Thermal Design Guide for Recom Power Modules

Introduction to Thermal Design

Proper thermal design is essential for reliable power supply operation. This guide covers thermal management for Recom DC-DC converters and AC-DC power supplies.

Understanding Power Dissipation

Calculating Power Loss

power dissipation in a DC-DC converter:

`` P_loss = P_out × (1/η - 1) `

Where:

• P_out = Output power (W)
• η = Efficiency (decimal)

Example: For 5W output at 85% efficiency: ` P_loss = 5 × (1/0.85 - 1) = 5 × 0.176 = 0.88W `

Efficiency Variations

note that efficiency varies with:

• Input voltage (typically lower at extremes)
• Output load (usually specified at 100% load)
• Temperature (may decrease at high temperatures)

Always use worst-case efficiency for thermal design.

Thermal Resistance Concepts

Thermal Resistance Model

heat flows from junction to ambient through thermal resistances:

` T_junction = T_ambient + P_loss × (R_thJC + R_thCH + R_thHA) `

Where:

• R_thJC = Junction to case thermal resistance
• R_thCH = Case to heatsink thermal resistance
• R_thHA = Heatsink to ambient thermal resistance

Recom Converter Thermal Characteristics

recom SIP/DIP package thermal resistances:

• R_thJC (junction to case): ~15-25°C/W
• R_thCA (case to ambient): ~40-60°C/W (without heatsink)

PCB Thermal Design

Copper Area for Heat Spreading

For converters without heatsinks, PCB copper area provides cooling:

Copper Area

Thermal Resistance

1 cm²	~80°C/W
5 cm²	~50°C/W
10 cm²	~40°C/W
20 cm²	~35°C/W

PCB Layout Guidelines

Thermal Vias: Use thermal vias under the converter

• 1.0-1.2mm diameter
• 0.3-0.5mm drill
• Copper filled for best conductivity

Copper Planes: Connect to internal copper planes

• Use multiple vias to spread heat
• Connect to ground and power planes

Component Placement: Allow airflow around converter

• Keep clearance for natural convection
• Avoid placing heat-sensitive components nearby

Heatsink Selection

When to Use Heatsinks

Heatsinks are typically not required for Recom converters at rated power up to 60°C ambient. Consider heatsinks when:

• Ambient temperature exceeds 60°C
• Power dissipation exceeds 1W
• Enclosure has limited airflow
• Reliability is critical

Heatsink Selection Process

Calculate Required Thermal Resistance:

` R_th_required = (T_case_max - T_ambient) / P_loss `

Select Heatsink: Choose heatsink with R_th less than required

Include Interface Resistance: Add 0.5-1.0°C/W for thermal interface

Example Calculation

given:

• Output power: 10W
• Efficiency: 86%
• Ambient temperature: 50°C
• Maximum case temperature: 105°C

Calculations: ` P_loss = 10 × (1/0.86 - 1) = 1.63W R_th_required = (105 - 50) / 1.63 = 33.7°C/W `

Without heatsink (R_thCA = 45°C/W): ` T_case = 50 + 1.63 × 45 = 123°C (TOO HOT!) `

With heatsink (R_thHA = 15°C/W + 1°C/W interface): ` T_case = 50 + 1.63 × 16 = 76°C (OK) ``

Thermal Interface Materials

Types of Interface Materials

Thermal Grease:

• Best thermal performance (0.1-0.5°C-cm²/W)
• Messy application
• Can pump out over time

Thermal Pads:

• Easy application
• Moderate performance (0.5-2.0°C-cm²/W)
• Good for gap filling

Phase Change Materials:

• Good performance (0.3-0.8°C-cm²/W)
• Easy application
• Long-term stability

Application Guidelines

• Apply thin, even layer
• Ensure full contact area coverage
• Minimize interface thickness
• Use mounting pressure per manufacturer spec

Enclosure and Airflow

Natural Convection

for natural convection cooling:

• Provide ventilation openings
• Orient enclosure for chimney effect
• Keep converter away from walls
• Use thermally conductive enclosure materials

Forced Air Cooling

for high-power or high-temperature applications:

• Use fans for airflow of 100-200 LFM
• Position intake away from exhaust
• Filter air to prevent dust buildup
• Consider fan reliability and noise

Sealed Enclosures

for sealed or IP-rated enclosures:

• Use external heatsinks with thermal vias
• Consider heat pipes for heat transfer
• Design for worst-case ambient temperature
• May require derating

Thermal Testing and Validation

Temperature Measurement

Thermocouples: Attach to case with thermal epoxy

Infrared Thermometer: Measure case temperature

Thermal Camera: Visualize hot spots

Testing Procedure

Measure ambient temperature

Apply maximum load

Allow 30 minutes for thermal stabilization

Measure case temperature

Verify within rated limits

Margin Requirements

design with margin for reliable operation:

• Keep case temperature 10-20°C below maximum
• Account for altitude (derate 1°C per 1000ft)
• Consider component aging effects

Special Considerations

High Altitude

at high altitude (>3000ft):

• Reduced air density decreases cooling
• Derate maximum ambient by 1°C per 1000ft
• Consider forced air cooling

High Humidity

in high humidity environments:

• Ensure adequate spacing for creepage
• Use conformal coating if required
• Consider condensation effects

Multi-Module Systems

For systems with multiple converters:

• Spread modules across PCB
• Avoid thermal shadowing
• Calculate total power dissipation
• Ensure adequate overall cooling

Design Examples

Example 1: PLC Power Supply

• Converter: R12P21503D (15W dual output)
• Ambient: 55°C in sealed enclosure
• Solution: Added 20cm² copper area, positioned near enclosure wall
• Result: Case temperature 78°C (within limits)

Example 2: Medical Device

• Converter: REM3.5-1205S (3.5W medical)
• Ambient: 40°C with natural convection
• Solution: Standard PCB layout with thermal vias
• Result: Case temperature 52°C (well within limits)

Conclusion

proper thermal design ensures reliable operation and long converter lifetime. Most Recom converters at rated power require only adequate PCB copper area for cooling. For high temperatures or high power, additional thermal management may be required. Contact our FAE team for assistance with challenging thermal designs.