results Output Power (W) Experimental efficiency graphs of the IBC prototype using SiC diodes Efficiency Graph - Efficiency Boost with SiC Diode - Experimental Results Output Power (W) Fig. 16 Efficiency difference between Figs. 14 and

7 TABLE V MEASURED TEMPERATURES T A T H,Diode T H,Switch T J,Diode T J,Switch Unit Si/Si * 61* C Si/SiC * 55* C *The value is estimated. Fig. 14 and Fig. 15 show the measured efficiency charts, where the x-axis is the output power of the system and the y-axis is the input voltage. Both graphs also show that the system provides higher efficiencies while it operates at high voltage and high power region. The reasons are the following, Low per leg current ripple: lower core loss of the inductors. Low dc current: lower core loss of the inductors and lower conduction loss and switching loss of the semiconductors. On the other hand, it is opposite to the system which operates at the low power and low voltage region. Since the system operates in the DCM region, the inductor still need to deal with the high amplitude current ripple, the core loss of the inductors dominates in the total loss of the system in that operating region. Fig. 16 shows the efficiency difference between the system using the two types of diode. It can be seen that the difference is generally around.4% to.8% in the whole input voltage range and the output power from 5 W to.5 kw. In other words, the SiC diodes helped the system increasing the efficiency by keeping the CoolMOS devices junction temperatures. Fig. 17-Fig. 19 illustrate the comparisons of the two systems in the European efficiency, the CEC efficiency and the Maximum efficiency. In these three efficiency comparisons, the system using the SiC diodes is better than that using the Si diodes by.4% to.8%, especially in the European efficiency. V. CONCLUSIONS A comparative study on using Si and SiC diodes in an interleaved boost converter for PV applications has been presented. The fair conditions such as a similar junction temperature of the semiconductors and using the same passive devices have been considered. The way to provide those conditions, a.5 kw interleaved boost converter with two optimized cooling systems has been implemented for testing the two types of diode. The results have shown that the converter performance, in terms of efficiency, volume and weight, using SiC diodes is better than that of the system using the Si diodes. The summary of the evaluation is shown in Table VI. The Si/SiC system provides significantly higher efficiency and higher power density by simple one to one diode replacement with system optimization. TABLE VI SUMMARY OF THE COMPARISONS Parameter Condition Si/Si Si/SiC Unit 15V % Heat Sink Volume cm 3 Heat Sink Weight g Junction A=3 C 61/69 55/48 C Inductor Volume cm 3 Inductor Weight g Fig. 17 Fig. 18 Fig. 19 Efficiency(%) Efficiency(%) Efficiency (%) CoolMOS + Si Diode CoolMOS + SiC Diode European efficiency Input voltage (V) European efficiency of the IBC. CoolMOS + Si Diode CoolMOS + SiC Diode CEC efficiency Input voltage (V) CEC efficiency of the IBC. CoolMOS + Si Diode CoolMOS + SiC Diode Maximum efficiency Input voltage (V) Maximum efficiency of the IBC.

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