Switched-Capacitor Multilevel DC/AC Inverters in Solar Panel Energy Systems
Switched-capacitor multilevel DC/AC inverters will be described in this chapter. For convenience, we call them switched-capacitor inverters or SC inverters (SCIs).
A switched capacitor (SC) is usually manufactured with a switch and a capacitor together [1–6]. It has been used in DC/DC converters for many years. It can be integrated into power semiconductor IC chips. Hence, SC converters have small size and work at high switching frequencies. This technique opened the way to building converters with high power density and attracted the attention of research workers and manufacturers. We were the first to use switched capacitors in DC/AC inverters.
A switched-capacitor DC/DC converter is shown in Figure 12.1a. It contains two SCs (C1 and C2), main switch S, two slave switches (S1 and S2), and three diodes. The main switch S and the slave switches are operated mutually exclusively; that is, when the main switch is on, the slave switches are off, and vice versa.
When the main switch S is on, the slave switches are off, and all diodes conduct. The equivalent circuit is shown in Figure 12.1b. Both SCs are charged by the source voltage E in the steady state. When the main switch S is off, the slave switches are on, and all diodes are blocked. The equivalent circuit is shown in Figure 12.1c. The voltages at the points 1, 2, and 3 are E, 2E, and 3E, respectively, in the steady state.
FIGURE 12.1
A switched capacitor.
12.2 Switched Capacitor Used in Multilevel DC/AC Inverters
In this section, we show how to apply the switched capacitor technique in multilevel DC/AC inverters.
A 5-level switched capacitor inverter is shown in Figure 12.2.
There are one DC voltage source E, one 3-position band-switch, and one change-over switch (2P2T) in the circuit. The slave switch S1 and the main switch S switch mutually exclusively; that is, when S is on, S1 is off, and vice versa. Capacitor C1 is a switched capacitor. When S is on and S1 is off, diode D1 conducts. Therefore, Capacitor C1 is charged to the voltage E in steady state. When S is off and S1 is on, diode D1 is blocked. The voltage at point 2 is V2 = 2 × E (V1 always equals E). Therefore, the operation status is as follows:
• Vout = 2E: 2P2T is on, the band switch is at position 2, and the main switch S is off.
• Vout = E: 2P2T is on, the band switch is at position 1, and the main switch S is on.
• Vout = 0: The band switch is at position 0 (i.e., N), and all switches can be on or off.
FIGURE 12.2
A five-level switched-capacitor inverter.
FIGURE 12.3
A five-level waveform.
• Vout = −E: 2P2T is off, the band switch is at position 1, and the main switch S is on.
• Vout = −2E: 2P2T is off, the band switch is at position 2, and the main switch S is off.
We have obtained a five-level output AC voltage. The output voltage peak value is two times the input DC voltage E. The waveform is shown in Figure 12.3.
A nine-level switched-capacitor inverter is shown in Figure 12.4.
There is one DC voltage source E, one five-position band switch, and one change-over switch (2P2T) switch in the circuit. The slave switches S1–3 and the main switch S switch mutually exclusively; that is, when S is on, all slave switches are off, and vice versa. Capacitors C1–3 are the switched capacitors. When S is on, all diodes conduct. Therefore, all SCs are charged to the voltage E in the steady state. When S is off and S1 is on, diode D1 is blocked. The voltage at point 2 is V2 = 2 × E; the voltage at point 3 is V3 = 3 × E; the voltage at point 4 is V4 = 4 x E; (V1 is always E). Therefore, the operation status is as follows:
• Vout = 4E: 2P2T is on, the band switch is at position 4, and the main switch S is off.
• Vout = 3E: 2P2T is on, the band switch is at position 3, and the main switch S is off.
FIGURE 12.4
A nine-level switched-capacitor inverter.
FIGURE 12.5
A nine-level waveform.
• Vout = 2E: 2P2T is on, the band switch is at position 2, and the main switch S is off.
• Vout = E: 2P2T is on, the band switch is at position 1, and the main switch S is on.
• Vout = 0: The band switch is at position 0 (i.e., N), and all switches can be on or off.
• Vout = −E: 2P2T is off, the band switch is at position 1, and the main switch S is on.
• Vout = −2E: 2P2T is off, the band switch is at position 2, and the main switch S is off.
• Vout = −3E: 2P2T is off, the band switch is at position 3, and the main switch S is off.
• Vout = −4E: 2P2T is off, the band switch is at position 4, and the main switch S is off.
We have obtained a nine-level output AC voltage. The output voltage peak value is four times the input DC voltage E. The waveform is shown in Figure 12.5.
12.2.3 Fifteen-Level SC Inverter
A 15-level switched-capacitor inverter is shown in Figure 12.6.
There is one DC voltage source E, one 7-position band switch, and one change-over switch (2P2T) switch in the circuit. The slave switches S1−6 and the main switch S switch mutually exclusively; that is, when S is on, all slave switches off, and vice versa. Capacitors C1−6 are SCs. When S is on and all slave switches are off, all diodes conduct. Therefore, all SCs are charged to the voltage E in the steady state. The voltage at point 2 is V2 = 2 × E; the voltage at point 2 is V3 = 3 × E; the voltage at point 4 is V4 = 4 × E, and so on, where V1 is always E. Therefore, the operation status is as follows:
FIGURE 12.6
A fifteen-level switched-capacitor inverter.
• Vout = 7E: 2P2T is on, the band switch is at position 7, and the main switch S is off.
• Vout = 6E: 2P2T is on, the band switch is at position 6, and the main switch S is off.
• Vout = 5E: 2P2T is on, the band switch is at position 5, and the main switch S is off.
• Vout = 4E: 2P2T is on, the band switch is at position 4, and the main switch S is off.
• Vout = 3E: 2P2T is on, the band switch is at position 3, and the main switch S is off.
• Vout = 2E: 2P2T is on, the band switch is at position 2, and the main switch S is off.
• Vout = E: 2P2T is on, the band switch is at position 1, and the main switch S is on.
• Vout = 0: The band switch is at position 0 (i.e., N), and all switches are on.
• Vout = −E: 2P2T is off, the band switch is at position 1, and the main switch S is on.
• Vout = −2E: 2P2T is off, the band switch is at position 2, and the main switch S is off.
• Vout = −3E: 2P2T is off, the band switch is at position 3, and the main switch S is off.
• Vout = −4E: 2P2T is off, the band switch is at position 4, and the main switch S is off.
• Vout = −5E: 2P2T is off, the band switch is at position 5, and the main switch S is off.
• Vout = −6E: 2P2T is off, the band switch is at position 6, and the main switch S is off.
• Vout = −7E: 2P2T is off, the band switch is at position 7, and the main switch S is off.
We have obtained a 15-level output AC voltage. The output voltage peak value is seven times the input DC voltage E. The waveform is shown in Figure 12.7.
12.2.4 Higher-Level SC Inverter
Repeatedly adding components (S1-C1-D1-D2) as shown in Figure 12.6, we can obtain higher-level inverters. We believe that readers of this book have understood how to construct higher-level inverters, for example, a 21-level SC inverter.
FIGURE 12.7
A fifteen-level waveform.
12.3 Simulation and Experimental Results
Switched-capacitor multilevel inverters in solar panel energy systems are examples for the simulation. The 17-level inverter’s simulation result is shown in Figure 12.8. Its corresponding experimental result is shown in Figure 12.9.
FIGURE 12.8
A seventeen-level simulation waveform.
FIGURE 12.9
A seventeen-level experimental waveform.
The 27-level inverter’s simulation and corresponding experimental results can be seen in Figures 12.10 and 12.11, respectively.
Furthermore, we use the switched-capacitor technique to produce 37-level and 47-level SC inverters for the solar panel energy system. Their output voltages have 37 and 47 levels, respectively. Their simulation and experimental results are shown in Figures 12.12,12.13,12.14,12.15.
We introduced switched-capacitor multilevel inverters in this chapter. All SC multilevel inverters have relatively simple structure, straightforward operation procedure, easy control, and higher output voltage (compared with the input voltage). We can use fewer components to construct more levels of the output voltage. We applied four SCIs from 17-level to 47-level of output voltage to a solar panel energy system and obtained satisfactory simulation and experimental results that strongly supported our circuit design. These SC multilevel inverters can be used in other renewable energy systems and industrial applications.
FIGURE 12.10
A 27-level simulation waveform.
FIGURE 12.11
A 27-level experimental waveform.
FIGURE 12.12
A 37-level simulation waveform.
FIGURE 12.13
A 37-level experimental waveform.
FIGURE 12.14
A 47-level simulation waveform.
FIGURE 12.15
A 47-level experimental waveform.
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