“UPS or battery backup units can provide uninterrupted power in a variety of critical and non-critical applications. UPS system can be divided into standby UPS, line interactive UPS and online or double converter UPS in the traditional sense. In the development of new categories of UPS, such as standby online hybrid or advanced ECO mode UPS, little progress has been made.

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This series of blog posts will be divided into two parts to study the use of bidirectional inverters in uninterruptible power supply (UPS), battery backup unit and energy storage system device applications.

UPS or battery backup units can provide uninterrupted power in a variety of critical and non-critical applications. UPS system can be divided into standby UPS, line interactive UPS and online or double converter UPS in the traditional sense. In the development of new categories of UPS, such as standby online hybrid or advanced ECO mode UPS, little progress has been made.

Figure 1 shows a block diagram of a traditional online UPS.

Figure 1: Block diagram of conventional online UPS

Figure 2: Block diagram of online UPS with bidirectional DC-DC converter

The high-efficiency small bidirectional reference design is a digitally controlled 2kW isolated bidirectional DC/DC converter designed to transfer power between a 300V to 400VDC bus and a 48V battery pack. The reference design has a full-bridge power stage on the high-voltage side and a current-fed full-bridge stage on the low-voltage side. In the presence of a DC bus (normal conditions), the design is designed to operate in a step-down mode and charge the battery with a constant current until the battery voltage is at a limit value. During a power outage, the design operates as a current-fed full-bridge converter, boosting the power of the 48V battery (36V to 60V input) to the 380VDC bus, supporting backup loading.

Figure 3: Block diagram of isolated bidirectional DC-DC converter

The reference design has a peak efficiency of 94% in the step-down mode (as a charger) and 95% in the step-up mode (during discharge), and the average efficiency is higher than 93%. This high discharge efficiency provides longer operating time for the battery. In the wide input voltage range of 36V to 60V, the current-fed full-bridge stage is used to achieve high efficiency in the discharge mode. It optimizes the design of the isolation transformer to minimize the root mean square (RMS) current in the winding while minimizing Ripple current through the input capacitor.

Working at a high switching frequency of 100kHz, the power level of this reference design is 2kW and the size is 185mm×170mm.

Figure 4: System efficiency in backup mode (boost mode)

The transition from the charging mode to the standby mode or the change over time is essential to ensure the continuity of the load power supply. The conversion time of the reference design is less than 100μs, thereby reducing the large capacitance required by the system to provide power during the conversion period.

In the second part of this series, we will study the working details of a 2KW, isolated DC-DC bidirectional reference design for 48V to 400V applications.