Understanding Power Supply De-rating for Engineers

Power supply de-rating is a critical concept in ensuring the safe and reliable operation of power systems, especially under non-ideal environmental conditions. This article will delve into the technical aspects of de-rating, exploring its types, causes, and practical implications for engineers designing systems that must endure high temperatures, varying voltages, and extreme environmental conditions.

What is Power Supply De-rating?

De-rating refers to operating a system or component below its maximum rated capacity to prevent failures and extend longevity. In power supplies, de-rating is often applied to output current, voltage, and power under specific conditions, such as high ambient temperatures or low input voltages. These de-rating curves are essential for engineers to understand the performance limitations of a power supply under varying conditions​.

Several factors influence power supply de-rating, including:

1. Thermal limits: Components within a power supply are sensitive to heat. As ambient temperature rises, components become less efficient and more prone to failure.
2. Input voltage: Lower input voltages increase the current demand, placing stress on internal components.
3. Environmental conditions: High altitudes, poor ventilation, or inadequate cooling can exacerbate the need for de-rating.

Thermal De-rating: Managing Heat Stress

Thermal de-rating is perhaps the most common and important form of de-rating engineers encounter. Power supplies generate heat during operation, and as the ambient temperature increases, the power supply must reduce its output to maintain safe operating conditions. Virtually all power supplies include thermal de-rating curves that plot the allowable power output relative to ambient temperature​.

For example, many power supplies are rated to deliver full power up to an ambient temperature of 50°C, but beyond that, output is reduced to protect internal components. In extreme environments, power output might be cut by as much as 50% if the temperature exceeds 70°C. The actual degree of de-rating varies depending on the design, cooling system, and component choices used within the power supply.

For example, here is the temperature de-rating curve for Micropower Direct’s MPM-20C AC/DC series. This curve applies to the 3V, 5V, and 9V models:

Managing thermal de-rating requires engineers to account for several factors:

• Cooling methods: Forced air cooling can help maintain higher power outputs at elevated temperatures. Power supplies may provide different de-rating curves based on whether natural convection or forced air cooling is used. For example, forced air cooling allows some supplies to operate at full load up to 70°C, compared to just 50°C with natural convection​.
• Component thermal limits: Each component in a power supply, such as the bridge rectifier, has its own thermal threshold. Exceeding this threshold will lead to accelerated wear and potential failure. Engineers can use the power dissipation and thermal impedance of components to calculate safe operating temperatures​.

Input Voltage De-rating: Coping with Power Fluctuations

Power supplies designed for global use typically support a wide input voltage range, such as 90-264VAC. However, low input voltage conditions necessitate higher current draw to maintain the desired power output, which puts additional stress on internal components like the input filter and rectifiers. Input voltage de-rating accounts for this by reducing power output when operating near the lower end of the input range​.

For instance, a power supply may provide full power at 120VAC, but when operating at 90VAC, it might only deliver 80% of its rated power. This de-rating curve helps ensure reliability, especially in regions where voltage fluctuations or brownouts are common​.

The MPM-20C specifically has an efficiency of about 82.5% at 85 VAC which increases to a maximum of 87%. These models have notable efficiency even at lower input voltages.

Managing input voltage de-rating involves:

• Voltage range considerations: Engineers need to ensure that their systems will perform reliably across a variety of input voltages. Power supplies designed for critical global applications should be chosen for their ability to maintain high power output even at low line voltages.
• Component stress: De-rating helps mitigate the effects of high input currents on power supply components, which would otherwise suffer from higher resistive losses and thermal stress at low input voltages.

Altitude and Environmental Considerations

In addition to temperature and voltage, environmental factors such as altitude can also influence the need for de-rating. As altitude increases, the density of the air decreases, which reduces the cooling efficiency of air-cooled power supplies. At altitudes above 2,000 meters, de-rating is typically required to prevent overheating​.

This is precisely the case for the MPM-20C series. These models require de-rating at 2,000 meters up to a maximum altitude of 5,000 meters, making this series an excellent choice for high altitude applications if appropriate power de-rating is applied.

Engineers should also consider other environmental factors such as mounting orientation and ventilation. Poor airflow can lead to localized overheating, further necessitating power reductions to maintain reliability. In such cases, designing systems with enhanced cooling or using power supplies with better thermal management capabilities can help alleviate the impact of these environmental stresses.

Reading De-Rating Curves

De-rating curves provide a visual representation of how a power supply's output changes based on external factors. These graphs typically plot ambient temperature, input voltage, or altitude against the maximum power output the supply can deliver. By consulting these curves, engineers can determine whether a power supply is suitable for a particular application, or whether additional cooling or power units are needed.

Conclusion: MPD’s Solutions for Reliable Power in Challenging Conditions

Power supply de-rating is an unavoidable factor in many designs, but it doesn't have to hinder system performance. MicroPower Direct (MPD) addresses these challenges head-on with a range of power supplies designed for robust operation in extreme environments. Our products feature detailed de-rating curves that allow engineers to design systems confidently, knowing how our supplies will perform under thermal, voltage, and environmental stress. Whether managing temperature extremes or operating in regions with unstable power grids, MPD’s solutions provide reliable power without compromise.

By selecting the right power supply and understanding its de-rating characteristics, engineers can ensure that their systems remain reliable, efficient, and safe—no matter the operating conditions.