Compact EV On-Board Charger in Urban Fleet Vehicle

Case Study 2: Compact EV On-Board Charger in Urban Fleet Vehicle

Scenario A municipal electric bus fleet operator upgraded onboard chargers to support 6.6 kW AC charging (dual-phase, SiC MOSFET-based). The charger is mounted in a tightly packed rear electronics bay with limited airflow (< 1 m/s natural convection), high vibration (ISO 10326-1 Class 2), and frequent thermal cycling (−20°C to +70°C ambient). Weight and volume are constrained: total heatsink mass < 1.2 kg; envelope ≤ 180 mm × 120 mm × 45 mm. Reliability target: > 15-year service life with no thermal derating.

Given Data

• Power Dissipation: 82 W (losses across PFC + LLC stages, measured at 94% efficiency)
• Maximum Junction Temperature: 125 °C
• Ambient Temperature: 65 °C (worst-case cabin bay temp during summer idling in traffic)
• Case-to-Heat Sink Thermal Resistance: 0.85 °C/W (low-compliance silicone pad, 0.5 mm thickness, 3.2 W/m·K)
• Spreading Resistance: 0.55 °C/W (due to asymmetric 18 mm × 12 mm SiC die layout on 1.6 mm thick AlSiC substrate)

Calculation

R_total = (125 − 65) / 82 = 60 / 82 = 0.7317 °C/W
R_heat_sink_required = 0.7317 − 0.85 − 0.55 = −0.6683 °C/W

Again, negative — but here, the issue is TIM selection, not power. Engineers switched to phase-change TIM (R_case_to_sink = 0.35 °C/W) and optimized mounting pressure (150 kPa) to reduce interfacial resistance. Spreading resistance improved via localized copper slug under die (R_spreading = 0.28 °C/W). Updated calculation:

R_heat_sink_required = 0.7317 − 0.35 − 0.28 = 0.1017 °C/W

Tool output:

• Required Heat Sink Thermal Resistance: 0.10 °C/W
• Recommended Heat Sink Size: 2,156,000 mm³ — exceeds volume constraint

Engineers then applied the tool’s size guidance iteratively: reducing target R_hs to 0.12 °C/W (accepting 1.2°C higher junction temp margin) yielded 1,680,000 mm³ — still too large. Final trade: used vapor chamber base (R_spreading = 0.12 °C/W) and high-emissivity black anodized fin array (ε = 0.82). With R_case_to_sink = 0.28 °C/W:

R_heat_sink_required = 0.7317 − 0.28 − 0.12 = 0.3317 °C/W

Tool output:

• Required Heat Sink Thermal Resistance: 0.33 °C/W
• Recommended Heat Sink Size: 642,300 mm³ (642 cm³ — fits 180 × 120 × 45 mm envelope with 1.5 mm fin thickness, 12 mm pitch, 32 mm height)

Result and Decision A vapor-chamber-integrated aluminum heat sink (642 cm³, 0.98 kg) was fabricated and validated. Thermocouple monitoring over 3-month field trial showed max junction temp = 122.4°C at 65°C ambient — meeting lifetime reliability targets. No thermal throttling observed across 12,000+ charge cycles.

Lesson Spreading resistance dominates in compact, high-power-density designs — investing in advanced baseplate solutions (e.g., vapor chambers or embedded heat pipes) often yields greater ROI than simply scaling fin volume. Always cross-check tool-recommended size against mechanical envelope before finalizing TIM and mounting strategy.