Dr L Ashok Kumar, Professor, Dept. of Electrical and Electronics Engineering, PSG College of Technology, Coimbatore, askipsg@gmail.com Preethi M R, Trainee Electrical Engineer, Yield engineering systems India Private Limited, Coimbatore
A new topology for high step-up nonisolated DC-DC converter using Silicon and Silicon Carbide devices for solar PV applications using LTSpice Softwares is presented in this paper. In order to use this DC – DC converter for high voltage anFig 2: characteristics of a Si and SiC MOSFETd high frequency applications, Silicon Carbide (SiC) device is the preferred choice. SiC power devices are preferred because of larger current carrying capability, higher voltage block capability, high operating temperature, and less static and dynamic losses than traditional silicon (Si) power switches. The proposed high-voltage gain converter topology has many advantages like low-voltage stress on the switches, high gain with low duty ratio, and a continuous input current. Comparison of Silicon MOSFET and SiC MOSFET for proposed converter is simulated and presented.
Keyword - Si and SiC Power devices, MATLAB, PLECS and LTSpice Softwares, PV application
I INTRODUCTION
Renewable energy source such as photovoltaic (PV) cell generate power from the sun light by converting solar power to electrical power with no moving parts, no maintenance. A single photovoltaic cell produces low level of voltage. In order to increase the magnitude of the voltage, dc- dc boost converter is used. The application of renewable energy sources for day-to-day life is increasing widely. This is due to clean and green nature of energy produced by renewable energy sources. Among the renewable energy sources, solar energy is most promising because of its pollution-free nature. But the output voltage obtained from the solar photovoltaic (PV) panels is very low, and therefore, it cannot be connected to high voltage load or grid directly. Therefore, a dc- dc conversion stage with high gain is required to interface the solar PV with high-voltage load or grid. Many high-gain DC-DC converters were reported in the last one decade. But the design improvement of these high-gain DC-DC converters with respect to increased voltage gain, component reduction, increased efficiency, and reduced losses is still being reported by many researchers. These high-gain DC-DC converters also find their applications in fuel cells, electric vehicles, battery energy storage, automotive industries, and uninterrupted power supplies.
Power electronics are fundamental components in consumer electronics and clean energy technologies. For several decades, silicon (Si) has been the primary semiconductor choice for power electronic devices. Due to the many decades dedicated to the development and fabrication optimization of Si devices, as well as the large abundance of material, the manufacturing capability is high, and the costs are extremely low. However, Si is quickly approaching its limits in power conversion. Wide band gap (WBG) semiconductors offer promise of improved efficiency, reduced size, and lower overall system cost. Of the various types of WBG semiconductors, silicon carbide (SiC) have proven to be the most promising technologies, with several devices already being sold commercially.
Alternatively, SiC has shown tremendous high temperature capability, as well as aptitude for high voltage applications. Furthermore, the cost of SiC devices has decreased within the last decade, and the performance has proven superior to that of conventional Si devices. Some of the potential application areas for these WBG devices include: transportation electrification and renewable energy. Transportation electrification is a major trend today, especially in the automotive industries. Further success of electric vehicle automation on advances in power electronics. Regarding renewable energy, in particular, WBG devices have been explored for replacement of Si insulated gate bipolar transistors (IGBTs) in photovoltaic inverters in order to improve efficiency. WBG semiconductors are well suited for these applications due to their high temperature capabilities, fast switching speeds, and low losses. The high temperature capability of these devices allows them to better withstand the harsh environmental conditions, and have relaxed cooling system requirements. In addition to this high temperature capability, the fast switching speed of these devices allows for higher frequency operation thereby resulting in the reduction of the passive components, which decreases the total size, weight, and cost of the system.
These benefits of WBG semiconductors such as SiC arise from their greater thermal conductivity, higher critical electric field for breakdown and saturation electron drift velocity, and lower intrinsic carrier concentration compared to Si. The higher thermal conductivity of SiC makes it superior to Si in terms of heat transfer to the environment, which allows for increased power densities. SiC has a breakdown electric field that is more than eight times higher than that of Si. This means that, for a given blocking voltage, SiC can be more heavily doped than Si, thereby reducing the on-state resistance. The higher saturation drift velocity makes SiC more suitable for high frequency applications. Also, due to the wider band gap of SiC, it has a lower intrinsic carrier concentration, which allows these devices to operate at higher temperatures without suffering from excessive leakage.
Alternatively, the larger critical electric field for breakdown of SiC allows it to have a greatly reduced drift region resistance for the same breakdown voltage. Furthermore, SiC MOSFETs
have the benefit of being unipolar devices, and thus typically experience faster switching than an IGBT. As a result, extensive work on the characterization of SiC MOSFETs, and comparison of their dynamic performance to Si IGBTs, has been carried out.
The initial fabrication of SiC MOSFETs was stifled by two main issues: (1) the quality of the SiC/SiO2 interface, and (2) the high electric field generation. The poor quality of the boundary between the SiC surface and oxide resulted in low mobility in the inversion layer, while the high electric field generated within the SiC caused degradation of the gate oxide. Significant improvement has been made to the manufacturing of SiC MOSFETs, and thus it has been demonstrated that these issues have been resolved. A common structure for SiC MOSFETs is the double-diffused or DMOSFET is shown in Figure 1, which allows for fast switching speed and high durability. In this structure, when no voltage is applied to the gate, a high voltage can be supported within the thick, lightly doped n drift region. Upon application of a positive gate bias, an inversion layer is produced at the surface of the p- well region underneath the gate electrode. This inversion layer provides a path for the flow of current from the drain to the source. This structure includes an intrinsic body diode, and allows operation in both the first and third quadrants. Current flows through the body diode when the MOSFET gate is off, and a positive drain bias exceeding approximately 0.7 V is applied. If instead a positive gate voltage is applied, and the drain is negatively biased, then the channel will conduct with current flowing from the source to the drain, resulting in third quadrant operation.
Fig 1. SiC MOSFET structure
The SiC power MOSFET is also capable of supporting high positive drain voltages. Furthermore, due to its greater critical electric field for breakdown, the doping concentrations in the drift region of SiC MOSFETs can be increased, thereby resulting in a lower drift resistance for a given blocking voltage. This relationship is shown by the following equation for the ideal on-state resistance Ron-ideal
Where BV is the breakdown voltage, εs is the dielectric constant of the semiconductor, μ n is the mobility of the drift region, and Ec is the critical electric field for breakdown. Moreover, SiC also features a higher saturation drift velocity, allowing for faster switching, and thus is suitable for high frequency applications.
II. CHARACTERIZATION AND MODELING OF A SIC MOSFET
A. POWER Si and SiC MOSFET
Power semiconductor device development has always been a driving force for power electronics systems. Silicon-based power devices have dominated the power electronics applications for a long time. As the requirements for electric energy continuously grow, silicon (Si) devices are coming to face some fundamental limits in performance due to the inherent limitations of Si material properties, which make them unsuitable for future demands, especially in high-voltage, high frequency, high-efficiency, and high-power-density applications. As an example, currently Si insulated- gate bipolar transistors (IGBTs) are able to handle high voltage over 5 kV and high current over 1000 A, however the bipolar nature of the device limits the switching frequency of converter systems below 100 kHz and hence the efficiency of the system. On the other hand, Si metal-oxide- semiconductor field effect transistors (MOSFETs), despite their high-switching-frequency capability in applications up to MHz, suffer from relatively high
on-state resistance and hence high conduction loss at higher blocking voltage, which effectively restricts their use to low voltage applications less than 600 V. In addition, the general 150_C limit of maximum junction temperature further constrains the use of Si devices in high-temperature and high- power-density situations.
The emerging silicon carbide (SiC) technology with its superior material properties compared to Si, brings solutions to all the above problems. As a wide bandgap material, SiC offers a critical electric field of 2.2 × 106V/cm, an order of magnitude higher than Si. This increases the voltage blocking capability of SiC power devices and allows them to be fabricated with much thinner and higher doped drift layers. As a result, the on-state resistance and conduction loss reduce significantly. The high thermal conductivity of SiC improves heat dissipation and, together with the wide bandgap energy (3.3 eV), allows high-temperature operation above 300_C. All of the above advantages of SiC material make it an attractive alternative to Si in the future for high voltage, high power, high frequency, high temperature, and high efficiency converter systems. Table 2.1 lists important material properties of Si and SiC.
Table 1. Comparison of Si and SiC material properties
The lower on-resistance makes SiC power MOSFETs an attractive choice in high power applications, offering similar conduction loss as Si IGBTs while operating at a much higher switching frequency. Due to lower device capacitance of SiC power MOSFET, for the similar voltage and current ratings, the switching loss of a SiC power MOSFET is much lower than Si IGBT or Si GTO. In inductive hard switching, SiC MOSFET body diode can be used if no external anti-parallel diode is connected.
B. STATIC I-V CHARACTERISTICS
According to the superior material properties of SiC compared with Si, the breakdown voltage of SiC can get almost 100 times larger than breakdown voltage of Si for the same doping concentration since SiC has almost 10 times larger critical breakdown electric field than Si.
Fig 2: characteristics of a Si and SiC MOSFET
III. PRINCIPLE OF OPERATION OF PROPOSED CONVERTER
A high-gain DC-DC converter with cross connected capacitors is proposed in this paper. Figure 3.1 shows the circuit configuration of the proposed converter. The proposed converter consists of three switches S1, S2, and S3, two inductors L1 and L2, four diodes D1, D2, D3, and D4, and three capacitors C1, C2, and Co. The duty ratio of S1 and S3 is d1 and that of S2 is d2. The cross connected capacitor structure used at the end of the converter circuit will double the output voltage; hence, increase in voltage gain is achieved in the proposed converter circuit. Depending on the switching operations of the switches S1, S2, and S3, there are three modes of operations present in the proposed converter during continuous conduction mode (CCM).
Figure 3.1: A high-voltage gain DC-DC converter in grid tied solar PV application.
Mode 1: in this mode, all the three switches S1, S2, and S3 are simultaneously turned on at t0. Here, the inductors L1 and L2 are parallelly charged from the input source Vi. The diodes D1, D2, and D3 are reverse biased. Diode D4 is forward biased. The load current is supplied by the capacitors C1 and C2. The voltage across both the inductors L1 and L2 is equal to the input source voltage.
Mode 2: when the switches S1 and S3 are turned off at t1, this mode of operation is established. Switch S2 still conducts. Here, the diodes D1 and D4 are forward biased, and other diodes are reverse biased. Both the inductors will get connected in series and are charged from the input source. The
voltage across each inductor is Vi/2. The output voltage Vo is equal to the sum of VC1 and VC2.
Mode 3: this mode of operation is established
when the switch S2 is turned off at t2. Diode D4 is reverse biased, and the other diodes are forward biased. The inductors will discharge the stored energy to the load with a voltage of (2Vi – Vo)/4.
Figure 3.2: Circuit diagram of proposed high-gain DC-DC converter.
By applying the volt-second balance principle for the inductor L1, the voltage gain expression is derived as
(2)
Table 2: Performance comparison of proposed and other existing converter topologies
A. Voltage Stress Analysis:
The voltage stress across the three switches S1, S2, and S3 during off state are given in
Equations (3), (4), and (5), respectively.
The voltage stress across the various diodes during reverse biased condition are given as follows:
VD1 = −Vi = -30 V (6)
VD2 = VD3 = VD4 = − Vo/2 = -210 V (7)
where VD1, VD2, VD3, and VD4 are the voltages across the diodes D1, D2, D3, and D4, respectively, during the reverse biased condition.
IV DESIGN AND SIMULATION :
Simulation of the proposed converter has been carried out in LTSpice for Si and SiC device. A. Inductor Design. The minimum value of inductance for both the inductors L1 and L2 are determined using
where ΔiL is the ripple current and is generally taken as 10% of the load current and f s is the switching frequency equal to 50 KHz.
B. Capacitor Design.
The value of capacitors C1, C2, and Co are determined using
where ΔVC is the ripple voltage and is usually assumed as 2% of the load voltage and Po is the load power.
Table3. Simulation Parameters
C. Simulation of DC-DC boost converter proposed topology in LTSpice
In Fig.4.2 SiC MOSFET C3M0045065K (CREE manufacturer) model is imported from Wolfspeed Website and used in LTSpice.
V. RESULTS AND DISCUSSION
Simulation of the proposed converter has been carried out using LTSpice. Input voltage is taken as 30V and the load resistance is 100 ohms. Various parameters of the proposed converter are measured, and the results are produced in this section.Duty ratio is taken as 40% (d1) for switches S1 and S3 and is 80% (d2) for S2. Figure 5.1 shows the output voltage and is observed as 360 V for both Si and SiC Devices. Therefore, the gain of the proposed converter is 12 and is closely matches with its theoretical value. Both the inductors are charged to a voltage equal to input voltage of 30V during mode 1. In mode 2, the voltage across the inductors is equal to half of the input voltage 15 V. During mode 3, when the switch S2 is turned off, the inductor discharges the stored energy, and the voltage across each inductor is observed as -143.5 V. Voltage stress across the switch S2 is measured as 335.4V which is half of the output voltage (Vo/2). Voltage stress across the switch S1 is measured as 191.7 V. Voltage stress across the switch S3 is measured as -143.6 V. This validates the theoretical performance calculations. Comparison of the simulated results of Si and SiC devices clearly shows that SiC has more advantages than Si MOSFET.
Fig 5.1 shows that Input Voltage is Boosted to 360V. Output Voltage obtained from the use of Si and SiC MOSFET are similar but there is significant difference found in the Voltage stress across the Si and SiC devices. SiC device have less Voltage Stress compared to Si Device.
A. Switching Losses of Si MOSFET and SiC MOSFET in LTSpice
By Comparing the Figure 5.4 and 5.5 switching losses in SiC MOSFET are less compared to Si MOSFET
B. IV CHARACTERISTICS OF Si AND SiC MOSFET in LTSPICE
C. Ron Vs Junction Temperature Characteristics Of Si And SiC MOSFET in LTSPICE
By Comparing the Figure 5.8 and 5.9 Ron Vs Temperature in SiC MOSFET are less compared to Si MOSFET.
D. Conduction Losses in Si and SiC MOSFET in LTSPICE:
By Comparing the Figure 5.11 and 5.10 conduction losses in SiC MOSFET are very very less compared to Si MOSFET.
VI. Conclusion
A high step-up nonisolated DC-DC converter was proposed, analyzed, and validated through the simulation results using Si and SiC Device in LTSpice software . There is double the Voltage stress across the Si MOSFET when compared to SiC MOSFET when simulated in LTSpice shown in Figure 5.3. On Comparing Si and SiC devices in this topology in LTSpice performance of converter is slightly improved in terms of voltage stress, temperature, switching loss when SiC Devices are used. So SiC Devices are preferred for High Voltage Application. The proposed converter used a cross connected capacitor structure which doubled the output voltage of the converter, and hence, the voltage gain is also doubled. The proposed converter was operated with d1=0.4 and d2=0.8 , and the voltage conversion ratio was theoretically calculated as 14. Also, the highest value of voltage across the switch is half of the output voltage, and therefore, the switching losses are reduced. Finally, the simulation results and waveforms were presented to validate the proposed converter. . In order to use this DC – DC converter for high voltage and high frequency applications, Silicon Carbide (SiC) device is the preferred choice from comparative results. SiC power devices are preferred because of larger current carrying capability, higher voltage block capability, high operating temperature, and less static and dynamic losses than traditional silicon (Si) power switches.
References
[1] Shanthi Thangavelu and Prabha Umapathy,”Design of New High Step-Up DC-DC Converter Topology for Solar PV Applications”, International Journal of Photo energy, Volume 2021, Article ID 7833628, 11 pages.
[2] Y. Zhang, H. Liu, M. Sumner, and C. Xia, “DC–DC boost converter with a wide input range and high voltage gain for fuel cell vehicles,” IEEE Transactions on Power Electronics, vol. 34, no. 5, pp. 4100–4111, 2019.
[3] A. Bharathi Sankar, Dr. R. Seyezhai , “Implementation of Sic Based DC-DC Boost Converter for Photovoltaic Applications”, International Journal of Engineering Research & Technology (IJERT), ISSN: 2278-0181 , TITCON-2015 ,Conference Proceedings.
[4] Jun, W., Z. Xiaohu. "10-kV SiC MOSFET-Based Boost Converter," IEEE Transactions on power electronics, 2008.
[5] M. Meraj, M. S. Bhaskar, B. P. Reddy, and A. Iqbal, “Nonisolated DC–DC power converter with high gain and inverting capability,” IEEE Access, vol. 9, pp. 62084–62092, 2021.
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