A new family of non‐isolated PWM DC–DC converter with soft switching
2018; Institution of Engineering and Technology; Volume: 12; Issue: 2 Linguagem: Inglês
10.1049/iet-pel.2018.5351
ISSN1755-4543
AutoresElham Gerami, Majid Delshad, Mohammad Reza Amini, Mohammad Rouhollah Yazdani,
Tópico(s)Multilevel Inverters and Converters
ResumoIET Power ElectronicsVolume 12, Issue 2 p. 237-244 Research ArticleFree Access A new family of non-isolated PWM DC–DC converter with soft switching Elham Gerami, Elham Gerami Department of Electrical Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, IranSearch for more papers by this authorMajid Delshad, Corresponding Author Majid Delshad delshad@khuisf.ac.ir Department of Electrical Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, IranSearch for more papers by this authorMohammad Reza Amini, Mohammad Reza Amini Department of Electrical Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, IranSearch for more papers by this authorMohammad Ruhollah Yazdani, Mohammad Ruhollah Yazdani orcid.org/0000-0002-9000-8684 Department of Electrical Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, IranSearch for more papers by this author Elham Gerami, Elham Gerami Department of Electrical Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, IranSearch for more papers by this authorMajid Delshad, Corresponding Author Majid Delshad delshad@khuisf.ac.ir Department of Electrical Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, IranSearch for more papers by this authorMohammad Reza Amini, Mohammad Reza Amini Department of Electrical Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, IranSearch for more papers by this authorMohammad Ruhollah Yazdani, Mohammad Ruhollah Yazdani orcid.org/0000-0002-9000-8684 Department of Electrical Engineering, Isfahan (Khorasgan) Branch, Islamic Azad University, Isfahan, IranSearch for more papers by this author First published: 01 February 2019 https://doi.org/10.1049/iet-pel.2018.5351Citations: 5AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat Abstract In this study, a new family of soft-switching pulse-width-modulated (PWM) converters with a lossless passive snubber is introduced. The switch is turned on under zero-current switching and turned off under zero-voltage switching and all diodes turn on and off under soft switching conditions. The proposed snubber consists of two resonant inductors a resonant capacitor and three diodes which can be applied to all non-isolated DC–DC converters. A buck converter with the proposed snubber is analysed and to verify theoretical analysis, a 100 W prototype is implemented and efficiency of converter compared with hard switching counterpart. Also to confirm the effectiveness of the proposed snubber to reduce conducted electromagnetic interference (EMI), the conducted EMI of a buck converter with proposed snubber cell is compared with conventional buck converter. 1 Introduction Low size and weight and high power density are the most important specifications in designing a power electronic converter. This goal is achieved by increasing the switching frequency. However, high switching frequency leads to high switching losses and electromagnetic interference (EMI). To solve these problems, soft-switching techniques are introduced. In DC–DC converters, various soft-switching techniques are presented such as resonant [1-3], active clamp [4-6], zero-current transition (ZCT) [7-10], zero-voltage transition (ZVT) [11-13], zero voltage zero current transition (ZVZCT) [14] converters and so on. In resonant converters [1-3], a resonant tank is added to the converter to achieve soft switching without any extra switch, but in these converters, output power is controlled by varying the switching frequency. Therefore, magnetic elements cannot be designed optimally. Active clamp techniques [4-6] are used extensively because of the low number of elements and the simplicity of the circuit operation. But they have high circulating current and a duty cycle loss which cause increased conduction losses and decreased voltage gain, respectively. In ZCT, ZVT and ZVZCT converters [7-14], the auxiliary circuit acts during switching intervals to provide zero voltage or current before switching instants. These converters use pulse-width modulation (PWM) to control the output voltage but they have at least two switches in their structure. Furthermore, in some of them, there are two or more than two auxiliary switches [10, 11] which complicate the converter operation and its control. Therefore, due to lack of an additional driver and the convenient of control circuit implementation, soft single switch PWM converters have been noticed [15-19]. In [15] a snubber with low number component for PWM DC–DC converters is introduced. The switch is turned on under zero-current switching (ZCS) condition and is turned off at almost zero-voltage switching (ZVS) condition. All diodes are soft switched but the proposed snubber has coupled inductors in its structure. The leakage inductance of such coupled inductors causes unwanted resonances in the circuit. A new lossless passive snubber with low switch stress is introduced in [16]. The converter switch operates similar to [15] but the number of elements increases and the leakage inductance problem still remains. In [17] a lossless passive snubber is added to the converters so all semiconductor devices turn on and off at soft switching conditions. This snubber can be applied to all isolated and non-isolated DC–DC converters. The energy of the snubber transfers to the output but the number of elements is increased. Also, the auxiliary diodes have current stress. A passive lossless snubber introduced in [18] reduces turn off losses in the boost converter. The added snubber creates a path for current at the turn off instant and the switch is consequently turned off in ZVS conditions. Also, the switch turns on at ZCS condition and all other semiconductor devices turn on and off at soft switching conditions. This snubber can be used only for a boost converter. A passive lossless snubber cell that provides soft switching conditions at on and off instants over a wide load range is presented in [19]. It does not impose extra voltage and current stress on the switch but the number of snubber elements is high in its structure. In this paper, a new family of non-isolated PWM DC–DC converters with soft switching is proposed. The proposed snubber does not have coupled inductors. The switch is turned on under ZCS and turned off under ZVS condition. Also, all diodes turn on and off under soft switching conditions and due to ZCS turn off the reverse recovery problem does not exist. The buck converter with the proposed snubber is analysed and its operating modes are described. The proposed snubber can be applied to the other converters such as boost, buck–boost, Cuk, SEPIC and Zeta. In Section 2, the circuit description and operating modes of the buck converter with proposed snubber are discussed. In Section 3, the design procedure of the converter is explained and in order to verify theoretical analysis, a 100 W prototype is implemented and its experimental results are illustrated in Section 4. In Section 5, the efficiency of the proposed buck converter compared with hard switching counterpart. In Section 6, conducted EMI of the buck converter with proposed snubber cell is evaluated. The other converters using the proposed snubber are introduced in Section 7. 2 Circuit description and operation The proposed soft-switching buck converter is shown in Fig. 1. The converter is composed of a buck converter and the proposed snubber circuit which consists of Lr1, Lr2, Cr, D1, D2 and Dr. For simplicity analysis of the converter, the following assumptions are made: Lo and Co are large enough so that they can be replaced by a current source (Io) and a voltage source (Vo), respectively. All semiconductor devices are ideal. Fig. 1Open in figure viewerPowerPoint Buck converter with a proposed lossless passive snubber The proposed converter has seven operating modes in one switching period. The main theoretical waveforms of the proposed buck converter are shown in Fig. 2 and the equivalent circuit for each operating interval is shown in Fig. 3. Fig. 2Open in figure viewerPowerPoint Key waveforms of the proposed buck converter Fig. 3Open in figure viewerPowerPoint Equivalent circuits during various operation modes of the proposed buck converter (a) Mode 1, (b) Mode 2, (c) Mode 3, (d) Mode 4, (e) Mode 5, (f) Mode 6, (g) Mode 7 Before the first mode, it is assumed that the output diode Do is on and all other semiconductor devices are off. Mode 1 [t0−t1]: At the beginning of this mode, the main switch S is turned on under ZCS condition due to slightly increase in current of Lr1 and Lr2. In this mode, diode Dr conducts and a resonance between Lr1 and Cr starts. Therefore, Cr voltage decreases and Lr1 current increases sinusoidally. Since the switch S and diode Do and diode D2 are on Vin is placed across Lr2 and the Lr2 current increases linearly with the slope of Vin/Lr2. The key equations of this mode can be obtained as follows: (1) (2) (3) (4)where V1 is Cr voltage at the beginning of this mode. The current of the switch is the sum of Lr1 and Lr2 current and its equation is as follows: (5)This mode ends when Vcr reaches Vin and diode D1 starts to conduct under ZVS condition. The duration of this mode is calculated as (6)The maximum current of Lr1 is I1 and Lr2 current at the end of this mode is I2 also I3 is a maximum current of the switch which are obtained according to the following relationships: (7) (8) (9)Mode 2 [t1−t2]: At t1, Vcr reaches −Vin and diode D1 starts to conduct under the ZVS condition and the Cr voltage is clamped at this level. In this mode, the Lr1 current decreases linearly because the constant voltage is placed across it. The Lr1 current is as follows: (10)This mode ends when ILr1 reaches zero and diode Dr turns off under ZCS condition so the duration of this mode is (11)Mode 3 [t2−t3]: This mode starts, when a current of Lr1 becomes zero and the diode Dr turns off under ZCS condition and ends when Lr2 current reaches Io and Do turn off under ZCS condition. The duration of this mode is calculated as follows: (12)Mode 4 [t3–t4]: At t3, the Lr2 current reaches Io and the diode Do current reaches zero, which causes to turn the diode Do off under ZCS. The operation of the converter in this mode is same as a conventional buck converter when its switch is on. Therefore, input energy transfers to the output. The duration of this mode is (13)Mode 5 [t4–t5]: At t4, switch S is turned off under ZVS due to the existence of Cr. In this mode, the output current discharges Cr linearly and this mode ends when Vcr reaches zero. The important equations of this mode are obtained as follows: (14) (15)Mode 6 [t5–t6]: In this mode, diode Do turns on under ZVZCS condition and a resonance between Lr2 and Cr starts. During this resonance, Vcr increases and iLr2 decrease. The equations of this mode can be written as follows: (16) (17) (18)The duration of this mode is 1/4 resonance cycle and V1 is Cr voltage at the end of this mode (19) (20)Mode 7 [t6–t7]: When the current of the diode Do reaches Io, diodes D1 and D2 turn off at ZCS condition. The operation of the converter in this mode is same as a conventional buck converter when its switch is off. This mode ends, when switch S is turned on. The duration of this mode is given in the following equation: (21) 3 Design procedure The output filter inductor Lo and the output filter capacitor Co are designed like a regular buck converter and just the proposed snubber circuit consists of Cr, Lr1 and Lr2 should be designed. According to (6), V1 should be greater than Vin, in practice, this value is considered 2Vin. Cr provides the ZVS condition for the switch at the turn off instant so its value selected similarly to snubber capacitor [20] (22)where tf is the switch current fall time, Is is the switch current before turn off and Vs is the switch voltage after turn off. To guarantee soft switching Cr is considered larger than Cr,min. Lr2 provides the ZCS condition for the switch at turn on instant so its value designed similar to snubber inductor [20] (23)where tr is the switch current rise time, Vs is the switch voltage before turn on and Is is the switch current after turn on. To guarantee soft switching, Lr2 is considered larger than Lr2,min. To decrease switch current stress, I1 should be considered smaller than Io so Lr1 must be larger than the following equation: (24)Load limitation is obtained according to the following equation: (25)The current and voltage stresses of the switch and diodes of the proposed converter are illustrated in Table 1 also the switch voltage and current stresses of the proposed converter have been compared with another converter which introduced in [15-17, 19] in Table 2. Table 1. Current and voltage stresses of switch and diodes Elements Voltage stress Current stress s Vin + V1 I3 Do Vin Io Dr Vin V1/Z1 D1 Vin + V1 Io D2 V1 Io Table 2. Compared of converter [15-17, 19] with a proposed buck converter Converter Voltage stress of switch Current stress of switch Number of auxiliary elements buck converter introduced in [15] 4 buck converter introduced in [16] 6 buck converter introduced in [17] 8 buck converter introduced in [19] 6 buck converter with proposed snubber 6 The control circuit of the proposed implemented buck converter is shown in Fig. 4. Fig. 4Open in figure viewerPowerPoint Control circuit of the proposed implemented buck converter 4 Experimental result A prototype of the buck converter with proposed snubber cell is implemented. The converter input voltage is 48 V, the output voltage is 24 V, output power is 100 W and the switching frequency is 100 kHz. The photograph of the implemented converter is shown in Fig. 5. According to Section 3, the value of Lr1, Lr2 and Cr are equal to 7, 16 µH and 6.8 nF, respectively. Furthermore, the value of the output filter inductor Lo and capacitor Co are 500 µH and 470 µF, respectively. IRF260 is chosen for the converter switch and all diodes are MUR820. The core of Lr1 is T50-26 and the core of Lr2 is T80-26. Fig. 6 shows the voltage and current of a switch. It can be observed that switch is turned on under ZCS condition and turned off under ZVS condition. As can be seen in the figure, there is a 20 V over shoot on the switch voltage that is because of resonant between switch parasitic capacitor and Lr2. Fig. 7 shows the voltage and current of Do, it can be seen that the diode turns on under ZVZCS and turns off under ZCS condition. Voltage over shoot is due to resonant between diode Do parasitic capacitor and Lr2. The voltage and current of Dr, D1 and D2 are presented in Fig. 8–10, respectively. As seen from the figures, diodes turn on and off under soft switching condition. Moreover, the voltage and current of the proposed converter in minimum load which is obtained from (25) and 75% of full load are presented in Figs. 11 and 12, respectively. As it can be seen, the soft switching conditions are also provided in these loads. Fig. 5Open in figure viewerPowerPoint Photograph of the implemented proposed buck converter Fig. 6Open in figure viewerPowerPoint (Top) voltage and (bottom) current of the switch (voltage: 50 V/div; current: 2 A/div; time scale: 1 µs/div) Fig. 7Open in figure viewerPowerPoint (Top) voltage and (bottom) current of the diode Do (voltage: 20 V/div; current: 2 A/div; time scale: 1 µs/div) Fig. 8Open in figure viewerPowerPoint (Top) voltage and (bottom) current of the diode Dr (voltage: 50 V/div; current: 1 A/div; time scale: 1 µs/div) Fig. 9Open in figure viewerPowerPoint (Top) voltage and (bottom) current of the diode D1 (voltage: 50 V/div; current: 2 A/div; time scale: 1 µs/div) Fig. 10Open in figure viewerPowerPoint (Top) voltage and (bottom) current of the diode D2 (voltage: 50 V/div; current: 2 A/div; time scale: 1 µs/div) Fig. 11Open in figure viewerPowerPoint (Top) voltage and (bottom) current of the switch in Io = 2 A (minimum load) (voltage: 50 V/div; current: 2 A/div; time scale: 1 µs/div) Fig. 12Open in figure viewerPowerPoint (Top) voltage and (bottom) current of the switch in Io = 3 A (75% load) (voltage: 50 V/div; current: 2 A/div; time scale: 1 µs/div) 5 Efficiency Elements losses of the proposed converter are given below. Coper losses of the inductor are as below equations: (26) (27) (28)where RLr1, RLr2 and RLo are resistances of inductors Lr1, Lr2 and Lo, respectively. Core losses of Lr1 and Lr2 are obtained as follows: (29)where B is the flux density in gauss and f is the switching frequency in hertz and core losses will be obtained in mW/cm3 which should be multiplied by the volume of the core to account losses in watt. Furthermore, Due to the current of Lo is in continuous conduction mode, core loss is negligible. Also, all semiconductor devices turn on and off in soft switching, so switching losses can be discarded (30) (31) (32) (33) (34)Also losses of conventional buck converter can be written as follows: (35) (36) (37) (38)The conductive losses of the switch are almost equal to (30). Also, the inductor losses and switching losses of Do in off instant can be obtained from (28) and (38), respectively. In order to compare the efficiency of the proposed converter with a conventional buck converter, the prototype converters are simulated by PSpice. The corresponding values of the equations above in the theory and according to the simulation results are presented in Table 3 that shows 5 W losses in the proposed buck converter and 7.8 W losses in its hard switching counterpart which matches with the simulation results Also the efficiency is measured at five different loads and as it can be seen in Fig. 13 efficiency of the proposed buck converter increased 4% in full load. The efficiency has >4% improvement at the lighter loads. Table 3. Losses of the proposed buck converter compared with the hard switching counterpart in theory and simulation Losses Theory, W Simulation, W Proposed buck converter Hard switching counterpart Proposed buck converter Hard switching counterpart Ploss,core L1 0.023 0 0 0 Ploss,coper L1 0.00006 0 0 0 Ploss,core L2 0.12 0 0 0 Ploss,coper L2 0.016 0 0.015 0 Ploss,core Lo 0 0 0 0 Ploss,coper Lo 0.812 0.812 0.9 0.9 Ploss,s 0.44 0.44 0.4} 3.5 Ploss,s,on 0 1.55 0 Ploss,s,off 0 1.39 0 Ploss,Do 1.65 2.25 1.5} 3.8 Ploss,Do,on 0 0.675 0 Ploss,Do,off 0 0.675 0 Ploss,D1 0.18 0 0.1 0 Ploss,D1,on 0 0 0 0 Ploss,D1,off 0 0 0 0 Ploss,D2 1.69 0 1.5 0 Ploss,D2,on 0 0 0 0 Ploss,D2,off 0 0 0 0 Ploss,Dr 0.095 0 0.1 0 Ploss,Dr,on 0 0 0 0 Ploss,Dr,off 0 0 0 0 Fig. 13Open in figure viewerPowerPoint Efficiency of the proposed buck converter compared with conventional hard-switching buck converter 6 Conducted EMI measurement To measure the conducted EMI a CISPR 22 LISN (Line Impedance Stabilization Network) inserted at the input of the converter [21] and GWINSTEK GSP-830 spectrum analyser placed on Rs as shown in Fig. 14. Resolution bandwidth was adjusted to 30 kHz and conducted EMI measured from the frequency range of 150 kHz to 30 MHz according to CISPR 22. The effectiveness of the proposed snubber cell to reduce conducted EMI is shown in Fig. 15. As it can see the buck converter with proposed soft-switching method reduces the EMI peak by 8 dB µV as compared to the conventional buck converter. Fig. 14Open in figure viewerPowerPoint CISPR 22 LISN model for conducted EMI measurement Fig. 15Open in figure viewerPowerPoint Measured conducted EMI of (a) Buck converter with proposed snubber, (b) Conventional buck converter. Vertical axis: 20–100 dB µV; horizontal axis: 150 kHz–30 MHz 7 Other converters using a proposed snubber The proposed snubber can be extended to other basic non-isolated DC–DC converters such as boost, buck–boost, Cuk, SEPIC and zeta. The proposed converters are shown in Fig. 16. The proposed snubber needs a reference voltage that voltage of Cr capacitor clamps on it. This value in buck converter is Vin. For the correct performance of the converter, this reference voltage in buck–boost and zeta converters are also equal to Vin. The reference voltage for the boost converter is equal to Vo and in Cuk and SEPIC converters are equal to capacitor voltage in their structure that is large enough so that it can be assumed that their voltage is constant. The operation of the snubber circuit is the same as the operation in the buck converter as discussed in Section 2. Fig. 16Open in figure viewerPowerPoint Other converter used a proposed snubber (a) Boost, (b) Buck–boost, (c) Cuk, (d) Zeta, (e) SEPIC 8 Conclusion In this paper, a lossless passive snubber cell is proposed to provide ZCS turn on and ZVS turn off condition for the power switch. The proposed snubber could be applied to basic DC–DC converters. All diodes in the proposed snubber operate under ZCS and therefore the reverse recovery problem does not exist. The proposed snubber is applied to the buck converter and analysed completely. The experimental results of the implemented converter confirm the theoretical analysis. The losses of the implemented converter theoretically calculated that shows 5 W losses in the proposed buck converter which matches with the simulation results and shows efficiency increased 4% in full load. Also measured conducted EMI of the implemented converter reduces the EMI peak by 8 dB µV as compared to the conventional buck converter. 9 References 1Emrani A., and Farzanehfard H.: 'Zero-current switching resonant buck converters with small inductors', IET Power Electron., 2012, 5, (6), pp. 710– 718 2Chung J.K., and Cho G.H.: 'A new soft recovery PWM quasi-resonant converter with a folding snubber network', IEEE Trans. Ind. Electron., 2002, 49, (2), pp. 456– 461 3Park C., and Choi S.: 'Quasi-resonant boost-half-bridge converter with reduced turn-off switching losses for 16 V fuel cell application', IEEE Trans. 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