Analysis of leakage current suppression mechanism of H5, Heric and H6 topology
1. H5 topology
Figure 1 is SMA SoLdr’s patented topology-H5 topology [8], this topology has a switch S5 in series in the main loop of the full bridge circuit. The H5 topology uses unipolar SPWM modulation, and its driving logic diagram is shown in Figure 2. In the power transmission phase, current flows through S5-S1-S4 (positive direction) and S5-S3-S2 (negative direction); in the freewheeling phase, S2, S4 and S5 are disconnected, and the photovoltaic panel is separated from the main circuit. That is, when the common mode voltage vCM changes at a high frequency, a large impedance is inserted in the common mode loop to achieve the purpose of leakage current suppression. Compared with other non-isolated topologies that use unipolar SPWM described later, this topology only adds a switch tube to achieve the purpose of suppressing leakage current, with fewer power devices and low cost. However, as shown in Figure 2, S5 is always in a high-frequency modulation state throughout the power grid cycle, and the conduction loss and switching loss are greater than other power devices, which increases the difficulty of device heat dissipation design. At the same time, the introduction of S5 makes the current flow through the three switching tubes during the power transmission stage, and the conduction loss is relatively large.
2.Heric topology
Figure 3 shows Sunways’ patented topology-Heric topology, which adds switch tubes S5 and S6 on the basis of the full-bridge circuit. Using the unipolar SPWM strategy shown in Figure 4, during the power transfer phase, current flows through S1–S4 (positive direction) and S3-S2 (negative direction); in the freewheeling phase, S1~S4 are disconnected, and the added S5 and S6 are used to provide a new freewheeling circuit, so that the photovoltaic panel is separated from the grid during the freewheeling phase. That is, a large impedance is connected in series in the common mode loop to achieve the purpose of leakage current suppression. Since the current only flows through two switching tubes in the power transmission and freewheeling phases, compared with the H5 topology, the conduction loss is small and the efficiency is high.
As the name implies, the Heric topology is also known as the AC-bypass topology.
3. H6 topology
Figure 5 is a kind of H6 structure topology inverter, adopting the unipolar SPWM strategy shown in Figure 6. In the power transmission phase, current flows through S1-S6-S4 (positive direction) and S3-S5-S2 (negative direction); in the freewheeling phase, the current flows through S6-D1 (positive freewheeling) and S5-D2 (negative freewheeling). This topology uses the newly added S5, S6, D1 and D2 for freewheeling, so that the photovoltaic panel is separated from the grid during the freewheeling phase, that is, a large impedance is connected in series in the common mode loop to achieve the purpose of leakage current suppression. Since the current needs to flow through three power tubes in the power transmission phase, compared to the Heric topology, the conduction loss is slightly larger. However, the current in the freewheeling phase of this topology can be freewheeled through diodes D1 and D2, instead of passing through S5 and S6 at the same time as in the Heric topology, so that the switch tube and diode can be optimized to achieve the purpose of efficiency optimization. However, to further improve the efficiency, more expensive devices, such as silicon carbide diodes, must be selected, and the cost of the devices has increased.
