On the other hand, thermal issues represent a significant challenge in the design of transmitters, especially in a combined PA/converter module. In configurations where the converter and the RF PA are co-packaged as a single module and installed at the top of the tower (alternative configurations keep the complete RF transmit stage at the base of the tower), even modest gains in efficiency can have tremendous performance advantages.

For example, Norton states that an improvement in converter efficiency from 90% to 92% can lead to total heat reductions of up to 20% in the combined module. The less heat produced by the converter, the more RF output power that can be generated by the RF PA, which is usually the driving parameter for the transmit path. Packaged as a half-brick, these converters are available with 28 V or 48 V outputs, and reach efficiencies up to 92%.

Digital power control might seem to hold the potential to introduce further single-digit percentages in gained efficiency. However, according to Harris, to emulate the equivalent operation of an analog control loop operating at 1 MHz (for example), the clocking frequency of a digital controller would have to be significantly higher, which could then hasten the arrival of potential RF interference with the RF PA in future systems.

Harris states that other advances, mainly in RF power device technologies such as SiC and GaN semiconductors, will have a greater impact on the overall efficiency of transmitters than digital power control. For now, converters with analog control (Norton states the PAH450S uses an analog control loop) represent a solid option for powering analog PAs. Harris also states that for PA applications, there are so many minor considerations that a generic digital power control platform might not be able to accommodate subtle hardware adjustments that are possible in a customized analog converter.

Power supplies needed for the PAs of next-generation base station applications are also faced with several other requirements. These include the ability to withstand extreme ambient temperature swings, compact size, and a wide output adjustment range to allow the user to optimise the amplifier’s performance. Lambda’s PAH450S series of 450 W dc-dc converter modules are designed to meet these demands.

For example, many cell repeaters are configured as three separate single-pole antennas, each 2 ft to 3 ft tall, mounted in a triangular configuration on a 200 ft to 300 ft tower. Each pole typically has a separate RF PA and dc-dc converter. A power bus is routed up the tower, and the wide input range spanned by the 48 V version of the converter (36 Vdc to 76 Vdc) is highly tolerant of the voltage drop that can occur along the length of the power bus, which is approximately 6 V to 7 V, according to Norton.