Three-terminal regulators like the LM7805 are simple but inefficient—their power loss turns into heat, making thermal performance critical for reliability. Since these devices lack internal thermal optimization, their package and PCB layout dictate how well they stay cool.
1. Key Metric: Thermal Resistance (θJA)
Thermal resistance (°C/W) measures how much the junction temperature rises per watt of dissipation. Key parameters:
θJC: Junction-to-case—how well heat moves from die to package surface;
θJA: Junction-to-ambient—the most practical value, including package, PCB, and airflow.
For example, a TO-220 might have θJA ≈ 50°C/W (drops to 10–20°C/W with a heatsink), while an SOT-223 on a minimal PCB can be 60–80°C/W. Lower = better cooling.
2. Calculate Junction Temperature
First, compute power dissipation: P = (VIN – VOUT) × IOUT
Then estimate junction temp: TJ = TA + P × θJA
If TJ exceeds the max rating (usually 125°C or 150°C), thermal design must improve.
3. PCB Layout Dramatically Affects θJA
Surface-mount packages (SOT-223, TO-263) rely entirely on the board for cooling. Crucially:
Datasheets specify θJA under standard copper area (e.g., “1 in²”);
Smaller copper = higher θJA;
Use 4–9 thermal vias under the exposed pad to connect to inner/ground planes—this can cut thermal resistance by 30%+.
4. Heatsinks & Forced Air Boost Performance
For TO-220, adding a heatsink is highly effective. Choose one with low thermal resistance (e.g., 5°C/W) and use thermal grease to minimize contact resistance. In enclosed systems, a small fan provides forced convection for extra margin.
5. Real-World Testing Is Essential
Calculations are estimates. Validate with infrared thermography or a thermocouple on the case. Estimate TJ as: TCASE + P × θJC. Test under worst-case conditions (high ambient + full load).
In Summary:
Evaluating thermal performance isn’t about the package alone—it’s a system-level task involving thermal specs, PCB copper, and real-world validation. Design the heat path wisely, and your three-terminal regulator will run reliably for years.
