Power supplies are vital components that provide the operating voltages and currents required by electronic circuits and systems. Selecting the appropriate power supply and mounting technique is an important part of the overall PCB design process.
Power supplies come in different forms like AC-DC wall adapters, DC-DC converter modules, and open frame power supplies. Most board-level DC-DC converters and open frame supplies are designed for direct mounting onto the PCB itself.
This article provides guidance on PCB design considerations, mounting approaches, thermal management, and best practices for integrating different types of power supplies onto printed circuit boards.
Power Supply Mounting Options
Power supplies meant for PCB mounting typically offer some combination of through-hole pins, surface mount pads, and mechanical mounting features. The best mounting method depends on parameters like:
- Available PCB area
- Power supply footprint and pinout
- Required mounting robustness
- Necessary heat dissipation capability
- PCB assembly process used
Some common power supply mounting schemes are:
Through-Hole Pin Mounting
The power supply connector/pinout matches a thru-hole pattern on the PCB for insertion and soldering. Typically requires wave soldering process. Provides strong mechanical coupling. Good for high vibration environments.
Surface Mount Pad Mounting
Surface mount footprints allow reflow soldering of the power supply. Requires less PCB area but heat transfer may be worse. Not ideal for heavy power supplies.
Screw/Standoff Mounting
The power supply fastens onto standoffs or spacers on the PCB using corner screws. Provides very robust mechanical attachment. Requires some manual assembly.
Panel Mounting
Power supply is mounted into a cutout on a metal bracket or chassis panel. Wiring then connects panel interface to the PCB. Useful when board space is unavailable.
DIN Rail Mounting
DIN rail clips allow mounting to standard DIN rails in industrial enclosures. Requires separate DIN rail allocation.
Adhesive Mounting
Suitable for lightweight supplies. Can accommodate uneven surfaces. Generally less robust and permanent.
Key Selection Considerations
Some important aspects when selecting a power supply for PCB mounting:
1. Board Area: Dimensional footprint – height, length, width. Avoid overlaps with adjacent components.
2. Mounting Holes: Spacing, diameter, pad type. Match PCB capability and assembly process.
3. Pinout: Layout of power, signal, control pins. Compatible with PCB layers and routing?
4. Connector Options: Terminal blocks, pin headers, wire leads. Easy integration?
5. Heat Dissipation: Will PCB area provide sufficient cooling surface?
6. Structural Strength: Weight, center of gravity. PCB can support without flexing?
7. Reliability: Vibration, shock resistance rating. Thermal stress on solder joints?
8. EMI/EMC: Noise emissions, susceptibility issues? Filtering or shielding needed?
9. Safety Standards: Applicable certifications – UL, CSA, CE, etc.
Analyzing requirements in each area ensures an optimal power supply selection for the design.
PCB Layout Considerations
Design the PCB layout to properly accommodate the power supply placement and pin interfaces. Key aspects to address:
1. Area: Reserve adequate board space for power supply footprint. Include keepouts.
2. Layers: Ensure sufficient layers to route power pins. Minimize crossovers.
3. Pin Mapping: Match board layers to power supply pin functions.
4. Pin Pads: Use large pad sizes for mechanical stability.
5. Routing: Provide sufficient trace widths for load currents.
6. Noise Coupling: Prevent coupling of noise into sensitive circuits. Assign ground planes carefully.
7. Constraints: Add height constraints for 3D modeling. Include keepouts for access space.
8. Thermal Pads: Connect to inner ground layers for heat sinking.
9. Test Points: Provide test points to validate voltage rails.
10. Legends: Identify power supply name and connections in silkscreen.
11. Safety: Follow spacing rules between hazardous potentials.
Advance planning of interface and layout details streamlines integration of the power supply.
Through-Hole Pin Mounting
Through-hole mounting with long solid pins inserted into plated PCB holes provides a very robust mechanical and electrical connection.
Benefits
- Strong solder anchor to PCB
- Withstands vibration and mechanical shock
- Easy hand assembly and servicing
- Good grounding via pin interface
Drawbacks
- Requires wave solder process
- Occupies more board space
- Limited routing under supply
- Not suitable for reflow process
Layout Guidelines
- Match hole locations to power supply pin spacing
- Use annular ring pads suited to hole size
- Allow sufficient pad clearance for wave soldering
- Connect thermal pad to ground plane
- Include silkscreen outline for alignment
Through-hole mounting is recommended for bulky, high power supplies or extreme vibration environments.
Surface Mount Pad Mounting
Soldering the power supply to surface mount pads on the PCB allows minimal footprint but requires attention to solder joint reliability.
Benefits
- Small footprint occupies less board space
- Permits easier routing of traces
- Enables reflow solder process
- Allows placement on both sides
Drawbacks
- Vulnerable to vibration and shock loads
- Requires careful soldering process control
- Heat transfer may be less effective
- Can’t easily be replaced or serviced
Layout Guidelines
- Specify pad dimensions to match power supply
- Allow sufficient pad spacing for solder mask web
- Assign pad shapes to facilitate inspection
- Connect thermal pads to inner ground layers
- Use generous fillets and thermal reliefs
- Apply no-fill zones under supply area
Surface mounting suits space-constrained designs using SMT assembly processes.
Screw/Standoff Mounting
Screwing the power supply onto threaded standoffs or spacers on the PCB provides a sturdy mechanical mounting option.
Benefits
- Robust against shock, vibration, acceleration
- Permits easy replacement or servicing
- Allows vertical stacking of boards
- Good electrical grounding via standoffs
Drawbacks
- Extra assembly step to install hardware
- Standoffs consume board space
- Cannot use reflow process
- Need to access both PCB sides
Layout Guidelines
- Position standoff footprints at screw holes
- Allow access space surrounding supply
- Use oversized pads for standoffs
- Standoff diameter should match screw size
- Include keepout under supply
- Allow margin for assembly clearances
Screw mounting is advantageous where mechanical stability is critical, at the cost of extra assembly work.
Thermal Management
Carefully managing power supply heat dissipation prevents shutdowns or damage from overheating.
Heat Sinking Approach
- Connect thermal pad to ground plane
- Ensure enough copper area for transfer
- Add thermal vias to inner layers
External Sink
- Attach sink plate/extrusion
- Electrically isolate if no grounding
- Use thermally conductive paste
Convection
- Orient for natural air flow
- Maintain clearance for airflow
- Add forced air cooling if needed
Conduction
- Use chassis or enclosure as sink
- Insert thermal gap pads
- Attach heatsink to outside
Layout Considerations
- Thermal pad size to match power supply
- Maximize copper fill around supply
- Minimize thermal obstacles
- Model heat dissipation to avoid overheating
- Monitor temperature using board sensors
Proper heat sinking design prevents power supply over-temperature failures.
Best Practice Design Tips
Follow these guidelines for reliable, optimized power supply integration:
- Select power supply early to guide layout
- Coordinate mechanical spacing and clearances
- Define interface requirements upfront – layers, pins etc.
- Reserve area on PCB prior to placement
- Minimize trace lengths for load connections
- Review layout before finalizing power supply
- Prototype board assembly to validate fit
- Assess vibration, shock datasheet ratings
- Verify electrical performance under load
- Test temperature using thermal probes
- Consider adding monitoring and protection
- Inspect solder quality and joints thoroughly
Careful planning of requirements, constraints, analysis, and testing ensures a robust power supply PCB implementation.
Conclusion
- Mounting power supplies directly onto a PCB saves space and cost compared to external modules.
- Through-hole pins allow robust mechanical attachment but occupy more area.
- Surface mounting is suitable for tight space but risks reliability.
- Screw mounting provides very strong connection with added hardware.
- Thermal design must dissipate heat adequately to avoid overload shutdown.
- PCB layout requires careful pin assignment, routing, and keepouts.
- Advance coordination of electrical, thermal, and mechanical factors helps avoid issues.
- Testing assembled boards under operating ranges validates the implementation.
In summary, mount power supplies securely onto the PCB with proper layout considerations to maximize performance and reliability of the overall system.
Frequently Asked Questions
Q: What are some typical voltage and current levels used for PCB power supplies?
Typical voltage and current levels:
- Voltage: 5V, 9V, 12V, 15V, 24V
- Current: 0.5A, 1A, 2A, 5A common; up to 15-20A
Higher voltages may require external power supplies for safety. Lower voltages can use on-board LDO regulators.
Q: How are through-hole pin power supplies secured mechanically before soldering?
Securing options before soldering:
- Press-fit insertion into plated holes
- Adhesive paste or tape on corners
- Mounting brackets around supply
- Fastening to spacers or standoffs
- Soldering critical pins only
This avoids stress on pins before the solder connects mechanically.
Q: What checks should be performed on the PCB after installing the power supply?
Recommended validation checks:
- Visual inspection of alignment, pin insertion
- Confirm supply is electrically powered on
- Verify output voltage rails with DMM
- Check for any short circuits or faults
- Measure ripple, noise on outputs
- Perform “wiggle” test for loose connections
- Assess heating of supply under load
This helps catch any integration issues before full system assembly.
Q: What is the typical height clearance needed above a PCB power supply?
General height clearance guidelines:
- At least 25-50% of supply height for air flow
- Greater for convection or forced air cooling
- 20-40mm typical; more for high power density
- Check height tolerances in 3D model
- Ensure excludes nearby tall components
- Heatsinks may require extra height
Leaving adequate vertical space prevents overheating issues.
Q: How can the reliability of soldered power supply joints be improved?
For enhanced solder joint reliability:
- Specify larger pad sizes to reduce strain
- Maintain solder mask web between pads
- Follow manufacturer recommended footprints
- Use thicker 2oz+ copper layers
- Implement annular ring breakout patterns
- Avoid tombstoning with unbalanced paste
- Assess thermal cycling performance in reliability tests
Robust solder joints prevent cracking over product lifetime.
