What is Power Derating?

At its core, power derating involves optimizing the performance of a power supply by balancing its various characteristics to ensure reliability. Essentially, it’s about making strategic trade-offs. Power supplies operate under multiple parameters. It’s impractical to push each one to its absolute limit without affecting others.

Derating serves a dual purpose: it safeguards power supplies and fine-tunes them within desirable operational ranges, even if it means compromising on less critical parameters.

Datasheets often mention nominal ratings, maximum ratings, and combined maximum ratings. These figures assume ideal conditions in your operating environment. Exceptional performance is achievable under perfect conditions, but when factors like temperature and altitude stretch beyond nominal boundaries, it’s crucial to remain within the safe operating area. This concept is what we refer to as rating. Understanding the difference between nominal and extreme conditions is essential.

All the products and systems are subject to some form of derating. Power supplies and filters are designed with this understanding in mind. Generally, lower input voltages lead to higher input currents, higher ambient temperatures result in higher junction temperatures, and greater outputs cause increased power dissipation. Any of these factors can degrade the performance or reliability of a power supply if not properly managed.

Derating mitigates these risks, enabling power supplies to function across a wider range of conditions. When we derate, we aim to maximize the utility of our power supply.

Derating isn't an automated function; it's a set of guidelines designed to help you achieve optimal performance. While we don’t utilize automatic systems to identify and rectify extreme conditions, you must understand and respect the operational boundaries. Certainly, there are built-in safeguards to detect issues like over-voltage or thermal peaks, but these do not cover all possible operational scenarios. It’s your responsibility to adhere to these guidelines. Every component has a defined maximum temperature threshold—semiconductors, for instance, can operate at temperatures ranging from 85°C to 125°C, based on their type. This principle extends to every part of the assembly, including the PCB, which also has its own thermal limits.

Thermal energy expulsion also varies under different conditions, such as natural convection or forced air. You must consider these factors when establishing a system-level thermal rating schema. Components can exceed their maximum temperatures due to ambient conditions or heat generated internally, leading to degradation and potential damage.

Magnetics also suffer at high currents and temperatures, generating waste heat and losing linear performance, affecting overall system performance and potentially causing cascading effects. Transistors have maximum current ratings and tend to derate at higher temperatures due to junction physics, generating additional waste heat. 

Every component on your board has thermal and electrical limitations, each contributing to derating in different ways. Users often struggle to navigate this complexity.

A board is an intricate assembly of components. Gauging the combined effect is nearly impossible without simulating all real-life conditions. Each element, including resistors, capacitors, magnetics, and even passive materials like PCBs and sheet metal, must operate within safe limits to ensure reliability, optimal performance, and safety, preventing overheating or the release of toxic chemicals.

We characterize these complex assemblies and present the data in a simple, readable format. We aim to simplify this for our partners. Using our testing and component knowledge, we provide derating specifications as guidelines to help you optimize your boards effectively.

Common Types of Power Derating

1. Thermal derating is the most prevalent, occurring 90-95% of the time. It involves trading off operating temperature with maximum power output and is expressed in terms of ambient or coolant temperatures, depending on whether the device is air or liquid-cooled.
2. Input voltage derating occurs when lower input voltages necessitate higher input currents. A decrease in input voltage requires larger input components, leading to trade-offs between lower voltages and output power.
3. Altitude derating affects air-cooled power supplies, as lower pressure at higher altitudes reduces the air’s ability to carry away heat, limiting the system’s power output.
4. Droop isn’t traditional derating but rather a trade-off between current and voltage when maximum power decreases, particularly in parallel setups.

Here’s an example of a power derating curve from a liquid-cooled power supply. The X-axis shows coolant temperature (10-60°C), and the Y-axes shows DC output in amperes and kilowatts.

De-Rating Curve

The power curve illustrates that the system can operate at maximum output from 10 to 35°C. Beyond 35°C, output power decreases linearly until 55°C, above which we don’t recommend operation. This curve serves as a guideline for selecting operating conditions. If exceeded, the system may shut down to prevent damage, so adhering to the derating curve is crucial. System planning should ensure coolant temperatures stay at or below 35°C for optimal performance.

It's important to note that the system isn't immune to errors, faults, or automatic shutdowns if it reaches an intolerable maximum temperature. For instance, if you attempt to run the system at 50°C at full power for an extended period, the coolant temperature will continuously rise. Eventually, it will reach a critical thermal threshold, forcing the system to shut down to prevent damage. To avoid such scenarios, adhering to the derating curve is essential for maintaining system stability and performance.

In your system planning, it's vital to ensure that the coolant temperature remains at or below 35°C to achieve maximum power output. Maintenance teams and installers of cooling systems can provide guidance on expected operating conditions and coolant temperatures. Therefore, when budgeting your system with a specified number of power supplies, you must account for the fact that higher coolant temperatures will reduce available power. This may necessitate purchasing additional power supplies to maintain the desired performance levels.

This underscores the importance of cross-checking with us, especially under complex conditions involving higher temperatures or elevated ambient temperatures. As mentioned earlier, if high current levels are also anticipated, the combination of these factors can be intricate and challenging to represent on a simple graph. If you're uncertain about operating conditions or planning a system and need to ensure its viability, we're here to help. Our field applications team and extensive engineering resources are at your service.

Now, let's delve into some practical examples to ground these concepts in real-world scenarios. Each example is based on actual situations we've encountered and hope they will be beneficial to you.

Case Study #1: Undertanding Temperature Derating with the MercuryFlex or LiquaBlade Programmable Power Supplies

Our first example involves a customer needing 12 kilowatts of AC to DC output power, which must fit into a 2U rack space (about 3.5 inches) with a maximum ambient temperature of 65°C. They require a 400-volt DC output and have a three-phase 480-volt AC input.

For this scenario, I would recommend the MercuryFlex™ system. The MercuryFlex™ is a 2U, 3-point system that allows for up to four units in a rack, providing 15 kilowatts of output power. It supports up to 450-volts DC output and accepts both single-phase and three-phase inputs. Given that this customer prefers an air-cooled solution and hasn't used liquid cooling before, the MercuryFlex™ is an ideal match for their needs.

However, they are attempting to operate it at a very high ambient temperature of 65°C, which is quite challenging. That's why it's essential to examine the derating curve in detail.

Here's a straightforward derating curve that illustrates the relationship between ambient air temperature (in degrees Celsius) and the maximum power output.

This specific curve pertains to an individual MercuryFlex™ unit. Typically, you can operate up to four of these units in a 2U rack.

For a single unit, you can see a consistent flat region in power output up to around 55°C. Beyond this point, the power output starts to decline, and at 65°C, a single MercuryFlex™ unit can deliver about 2,900 watts.

If you parallel four of these units in a 2U rack, the combined output is approximately 11.6 kilowatts—just shy of the 12 kilowatts the customer requires.

While this shortfall is relatively minor, it's still a deficit. At this stage, we would consult with the customer to understand their exact needs. Do they require precisely 12 kilowatts, or is there some flexibility in their voltage and current requirements? Specifically, can they tolerate slight dips under worst-case scenarios, such as when the ambient temperature reaches 65°C?

Another option might be to use an additional rack, though this could complicate their application. We could suggest using the MercuryFlex™ units while carefully monitoring operations to avoid entering the high-temperature region near 65°C.

Note: In the real world, different brands make varying claims. Comparing products using their actual performance curves—what we call derating—is crucial. It's not about the limitations of the product but the underlying physics. A product’s face value or label might say one thing, but real-life performance can differ. Robust designs often feature more graceful derating curves, and it's vital for users to recognize this. Not all brands and datasheets are created equal, so delving into these details is essential to understand the real available throughput.

Let's consider a second option for our customer. As discussed, if the customer can tolerate slight deviations below 400-volts DC and 30 amps, the MercuryFlex™ units should suffice. However, if their requirements are stringent and they need to maintain exactly 400-volts and 30 amps across the entire temperature range, we have another solution: the LiquaBlade™.

It's important to note that derating still applies to the LiquaBlade™, but as a liquid-cooled system, it is less affected by ambient air temperature. The primary factor limiting its output power is the coolant temperature, which is controlled by the chiller used.

We can present the LiquaBlade™ as an alternative solution. It offers a significant headroom with a 16.5-kilowatt output and up to 500 volts, ensuring it meets their exact voltage and current requirements. Additionally, it fits into a 1U rack space instead of 2U and supports three-phase input, aligning with their specifications.

While transitioning to a liquid-cooled solution might be a significant change for customers accustomed to air-cooled systems, the LiquaBlade™ offers robust performance and reliability. This option, though different in its derating characteristics, could better suit their needs.

What Percentage Should We Consider for Operational Headroom in Power Supply Design?

This can vary, but generally, for normal operating conditions as specified in the datasheet, it's recommended to have around 20% headroom. This buffer ensures reliability under standard conditions. However, for specific edge cases or more demanding scenarios, consulting with a field applications engineer (FAE) can provide more tailored guidance.

However, when you have an in-depth understanding of all operating conditions at any given time, the need for a buffer diminishes. In such precise scenarios, our data specifications are fully met. The 20% buffer is more about preparing for the unexpected. For example, based on the curve, you might predict that temperatures will stay below 50°C in 99% of cases, allowing you to operate close to the limit without problems. But in reality, there's always that 1% chance where temperatures could surge to 60°C or even 70°C. This is where the 20% buffer becomes essential. It ensures your system remains reliable even when operating conditions deviate from expectations. So, unless you're absolutely certain about your power supply’s operating conditions in every scenario, incorporating this buffer is highly recommended.

Case Study #2: Evaluating Derating Curve Under Force Air vs Convection Cooling in Power Supplies 

 Suppose we have a requirement for 160 watts DC output at 28-volts DC, with a single-phase 90-volts AC input, and a maximum ambient temperature of 40°C, operating solely on convection airflow.

The customer is interested in the ASM201 series of power supplies, which claims a maximum output of 200 watts and is medically rated at 2 x 4 x 1.1 inches. The key factor here is the convection airflow. With air-cooled supplies, there are different ratings: one for forced air (active airflow) and another for natural convection (passive airflow). Forced air can remove heat more efficiently, while convection relies on the slower process of natural air movement.

The customer might see the 200-watt rating and think that a 20% buffer would be sufficient for their 160-watt requirement. However, this isn't necessarily true if their operating conditions differ from those specified in the datasheet.

Here, we have two sets of curves for the ASM 201series. On the left, the curve shows performance with 14 cubic feet per minute of airflow for a 24 to 36 volts DC range, which matches our 28-volt system with a 90-volts AC input. At 40°C, this setup hits 200-watts without issues.

However, the right curve shows performance under convection. At the same 40°C, the maximum output drops to 140-watts due to the less efficient heat removal. This means the ASM 201 won't meet the 160-watt requirement under convection.

Additionally, we offer the ASM202 series, which has a higher capacity with forced air but similar performance under convection. If the customer needed 220-watts with forced air, the ASM202 would be ideal. But since the requirement is under convection, we need to explore alternative solutions.

Note: If you're wondering why we're discussing various input voltages, it's because real-world conditions vary significantly across different regions. For example, in the U.S., we commonly refer to our grid voltage as 110-volts, but it often measures around 115-volts. In contrast, countries like Japan and Taiwan have a nominal voltage of 100 volts, and in densely industrialized areas, it can drop to as low as 90-volts.

These differing conditions underscore the importance of tailoring solutions to specific market requirements. For engineers selecting components for systems used in varying international environments, it's crucial to account for these differences to ensure optimal performance. This is why we meticulously publish real-life data to guide your design choices.

It's essential to look beyond headline specifications and consider the context of your specific application. While your system might operate well under standard conditions, unexpected scenarios could arise, making it wise to consult datasheets or speak with experts for more nuanced applications.

Moreover, a quick note on airflow: When we discuss airflow, we often refer to a fan of specific dimensions blowing air through a defined cross-sectional area, which we call a duct. In practice, the duct might not exist, but the concept helps us measure airflow in cubic feet per minute (CFM). Other manufacturers might present airflow in linear feet per minute or meters per second, depending on regional practices. These measurements are convertible, so if you need clarification, please feel free to reach out to us.

Now, regarding the ASM 201: Unfortunately, it derates to 140 watts under convection, which doesn't meet our requirement. So, what's the next step? We typically move to a higher-rated system to ensure it meets the necessary power output under the given conditions. We then considered the ASM202, but it also didn’t suffice under convection.

The next viable option is the ASM400. As its name suggests, it can handle up to 400-watts with full airflow. It measures 5 x 3 x 1.32 inches, slightly larger than the ASM201, and it meets medical standards, which could be an added benefit for some applications.

Even under the worst-case scenario of natural convection with a 90-volts AC input, the ASM400 provides up to 200-watts for 15-54V models and 180-watts for 12V models. Given our 28V requirement, the blue curve on the rating chart shows that at 40°C, the ASM400 should meet our needs.

The ASM400 should deliver up to 200 watts at 40°C, providing ample headroom for the customer's needs. Even if their system's temperature rises to 45°C or even 50°C, it would still comfortably exceed their 160-watt requirement. Thus, the ASM400 is more than capable of meeting their demands. Although it’s slightly larger, it offers a much better fit for their specifications.

Case Study #3: Real-World Altitude Power Derating with the HermesFlex™

Now, let's delve into a more complex scenario—altitude. This aspect might not typically impact our systems' real-world operation, but understanding how to calculate altitude derating is beneficial. While it's less common, it does come up occasionally.

Always refer to the datasheet to check the altitude specifications. If your requirements are unusual, don't hesitate to reach out to us. Our engineering team can perform the necessary calculations and testing to ensure your system operates optimally.

Here's an example of altitude-related derating. We have a curve for pressure versus altitude, calculated as:

At 5 kilometers, the pressure drops to about 55% of sea level, meaning air density—and thus heat-carrying capacity—is also reduced by 45%.

In this example, we’ll simplify calculations by assuming a 45% derating, although air-cooled supplies with fans can adjust for thinner air. For a 3.5-kilowatt, 60-volts DC output with a single-phase 220 AC input, fitting into a single 1U rack and functioning up to 5 kilometers, the HermesFlex™ is the best option. Each HermesFlex™ unit provides 2.2 kilowatts up to 500-volts, with three units fitting into a 1U rack, totaling 6.6 kilowatts at sea level.

Assuming a 45% derating at 5 kilometers, the capacity drops to 2.4 kilowatts per unit. To meet the 3.5-kilowatt requirement, we need to compensate for the reduced air density. Dividing 3.5 kilowatts by 0.55 gives us approximately 6.4 kilowatts, which aligns with using three HermesFlex™ units. This configuration ensures that even at high altitudes, the system will meet the required power output.

When the 6.6-kilowatt capacity of the HermesFlex™ system is elevated to 5 kilometers, it derates to approximately 3.6 kilowatts. This configuration should meet all the customer requirements even at high altitudes.

These examples illustrate that while we aim to promote our most compact and attractive solutions, customers often prefer starting with the smallest device available. As we better understand their operating conditions, it may become necessary to transition to a higher-rated product. Fortunately, we have thoroughly characterized our products and have all the information readily available. By sharing your specific application details with us, we can explicitly determine the best solution for your needs.

One common question we receive is about the optimal positioning of fans for cooling power supplies. For low-power supplies without built-in fans, we provide recommendations on fan positioning, specific fan models, operating speeds, and even the orientation of the power supply itself. For instance, in convection cooling, the orientation significantly impacts the convection experience because it changes the cross-sectional area through which air can flow. A power supply positioned with a wide XY plane and a narrow Z plane will have more area for air to escape compared to one positioned differently.

Moreover, the direction in which hot air is expelled—whether below, to the side, or above the power supply—can affect its thermal performance. Hot air rising back into the power supply can increase its temperature, so these factors must be carefully considered.

In summary, there are numerous considerations to keep in mind, and we encourage you to reach out for personalized advice tailored to your specific application. Should you have any inquiries about derating or need expert guidance, don't hesitate to reach out to us at any stage of your design journey.