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NOVEL 500 MHz SOLID STATE POWER AMPLIFIER MODULE DEVELOPMENT AT SIRIUS M. H. Wallner ∗ , R. H. A. Farias, A. P. B. Lima, F. S. de Oliveira, CNPEM, Campinas, Brazil Abstract A new solid state power amplifier (SSPA) module is being developed at the Brazilian Center for Research in Energy and Materials (CNPEM) to drive one of the superconducting RF cavities to be installed at Sirius, its new 3 GeV fourth generation synchrotron light source. Several prototypes have been built and tested in-house, and a planar balun was de- signed to achieve a push-pull configuration at deep class AB operation. Efforts to optimize heat exchange in various ways have been made. Results obtained thus far are presented and the next steps concerning development are discussed. INTRODUCTION Since 2010 the Brazilian Synchrotron Light Laboratory (LNLS) has employed solid state power amplifier (SSPA) modules on UVX’s storage ring, its late second generation synchrotron light source located at Campinas, Brazil. The 476 MHz amplifier design was conceived at SOLEIL, and the modules were put together on 50 kW amplifiers. For Sirius’ first phase of operation, new 500 MHz modules were developed and utilized to build its 40 kW booster amplifier and two 60 kW storage ring amplifiers, with 2 more ampli- fiers being assembled to drive one of the superconducting cavities that are expected to be installed in 2023 [1]. For the remaining amplifiers needed to power the second superconducting cavity, a new amplifier module is being designed and tested. This development intends to improve some characteristics of the currently employed amplifier modules, like RF output power and efficiency, effectively reducing production and maintenance costs. Part of the motivation also comes from the fact that the project has a long time span and, in the meanwhile, new LDMOS devices have been released on the market. DESIGN PROCEDURE The first design step was to choose the transistor that would amplify the RF signal. In that regard, Ampleon’s 9th generation LDMOS device BLF978P seemed like a reason- able candidate, for it is the successor to Ampleon’s BLF578, which is already used on Sirius’ current amplifier modules and has proven its reliability over the years. BLF978P has higher gain and is able to achieve higher output power lev- els [2]. The device is meant to be operated in a class-AB mode with a push-pull configuration. Table 1 summarizes the main characteristics and goals of this amplifier modules’ design. It was possible to perform load-pull analysis on this de- vice’s nonlinear model provided by the manufacturer and ∗ [email protected] Table 1: Module Specifications and Goals Description Goals LDMOS device BLF978P Class of operation AB Output power > 800 W Efficiency > 65 % Drain-source voltage 48 V Quiescent drain current 20 mA extract input and output impedances for matching circuits. The next subsections will be dedicated to describe some of the module’s elements. Balun To achieve a push-pull configuration of the amplifier it is needed to transform the single-ended RF input signal into a differential pair. Conversely, the amplified RF signal needs to be brought together again into a pair of conductors to exit the amplifier. A balun was employed to this purpose. To improve repeatability and reduce production costs, an effort was made to design a planar structure, without the need to solder coaxial cables directly onto the PCB. This concept has been developed on previous works by NSRRC and the same principles were employed on this case [3]. A view of the balun’s simulated structure on HFSS can be seen on Fig. 1. The input reflection coefficient, ampli- tude and phase balance values obtained are presented on Fig. 2. The assymetry that is intrinsic to the structure can be compensated through a stub on one side of the balun and tweaking of the central slot. Altough this version was not employed on the prototype, it serves to show this structure’s capabilities and potential. Figure 1: Input balun structure simulated on HFSS. Circulator and RF Termination A circulator and a RF termination are utilized after the output balun to isolate the device from the subsequent com- bining stages of the amplifier. This is important for two reasons: it prevents impedance modulation on the output 12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW Publishing ISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-TUPAB358 MC7: Accelerator Technology T08 RF Power Sources TUPAB358 2349 Content from this work may be used under the terms of the CC BY 3.0 licence (© 2021). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI
Transcript of MQp2H 9zz K?y bQHB/ bi i2 TQr2` KTHB7B2` KQ/mH2 ...
NOVEL 500 MHz SOLID STATE POWER AMPLIFIER MODULEDEVELOPMENT AT SIRIUS
M. H. Wallner∗, R. H. A. Farias, A. P. B. Lima, F. S. de Oliveira, CNPEM, Campinas, Brazil
AbstractA new solid state power amplifier (SSPA) module is being
developed at the Brazilian Center for Research in Energyand Materials (CNPEM) to drive one of the superconductingRF cavities to be installed at Sirius, its new 3 GeV fourthgeneration synchrotron light source. Several prototypes havebeen built and tested in-house, and a planar balun was de-signed to achieve a push-pull configuration at deep class ABoperation. Efforts to optimize heat exchange in various wayshave been made. Results obtained thus far are presented andthe next steps concerning development are discussed.
INTRODUCTIONSince 2010 the Brazilian Synchrotron Light Laboratory
(LNLS) has employed solid state power amplifier (SSPA)modules on UVX’s storage ring, its late second generationsynchrotron light source located at Campinas, Brazil. The476 MHz amplifier design was conceived at SOLEIL, andthe modules were put together on 50 kW amplifiers. ForSirius’ first phase of operation, new 500 MHz modules weredeveloped and utilized to build its 40 kW booster amplifierand two 60 kW storage ring amplifiers, with 2 more ampli-fiers being assembled to drive one of the superconductingcavities that are expected to be installed in 2023 [1].
For the remaining amplifiers needed to power the secondsuperconducting cavity, a new amplifier module is beingdesigned and tested. This development intends to improvesome characteristics of the currently employed amplifiermodules, like RF output power and efficiency, effectivelyreducing production and maintenance costs. Part of themotivation also comes from the fact that the project has along time span and, in the meanwhile, new LDMOS deviceshave been released on the market.
DESIGN PROCEDUREThe first design step was to choose the transistor that
would amplify the RF signal. In that regard, Ampleon’s 9thgeneration LDMOS device BLF978P seemed like a reason-able candidate, for it is the successor to Ampleon’s BLF578,which is already used on Sirius’ current amplifier modulesand has proven its reliability over the years. BLF978P hashigher gain and is able to achieve higher output power lev-els [2]. The device is meant to be operated in a class-ABmode with a push-pull configuration. Table 1 summarizesthe main characteristics and goals of this amplifier modules’design.
It was possible to perform load-pull analysis on this de-vice’s nonlinear model provided by the manufacturer and
Table 1: Module Specifications and Goals
Description Goals
LDMOS device BLF978PClass of operation ABOutput power > 800 WEfficiency > 65 %Drain-source voltage 48 VQuiescent drain current 20 mA
extract input and output impedances for matching circuits.The next subsections will be dedicated to describe some ofthe module’s elements.
BalunTo achieve a push-pull configuration of the amplifier it is
needed to transform the single-ended RF input signal into adifferential pair. Conversely, the amplified RF signal needsto be brought together again into a pair of conductors to exitthe amplifier. A balun was employed to this purpose. Toimprove repeatability and reduce production costs, an effortwas made to design a planar structure, without the need tosolder coaxial cables directly onto the PCB. This concepthas been developed on previous works by NSRRC and thesame principles were employed on this case [3].
A view of the balun’s simulated structure on HFSS canbe seen on Fig. 1. The input reflection coefficient, ampli-tude and phase balance values obtained are presented onFig. 2. The assymetry that is intrinsic to the structure canbe compensated through a stub on one side of the balun andtweaking of the central slot. Altough this version was notemployed on the prototype, it serves to show this structure’scapabilities and potential.
Figure 1: Input balun structure simulated on HFSS.
Circulator and RF TerminationA circulator and a RF termination are utilized after the
output balun to isolate the device from the subsequent com-bining stages of the amplifier. This is important for tworeasons: it prevents impedance modulation on the output
12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-TUPAB358
MC7: Accelerator Technology
T08 RF Power Sources
TUPAB358
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that could be caused due to mismatches on the combiningstages and protects PCB components in case of power beingreflected back to the amplifier module. Given its narrowbandnature, it can also decrease harmonics power level on theoutput.
PROTOTYPE PERFORMANCESome prototypes were built in-house, with soldering and
assembly done by hand. Figure 3 presents gain and effi-ciency curves with respect to output power. At the 1 dBcompression point, it was possible to achieve 886 W outputpower and 68.9% efficiency.
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Figure 3: SSPA gain and efficiency curves with respect tooutput power.
Temperatures over key components of the prototype werealso measured with an infrared camera. Results may befound in Fig. 4. Only the hottest capacitor’s temperatureis shown. The transistor temperature indicates that it is ona very safe operation point regarding its mean time beforefailure (MTBF). Capacitors are well below operation limits.
THERMAL MANAGEMENTLNLS has employed amplifier modules with direct water
cooling in the past, but due to some issues (water leakage,assembly dificulties) the amplifier modules at Sirius areinstead mounted on top of heat sinks, composed of aluminumplates with embedded copper tubes.
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Figure 4: Temperature of several components with respectto output power.
Between the amplifier module and the heat sink a thingraphite sheet is placed to compensate for any ruggednessboth surfaces may have. The amplifier module’s case ismade out of aluminum, with a copper cylinder pressed on ahole with almost the same diameter. A cavity is milled inthe copper cylinder to mount the transistor. A silver thermalgrease that ensures good thermal and electrical contact isused between the transistor’s flange and copper cylinder.
Figure 5 depicts a cross section view of the amplifiermodule mounted on the heat sink. Graphite sheet and silvergrease thicknesses are oversized for better understanding.Water flows through the copper tubes and exchanges heatwith the LDMOS device. On the next subsections someefforts made to improve this feature are detailed.
Figure 5: Amplifier module cross section view.
Thermal Gap FillerThe output balun can exhibit relatively high temperatures,
so it was attempted to fill its cavity with a thermal gap filler.This compound has a dielectric constant of 3.2 and must betaken into consideration when optimizing the balun. Fig. 6presents a picture of the amplifier module case with thethermal gap filler inside the output balun cavity.
The impact on the balun temperature was evaluated bycomparing two prototypes, one with the thermal compoundand the other without it, and the result is presented in Fig. 7.Close to the 800 W output power level, the temperaturedifference can reach up to 38 °C.
Module CaseThe graphite sheet’s thermal impedance is inversely pro-
portional to the pressure applied between the surfaces on
12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-TUPAB358
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MC7: Accelerator Technology
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Figure 6: Thermal gap filler (yellow cylinder) inside theoutput balun cavity on the amplifier case module.
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Figure 7: Output balun temperature with and without ther-mal gap filler.
which this thermal compound is located. This pressure isenforced by fastening screws through the sides of the am-plifier case on the heat sink. However, the force transferredfrom the screws to the bottom surface of the amplifier caseis concentrated on a rather small area around the screw. Anidea to solve this issue was to mill the bottom surface of themodule’s case sides, so that the area of contact to the heatsink decreases. Simulations were done in Ansys Mechanicalto assess the values of stress on the bottom surface on bothcases.
Altough the temperatures obtained with the milled casewere lower than the flat one, the amplifier’s operation pointwere different from each other, so a conclusive statementon which one is better cannot be made. A more definitetest remains to be done. However, a possible drawbackalready observed by milling the case was that the center ofthe bottom surface detaches itself slightly from the heat sinkwith increasing screw torque.
To assess this feature of the mounting procedure, the LD-MOS’s case temperature was measured with respect to thescrews fastening torque on the milled case. The result canbe found on Fig. 8. It suggests that there is a optimumtorque for which heat exchange is best, but up to 1 Nm theypresented almost the same temperature values.
DEVELOPMENT NEXT STEPSFor the next stages of the SSPA module development, a
review of the amplifier PCB is foreseen, with improvementsto the input and output balun. To ensure repeatability of theresults presented on this paper, a small automated production
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batch will be provided and tested. In particular, gain andphase stability are important for the efficiency of the ampli-fier’s power combining stages. Moreover, fatigue tests willbe performed to assess the device’s robustness to repeatedand abrupt RF turning on and off sessions.
Finally, full reflection operation tests at nominal outputpower will be performed to assure the RF termination in-stalled on the amplifier module can handle this condition.Currently, a 80-way combiner cavity is also being designedat CNPEM and simulations have shown that failing modulesconnected to it can experience this level of power enteringits output port [1]. This, in fact, is the topology expectedfor the next four 60 kW RF amplifiers, along with properfeeding of the amplifier modules by 8 and 10-way powerdividers that are also being developed and tested in-house.
CONCLUSIONSA RF SSPA module was designed and prototypes were
assembled and mounted in-house. The goals defined onTable 1 were succesfully achieved through careful tuningof the input and output impedance matching circuits on theworkbench. Some tests to improve thermal management ofthe device were carried, with positive results on the aver-age temperature of the amplifier’s components. Automatedproduction of a small batch of amplifier modules will besupplied and future tests shall validate the design.
REFERENCES[1] A. P. B. Lima et al., “Status of Sirius Storage Ring RF System”,
presented at 12th Int. Particle Accelerator Conf. (IPAC’21),Brazil, May 2021, paper THPAB347, this conference.
[2] Ampleon, https://www.ampleon.com/products/general-purpose-wideband/50-v/BLF978P.html
[3] T. C. Yu et al., “Planar Balun Design With Advanced Heat Dis-sipation Structure For kW Level Solid-State Amplifier ModuleDevelopment”, in Proc. 4th Int. Particle Accelerator Conf.(IPAC’13), Shanghai, China, May 2013, paper WEPFI060, pp.2830-2832.
12th Int. Particle Acc. Conf. IPAC2021, Campinas, SP, Brazil JACoW PublishingISBN: 978-3-95450-214-1 ISSN: 2673-5490 doi:10.18429/JACoW-IPAC2021-TUPAB358
MC7: Accelerator Technology
T08 RF Power Sources
TUPAB358
2351
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