Thermal Management in EV Chargers: From Junction to Ambient

Understanding the Thermal Chain

Heat generated in a semiconductor device must travel from the junction to the ambient environment. Every interface has a thermal resistance.

$T_j = T_a + P imes (R_{ heta jc} + R_{ heta cs} + R_{ heta sa})$

Where:

Tj: Junction temperature

Ta: Ambient temperature

P: Power dissipated

Rθjc: Junction-to-case resistance

Rθcs: Case-to-heatsink resistance

Rθsa: Heatsink-to-ambient resistance

Calculating Power Dissipation

MOSFET Losses

$P_{total} = P_{cond} + P_{sw} = (I_{rms}^2 imes R_{dson(Tj)}) + ((E_{on} + E_{off}) imes f_{sw})$

Critical: Always use Rdson at operating junction temperature (e.g., 150°C), not the 25°C datasheet value. SiC Rdson can increase 2–3× over this range.

Thermal Interface Materials (TIM)

Heatsink Sizing

For passive cooling (natural convection), the heatsink must handle the total system loss (e.g., 151W for a 3.3 kW OBC). This often requires a machined aluminium cold plate or a large extruded fin structure integrated into the enclosure.

Key Takeaways

Calculate dissipation for EVERY component.

Use operating Tj for calculations.

Target $T_j < 125^circ C$ at $60^circ C$ ambient for reliability in harsh climates.

Validate with thermal imaging of the first prototype.