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Total Power Loss
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Conduction Loss
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Switching Loss
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Gate Charge Loss
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Required Heatsink Thermal Resistance
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Junction Temperature
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MOSFET power dissipation is the electrical power converted to heat during operation. It includes conduction losses when the MOSFET is ON, switching losses during transitions, and gate charge losses. Understanding power dissipation is crucial for thermal management and preventing device failure.
Conduction losses are calculated using P_cond = I²DS × RDS(on) × D, where IDS is the drain current, RDS(on) is the on-state resistance, and D is the duty cycle. This represents power lost due to resistance when current flows through the MOSFET.
Switching losses occur during the transition between ON and OFF states. They're calculated as P_sw = 0.5 × VIN × IDS × (tr + tf) × fsw, where tr and tf are rise and fall times. Higher switching frequencies increase these losses significantly.
Calculate heatsink thermal resistance when the total power loss could cause the junction temperature to exceed its maximum rating. The heatsink helps dissipate heat to keep the MOSFET within safe operating temperatures.
Higher switching frequencies increase both switching losses and gate charge losses proportionally, as both occur once per switching cycle. Conduction losses remain unchanged. This is why high-frequency applications require careful thermal design.
RDS(on) is the drain-to-source resistance when the MOSFET is fully turned ON. Lower RDS(on) values reduce conduction losses, making the MOSFET more efficient. This parameter varies with temperature and gate voltage.
Maximum junction temperature is specified in the MOSFET datasheet, typically 150°C to 175°C. Operating below this temperature ensures reliability and prevents thermal runaway. Always include safety margins in your thermal design.
Gate charge losses depend on gate charge (QG), gate drive voltage (VGS), and switching frequency. The formula is P_gate = QG × VGS × fsw. Using lower gate drive voltages can reduce these losses but may affect switching performance.