Massive Inrush Current? Fix It With One MOSFET

Massive inrush current can destroy power supplies, stress MOSFETs, and cause unpredictable startup behavior in real circuits. In this video, I show how to reduce large startup surges using a simple MOSFET soft-start circuit. Using LTspice simulations, we walk through a real design scenario with large bulk capacitance (10,000 µF) and explore why this creates huge inrush currents at power-up. Then we design a practical PMOS soft-start solution that dramatically reduces startup current while keeping MOSFET stress within the safe operating area (SOA). Topics covered in this video: • Why large capacitors create dangerous inrush current • How to simulate motor inrush in LTspice • Designing a MOSFET soft-start circuit using an RC gate ramp • How resistor and capacitor values affect startup current • Checking MOSFET Safe Operating Area (SOA) correctly • Why small MOSFET packages often fail in soft-start designs • When to choose larger packages like DPAK / D2PAK • Reverse polarity protection using a second MOSFET • TVS diode protection for power connectors • Why some “improved” soft-start circuits can actually create new problems This approach works well for systems with large capacitive loads such as: • Motor controllers • H-bridge drivers • Power supplies with large bulk capacitors • Embedded systems with high startup current If you're designing power electronics, motor drivers, or embedded hardware, controlling inrush current is critical to building reliable systems. #electronics #powerelectronics #mosfet #pcbdesign #embeddedengineering 00:00 The inrush current problem 01:10 Why large capacitors cause huge startup current 02:45 Simulating motor inrush in LTspice 04:20 Designing the MOSFET soft-start circuit 07:10 Choosing resistor and capacitor values 10:05 Checking MOSFET Safe Operating Area (SOA) 12:30 Why small MOSFET packages fail 14:00 Adding reverse polarity protection 15:30 TVS protection and final design 17:00 A dangerous soft-start variation 18:30 Final thoughts