Once 5G Hits, DC-DC Converters May Be RF Power Amplifier’s Saving Grace

With the 5G rollout imminent, engineers have been tasked with redesigning much of the cellular infrastructure that will support the high frequencies offered by 5G. One of the major challenges of this design is to minimize power consumption; lower power consumption means longer battery life in phones and less money in base stations. 

 

Breakdown of power consumption by component in RF systems. Image used courtesy of Mouser

 

As Texas Instruments' Timothy Hegarty points out, in both base stations and in cellular devices, power amplifiers are the main consumers of energy. For this reason, engineers have honed in on this component with the hopes of maximizing its power efficiency.

Interestingly, DC-DC conversion technologies may be the key to optimizing power efficiency in RF power amplifiers. Recognizing this reality, Flex Power Modules recently teamed up with both the OpenRAN and O-RAN Alliance to expand its range of DC-DC converters for RF power amplifiers. 

 

One of Flex Power Modules fully-regulated analog DC-DC converters, the PKM4516AD. Image used courtesy of Flex Power Modules

 

“We are leveraging our interaction with OpenRAN members to provide companies with a wide selection of DC-DC parts that are ideal for RFPA applications," explains Flex Power Modules' head of sales APAC, James Zhang.

An overview of RF power amplifiers and their power efficiency can help us size up DC-DC converters as a solution, especially in the face of 5G. 

 

What is an RF Power Amplifier? 

RF power amplifiers (RFPAs), put simply, are devices that convert a low-power radio frequency signal into a high-power radio frequency signal.

 

Block diagram showing where, sequentially, an RFPA operates. Image used courtesy of Analytical Graphics

 

These components are used in the transmitter section of an RF system since they are required to drive the antenna and transmit the signal. 

 

Why are RF Power Amps Inherently Inefficient?

Due to fundamental laws of thermodynamics, it is physically impossible for electronic devices to achieve 100% power efficiency. RF applications suffer especially poor power efficiency due to the many impediments of converting DC power to RF power. These causes include signal path loss, high operating frequencies, and inherent shortcomings of devices. 

 

Gain compression is shown graphically. Image used courtesy of Keysight

 

In an RFPA, linearity is of equal importance to power efficiency. Linearity is critical since the modulation schemes used in cellular communication create amplitude-modulated signals that exhibit a non-constant amplitude envelope. 

Unfortunately, this presents a tradeoff with power efficiency because efficient PA operation occurs in gain compression (when amplifiers don’t exhibit a linear relationship between input and output power). Hence, power amplifiers are forced to operate less efficiently for the sake of linearity.

 

Other Causes of RFPA Inefficiency and Solutions

Historically, in a cellular device, the power amplifier is powered directly from the device’s battery. In this way, it becomes a source of inefficiency because, as previously mentioned, cellular modulation schemes cause signals to have non-constant amplitudes. Essentially, we are providing a constant amount of power to an amplifier, even though the actual amount of power needed varies. 

 

Traditional PA operation resulting in wasted energy. Image (modified) used courtesy of Digi-Key

 

Creatively, engineers came up with techniques such as average power tracking (APT) and envelope tracking (ET) to resolve this issue. 

 

Energy savings given by APT and ET vs. fixed voltage. Image used courtesy of Texas Instruments 

 

Using these techniques, the power supplied to the amplifier is determined by either the average power of the signal or the signal’s envelope. 

 

How a DC-DC Converter Can Help

A DC-DC converter can ameliorate some of these power inefficiencies in RF power amplifiers. To provide real-time power tracking and dynamically adjust the DC supply voltage, a DC-DC converter is necessary. 

 

How a DC-DC converter can be used to save energy in RFPAs. Image (modified) used courtesy of Digi-Key

 

For RF applications, a linear regulator is seemingly the best choice for a converter because it generates very little noise and there’s no high-frequency switching that could potentially interfere with the RF signal.

However, since power is a huge concern, linear regulators are not chosen due to their inefficiencies. This has led companies to design DC-DC converter modules that are specifically optimized for RF power amplifiers.

 

With 5G, Power Efficiency is Key

For 5G to be successful on a large scale, it will need to be power efficient. Power efficiency saves cost in energy and in cooling solutions. It’s for these reasons that companies like Flex Power Modules have been leveraging O-RAN and OpenRAN to grow their presence in the RFPA market.

The continual development of RFPA-specific DC-DC converters will hopefully lead to scalable and affordable 5G solutions to support cellular communication and IoT applications. 

 

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If you work with cellular infrastructure for 5G, how have you accounted for power efficiency in your design? Share your experiences in the comments below.