Over 10MHz Bandwidth Envelope-Tracking DC/DC converter for Flexible High Power GaN Amplifiers Nicolas Le Gallou 1 ,David Sardin 2 , Christophe Delepaut 1 , Michel Campovecchio 2 , Stéphane Rochette 3 1 ESA/ESTEC, Keplerlaan 1, 2200 AG Noordwijk, The Netherlands, Nicolas.Le.Gallou@esa.int 2 XLIM – UMR 6172, Université de Limoges/CNRS, 123 Avenue A. Thomas, 87060 Limoges, France. 3 THALES ALENIA SPACE, 26 Avenue J.F. Champollion 31037 Toulouse, France Abstract — This paper describes a fast envelope-tracking circuit capable of 10 MHz (up to 17.5 MHz) bandwidth based on RF GaN switching devices and 50 MHz switching frequency. The efficiency of the VHF converter is 84% to 90% which is comparable to a conventional DC/DC converter for spaceborne application. A demonstrator has been built and mated with a RF GaN HEMT output stage. The C/I measurement for 10-12W RF output power close to saturation show a linearity improvement by 5-8dB and an efficiency improvement of up to 8 points when compared to the case of no tracking circuit. Index Terms — SSPA, HPA, envelope-tracking, HPA, GaN technology, DC/DC converter. I. INTRODUCTION Typically amplifiers for Spaceborne Telecom or Navigation face similar requirements than amplifiers developed for ground applications. Primarily efficiency and linearity are key requirements. The Fig.1 gives an overview of a typical S-band Spaceborne Solid Sate Power Amplifier (SSPA). In such RF equipment, the DC/DC converter is integrated in the unit to interface with the satellite platform (BUS) which provides 50V or 100V. The converter (Electronic Power Conditioner) transforms the BUS voltage to the required value needed for each RF module (9V or 50V for HPA power lines). The EPC efficiency is often above 90%. Since most of Spaceborne SSPAs are used in “transparent” payloads, the signals are generally not known in advance and no baseband signal is available. This discards the digital pre- distortion techniques due to the cost and energy necessary to access the signal (down/up conversion, signal processing). Still, power flexibility (constant efficiency over 4dB power range), improved linearity and efficiency are required and various techniques to improve the performance of SSPAs operating with modulated signals are being investigated. With the presence of a DC/DC converter in a spaceborne SSPA, the envelope-tracking technique [1] seems to be a very well suited approach to allow power flexibility and improved performance. However the bandwidth required for L & S band SSPA is typically 36-40 MHz. For such a bandwidth the envelope-tracking system should ideally provide an efficiency of 90% to avoid performance degradation. This paper describes a demonstrator of a fast DC/DC converter associated to a GaN HPA capable to handle signal bandwidths of 36-40 MHz with good efficiency. Fig. 1. Spaceborne SSPA block diagram. II. BANDWIDTH REQUIREMENTS AND SWITCHING FREQUENCY The envelope-tracking allows an improved performance compared to the conventional fixed V ds approach. Fig. 2. Multi-carrier simulation – Fixed V ds / Envelope-tracking COM B HPA(s) RF In - Power, Voltage, Temperature Telemetries - Temperature compensation - Overdrive protection, Gain Control MLA EPC ATT+ LLA ATT DRIV BUS TM/TC ISO DIV Φ Phas RF Out PAE Fixed Vds PAE Env Track. NPR Fixed Vds NPR Env Track. Efficiency & Linearity both improved at High Pout Copyright © 2011 IEEE. Reprinted from 2011 IEEE MTT-S International Microwave Symposium, Baltimore, MD, USA. This material is posted here with permission of the IEEE. Such permission of the IEEE does not in any way imply IEEE endorsement of any of Cree’s products or services. Internal or personal use of this material is permitted. However, permission to reprint/republish this material for advertis- ing or promotional purposes or for creating new collective works for resale or redistribution must be obtained from the IEEE by writing to pubs-permissions@ieee.org By choosing to view this document, you agree to all provisions of the copyright laws protecting it.