Comparative Study Of Topologies And Modes Of Operation Of Flyback Converters In PV Applications

Operation Principle of Flyback Converters

The flyback converter refers to a buck-boost converter used for both AC-DC and DC-DC conversion where a galvanic isolation is implemented between the input and output. The inductor is split so that a transformer is created and operates in such a way that the voltage ratios are multiplied and also isolated at the same time, essentially achieving two results of stepping up and isolation using a single architecture (Whitaker, 2005). This section compares in-depth, the various topologies in the context of their efficiency, availability, compactness, cost effectiveness, and ease of implementation. Based on these comparisons and simulations for supporting the analyses, several topologies are recommended as potential candidates for practical implementation and modification of topology.  This chapter also provides a comprehensive literature review of the existing MIC topologies and the flyback topology in PV applications, as a theoretical foundation for the design of a step up DC-DC flyback converter step up applications.

The flyback converter operation principle is such that closing the switch causes the transformer primary to be connected directly to the input voltage source, resulting in an increase in the magnetic flux and primary current of the transformer; this causes the transformer to store energy. The secondary transformer winding gets a negative current induced to it reversing the diode and the output load is given energy by the output capacitor. Opening the switch causes a drop in the primary magnetic flux resulting in a positive secondary voltage making the diode forward-biased. This allows current flow from the transformer and the transformer energy recharges the capacitor, supplying the load with energy (Neacsu, 2014). Energy storage in the transformer before being transferred to the output converter allows multiple outputs to be generated by the topology with minimal extra circuitry albeit with the need for the output voltages have to be matched through the turns ratios. A controlling rail has to be loaded before applying load to the uncontrolled rails to allow the pulse width modulation opening up and to supply sufficient energy to the transformer. The flyback converter, according to Rashid (2007), is essentially  an isolated power converter with two control schemes of current and voltage mode control.

DC-DC flyback converters operate based on the principles of a switching regulator, which is a circuit that utilizes an inductor, a power switch, and a diode for the transfer of energy from an input to an output. The basic switching circuit components can be rearranged to create a flyback inverter, a boost (step-up) converter, and a buck (step-down) converter (Wens & Steyaert, 2011), (Chen, Huang & Yu, 2013). Micro converters (MIC) or AC modules are important components of photovoltaic power systems. MIC topologies can be classified as (1) MIC with DC link, (2) DC link-less MIC, and (3) MIC with current unfolding. Flyback converter topology is considered a popular and highly suitable solution for PV (photovoltaic) applications; the Flyback topology has three main mods of operation; the discontinuous mode (DCM) , the boundary conduction mode (BCM) , and the continuous conduction mode (CCM) (Corradini, 2015), (Ma, Tsui & Mok, 2003).

Advantages of Flyback Converters in PV Applications

Flyback converters are very popular for use in PV applications because of their simple design and also provide galvanic isolation that tackles ground separation issues and ground capacitor. Flyback converters have been proposed for use in PV applications as a module integrated converter (MIC) to increase PV systems energy capture (Linares, 2009). PV cells generate DC (direct current) power at a volt fraction, while most utilities use AC (alternating current) power with voltages above 100 V. as such PV installations connected to the grid need an inverter to convert power to AC from DC. Operating PV cells in string mode is inevitable due to non uniform radiation; this causes a problem termed the ‘string problem’. (Linares, 2009). Because of the wide variations in the levels of voltages when operating in ‘string mode’, an extra DC-DC converter to regulate the input DC voltage to the inverter is required. The string problem can be tackled and mitigated by using the MIC which tackles the string problem through the addition of DC-DC inverters at each PV panel output, resulting in each panel not being affected by its neighbors operating conditions. The MIC concept is used to transfer energy yields directly into the grid with only a flyback converter being installed between the source and the grid, a concept that has been researched considerably (Li & Wolfs, 2008), (Araujo, Zacharias, Sahan, Torrico-Bascope & Antunes, 2007), (Liang, Huang, Guo & Li, 2011), with the results showing the approach is feasible.

The flyback converters, using the fluctuation model, has three main modes of operation, including the DCM (discontinuous mode of operation, the BCM (binary conduction mode), and the CCM (continuous conduction mode). The difference between the three is the current level at the end of every steady state period. The DCM has a simple control loop and increases efficiency when load conditions are high. If current drops to zero before another period is reached, it is considered DCM. The DCM is feasible only in situations of low power PV applications and power losses occur only during turn-off transition. The main current harmonics does not depend on ambient temperature changes or irradiation and makes filter design easy (Erickson & Maksimovic, 2012). BCM occurs when current zero when the switch is activated; this makes it a feasible design for higher power levels. BCM has more complicated controls since it employs variable switching frequency. Both transformer current paths must be sensed meaning that this design must have more components, requires more complicated algorithms; these make it less reliable but more expensive (Belu, 2013). The design of the output filter gets more complicated since the required cut-off frequency must cater for the load conditions.

Modes of Operation of Flyback Converters

However, the BCM design doubles processed inverter power meaning that higher energy densities are higher. This is a good fit for PV applications where small volumes are desired. The design of the converter is very crucial since transformer current is not measured meaning that the flyback may enter into the CCM; CCM is not desirable for PV flyback operation. For DC-DC converter operation, a constant value is used as the reference; however, in inverter application, the reference is alternating signal following the desired frequency. Designing the controller becomes more complex for the inverter because the controller must handle varying reference signals.  Hardware design is a challenge; however, a proposed solution by Nanakos, Tatakis & Papanikolaou (2012) entails gathering all sources of losses and then minimizing them; however, this poses a challenge for optimization taking into account the cost aspects. The capacitor design section is also another challenging area in flyback converter hardware design; the flyback inverter in DCM mode has four harmonics sources; control signal, switching, DC link voltage ripple, and grid voltage distortion.  Switching is inherent in the flyback topology and is in high frequency and as such, needs to be filtered via the output CL filter. Control signal harmonics appear when the lookup table resolution is poor and this can be solved by increasing the table size. The input voltage ripple should not be more than 5% so that the THD limit can be maintained according to the IEC standard 61000-3-2 when in DCM mode . the use of open loop control results the harmonics effects from the ripple

The ILFI (interleaved flyback inverter) is a modification of flyback design procedures and is attained by paralelling two (or more) flyback converter sharing resource sets; it has a pseudo DC link at the common output coupling point.  A H-bridge connects the grid to the DC link through a simultaneous activation of  two switches so that a positive voltage is maintained at the transformer terminal and only the flyback converters do the conversion. The resulting inverter flips polarity based on grid voltage so proper operation is maintained. The performance and load sharing attributes are greatly improved using the interleaving flyback converters that enable master -slave / multi phase control schemes. The input capacitance of the system is also improved through the phase control attribute’ interleaving a series of flyback controllers have been used to develop a 2kW rated ILFI (Li & Oruganti, 2012). In designing DC-DC flyback converters, the chosen mode of flyback operation is the DCM (Li & Oruganti, 2012); the transfer function linearity generally makes the approach to control easy while also reducing losses due to switching to zero because ZCS (zero current switching) can be attained.

Hardware Design Challenges of Flyback Converters

The approach of using DCM in flyback control also results in fast dynamics, the need for a small transformer, and eliminates reverse recovery issues. While this mode has its limitations, most can be tackled during the design stage (Adib & Farzanehfard, 2008) ; the dynamic model is very similar to CCM. Because of a linear increase in current with voltage, the flyback in DCM works perfectly without the need for a current sensor (Li & Oruganti, 2012); however, the input capacitor must be large enough to reduce ripple, which offsets the cost reduction from eliminating sensors. Taking into consideration voltage, this topology allows the ripple magnitude to be under 20% that produces current with 3.5% THD. A stronger ripple results in a sharp rise in harmonics. Ripples cause lower power use levels, apart from affecting harmonics. To ensure proper MPPT, just 10% can be tolerated because the result is a 1% utilization loss that is acceptable. The flyback converter in DCM is always characterized by ZCS because the current starts rising from zero at each period so the control system must raise the current level to be higher than other design modes so the same energy can be delivered. Apart form the magnetizing inductance, the flyback transformer has leakage inductance that prohibits any sudden current changes.

Otherwise, the main switch would burn from the energy stored in the device if there was no snubber circuit installed. The efficiency of the system will be worsened if all leakage energy is wasted on a snubber circuit, and proposals have been made to improve efficiency, such as by having a forward snubber circuit. The snubber circuit is composed of a switch and capacitor; the capacitor takes energy automatically once the MOSFET body diode main switch is deactivated. Once all leakage energy is transferred to the capacitor, the direction of current switches although this approach has not resulted in significantly high efficiencies when tested.  Another proposed approach is an ACC (active clamp circuit) combined with soft switching that allows bi-directional current flow where only a rectifying MOSFET is placed at the secondary terminal of the transformer. To act a s a low power clamp system, a capacitor is placed parallel to the main switch. Another proposed approach is the combining of the flyback converter with SEPIC topology to reduce ripple in the input voltage through a two step process. In this setup, energy is drawn from the SEPIC capacitor rather than directly from the input result in input voltage with less ripple when compared to the one stage system.

Interleaved Flyback Converters (ILFI)

The figure below shows the general MIC with DC link topology; the DC-DC step-up converter raises the voltage of the PV module (usually in the 22v DC to 45v DC range) to a voltage exceeding the peak grid voltage. This DC voltage is converted by an inverter to the AC-line voltage through SPMW (sinusoidal pulse width modulation). Energy is stored by the DC link capacitor and is responsible for the decoupling of power. The DC-DC converter topologies vary, based on each case; the following section discusses four DC-DC converter configurations.  

Fig 1

The inverter stage uses a technology referred to as ZVRT (zero voltage resonant transition) that operates at a 25 kHz switching frequency. The strategy for control is optimized to reduce inverter loss. The prototype has a 200 W rating and demonstrates a 96% peak efficiency including control logic and drivers; however, this topology doesn’t provide galvanic isolation (Li & Wolfs, 2008). High efficiency DC-DC boost converters for MIC that are transformer-less is proposed by Woo-Young, Ju-Seung & Jae-Yeon, (2011); The proposed topology is based on the ZETA boost converter, however, the coupled inductor replaces the input inductor. This topology has one main advantage that it is effectively possible to recycle the energy stored in the leakage inductances. This helps improve efficiency and in addition, has the direct consequence of lowering the voltage stresses at switch Q1. A 97.3% peak efficiency was achieved with a 250 W prototype switched at 50 kHz for a 50W load; for the nominal load reports an efficiency of 94.8%. A continuous conduction mode (CCM) is a topology proposed by Zhao, Lee & Tsai (2002); the CCM flyback DC converter stage that increases the low DC input voltage to higher than the peak grid voltage level. 

The DC voltage is then converted by a full bridge inverter to the alternating grid voltage through the use of SPWM. Both converter stages have a switching frequency of 25 kHz Flyback converters in PV applications. The major benefits of this topology is that the transformer is isolated; this allows the AC and DC sides grounding and the low component count that can reduce system cost and improve reliability. Watson, Lee & Hua (1996) discuss an analogous topology which has an LC filter added to it at the full bridge converter input stage, while Kim & Kwon (2010) propose a half-wave zero voltage switching resonant converter. The major contribution of this study is in the DC-DC conversion that can operate in discontinuous as well as continuous conduction modes. The DC-DC conversion stage reports an efficiency of 90% for an 85 MW prototype in CCM operation, and a 1 MHz switching. The lower level of efficiency shows that despite soft switching being an important strategy for obtaining a high efficiency MIC, other factors need to be taken into consideration. An overall system-design approach in essential, including the choice of switching frequency and switching components, design, and PCB layout.

The  MIC with a pseudo DC-link is discussed by Li & Wolfs (2008) and Edwin, Xiao & Khadkikar (2014), the current unfolding MIC’s normally have a minimum of two stages; Stage one generates a high frequency sine-wave rectified current in a two line-frequency envelope. In stage two, the current is unfolded and then injected into the grid at line frequency. The figure 2 below shows a topology consisting of a Flyback converter stage that raises the module voltage to a grid compatible level. Sine rectified DC current is produced using the PWM and the DC current converted to AC current by the current unfolding stage, before being injected into the grid. The topology can operate either in BCM (boundary conduction mode) or DCM. This topology has been extensively covered in several papers (Li & Wolfs, 2008), (Rodriguez & Amaratunga, 2008), (Kjaer, Pedersen & Blaabjerg, 2005).

Fig 2

Topologies have also been proposed in PV flyback converter design by Edwin, Xiao & Khadkikar (2014) where the MIC topologies are made up of at least two stages of power, the first stage converts the input DC voltage to a high frequency AC voltage that a high frequency transformer steps up. The second stage consists of a cyclometer that coverts the high frequency AC to line-frequency AC current injected into the grid.

A topology is presented by Edwin, Xiao & Khadkikar (2014) that consists of a DC-AC converter hard switched at 20 kHz, a high frequency transformer that steps up the voltage to a level compatible with the grid and a then a cyclo-converter with converts down the high frequency AC to grid-frequency current; the cyclo-converter is, however, soft switched. Another topology is also presented by  Edwin, Xiao & Khadkikar (2014); it consists of a push-pull stage switching at 40kHz and a high frequency transformer that raises the input voltage to the level that is required. A forced commutated cyclo-converter is found at the output stage; a prototype of 300W is implemented. The third topology presented by Edwin, Xiao & Khadkikar (2014) is made up of three main stages: a  full - bridge   voltage   source   inverter,   an   impedance - admittance   conversion   circuit  (immittance circuit) comprised of LC components, and a cyclo-converter comprised of  bidirectional  switches  and  a  center - tapped  transformer.  The  immittance  conversion  circuit converts a voltage source to a current source. The switching frequency is rated  at 20 kHz for a 30W prototype as shown in Fig 3 below; 

Fig 3

Another  novel DC link - less MIC topology is proposed where the converter is made up of  two  main  power  conversion  stages;  the first  stage  is  a  zero  voltage  resonant  transition  full - bridge  converter  employing  a  series  resonant  tank.  This stage produces a quasi - sinusoidal current. A high frequency transformer steps up  the  voltage  to  a  level  compatible  with  the  grid  voltage.  The  second  stage  is  a  half  wave  cyclo-converter  which  injects  current  in  to  the  grid.  The  role  of  the  capacitor  C block at  the  transformer  secondary  is  to  filter  out  or  stop  DC  currents  from  flowing  through  the  transformer  winding  as  this  could  cause  a  short  circuit (Edwin, Xiao & Khadkikar, 2014)

Conclusion

This chapter reviewed previous research regarding the  flyback inverter for PV  applications and a state-of-the-art in MIC technology, examining some of the  most recent MIC topologies.  The Flyback Inverter 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  between  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 MIC topologies are classified in to MICs with a DC - link, MICs with a current - unfolding circuit, and finally in to MICs without a DC - link.  T he   chapter   shows   that   the   MIC   topologies   with   current - unfolding   and   more 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. The evaluation of the  proposed system is then partitioned into two chapters; theoretical and experimental.

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