How to Select a DC-DC Converter for Space Applications?

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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.

  • Radiation Hardness: Space environments expose electronic components to high levels of radiation. Select DC-DC converters specifically designed to withstand radiation, such as those using radiation-hardened (rad-hard) or radiation-tolerant components.
  • Temperature Range: Space environments experience extreme temperature variations. Ensure that the selected converters can operate within the specified temperature range of the mission. Some components may require an extended temperature range or special thermal management techniques.
  • Voltage and Power Requirements: Determine the voltage and power requirements of your space mission. Select converters that can handle the input and output voltage ranges needed for your application. Consider power efficiency to minimize energy loss.
  • Efficiency: Efficiency is critical in space applications where power resources are limited. Select DC-DC converters with high efficiency to minimize power consumption and reduce the need for additional cooling systems.
  • Size and Weight: Space missions often have strict size and weight constraints. Select converters that are compact and lightweight while still meeting the power requirements of the mission. This is especially important for small satellites and other space systems with limited payload capacity.
  • Reliability and Redundancy: Reliability is crucial in space applications where repairs or replacements are often impossible. Choose converters from reputable manufacturers with a proven track record in space missions. Consider incorporating redundancy in your power system to enhance reliability.
  • EMI/EMC Compliance: Ensure that the DC-DC converters comply with electromagnetic interference (EMI) and electromagnetic compatibility (EMC) standards. This is essential to prevent interference with other systems on the spacecraft.
  • Input and Output Isolation: In some applications, it's important to have galvanic isolation between input and output to prevent ground loops and ensure the integrity of the spacecraft's electrical system.
  • Flight Heritage: Consider using components that have a flight heritage. Flight heritage are those components that have been successfully used in previous space missions. This can provide confidence in the reliability and performance of the chosen DC-DC converters.
  • Testing and Qualification: Ensure that the selected converters have undergone thorough testing and qualification processes. This includes testing under simulated space conditions to verify their performance and reliability.
  • Customization and Flexibility: Choose converters that offer customization options to meet specific mission requirements. Some missions may have unique power distribution needs that require flexible converter configurations.

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:

  • EMI Suppression: Its primary role is to suppress electromagnetic interference generated by the converter during its operation. EMI can interfere with the performance of other electronic devices in the system or surrounding environment. The filter reduces the amplitude of conducted and radiated EMI emissions, ensuring compliance with electromagnetic compatibility (EMC) standards.
  • Compliance: It helps the converter meet regulatory requirements and standards for EMI emissions, ensuring that the device does not cause interference with other electronic equipment and meets electromagnetic compatibility standards such as FCC, CISPR, or military standards.
  • Protection: The EMI filter protects both the converter and other sensitive electronic devices in the system by preventing external interference from affecting their operation. It acts as a barrier, blocking unwanted noise and disturbances from entering or leaving the converter.
  • Improved Reliability: By reducing EMI emissions and providing protection against external interference, the inbuilt EMI filter contributes to the overall reliability of the hybrid DC-DC converter and the system it is integrated into. It helps prevent malfunctions or damage caused by electromagnetic interference.
  • Enhanced Performance: A well-designed EMI filter can improve the performance of the converter by ensuring stable operation without interference from external sources. It helps maintain clean power output and reduces the risk of voltage spikes or noise that could affect the performance of connected electronic devices.

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:

  • Down-lead package (with and without flanges)
  • Side-lead package

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:

  • Selection of the control strategy (e.g., Voltage mode control (VMC) or Current mode control (CMC))
  • Design of the control loop, including the compensator and feedback network
  • Tuning of the controller parameters to achieve the desired dynamic response and stability margins 

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:

  • Thermal Testing: Thermal Vacuum Testing - Simulate the extreme temperatures of space and vacuum conditions to ensure the converter can operate within specified temperature ranges. Temperature Cycling - Subject the converter to temperature variations to assess its performance under thermal cycling conditions.
  • Radiation Testing: Total Ionizing Dose (TID) Testing - Expose the converter to ionizing radiation to evaluate its response to cumulative radiation dose. Single Event Effects (SEE) Testing - Assess the converter's susceptibility to single-event phenomena such as latch-up, single-event upsets (SEUs), and single-event transients (SETs).
  • Vibration and Mechanical Shock Testing: Random Vibration Testing - Simulate the vibrations encountered during launch and other operational conditions. Mechanical Shock Testing - Expose the converter to mechanical shocks to ensure its resilience to launch and deployment forces.
  • Electromagnetic Compatibility (EMC) Testing: Radiated EMI/EMC Testing - Evaluate the converter's susceptibility to and emission of electromagnetic interference in space environments. Conducted EMI/EMC Testing - Assess the conducted emissions and susceptibility of the converter.
  • Input/Output Voltage and Load Range Testing: Input Voltage Range Testing - Verify the converter's performance across the specified input voltage range. Output Voltage and Load Range Testing - Evaluate the converter's ability to handle different output loads and voltages.
  • Reliability and Endurance Testing: Life Testing - Subject the converter to prolonged operational conditions to assess its long-term reliability. Cycle Testing - Perform repeated power cycles to evaluate the converter's durability.
  • Transient Response Testing: Transient Response Analysis: Evaluate how well the converter responds to sudden changes in load or input conditions.
  • Redundancy and Fault Tolerance Testing: Redundancy Verification - Ensure that redundant systems function as intended. Fault Tolerance Testing - Assess the converter's ability to detect and recover from faults.
  • Isolation Testing: Isolation Voltage Testing - Verify the isolation capabilities of the converter to prevent unwanted currents between input and output.
  • Functional Testing: Functional Verification - Confirm that the converter meets its specified performance parameters under normal operating conditions.
  • Software/Firmware Testing: Firmware Verification - Ensure the reliability and correctness of any embedded software or firmware controlling the converter.

Click here to learn more about DC-DC Converters that are used for Space Applications.