Flyback inverter is a device that converters the DC into AC with the use of flyback transformer. For this inverter topology no dc-dc converter is required as the dc voltage is not related to the operation.  It is simple, has less component count hence reduced cost and has advantage of isolation through transformer[4]. Let's see how it works.



Basic topology of a flyback inverter is shown below.

Fig. 1 Fundamental topology of flyback inverter

As shown in Fig.1 the fundamental topology has 3 bi directional switches, a center tapped transformer, a low pass filter and an input capacitor to handle the power difference. Lets have a look on to the operation of this inverter

The switch S_m is the main switch that is connected on primary side. It is switched with a train of pulses such that the transformer stores energy in its magnetizing inductance and releases it when the secondary switches are operated. For easy operation usually the switch Sm is connected on the lower side of the primary winding. The envelope of the primary current is sinusoidal and is in phase with the ac utility grid line voltage [4].

The secondary side switches S_ac1 and S_ac2 operates reciprocally during each half cycle of the Vac. Therefore, for positive half cycle of the grid voltage V_ac, the switch S_ac1 operates and for the negative half cycle S_ac2 is turned on. The operation of the secondary switches is thus to unfold the rectified sinusoidal voltage.

Low pass filter is required to inject pure sinusoidal current in the grid. Figure 2. below shows the timing diagram of the fundamental flyback inverter. As seen in Fig.2 the reference waveform is a rectified sine waveform that is obtained by full wave rectification of the grid voltage. It is then normalized using a gain block and multiplied by the MPPT output. This means that the amplitude of the reference waveform is variable in order to track the MPP of the PV array. S_ac1 and S_ac2 are switched at line frequency whereas the switching frequency of the S_m is defined  by the triangular high frequency waveform.

Advantage of using a flyback inverter is that in case of parallel operation it enables the realization of current sharing operation without any current sharing controller [3].

Fig 2. Timing diagram

Fig 3 : Current waveform (Modified and adopted from reference [1])

Some points to ponder.

1. In order to inject the active power into the grid the peak currents of the primary side outline a sinusoidal waveform as shown in Fig. 3 [1].
2. Leakage inductance of the transformer contributes towards the system loss.
3. Transformer design is very important for correct results. The design of transformer is discussed here. 
4. The transformer turns ratio affects the stress across the power switches. 
5. Usually the frequency of switching is as high as 100-300 kHz.
6. The proper operation of this inverter means a unity power factor operation.
7. There is always a significant difference in the amplitude of the ac current before and after the low pass filter.

References

[1] Nanakos, A.C.; Tatakis, E.C.; Papanikolaou, N.P., "A Weighted-Efficiency-Oriented Design Methodology of Flyback Inverter for AC Photovoltaic Modules," Power Electronics, IEEE Transactions on , vol.27, no.7, pp.3221,3233, July 2012
[2] Mingzhi Gao; Min Chen; Chi Zhang; Zhaoming Qian, "Analysis and Implementation of an Improved Flyback Inverter for Photovoltaic AC Module Applications," Power Electronics, IEEE Transactions on , vol.29, no.7, pp.3428,3444, July 2014
[3] Shimizu, T.; Wada, K.; Nakamura, N., "Flyback-Type Single-Phase Utility Interactive Inverter With Power Pulsation Decoupling on the DC Input for an AC Photovoltaic Module System," Power Electronics, IEEE Transactions on , vol.21, no.5, pp.1264,1272, Sept. 2006
[4] Kasa, N.; Iida, T.; Liang Chen, "Flyback Inverter Controlled by Sensorless Current MPPT for Photovoltaic Power System," Industrial Electronics, IEEE Transactions on , vol.52, no.4, pp.1145,1152, Aug. 2005