Thursday, March 5, 2020
Wide-bandgap (WBG) semiconductors, such as gallium nitride (GaN) and silicon carbide (SiC), are proving to be the most promising materials in the field of power electronics since silicon was introduced. These materials have several advantages compared to traditional silicon-based technology, such as the ability to manage high power levels, insensitivity to radiation, high-temperature operation, high switching frequencies, low noise, low power losses, and high efficiency.
WBG semiconductors are, therefore, of strategic importance to the development of next-generation space-borne systems. Gallium nitride, in its enhanced-mode version (eGaN), is widely used in the development of FETs and HEMTs for space applications.
Effects of radiation on power devices
The space environment has particular conditions that can influence and, in some cases, degrade the mechanical characteristics of space-based materials, which can negatively influence the overall behavior of a system’s operation. Space radiation consists primarily of 85% protons and 15% heavy nuclei. The effects of radiation can lead to degradation, interruptions, and discontinuities in the performance of devices.
The main requirement for space-qualified components is the ability to ensure reliable long-term operation. A radiation-hardened, or rad-hard, design determines the requirements of an electronic component to withstand the effects of radiation. It can be one of the most expensive and time-consuming approaches, but it is sometimes the only solution for electronic components in order to protect human lives or safeguard important orbital missions in space.
Electronic components used in space-borne applications are primarily subjected to space radiation, known as single-event effect, or SEE, caused by electrons and protons trapped in Earth’s magnetic field. Another important effect of space radiation is the total ionizing dose (TID). The difference between the two concepts is very simple: SEE is the result produced by a single high-energy particle that hits the device, while TID measures the effects produced by prolonged exposure to ionizing radiation.
The TID exposure is measured in radiation-absorbed doses (rads), which quantifies the total exposure of a material to radiation. Given a specific device, the total dose radiation threshold is the minimum level of rad that will cause device failure. Most rad-hard commercial devices can withstand up to 5 krads before functional failure occurs. The SEE indicator becomes particularly significant in applications such as satellites and spacecrafts. The high density of protons and ions present in the environment in which these systems operate can cause a series of different SEEs in electronic circuits, including single-event upset (SEU), single-event transient (SET), single-event functional interrupt (SEFI), single-event gate rupture (SEGR), and single-event burnout (SEB). SEE events can cause a degradation of system performance, up to total destruction. In order to ensure a high degree of reliability, it is necessary to sele
ct components in which the effects produced by radiation have been measured and declared.
WBG advantages in space-borne systems
Reduced weight and size, together with high efficiency and reliability, are fundamental requirements for components intended for use on spacecraft. GaN power devices provide the highest level of efficiency in the smallest footprint available today. Gallium arsenide also has excellent characteristics in terms of electromagnetic compatibility (EMI): The reduced parasitic capacitance decrease the energy stored and released during the switching cycles, while the reduced footprint improves the loop inductance, particularly insidious as it acts as a transceiver antenna.
Power devices used in critical applications such as space missions, high altitude flights, or strategic military applications must be resistant to failures and malfunctions caused by ionizing radiation. Commercial GaN power devices offer significantly higher performance than traditional rad-hard devices based on silicon technology. This allows the implementation of innovative architectures for applications in satellites, data transmission, drones, robotics, and spacecraft.
Enhanced GaN HEMT
Rad-hard MOSFETs have reached their technology limits with large die sizes and a performance figure of merit (FoM), expressed by the formula FoM = RDS(ON) × Ciss, which is much higher than that of an eGaN transistor. The FoM is a very important parameter — the smaller the value, the better the efficiency of the system.
In addition, eGaN HEMTs are easier to drive, as they require 10× to 40× less gate charge than the best rad-hard MOSFETs. GaN devices can also be mounted directly on the ceramic substrate without requiring any external package. It is possible to eliminate wire bonds and related inductance, reaching very high switching rates. The eGaN switching speeds are determined only by the resistance and capacitance of the gate and drain nodes.
Switching times can easily reach sub-nanosecond levels, so particular attention should be paid to both the design and PCB layout phases of development when using these high-performance devices.
Rad-hard GaN solutions
Renesas Electronics, a leading supplier of advanced semiconductor solutions, has developed the industry’s first rad-hard 100-V and 200-V GaN FET power solutions, suitable for enabling primary and secondary DC/DC converter power supplies in space-borne systems. These GaN FETs have been characterized for destructive single-event effects and tested for TID radiation. The ISL7023SEH 100-V, 60-A GaN FET and ISL70024SEH 200-V, 7.5-A GaN FET provide up to 10-orders-of-magnitude-better performance than silicon MOSFETs while reducing package size by 50%.
They also reduce power supply weight and achieve higher power efficiency with less switching power loss. At 5-mO RDS(ON) and 14 nC (QG), the ISL70023SEH enables the industry’s best figure of merit. Fig. 1 shows the very low RDS(ON).
VPT Inc. offers the SGRB series of DC/DC converters, specifically designed for harsh radiation environments in space applications. Based on advanced GaN technology, the SGRB series provides high efficiency, resulting in reduced system size, weight, and cost.
With up to 95% efficiency, the GaN technology results in greater efficiency compared to traditional radiation-hardened silicon products. It has been designed specifically for space-borne telecommunications in which high efficiency, low noise, and radiation tolerance are imperative (Fig. 2).
Freebird Semiconductor offers a wide selection of high-reliability GaN HEMT discrete devices integrated into GaN adapter modules (GAMs), creating the patented circuitry found in its multi-function power module series. These universal GaN adapter modules (Fig. 3) incorporate eGaN switching power HEMTs with GaN-based high-speed gate drive circuits for use in commercial satellites.
The rad-hard FBS-GAM01-P-C50 single low-side power development driver module incorporates GaN switching power HEMTs in a nine-pin SMT over-molded epoxy package. Integrated devices include Freebird’s FDA10N30X output power eGaN HEMT switch and an output clamp Schottky diode, optimally driven by high-speed gate drive circuitry consisting entirely of eGaN switching elements. It also includes 5-V input VBIAS overvoltage clamping protection with VBIAS undervoltage driver disable and reporting. The SMT over-molded epoxy package provides an engineering development platform for the FBS-GAM01-P-R50 flight unit version.
A reliable, continuous power supply is essential to the success of a space mission. In real-world applications, the main adadvantage of switching to SiC- or GaN-based broadband semiconductors is the increased power conversion efficiency.
The ability of SiC- or GaN-based broadband semiconductors to operate at high temperatures also has significant advantages. Not only can these devices be used in higher heat environments, they require less overall cooling, reducing the space and cost of cooling components in the power converter.
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