# Three-switch bridge arm circuit/double buck circuit/three-level full bridge circuit

1. Three-switch bridge arm circuit

The design idea of ​​this circuit is also derived from the HERIC circuit [121], as shown in Fig. 1(a) and (b). Obviously, the efficiency of this circuit is reduced due to the higher conduction loss than the HERIC circuit. The full-load efficiency of the whole machine is about 97% [122]. Its working principle is similar to that of the HERIC derivative circuit, and will not be repeated here.

1. Dual BUCK circuit topology

The circuit consists of two buck circuits and two power frequency switches. Among them, S1, VD1, and L1 form a positive half-cycle buck circuit, and S2, VD2, and L2 form a negative half-cycle buck circuit [123], as shown in Figure 2(a) and ( b) shown. Taking Figure 2(a) as an example, the working principle is described as follows:

In the positive half cycle of the grid voltage, the S1, VD1, L1 and switch S3 of the positive half cycle buck circuit work, the switch S3 is always on, the negative half cycle buck circuit and switch S4 do not work, and the switch S1 is turned on and off by PWM.

When the switches S1 and S3 are turned on at the same time, the current generated by the photovoltaic array flows through the switch S1, the filter inductor L1, the single-phase power grid, and the switch S3. When the switch S1 is turned off, the current on the line passes through the filter inductor L1, the single-phase grid, the switch S3, and the diode VD1 for freewheeling.
During the negative half cycle of the grid voltage, S2, VD2, L2 and switch S4 of the negative half cycle buck circuit work, the switch S4 is always on, the positive half cycle buck circuit and switch S3 do not work, and the switch S2 is turned on and off by PWM.

When the switches S2 and S4 are turned on at the same time, the current generated by the photovoltaic array flows through the switch S2, the filter inductor L2, the single-phase grid and the switch S4. Similarly, when the switch S2 is turned off, the current on the line is freewheeling through the filter inductor L2, the single-phase grid, the switch S4 and the diode VD2.

It can be seen that during the positive and negative half cycles of the power grid, only one filter inductor works. Therefore, compared with a circuit in which two inductors play a filtering role at the same time, in the case of the same switching frequency, in order to achieve the same filtering effect, the circuit’s The filter inductance should be doubled. At the same time, it can be seen that during the current freewheeling period, there is no energy transmission between the photovoltaic array and the single-phase power grid, thus avoiding the high-frequency voltage component appearing on the DC bus, so that the high-frequency common mode current is effectively suppressed, and the maximum efficiency of the circuit is is 99% [123].

1. Three-level full bridge circuit

The three-level full-bridge circuit is shown in Figure 3. Now take Figure 6.28 as an example to illustrate its working principle as follows:

During the positive half cycle of the grid voltage, the switches SP1 and SN1 work at high frequency, the switches SP2 and SN2 are always on, and the rest of the switches are off. When SP1 and SN1 are turned on, the current generated by the photovoltaic array flows through the switches SP1 and SP2, the filter inductor L1, the single-phase grid, the filter inductor L2, and the switches SN1 and SN2. When SP1 and SN1 are turned off, the current on the line is freewheeling through switch SP2, filter inductor L1, single-phase power grid, filter inductor L2, switch SN2 and the anti-parallel diodes of switches SN3 and SP3.

During the negative half cycle of the grid voltage, the switches S7 and S8 work at high frequency, the switches SP3 and SN3 are always on, and the rest of the switches are off. When S7 and S8 are turned on, the current generated by the photovoltaic array flows through switch S8, filter inductor L2, single-phase power grid, filter inductor L1, and switch S7. When S7 and S8 are turned off, the current on the line is freewheeling through the anti-parallel diodes of switches SN2 and SP2, filter inductor L2, single-phase power grid, filter inductor L1, switches SP3 and SN3.
The common-mode voltages in the above working states are all 0.5VPV, which remains unchanged, so the conditions for basically eliminating leakage current are met, and the maximum efficiency of the circuit is 97.71%.

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