Buck and Boost Converter Waveforms in an RF Power SupplyBy ZM Peterson • Aug 17, 2021
All electronics need some source of power, whether it’s a simple DC power supply, battery, or electromechanical AC generator. RF power systems don’t use power supplies running at high frequencies or sourcing an output waveform directly. Instead, they run on DC buck or boost converters that provide DC power to an amplifier stage, and a modulated signal is used to modulate power delivery through the amplifier.
Once power is delivered to the load, it should match the waveform of the modulating signal as long as the FET in the amplifier stage is not driven to saturation. Another important question relates to the output buck or boost converter waveform used in the regulation stage internal to the RF power supply. For an RF power supply what do these waveforms look like, and how do they compare to the waveform from a simple DC power supply? In this article, we’ll explore these differences and show what designers can expect from a high-performance RF power supply running at GHz frequencies.
In this article, I want to give an overview of the typical topology used in an RF power supply with a buck or boost converter to ensure highly efficient power conversion and regulation, as well as accurate tracking of the output power such that it closely follows the modulated signal. This can cause these converters to become very complex, but this is critical to ensure high efficiency in converters and transmitters running at high output power. In particular, we’ll look at a few important stages in the design and how they affect the output waveforms:
- The output filter stage and its purpose in shaping the output power waveform
- How an advanced multiphase converter topology influences regulation
- How feedback can be implemented in these designs to ensure nominal power output is set to the desired average value.
RF Power Supply Topology
An RF power supply is really a DC power supply that is capable of tracking the modulated signal envelope at the baseband frequency, giving an output power waveform that looks very different from a typical switching DC-DC converter. Buck, boost, buck-boost, or any other power supply topology can be used to supply power to RF components as long as it can meet these basic requirements. Some simple methods are used in an RF power supply to ensure power conversion and delivery is highly efficient, and these methods are reflected in the waveform delivered to the load component.
The general topology of an RF power supply is shown below, where feedback is implemented to provide precise regulation of the power supply. In this topology, we generally consider an isolated power converter (AC to DC), but not necessarily an isolated switching converter (e.g., an LLC resonant converter design).
This block diagram shows the path from an AC input that gets rectified, converted, and regulated down to low-level DC to supply the power to an RF power system, such as an amplifier.
The above architecture can be a multiphase design or single-phase design, although a multiphase design is the best choice for an RF power supply running at high GHz frequencies. At the input on the DC side, we have a typical EMI filter design intended for removing common-mode and differential-mode noise with a low-impedance connection to earth through the chassis. In a small SMPS design for an RF power supply running in a mobile system with an oscillator, this may not be the case, and instead any metal elements in the enclosure will need to be tied back to the ground plane on the input side.
On the output side, we have more sophisticated architecture that produces the required output waveforms from the RF power supply. The SMPS section could have boost or buck topology, or possibly an isolated topology like flyback. The typical output capacitor and inductor pairing in a buck or boost converter, as well as the typically low frequencies in standard designs (~100 kHz), do not provide the efficiency required in high-frequency RF power supplies and amplifiers. Next, we want to look at the filter stage as this will determine the shape of the output power waveform.
Envelope Tracking With the Output Filter
The output filter on a multiphase RF power supply is normally a higher-order low-pass filter with bandwidth just above the baseband frequency. To see why this setting matters, take a look at the topology below, where an RF power amplifier is being driven by an RF power supply:
This block diagram shows the path from the RF power supply output and buck or boost converter waveforms for output power in the case of envelope tracking in the time domain or frequency domain (assuming continuous conduction mode).
In a multiphase converter, the output LC filter stage and the inductors on each switching stage will typically form a 4th order or higher filter. This envelope tracking filter on the output ensures that our buck or boost converter waveform is only modulated at the baseband frequency when the driving signal is given to the gate on the amplifier. The total power delivered to the load oscillates due to modulation by the carrier, while the frequency at which power is output oscillates within the passband of the output filter.
Note that this thing we call "envelope tracking" is not really an envelope tracking filter when we are dealing with a frequency-modulated signal. However, the same type of filter can be used with amplitude modulation as long as the baseband frequency falls within the filter’s passband. In both cases, the average power delivery has a well-defined value that is determined by the baseband frequency.
Switching Converter Stage
In general, multiphase converters are preferable when working at high GHz frequencies due to their ability to mimic higher switching frequencies. Consider a system with N separate driver stages: by applying a phase difference between each driver stage, current is delivered at a rate that is equal to the switching frequency multiplied by 2N>. This then allows physically smaller components to be used in the design while ensuring the design meets a target ripple value, something that is valuable for the types of small SMPS devices we design for RF systems. To learn more about multiphase converters, read this recent article.
The image below shows a full comparison of the output boost converter waveforms operating in the continuous mode; similar waveforms would be seen in a buck converter. In this image, the PWM drivers have a low duty cycle (less than 50%) to allow for some turn-off time between switching events in the output inductor and filter. Here, we’re only showing the output assuming DC power draw; running with a modulated signal at the amplifier would modify these waveforms due to power being pulled at the baseband frequency.
Boost converter waveforms in an RF power supply operating in continuous conduction mode. Note that buck converter waveforms are very similar, depending on topology.
If precise regulation is needed, such as locking the output voltage/current to some required level within the limits of the converter, feedback is applied on the output side in the switching section and the PFC section. Both sections are needed to ensure highly precise regulation and efficient power conversion within the limits of the converter (depending on the input voltage range). The block diagram below shows a sense and control block added to our power converter, which is responsible for implementing the control algorithm.
Feedback is implemented in the PFC section and in the switching section to ensure accurate regulation to a desired nominal output power level.
The sense and regulation section could be as simple as a current-sense amplifier and an MCU. In particular, an MCU with multiple PWM outputs could be used as the driver as long as the FETs in the design could be driven at logic levels. For high power designs, this won’t be the case, and gate driver ICs will be needed to modulate the FETs in the switching section. Fortunately, there are some multiphase controller ICs available from semiconductor manufacturers that are compatible with many high-power MOSFETs, including GaN FETs that are needed in GHz RF power supplies.
When you’re ready to start your next RF power supply design, you should work with a PCB design firm that can help your company push the limits of new technology. 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 rf power supply is fully manufacturable at scale. Contact NWES for a consultation.