For non-isolated applications, a bidirectional DC/DC converter can be used to have the possibility of battery energy storage system (BESS). Bi-directionality is important for the DC/DC converter to act like a battery charger (in buck mode) and discharging the battery (in boost mode) to provide a higher and stable output voltage at the DC link. This can then be transferred into AC power towards the grid by means of a DC/AC converter in conditions like low irradiance scenarios or night-time conditions.

Solar energy has daily, seasonal and yearly fluctuations, so it is not the most reliable source. In a grid connected system, maximum power is delivered to the grid during noon, while in the morning and evening it is less. In many regions world-wide, the price of power is demand-dependent – the price (cost per watt) is higher when demand is high (like evenings and mornings), and it is lower when demand is low (noon, late night). So a consumer with solar installation gets a lower price for the power he produces during the noon time, while paying higher for the power he consumes off the grid in the evening. Also, if there is any fault in the grid, all the power produced goes to waste as the grid is not accessible. Due to such issues, the trend is to have some local energy storage so that energy can be stored and released to the grid when it is accessible and when demand is high.

To increase the power level of this stage and to reduce the current ripple, interleaving of branches can be carried out. Interleaving helps in reducing output current ripple, output noise, decrease of the size of the EMI filter, and increase power density. It is relatively simple to implement and control, however for optimum performance it is recommended to have the interleaved half-bridges as symmetrical as possible, to have balanced current flow between the them. Such multiphase designs also enable multiples of switching frequency across the output EMI filter, thus making the design smaller. A typical application of such a non-isolated topology can be seen in TIDA-010938 in Figure 2-3. Here, we can see two interleaved stages with an applied phase difference of 180^o between each other.

Figure 2-3 DC/DC Stage for Battery Block Diagram

Isolation comes into play when we talk about systems relating to automotive applications or battery with lower voltage ratings. Most popular topologies in this regard include the Dual Active Bridge with Extended Phase Shift (for example in TIDA-010054) which deals with a primary voltage of 700V to 800V DC, and secondary voltage of 350V to 500V DC (single-phase-shift SPS) or 250V to 500V (extended-phase-shift EPS) for power levels up to 10 kW, Phase-shifted Full-Bridge (for example in PMP22951) which deals with a voltage of 400V down to 54V and a power level of 3kW or CLLLC Dual-Active Bridge (for example in TIDM-02002) which deals with a primary voltage range of 380–600V to a secondary voltage range of 280–450V and power levels up to 6.6kW. Depending on whether it is a three-phase or a single-phase application, the drain-source voltage rating of the devices would change. This translates to 650V drain-source voltage rating for single-phase and up to 1200V rating for three-phase application (higher drain-source voltage rating required for systems with higher DC bus voltage).

Though Lead-acid type batteries are very popular in energy storage systems, newer systems are increasingly moving to various types of Lithium batteries. The battery voltage depends upon the system power level. Lower power single phase systems commonly use 48V battery, while higher power three phase systems use 400V battery. Systems with even higher power range of string inverters could use 800V battery for storage. This may vary depending on the application and use case.

A more detailed block diagram of Energy Storage Power Conversion System is available on TI's Energy storage power conversion system (PCS) applications page.