7 CHAPTER 2 2. 1 Literature Review 2.1 Introduction In this chapter , a comprehensive review of the existing MIC topologies is conducted. Simulation is used for in - depth analysis and comparison. Following the work in [ 22 ] and [ 23 ] , t he MIC topologies are clas sified in to three main categories: (1) MIC with DC link; (2) MIC with current - unfolding; (3) DC link - less MIC. The study compares various topologies in - depth with respect to efficiency, compactness, reliability, cost effectiveness and ease of implementati on. Based on the comparison, several topologies are recommended to be the potential candidates for practical implementation. Simulation is adopted to support the analysis. 2.2 MIC with DC Link The general topology of a MIC with DC link is shown in Fig. 2 . 1 . The DC - DC step up converter increases the PV module voltage (usually in the 22Vdc – 45Vdc range) to a 1 A version of this chapter has been published in the IEEE IECON Conference Proceedings. F. Edwin, W. Xiao, V. Khadkikar, “Topology Review of Single Phase Grid - Connected Module Integrated Converters for PV Applications,” IEEE Industrial Electronics Conferen ce IECON, Oct. 2012, pp. 821 – 827. CHAPTER 2: LITERATURE REVIEW 8 voltage above the peak grid voltage. An inverter then converts this DC voltage to the ac - line voltage through sinusoidal pulse width modulation (SPWM). The DC link capacitor stores energy and is responsible for the power decoupling. The topology of the DC - DC converters varies from case to case. Four configurations are studied in the following section. = = = G r i d D C - D C S t e p u p c o n v e r t e r D C - A C I n v e r t e r D C - l i n k S o l a r P V m o d u l e Fig. 2 . 1 : MIC with DC link The topology in Fig. 2 . 2 was proposed in [ 24 ] . The inverter stage utilizes a technology called zero - voltage resonant transition (ZVRT) operating at a switching frequency of 25 kHz. The control strategy is optimized to minimize the inverter loss. The prototype is rated at 200W and dem onstr ates a peak efficiency of 96% including drivers and control logic. However, this topology does not offer galvanic isolation. = = G r i d D C - D C S t e p u p c o n v e r t e r D C - l i n k S o l a r P V m o d u l e Q b C r L b L a Q a L d Q 1 Q 4 Q 3 Q 2 Fig. 2 . 2 : Topology proposed in [ 24 ] In [ 25 ] , a high efficiency transformerless dc - dc boost converter for MIC is proposed, as shown in Fig. 2 . 3 . The topology is based on a ZETA boost converter, but the input inductor is replaced by a coupled inductor. The main advantage of this topology is CHAPTER 2: LITERATURE REVIEW 13 reliability and efficiency. A conversion efficiency of 70% is reporte d for the 100W prototype. Q 1 C 1 T x D 1 C 2 Q 2 D 2 Q 3 L f G r i d D 1 C x Q x Fig. 2 . 8 : Flyback inverter with second - harmonic injection proposed in [ 32 , 33 ] The topology shown in Fig. 2 . 9 is an interleaved flyback with current - unfolding [ 34 ] . The current ripple through the source is reduced as a result of the interleaving action. Thus the cu rrent ripple through the decoupling capacitor is reduced. This could improve system reliability. A peak efficiency of 94% is reported for a 195W prototype switching at 172 kHz. Q 1 C 1 T x D 1 C 2 D 2 L f G r i d Q 2 T x C u r r e n t - u n f o l d e r Fig. 2 . 9 : Interleaved flyback with current - unfolding [ 34 ] CHAPTER 2: LITERATURE REVIEW 14 In [ 35 ] , a three - stage MIC with current - unfolding is presented and shown in Fig. 2 . 10 . The first stage of this topology is a current source push - pull converter which raises the voltage. A full - wave rectifier at the secondary feeds a sine - wave modulate d buck converter. The current - unfolding circuit subsequently injects the current at line frequency in to the grid. The reported prototype achieves an 80% overall converter efficiency including the control circuitry. Table 2 . 2 summarizes the comparison of various MIC topologies with current - unfolding circuit. Q 1 D 1 C 2 D 2 L f G r i d C u r r e n t - u n f o l d e r L 1 Q 2 C 1 Q 3 L 2 B u c k c o n v e r t e r P u s h p u l l b o o s t c o n v e r t e r D 3 Fig. 2 . 10 : MIC with current - unfolder proposed in [ 35 ] Table 2 . 2 : Comparison of MIC topologies with current - unfolding MIC Topology Fig. 2 . 7 Fig. 2 . 8 Fig. 2 . 9 Fig. 2 . 10 Power rating 200W 100W 195W 300W Number of active switches 3 4 6 6 Number of discrete diodes 2 3 2 4 Number of copper windings 3 3 5 7 Number of magnetic cores 1 1 2 1 Soft - switched? No No No No Switching frequency 32kHz 20kHz 173kHz 20kHz Maximum reported efficiency 96% 70% 94% 80% CHAPTER 2: LITERATURE REVIEW 19 expectancy of MIC systems. Therefore, an improved circuit topology has been proposed in [ 42 , 43 ] to minimize the decoupling capacitance and expect high - power density and long life time. Research also focuses on developing new topologies to improve conversion efficiency [ 25 , 26 , 29 , 39 , 44 , 45 ] . Others aim at the objectives of reducing total harmonic distortion (THD) [ 46 ] , a nd/or improving power factor [ 47 ] . The MIC topology review and comparison have been conducted in [ 4 , 22 , 23 , 31 ] . The study in [ 22 ] summarizes power rating, component count, efficiency, and PCB size of different inverter topologies. In the conclusion, the pseudo dc - link and dc l ink - less inverters are considered to possess a strong potential for future research interest. Furthermore, a comprehensive comparison of MIC topologies is reported in [ 23 ] including power rating, component count, switching frequency, soft - switchi ng capabil ity, and efficiency. It emphasizes that the flyback MIC (FMIC) topology with current-unfolding shows low component count and potential for high efficiency and reliability. Based on the previous studies, and for the reasons expounded above, the author decid ed to focus attention on the flyback MIC with current - unfolding. 2.6 Analysis and S imulation of F lyback MIC with Current - Unfolding in DCM In this section , a more in - depth analysis , a nd simulations of the flyback inverter with current - unfolding will be conducted. The converter is shown in Fig. 2 . 7 and it operates in DCM . The basic operation of the flyback inverter is as follows. With respect to Fig. 2 . 7 , t he m ain switch Q1 operates with variable duty cycle. The d uty cycle varies in a sinusoidal manner with modulation index proportional to the MPPT . D uring the positive grid half - cycle, Q2 is switched on (Q3 off) and the upper half of the unfolding circuit operates, injecting current in to grid. During the negative half - cycle, Q3 is switched on (Q2 off) and the lower half of the unfolding circui t CHAPTER 2: LITERATURE REVIEW 20 operates, injecting current in to grid in the reverse direction. The inverter operate s in DCM, but it has been shown in [ 48 ] that the inverter can also operate in CCM. Fig. 2 . 16 : Magnetizing inductance current in DCM The condition for DCM, as shown in Fig. 2 . 16 , is that: f sw on t T T  ( 2 . 1 ) Where t f is the switch off - time (s), T s w is the switching period (s), and t on is the switch on - time occurring for ω t = π /2 , with ω being the grid angular frequency (rad/s). Following the derivations in [ 30 ] during any switching period, the relation between the peak current through the primary winding and the duty cycle is expressed as:   , () pv prim pk m sw V i t d t Lf  ( 2 . 2 ) where i prim,pk (t) represents the set of points forming the primary winding current waveform envelop, as shown in Fig. 2 . 16 ; V pv is the PV module voltage (which is assumed constant) (V); L m is the magnetizing inductance of the flyback transformer (H ); f sw is the switching frequency (Hz), and d(t) is the instantaneous duty cycle which can be defined as:   ( ) sin 2 p sw m avg p pv d t d t f L P d V           ( 2 . 3 ) 1/2 1 3/2 2 0 Ip  t (  ) i prim (t) T o n T f T s w T s w > T o n + T f CHAPTER 2: LITERATURE REVIEW 27 particularly the flyback MIC show strong potential for future development and applications. The flyback MIC in a DCM scheme is examined in a more detailed fashion and simulated. Its o pen loop current control loop shows good performance. Unlike the DCM, the CCM scheme could show more modeling and control complexity; however CCM could lead to higher efficiencies and lower switch stress than in DCM.