Efficient Voltage Conversion from an Intermediate Voltage Rail
摘要
There are different switch-mode power supply topologies to convert an intermediate voltage rail into lower voltages to power different loads in various applications. If the intermediate voltage rail voltage is relatively high, such as 48 V, and if the output voltages need to be relatively low, such as 12 V or 5 V, a new topology—the hybrid converter—offers higher power conversion efficiency than simple heritage buck regulators. This article introduces the innovation of a hybrid converter as well as a practical solution with a μModule® regulator.
Introduction
DC link voltages are used in many systems. Often these voltages are 24 V, as in the industrial sector, or 48 V, like in the automotive sector. Modern server and data center applications also use 48 V and sometimes 52 V. Different voltage converter topologies can be used to convert this DC link voltage to 12 V or 5 V. A conversion according to the buck control concept is particularly common if no galvanic isolation is required. However, a buck regulator has only moderate efficiency with a voltage conversion from a high DC link voltage, such as 48 V to a low output voltage of 5 V. This is due to the necessary operation at a low duty cycle of 9.6%; for example, with a conversion from 48 V to 5 V. See Figure 1.
With a transformer-based architecture, such as a push-pull or forward converter, the duty cycle can be adjusted by the winding ratio of the transformer. This would allow the conversion to take place more efficiently if it weren’t for the additional losses caused by the transformer. Especially in applications that do not require galvanic isolation, one would like to do without a transformer.
A convincing solution here is a hybrid converter. It uses an innovative power conversion topology that connects a charge pump topology to a buck regulator. This converter uses a total of four switches that are used to halve a supply voltage with charge pump action. In addition, the lower two switches are used together with an inductor to convert the halved supply voltage to the desired output voltage. Figure 2 shows the circuit concept of a hybrid voltage converter. A hybrid converter is therefore the fusion of a charge pump with a downward-converting buck regulator.
The circuit from Figure 2 can be built with a hybrid controller IC, such as the LTC7821.
For a very compact solution, the LTM4654 is a good choice. This is a building block from the μModule® family from Analog Devices. This solution can convert a DC link voltage up to 55 V to a lower, adjustable voltage of 5 V or 12 V, for example. The converted power can be up to 300 W continuously. Figure 3 shows a solution with the highly integrated LTM4654. This circuit requires very few external components and allows for a conversion efficiency of 96.7% at a conversion of 48 V input voltage to 9 V output voltage at 15 A load current.
This 300 W module is only 16 mm × 16 mm and has a height of 8.96 mm.
In addition to pure voltage conversion, the module can be used both as a current source and as a current sink. This means that bidirectional operation can take place to work with energy efficiently in a system. The LTM4654 can also be used in the negative voltage range; for example, to convert +30 V to –7 V. This conversion takes place in a similar way to a buck regulator in the inverting buck-boost mode.
Furthermore, it is possible to operate several of these hybrid converters in parallel. With two converters, twice the current and thus twice the power can be converted.
Conclusion
For efficient voltage conversion, there are not only the usual circuit topologies that are widely used, such as the buck converter or various transformer-based solutions. Innovative, new solutions such as hybrid converters offer important advantages. These create higher efficiency, especially in bus converters in various applications, and require little space on a printed circuit board.
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