Power Tip Converter Topologies for Isolated Energy Storage Systems

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Four single-stage converter topologies are commonly used for energy storage. With their wide voltage range and dynamic response, the DAB and SR-DAB topologies are the most suitable. A comparison of the topologies.

Due to the growing prevalence of PV systems and electric vehicles, energy storage systems (ESS) are increasingly being installed in private households, and bidirectional, isolated DC/DC converters are required to connect them. Various topologies are suitable for this purpose.

An MPP tracker (Maximum Power Point) feeds the energy from the PV panels into a high-voltage DC bus, from which the energy is then either fed into the power grid via an inverter or supplied to the batteries of an ESS via a bidirectional DC/DC converter. Traditionally, this converter consists of an unregulated LLC converter and a regulated buck/boost converter.

This solution is characterized by simple control and a wide voltage range; however, the use of two stages reduces efficiency and increases costs. A single-stage solution would clearly be more efficient and cost-effective, but it requires a more complex control scheme.

Bidirectional CLLLC Resonant Transducer

While the pure LLC topology is primarily used for unidirectional converters, adding resonant elements on the secondary side allows for the configuration of a bidirectional CLLLC resonant converter. The resonant network here is symmetrical and proportional to the transformer’s turns ratio. Soft switching is supported in both the forward and reverse directions. Another key advantage of this topology is its suitability for bidirectional power transfer.

The drawbacks of the CLLLC converter are its narrow voltage range and the use of five resonant elements, which increase the system's space requirements and also drive up costs. In addition, the additional inductance results in increased losses.

One alternative is the DAB (Dual Active Bridge) topology shown in Figure 1, which consists of a full bridge with active switches on both the primary and secondary sides. In addition to bidirectional operation, the advantages of this variant include faster dynamic response and a wider voltage range than those of the LLC and CLLLC topologies.

Using a simple PLC control (Single-Phase-Shift), the polarity and magnitude of the phase angle between the primary- and secondary-side bridges can be varied to determine the direction and extent of energy transfer. However, the peak currents and circulating current are high in SPS control, resulting not only in high switching losses but also in circulating losses.

Zero-voltage switching (ZVS) also poses a challenge at low loads. By using more sophisticated modulation techniques, a DAB converter can theoretically support ZVS across the entire operating range, and it is also possible to optimize peak currents and circulating current. However, this requires a significantly more complex system analysis.

Inserting a series resonant circuit into the DAB converter results in the SR-DAB topology (Figure 2). Similar to a series resonant converter, the SR-DAB converter must also be operated above the resonance frequency so that it operates in the inductive region, which in turn is a prerequisite for zero-voltage switching. Given the circuit’s resonant characteristics, the coil current is nearly sinusoidal, which can result in lower off-state currents and a lower RMS coil current compared to a traditional DAB configuration. 

*Guangzhi Cui is a Power Systems Engineer at Texas Instruments in Dallas, USA.