6 CHAPTER 2 2 Literature Review This chapter provides a literature review , focusing on flyback topology in PV applications. F lyback converter topology is considered a popular solution for photovoltaic (PV) applications and the topology fundamentally has three operation modes: discontinuous conduction mode (DCM) [23 - 28, 31] , boundary conduction mode (BCM) [32 - 34] , and continuous conduction mode (CCM) [29, 35, 36] . Th is study gives an insight into the propose d modification of topology and compares them with respect to practical complexity, reliability, and efficiency. 2.1 Flyback converters in PV applications Fig. 2 . 1 shows the basic configuration for a flyback converter . Apart from the input and output capacitors, the topology only has one transformer, one power switch, and one unidirectional switch or a diode. Its brief operation funda mentally ha s two operation scene s, depending on the switch status . Firstly , the switch is activated. Current from the power source rushes into its path, going through the transformer and the switch. As the switch conducts, the current rises linearly in proportion to magnetizing inductance of the transformer . 7 Once the switch has been turned off , the current level through the transformer cannot simply disappear. Shown in Fig. 2 . 1 , the magnetizing inductance is represented as an inductor paralle l to the ideal transformer. The current being uphe ld by the inductance starts to circulate into ideal transformer’s negative terminal instead. As the current flows out from the transformer’s secondary negative terminal, the diode conducts and thus delivers the energy to the output side . As the transformer is connected to the output, the output terminal voltage reflects onto the transformer , and thereby the magnetiz i ng inductor as well. The secondary side current level then decreases linearly. INPUT TERMINAL OUTPUT TERMINAL POWER DIRECTION Fig. 2 . 1 : Fundamental flyback converter. Working with this fluctuation of curr ent in the transformer , there are three modes of operation: discontinuous conduction mode (DCM), boundary conduction mode (BCM), and continuous conduction mode (CCM). The difference is the level of current at the end of each period during a steady state. It is considered to be in DCM if it drops to zero before reaching another period, and CCM if otherwise. BCM is when the current reaches zero as the switch is activate d . Flyback converters are po pular in PV applications for their simp licity in hardware structure. They also provide galvanic isolation, solving ground capacitor and line 11 achieved . Additionally, it has fast dynamic, has no reverse recovery issues, and requires a smaller transformer [37] . Although there are disadvantages associat ed with the mode, most of them can be handled in the design stage [37] . In contrast to common belief in its model, the dynamic model is actually the same as that of the CCM [44] . Since the current increases linearly to the input voltage, the flyback inverter in D CM works perfectly without a current sensor [23] . Although it reduces the cost on sensor devices, the input capacitor has to be large enough to minimize the ripple. As discussed in the study, there is no need to use it to perform MPPT. This issue, however, could be fixed by including voltage into the control signal calculation [43] . With the voltage t aken into consideration, the topology allows the magnitude of the ripple under 20% , which can produce the output current with 3.5% THD. The harmonics sharply increase with a stronger ripple. However, ripples do not affect only harmonics, but also cause a l ower power utilization level. In order to ensure proper MPPT, only 10% is tolerable as it results in an acceptable 1% utilization loss. Regarding studies in control issues, Zhang et al [33] suggest that, for ILFI, DCM has higher efficiency than BCM for lower than 200W instantaneous power . It is studied under the assumption that BCM has to vary its switching frequency to keep it in the mode and often has to use high frequency. It is also better to utilize only one set of flyback converter s when the instantaneous power is lower than 125W . This is because the switching loss is higher compared to the efficiency gain from sharing the loads. After the system pass es 125W points , th is study proposes the second flyback converter set with its own controller be used . The distribution of the reference signal, however, is not half by half. The study proposes that, if the total amplitude is 2, the reference amplitude for each 13 Kim et al [34] takes the active forward snubber, or active clamp circuit (ACC), idea and combines it with soft - switching. The idea is to allow bi - directional flow of current by putting only a rectifying MOSFET at the transformer’s secondary terminal. Unlike the previous proposal, this study activates the ACC only at high power to preserve energy. A capac itor is put in parallel with the main switch to serve as a low power clamp system. The rectifier switch has two functions: to provide low resistance path for the secondary current, and to perform soft - switching. Its first duty is done by activating the swi tch until the secondary current reaches zero. This is estimated by sensing the input voltage for accumulated energy and output voltage for the degrading rate. The switch is activated again as the system reaches another period and is activated for a short t ime depending on the line voltage. As the rectifier switch activat es , the current flows from the grid into the transformer. Thus the main switch of the body diode is force - activated to allow reverse direction current flows. With the diode activated, the vo ltage across the main switch disappears. Thus, soft switching is achieved and t h is work confirms that the scheme can improve low power region efficiency [24] . Fig. 2 . 4 shows the three - port flyback inverter proposed by Hu et al [28] . This topology exploit the idea of the differences in instantaneous power level between the DC sourc e and the AC output. The control system divides the operation into two modes: when the DC power is higher than the output and vice versa. The decoupling capacitor (Cd) automatically takes energy from the leakage inductance when it is in the first mode. The system then releases the energy from the capacitor by activating S2 and extending S1 operation after its presumed operation. In order to force deactivation of D2, a voltage control loop is required to ensure hig h voltage across the capacitor. 14 g V D 1 D 2 D 3 D 4 D 5 S 1 S 2 S 3 S 4 Cd C d Fig. 2 . 4 : Three - port flyback inverter proposed by Hu et al [28] . Another solution proposed is to combine the flyback converter with SEPIC topology [25] . This is a semi two - stage system proposed to reduce the input voltage ripple. With the combination of a SEPIC and a flyback inverter, the energy is drawn from SE PIC’s capacitor instead of directly from the input. Thus , the input voltage does not have as high a ripple as the one stage system , since the power flow from the source is smoother. As a result, this type of system allows lower capacitance values and makes the film capacitor a feasible choice. In this work, a snubber circuit consist ing of only a capacitor installed across the flyback switch is proposed. The leakage energy is taken into the capacitor once the flyback switch is automatically deactivated and g iven back to the linking capacitor. Although the resulting averaged efficiency is not as high as most of the studies, this topology has a considerable less complex control system compared to other clamping proposals . 2.4 Flyback converter in BCM BCM is the bor der mode between DCM and CCM. The current rises from zero and drops to zero in every period. Thus, like DCM, ZCS is done automatically. Although this mode 15 does not require a closed - loop control system to shape the current like DCM, its control system is ge nerally much more complicated. Modeling for BCM consists of two independent equations instead of one , since the control system has to vary both the duty percentage and switching frequency [45] . It also has issues about switching loss since it usually operates in varying high frequency; thereby its power density is lower than the DCM in low power levels [26, 32, 46] . Thus , a flyback inverter which operates BCM operation has been impractical until recently. [47] . It is more accep table , however, if it operates in a mixture with DCM with the power level in consideration [26, 46, 48] . Gao et al [47] offers a detailed analysis on an ILFI under BCM operation. The mathematical models are de scribed in detail , along with other information related to the operation. System hardware design procedures f or each of the parameters are also derived. Regarding the control, the operation is divided into three regions. The first region is the blank mode. This is due to the fact that BCM operation utilize s high switching frequency , especially in low power , in or der to remain in BCM. Th erefore, the required frequency is not feasible when the instantaneous power is very low. The second zone is to utilize one flyback set under BCM. This mode goes on until the reference current crosses another value. This value could well be half - rated power as the next mode is activating another flyback set. The reference signal is divided by half for each flyback set with a master - slave relationship between their controllers. In this type of relationship , the system can utilize 2 - phase control by activating only one switch at a time to ease discontinuity in the output. The proposed control system utilizes the peak current mode (PCM) control by using comparators and flip - flops logic guided by reference and calculated by a processor embedded with the other necessary calculations. Th is work also 16 discover ed that the reference signal should not vary alone by a sinusoid term, but by two combined. The e xperimental stage shows promising results with high overall ef ficiency and also satisfying regulations. There are other works focusing on mixing DCM and BCM together in order to reach optimal operation. Kyritsis et al [26] weigh s up the advantages and disadvantages of the two modes. The study shows that DCM is preferable , si nce BCM complicates the system while still have the same disadvantages of DCM. The only point in which DCM is inferior to BCM , is in its ability to perform conversion for high power applications. BCM operation extends the limit of the devices and thus has higher capacity in terms of rated power. Considering power density, however, only in high power range is BCM better than DCM. This work claims BCM could have as high as 97% in the higher zone. Zhang et al [46] continue d the study on this optimization by further analyzing the control schemes for BCM operation. There are two major control operations for BCM: open - loop and closed - loop. The open - loop control scheme r elies heavily on predefined calculations and parameters. This is barely practical , especially if soft - switching is desired. Since BCM is the borderline between CCM and DCM operations , a minor mistake on the parameters, especially magnetizing inductance, co uld leap the system into instability if CCM is somehow achieved. Delay time is used to ease the concern, but at the cost of efficiency. The closed - loop scheme is the aforementioned PCM control. This type of control scheme takes the raw current value and co mpares it with the reference signal calculated from the MPPT scheme. In addition, the author proposes the phase synchronization scheme in case of an inductance mismatch by using the comparators output and calculat ing the phase error from the model mismatch. In this case , the system 18 st udies have been carried out on this, until recently [29, 35, 36] when a proposal for creating a low cost AC module without complex procedures of programming was originated . Li an d Oruganti [29] propose d design steps for a flyback inverter including control system design in detail. The control system, excluding the MPPT, consists of only an analogue representation of a type - 2 compensator and a rough mathematical model derivation is given. Unlike DCM and BCM operations, this control system for CCM works well in any modes without any stability issues. The experimental results show promising a future for the CCM flyback inverter as such a simple system could achieve high efficiency , while at the same time me et the requirements of the grid codes. Thang et al [36] shift the focus on detailed modeling for better control design and propose a DC current rejection system to cancel the DC curr ent injection , in case the system produces one. The resulting efficiency stays steadily on the 90% line , even in the lower power range. However, th is proposed is still composed of only analogue devices, providing no room for adjustments in functionality. I t is well known that controlling the output current is a better way of controlling flyback converter in CCM , due to its highly discontinuous nature of primary current [49] . The output current, however, could be sensed after the output filtering part, and therefore could be also implemented for discrete control system. Edwin et al [35] offer a mathematical model fo r the ILFI as a second order model. With th is model, a PI controller should be enough. The author claims, however, that the second order is not enough to control the output current since the sensed current contains the dynamics of the CL filter. Thus , the fourth order system is proposed and the type - 2 compensator is thereby required. The resulting efficiency, similar to the aforementioned works, is high , 1 9 with the value mostly running around 90% with the claimed peak at 95.7%. Sensing the output current, however, ceases the possibility of sharing the unfolding bridge with other MICs as proposed in [36] . 2.6 Summary This chapter include d a review of previous research regarding the flyback inverter for PV applications. The topology is organized by its operation modes into DCM, BCM, and CCM. While the early works focused on the DCM operation , BCM and CCM has also recently been gaining in attention . The DCM provides simplicity with high efficiency at low power range, but with high device stresses. The BCM increases the control difficulties in calculation and the DSP process, but being in betwe en CCM and DCM means that the mode also shares both of their advantages and disadvantages. Recently initiated, CCM flyback for PV application introduces itself to be another appealing choice , since it achieves high efficiency across all the power range, bu t it also has control issues . In addition, soft switching is not easily realized. Acknowledging this, the next chapter proposes a CCM flyback topology and its control scheme. The evaluation of the proposed system is then partitioned into two chapter s : theo retical and experimental.