Step-Up DC-DC Switching Converter With Single Switch and Multi-Outputs Based on Luo Topology

In this paper, a step-up DC-DC multi-output converter is introduced by integrating a super-lift Luo converter, flyback topology, and coupled inductor concept. The proposed multi-output converter has positive output super-lift structure while simultaneously generating step-up voltages in its outputs. The proposed step-up converter has two non-isolated and one isolated output with a simple structure using one switch and one magnetic core. There is no voltage spike by the leakage inductance of the coupled inductor across the switch in the proposed converter. Therefore, the switch has low-stress voltage. The energy in the leakage inductor is recycled leading to higher efficiency in comparison to similar converters with the coupled inductor. The operating principles and the characteristics of the proposed converter are analyzed and discussed. The experimental results of 110W prototype verify the theoretical analysis and the benefits of the proposed converter in comparison to similar multi-output converters. The conducted electromagnetic interference evaluation of the proposed converter is presented and it is reduced using a common-mode choke.


I. INTRODUCTION
Over the last few decades, power DC-DC converters have been the subject of great concern due to their wide increase of utilization in different applications [1].
Stepup DC-DC converters are widely used in numerous power conversion applications and converting lower power DC levels to higher levels. The energy can be stored either in magnetic components as single/coupled inductor or in electric field components as capacitors by active or passive power switching elements.
Step-up DC-DC converters are used in portable device applications, aerospace, satellite application, hybrid electric vehicles, and photovoltaic systems [2]- [5].
Step-up DC-DC converters can be divided into isolated and non-isolated converters. Boost converter as a regular nonisolated converter provides high-voltage gain with working at a higher duty ratio. Different topologies of step-up DC-DC non-isolated converters are proposed to improve voltage gain and efficiency. In some applications, isolated converters are chosen to meet the safety standards, provide galvanic isolation, and enhance noise immunity. Flyback converter as The associate editor coordinating the review of this manuscript and approving it for publication was Zhilei Yao. a conventional isolated step-up converter is used in low power applications together with multiple outputs due to the low number of elements, modest construction, and low cost [6]. In [7], an integrated topology with flyback converter in a single-stage structure is presented to suggest flexible stepup DC-DC converters. Coupled inductor is often selected due to its reduced component count, better integration, and lower inductance requirement compared to uncoupled inductor [8], [9]. However, coupled-inductors can provide a high step-up ratio with a low operating duty cycle using desire turn ratio, and consequently increasing power converter efficiency considerably [10], [11]. Flyback converter can be integrated with other topology using coupled inductor; highvoltage gain of these converters can be achieved by adjusting turns ratio of coupled inductor [12], [13].
The power converter of a single input source and multiple outputs with various voltage is known as a singleinput multiple-output converter (SIMO) [14]. The singleinput multi-output converter is becoming the cost-effective choice instead of hiring multi single-input single-output converters [2]. SIMO can be isolated type or non-isolated; isolated-type has multiple winding transformers to employ for multiple loads [15]. Generally, multi-output converters can be widely used in renewable energy [16]- [18], VOLUME 10, 2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ electric vehicles [19], [20], DC Nano-grids [21], small satellites [22], power charger-supply applications [23], and LED driver [24]. In [24], a multi-output converter for the LED driver is proposed using two transformers and three switches. However, there are voltage spikes across the switches yielding electromagnetic emissions and higher switch stress. A switched boost dual output converter is designed for solar power applications but three switches are needed for dual outputs [15]. A single-input three-output converter with low voltage stresses on semiconductors, for low power applications of an electric vehicle or in Green houses is presented in [19]. However, many components have been made a complex circuit along with decreasing efficiency in the mentioned converter.
Step-up multi-output converters can employ boost, SEPIC, Cuk, and Luo topologies. Luo converters are a series of DC-DC step-up developed-type converters with a simple circuit structure, high efficiency, and low current and voltage ripple. Luo converters with the voltage-lift (VL) technique have a higher voltage gain than the regular boost converter. Super lift Luo converter significantly increases voltage transfer gain stage by stage [25], [26]. Positive output super lift converters are divided into some series based on voltage increment, number of the inductors, and capacitors. Each circuit of the main series has one switch, n inductors, 2n capacitors, and (3n-1) diodes for the n th stage circuit [27]. The voltage-lift and coupled inductor techniques are good combinations to meet high step-up voltage gain for a converter [28]. Interleaved Luo converter deals with reducing circuit component count in addition to power quality enhancement. Using Luo converter along with conventional step-up converters would improve both circuit limitations; lowering switch stresses as well as expanding the area of operating in continuous conduction mode (CCM) [29], [30].
One way to obtain a proper step-up ratio in converters is to use a positive output super-lift Luo DC-DC converter (POSLLC). POSLLC is more powerful than boost, CUK, and SEPIC converters, It has higher efficiency, good power density, higher voltage transfer gain, and large voltage amplification [31]. Generally, the positive super-lift Luo converter shown in Fig. 1 operates in continuous conduction mode (CCM), with two different work states during one switching period.
In this paper, a single-switch step-up converter is proposed with multi-outputs based on positive output super-lift Luo topology to provide two non-isolated outputs and one isolated output. Multiple outputs of the proposed converter are achieved by integrating a flyback converter, super lift Luo converter, and coupled inductor with a single switch. Since all inductors are on one core, the size of the converter is reduced in comparison to multi-output converters with two transformers. Despite regular coupled inductor converters, the energy of the leakage inductances is recycled to eliminate voltage spikes across the switch leading to higher efficiency and lower electromagnetic emissions. The proposed converter can be utilized for supplying midlevel voltage dc loads in some  multi-output applications like electric vehicles or renewable energy systems. This paper is organized as follows. Section II presents the theoretical analysis and operation intervals of the proposed converter. In section III, the design considerations and dynamics of the proposed converter are discussed. Section IV presents the results to show the experimental waveforms with a comparison among step-up converters. The conducted electromagnetic interference (EMI) of the proposed converter is measured in section V. Finally, concluding remarks are offered in section VI.

II. OPERATING PRINCIPLES
The proposed converter is shown in Fig. 2. The positive output super-lift Luo DC-DC converter (POSLLC) in Fig. 1, is integrated with the flyback topology and coupled inductor. It poses three outputs with one single switch and one coupled inductor. As shown in Fig. 2, the proposed converter involves input source V in , three coupled inductor L 1 , L 2 , and L 3 on one core, capacitors C 1 , C O1 , C O2 , and C O3 and diodes D 1 , D 2 , D 3 , and D 4 with a switch, S. The regular pulse-widthmodulation (PWM) can be used for the converter control scheme.
To analyze the proposed converter, the following conditions are supposed: 1) The converter operates under CCM.
2) Forward voltage drop of the diodes, the equivalent series resistance of the coupled inductor and output capacitors are neglected. 3) Equivalent series resistance (ESR) of C 1 , is modeled by r c1 . 4) Capacitors Co 1 , Co 2 , and Co 3 are assumed constant. 5) A magnetizing inductor L m , leakage inductances L lk1 , L lk2 , and L lk3 are used to model coupled inductor and L m is large enough. 6) The primary coupled inductor L 1 has n 1 turns; n 2 and n 3 are the number of turns for L 2 and L 3 respectively. Coupled inductors turn ratios are: The proposed converter has three basic modes, which are illustrated in Fig. 3. The typical key waveforms at the steadystate condition are also shown in Fig. 4. The operating modes are described as follows: Fig. 3a: this mode starts by turning on the switch at t 0 . The current flows through L lk1, L m , and C 1 . In this mode, the voltage across D 1 reaches zero and it turns on. Diode D 2 , D 3 , and D 4 is reverse biased and the magnetizing inductor L m is charged.
The important equations of this mode are as follows: The switch current I s (t) is: The C 1 charging current appear exponentially in this mode. The initial value of the charging current depends on the initial voltage across C 1 , the ESR of C 1 , and the resistance of the switch. r C1 as a major parasitic component serves to limit the peak amplitude of the charging current. At the end of this mode, the voltage across capacitor C 1 reaches about V in , and diode D 1 is turned off. The energy is stored in the magnetizing inductance L m in this interval. The loads R O1 , R O2 , and R O3 are supplied through the discharging output capacitors C O1 , C O2 , and C O3 respectively. Mode II, [t 1 , t 2 ]; Fig. 3b: The switch is turned off at t 2 . Currents of leakage and magnetizing inductance L lk1 and L m decrease linearly. Diodes D 2 , D 3 , and D 4 conducts due to their forward bias and the power is transferred to outputs via coupled inductors L 2 and L 3 . Since D 2 is on, D 1 remains off at this mode. The stored energy of C 1 starts to transfer the output branch C O1 and R O1 . The flyback section consists of D 3 , L 2 , C O2 , and R O2 , where the energy is delivered to the load by L 2 and leakage inductor L lk2 . C O3 has become positive and starts to be charged via L 3 and leakage inductor L lk3. VOLUME 10, 2022 Stored energy in the leakage inductance L lk1 absorbs by capacitor C 1 so the voltage spikes on the switch are escaped which is an important feature of the proposed converter. The currents of the leakage and magnetizing inductor are expressed by the following equations: where:

III. ANALYSIS OF THE PROPOSED CONVERTER
In order to design the converter, selecting the appropriate values of L m , N A , N B , C o , and the value of duty cycle (d) are vital. Design consideration is classified from conversion gains to losses as well as dynamics.

A. CONVERSION GAINS
The voltage conversion ratio is calculated by volt-second balance of the inductors: Consequently, voltage gains are presented in (14), (15), and (16): For a better comparison, Fig. 5 illustrates a plot of three voltage gains versus duty cycle of the proposed converter for the turns-ratio N A and N B equal to two. Fig. 6 shows plot of the voltage gain of flyback part in the proposed converter versus duty cycle considering different turns ratios. This output provides an isolated one and can be adjusted by turns ratio N B . In addition, voltage gain and duty cycle curves under various turns ratios are plotted in Fig. 7 for output 3.

B. DESIGN OF COUPLED INDUCTOR
Turn ratios of the coupled inductor can be determined according to desired gains using (14), (15), and (16). In the proposed converter, transferring energy to the output stage rests on the magnetizing inductance. To determine the current and value of the magnetizing inductance, the following procedure is used.   The voltage across L m is: V in is applied across L m , so: 16874 VOLUME 10, 2022 In order to guarantee the operation of in CCM, the average current through the magnetic inductance, L m , should be more than half of the inductor current ripple. The variation ratio of current i Lm through L m is: In the proposed converter, ξ is designed much lower than unity, to confirm that it normally works in the continuous mode. Moreover, the average current of the magnetizing inductance L m is achieved as given in (20): The stability of the magnetizing current is also considered in the design of the magnetizing inductance. Therefore, by using (18) and (20), the magnetizing inductance for the presented converter is obtained as follows: For known turn ratio considering desired gains, the inductors ratios are determined with approximation as below: C. OUTPUT CAPACITORS Based on two main modes, the appropriate value of C 1 is determined considering discharging time interval of the capacitor, that is, (1−d)T. The equality in (24) should be satisfied with a peak-to-peak voltage ripple of C 1 : (24) In view of the effect of the capacitor voltage drop, the output voltage ripples should be attended. Therefore, Ro 1 , and the C 1 value should not be too small moreover, f s should not be too low. C O1 , C O2 , and C O3 can be found using the amp-sec balance principle:

D. COMPONENT VOLTAGE AND CURRENT STRESSES
In the first mode, the switch is on, so the following equations can be written for capacitors' currents based on Fig. 3: I CO1,CO2,CO3−on = I O1,O2,O3−on (27) At mode II, the switch is off and the capacitors' currents can be expressed as follows: The amp-sec balance to all capacitors, the equations can be derived as: Considering that the load is resistive: I out−on = I out−off = I out (34) By substituting (26)-(31), into (32) the current of capacitors in mode I can be deduced as follows: In addition, the currents of capacitors in mode II are: Considering the above equations, the mean stress of semiconductors' currents during two modes can be obtained. The current and voltage on diodes and the switch are listed in Table 1. It can be concluded that the switch voltage stress of the proposed converter has no relation with the turn-ratio of the coupled inductor and can be related to the duty cycle and input voltage; however, in the single switch multi-output converters there are some design limitations due to a unique duty cycle and turn ratios.

E. RMS CURRENTS AND POWER LOSSES
The efficiency value of the proposed converter is achieved considering losses. Power losses are estimated supposing the inductor currents are ripple-free. RMS currents are shown in Table. 2.
In order to calculate the efficiency of the proposed converter, main losses must be included in power loss analysis.
Therefore, power losses of the proposed converter can be considered into four sets: (i) switch loss, (ii) diodes loss, (iii) (ESR) (r c ) loss, and (iv) magnetic components loss. In this part, it should be mentioned that capacitors ESRs, are supposed to be equal.
The power losses of power switch are equal to the summation of switching and conduction losses. Considering r on as on-resistance of the switch, conduction losses is: Considering switch output capacitance C s as a linear element, switching loss is calculated by: Total power loss in the switch without drive power loss is: The total power losses in the diodes are the summation of power losses in forward resistance r f and threshold voltages V F : The losses of coupled inductor are divided into core and copper losses. Core losses considerably contribute to the temperature rise of coupled inductor, so it is not neglected in the power loss calculations; Winding coil losses or copper losses due to skin effect, proximity effect, the effect of eddy currents in the windings, and edge effects may be significant. Although, for simplicity, these additional winding losses are ignored and just RMS value of the coupled inductor current is estimated for copper losses.
The ESR of the inductor L also causes some power loss.
Since the current ripples across the inductors are not considered, the RMS value of the coupled inductor current is considered equal with its average value so the copper losses in windings are expressed in (49), (50), and (51): The capacitors' lead inductance is ignored and it is not considered as a part of the circuit. In addition, the power losses in capacitor ESRs are given by: Eventually, the overall power loss is estimated by the following equation: P Loss = P S−Loss + P D−loss + P L−Loss + P C−Loss (54)

F. DYNAMIC PERFORMANCE
To obtain transfer functions, the average state-space method is utilized. The state equations and output variables are given in state-space form as follows: The state and input variable vectors are specified as: The matrixes related to (55) are given as follows: The dynamic performance can be analyzed with smallsignal frequency response. Bode plots responses are presented in Fig. 8. PWM method is used to control the proposed converter like single-switch multiple-outputs converters in [23], [32]- [35]. The PWM control diagram is shown in Fig. 9 which is used to regulate three output voltages. The controller directly senses and regulates V O2 , and LUO side output voltage (V O1 ) and boost side output voltage (V O3 ) are indirectly regulated. In the proposed converter, coupled inductor at the output side is connected to three output ports. So, the outputs in the proposed converter are related to each other and it is enough to feedback from only one output. This is another advantage of the proposed converter with respect to some multi-output converters in [2], [21], [34].

IV. EXPERIMENTAL RESULTS
In order to confirm the theoretical analysis, a 110W CCM prototype is implemented. Fig. 10 shows the prototype of the proposed converter. The 110W proposed converter is assumed to operate with 12V input voltage 48V, 36V, and 24V   V O3 = 24V, turn ratios of N B and N A is determined based on (15), and (16) respectively. According to (21), minimum amount of magnetizing inductance (L m ) is obtained. To guarantee CCM operation, 197µH magnetizing inductance is implemented. By known N A , N B , and L m , the value of L 2 and L 3 is calculated using (22) and (23). The converter components and specifications are showed in Table 3. In order to select right semiconductors, the corresponding voltage, and current stresses is considered using Table 1. Therefore, IRFP260, for MOSFET, and BYV32-200 for diodes, is selected. Fig. 11a shows voltage and current waveforms of the switch S. The L 1 current in Fig. 11b, D 1 current and voltage in Fig. 11c, voltage and current of C 1 in Fig. 11d, D 2 , D 3 , and D 4 are in Figures 11e, 11f, and 11g respectively verifying theoretical waveforms. The output voltages V O1 and V O2 are shown in Fig. 11h and the isolated output flyback part is presented in Fig. 11i.
Since the energy stored in the leakage inductance is released through the diode D 1 and C 1 in switching turn-off instant, there is no voltage spike across the switch (Fig.11a), which is an advantage of the proposed converter leading to low switch stress and capacitive turn-on losses. Meanwhile, the Luo structure of the proposed converter minimizes the output current ripple and maintains the conversion efficiency at high power levels. In addition, the super lift Luo converter section, donates high-power density, higher voltage gain, and a less complicated structure as well. A single core is employed for three-coupled inductor as another advantage of the converter. Moreover, the three outputs connected to one coupled inductor and it is sufficient to have a feedback only from one sensitive output unlike the control circuit in [34] in which feedbacks from all outputs are needed. Power losses of the proposed converter are displayed in Fig. 14. The most power losses percentage belongs to diodes. Fig. 15 shows the efficiency curve of the proposed converter in comparison to [23], which is a three outputs SEPIC-Cuk-Boost converter at a similar condition. The efficiency is about 92.5% at the nominal output power for the proposed converter. Table 4 compares the features of the proposed SIMO converter with similar previous papers. The efficiency of     converters has also shown in Table 4 at the nominal output power equal to 110W [2] requires three switches and voltage  spike on one switch with two outputs. Although [21], [36] with two outputs possess has low number of elements, the voltage gain is low. [14] and [16] have high gain in one output and better efficiency by using soft switching technique. However, there are spike voltages and the switch has high voltage stress. A converter without spike on its two switches is introduced in [19], but a large amount of passive components is used and the voltage gain may be sensitive to parasitic parameters. Reference [23] has insignificant voltage gain in all outputs considering its component counts. Compared to other three outputs SIMO converters, the proposed converter has a low number of elements without voltage spike on its single switch even using coupled inductor.

V. CONDUCTED EMI MEASUREMENT
Conducted EMI of the proposed converter without and with 1mH common-mode (CM) choke are measured using CISPR 22 line impedance stabilization network (LISN) as shown in Fig. 16. In Figures 17 (a) and 17(b), the main conducted EMI peak is compared which verified the benefit of applying CM choke as a cost-effective and simple solution for the proposed converter. In most frequencies, the EMI main peak is significantly reduced. The reduction value reaches around 15dB at 3-18 MHz frequency band as is clear in Fig. 18.

VI. CONCLUSION
In this paper, a single switch step-up DC-DC converter with three outputs based on Luo topology is proposed. The operation analysis, design considerations, dynamic response, and losses are discussed. The 110W prototype verifies the theoretical analysis such as low switch voltage stress and low output current ripple with three step-up outputs with one switch and magnetic core. By recycling leakage inductance energy, there is no voltage spike across the switch, which is the main advantage of converters with coupled inductor. The proposed converter provides efficiency around 92.5% at full load. The conducted electromagnetic interference (EMI) is also evaluated and reduced by a CM choke as a simple and cost-effective method.