Editorial Team - everything PE
Jan 23, 2024
DC-DC converters play a crucial role in the power systems of spacecraft by receiving power from either solar arrays or batteries. The accepted voltages vary depending on the power bus design and mission conditions. Subsequently, these converters provide voltage-regulated outputs tailored to meet the specific requirements of diverse digital and analog electronics on board.
In order to ensure reliability, efficiency, and performance under the severe circumstances of space environment, various factors must be taken into consideration while selecting a DC-DC Converter.
Hybrid DC-DC Converters
Traditional DC-DC converter ICs are low-voltage regulators and are relatively large when compared to contemporary electronic packaging standards. Recently hybrid DC-DC converters have been employed for higher-power applications such as spacecraft buses. Implementing hybrid microcircuit manufacturing in the design of these circuits can significantly reduce the system's footprint, potentially enhancing efficiency and mitigating common-mode noise.
A hybrid DC-DC converter is a power converter that combines features of different types of DC-DC converters. There are several types of DC-DC converters, including buck converters, boost converters, buck-boost converters, and flyback converters, among others. A hybrid DC-DC converter may combine elements from different converter topologies to optimize efficiency, reduce size, improve transient response, or address specific requirements of space missions such as guaranteed performance under harsh environmental conditions.
Functional Block Diagram of Hybrid DC-DC Converter
Planar transformer replaces wires with printed wiring board and has no micro-soldering and wiring. Analog ASICs enable footpoint reduction from equivalent discrete devices.
Designing space-qualified DC-DC converters involves careful consideration of various factors to ensure their reliability and performance in the harsh environment of space. Some key design considerations are discussed below.
Inrush Limiter: An inrush limiter in a DC-DC converter limits the initial surge of current that occurs when power is first applied to the converter. This surge, known as inrush current, can result in unpredictable system operation, blown protection fuses, and can harm the converter.
The inrush limiter helps mitigate this issue by restricting the flow of current during startup, effectively smoothing out the transition from no load to full load conditions. By doing so, it prevents excessive stress on the converter components, reduces the likelihood of blown fuses or circuit damage, and ensures more stable operation. In addition to protecting the converter itself, an inrush limiter can also prevent disruptions to the overall system, minimize electromagnetic interference (EMI), and enhance reliability.
EMI Filter: The inbuilt EMI (Electromagnetic Interference) filter in a hybrid DC-DC converter serves several important roles:
Radiation Consideration
Radiation exposure accelerates the deterioration of electronics, resulting in a swift performance decline of devices. Even minor damage at the component level can trigger malfunctions in larger systems. For example, the breakdown of a Schottky diode, employed in hybrid DC-DC converters not only compromises the converter's performance but also affects the integrity of the entire system, potentially leading to widespread failure in satellite power distribution.
Radiation hardness assurance (RHA) is a procedural safeguard employed to guarantee that materials and electronics utilized in space systems maintain their efficacy even under diverse radiation intensities. This methodology includes delineating system prerequisites, defining environmental conditions, selecting components, conducting rigorous testing, implementing shielding measures, and designing for radiation tolerance. Assessing the performance of components under radiation threats like Total Ionizing Dose (TID) and Single Event Effects (SEE) is an essential aspect of Radiation Hardness Assurance (RHA) for DC-DC converters. Total Ionizing Dose (TID) refers to the gradual degradation over time experienced by a device when subjected to ionizing radiation. Single-event effects (SEEs) are discrete incidents where a single ionizing particle imparts sufficient energy to induce an effect in a device.
SEE can be categorized into soft errors, involving Single Event Upsets (SEUs) and Single Event Transients (SETs), or hard errors, involving Single Event Latchup (SEL), Single Event Burnout (SEB), and Single Event Gate Rupture (SEGR). SEUs may manifest as transient pulses in logic circuits or as bitflips in memory cells, while SEL induces elevated operating currents beyond device specifications. SEB may manifest as the burnout of power MOSFETs, necessitating the selection of MOSFETs resilient to this phenomenon and the implementation of proper voltage derating in DC-DC converter designs to mitigate its impact.
Additionally, enhanced low dose-rate sensitivity (ELDRS) effects are taken into account for all bipolar integrated circuits (ICs) utilized in hybrid converters. It refers to a phenomenon observed in certain semiconductor devices, particularly those used in space applications. This phenomenon describes an increased susceptibility to radiation-induced degradation when the device is exposed to low dose rates of ionizing radiation over an extended period.
MIL-PRF-38534 Qualifications
MIL-PRF-38534 is a military performance specification that outlines the requirements for the assembly, testing, and quality assurance of microelectronic devices, particularly those intended for use in high-reliability applications within aerospace and defense systems. This specification ensures that the components meet the stringent standards necessary for reliable operation in harsh environmental conditions and under demanding performance requirements. It is recommended to use hybrid converters qualified to MIL-PRF-38534 Class H and Class K.
Converters that meet the Class H requirements are designed to withstand harsh environmental conditions, including temperature extremes, mechanical shock, vibration, and radiation exposure. They undergo rigorous testing and screening processes to ensure consistent performance and reliability over their operational lifetimes. Class K specifically denotes a level of quality and reliability within the MIL-PRF-38534 specification.
Mounting and Thermal Consideration
The bare semiconductor die is mounted to a thick film ceramic substrate which is mounted to the header usually made of steel. Thermopad in each package is through the base plate. Hence base plate has to be maintained at 1250C or below. There is no thermopad to the lid of the package.
Hybrid DC-DC converters are available in two types of packages:
These converters are mounted on the PCB near the edge of the board as their mass is significant compared to other components of the PCB. This increases the resonant frequency of the circuit board assembly and results in a more rugged vibrational-resistant design. The flange-type package is more suitable for high-vibrational environments.
Hybrid converters can also incorporate advanced control techniques, such as digital control, to enhance performance and adapt to changing operating conditions. The controller design process involves the following steps:
Additionally, the converters may include features like soft-switching techniques, resonant converters, or other innovative technologies to minimize switching losses and improve overall efficiency.
Different Types of Testing for DC-DC Converters in Space Applications
Testing DC-DC converters for space applications is crucial to ensure their reliability and performance in the harsh conditions of space. Here are various testing scenarios that need to be undertaken:
Click here to learn more about DC-DC Converters that are used for Space Applications.
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