Ensuring reliability: DC-DC converters for harsh environments

In today's fast-paced technological world, ensuring the reliability and durability of power systems is more important than ever.

High-reliability DC-DC converters play a crucial role in many applications, providing stable and regulated voltage outputs from unregulated inputs. Selecting the right converters for harsh environments requires careful consideration of key factors to optimize performance, durability, and safety. This article delves into the critical electrical, thermal, mechanical, and environmental criteria that engineers and industry professionals need to evaluate. By understanding these essential selection criteria, you can make informed decisions to enhance the efficiency and longevity of your systems, even in the most challenging conditions.

Electrical requirements

When designing a high-reliability DC-DC converter for harsh environments, several critical electrical requirements must be considered to ensure proper function and extended lifespan. Here's a breakdown of essential areas.

• Input and Output voltage range: High-reliability DC-DC converters for harsh environments require a wide input voltage range (e.g., 9V-36V for aerospace, 18V-75V for industrial) with protections like over-voltage protection (OVP), under-voltage lockout (UVLO), and surge protection. Additionally, they need tightly regulated output voltages (±1% or better) that may be adjustable to meet varying load requirements.
• Output power and efficiency: High-efficiency DC-DC converters are crucial for reducing energy consumption and costs, extending device lifespan, and supporting environmental sustainability. They minimize power losses, leading to lower electricity costs, especially in high-demand environments like data centres. Higher efficiency results in less heat generation, which extends the lifespan of electronic devices and ens ures reliable performance in critical applications. While 100% efficiency in DC-DC converters is ideal, real-world efficiencies typically range from 70% to 95%. Achieving this involves using switched-mode circuits with minimal power dissipation and employing pulse-width modulation (PWM) for precise output voltage control. In Fig 1. a buck converter in switched mode DC-DC converter consists of a switch network that reduces the DC component of voltage and a low-pass filter that removes the high-frequency switching harmonics, resulting in high efficiency.

Figure 1: The Buck converter (Source)

• Ripple and Noise: In a DC-DC converter, switching buck regulators inherently pass low-frequency ripple and high-frequency noise to the output. A practical solution for filtering these is using a power module with a post-filter. The effectiveness of an LDO (low-dropout linear regulators) and a second-stage LC filter for noise reduction makes them perfect for high reliability in harsh conditions.
  A second-stage LC filter reduces LF ripple and HF noise without regulation penalties if inside the feedback loop, though it requires careful design for stability and parasitic elements. It offers a lower BOM count and minimal power loss. An LDO is simpler for filtering but comes with trade-offs like higher power dissipation and larger solution size due to additional components.

Figure 2: Second-Stage LC Filter with Parallel Damping Branch (Source)

Figure 3(a) shows a second-stage LC filter with an inductor and series resistor (DCR), bypass capacitor, and damping branch cascaded to the first-stage output capacitors. This LC filter creates high impedance, dissipates noise as heat, and forms a low-pass network with shunt capacitors to filter high-frequency noise effectively.

Figure 3: (a)Simplified adjustable LDO block diagram of LDO shows internal noise sources(Source)

Figure 3: (b) Simple RC network reduces noise gain an adjustable output (Source)

Figure 3(b) shows an adjustable-output LDO with R1 and R2 setting the output voltage, while R3 and C1 reduce the error amplifier's AC gain.

• Isolation: Insulated (galvanically) DC/DC converters have an isolation barrier, ensuring complete electrical separation between input and output and protecting connected devices from disturbances and voltage spikes. In highly reliable DC-DC converters, a transformer provides isolation using magnetic coupling, preventing high-frequency noise and common-mode voltage transfer while ensuring safety by eliminating direct current paths between circuits at different voltage levels.

Thermal Management

Efficient heat management is crucial for DC-DC converter reliability and longevity, as excessive heat degrades performance and shortens lifespan. Cooling methods include passive cooling, heatsinks, and active cooling with fans, chosen based on power output, efficiency, ambient temperature, and temperature fluctuation tolerance. High-power applications or environments with limited airflow may require converters with integrated fans for adequate cooling.

• Operating temperature range: DC-DC converters without derating offer higher performance and reliability by providing full power even at high temperatures. This capability is essential for compact designs and extended use in harsh environments, which include extreme temperature variations, dust, combustibles, moisture, vibrations, impacts, and power surges from natural or artificial sources. Their robust design and full-power operation enhance the lifespan of battery or supercapacitor-powered systems. High-reliability DC/DC converters in harsh environments can deliver full power up to 85°C without derating, eliminating the need for extra heat sinks. With innovative designs and advanced components, they efficiently dissipate heat through the base plate, improving the operating range from -35 °C to 185 °C.

Figure 4: Typical efficiency performance concerning operating temperature (Source)

• Thermal protection: Effective thermal management is essential to maintain the junction temperature (TJ-max) at or below a safe value, typically 125°C or 150°C for automotive devices, under all operating conditions. Ensuring the average semiconductor die temperature remains below the maximum safe limit (TJ-max) during operation is crucial for device reliability. High current density on the IC die limits TJ-max, and exceeding it usually triggers thermal shutdown protection. Users must decide on a reasonable TJ below TJ-max, as lower TJ enhances device reliability. At the same time, lower TJ improves reliability and restricts maximum ambient temperature or power dissipation. Eq 1 shows the calculation of junction temperature.
  T_J = T_A + P_D X θ_JA ------- Eq 1
  Where T_J is the Regulated junction temperature.
  T_A is the ambient temperature.
  P_D is power dissipation, and θ_JA is thermal resistance.

Figure 5: PCB Area vs. Thermal Resistance Plot for Package Characterization

Power modules in DC/DC switching regulator designs simplify power solutions and enhance efficiency, size, and EMI compliance. Thermal design with all power-dissipating components in the module is more straightforward if the package is well-characterized, allowing for precise thermal resistance-to-ambient plotting based on board copper area.

Cooling Mechanisms:

Proper thermal management in DC-DC converters enhances reliability, reduces size, and cuts lifecycle costs. This overview covers standard cooling techniques for DC-DC converters, focusing on conduction and convection methods.

• Heat sink: These sinks transfer heat through direct contact and come in two types: finned and unfinned (cold plates). Finned heat sinks, often used with forced air cooling, offer rapid heat dissipation but are typically limited to medium-large applications due to space constraints.
• Cold plate: These use a thick metal plate to draw heat away from a unit, effective in compact designs. However, additional measures such as natural airflow, thermal compounds, or forced air are necessary to dissipate the heat.
• Liquid cooling: The process employs a cold plate with an embedded tube circulating a water-glycol mix, maintaining a steady base plate temperature, ideal for tight spaces like aircraft and ground vehicles, but is more complex and costly due to additional materials and engineering.
• Thermal compound: These, also known as thermal paste or grease, enhance heat transfer between a unit and its cooling component, ensuring adequate adhesion and heat conduction.
• Natural air flow: This method uses surrounding air to dissipate heat. It is low-cost and reliable due to the absence of mechanical parts, but it has limited temperature ranges and is unsuitable for high-energy applications.
• Forced air: An enhanced natural airflow method, incorporates fans to maximize heat dissipation. While practical, fans introduce inefficiencies and can increase internal dust.

Figure 6: Thermal impedance (Source)

Mechanical robustness

High-reliability DC-DC converters for harsh environments must exhibit exceptional mechanical robustness to endure demanding conditions. With stringent standards, secure mounting mechanisms, and robust encapsulation, these converters ensure uninterrupted operation and extended service life, even in the most demanding applications and hostile environments.

• Vibration and shock resistance: These converters comply with stringent standards, such as MIL-STD-810G, ensuring they can withstand intense vibrations and shock. This military-grade standard is a testament to the converter's resilience, enabling it to operate reliably even in the most challenging conditions.
• Mounting and encapsulation: These converters feature secure mounting options and robust encapsulation, protecting against mechanical stress, moisture, and contaminants. The encapsulation protects the internal components from external elements that could otherwise compromise the converter's performance or longevity. This level of protection is crucial in harsh environments where the converter may be exposed to extreme temperatures, dust, or other environmental hazards.

Figure 7: The HQA series of encapsulated DC-DC converters from TDK (Source)

• Environmental considerations: High-reliability DC-DC converters designed for harsh environments address several key environmental considerations to maintain optimal performance.
• Humidity Protection: They are rated for high-humidity environments, often employing conformal coating or potting for additional moisture protection.
• Dust and Corrosion Resistance: In dusty or corrosive atmospheres, selecting converters with appropriate Ingress Protection (IP) ratings is crucial. For instance, DC-DC converters with IP67 ratings are essential for machines or vehicles in extreme environments. These converters offer protection against dust and sand ingress and can withstand water immersion up to a depth of 1 meter for 30 minutes. While IP21-rated converters are often sufficient and more cost-effective, IP67 is crucial for automotive and military applications where waterproofing and resistance to harsh conditions are necessary.
• High Efficiency and Robust Design: These converters maintain high efficiency (>96%) and are designed to endure harsh environments with low heat buildup and ruggedized housing for heavy vibrations and wet conditions.
• High Altitude Operation: High-altitude operation poses challenges such as reduced cooling efficiency and potential corona discharge. It is essential to check the manufacturer’s derating curves for performance adjustments to ensure converters remain reliable and efficient in extreme conditions.

MTBF (Mean Time Between Failures) and Reliability Data

Reliability is often specified in terms of Mean Time Between Failure (MTBF), with values extending into the millions of hours. The ambient or case temperature at which the specification applies is equally essential.

Figure 8: MTBF versus ambient temperature graph (Source)

• Increased reliability: Improving Mean Time Between Failure (MTBF) offers many industry advantages. It boosts reliability by slashing downtime, enhancing productivity, and curbing safety risks.
• Customer satisfaction: Prolonged operation times and fewer breakdowns foster loyalty and repeat business.
• Cost benefits: Smarter maintenance strategies reduce overall maintenance expenses.
• Longer equipment lifespan: Improved MTBF, minimizes capital expenditures and enhances asset utility.
• Quality control: Mitigating defects during manufacturing enhances product quality.
• Industry benefits: Industries like aerospace, defence, and healthcare significantly benefit from improved MTBF, which enhances safety by mitigating the risk of breakdowns.

Protection features

DC-DC converters implement various protection mechanisms to guard against faults, including:

• Over Voltage Protection (OVP): Prevents the output voltage from exceeding a set value due to internal faults or external overvoltage. This causes the converter to shut down or enter 'hiccup' mode.
• Overcurrent Protection (OCP): protects the converter from damage caused by excessive load current. It triggers a shutdown when temperature limits are exceeded. The converter may reset once the temperature drops or requires restarting.
• Short Circuit Protection (SCP): Protects against output short circuits. The converter can recover when the brief is removed or may require a restart. Long connection shorts may rely on OCP or OTP for protection.
• Over Temperature Protection (OTP): Prevents component overheating by shutting down the converter due to overload or high ambient temperatures, with recovery upon cooling or needing a restart.
• Under Voltage Protection (UVP): Ensures operation only when the input voltage is within a valid range.

Table 1: Different application areas of harsh environments with corresponding types of DC-DC converters

As a global distributor, we partnered with top suppliers to offer an extensive range of products, including isolated chassis mount, front-end rack mount, DIN rail mount, surface mount, and through-hole options for diverse industrial settings.

Conclusion

High-reliability DC-DC converters provide robust power conversion for critical applications. They offer superior electrical fault protection, thermal management, and mechanical resilience for extreme temperatures, altitudes, and harsh environments. Key benefits include high efficiency, low noise, vibration/shock resistance, and secure packaging. These features are essential for military, aerospace, industrial, medical, and renewable energy applications. Farnell offers a comprehensive range of high-reliability DC-DC converters designed for harsh environments, including Isolated Chassis Mount, Front End Rack Mount, DIN Rail Mount, Surface Mount, and Through-hole DC-DC Converters, ensuring stable, efficient, and reliable power conversion in challenging conditions.