Photovoltaic miniature inverter produced by American Phase Energy Co., Ltd. is shown in Figure 7.15. The front stage uses an active clamp interleaving flyback converter for sinusoidal PWM wave modulation, forms a power frequency sinusoidal half wave envelope through a high-frequency flyback transformer and a fast recovery diode rectifier circuit, and the rear stage uses a bridge inverter circuit composed of silicon controlled rectifier for power frequency polarity reversal, and injects the sinusoidal current synchronized with the grid voltage into the grid through the CL filter circuit. The circuit adopts electrolytic capacitor for power decoupling, and its peak efficiency is 96.3%.
Inductive current critical control mode (BCM) and intermittent control mode (DCM) control strategies are widely used in interleaved flyback micro inverters. The main losses of the circuit under heavy load include the conduction loss of the power switch and rectifier diode of the flyback converter, the core loss and copper loss of the transformer; The main losses of the circuit under light load include the off loss and drive loss of the power switch of the flyback converter, and the core loss of the transformer. Therefore, when the load is light (the output power is less than 200W), the circuit efficiency of DCM control strategy is slightly higher than that of BCM control strategy. In order to improve the efficiency of micro inverters, most of them adopt DCM control strategy. For the interleaved flyback micro inverter composed of two flyback converters with the same capacity, there are usually four control strategies in half a grid cycle [168-171].
Mode 1:1 flyback converter has been working independently in DCM mode, while No. 2 flyback converter has not been working. The reference currents i1ref and i2ref of flyback converter are shown in Formula (7.1) and Figure 7.16 (a).
Mode 2: In half a grid cycle, when the output instantaneous power is small, No. 1 flyback converter operates in DCM mode; When the output instantaneous power is large, No.1 flyback converter operates in BCM mode by frequency conversion, No.2 flyback converter does not work directly, i1ref and i2ref are shown in Formula (7.1) and Figure 7.16 (b) [172-176].
Mode 3: Two flyback converters shift 180 phases in a power sharing and switching cycle. The mode always works in DCM mode alternately. i1ref and i2ref are shown in Formula (7.2) and Figure 7.16 (c).
Mode 4: In half a grid cycle, when the output instantaneous power is small, No. 1 flyback converter operates in DCM mode independently, and i1ref and i2ref are shown in Formula (7.1); When the output power is large, No. 2 flyback converter is put into operation. At this time, the two flyback converters work alternately in DCM mode in the way of power sharing and 180 ° phase shifting in the switching cycle. At this time, i1ref and i2ref are shown in Formula (7.2). Finally, No. 1 flyback converter still works independently in DCM mode. At this time, i1ref and i2ref are shown in Formula (7.1), and the whole process is shown in Figure 7.16 (d).
Four effective hybrid control modes can be obtained from the above four control strategies.
① The first is the organic combination of mode 1 and mode 3. When the output power of the hybrid control mode is small, mode 1 is selected; Select mode 3 when the output power is high. This situation is applicable to the conventional control of micro inverters,
② The second is the organic combination of mode 1 and mode 2. When the output power of the hybrid control mode is small, mode 1 is selected; Select mode 2 when the output power is high. The mode is mostly applied to the micro inverter composed of a single flyback converter.
③ The third is the organic combination of mode 1, mode 2 and mode 3. When the output power of the hybrid control mode is small, mode 1 is selected; Select mode 2 when outputting medium power; Select mode 3 when the output power is high. This mode is the most complex to control and is generally not used.
④ The fourth is the organic combination of mode 1 and mode 4. When the output peak power of the hybrid control mode is small, mode 1 is selected. When the output peak power is large, select mode 4, that is, in half a grid cycle, when the instantaneous power is small, only No. 1 flyback converter works and operates in DCM mode; When the instantaneous power is large, No. 2 flyback converter works [171]. After the hybrid control, the overall efficiency of the interleaved flyback micro inverter will be improved by 0.5% [168] on the basis of the traditional interleaving control.
