Figure 5-4 shows the typical block diagram of string inverter with current sensing and SPDs. The legends are:

1. Surge applies on PV+ and PV-
2. PV string current sampling
3. DC side SPD
4. MPPT Boost current sampling
5. 3-phase current sampling
6. AC side SPD

Figure 5-4 Typical Block Diagram of String Inverter with Current Sensing and SPDs

Surface Mount Device (SMD) in-package Hall-effect sensing designs can meet the surge current capability challenges in solar inverter systems even compared to through-hole mounted current sensing designs. The latter naturally has a high surge current capability because their construction allows for a very large insulation distance, while the former depends on the limited insulation performance between the leadframe and the die.

Comparing the difference between 2) PV string current sampling, 4) MPPT Boost current sampling and 5) 3-phase current sampling, AC side has SPD between AC current sampling and grid; MPPT is inside the inverter and is not directly affected by lighting surge. The DC side SPD is after PV string current sampling and there has no protection for PV string current sensors, which is very challenging for the in-package Hall-effect current sensors.

The PV string current sensing is directly exposed to the foremost position of the inverter. When DC switches are turned ON (PV strings are connected to the inverter), DC SPD can protect the PV string current sampling sensors. However, when DC switches are turned OFF in some situations (such as shutdown at night, during maintaining work and so on), meaning that DC SPD is not connected to the PV string current sampling circuit and cannot provide protection. Lighting can directly destroy the in-package Hall-effect current sensor if it doesn’t have enough surge current and insulation capabilities.

Apart from selecting the in-package Hall-effect current sensor with high surge current capability (≥ 10kA) and doing lightning test in the whole system to verify reliability, there is another way to use in-package hall-effect current sensor if it doesn’t have enough surge current capability.

Figure 5-5 shows the comparison between traditional inverter system architecture and new proposed architecture to allow < 10kA lower surge current capability. The main difference is the location of SPDs. For traditional architecture, the SPD circuits are placed after DC switch and on the main power board. So, when DC Switch turns OFF, SPD circuits do not function. For new architecture, SPD circuits move to the string current sampling board and before the DC switch. Therefore, SPD circuits always work in either DC switch turn OFF or turn ON.

This design works in an inverter with one PV string to one MPPT, such as low power (≤ 25KW) residential inverter or hybrid inverter. For string inverter with 2-string or multi-string per MPPT, the traditional architecture has a 2-string or multi-string connected in parallel with one SPD deployed. The new design requires an SPD installed in each corresponding string.

Figure 5-5 Inverter System Architectures Comparison