M. Vasic

Optimum design of an envelope tracking buck converter for RFPA using GaN HEMTs

In this paper, filter design methodology and application of GaN HEMTs for high efficiency Envelope Amplifier in RF transmitters are proposed. The main objectives of the filter design are generation of the envelope reference with the minimum possible distortion and high efficiency of the amplifier obtained by the optimum trade-off between conduction and switching losses. This optimum point was determined using power losses model for synchronous buck with sinusoidal output voltage and experimental results showed good correspondence with the model and verified the proposed methodology. On the other hand, comparing to Si MOSFETs, GaN HEMTs can provide higher efficiency of the envelope amplifier, due to superior conductivity and switching characteristics. Experimental results verified benefits of GaN devices comparing to the appliance of Si switching devices with very good Figure Of Merit, for this particular application.

Hybrid behavioral-analytical loss model for a high frequency and low load DC-DC buck converter

This work presents a behavioral-analytical hybrid loss model for a buck converter. The model has been designed for a wide operating frequency range up to 4MHz and a low power range (below 20W). It is focused on the switching losses obtained in the power MOSFETs. Main advantages of the model are the fast calculation time (below 8.5 seconds) and a good accuracy, which makes this model suitable for the optimization process of the losses in the design of a converter. It has been validated by simulation and experimentally with one GaN power transistor and three Si MOSFETs. Results show good agreement between measurements and the model.

Interleaved multi-cell isolated three-phase PWM rectifier system for aircraft applications

Recently there has been an important increase in electric equipment, as well as, electric power demand in aircrafts applications. This prompts to the necessity of efficient, reliable, and low-weight converters, especially rectifiers from 115VAC to 270VDC because these voltages are used in power distribution. In order to obtain a high efficiency, in aircraft application where the derating in semiconductors is high, normally several semiconductors are used in parallel to decrease the conduction losses. However, this is in conflict with high reliability. To match both goals of high efficiency and reliability, this work proposes an interleaved multi-cell rectifier system, employing several converter cells in parallel instead of parallel-connected semiconductors. In this work a 10kW multi-cell isolated rectifier system has been designed where each cell is composed of a buck type rectifier and a full bridge DC-DC converter. The implemented system exhibits 91% of efficiency, high power density (10kW/10kg), low THD (2.5%), and n-1 fault tolerance which complies, with military aircraft standards.

An overview of fast DC-DC converters for envelope amplifier in RF transmitters

Over the past few years, RF power amplifiers are taking advantage of the switched dc-dc converters to use them in several architectures that may improve the efficiency of the amplifier, keeping a good linearity. The use of linearization techniques such as envelope elimination and restoration and envelope tracking requires a very fast dc-dc power converter to provide variable voltage supply to the power amplifier, but theoretically, the efficiency can be much higher than using the classical amplifiers belonging to classes A, B, or AB. The purpose of this paper is to analyze the state of the art of the power converters used as envelope amplifiers in this application where a fast output voltage variation is required. The power topologies are explored and several important parameters such as efficiency, bandwidth, and output voltage range are discussed. Some of these topologies are compared in terms of efficiency for a particular set of specifications.

Envelope amplifier based on a hybrid series converter with the slow-envelope technique

High frequency dc-dc switching converters are used as envelope amplifiers in RF transmitters. The dc-dc converter should operate at very high frequency to track an envelope in the MHz range to supply the power amplifier. One of the circuits suitable for this application is a hybrid topology composed of a switched converter and a linear regulator in series that work together to adjust the output voltage to track the envelope with accuracy. This topology can take advantage of the reduced slew-rate technique where switching dc-dc converter provides the RF envelope with limited slew rate in order to avoid high switching frequency and high power losses, while the linear regulator performs fine adjustment in order to obtain the exact replica of the RF envelope. The combination of this control technique with this topology is proposed in this paper. Envelopes with different bandwidth will be considered to optimize the efficiency of the dc-dc converter. The calculations and experiments have been done to track a 2MHz envelope in the range 0-12V for an EER RF transmitter.

Comparison of three-phase active rectifier solutions for avionic applications: Impact of the avionic standard DO-160 F and failure modes

In aircraft applications, there has been an increasing trend related with the More Electric Aircraft (MEA), which results in rapid rise in the electrical power demand on-board. One of its goals lies in minimizing weight and volume of the electrical subsystem while maintaining good power quality and efficiency. The main purpose of this paper is to present and analyze an electrical design of a three-phase Boost rectifier, a three-phase Buck rectifier and a three-phase Vienna rectifier for output power level of 10 kW and compare them in terms of weight, volume, efficiency etc. Moreover, the design is obliged to comply with specific sections of DO-160 standard for avionic equipment with 230 Vac, 360–800 Hz grid conditions. Even though all proposed solutions satisfy the standard requirements, it will be shown that the Vienna rectifier has the lowest volume and not considering failure modes, the better solution overall. However, due to increased number of semiconductors and additional circuitry required for soft start-up, the Buck rectifier would prove to be the more robust solution failure-wise.

Optimized design of GaN switching capacitor based envelope tracking power supply for satellite applications

In modern communication system, both the bandwidth and peak-to-average power ratio of the transmitted signal are increasing rapidly. As a result, the power amplifiers based on linear power amplifiers such as class A, B or AB suffer from very low efficiency. The most promising solutions are the Envelope Tracking (ET) and Envelope Elimination and Restoration (EER) techniques, which can greatly improve the efficiency by employing a dynamic power supply that tracks the envelope of the transmitted signal. This paper presents an implementation of the ET power supply based on a multilevel converter in series with a linear regulator for a satellite application. The multilevel converter consists of a switching capacitors based converter in order to have highly efficient and light solution because the weight of power supply, together with its efficiency, is of the outmost importance for the application. A prototype capable of following a 5MHz RF envelope signal with maximum output power of 40W is fabricated. The experimental results verify the effectiveness of the proposed solution with 96.15% efficiency for the multilevel converter and 75% for the ET power supply. Furthermore, an experimental comparison between Silicon and GaN transistors is conducted in order to verify the benefits of GaN transistors.

High efficiency power amplifier applying envelope elimination and restoration technique with a single stage envelope amplifier with ripple cancellation network

This paper presents the integration of a single stage envelope amplifier with a switched class E amplifier using the Kahns technique or envelope elimination and restoration (EER). This technique is based on the combination of a switched and high efficient power amplifier supplied by a hi gh efficient envelope amplifier to obtain a linear and highly efficient RF amplifier. This work is focused on the envelope amplifier, a synchronous buck converter with ripple cancellation network, and on the integration with the high efficiency switched class E amplifier. With the proposed design of the synchronous buck converter with ripple cancellation circuit it is possible to decrease the ratio between the switching frequency and the large signal bandwidth of the envelope amplifier, which is one of the most limiting design factors of this application. The predistortion technique has been applied to increase the linearity and the improvement has been quantified experimentally. A high total efficiency has been obtained both for a quadrature amplitude modulation (QAM) signal and for an orthogonal frequency-division multiplexed (OFDM) modulation, with higher peak to average power ratio(PAPR).

Application of GaN FET in 1MHz large signal bandwidth power supply for radio frequency power amplifier

In this paper, implementation and testing of non-commercial GaN FET in a simple buck converter for envelope amplifier in ET and EER transmission techniques has been done. Comparing to the prototypes with commercially available EPC1014 and 1015 GaN FETs, experimentally demonstrated power supply provided significantly better thermal management and increased the switching frequency up to 25MHz. 64QAM signal with 1MHz of large signal bandwidth and 10.5dB of Peak to Average Power Ratio was generated, using the switching frequency of 20MHz. The obtained efficiency was 38% including the driving circuit and the total losses breakdown showed that switching power losses in the FET are the dominant ones. This implies that minimization of the gate charge is the key issue in the optimization of the device for this kind of application.

Comparison of two multilevel architectures for envelope amplifier

Modern transmitters usually have to amplify and transmit signals with simultaneous envelope and phase modulation. Due to this property of the transmitted signal, linear power amplifiers (class A, B or AB) are usually used as a solution for the power amplifier stage. These amplifiers have high linearity, but suffer from low efficiency when the transmitted signal has low peak-to-average power ratio. The Kahn envelope elimination and restoration (EER) technique is used to enhance efficiency of RF transmitters, by combining highly efficient, nonlinear RF amplifier (class D or E) with a highly efficient envelope amplifier in order to obtain linear and highly efficient RF amplifier. This paper compares two solutions for the envelope amplifier based on a combination of multilevel converter and linear regulator. The solutions are compared regarding their efficiency, size and weight. Both solutions can reproduce any signal with maximal spectral component of 1 MHz and give instantaneous maximal power of 50 W. The efficiency measurements show that when the signals with low average value are transmitted, the implemented prototypes have up to 19% higher efficiency than linear regulator that is used as a conventional solution.