RF Power Supply Design

Key Takeaways

• In addition to careful consideration of high-frequency loss and thermal properties, effective regulation and filtration are required to maintain signal integrity and manage heat generation.

• Strategic component placement and routing are essential to minimize parasitic effects and electromagnetic interference, especially in high-power applications.

• The layout demands attention to aspects like power inlet design with filter capacitors, cable layout to prevent electromagnetic interference, and the selection of suitable materials and techniques for stack up.

RF power supply design topology that utilizes an RF amplifier.

RF power supply design is a complex and nuanced process, demanding meticulous attention to detail for best performance. These power supplies must ensure that different sections of an RF system operate harmoniously without the detrimental transfer of excessive noise, a challenge that intensifies with the higher power levels inherent in such systems. 

Aspects of RF Power Supply Design At a Glance

RF Power Supply Design Considerations

High power applications present multiple challenges for RF power supply design. 

• Strategic component placement and routing can optimize system integrity by combating against power inadvertently seeping into adjacent circuits through the feedline. 

• Managing heat output from FETs and amplifiers is crucial—a challenge that has led to the adoption of sophisticated semiconductor materials like Gallium Nitride (GaN) on Silicon Carbide (SiC) for efficient RF amplification. 

• RF power systems operating at extremely high power require the components to be assembled within a substantial housing or cabinet. In contrast, lower power systems can be integrated into a printed circuit board. 

Filtering and Regulations

Careful regulator selection and filter design are equally important for RF power supply design. 

• Regulators ensure that the voltage differential is not excessively high. A high differential can result in significant power loss, leading to rapid heating that may exceed the component's temperature rating. 

• Higher-order filters and a switching regulator that can adapt to the dynamics of the modulated signal are both crucial for effective RF power supply performance, especially in modulation scenarios. This is particularly relevant for an amplitude-modulated output, where the regulator and filter must track the signal's envelope, and for a frequency-modulated output, where modulation at the baseband frequency is required. 

The requirement of filters and regulators necessitates a complex, multiphase, and multilevel topology in the design. Power decoupling and filtering further involve isolating power cables from each other and ensuring that the power line is kept separate from lines prone to strong interference, such as clock lines. 

RF Power Supply Design Tips for Working with Ground

In mixed signal RF power supply designs, it's crucial to avoid a common pitfall: attempting to galvanically isolate the output of a switching regulator (if used) from the RF section by routing it across split grounds. Such an approach can lead to electromagnetic interference (EMI) issues, similar to those encountered when routing a single-ended digital signal over a discontinuous ground plane. This mistake is frequently seen in mixed signal systems and can easily occur in RF power supply designs if one mistakenly assumes that the output from the switching regulator is purely DC. However, even when employing envelope tracking, the power drawn is still time-averaged at the baseband frequency, which might range from tens to hundreds of MHz.

For the power supply wiring of a small signal amplifier, it's important to implement isolation using ground copper planes and via stitching. This technique is essential to shield the amplifier from external EMI interference, thereby preventing any degradation in signal quality. Such isolation is a key factor in maintaining the integrity and performance of the RF system, ensuring that the small signal amplifier functions optimally without interference from external noise sources.

Stackup and Material

In the process of determining the stackup for your RF power supply design, it's essential to take into account the frequency at which your regulator operates and the length of the RF lines on your PCB. For designs that incorporate extended RF lines, selecting a low-loss laminate, such as a PTFE-based material is advisable to minimize signal degradation. However, in scenarios where RF lines are not extensively routed, or loss considerations are not a primary concern, standard FR4 laminates can be a suitable and cost-effective choice for stackup materials. An effective compromise between achieving low loss and maintaining affordability is to opt for a hybrid stackup. This approach involves using a low-loss dielectric material specifically for the areas supporting your RF lines, thereby balancing performance with budget constraints.

Placement and Routing