How can we meet the growing power requirements of the future? 5G network deployments necessitate high-efficiency power conversion solutions capable of handling increased power demands without sacrificing reliability or compactness. In this post, we'll introduce a scalable, efficient, and advanced solution—the multiphase inverted buck-boost power conversion—and discuss why it's perfect for the challenges presented by modern telecom infrastructure.
Introducing a Multiphase Inverted Buck-Boost Controller
Multiphase inverted buck-boost controllers, such as the MAX15258 from Analog Devices, are revolutionizing telecom power design. Unlike traditional forward converters, these controllers use multiple phases operating in parallel, significantly reducing ripple currents, lowering switching losses, and enhancing efficiency dramatically (often beyond 98%).
Figure 1 shows a typical high-level block diagram of the power supply for a 5G macro or Femto remote radio unit (RRU) board. A hot swap controller is almost universally placed in front of the -48 VDC converter. Examples of fully featured -48 VDC hot swap power managers are the ADM1073 and LTC4284, which are a great fit for these applications.
Figure 1: A Power Supply for a 5G Macro Base Station Block Diagram
The MAX15258 is a high-voltage multiphase boost controller with an I2C digital interface, designed to support up to two MOSFET drivers and four external MOSFETs in single-phase or dual-phase boost/inverting-buck-boost configurations. The device monitors the low-side MOSFET current of each phase, via RSENSE, and uses a differential current-sense signal to ensure a proper active phase current-balance behavior when two MAX15258 ICs are stacked up in a host-node configuration. The current imbalance is applied to the cycle-by-cycle current sensing circuitry as feedback, helping to regulate the load current so that it is evenly shared between the two phases. Two controllers can be stacked for a 3-phase or a 4-phase configuration. The device drives the phases with the right amount of phase shift to obtain maximum ripple cancellation. Figure 2 shows the simplified block diagram of an interleaved two-phase inverted buck-boost implementation.
Figure 2: A Simplified Block Diagram of the Two-Phase Interleaved Inverted Buck-Boost
The MAX15258 is fundamentally a boost converter running at a relatively low frequency. This naturally reduces the switching losses, which are the most important contributors to power loss in these converters. The device is designed to support up to 1 MHz switching frequencies. In multiphase operations, the phases run in parallel, and they all run at the same frequency (but interleaved). The total equivalent frequency is N × Freq where N is the number of phases, but the losses are those at frequency for each converter. The interleaving implementation results in some cancellation of the ripple current seen by the output capacitor. This is the trick that makes the MAX15258 a front-runner solution for -48 VDC conversion.
Scalability and Efficiency in High Power Applications
The active-clamp forward converter requires too many complicated steps to ensure that the transformer does not saturate. On the other hand, the MAX15258 controller is designed to scale easily, supporting up to four phases for seamless high-power applications, while allowing designers to set the output voltage dynamically through a digital serial interface. Its capability to operate at lower switching frequencies per phase reduces overall system losses, while interleaving phases provides excellent ripple cancellation, resulting in smaller inductors and capacitors. Advanced telemetry, such as current balancing and digital interface options, allows precise monitoring and dynamic adjustments, further boosting reliability and operational efficiency.
Conclusion
With the telecom industry's rapid shift towards denser, more power-hungry networks, network operators are going to have to install more small cells in more places faster than ever before. And of course, the point of load (PoL) in these products needs to be very efficient, with ratings of at least 98% power conversion efficiency. The MAX15258 high-voltage inverting buck-boost controller design is cost-effective, efficient, and scalable, allowing easy addition and removal of phases on the same PCB layout. These benefits allow power converter designers to extend power conversion efficiency. ADI will continue to respond to these and similar challenges by developing more -48 VDC high-power conversion solutions designed for the 5G market while drawing on considerable expertise in power supply architecture.
Read all the blogs in the Inside the Negative 48V Revolution series.