Using a Multiphase Buck Converter in 5G RF Power SuppliesBy ZM Peterson • Jul 24, 2021
It’s well-known that 5G systems and components produce a lot of heat. The RF power amplifier stage in everything from handset modems to large transmitters used in base stations suffers from this problem. As a result, cooling measures are needed for RF power amplifiers as they produce significant heat when powered with a DC supply voltage.
In a base station, the cooling equipment typically takes up a substantial portion of the RF transmitter system. In a handset or other small device, designers might resort to mounting directly on the enclosure or using thermal interface materials, while slightly larger form factors might allow for a fan or unique finned enclosure to move heat. As devices get larger, we have to look back to the electrical side to see how we can implement cooling measures on the device and ensure everything is kept at sufficiently low temperature. In larger systems that contain an RF power amplifier, additional cooling measures are needed to ensure the design does not overheat.
Changing to an alternative regulator topology is one answer to this problem. The regulator in an RF system needs to supply very low noise power to the amplifier while also tracking the envelope of the device to ensure a high power factor. In this article, we’ll look at the advantages of using a multiphase buck converter in these envelope tracking power supplies, as well as some important simulation results showing how these systems operate.
RF Power Supply Design Considerations
The primary goal in designing RF power supplies to support amplification and RF power delivery, potentially from low MHz to high GHz frequencies. Envelope tracking is a standard method used in power supplies and it is even used in specialty ICs used for controlling PWM driving in buck converters. In switching power supplies used for RF designs, there are a few places to start in terms of component selection and signal properties:
- Bandwidth. Just like any other RF system, the bandwidth to be regulated matters. When a FET-based amplifier is driven into the ON state, it will pull power from the amplifier such that, ideally, the output power matches the input waveform in the time domain. This waveform will have some defined bandwidth due to modulation in the driving signal, and the regulator needs to be able to respond within that bandwidth.
- FET material platform. The FETs you use matter as they will limit the useful frequency response, which leads to a delay in tracking the envelope of high-frequency modulated signals. GaN on SiC is a superior material platform for FETs in multiphase buck converters and RF front end power amplifiers for GHz frequencies.
- FET array. This point is the key to using a multiphase buck converter successfully. The FET array needs to be designed carefully to ensure high power delivery from the device at the desired voltage current. Series or parallel arrangements could be used, although parallel arrangements are less preferred as they form a set of strongly nonlinear coupled oscillators, potentially with positive feedback. This would lead to transients and peaking such as you might see in amplifier stability problems.
- Zero-voltage switching (ZVS). This point relates to controlling when the current delivered through a switching element rises above 0 A. By enforcing some delay in the rise time, you eliminate losses that would normally accumulate on the falling edge of the drain-source voltage in your FETs.
Only the first point in the above list falls outside the purview of the regulator design. The other points can be designed or controlled with a multiphase buck regulator. Note that boost versions are available, but the typical RF system will use a buck converter to step down rectified mains power to supply a power amplifier. The image below shows a typical flowchart used in envelope tracking power supplies.
Note that the feedback loop is not strictly required. While an active control strategy could be used, you can also implement passive envelope tracking with a high-order filter on the regulator output.
The main idea behind envelope tracking is to ensure the power supply only provides the exact power needed, and no less. This means peak-to-average power ratio (PAPR) is closer to 1 for these converters. Finally, if ZVS can be achieved, switching losses in the system will be lower as current will only be flowing when the device is truly active, rather than at the tail-end of the modulating gate pulse.
Multiphase Buck Converter Design
The example below is an excellent illustration of a topology that could be extended to an N-phase buck converter design. I’ve adapted this example from a recent article by Maurizio Di Paolo Emilio in Power Electronics News, and hopefully this will nicely illustrate some of the advantages of this kind of buck converter. In general, we could have N phases (i.e., N FET arrangements), although 2 or 3 are typical. The FET arrangement used here needs to be carefully designed to ensure that total dropout doesn’t occur over the first MOSFET in the series array. Generally, the flying capacitor across FETs 2 and 3 aids regulation and helps prevent dropout.
This converter was originally designed to be a 3-level, 2-phase buck converter. However, this design could be extended to any number of phases.
Role of Switching in Each Phase
In this design, we have two repeated switching sections in parallel implemented with a group of FETs in each stage. These FETs are designed to switch independently, meaning switching in each section is delayed by some phase value. The phase difference between each FET array is 360/N degrees. This means that the current flowing into the output Pi filter (which helps smooth the output) is equal to the current through each of the output inductors L1. The value of L1 will determine the ripple in each stage, as well as the equivalent ripple in the final converter design.
If we look at the waveforms for a multiphase converter, we would see that each switching stage will modulate the FET arrays 2N times during a PWM period. Therefore, an ideal duty cycle for these converters would be less than 1/N, although technically these designs could be run at any duty cycle up to approximately 0.9. Because the system acts like it is running with a much larger frequency, the total ripple value will be smaller and the range of allowed inductors is larger. In order to ensure ZVS in each stage of the converter, the value of L1 should be selected such that the peak-to-peak ripple in each stage is more than double the average current.
Advantages of Multiphase Buck Converters
To summarize, there are several advantages to using a multiphase buck converter with passive envelope tracking, particularly for RF power supply designs:
- The output ripple is lower than in a standard single-phase topology that runs at higher frequency. This is because a multiphase buck converter with N stages acts like a single-phase converter that runs at a factor 2N larger frequency.
- Physically smaller inductors and lower duty cycle could be used to reach the same level of ripple control, again thanks to the artificially increased frequency in these devices.
- Complex control circuitry that must run at very high frequency is eliminated thanks to passive envelope tracking. The ability to passively tracked the output power envelope helps reduce system size and causes the power output to be adjusted in response to modulation.
- If ZVS can be achieved, then the efficiency of these power supplies will be much larger (higher PAPR value) as switching losses are reduced. Switching losses can also be reduced by using GaN FETs in these systems.
If it is determined a new design will need a buck converter, there are several ICs that can be used in the design that will provide a highly integrated platform for low power systems. Higher power systems (generally greater than about 10-20 A) will need discrete components to implement the switching stage designs required in these products. Due to the implementation of multiple capacitive switching nodes, it is likely that capacitive coupling can occur between the switching nodes and other portions of the system, including the output stage components in dense designs. Steps to reduce parasitic capacitance should be implemented to ensure these designs do not have excessive noise coupling at low-to-moderate frequencies.
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