Why Temperature and Load Conditions Matter
Power supply units (PSUs) are the unsung heroes of modern electronics, quietly converting mains voltage into stable, usable power. In industrial control panels and automation systems, DIN rail-mounted power supplies are the go-to solution for reliable and efficient power delivery. Like any electronic component and device, PSUs do not work in a vacuum. Environmental factors, most notably ambient temperature, can influence the performance of PSUs.
Thermal derating is a common support topic in power supply design for industrial and automation applications. This blog introduces the fundamentals of thermal derating, including ambient temperature, load derating, convection versus forced-air cooling, and the importance of PCB layout in PSUs.
What is Ambient Temperature?
Let’s start with the basics. The term “ambient temperature” refers to the temperature of the air surrounding a device. For a DIN rail power supply mounted inside an electrical cabinet, it’s the temperature inside the cabinet, not the temperature of the PSU casing or of the room outside. This is a crucial distinction because a PSU’s ability to dissipate heat depends on the temperature difference between its components and the surrounding air. If the ambient temperature is high, the PSU cannot cool itself as effectively, leading to thermal stress.
What is Thermal Derating?
The term “thermal derating” refers to reducing the maximum output power of a power supply as the ambient temperature increases. It’s a safeguard against overheating. Every PSU has a thermal envelope in which it can safely operate. For example, a unit might provide 100% of its rated output up to 50°C. Beyond that, it may need to be derated, perhaps linearly down to 50% at 70°C. Ignoring thermal derating can lead to overheating, reduced lifespan, thermal shutdown, and even catastrophic failure.
What is Load Derating?
Load derating is a broader term that includes thermal derating but can also involve reductions in output power because of other factors, including the following:
- Altitude (less air = less cooling)
- Poor ventilation
- Input voltage (e.g., some PSUs provide lower output current if their input voltage drops)
In essence, thermal derating is a type of load derating specifically triggered by elevated temperature.
Convection vs. Forced-Air Cooling
There are two main ways to cool electronic components:
- Convection cooling: Relies on natural airflow. This form of cooling is silent, simple, and involves no moving parts.
- Forced-air cooling: Uses fans to push or pull air over components. This form of cooling is more effective in removing heat, but it has drawbacks.
Fans can be noisy. They introduce moving parts that can fail, and, in the absence of suitable filters (which themselves need to be cleaned or otherwise maintained), they tend to suck or blow dust into the system. That dust can settle on components and insulate them, reducing cooling efficiency and potentially causing long-term damage. Though fans may be installed in the overall enclosure, the DIN rail PSUs should avoid needing internal fans, because this simplifies installation, reduces costs, and increases reliability.
What is the Chimney Effect?
The chimney effect, also known as the stack effect, is a phenomenon in which air movement is driven by temperature differences between two vertical regions, typically within a chimney, building, or electronic enclosure. Here’s how it works. Because it is less dense than cool air, warm air naturally rises. As warm air escapes from the top of an enclosure (or chimney), it creates a low-pressure zone at the bottom. This low pressure draws in cooler air from below, creating a continuous upward airflow.
In PSUs, the chimney effect can be leveraged for passive cooling. Heat-generating components, such as power transistors and transformers, cause the air inside the PSU to warm up and rise. Vents at the top allow the hot air to escape, while vents at the bottom allow cooler air to be drawn in, thereby maintaining a natural convective airflow through the device. By reducing dependency on active components that can fail over time, such as fans, passive cooling based on the chimney effect improves maintainability and reliability. By removing excessive heat without fan use, the chimney effect reduces energy consumption and noise.
What is the Role of PCB Layout?
The printed circuit board (PCB) layout has a significant impact on how effectively a power supply unit (PSU) manages heat. A strong thermal design includes:
- Strategic placement of hot components.
- Wide copper pours for heat spreading.
- Adequate spacing between heat-generating parts.
- Thoughtful component height and orientation to facilitate the chimney effect and encourage airflow.
Together, these choices improve passive cooling and prolong component lifespan.
Additional Resources
The
RECOM AC/DC, DC/DC, and EMC Books of Knowledge provide an extensive collection of experience and learnings necessary to implement successful AC/DC power supply designs.
RECOM’s
Energy Efficiency in AC/DC Power Supplies: Quick Guide to the Essentials blog provides a helpful introduction to ecodesign regulations, a set of standards and guidelines established by governments or regulatory bodies to ensure that products are designed to minimize their environmental impact throughout their entire life cycle. These regulations primarily focus on improving energy efficiency and promoting sustainability by reducing waste, emissions, and resource consumption.
The
RACPRO1 page provides more information about this exciting new DIN-rail mounted power supply family.
