MOSFET Thermal Impedance Design – Keep It Cool & Efficient!
🧯 The Problem
Many hobbyists and prototypers in India overlook thermal impedance when using MOSFETs. When a MOSFET handles high current, it heats up. If that heat isn’t managed, it can exceed the safe junction temperature and fail prematurely. The key to safe operation is understanding and managing thermal impedance from junction to ambient.
🛠️ The Solution
To avoid overheating, use heat sinks, copper pours on PCBs, or thermal pads to reduce RθJA (thermal resistance junction-to-ambient). Choose MOSFETs with low RDS(on) and calculate thermal rise based on power dissipation.
🧪 Practical Example
Imagine your MOSFET switches 5A at 12V and has an RDS(on) of 50mΩ.
Power dissipation = I² × RDS(on) = 5² × 0.05 = 1.25W.
If the thermal impedance is 40°C/W:
Temperature rise = 1.25W × 40°C/W = 50°C.
If ambient is 35°C, junction temp hits 85°C—safe! But if you skip cooling, temps can spike to 120°C+!
📐 Sample Calculation
Say you upgrade to a MOSFET with RDS(on) = 20mΩ:
Power = 5² × 0.02 = 0.5W,
Temp rise = 0.5W × 40 = 20°C → Much cooler!
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