Two DC/DC converter combinations designed to reduce system cost, component count and size
With growing focus on compact and efficient power systems, bidirectional converters are gaining more attention. A bidirectional DC/DC converter enables power flow in both directions, replacing two separate DC/DC converters typically used for battery charging and backup operations. This not only reduces system cost and component count but also minimizes the overall size, making it ideal for modern applications.
This blog series will explore the use of bidirectional converters in uninterruptible power supplies (UPS), battery backup systems, and energy storage devices, in two parts. In the first part, we'll look at how these converters enhance traditional UPS configurations and improve efficiency.
UPS or battery backup units play a crucial role in providing continuous power in both critical and non-critical environments. Traditional UPS systems include standby, line-interactive, and online or dual conversion models. While there has been limited innovation in new categories like hybrid standby-online or advanced ECO mode UPS, the demand for more efficient and flexible solutions continues to rise.
Figure 1 shows a block diagram of a conventional online UPS. During normal operation, the main DC bus is maintained between 300V and 400V through the AC grid. The battery serves as an energy storage unit and can be charged via an AC source or DC bus using dedicated AC/DC or DC/DC converters. 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, compact 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. In normal conditions, the design 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 (36V–60V input) to a 380V DC bus to support backup loads.
This reference design achieves a peak efficiency of 94% in buck mode (as a charger) and 95% in boost mode (during discharge). With an average efficiency over 93%, it offers extended battery runtime. Its high current efficiency in boost mode, across a wide input range of 36V–60V, optimizes the isolation transformer design by minimizing RMS current in the windings and reducing ripple current in the input capacitor.
Operating at a high switching frequency of 100kHz, the design delivers 2kW of power in a compact size of 185mm x 170mm. 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 supply. The reference design achieves this in under 100μs, significantly reducing the need for large capacitors or other components to sustain the load during the switch. Figure 5 highlights the fast mode transition from battery charging to standby.
In the second part of this series, we’ll dive deeper into the workings of this 2kW isolated DC/DC bidirectional reference design, specifically tailored for 48V to 400V applications.
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