Two DC/DC converter combinations designed to reduce system cost, component count and size
With growing focus on more compact and efficient power systems, bidirectional converters are gaining significant attention. A bidirectional DC/DC converter enables power flow in both directions, replacing the need for two separate DC/DC converters used in battery charging and backup operations. This integration reduces system cost, component count, and overall size, making it an attractive solution for modern power applications.
This blog series will explore the use of bidirectional converters in uninterruptible power supplies (UPS), battery backup units, and energy storage systems, in two parts. In the first part, we'll look at how these converters enhance traditional UPS designs and improve efficiency in critical power applications.
UPS or battery backup units play a vital role in providing continuous power in both critical and non-critical environments. Traditional UPS systems include standby, line interactive, and online (or dual conversion) types. However, there has been limited innovation in newer categories like hybrid standby-online or advanced ECO mode UPS systems.
Figure 1 shows a block diagram of a conventional online UPS. During normal operation, the main DC bus is regulated between 300V and 400V using the AC grid. The battery serves as an energy storage unit and can be charged via an AC source or DC bus using a dedicated AC/DC or DC/DC converter. A second DC/DC boost converter transfers power from the battery to the DC bus during a power outage. Figure 2 illustrates an online UPS with a bidirectional converter.
The highly efficient, small-scale bidirectional reference design is a digitally controlled 2kW isolated DC/DC converter designed to transfer power between a 300V–400V DC bus and a 48V battery pack. It features a full-bridge power stage on the high side and a current-fed full-bridge stage on the low side. Under normal conditions, it operates in buck mode, charging the battery at a constant current until it reaches its voltage limit. During a power failure, it switches to boost mode, stepping up the 48V battery (with an input range of 36V–60V) to a 380V DC bus, supporting backup loads.
This design achieves peak efficiencies of 94% in buck mode (as a charger) and 95% in boost mode (during discharge), with an average efficiency over 93%. The high efficiency in boost mode extends battery run time. Additionally, the current-fed full-bridge stage ensures high efficiency across a wide input voltage range, optimizing the isolation transformer design to reduce RMS current in the windings and minimize ripple current in the input capacitor.
Operating at a high switching frequency of 100 kHz, the reference design delivers 2 kW of power within a compact size of 185 mm x 170 mm. Figure 4 shows the system efficiency in backup mode (boost mode).
The transition from charging mode to standby mode must be seamless to ensure uninterrupted power delivery. The reference design achieves a conversion time of less than 100μs, minimizing the need for large capacitors to support the load during the switch. Figure 5 highlights this fast mode transition from battery charging to standby mode.
In the second part of this series, we’ll dive deeper into the working principles of the 2kW, isolated bidirectional DC/DC reference design for 48V to 400V applications.
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