As 5G and telecom technology continue to advance at breakneck speed, power demands for network equipment are soaring to new heights. Traditional power conversion methods, such as the active-clamp forward converter, face new challenges. The active-clamp forward converter remains the industry’s most utilized point of load (topologies for converting -48 VDC to positive voltages. Let's explore why these conventional methods struggle to meet today's telecom requirements and what's needed for future-proofing.
Challenges with Active-Clamp Forward Converters
Now that we understand where -48 VDC comes from, let’s discuss one of the industry’s most utilized PoL topologies for converting -48 VDC to positive voltages. Many telecom PoL designers use an active-clamp forward converter to implement their inverted buck-boost design. Other circuit versions that are also used are push-pull, half-bridge, or full-bridge converters. The benefits are that most of the transformer leakage energy is reclaimed via its nearly lossless recovery method. It is important for the PoL designer to first understand the basic timing that is default to the active clamp reset. In fact, a mis-sizing of the clamp capacitor can lead to an increase of the PoL’s duty cycle, which can result in transformer saturation and cause a long-term reliability effect on the main switch. Figure 1 shows a conventional low-side transformer reset active-clamp forward converter circuit design. The transformer reset mechanism includes CCLAMP and Q1.
Figure 1: A Conventional Low-Side Transformer Reset Clamp Active-Clamp Forward Design
Active-clamp forward converters have been reliable for years, but they exhibit notable drawbacks when scaling beyond 500 W. Specifically, these converters require precise sizing of the clamp capacitor, making them vulnerable to transformer saturation and increased voltage stress—potentially catastrophic scenarios for network reliability.
A few of the disadvantages associated with the active clamp include the need to precisely size the clamp capacitor. A large capacitor value results in less voltage ripple but introduces a transient response limitation. The active-clamp forward topology necessitates an advanced control technique to synchronize the delay timing between the active clamp and the main switch gate drive. Another disadvantage associated with the active clamp is that if not clamped to some maximum value, an increased duty cycle can result in transformer saturation or additional voltage stress on the main switch, which can be catastrophic.
Why Multiphase Matters
Power demand in telecom networks is increasing rapidly due to several converging factors. As technologies like 6G replace 5G, the need for higher data throughput significantly increases energy consumption. Additionally, some infrastructures are becoming more dense, with the deployment of numerous small cells and edge data centers to meet the requirements of low latency and high bandwidth. This densification is coupled with an increase in computational loads at the network edge, for example, on the remote radio unit (RRU). Furthermore, each telecom site now houses more active equipment, all of which contributes to the growing power demands.
Single-phase converters quickly lose efficiency and become impractical as telecom power demands rise. Multiphase designs, by contrast, distribute loads efficiently, reduce ripple current, and significantly increase operational reliability and efficiency. The active-clamp forward converter discussed above is a single-stage DC-to-DC converter.
As the power level increases—for example, 800 W equipment in 5G systems is becoming the norm—a multiphase design will exhibit more advantages for these power-hungry applications. A single-phase converter misses out on all the benefits that come with using a multiphase interleaved operation. Also, an active-clamp forward design cannot be scaled to higher output power with similar results as a design with lower output power.
Conclusion
Forward converters have served telecom well, but as we advance into the era of power-intensive 5G, the need for innovative power conversion solutions becomes evident. Embracing multiphase designs will become essential for achieving high reliability and efficiency in next-generation telecom equipment. In the next blog, we will discuss a multiphase solution with interleaved operation to accommodate such applications.
For more on this, see the technical article Building a Better -48 VDC Power Supply for 5G and Next Generation Telcom Equipment
Read all the blogs in the Inside the Negative 48V Revolution series.