With increasing emphasis on more compact and more efficient power systems, two-way converters are receiving increasing attention. A bidirectional DC/DC converter with bidirectional power flow combines the two DC/DC converters traditionally required for battery charging and backup operations, reducing overall system cost, component count and size.
This series of blog posts will examine the use of bidirectional converters in uninterruptible power supplies (UPS), battery backup units, and energy storage system device applications in two parts.
UPS or battery backup units provide uninterruptible power in a variety of critical and non-critical applications. UPS systems can be divided into standby UPS, line interactive UPS and online or dual commutation UPS in the traditional sense. Little progress has been made in new categories of UPS development, such as standby online hybrid or advanced ECO mode UPS.
Figure 1 shows a block diagram of a traditional online UPS.
Figure 1 : Block diagram of a conventional online UPS
In normal operation, the main DC bus is regulated between 300V and 400V via an AC grid source. The battery acts as an energy storage unit and can be charged via an AC power source or DC bus using a dedicated AC/DC or DC/DC converter. Another DC/DC boost converter is used to transfer battery power from the battery to the DC-Bus during standby power down (standby operation). Figure 2 shows a block diagram of an inline UPS with a bidirectional converter.
Figure 2 : Online UPS block diagram with bidirectional DC-DC converter
The highly efficient 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 side and a current fed full bridge stage on the low side. In the presence of a DC bus (normal condition), the design operates in buck mode and charges the battery at a constant current until the battery voltage is at a limit. During power-down, the design operates as a current-fed full-bridge converter that boosts the power of a 48V battery (36V to 60V input) to a 380VDC bus, supporting backup loading.
The reference design has a peak efficiency of 94% in buck mode (as a charger), 95% in boost mode (during discharge), and an average efficiency of over 93%. This high discharge efficiency provides longer run times for the battery. High current efficiency in discharge mode using a current-fed full-bridge stage over a wide input voltage range of 36V to 60V, which optimizes the design of the isolation transformer to minimize the root mean square (RMS) current in the winding while minimizing Pass the ripple current of the input capacitor.
Operating at a high switching frequency of 100 kHz, the reference design has a power level of 2 kW and a size of 185 mm x 170 mm.
Figure 4 : System efficiency in backup mode (boost mode)
The transition from charging mode to standby mode or over time is critical to ensure continuity of load power. The conversion time of the reference design is less than 100μs, which reduces the large capacity of the system required to provide power during the conversion.
Figure 5 : Fast mode switches from battery charging to standby mode
In the second part of the series, we will study the details of the work of a 2KW, isolated DC-DC bidirectional reference design for 48V to 400V applications.
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