A COMPARATIVE STUDY OF SEPIC, CUK AND ZETA CONVERTERS

Transcription

1 cientific Bulletin of the Electrical Engineering Faculty 2008 A OMPARATIVE TUY OF EPI, UK AN ZETA ONVERTER Florian ION, Gabriel PREUA 2 Abstract: In this paper a comparative study of - converters is presented. The EPI, uk and ZETA converters in applications are detailed. Also are presented the operating simulation for these converters. The results of simulations are compared with the measurements done for a ZETA converter with an output of 3.3V and different output currents. Keywords: - converters, EPI converter, uk converter, ZETA converter 2.. OPERATING PRINIPLE 2. Fundamental - onverters From the viewpoint of input and output voltages V and V o, the fundamental converters are: () step-down or buck converters, (2) step-up or boost converters, (3) step down/up or buck-boost converters. Figure shows the principle diagrams of these topologies.. INTROUTION It s well known that to have the maximum efficiency of the solar panels, the load must be connected to the solar panel through a - converter. The topologies and the operation of these converters are very well described in the literature. A classification of these converters is presented in []. The authors of [] consider the - converters in six decades: () classical/traditional converters, (2) multiple-quadrant converters, (3) switched component converters, (4) softswitching converters, (5) synchronous rectifier converters, (6) multiple energy-storage elements resonant converters. The classical/traditional converters are divided in five categories: () fundamental converters, (2) transformer-type converters, (3) developed converters, (4) voltage-lift converters, and (5) super-lift converters. The converters studied in this paper are classical developed converters well known in literature like converters in EPI (ingle Ended Primary Inductance onverter) topology, uk and ZETA (Positive Output Luo onverter). The developed-type converters derived from fundamental converters by addition of a low-pass filter. In [2] these converters are considered like a MATER converter switched by a PWM signal, and a LAVE converter achieved with passive components. Because the great usage of converters in the topologies mentioned above in applications, we found opportunity for a short presentation of operating principles, simulations and some experimental results in this work. The equations of main operating parameters, advantages and disadvantages of each topology are presented in chapter 2 of this paper. The simulations and the measurements are presented in chapter 3, and the final conclusions in chapter 4. a) b) c) Figure. Fundamental - onverters: a) buck converter, b) boost converter, c) buck-boost converter. In ideal operating conditions (no voltage loss on the switch, the average voltage across inductors L at steady state zero, no current loss on capacitors, and no voltage loss on diode at forward conduction) the equations of ratio V o /V are: for the buck converter (): V o = () where is the duty cycle of PWM signal of switch, with the meaning from equation (2), ton = (2) T o o o Valahia University/Electronic epartment, Targoviste, Romania, flion@valahia.ro 2 Valahia University/Electronic epartment, Targoviste, Romania, gpredusca@valahia.ro 7

2 cientific Bulletin of the Electrical Engineering Faculty 2008 where t ON is the conduction time of switch and T is the period of PWM signal. for the boost converter (3): V o = (3) V for the buck-boost converter (4): V o = V (4) In all these equations the internal resistance of power supply V was considered zero [3]. a) o o 2.2. eveloped - converters b) Figure 2 shows the topologies of developed - converters. These topologies have few similitudes: The equation of the transfer function of these converters is (4), the same with that one of buck-boost converter, if the conditions of ontinuous onduction Mode M are assured. These converters are used in different applications, such as with solar panels, in systems supplied with electrical energy where the output voltage V o of converter can be superior or inferior of the input voltage V of converter. The fundamental converters don t accept this situation. These converters are integrated in the MPPT of solar panels. The capacitor assures the galvanic insulation between input and output. The short-circuits or others breakdown of the load don t affect the power supply solar panels. The output voltage becomes zero if the PWM control signal of switch is missing. The diode can be replaced by a transistor switched synchronal with the main switch in the synchronous converters. The differences between these topologies are: EPI and uk converters became from the boost converter, and ZETA converter from the buck-boost converter. The ripple current in the load is greater for uk and ZETA converters than EPI, because the EPI converter has an inductor L 2 that smooth the current spikes. The switch of EPI ad uk converters is a N channel MO transistor that needs a Low ide driver, when the ZETA converter has a P channel MO transistor that needs a High ide driver. Because, these three topologies have many advantages mentioned above, these things make enable their integration in applications with a great efficiency of using the solar energy in solar panels with MPP trackers. c) Figure 2. eveloped - onverters: a) EPI converter, b) uk converter, c) ZETA converter 2.3 Integration of convertors in MPPT systems The perturb-and observe (PAO) method for the MPPT is an iterative approach. The MPP is obtained by making the derivate of power equal with zero in the feedback circuit that commands the duty cycle of switch. This is very useful because doesn t need the disconnection of panels from the load. Through this method can be reached good results if it is compared the instantaneous conductance of panel with the incremental conductance of panel the method is known as Incremental onductance Technique (IT) [4]. If it is considered the equivalent circuit of the solar panel like in Fig. 3, with v i the input voltage of panel and r i the equivalent input resistance of panel, P i the input power, P o the output power (5), the P = 0 means (6). 2 vi Pi = Po = (5) ri vi Vi = (6) ri 2Ri The method proposed in [4] resides in the connection of a EPI or uk converter between the solar panel and load. The converter works in continuous current mode (M) through inductor L Figure 2 a), but with discontinuous voltage (V) on the capacitor. The duty cycle of PWM signal of switch is adjusted in a proper way to achieve the input resistance of converter equal with the output resistance of solar panel. Figure 3 shows the equivalent circuit of solar panel and converter. o 8

3 cientific Bulletin of the Electrical Engineering Faculty 2008 Figure 3. Equivalent circuit of a solar panel and converter [4] The operating equations of EPI converter in V mode are the next: (7a) the voltage on capacitor, (7b) the voltage on diode. v I( d) T V ( t) = Vo, I ( t dt ) V o o, I 2 t, 0 < t < dt d T < t < dt dt < t < T (7a) v V o + v ( t), 0 < t < dt ( t) = (7b) 0, dt < t < T where I and I 2 are the inductor currents - assumed to be constant, dt is the conduction time of switch, d T is the conduction time of diode, and T is the period of PWM signal of switch - T =, f frequency of f PWM signal. The three sequences in one switching cycle are shown in Figure 4. Because the voltage of capacitor at d T I is v ( d T ) = V o, the duty cycle is d = ( d). I In the steady state the voltage on the inductor L 2 is zero. From this moment the output voltage V o is equal with the average voltage of diode (8). V o dt T = v ( t) dt I( d) d T = (8) 0 2 Moreover, the voltage stress on the switch is given by (9). In the same way can be determined the operating equations of uk and ZETA topologies. This is shown in [4]. I v stres = v ( T ) + Vo = ( d) T (9) 2 Figure 4. Operating principle of the EPI converter. a) equivalent circuits, b) theoretical waveforms [4] 3. imulations In this chapter it will be presented few representative waveforms of each topology. The simulations were done in OrA, in the next conditions: input voltage V =2V, output voltage V o =3.3V, load resistance R =3.3Ω, duty cycle =0.22, switching frequency f =500kHz, coupling capacitor =47μF, output capacitor o =00μF, inductors L =L 2 =6.2μH. 3. The EPI converter To simulate the operation of EPI converters it was used the diagram from Figure 2 a). In Figure 5 a), b), and c), is shown the output voltage V o, the ripple of 9

4 cientific Bulletin of the Electrical Engineering Faculty 2008 output voltage ΔV o, and the voltage stress V stress of switch. At steady state after.5ms, these values are: V 0 =3.5V, ΔV o =6mV pp, and V stress =6V s 0.5ms.0ms.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms a) V o output voltage s 0.5ms.0ms.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms a) V o output voltage -3.65V V ms ms ms.0000ms.0500ms.000ms b) ΔV o output ripple V 2.960ms 2.962ms 2.964ms 2.966ms 2.968ms 2.970ms 2.972ms 2.974ms 2.976ms 2.978ms b) ΔV o output ripple 5V 2 5V 5V us us us us 88.00us us us us V(M2:d) c) V stress voltage stress on switch 5V ms ms ms ms ms ms V(M2:d) c) V stress voltage stress on switch Figure 5. imulated waveforms of EPI converter: a) output voltage, b) output ripple, c) voltage stress of switch 3.2. The uk conveter To simulate the operation of uk converter it was used the diagram from Figure 2 b). Figure 6 a), b) and c) shows the waveforms of output voltage, output ripple and voltage stress of switch in the same conditions. At steady state after 0.5ms, these values are: V 0 =3.5V, ΔV o =28mV pp, and V stress =6V The ZETA converter To simulate the operation of ZETA converter it was used the diagram from Figure 2 c). Figure 7 a), b) and c) shows the waveforms of output voltage, output ripple and voltage stress of switch in the same conditions. At steady state after 0.5ms, these values are: V 0 =3.4V, ΔV o =26mV pp, and V stress =6V. Figure 6. imulated waveforms of uk converter: a) output voltage, b) output ripple, c) voltage stress of switch onclusions on simulations are in Table. Table. imulation Results Voltage Topology EPI uk ZETA V o [V] ΔV o [mv pp ] V stress [V] Experimental verifications An experiment has been performed using a ZETA topology on a LT622 a urrent Mode tep-own / converter of Linear Technology [5]. The schematic diagram is the typical application proposed by the producer and is shown in Figure 8 [6]. In Figure 9 are the waveforms in the next conditions: V =8.3V, V o =3.3V, R =20Ω (I o =65mA). The channels of oscilloscope represent: H V G PWM signal on the gate of MO transistor, H2 V drain voltage of MO transistor, H3 V voltage 20

5 cientific Bulletin of the Electrical Engineering Faculty 2008 on positive pin of capacitor, H4 ΔV o output voltage ripple. In Figure 0 are the waveforms in these conditions: V =4.9V, V o =3.4V, R =0Ω (I o =340mA). It can be observed other values for switching frequency and duty cycle of PWM signal systems where these conditions are reached very often like solar panels in different levels of solar radiation. The results of measurements are in Table 2. The circuit LT622 changes the switching frequency Figure 9, 0,, in a wide range from 265 khz to 024 khz. Also, the duty cycle of PWM signal in the gate of the MO transistor is changed according with the work conditions input and output voltage, and the output current, to insure a constant value of output voltage. The ripple of output voltage measured in real conditions is few times greater than that one obtained in ideal conditions of simulations. 0s 0.5ms.0ms.5ms 2.0ms 2.5ms 3.0ms 3.5ms 4.0ms V(R8:2) a) V o output voltage 3.62V 3.6V V 4.65ms 4.70ms 4.75ms 4.80ms 4.85ms 4.90ms 4.95ms 5.00ms V(R8:2) b) ΔV o output ripple 2 5V Figure 9. Waveforms of V G, V, V, ΔV o in conditions: V =8.3V, V o =3.3V, R =20Ω 5V 4.650ms 4.652ms 4.654ms 4.656ms 4.658ms 4.660ms V(M:s)- V(M:d) c) V stress voltage stress on switch Figure 7. imulated waveforms of ZETA converter: a) output voltage, b) output ripple, c) voltage stress of switch Figure 8. ZETA converter with LT622 [9] The experimental waveforms in the conditions: V =7.V, V o =3.4V, R =5Ω (I o =680mA) are shown in Figure. It can be seen that the ZETA converter build with the LT622 integrated circuit works well with input voltages less than output voltage and at an input voltage over the output voltage. This advantage can be used in Figure 0. Waveforms of V G, V, V, ΔV o in conditions: V =4.9V, V o =3.4V, R =0Ω 2

6 cientific Bulletin of the Electrical Engineering Faculty 2008 REFERENE Figure. Waveforms of V G, V, V, ΔV o in conditions: V =7,V, V o =3,4V, R =5Ω [] F. L. Luo, H. Ye, Advanced / onverters, R Press, [2]. Maniktala, lave onverters Power Auxiliary Outputs, EN Magazine, Elsevier, [3] P. onstantin, et al., Electronică industrială, Editura idactică şi Pedagogică, Bucureşti, 983. [4] H. -H. hung, et al., A Novel Maximum Power Point Tracking Technique for olar Panels Using a EPI or uk onverter, IEEE Transaction on Power Electronics, Vol. 8, No. 3, May [5] M. obre, F. Ion - supervisor, orecţia factorului de putere în convertoarele în comutaţie cu reţele de comutare de ordin zero, Proiect de diplomă, Universitatea Valahia, Târgovişte, iulie ***, LT622 Low Voltage Input urrent Mode tep- own / ontroller, ata heet, Linear Technology, 998. Table 2. Measurement results ZETA converter with LT622 with output voltage 3.3V Voltage V =8.3V, V =4.8V, V =3.2V, I o =65mA I o =65mA I o =65mA ΔV o [V pp ] f [khz] Voltage V =8.3V, V =4.9V, V =8.3V, V =7.V, I o =340mA I o =340mA I o =680mA I o =680mA ΔV o [V pp ] f [khz] ONLUION In this paper was presented a comparative study of - converters in EPI, uk and ZETA topologies. It was studied the fundamental converters and developed converters in the topologies mentioned above. The operation equations of main parameters were presented. Moreover, it was presented the simulations of these converters in the same work conditions. The waveforms that have seen on a ZETA converter with a constant output voltage and variable input voltage and load confirmed the simulations results of that converter. 22