Lesson 12/1392%
CAPSTONE PROJECT 2
Power Supply Heat Sink Design
Design thermal management for a 500 W half-bridge DC-DC converter with high-side and low-side MOSFETs. Compare natural convection and forced-air cooling approaches and select the production solution.
For a power MOSFET in a switching converter, total loss = conduction loss + switching loss:
Pcond = IRMS2 × RDS(on)
Psw = 0.5 × VDS × ID × (tr
+ tf) × fsw
Reading the MOSFET Datasheet for Thermal Parameters: key
sections in the Infineon IPP110N20N3 datasheet: Page 1 Key Parameters (RDS(on)
= 11 mΩ, VDS = 200V, ID = 88A); Thermal
Characteristics Table (RθJC = 0.7 °C/W, RθJA
= 62 °C/W); SOA diagram for safe operating area limits; transient thermal
impedance ZθJC(t) curves for pulsed operation. The TO-220
package mounts directly to a heatsink via the metal tab (drain connection) — an
electrical insulator (mica washer or silicone pad) is required for non-isolated
mounting.
MOSFET Power Loss Calculator
Conduction Loss (Pcond)
Switching Loss (Psw)
Total Loss per MOSFET
Heatsink Requirement Calculator
Thermal Budget (TJ - TA)
ΔT used by RθJC (per device)
ΔT used by RθCS (per device)
Available ΔT for heatsink
Total power to heatsink
Required RθSA
Important: When multiple devices share a heatsink, the total
power is the sum of all devices, but RθJC and RθCS
apply per device. The heatsink must handle the total heat from all devices.
Required RθSA vs. Power per Device
Natural Convection Heatsink Sizing
Optimal Fin Spacing
Number of Fins
Total Surface Area
Estimated RθSA
Meets Requirement?
Challenge: With 36 W total and natural convection only,
achieving RθSA < 2 °C/W requires a large heatsink
(typically >150 cm² surface area in vertical orientation). This may
exceed available board space.
Natural Convection: RθSA vs. Fin Height
Adding a fan dramatically reduces thermal resistance. Adjust air velocity and heatsink size to compare natural vs. forced convection performance.
Fan-Cooled Performance
Estimated RθSA
Predicted TJ
Temperature Margin
Improvement vs Natural
Natural vs Forced Convection Comparison
Natural Convection
RθSA ≈ 2.5–4 °C/W · Requires large heatsink (100×100×50mm) · Silent, no moving parts · May not meet thermal requirement
RθSA ≈ 2.5–4 °C/W · Requires large heatsink (100×100×50mm) · Silent, no moving parts · May not meet thermal requirement
Forced Convection (2 m/s)
RθSA ≈ 0.8–1.5 °C/W · Compact heatsink (80×80×35mm) · Fan noise ~25-35 dBA · Easily meets thermal requirement ✓
RθSA ≈ 0.8–1.5 °C/W · Compact heatsink (80×80×35mm) · Fan noise ~25-35 dBA · Easily meets thermal requirement ✓
Automated Design Review Checklist
☑ Thermal Performance: TJ < TJ,max
with ≥20°C margin
☑ Reliability: MTTF > 100,000 hours at operating
temperature
☑ Manufacturability: Standard TO-220 mounting,
commercially available heatsink
☑ Cost: Aluminum extrusion + standard fan within BOM
target
☑ Weight: Total heatsink assembly < 300g
☑ Mechanical: Screw mounting with proper torque (0.5
N·m)
☑ Electrical Isolation: Insulator pad rated for VDS
voltage + margin
☐ Vibration: Verify heatsink retention under
shock/vibration (if applicable)
☐ Dust: Evaluate fin spacing for dust accumulation in
operating environment
Final Design Summary
══════════════════════════════════════════════════════
POWER SUPPLY HEAT SINK DESIGN REPORT Capstone Project 2 — Rising Edge
Training
════════════════════════════════════════════════════
── APPLICATION ── Converter: 500W Half-Bridge DC-DC (48V
→ 12V) Topology: Synchronous buck with half-bridge Devices: 2×
Infineon IPP110N20N3 (TO-220) Total Loss: 36W (18W × 2 devices)
── THERMAL PARAMETERS ── R_θJC: 0.7 °C/W (per
device) R_θCS: 0.5 °C/W (with silicone insulator) T_J,max: 150°C
(derated from 175°C) T_ambient: 50°C Required R_θSA: 1.94 °C/W
── NATURAL CONVECTION ANALYSIS ── Required Heatsink:
100×100×50mm (vertical orientation) Estimated R_θSA: 2.5
°C/W Status: ✗ INSUFFICIENT (T_J = 158°C exceeds limit)
── FORCED CONVECTION SOLUTION (RECOMMENDED) ── Heatsink:
80×80×35mm extruded aluminum Material: Aluminum 6063-T5, black
anodized Fan: 60mm axial, 3000 RPM, 15 CFM Air Velocity: ~2 m/s across fins
Achieved R_θSA: 1.1 °C/W Predicted T_J: 93.6°C Margin: 56.4°C
below maximum ✓ ── MOUNTING DETAILS ──
Attachment: M3 screws with spring washers Torque: 0.5 N·m TIM: Silicone
insulator pad (0.5 °C/W) Electrical: 2.5kV isolation rating ──
STATUS: PASS (Forced Convection) ──
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