When designing circuits with MOSFETs, you might notice your devices struggling with a sudden power-on surge. This issue is called “inrush current,” which can cause unwanted voltage spikes and potentially damage your components. But don’t worry! A MOSFET soft-start circuit can help smooth out this problem.

The Problem :

A MOSFET soft-start circuit gradually ramps up the current supplied to your load when the device is powered on. By doing so, it reduces the shock of high inrush current, protecting your circuit from damage and ensuring a more stable performance.

Example :

Think of turning on a heavy-duty appliance like a fan. If you were to flick the switch on instantly, the motor could experience a large, sudden current surge that stresses the internal components. A soft-start circuit prevents this by gently increasing the power over a set period.

Sample Calculation :

Suppose you’re using a resistor of 1kΩ in series with a 10µF capacitor for your soft-start circuit.

The time constant (τ) is calculated as:

τ = R × C = 1000Ω × 10µF = 10 milliseconds
This means the capacitor will take about 10 milliseconds to charge, ensuring a gradual rise in current.

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