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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 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


Detailed Description

Regulation and Filtration Stages

Incorporate effective regulation and filtration stages.

RF Circuit Board Compatibility

Align the RF power supply with typical RF circuit board requirements to maintain signal integrity at high frequencies. Use components and materials with high-temperature ratings and effective thermal properties to handle the heat generated during operation.

Power Inlet Design with Filter Capacitors

At the power inlet of the RF PCB, three filter capacitors (e.g., 10µF, 0.1µF, 100pF) are commonly employed in parallel. This arrangement capitalizes on the unique filtering capabilities of each capacitor to target low, medium, and high frequencies on the power line. They should be strategically placed close to the input pins of the power supply in descending order of their capacitance values, optimizing their effectiveness in noise reduction across the spectrum.

Cable Layout Considerations

Ensure that the input and output power cables do not cross paths to reduce the risk of electromagnetic interference between the cables, which could compromise the performance and stability of the power supply.

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



Grounded Coplanar Waveguide Routing

Use grounded coplanar waveguides for RF traces routing. Avoid guard traces for shielding; prefer via fences for coplanar lines. Consider backdrilling for internal layer routing.

Conditioning the RF Output

Conditioning depends on the supply's function. Use filters like BAW or SAW if routing to an antenna or coaxial line. Be cautious with high RF power supplies and component selection.

Incorporating an Isolator/Circulator

Add an isolator/circulator on the RF Out net to prevent reflections. SMD isolators are recommended for power over RF or direct RF output to an antenna.

Transmission Line Design

Account for copper roughness and dielectric dispersion. Maintain 50 Ohms impedance in RF transmission lines.

Minimizing Electromagnetic Radiation

Reduce loop area in the primary, secondary, and load sides of the DC-DC module. Minimize large current loops and place power and ground cables closely.

Switching Power Supply Layout

Layout peripheral devices for the shortest power backflow path in switching power supplies. Place filter capacitance near the supply pin.

Board Power Line Routing

Avoid long-distance power lines near cascade amplifier terminals. Do not use power cables for RF signal transmission. Use high-frequency filter capacitors at both ends and potentially in the middle of long-distance power lines.

As you delve into the complexities of RF power supply design, consider the powerful capabilities of Cadence AWR software. This tool is designed to simplify and optimize your design process, providing assistance in simulating some of the issues that we’ve discussed. Whether it's managing regulation and filtration stages, or ensuring RF circuit board compatibility, Cadence AWR software offers advanced features that can help.

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