Magnetic Material Assessment of a Novel Ultra-High Step-Up Converter with Single Semiconductor Switch and Galvanic Isolation for Fuel-Cell Power System.

In this paper, a novel step-up converter is proposed, which has the particular features of single semiconductor switch, ultra-high conversion ratio, galvanic isolation, and easy control. Therefore, the proposed converter is suitable for the applications of fuel-cell power system. Coupled inductors and switched capacitors are incorporated in the converter to obtain an ultra-high voltage ratio that is much higher than that of a conventional high step-up converter. Even if the turns ratio of coupled inductor and duty ratio are only to be 1 and 0.5, respectively, the converter can readily achieve a voltage gain of up to 18. Owing to this outstanding performance, it can also be applied to any other low voltage source for voltage boosting. In the power stage, only one active switch is used to handle the converter operation. In addition, the leakage energy of the two couple inductors can be totally recycled without any snubber, which simplifies the control mechanism and improves the conversion efficiency. Magnetic material dominates the conversion performance of the converter. Different types of iron cores are discussed for the possibility to serve as a coupled inductor. A 200 W prototype with 400 V output voltage is built to validate the proposed converter. In measurement, it indicates that the highest efficiency can be up to 94%.


Introduction
The power generation coming from petro-chemical, coal, and liquefied natural gas (LNG) results in a series of damages like environmental destruction, climate change, and aerial contamination. Therefore, renewable resources have been growing rapidly in recent years to overcome the mentioned problems. Nowadays, green energy is adopted in many fields from household use to industry application. In some applications, such as electric vehicle (EV) and grid-tied renewable-energy system, a high DC voltage to be in the range of 380 to 420 V is required [1]. Accordingly, incorporating a high step-up DC/DC converter to boost a low-voltage distributed energy resource becomes essential in green energy system. A high step-up converter developed for photovoltaic (PV) module, wind turbine, or fuel cells is able to boost up a low voltage to a much higher level [2][3][4][5][6][7][8][9][10], thus, which can accomplish power injecting into a DCbus [11,12]. Magnetic inductor is a main component that is much more important than any other devices in a high step-up converter. A theoretical model of magnetically coupling inductor is studied to analyze the saturation condition [2]. Utilizing parallel paths to share input current for higher power processing is proposed in [3,4]. An extra high gain can be achieved by means of voltage multipliers, which has been presented in [5]. Some researchers derive new topologies from boost-type configuration [6,7], while the literature [8] concentrates on interleaved operation and leakage energy recycling. In [9], an isolated step-up converter based on quasi Z-source is presented to lower the component count. Combination of boost configuration, coupled inductor and interleaving control is discussed in [10]. For the achievement of high voltage gain, the method that a high step-up converter incorporates coupled inductor [13] and/or switched capacitor [14] is a common approach. However, the shortcomings of galvanic isolation, more active switches, and without high enough voltage gain for direct grid-tied application still exist. For this kind of converter, magnetic device, that is, the coupled inductor, will dominate the power conversion performance of a high step-up converter. The characteristics of a magnetically coupling component should be discussed. Unfortunately, which type of iron core should be feasible and adopted is missed in most literature.
Magnetic core, which includes Ferrite, Molypermalloy Powder (MPP), High Flux, and Kool Mµ, are widely used in switching-mode power converters. The characteristics of these magnetic materials, such as flux density, biasing capability, permeability, and core loss, are different. The core loss is of vital importance in power converter design. The following literature discussed core loss from various aspects. A simplified model is proposed in [15] to estimate core loss, which considered the effects of both frequency and temperature. Another one by means of investigating the toroidal cores of Ferrite material explored core loss [16]. In [17], the authors introduced a solution to calculate the core loss of a transformer in switching the power supply with realistic measurements. As for non-sinusoidal input, the literature [18] presented the core loss estimation covering the influences of frequency and duty cycle.
This paper proposes a single-switch isolated ultra-high step-up converter (SIUSC), which can achieve an ultra-high voltage gain for low-voltage input. Its voltage conversion ratio is much higher than that of any other step-up converter. In addition, the SIUSC inherently has the particular features of only one semiconductor switch needed, galvanic isolation, being suitable for any low voltage source, and easy control. In order to characterize a better performance, different materials of magnetic cores are considered and then valued. The converter power stage is depicted in Figure 1. The SIUSC includes several parts: one boost cell, three forward-flyback cells, and one flyback cell, in which the coupled inductor T 2 performs galvanic isolation. For a better understanding, the advantages of the proposed SIUSC are summarized as follows: (1) Only one active switch is utilized.
(2) The energy stored in leakage inductance can be recycled.
(3) The voltage conversion ratio is high enough so that SIUSC is capable of dealing with low voltage input. (4) The structure of the SIUSC has galvanic isolation. This paper is organized as follows. After the introduction in Section 1, Section 2 details the steady-state analysis of the proposed converter followed by the voltage gain derivation is presented in Section 3. After the inductance design of the coupled inductor operated in continuous conduction mode (CCM) in Section 4, the evaluation of coupled-inductor loss for different magnetic materials is given in Section 5, providing an appropriate selection of magnetic core. Finally, the experimental results measured from a 200-W prototype are given in Section 6 to validate the proposed converter, while the conclusion is summarized in Section 7. Figure 1 is the configuration of the converter, in which symbol definitions are summarized in the following. The V in and V o signify the terminal voltages at low-and high-voltage sides, respectively. S 1 is the semiconductor switch, while D 1 -D 5 , D lk , and D o are power diodes. C 1 -C 5 , C lk , and C o are capacitors that are employed in the power stage. The magnetically-coupled device T 1 has N 1 turns at the primary, N 2 turns at the secondary, magnetizing inductances L m11 and L m12 , and leakage inductances L lk11 and L lk12 . Similarly, N 11 and N 22 represent the primary and the secondary turns of T 2 , L m21 and L m22 denote the primary and secondary magnetizing inductances, respectively, and L lk21 and L lk22 express the leakage inductances. Before the description of converter operation, some assumptions are made as follows:

Operation Mode of the Proposed Converter
(1) In Figure 1, all of the coupled inductors are in CCM.
(2) Parasitic input capacitance of the main switch, C iss , is neglected, and all of the diodes are considered ideal. (3) The capacitances of all capacitors are considered large enough to ignore the ripples across them.
Thus, their voltage can be regarded as constant during one switching period. (4) The turn ratios of T 1 and T 2 are both equal to n. That is, n 1 = n 2 = N 2 /N 1 = N 22 /N 11 .
The operation of the proposed SIUSC can mainly be divided into six operation modes, whose corresponding equivalents are represented in Figure 2. Key waveforms of the SIUSC over one switching cycle are depicted in Figure 3. The operation of the converter is described mode by mode in the followings.

Mode 1 [t 0 , t 1 ] (see Figure 2a):
The converter operation over one switch cycle starts at Mode 1, in which the switch S 1 is turned ON at t = t 0 . The magnetizing inductors L m11 and L m21 absorb energy from V in and C 4 , respectively. Meanwhile, capacitor C 1 and the secondary of T 1 release energy to capacitor C 2 via D 2 and S 1 . Once the switch S 1 is turned OFF, operation of the converter enters into the next mode. Mode 2 [t 1 , t 2 ] (see Figure 2b): In Mode 2, switch S 1 is in OFF state. During this time interval, the energy of L lk11 is forwarded to capacitor C 1 via diode D 1 . Similarly, the energy of L lk21 is dumped to C lk via D 3 . When i D2 drops to zero, this mode ends. Figure 2c): The S 1 remains the same status as in Mode 2. The energy of L m11 is forwarded to capacitor C 1 . Meanwhile, the capacitor C 3 is charged via the loop of V in -N 1 -C 2 -N 2 -D 3 . When the capacitor C lk stops absorbing energy from L lk21 , this mode ends. Figure 2d): During the interval of Mode 4, switch S 1 is in OFF state. Input voltage V in and magnetizing inductance L m11 proceed with energy releasing toward C 1 and C 3 via D 1 and D 3 , respectively. In addition, the L m21 begins pumping its energy to output capacitor C o via the loop of C 4 -N 22 -C 5 -D o -C o . Since C 4 and C 5 are in series at this energy-pumping loop, both capacitors also forward their energy to C o . The current following through D 1 , i D1 , decreases. Mode 4 ends when i D1 is equal to zero. Mode 5 [t 4 , t 5 ] (see Figure 2e): The switch has the same status as in Mode 4. Since the capacitor C 1 has been fully charged, the L m11 and L m21 pump energy to C 3 and output, respectively. This mode will end as current i Do falls to zero. Figure 2f): In this mode, switch S 1 is in ON-state again. The energy stored in capacitor C lk is drawn out via S 1 ; that is, the leakage energy of L lk21 is successfully recycled. This mode ends when i Dlk starts to increase, and converter operation over one switching cycle is completed.

Voltage Gain Derivation
In this section, the voltage conversion ratio of SIUSC will be derived. For high power applications, the SIUSC is designed to operate in CCM. Furthermore, the assumptions made in the previous section are also considered. In addition, the phenomenon that occurs during switching transient is ignored.
It can be found that the output voltage V o is equal to the sum of V C4 , n 2 V Lm21 , and V C5 . Hence, in order to determine the voltage ratio of V o to V in , the voltages of V C1 , V C2 , V C3 , V C4 , V C5 , V Clk , and V Co have to be obtained in advance. Firstly, the voltage across C 1 can be determined by applying volt-second balance criterion (VSBC) to L m11 , which can yield.
As for V C2 , the voltage across C 2 is the sum of V C1 and n 1 V Lm11 when S 1 is in ON state. Thus, V C2 can be found by The V C3 is equal to the series voltage of V in , V Lm11 , n 1 V Lm11 , and V C2 , accordingly, which can be The capacitors C 4 and C 5 are charged simultaneously while S 1 is closed. Thus, both voltages V C4 and V C5 are identical and equal to n 2 V Lm21 , and then the following relationship holds: With respect to the voltage across capacitor C lk , the voltage V Clk can be determined from V Lm12 , V C2 and V Lm21 . It is given by Since the output voltage Once the voltages V C1 , V C2 , V C3 , V C4 , V C5 , V Clk and V Co are given in terms of V in , the conversion ratio of output to input voltages, V o /V in , can readily be found by

Inductance Design for CCM
The boundary condition of i Lm1 is designed at 20% of full load for overall efficiency consideration. That is, the output resistance is 4000 Ω at boundary. The maximum and minimum currents of i Lm1 , denoted as I Lm1,max and I Lm1,min , respectively, can be calculated by and where I Lm1,avg is the average value of i Lm1 and ∆i Lm1 stands for the current change on the magnetizing inductance over switch ON or OFF interval. To make sure the magnetizing inductance L m1 is in CCM, the current I Lm1,min should be greater than zero. Additionally, the ∆i Lm1 and I Lm1,avg can be determined as follows: and where f s represents the switching frequency in hertz. Thus, By rearranging the Equations (9)-(12), the minimum value that makes coupled inductor T 1 operate in CCM is obtained as:

Magnetic Core Selection
There are three types of toroidal cores, MPP, High Flux, and Kool Mµ, which are widely adopted in switching power supply. They are also considered to serve as the magnetic core of the coupled inductors in SIUSC and are discussed in this section. Based on (13), if given that D = 0.47, f s = 50 kHz, n 1 = n 2 = 1, and R o = 4 kΩ, the inductance of L m1 will be 70.4 µH. In addition, in order to process a power of 200 W, the I Lm1,avg should be 8.33 A. At this situation, current ripple ∆i Lm1 is limited within 2 A. As a result, the maximum current of L m1 , I Lm1,max is calculated as The maximum increment on flux density of a specific magnetic material, ∆B max , has to be known to avoid a coupled inductor from going saturation. The ∆B max can be found from the manufacturer's datasheet. With the values of maximum magnetizing current, maximum flux change, switching frequency, and volume of core, it can benefit a designer to assess the loss of a coupled inductor and then to choose an appropriate magnetic core [19].

MPP Core
The core loss of MPP can be calculated by P core loss−MPP = 53.05 B 2.06 f s 1.56 , (15) in which the B is flux density and f s is operation frequency. A corresponding plot is shown in Figure 4, in which the permeability equals 125 µH/m. In general, the B will fall within the range from 0.2 to 0.25 Tesla. If an MPP core with part No. 55324-A2, whose volume is 6.088 cm 3 , is considered, the associated power loss density will be 861.39 mW/cm 3 under the design conditions that B is given as 0.
2 Tesla and f s = 50 kHz. As a result, the core loss is computed as 5.244 W. Based on the given values of L m1 , I Lm1,max , B, and the effective cross-section area of 0.678 cm 2 for MPP 55324-A2, the turns of primary side can be determined as follows: Since turns ratio should be an integer, N MPP is chosen as 26 turns. An average length of one turn is 3.44 cm, which yields the total length of the winding at the primary is l MPP = 26 × 3.44 = 894.4 mm (17) A copper wire with cross-section area of 0.518 mm 2 is adopted for a maximum carried current of 11 A. The resistivity of copper, ρ, is 2.3 × 10 −6 Ω-m. Thus, resistance of the primary winding is calculated by Suppose that the turns ratio of coupled inductor is unity. The total copper loss is calculated as

High Flux Core
With respect to the core loss of the magnetic material of High Flux, it can be estimated by (20) Figure 5 illustrates the relationship of core loss versus flux density under different switching frequencies, while permeability is 125 µH/m. If 0.2-Tesla flux density is the operation point and the core of High Flux 58324-A2 is chosen, then the associated core loss density will be 2136.57 mW/cm 3 under 50-kHz switching frequency. Since the volume of the core 58324-A2 is 6.088 cm 3 , core loss of the High Flux magnetic coupled inductor is 13.007 W. For High Flux 58324-A2, the allowed maximum variation at flux density is up to 0.75 Tesla. With the same conditions in MPP, the turns of primary side can be determined as follows: Since turns ratio should be an integer, N High Flux is chosen as 13 turns. An average length of one turn is 3.44 cm for High Flux 58324-A2. Therefore, the total length of the winding at the primary is Resistance of the primary winding is calculated as Then, the total copper loss of the coupled inductor with High Flux core is P copper−High Flux = 9.33 2 × 1.985 × 2 = 345.58 mW (24)

Kool Mµ Core
Another magnetic core widely adopted in switching power circuit is the type of Kool Mµ. Its core loss can be determined by the following equation. Based on (25), a corresponding plot is presented in Figure 6, while permeability equals 125 µH/m. A Kool Mµ core with part no. 77324-A2 is chosen, whose volume is 6.088 cm 3 . Its associated power loss density will be 1560.94 mW/cm 3 under the conditions, B = 0.2 Tesla and f s = 50 kHz, which results in a core loss of 9.503 W.
Thus, N Kool Mµ is chosen as 18 turns. Because an average length of one turn is around 3.44 cm, the total length of the primary winding is 619.2 mm. The resistance of the primary winding is calculated as In addition, the copper loss of a coupled inductor with unity turns ratio can be estimated as P copper−Kool Mµ = 9.33 2 × 2.749 × 2 = 478.59 mW (28)

Experimental Results
To validate the proposed SIUSC, a 200-W prototype is built with the specifications and components that are summarized in Table 1, while the measuring equipment are shown in Table 2 Figure 7 illustrates the input current of the proposed converter, which verifies that the SIUSC indeed operates in CCM. Figure 8 shows the measurements from active switch S 1 , in which it can be observed that the v ds1 can be clamped to a lower voltage stress. This voltage-clamped effect also demonstrates the feature of leakage energy recycling. Figure 9 shows the transient response of the converter under step load change, which confirms that the SIUSC can intrinsically achieve excellent performance. Figure 10 depicts the measured efficiency of the prototype, where the peak efficiency is around 94% at 80 W. Figure 11 shows the photo of test bench, where the equipment for practical measurement have been listed in Table 2. In Figure 11, the length, width, and height of the main circuit are around 13.8, 8.04, and 3.4 cm, respectively.

Conclusions
This paper proposes a novel single-switch isolated ultra-high step-up DC/DC converter, which is applicable to fuel cells, PV module, and battery system. The main contribution of this paper is that an ultra-high voltage conversion ratio can be easily achieved even under a low turns ratio and duty ratio. This outstanding performance make the SIUSC be much more suitable for any low voltage source to boost its input voltage. In addition, the loss of magnetic components is assessed and discussed for three different types of toroidal cores. A 24/400 V 200 W prototype has been built and examined to verify the feasibility of the SIUSC. The measurements indicate the converter having a maximum efficiency of 94%.