Temperature Stability Assessment of GaN Power Amplifiers with Matching Tantalum Capacitors
Wide bandgap GaN and SiC devices are expected to experience high levels of growth in applications ranging from power conversion to RF transistors and MMICs.
Wide bandgap GaN and SiC devices are expected to experience high levels of growth in applications ranging from power conversion to RF transistors and MMICs. End users recognize the advantages of GaN technology as an ability to operate under higher currents and voltages. RF GaN market is expected to grow at 22.9 % CAGR over 2017-2023, boosted by implementation of 5G networks. 
During the past years, wideband semiconductors have reported an achievement of >1000 V BDV that opens new challenges for high power industrial applications such as electric traction systems in trams, trolleybuses, or high-speed trains.
Decoupling and BIAS matching tantalum capacitors are used due to their stability of capacitance value over a wide temperature range, stable capacitance with BIAS, and no piezo noise sensitivity in small, low-profile case sizes. They are not prone to the wear down associated with aluminum electrolytic capacitors and exhibit high reliability and stability across temperature, voltage, and time.
Figure 1. RF GaN Power Amplifier. Image credit: Cree.
GaN Power Amplifiers with Matching Tantalum Capacitors
GaN RF Power Amplifiers
Requirements for the best linearity of RF GaN power amplifiers, as one of the key parameters, can be achieved in two ways:
- Use of optimum output impedance of the optimum linearity, this could, however, limit the output power and decrease efficiency.
- Use of optimum output impedance for maximum power output and define the working linearity region by the proper BIAS point setting and optimization. This requires a proper design of BIASing circuits and their stability in wide operating conditions . Tantalum capacitors in the number of reference signs are used to keep the working point within the high linearity region.
RF GaN reference designs with tantalum capacitors:
- Nitronex NPTB00004 GaN 28V, 5W RF Power Amplifier for CW, pulsed, WiMAX, W-CDMA, LTE, DC to 6 GHz. 10μF 16V gate decoupling capacitors.
- Qorvo QPD1008 125W, 50V, DC – 3.2 GHz, GaN RF 10μF 16V gate BIAS decoupling capacitors.
- QPD1008L DC - 3.2 GHz, 125 Watt, 50V GaN RF Power Transistor
- QPD1009 DC - 4 GHz, 15 Watt, 50V GaN RF Transistor
- QPD1010 DC - 4 GHz, 10 Watt, 50V GaN RF Transistor
- QPD1015L DC - 3.7 GHz, 65 Watt, 50V GaN RF Power Transistor
- Cree / Wolfspeed CGHV50200F 200W, 4400 - 5000 MHz, 50-Ohm Input/Output Matched, GaN HEMT with 10μF 16Vcapacitors
- CGH40006P 6 W, RF Power GaN HEMT
- CGH40010 10 W, DC - 6 GHz, RF Power GaN HEMT
- CGH40025 25 W, RF Power GaN HEMT
- CGH40045 45 W RF Power GaN HEMT
- CGH55030F1 / CGH55030P1 30W, 5500-5800 MHz, 28V, GaN HEMT for WiMAX
GaN High Power PFC Compliant Systems
The main power supplies used in telecom, server, and industrial power supply unit (PSU) systems convert AC line power to an isolated constant DC voltage output suitable for the power loads—typically, 1kW to 5kW, 12V for server PSUs, 48V for telecom rectifiers, and 24V for industrial PSUs. These systems require a front-end power factor correction (PFC) circuit to shape the input current of the power supply to meet the power factor and current total harmonic distortion (THD) norms defined in IEC610002-3.
The requirements for PFCs to meet >80% of the industry standards call for very high efficiency over wide operating ranges of input and output. This need has generated interest in bridgeless PFC topologies that push the efficiency above 99%. 
Capacitors are among the critical components that, in the case of SC failure, may cause a fatal error. Tantalum capacitors provide high capacitance efficiency in small dimensions with stable electrical parameter over a long lifetime include:
- Texas Instrument TIDA 00961 GaN 12V, 1.5kW for telecom, servers and industrial power supplies 100μF 16 V, 220μF 16 V tantalum capacitors for bulk 12V line stabilization , 4.7μF 10V on 3.3V output stabilization.
- Texas Instrument LMG3410 600V 12A Integrated GaN Power Stage for solar power, battery chargers with 33μF 16V as 5V output capacitor.
GaN Hi Point of Low Controller
TI's half-bridge point-of-load 80V 10A GaN controller LMG5200 evaluation board implements the 48V to 1V converter as a single-stage hard-switched half-bridge with a current-doubler rectifier. This topology efficiently supports a high step-down ratio while providing significant output current and fast transient response. GaN offers superior switching performance to traditional silicon MOSFETs due to its lack of reverse-recovery effect and reduced input and output capacitance. By using a GaN module, this application achieves high efficiency while operating in a hard-switched configuration.
Four low ESR B case 150μF 6.3V polymer tantalum capacitors are used as high efficiency, high power filtering output small size capacitors.
GaN Board Temperature Stability Assessment
GaN Board Selection
ORVO QPD1008 has been selected as the typical representative board with input tantalum capacitors subjected to a temperature stability measurement at RICE, University of West Bohemia. The QPD1008 is a 125W (P3dB) wideband unmatched discrete GaN on SiC HEMT which operates from DC to 3.2GHz with a 50V supply rail. The device is in an industry standard air cavity package ideally suited for military and civilian radar, land mobile and military radio communications, avionics, and test instrumentation. The device can support pulsed, CW, and linear operation. 
In measurement setups, this transistor connected to the test board as an amplifier for a frequency range from 0.96GHz to 1.215GHz. The aim was to observe an influence of the C2 capacitor on the transistor behavior for different ambient temperature level. The C2 capacitor is a tantalum capacitor (10 μF/16 V) recommended by the manufacturer in its reference datasheet (see Figure 2 and Figure 3).
Figure 2. Circuit diagram of GaN QPD1008 connection as an amplifier for a frequency range from 0.96 GHz to 1.215 GHz.
Figure 3. GaN QPD1008 evaluation test board with C2 tantalum capacitor.
The measurement was performed in the standard conditions within the referenced datasheet operating conditions recommendations.
Bias and Voltage Setup According to the Datasheet
- Input signal: sin wave, a frequency of 1GHz, power of 15dBm
- Ambient temperature: -30°C; +25°C and +70°C
- Measurement equipment:
- Spectrum analyzer Agilent N9320B
- Oscilloscope Tektronix MSO4104B
The spectrum analyzer was connected to the test board output via the 30dB attenuator. C2 tantalum capacitor waveforms were measured by an oscilloscope and compared at different temperatures. The test board was inserted into the climatic chamber. Before the measurement, the test board was conditioned at least 30 minutes at the desired temperature (-30°C, +25°C, and +70°C). The spectrum of the output signal was measured and compared.
Figures 4, 5, and 6 present the measured waveforms and their FFT analyses on C2 tantalum capacitor position at temperatures -30°C, 25°C and +70°C. Figure 7 shows the spectrums of the output signals on the GaN test boards.
Figure 4. The waveform on a C2 tantalum capacitor at -30 °C and its FFT analysis.
Figure 5. The waveform on a C2 tantalum capacitor at +25°C and its FFT analysis.
Figure 6. Waveform on C2 tantalum capacitor at +70 °C and its FFT analysis – tantalum capacitor.
Figure 7. Spectrums of the GaN test board output signals for different temperatures.
There are practically no measurable differences in output signal stability of the tested RF amplifier board at +25°C or +70°C. Some output signal drops about 5dBm test frequency attenuation is visible in the measured spectrums at the ambient temperature of -30 °C. Nevertheless, such a shift within 5dBm can still be considered as a very good stability performance of the tested GaN RF power amplifier at the referenced conditions.
Utilizing GaN in RF Systems and More
The rate of improvement in conventional MOSFETs has leveled off. Their performance is now close to the theoretical limits determined by the underlying fundamental physics of these materials and processes.
GaN features a higher critical electric field strength than silicon, resulting in a smaller size for a given on-resistance and breakdown voltage than a silicon semiconductor. GaN also offers extremely fast switching speed and excellent reverse-recovery performance, critical for low-loss, high-efficiency performance.
The above feature poses GaN as an ideal choice for RF systems and the upcoming fast growing 5G networks development as well as growing market of high power ~1kV industrial applications such as renewable energy, EV/HEV vehicles, power traction systems, and servers. GaN advantages can also bring new applications that have not been possible to make within such small size and simplified architecture so far such as the 48V to 1V single-stage hard-switched converter.
Stability at wide temperature range and harsh conditions are one of the design challenges for the number of industrial applications. Linearity, efficiency, stability, and high-power outputs are mostly driven off a good impedance matching and stable gate BIAS working point setting. Tantalum capacitors have been the favorite solution for BIAS decoupling circuits in the latest GaN power amplifiers.
The reference measurement on GaN RF power amplifier Qorvo QPD 1008 with tantalum 10μF 16 V decoupling capacitor confirmed its very good stability at the whole temperature range from -30°C to +70°C.
 Texas Instrument TIDA 00961 GaN 12V, 1.5KW design-in note; January 2018; http://www.ti.com/lit/ug/tidudt3/tidudt3.pdf
 Markos.A.,Z.; “Efficiency Enhancement of Linear GaN RF Power Amplifiers Using the Doherty Technique”; 2009; Kassel university press GmbH; ISBN online: 978-3-89958-623-7
 Qorvo QPD1008 125W, 50V, DC – 3.2 GHz, GaN datasheet; https://www.qorvo.com/products/p/QPD1008
About the Author
T. Zednicek worked for European Passive Components Institute at Lanskroun, Czech Republic.
R.Demcko, M.Weaver, and D.West worked for AVX Corporation at Fountain Inn, SC, USA.
T. Blecha, F.Steiner, J.Svarny, and R.Linhart worked for RICE - University of West Bohemia at Pilsen, Czech Republic.