This article appeared in Electronic Design and has been published here with permission.
This article is part of our IMS 2023 coverage.
NXP rolled out a family of gallium-nitride (GaN)-based RF power amplifiers (PAs) that uniquely leverages top-side cooling to reduce the thickness and weight of radios in 5G base stations by more than 20%. The new series of RF modules unites the company’s LDMOS and GaN technologies in multichip modules (MCMs). Top-side cooling handles the higher power levels of 5G base stations without overheating.
The new RF offerings feature a high gain of more than 30 dB and high efficiency of 46% over 400 MHz of instantaneous bandwidth. Per NXP, they’re ideal for 5G radios that cover frequency bands from 3.3 to 3.8 GHz. The LDMOS-based power IC acts as the driver in the dual-stage power amp, while the GaN device plays the role of the secondary power stage, giving signals a final push through the RF signal chain.
Telcos are buying base stations by the millions to expand 5G networks around the world. As a result, NXP and other semiconductor firms are fighting to supply more of the RF chips at the heart of the systems.
In 2020, NXP ramped up mass-production of RF GaN at its U.S. fab to meet rising demand for 5G chips, which must handle millimeter waves and other high-frequency bands, where LDMOS on its own hits the wall.
Power-Handing Properties
GaN, which belongs to a class of wide-bandgap semiconductors, can handle 10X higher breakdown voltages than alternatives, while it beats silicon in terms of efficiency, too, leading to less power loss.
Other unique properties of GaN are very high electron mobility and a reduction in temperature coefficient, which keeps on-resistance (RDS(on)) in check. Thus, it improves performance in cases where conduction losses tend to dominate.
GaN can also run at high switching frequencies along with large voltage transients (dV/dt) due to improvements in parasitic capacitance. The result is that GaN power devices lose less power during turn-on and turn-off. Reverse-recovery charge (QRR) is limited or even wiped out completely as well.
The power-handling qualities of GaN are giving it the edge in power electronics. But they’re also relevant to RF power amplifiers, one of the core building blocks of a base station that boosts RF power signals.
The latest 5G base stations are equipped with from 32 to 64 antennas, giving them more lanes to communicate with devices on the cellular network at once. This translates to faster data transfers and wider coverage.
These antennas are also required to tap into another technology: massive MIMO. Using it, 5G radio units can use beamforming to connect many more devices, which in turn boosts the capacity of a base station.
But bundling all of these antennas into a single box requires the use of more RF power amps. As a result, challenges emerge when it comes to managing power and heat in the radio unit, which can consume from 200 W to more than 300 W.
High thermal conductivity means that RF GaN power devices themselves are relatively easy to cool. But care must still be taken to transfer heat that can impact performance or lead to faults away from the chip. The amount of heat encountered by the device can vary significantly, depending on how it’s packaged and how it’s mounted on a circuit board (PCB). Thus, it becomes mandatory to correctly manage the removal of heat.
Staying Cool for 5G
For the most part, RF GaN power devices are housed in bottom-cooled packages. The heat is dissipated through the underbelly of the package and then the PCB acts as a heatsink, resulting in excess loss.
But there’s a limit to the amount of heat that can be taken out from under the RF module. When handling the higher power density of 5G radios, staying cool is a serious challenge without using bulky packaging.
To better manage thermals, NXP is bringing high-side cooling technology to RF power devices for the first time (see figure). The first products in the new family are the A5M34TG140-TC, A5M35TG140-TC, and A5M36TG140-TC.
