Thermal Considerations When Selecting Power Supplies
For system integrators, one of the most common and critical topics during the design-in of a power supply component is thermal management of the overall system. In real-world applications, significant heat can be generated by CPUs, semiconductors, RF modules, and other active components. If the power supply is also integrated into the application, thermal dissipation becomes even more critical.
For many system components, developers have little influence on thermal optimization—after all, a processor is a processor, and a MOSFET is a MOSFET. This significantly limits optimization potential.
Power supplies, however, are a different story. Here, maximum efficiency is key, as lower efficiency directly translates into additional heat that must be managed. Efficiency data can typically be found in the power supply datasheet, allowing a quick assessment of the fundamentals.
This raises several important questions:
- Which power supply should I select?
- How can the efficiency of a PCB-mounted solution be compared fairly with a plug-and-play solution?
- How can such a comparison be structured to enable a meaningful efficiency assessment?
Key Considerations When Selecting a PSU
Selecting the right power supply involves several challenges that must be carefully evaluated, including:
- Required product standards
- EMC requirements
- Available installation space
- Mechanical mounting options
- Budget constraints
- Availability of in-house development resources
- Expected efficiency (i.e., how much heat must be dissipated)
- Availability and project timeline
…and many additional factors that may apply depending on the application.
How to Compare Solutions Correctly
Power supply solutions can generally be divided into:
- Fully integrated plug-and-play solutions
- Partial solutions, such as PCB-mounted DC/DC converters (“bricks”)
Additionally, we distinguish between COTS (commercial off-the-shelf) products and fully custom solutions.
In the following, we focus on COTS products.
To ensure a fair comparison, three different solutions are evaluated in a real application environment:
- A fully filtered plug-and-play solution for defense applications (60 W)
- Two brick-based solutions (one industrial converter and one defense-grade converter)
Brick Solutions and Required Additional Components
A PCB-mountable module (“brick”) always represents only a partial power supply solution. Additional filtering components are typically required to meet system requirements.
Key requirements beyond high efficiency include:
- Filtering according to
- MIL-STD-461G (Navy Mobile and Army)
- MIL-STD-1275E, including load dump as per DEF STAN 61-5 Part 6
- Reverse polarity protection
- Inrush current limitation
In this example, the brick only performs the voltage conversion from 24 V to 12 V. However, without additional circuitry, the converter will not operate. External components are required to protect against surge and burst disturbances. These include capacitors for energy buffering as well as filtering elements to reduce conducted and radiated emissions and ensure EMC compliance.
Additional Filtering Requirements and Losses
The following components are typically required:
- Extended input range (brick: 4:1 vs. ACM60: 6.7:1) leads to approx. 1 W additional loss due to higher-voltage MOSFETs with increased RDS(on)
- Three capacitors (“Strokos”) for high- and low-frequency noise suppression (approx. 3.6 W loss)
- Snubber networks and MOSFETs with higher resistance for smoother switching edges and improved EMC performance
- Series choke for further EMC reduction (approx. 0.8 W loss)
- Surge protection (MIL requirements) adding approx. 1–1.5 W loss
- Active reverse polarity protection using efficient transistor-based solutions instead of diodes
Summary of Losses Due to Filtering:
| Component | Power Loss | Reason |
|---|---|---|
| Additional input filtering | ~2 W | High RDS(on) MOSFETs, incl. load dump |
| Capacitors (“Strokos”) | ~3.6 W | 3 capacitors at ~1.2 W each |
| Two MOSFETs | ~2.5 W | High-resistance MOSFETs |
| Series choke | ~0.8 W | EMC filtering |
| Miscellaneous components | ~0.4 W | Minor components |
| Total | ~9.3 W | Total loss due to filtering |
Efficiency Comparison
- Modern industrial bricks: ~92.5% efficiency at full load (25 °C)
- Newer PCB-mount military converters: ~86–87%
- Small external filter modules: ~98% efficiency
This results in:
- Industrial brick with partial filtering: approx. 90% (0.925 × 0.98)
- Military brick with partial filtering: approx. 85% (0.87 × 0.98)
- ACM60 plug-and-play solution: 84% (no additional filtering required)
| Converter | DC/DC Efficiency | Filter Module Efficiency | Additional Losses | Overall Efficiency |
|---|---|---|---|---|
| Industrial brick | 92.5% | 98% | ~5.5 W | 84% |
| Military brick | 87% | 98% | ~5.5 W | 80.5% |
| ACM60 | 84% | Not required | Not required | 84% |
Conclusion
A purely datasheet-based comparison—without considering the additional design effort—can be misleading. The seemingly lower efficiency of 84% for a fully integrated solution becomes much more competitive when real-system conditions are taken into account. From both an economic and physical perspective, this represents an optimized balance.
When deciding between a plug-and-play DC/DC converter and a custom power supply design based on bricks and discrete components, the following aspects should be considered:
- Significant engineering resources may be tied up for extended periods
- A plug-and-play solution offers comparable or even better real-world efficiency using proven technology
- Responsibility for performance and compliance is transferred to the supplier
- A custom solution allows greater flexibility in layout, placement, and potentially cost optimization in production
