Parallel MOSFET Design: Practical Solutions for Current Sharing & Thermal Management

1. Why Parallel MOSFETs in High-Power Designs?

Key Advantages:

• Linear Current Scaling: N parallel MOSFETs can theoretically handle N× rated current (e.g., 4× 25A MOSFETs for 100A applications)
• Thermal Load Distribution: Spread power dissipation to avoid single-device overheating
• Design Flexibility: Use standard components instead of rare high-current MOSFETs

Typical Applications:

• Industrial motor control (VFDs)
• Solar inverter power stages
• High-power DC-DC converters

2. Core Design Principles: Get the Basics Right

Circuit Topology:

1. Gate (G) Parallelization: Connect all gates to a common driver with low output impedance (e.g., TI’s UCC27534)
2. Source (S) & Drain (D) Connections: Short, symmetrical traces to minimize inductance differences
3. Layout Golden Rule: Keep gate traces <2cm with matched length/width (±5%)

Drive Circuit Essentials:

• Gate Resistors: 5-10Ω per MOSFET to control di/dt and dampen oscillations
• Driver Selection: High-current output (2A+ peak) to ensure sub-10ns switching sync

3. Critical Challenges & Engineering Solutions

️ Current Sharing Imbalance: The #1 Failing Point

Root Causes:

• Rds(on) Variability: Batch-to-batch differences up to 15% (e.g., 10mΩ vs 11.5mΩ)
• Layout Asymmetry: Uneven trace inductance causes transient current spikes

Mitigation Strategies:

1. Device Screening: Use a TI DMM6500 to measure Rds(on) at 25°C/100°C; match within 5%
2. Star Topology Layout: Route drain traces from a common input capacitor point (see Figure 1 in Appendix)
3. Source Balancing Resistors: Add 20mΩ/1% resistors (e.g., Ohmite MPR-10) for dynamic current sharing

️ Thermal Management: Design for Heat from Day 1

Passive Cooling Techniques:

• Heat Sink Calculation: Use θJA = (Tj max - Ta)/Pd; select sinks with <1.5°C/W for 50W dissipation
• Thermal Interface Materials: Bergquist Gap Pad 1000 with <0.25°C·cm²/W resistance
• PCB Optimization: 2oz copper pours with 8+ thermal vias (0.8mm) per MOSFET

Active Cooling for Demanding Applications:

• Forced Airflow: 80mm fan producing 1.2CFM over heat sinks
• Liquid Cooling: Cold plates with <0.1°C/W thermal resistance for EV drivetrains

4. Practical Design Example: TI-Based 3-MOSFET 75A Stage

Bill of Materials (BOM):

Layout Checklist:

1. Gate traces: 50mil width, 1.8cm length, routed over ground plane
2. Drain connections: 120mil copper pours with star topology from input cap
3. Thermal vias: 10x 0.8mm vias under each MOSFET pad

Testing Protocol:

1. Use a TI DPO4054B to measure Vgs skew (<12ns)
2. Clamp meter (Fluke 376) to verify <8% current imbalance at full load
3. IR thermography to confirm Tj <110°C under continuous operation

5. TI Component Selection Guide

MOSFET Key Specs:

• Rds(on) Temperature Coefficient: Positive TC for self-balancing (e.g., CSD18532KCS has +0.8%/°C)
• Gate Charge (Qg): <45nC for 20kHz+ switching (e.g., CSD19536KCS: Qg=38nC)
• Package Thermal Performance: D2PAK-7 (TO-263-7) offers 25% better θJA than TO-220

6. e2e Community FAQs & Troubleshooting

7. Designer’s Checklist for Success

 [ ] Screen MOSFETs for Rds(on) matching at operating temperature
 [ ] Implement star-topology layout for drain/source traces
 [ ] Include gate resistors and driver snubber networks
 [ ] Model thermal performance with TINA-TI or PSpice
 [ ] Test current balance under 25°C/100°C ambient conditions

8. e2e Exclusive: TI Reference Design Link