Nonlinear Electro-Thermal Modelling of Packaged Power GaN HEMTs for Adaptive SSPAs Dedicated to Reconfigurable Payloads
SARDIN, D¹; REVEYRAND, T¹; CAMPOVECCHIO, M¹; BOUYSSE, P¹; LE GALLOU, N²; ROCHETTE, S³; FORESTIER, S³; VILLEMAZET, J.F.^3
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The development of a new generation of transmitters at payload level operating with complex modulation scheme for telecom applications (i.e. multi-carrier operation) require to improve the efficiency of SSPA and implement an high level of flexibility especially in terms of output power. In this paper, strategic power technologies are addressed (i.e. GaN technology) identified as the critical technology for RF payload applications. Power flexibility is a critical identified need in transmitters at payload level since spacecrafts are built for a quite long period of 15 spaceflight years during which they are required to operate in non-constant traffic beams and variable coverage areas. This topic is extremely sensitive in power consumption of the global payload and researches focus on efficiency enhancement of SSPA assuming an output power dynamic range; as an example improving SSPA efficiency from 30% to 35% at constant output power would save DC power and decrease by ~14% the power consumption at payload level. In order to implement such power flexibility but also meeting the constraints of high efficiency and good linearity, smart design approach of SSPA have to be investigated in conjunction with the strategic GaN technology. Nevertheless, the design of adaptive power amplifiers requires efficient and suited non linear model of the GaN devices. Moreover, given the power density of GaN technology, self heating effects have to be implemented in a non linear electro-thermal model. Therefore, the first part of this paper will be dedicated to the presentation of the specific non linear modelling process of commercial GaN technology using specific pulsed measurement techniques and 3D electromagnetic simulation of device packages while the second part will be dedicated to the presentation of some smart power architectures (Doherty, Envelope Tracking) with their respective advantages and disadvantages.
The first part deals with the non linear modelling results of 10W packaged GaN devices. The modelling process is divided into two main tasks. The first modelling task deals with the electromagnetic analysis required to derive the linear extrinsic model of the RF package mounted on a flange while the second modelling task is devoted to the circuit model extraction of the non linear intrinsic part and of thermal behaviour.
The first modelling task describes the extraction of the package model. Commonly used RF power package presents different stacked metallization layers associated to array of bonding wires into specific dielectric enclosure forming an interior air chamber. Therefore, lot of coupling effects has to be computed and make a 3D EM simulator (i.e. HFSS) required for this work. The second modelling task is based on isothermal pulsed I-V and pulsed S-parameters measurement in order to identify the main non linearity and assess the importance of gate and drain trapping effects. Moreover, a specific pulsed measurement technique called "coincidence method" has been implemented to derive the thermal behaviour of the 10W device. Finally, the electro-thermal model implements the temperature dependence of each non linear element equations (Ids, Cgs...) using multiple thermal equivalent circuits (Rth, Cth) and the calculated dissipated power. At the end of the different modelling tasks, the model accuracy has been checked by the comparison with load-pull measurements at 2.18GHz demonstrating more than 10W output power and up to 65% PAE for the optimum load conditions at the first and second harmonic frequencies.
The last part of this paper will be dedicated to the presentation of smart design techniques of SSPA. Usually, SSPA on-board telecom payloads are based on conventional design (i.e. operation close to saturation for optimum efficiency). If such operation is the best compromise for constant envelope signal, this is not the case for non constant envelope telecom signals demonstrating large peak-to-average ratio. This paper will deal with the way of adapting continuously the operating point of the amplifier to the envelope variations of the input RF signal in order to linearly amplify each signal sample with the optimum efficiency.