Designing a Compact 3.3 kW On-Board Charger Using SiC MOSFETs for a Two-Wheeler EV Platform
How Edrift Electric engineered a high-efficiency, thermally optimized OBC with 40% reduced form factor for integration into a production 2W EV platform.
01. Client Overview
Strategic Partnership & Objectives
A fast-growing Indian electric two-wheeler manufacturer approached Edrift Electric with a clear requirement: a compact, efficient, and cost-optimized 3.3 kW on-board charger that could integrate directly into their next-generation 2W EV platform without requiring an external cooling system.
02. Engineering Challenge
Overcoming Technical Limitations
The client's existing charger design — based on a conventional IGBT topology — was occupying a volume of 2.1 litres within the vehicle chassis. This was creating packaging conflicts with the battery management system and motor controller, forcing the mechanical team to make uncomfortable structural compromises.
Critical Bottlenecks Identified:
03. Technical Constraints
Hard Design Constraints
Electrical Requirements
Mechanical & Environmental
04. Design Approach
Multi-Stage Topology Optimization
Edrift's engineering team structured the design into three primary power conversion stages, each selected and optimized specifically to meet the thermal and volumetric constraints.
Stage 1 — Totem-Pole PFC
The first stage uses a bridgeless totem-pole PFC topology implemented with GaN HEMTs. This eliminates the conventional diode bridge rectifier — the single largest source of conduction loss. Operating at 140 kHz, it achieves a power factor of 0.99 and THD of 3.2%.
Stage 2 — LLC Resonant DC-DC
The isolation stage uses an LLC resonant converter with SiC MOSFETs. ZVS on primary MOSFETs eliminates switching losses across the full load range. Operating frequency: 120 kHz – 200 kHz, enabling significant reduction in transformer core volume.
Magnetics Design
Custom-designed planar E-core topology in N87 ferrite. Planar construction achieved winding height < 8mm and improved thermal coupling. Resonant inductor integrated as a controlled leakage element.
Gate Driver Design
Active Miller clamp circuit and independent gate resistors for turn-on (10Ω) and turn-off (2.2Ω). Isolated power supply per switch: 3kV isolation rating.
05. Semiconductor Selection
Power Switch Matrix
The selection of switching devices was the most critical design decision — directly determining efficiency, switching frequency, and thermal performance.
| Parameter | Si IGBT | Si MOSFET | SiC MOSFET | GaN HEMT |
|---|---|---|---|---|
| Vds rating | 600V | 600V | 650V | 650V |
| Rdson (typ) | — | 85 mΩ | 28 mΩ | 18 mΩ |
| Max freq. | 65 kHz | 100 kHz | 300 kHz | 500 kHz |
| Switching loss | High | Medium | Low | V. Low |
| Thermal Rth | 0.42 | 0.38 | 0.29 | 0.24 |
Selection Rationale:
- SiC MOSFETs enabled low conduction loss at 58A output.
- GaN HEMTs minimized conduction loss in PFC switches with zero reverse recovery.
- Automotive-grade qualification matched OEM requirements.
Final Device BOM:
06. Efficiency Performance
Measured System Efficiency
Efficiency was measured across the full load sweep from 10% to 100% rated power.
| Load % | Baseline | Edrift Design |
|---|---|---|
| 10% | 83.4% | 88.2% |
| 25% | 88.8% | 92.9% |
| 50% | 91.3% | 94.1% |
| 100% | 91.2% | 94.8% |
Impact Summary
07. Thermal Analysis
Heat Management & Junction Temperatures
Passive cooling demanded exceptional thermal management from the PCB layout upward.
The chassis doubles as the heatsink. Aluminium extrusion profile forms the base plate with SiC MOSFETs mounted directly using 0.1mm TIM.
| Device | Baseline | Design | Delta |
|---|---|---|---|
| PFC Switch | 102°C | 64°C | −38°C |
| LLC MOSFET | 85°C | 52°C | −33°C |
| LLC Diode | 91°C | 48°C | −43°C |
Thermal Success Criteria Met
All semiconductor junctions operate below 65°C even at maximum ambient temperature, providing 110°C margin.
08. Power Density & Form Factor
Volumetric Comparison
Significant volume and weight reduction achieved through high-frequency operation and planar magnetics.
Volume Recovery Vectors
- 1Planar transformer: 61% lower height
- 2Higher switching frequency: 65kHz → 140kHz
- 3Elimination of diode bridge: -18cc volume
- 4Integrated resonant inductor: -24cc
Mechanical Outcome:
Resolved chassis packaging conflict and eliminated active cooling, saving 380g weight.
09. Testing & Validation
Rigorous Automotive-Grade Verification
Comprehensive electrical and environmental testing matching Indian and international standards.
10. Deployment Outcome
Field Results & Scaling
The SiC OBC entered pilot production in Q3 2024 with 120 units deployed across 3 Indian cities.
Ready to Optimize Your Power System?
Our engineering team works with Tier-1 OEMs to deliver automotive-grade SiC and GaN solutions that scale.