Power Dissipation in Electronics: From Calculation to PCB

Power lost in a component appears mostly as heat. Estimating that loss and the path from the component to ambient helps identify where thermal validation is needed.

The Fundamentals

Power dissipation in a resistive element is defined by:

P = I² × R  =  V² / R  =  V × I

For a 10 V to 5 V linear regulator supplying 1 A, the IC dissipates about 5 W. A buck converter delivering the same 5 W output at an assumed 90% efficiency would dissipate about 0.56 W. Actual efficiency depends on the device and operating point.

Where Power Actually Goes

In a real system, losses appear across multiple domains:

Domain	Primary Loss Mechanism
Linear regulators	Voltage drop × current
MOSFETs	I²×Rds(on) conduction + switching losses
Inductors	I²×DCR + core losses
PCB traces	I²×R_trace
Connectors	Contact resistance × I²

Trace Resistance

Using bulk copper resistivity as a first estimate, a 10 mm trace that is 0.3 mm wide and 35 µm thick has:

R = ρ × L / A
R = (1.72×10⁻⁸) × 0.01 / (0.3×10⁻³ × 35×10⁻⁶)
R ≈ 16.4 mΩ

At 3 A, that is about 147 mW. Actual resistance also depends on copper thickness, temperature, and geometry.

Thermal Resistance Chain

With a heatsink, a simplified steady-state thermal path is:

T_junction = P_dissipated × (θ_jc + θ_cs + θ_sa) + T_ambient

Where:

θ_jc — Junction-to-case (from datasheet)
θ_cs — Case-to-heatsink (depends on TIM/pad)
θ_sa — Heatsink-to-ambient (heatsink selection)

Without a heatsink, use the package’s junction-to-ambient estimate instead of θ_sa. For example, 8 W with θ_ja = 60°C/W would imply a 480°C temperature rise in this simple model, far beyond a practical operating limit. With a heatsink and illustrative values θ_jc = 2°C/W, θ_cs = 0.5°C/W, and θ_sa = 5°C/W, the same 8 W at 40°C ambient gives an estimated junction temperature of 100°C. These values must come from the chosen device, mounting method, heatsink, and airflow. See TI’s thermal model guide for the distinction between θ_ja and a heatsink path.

My Verification Workflow

Schematic phase: Annotate every power path with expected I, V, and P
Layout phase: Check resistance and temperature rise on relevant power traces
Thermal simulation: Build a thermal model in MATLAB or LTspice
Lab validation: Thermistor + thermal camera after 30-min steady state soak

Estimate the losses, measure temperatures under representative loads, and update the model where measurements differ.