Power Tips #155: How to choose the best topology for isolated energy storage systems

With the increasing popularity of renewable energy and electric vehicles, the large-scale application of bidirectional isolated DC/DC converters is growing. This article will focus on energy storage systems (ESSs), with an overview of the isolated bidirectional topologies used in ESSs.

Figure 1 shows a typical residential ESS. A unidirectional DC/DC converter, also known as a maximum power-point tracker, converts the energy generated by solar panels and then feeds that energy to a high-voltage DC bus. An inverter may transfer this energy to the electric grid, or the energy may be stored in the battery system, where a bidirectional DC/DC converter charges and discharges the battery.

Figure 1 This block diagram shows the constituent pieces of a residential ESS system. Source: Texas Instruments

Because of safety considerations, residential ESSs use low-voltage batteries, such as 48V. Therefore, the DC/DC converter must provide galvanic isolation between the high-voltage DC bus and the battery. Because the DC bus and battery vary from the voltage ranges shown in Figure 1, one of the design priorities for the converter is to have a wide voltage-adjustment range.

The traditional solution is usually an open-loop inductor-inductor-capacitor (LLC) and a closed-loop buck/boost converter. The benefit of this solution is that it is easy to control and has a wide voltage-adjustment range, but a two-stage converter results in lower total system efficiency and higher costs.

To optimize the performance of the bidirectional DC/DC converter, a one-stage solution is preferable. Compared to a two-stage solution, a one-stage solution has higher efficiency and lower costs, but it also needs a more complex control scheme to achieve high performance. The most popular one-stage solutions are LLC, capacitor-inductor-inductor-inductor-capacitor (CLLLC), dual active bridge (DAB) and series resonant DAB (SR-DAB).

As shown in Figure 2, an LLC resonant converter can achieve very low switching losses. When operating near or below the resonant frequency, the LLC can achieve zero voltage switching (ZVS) on active bridges and zero current switching on rectifier bridges. When operating above the resonant frequency, an LLC can only achieve ZVS on active bridges.

Figure 2 A LLC resonant converter, shown in this schematic, can achieve very low switching losses. Source: Texas Instruments

Since this converter soft switches during turnon and turnoff, the electromagnetic interference emissions are comparatively low to other converters without soft switching. But designers generally only use the LLC resonant converter for unidirectional power transfer. It is not good for wide-gain-range applications such as ESSs because the voltage gain curve is flat when the switching frequency is higher than the resonant frequency, as shown in Figure 3.

Figure 3 This LLC voltage gain curve is flat when the switching frequency is higher than the resonant frequency. Source: Texas Instruments

Adding resonant elements on the secondary side of the transformer will form a bidirectional CLLLC resonant converter. As shown in Figure 4, the converter resonance network is symmetrical and proportional to the transformer’s turns ratio, so this converter maintains the resonance law and functionalities, such as soft-switching features in both forward and reverse operation.

Figure 4 A bidirectional CLLLC, shown in this schematic, is symmetrical and proportional to the transformer’s turns ratio. Source: Texas Instruments

A major advantage of this topology is the ability to provide symmetrical bidirectional power transfer. The effect of the secondary resonant tank may cause the voltage gain curve to have multiple crests, however, as shown in Figure 5. Similar to an LLC converter, a CLLLC also has a narrow voltage adjustment range and consists of five resonant elements, which increases the size and cost of the system, while the additional inductor also introduces additional losses.

Figure 5 The effect of the secondary resonant tank, shown in this gain curve of a CLLLC, may cause the voltage gain curve to have multiple crests. Source: Texas Instruments

Figure 6 shows the basic DAB topology, which consists of a full bridge with active switches on both the primary and secondary sides. The main advantages of the DAB converter are its inherent bidirectional capability, faster dynamic response, and wider voltage-adjusted range compared to the LLC and CLLLC topologies.

Figure 6 The basic DAB topology, shown in this schematic, consists of a full bridge with active switches on both the primary and secondary sides. Source: Texas Instruments

Through the most basic single-phase-shift (SPS) control, it is possible to adjust the polarity and magnitude of the phase-shift angle between the primary and secondary bridges, thereby controlling the magnitude and direction of its transmission power. The peak current of transistors and the circulating current are large with SPS control, however, which results in high turnoff losses as well as circulation losses.

ZVS is also difficult to achieve at light loads. With advanced modulation schemes such as triple-phase shift, a DAB converter can theoretically achieve ZVS over the entire operating range and could optimize high peak current and circulating current, but will require significantly more complex system analysis.

Inserting a series resonant tank into the DAB converter forms an SR-DAB, as shown in Figure 7. Similar to a series resonant converter, an SR-DAB needs to operate above the resonant frequency in order to ensure that the converter is in the inductive region, which is a necessary condition to realize ZVS. Given the resonance features of the circuit, the inductor current of an SR-DAB is almost sinusoidal, which could have a smaller turnoff current and root-mean-square inductor current compared to a traditional DAB.

Figure 7 Inserting a series resonant tank into the DAB converter forms an SR-DAB, as shown in this schematic. Source: Texas Instruments

Table 1 summarizes the primary features of the four isolated bidirectional DC/DC topologies.

	LLC	CLLLC	DAB	SR-DAB
Voltage adjustment range	Narrow	Narrow	Wide	Wide
Transient response	Slow	Slow	Fast	Fast: phase shift plus fixed frequency Slow: phase shift plus variable frequency
ZVS range	Wide (full-range ZVS）	Wide (full-range ZVS）	Narrow (no ZVS at light loads)	Medium
Turnoff current	Low	Low	High (with SPS)	Medium (with SPS)
Control complexity	Moderate	Moderate	Simple (SPS) Complex (TPS)	Simple (SPS) Complex (TPS)

Table 1 This table summarizes various DC/DC topologies.

All four topologies provide isolation and bidirectional power transfer functions; however, DAB and SR-DAB are more suitable for ESS applications given their wide voltage-adjustment range and faster dynamic response. In addition, an SR-DAB has a lower turnoff current and a wider ZVS range compared to a DAB, with higher efficiency than a DAB as well.

For more details about the DAB topology, see the Texas Instruments Power Supply Design Seminar paper, “Dual Active Bridge Topology Overview.”

Guangzhi Cui is a power systems engineer at Texas Instruments. In this role, he handles industrial power system design and provides semiconductor power system solutions and customer technical support.

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