Optocoupler devices in home energy storage systems

New energy and energy storage, a hot field nowadays, has become one of the most important development directions under the wave of development to achieve "carbon neutrality". New energy requires the use of energy conversion equipment to convert the potential energy of solar, wind, heat or water into electrical energy, which is then transmitted to each terminal. Energy storage systems, on the other hand, store the electricity produced by irregular and intermittent production of electricity from wind and photovoltaic power generation systems to stabilize the output of new and renewable energy sources by charging and discharging them.

Optocoupler application in typical PV inverter

A typical grid-connected PV inverter system in a high-power centralized power generation site consists of several major components: PV string combiner, DC/DC and DC/AC inverters, and field bus communication.

In such a complex application scenario, optocouplers are used in many places. For example, on voltage sensors for monitoring PV arrays in PV string combiners, on voltage sensors for measuring bus voltage in DC/AC inverters, on voltage sensing + photo MOS for monitoring insulation resistance, on current sensing in PV string combiners and inverters, and on gate drive optocouplers in DC/DC and DC/AC converters as well as digital optocouplers in fieldbus communication.

What are the requirements for the use of home ESS optocouplers for integrated photovoltaic power generation

Energy storage systems are devices that store electricity when demand is low and export the stored electricity when demand is high, improving the energy efficiency of power plants and stabilizing grid operations. The integration of renewable energy into the grid is driving the market progress of ESS. There are various options for energy storage systems, one of which is to store energy in the form of a battery bank. The main components of this system are panel array, charge controller, bi-directional inverter, smart meter, grid, and battery bank.

The combination of PV power generation and battery ESS system is a common configuration for home ESS today. The main power supply comes from the grid, and the renewable energy is transferred from the PV panel to the battery and inverter to the AC load, and if the PV power is insufficient, the power is supplemented from the grid, and when the grid power supply is insufficient, the power is obtained from the PV panel and supplemented by the battery pack power. Somewhat similar to the application of optocouplers in a typical grid-connected PV inverter, there are quite a few optocouplers in this ESS system. The optocouplers are mainly selected because they provide enhanced insulation and less noise for EMI or CMR compared to capacitive and magnetic couplers in digital isolation, the biggest advantage of optocouplers.

ESS DC/DC stage and inverter are the application segments with large demand for optocouplers, among which gate drive, current sensing and digital communication have different demands on optocouplers.

In order to achieve fast and reliable gate drive, higher peak output current is required, lower propagation delay and higher frequency are essential for fast switching, in order to prevent mis-driving in noisy environment, CMTI should be as high as possible (at least 100kV/µs or more), and lower ICC is beneficial to achieve Low power consumption and bootstrap operation. Active Miller clamping is also a plus for optocoupler devices, especially in high power applications such as photovoltaic inverters, where the switch cannot withstand negative gate voltages at certain times, and active Miller clamping ensures that it remains cutoff.

The key features of optocouplers in current/voltage sensing are gain accuracy, bandwidth and linearity. From the industry leading optocoupler manufacturer Broadcom's current parameters, gain accuracy can already be ±0.5%, bandwidth of 200KHz, and fast response detection of 1.6us. The higher the CMTI, the better, to prevent false sensing in noisy environments.

In digital communication, the first thing to look at is the speed, and from a technical point of view the high speed limit that digital optocouplers can achieve is certainly impossible to compare with magnetic coupling technology. As mentioned above, the biggest advantage of optocoupler is that it can provide enhanced insulation. With enhanced isolation, the speed of digital optocoupler is also increasing as much as possible, from 1MBd to 5MBd, 8MBd, 10MBd, 20MBd to 50MBd, digital optocoupler has increased the speed a lot. Another advantage or skill of digital optocoupler is that it can be directly driven by placing resistors at the front of the LED, and the CMTI can be optimized by configuring the resistor resistance distribution.

Summary

Optocouplers can provide precise signal isolation and safe and reliable enhanced electrical insulation performance, and have important applications in these energy conversion and energy storage devices, whether it is the gate drive of power devices, current and voltage detection or IPM interface, digital signal isolation, etc. Optocouplers protect the whole home ESS energy storage system, and the role of leakage prevention and filtering greatly improves the safety of the energy storage system.