Analytical and Simulation Comparison of Losses in Non-Isolated DC/DC Converter Using Si and SiC Switches for PV Application ^†

Abstract

With the growth of renewable energy sources around the world, the demand for cost-effective and efficient converters that can operate at high frequency and have less switching and conduction loss has grown. High efficiency is one of the most difficult goals to attain in power electronic converters. Wideband switches can be used to achieve this purpose, although they add to the system’s cost. In this paper, a comparison between SiC MOSFET and Si MOSFET switches was carried out for a 3 KW I-IIB Buck-boost/Boost non-isolated reduced redundant converter for the photovoltaic system with a wide input voltage range. Mathematical calculations were used to investigate switching and conduction losses, and software simulations in PSIM were used to verify their authenticity. In high-frequency power applications, the results suggest that SiC MOSFET can work more efficiently than Si MOSFET. Si MOSFETs, on the other hand, are still preferred for small voltage and low power applications due to their lower costs.

1. Introduction

Renewable energy sources are rapidly used all around the world due to its plentiful, environment-friendly, and broadly distributed nature. Renewable energy sources, specifically solar energy, have gained more attention and become the most important source to meet energy challenges. The energy produced from PV systems is non-linear because the magnitude of voltage and current coming from PV varies due to weather conditions such as temperature and solar irradiance. Thus, these systems require converters that will provide a constant output, be able to work with a wide input voltage range, possess a high power range, and have high efficiency. In renewable energy sources specifically for photovoltaics systems, DC/DC converters have more significance and play a vital role. Due to the non-linear behavior of PV systems, DC/DC converters are necessary for them. The basic goal of a DC/DC converter is to provide smooth and constant output voltage with great efficiency regardless of the input voltage generated by PV panels [1,2].

In the selection and design of converters, efficiency is a critical consideration. Most commercially available converters are constructed on silicon devices. Switching devices play important role in converter designing. There are two types of switching devices SiC and Si MOSFETs mainly used for power converters. Si MOSFETs are widely used as switching devices due to their low cost but SiC MOSFETs have gained more attention in the last few years due to their high efficiency, ability to work with high temperature and frequency, and high-power density [3]. Hence, the loss analysis of switching devices should be evaluated for converters and accompanies the low switching loss device.

In the literature, various types of converter topologies have been proposed for reducing losses; however, this has increased the amount of switches and the total complication of the system [4]. SiC and Si MOSFETs are the primary switching devices of converters. When compared to Si switches, wide bandgap switching devices such as silicon carbide switches are able to perform better with high power, high frequency, and high voltage applications. The loss comparison of SiC and Si MOSFET for half bridge LLC converters is reported in [5,6]. The efficiency, size, and power density of various Silicon carbide MOSFETs are compared in [7]. In comparison to Si MOSFETs, it is proposed that SiC MOSFETs can function at higher frequencies with improved efficiency [8,9,10].

This paper includes an analysis of the I-IIB Buck-boost/Boost non-isolated reduced redundant power-processing converter with a Si MOSFET compared to a model with a SiC MOSFET. Conduction and switching losses were simulated and calculated using the simulation software PSIM for a wide input voltage range, and both versions were compared under various loads; conduction and switching losses were calculated.

2. Proposed Converter

The proposed topology is the “I-IIB Buck-boost/Boost reduced redundant power-processing converter” for PV applications, as shown in Figure 1. The design parameters of topology for PV application are given in

Table 1. Design parameter of the proposed converter.

Parameter	Values	Unit
PV power range	3	kW
Input Voltage range	100–300	V
Output Voltage	400	V
Switching Frequency	50	kHz
$\mathrm{Inductance}(L_1$$,L_2$)	400µ	Henry
Ro	53.3	Ohm

. The proposed converter has a simpler design and the lowest number of switching components. The converter consists of the input voltage source, two inductors ($L_1$ and $L_2$), two transistors (MOSFETs switch $S_1$ and $S_2$), two diodes, load, and two capacitors from which one is used as the storage element referred to the output capacitor, and other is used as a buffer capacitor to reduce the overvoltages and oscillations. The working principle of topology is that when both the switches are turned on, then both inductors named $L_1$ and $L_2$ start charging in parallel from dc source. The power demanded by load is supplied by the output capacitor.

In this paper, a 3 KW I-IIB Buck-boost/Boost non-isolated reduced redundant converter is modeled for Si MOSFET (STW45NM50) and SiC MOSFET (C3M0030090K) to evaluate their functioning. The proposed converter works with varied input voltage ranges and the converter output is controlled by adjusting the duty cycle.

Table 2. SiC MOSFET compared to Si MOSFET.

Properties	SiC MOSFET C3M0030090K	Si MOSFET STW45NM50
Manufacturer	CREE (Durham, NC, USA)	ST Microelectronics (Geneva, Switzerland)
VDs in (V)	900	500
Ron [Ω] a 25 °C	0.03	0.08
IDs (A)	63	45
Maximum Junction temperature (°C)	150	150
tdon (s)	15	35
tdoff (s)	32	23
Qrr (nC)	545	12700
VGS(th) (V)	2.4	4
Cost per Unit (USD)	29	7.6

shows a comparison of Si (STW45NM50) and SiC (C3M0030090K) MOSFETs. At high temperatures, SiC MOSFETs have a better reverse-charge recovery characteristic. The cost of Si MOSFET, on the other hand, is significantly cheaper than that of a SiC MOSFET.

3. Mathematical Modelling of the Proposed Topology

Mathematical computations are used to estimate MOSFET losses. In both of our suggested models, Equations (1)–(7) are utilized to calculate losses across all MOSFETs [7].

Input Current:

$$Iin=\frac{Pout}{Vin}$$

(1)

Average current for converter:

$$Iavg=Iin$$

(2)

Approximate RMS current:

$$Irms=1.11\ast Iavg$$

(3)

Peak Current:

$$Ipeak=1.414\ast Iavg$$

(4)

Conduction losses across each MOSFET:

$$Pcond=Irm{s}^2\ast Rds\ast Duty cycle$$

(5)

Switching Losses for one MOSFET:

$$Psw=\left(\frac{Vin}{2}\right)\ast Ipeak\ast \left(toff+ton\right)\ast fsw$$

(6)

Total Losses for all MOSFETS:

$$Ptotal=2\ast \left(Pcond+Psw\right)$$

(7)

Here, the SiC MOSFET-C3M0030090K is utilized for one model and the Si MOSFET-STW45NM50 is used for another. Only one operation mode is investigated, with the duty ratio kept constant, to highlight the difference in components.

4. Simulations and Results

For the analysis of converter for PV applications, a 3 kW prototype of the proposed converter is constructed for both models in PSIM software. SiC MOSFET- C3M0030090K is used for one prototype and Si MOSFET-STW45NM50 is used for another prototype. The converter is designed to operate well with a wide input voltage range and maintains a constant 400 V at the output. The PSIM simulation model of I-IIB Buck-boost/Boost reduced redundant power-processing converter is shown in Figure 2. Figure 3a shows the efficiency of the converter for a wide input voltage range for both the Si and SiC MOSFETs. The result shows that the efficiency of Si MOSFET is low compared to SiC MOSFET. Figure 3b shows the switching loss for changes in the input voltage. The result shows that the SiC MOSFET’s switching loss is lower than the Si MOSFET’s switching loss.

Figure 4a,b show switching and conduction loss for Si and SiC MOSFETS with different input voltages. The result show that as we increase the input voltage, the switching loss also increases. At low inputs, voltage conduction loss is high. The switching loss of SiC MOSFETs is much lower than Si MOSFET’s. Figure 5a–d show the loss comparison of switches at different power levels for full loads at 75%, 66.6%, and 50% load. SiC MOSFETs have substantially lower switching and conduction losses than Si MOSFETs.

5. Conclusions

In this paper, an analysis of the I-IIB Buck-boost/Boost non-isolated reduced redundant converter for Si (STW45NM50) and SiC (C3M0030090K) MOSFETs is designed in this study. Si and SiC properties were compared. PSIM was used to simulate switching and conduction losses. The findings were compared across a wide input voltage range, switching frequency, and for power. The findings reveal that silicon carbide (SiC) MOSFETs are more efficient in high-frequency power applications than silicon (Si) MOSFETs. Si MOSFETs, on the other hand, are still preferred in low-voltage and low-power applications due to their lower costs.