Calculate P = (Vin - Vout) × I and Estimate Junction Temperature
Regulator Presets
Vin (V)
Vout (V)
Load I (A)
Thermal Parameters (optional)
RθJC (°C/W)
RθCS (°C/W)
RθSA (°C/W)
Ta (°C)
Result
Power Dissipation
Efficiency
Junction Temp Tj
Thermal R total
Step-by-Step Calculation
Regulator Power Formulas
P_diss = (Vin - Vout) × I_load
Efficiency η = Vout / Vin × 100%
Tj = Ta + P_diss × (RθJC + RθCS + RθSA)
P_out = Vout × I_load
Linear regulators dissipate excess voltage as heat. The power dissipated is simply the voltage drop across the regulator times the load current. This makes them inefficient when the input voltage is much higher than the output. The wasted power (heat) must be removed to keep the regulator within its maximum junction temperature. Proper heatsinking is essential for reliable operation.
⚠Maximum junction temperature for most regulators is 125-150°C. Always derate (use <80% of max). Example: 7805 max Tj=125°C. For 7W dissipation with RθJA=65°C/W (no heatsink): Tj=25+7×65=480°C → immediate failure! Heatsink is mandatory.
Linear Regulator Thermal Design
Thermal management is critical for linear regulators. The total thermal resistance from junction to ambient includes: junction-to-case (RθJC, internal), case-to-sink (RθCS, thermal interface material), and sink-to-ambient (RθSA, heatsink). The junction temperature must stay below the maximum rating. If Tj exceeds the limit, use a larger heatsink, reduce input voltage, or switch to a switching regulator.
Linear: η=Vout/Vin. 5V from 12V = 41.7%. Switching: η=80-95% regardless of ΔV. Buck converter: 90% from 12V to 5V.
Thermal Pad Types
Thermal grease: RθCS≈0.5-1°C/W. Silicone pad: 1-2°C/W. Mica + grease: 0.5-0.8°C/W. No insulator: 0.2-0.5°C/W but electrically connected.
Teaching Example: 7805: 12V→5V @ 1A, TO-220 with heatsink.
P = (12-5)×1 = 7W. Efficiency = 5/12 = 41.7%.
RθJC=5, RθCS=1 (grease), RθSA=10 (heatsink) → Total R=16°C/W.
Tj = 25 + 7×16 = 137°C. Max Tj=125°C → OVERHEAT! Need better heatsink (RθSA<7.1°C/W).
Applications
Power Supply DesignAudio CircuitsSensor PowerMicrocontrollerRF/Analog
Frequently Asked Questions
How to calculate regulator power?▼
P = (Vin-Vout)×I. 12V→5V@1A = 7W dissipated as heat. Efficiency = Vout/Vin = 41.7%. For lower dissipation, reduce ΔV or use switching.
What happens if regulator overheats?▼
Most regulators have thermal shutdown (~160°C). It cycles on/off (pulsing output). Continuous overheating damages the regulator. Always heatsink properly.
Do I need a heatsink?▼
Rule: if P > 1W, use a heatsink. TO-220 without heatsink: RθJA≈65°C/W. 12V→5V @ 100mA = 0.7W → Tj=25+45.5=70.5°C (OK without).
Linear vs switching?▼
Linear: simpler, lower noise, inefficient for high ΔV. Switching: efficient, complex, more noise. Use linear for ΔV<3V or I<1A. Use switching for >3VΔ or >1A.
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