Achieving efficient voltage conversion through an intermediate voltage rail

DC bus voltages are employed in numerous systems. Such voltages typically operate at 24 V (e.g., in industrial applications) or 48 V (e.g., in automotive contexts). Modern server and data centre deployments also utilise 48 V, with 52 V employed in certain scenarios. Multiple voltage converter topologies may be selected when converting this DC bus voltage to 12 V or 5 V. Where electrical isolation is not required, conversion methods based on the buck control concept are particularly prevalent. However, when a buck regulator converts a high DC voltage (such as 48 V) to a low output voltage (such as 5 V), it must operate at a low duty cycle of 9.6% (for the 48 V to 5 V example), resulting in only moderate efficiency. See Figure 1 for details.

Figure 1. Power supply architecture employing a 48 V DC bus voltage

For transformer-based architectures such as push-pull or forward converters, the duty cycle can be adjusted by altering the transformer's winding ratio. While this theoretically enhances conversion efficiency, the transformer itself introduces additional losses, limiting the efficiency gains. Particularly in applications where electrical isolation is unnecessary, omitting the transformer is often preferred.

Hybrid converters provide an effective solution to this challenge. Employing an innovative power conversion topology, it combines a charge pump configuration with a buck regulator. This converter utilises four switches: the charge pump halves the input voltage, after which the two lower switches, in conjunction with an inductor, convert this halved voltage to the required output level. Figure 2 illustrates the circuit concept of the hybrid voltage converter, thus demonstrating that the hybrid converter can be regarded as a fusion of a charge pump and a buck regulator.

Figure 2. Topology of the hybrid converter

The circuit shown in Figure 2 can be constructed using hybrid controller ICs such as the LTC7821.

For achieving exceptionally compact solutions, the LTM4654 presents an excellent choice. This device belongs to ADI's μModule® series, capable of converting DC bus voltages up to 55 V into adjustable lower voltages such as 5 V or 12 V, delivering continuous output power up to 300 W. Figure 3 illustrates a solution employing the highly integrated LTM4654. Requiring minimal external components, this circuit achieves 96.7% conversion efficiency when converting a 48 V input to a 9 V output at a 15 A load current.

Measuring just 16 mm × 16 mm with a height of 8.96 mm, this 300 W module offers exceptional compactness.

Figure 3. Highly Integrated μModule Voltage Regulator in Power Bus Converter Applications (Simplified Schematic Diagram)

Beyond pure voltage conversion functionality, this module may also serve as a current source and current sink. This enables bidirectional operation, facilitating efficient energy management within the system. Furthermore, the LTM4654 is suitable for negative voltage conversion scenarios, such as converting +30 V to -7 V. Its operating principle resembles the reverse buck-boost mode of a step-down regulator.

Multiple hybrid converters may also operate in parallel. When two converters operate in parallel, the current handling capability doubles, with the power conversion capability correspondingly increasing by a factor of two.

Conclusion

In the field of high-efficiency voltage conversion, alongside widely adopted common topologies such as buck converters and various transformer solutions, innovative approaches like hybrid converters also offer significant advantages. These novel solutions enhance efficiency—particularly for bus converters—while occupying minimal printed circuit board space.