Most high-power power supplies use forced-air cooling to dissipate the generated heat; however, this approach is impractical in a fully sealed unit. Therefore, motherboard-cooled converter modules have become a popular solution for these applications.
What are motherboard-cooled converter modules?
Motherboard-cooled power converter modules, also known as bricks, provide modular power solutions for integration into end-user equipment. These high-power-density solutions are ideal for sealed outdoor enclosures, transportation, and defense applications, where their robust construction and conduction cooling are particularly advantageous. They are also suitable for high-density, forced-air-cooled applications, where their compact size can be leveraged.
Furthermore, they serve as building blocks for low-risk, customized power solutions, whether designed by the end-user equipment manufacturer with technical support from the module manufacturer or developed directly by the manufacturer as bespoke solutions, with low development costs, reduced risk, and accelerated time to market.
What applications are they compatible with?
Motherboard-cooled converters are component-level power solutions, rather than off-the-shelf products, which typically require additional design resources and components for proper operation, electrical safety, thermal management, and electromagnetic compatibility (EMC). They are designed for both DC and AC input applications, with input ranges designed to cover DC battery and vehicle supplies, as well as higher-voltage, rectified, or power factor corrected (PFC) AC supplies.
The industry has developed standard sizes for these bricks, described as eighth bricks, quarter bricks, half bricks, and full bricks. Power ratings of up to 600 or 700 W can be achieved in a standard 2:1 input full brick, although power density decreases as the input range expands to 4:1, 8:1, or even 12:1 to accommodate multiple battery supply ratings in an attempt to standardize system design for multiple power platforms. Baseboard-cooled AC-input converters are also available as a more complete solution providing low-voltage AC-to-DC conversion or as PFC modules with high-voltage outputs (typically around 400 VDC) to power high-voltage input DC-DC bricks.
Most of these AC input products also require additional design and components, including high-voltage electrolytic capacitors and components for EMC, with one or two exceptions, such as XP Power's ASB series of complete 75, 110, and 160W AC-DC brick solutions, which only require thermal management and include all other parts.
Key elements of power module integration
Thermal management
Thermal management is a key element in the integration of power modules designed to be cooled by the motherboard. The module is designed so that the power-dissipating components, such as the power semiconductors and transformer, are thermally bonded to the motherboard, which must be kept below a maximum operating temperature under the worst-case application conditions.
The cooling system's thermal resistance must be matched to the power required by the load or end equipment, the module's efficiency (which determines the power dissipated in the converter block), and the maximum operating temperature of the equipment. The power dissipation (in watts) can be determined from the module's efficiency specification under worst-case operating conditions, although it's important to consider the actual operating load and the lowest applied input voltage, rather than the efficiency listed on the datasheet.
Figure 1: Example of the variation of efficiency with input voltage and load
Once the efficiency at the most unfavorable operating point is established, the residual power dissipated as heat is calculated as follows:
Figure 2: Calculation of the residual power dissipated as heat
Once the power/waste heat is determined, the simple model shown below determines the thermal resistance required for operation, with ∆T defined as the difference between the maximum operating temperature of the equipment and the maximum temperature of the power block's motherboard. The thermal resistance between the case and the heatsink is typically 0.1 °C/W when using a thermal pad or grease.
Figure 3: Thermal model of the power supply block and heat sink
The thermal resistance of the heatsink to the environment depends largely on the available airflow, meaning that in convection-cooled applications, its physical size will be much larger than in a comparable power system with forced air or liquid cooling. When using multiple blocks connected to a common heatsink or a cold wall, the sum of the power dissipated by each block in the system under the worst-case conditions determines the total thermal resistance required.
Electromagnetic compatibility
In addition to the thermal management described above, motherboard-cooled modules require additional external components for proper operation, reverse polarity protection, and noise emission control, as well as for protection against spikes and surges as defined in the application's susceptibility requirements. This means that a network of capacitors for noise mitigation and source impedance reduction, inductors, and surge suppression components must be installed within the final application.
Fuses must also be implemented to ensure safety in the event of a catastrophic failure causing a short circuit in the supply. The power module datasheet and application notes will specify the required component values, although it is the design engineer's responsibility to implement them, following good design practices for any creepage distance and clearance requirements, and minimizing parasitic inductance to comply with EMC regulations.
Figure 4: Schematic of DC input systems
FS1 provides short-circuit fault protection at the input, and D1 provides reverse polarity protection. L1, C1, and C2 form a pi filter to mitigate differential noise created by rapid current changes in the power switching stage, and L2, C4, and C5 form a common-mode filter to mitigate noise created by rapid voltage changes in the power stage. C3 provides a low-impedance source for the power converter's switching current demand, and TVS1 is a bidirectional transient surge suppressor to protect against spikes and surges. C6 and C7 reduce output common-mode noise, and an additional differential filter can be added at the output for applications requiring very low noise.
In general, the decoupling capacitors (C4, C5, C6, and C7) should be as close as possible to the pins and the chassis-to-motherboard connection to keep the loop as short as possible. The input electrolytic capacitor (C3) and the transient voltage suppressor (TVS1) should be physically close to the module's input pins with the loop as small as possible, and traces under the power module should be avoided.
There are a limited number of application-specific filter modules available for abnormal surges found in DC ingress transmission and defense applications. These typically also require some additional components for full compliance.
Figure 5: Schematic for a CC entry defense application
In this case, the DSF filter module contains all the active surge protection circuitry necessary to meet the requirements of a nominal 28 V power supply for military vehicles, as well as the filter inductors. C1 completes the differential filter stage, and C3 and C4 complete the common-mode filter stage. C2 provides a low-impedance source for the MTC series DC/DC converter. When using a PFC module solution for an AC input power system, similar EMC components are required, along with a high-voltage (450 VDC) electrolytic capacitor (C6), as shown. The capacitor value is determined by the system's maintenance or continuity requirements.
Figure 6: Schematic of AC input systems
AC input solutions
Some AC input solutions combine the PFC and DC-DC sections into a single unit, with connections available for the large-capacity capacitor. Additional requirements exist for AC input systems regarding creepage and clearance distances between line and neutral, and between line, neutral, and earth, which must be met during the power system implementation. These creepage and clearance distances are described in the relevant safety standard for the end application.
The additional components described above must also be carefully selected and thermally managed to remain within the safe thermal limits of the inductors and filter capacitors, and to ensure that the electrolytic capacitors used achieve the desired lifespan throughout the final equipment's mission profile. Heat from the inductors can be dissipated to the equipment's cold wall using suitable insulating thermal pads, and other components are often mounted on the back of the printed circuit board and kept away from any higher-temperature parts.
The leading manufacturers of these motherboard-cooled, high-density converters provide application information and electromagnetic compatibility (EMC) data to support the design process. They also have experienced application engineers who assist users during the design and compliance testing phases, and offer pre-compliance testing against the most common industry, communications, transportation, and defense EMC standards.
The manufacturer will also have used the modules in a wide variety of application-specific custom designs and will have gained invaluable experience in their application and in complying with safety and EMC standards. Turning to the module manufacturer to design and produce a custom block-based power system remains a popular option, as it eliminates the need for additional in-house design and testing resources and allows the original equipment manufacturer to focus on core system design.
Author: Andrew Bryars
Andrew Bryars graduated with honors in Electrical and Electronic Engineering and has been working in the power supply industry since 1990. He has been with XP for over 25 years and has held various engineering, sales, and management positions, culminating in his current role as Senior Product Manager.
