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.
Envelope tracking as used in the RF power supply design with a multiphase buck converter ensures high peak to average power ratio (PARP). Implementation of ZVS helps reduce switching losses, bringing the efficiency of these supplies above 95%. To see how a multiphase buck converter works, let's look at an example with a 2-phase regulator.
Multiphase Buck Design From Start to Finish
First, let's look at the example circuit shown below. 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.
The power stage of this two-phase buck converter and the ET application's ZVS fourth-order output filter is precisely shown below. The behavior of this RF power converter is in part determined by the resistive load RL. In the MATLAB simulation results shown below, we can see how voltage control across the flying capacitor at Vin/N; the switch-node voltage of every phase known to change between 0 and Vin/2 for 0 < D < 0.5, and between Vin/2 and Vin for 0.5 < D < 1. One can see that the total current has a ripple frequency that is a factor 2Nf larger than the switching frequency used in each phase. The phase difference between each FET array is 360/N.
Switching waveforms of our example converter at a duty cycle ranging from 0 < D < 0.5 (left) and 0.5 < D < 1.0 (right).
The low-side MOSFETs (LSMs) are comprised of the top two devices S1x and S2x, which connect inductor L1 to the input DC bus/capacitor positive terminal (high-side MOSFETs, or HSM). The lower two devices S3x and S4x connect the inductor L1 to the flying capacitor's ground/negative terminal. ZVS turn-on in the LSMs can be facilitated by introducing an appropriate delay in the gate signals to the low-side devices. Dissipation is observed during the turn-on of the high-side device due to a lack of negative inductor current to charge or discharge the parasitic capacitance across the devices. The proposed design achieves ZVS turn-on of the HSM by selecting the L1 inductor value to carry a peak-to-peak ripple current that is more than twice the value of its average current. The phase currents are also balanced without the need for current control loops by having an appropriately designed value of L1.
To simulate the device, we need real component models for the FET arrays. I've chosen EPC800 series eGaN FETs based on their ultra-small footprint, zero rate of reverse recovery, and low switching losses. In the graph below, we can see how the proposed three-level design with a maximum power rating of 115 W outperforms its conventional counterpart at switching frequencies reaching up to 50 MHz.
Efficiency curves for our multiphase buck converter with 2 phases (left) and 3 phases (right).
For ZVS operation, the duty cycle range is set between 0.1 to 0.8, and L1(max) is determined at D = 0.8. The remaining filter components are built for 20 MHz bandwidth and 50 dB switching frequency ripple attenuation via bode plot approach, with L1 fixed at a value less than L1(max). The values for the fourth-order ZVS filter components for a load resistance of 6.6 Ω are shown in the table below.
These values can now be used in a simulation to determine the operational limits and effects of multiple stages on regulation.
Finally, the switch node voltages and inductor currents of the proposed two-phase three-level buck converter are calculated during tracking of randomly-generated waveform with 20 MHz bandwidth and its envelope signal. Depending on the input envelope command, the switch node voltages alternate between either (i) 0 V and 15 V or (ii) 15 V and 30 V. As a result, voltage stress across GaN MOSFETs is restricted to half of the input voltage. Furthermore, because the high side FETs are turned to with ZVS, the currents supplied by each phase are naturally balanced.
An efficiency comparison between conventional and multiphase buck converters (EPC8004 and EPC8009 GaN MOSFETs) is shown below. The proposed design outperforms a conventional buck design in terms of overall efficiency. Furthermore, due to partial ZVS turn-on of the GaN MOSFETs, there is a decline in efficiency for a certain output voltage range. The proposed converter has a peak efficiency of 97.5 percent at 115 W and total average efficiency of 94.5 percent at 26 W. Furthermore, the proposed design's efficiency is greater than 90% for the majority of the operating range and has a PAPR of 10 dB.
Efﬁciency vs. output voltage comparison.
In the above example of a multi-phase three-level buck converter, a ZVS low pass filter was constructed to track a 20 MHz LTE envelope signal while also maintaining inherent phase current balance. For the designed rating and PAPR, efficiency comparisons demonstrate that the proposed three-level buck converter has a higher average power efficiency than the two-level buck option. Furthermore, the proposed design is easily scalable for high-power ET applications, allowing for greater bandwidth and PAPR.
The advantages of a multiphase buck converter should be clear. By combining two or more phases into the converter topology, it's possible to achieve a larger switching frequency at the output inductor, which decreases ripple. In other words, the converter behaves as if it were being driven with a higher frequency input signal, as well as if it had larger inductance. This allows a designer to use physically smaller components if needed in order to hit footprint goals.
When you need to design high-efficiency RF power supplies for your 4G LTE or 5G system, work with a company that helps clients push the limits of new technology. NWES is an experienced PCB design firm that understands how to build and use multiphase buck converter designs. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your next high speed digital system is fully manufacturable at scale. Contact NWES for a consultation.