Please enter all required values.
RESULTS
Input Parameters Specification
FET Gate CapacitanceMOSFET gate capacitance in pF used with the soft-start capacitor path for inrush current limiting.
Optional Capacitance C1External capacitance in µF added to the gate timing network to slow load switch turn-on.
Voltage Dip DurationUn-mitigated dip duration in microseconds before adding load switch inrush control.
Voltage Dip MagnitudeOriginal and desired maximum voltage dip values used to stretch the soft-start time constant.
Practical Operational Examples
Load Switch Soft Start
Use this calculator when a MOSFET load switch causes input rail voltage dip due to capacitor charging current.
Gate Resistor Selection
The calculated R3 gate resistance helps slow MOSFET turn-on and reduce inrush current in power distribution circuits.
Hot-Plug Rail Protection
Useful when USB rails, battery packs, DC adapters or module supplies dip because a downstream capacitor charges too quickly.
Soft-Start Tuning
Increase C1 or Rgate to extend the time constant and reduce sudden startup current through the MOSFET switch.
Diagrams & Theory
This load switch soft-start circuit uses a proper MOSFET symbol, red gate resistors and green timing capacitor to slow startup and reduce inrush current.
Formulas & Mathematical Logic
Step 1: Convert Cgate from pF to F: Cgate(F) = Cgate × 1e-12.
Step 2: Convert C1 from µF to F: C1(F) = C1 × 1e-6.
Step 3: Convert Tdip from µs to seconds: Tdip(s) = Tdip × 1e-6.
Step 4: Total capacitance: Cpar = C1 + Cgate.
Step 5: Stretch factor: stretch = Vdip / Vdipmax.
Step 6: Time constant: TC = stretch × Tdip.
Step 7: Gate resistance: Rgate = TC / Cpar.
Step-by-Step Example
Example: Cgate = 2000 pF, C1 = 0.1 µF, Tdip = 50 µs, Vdip = 1 V, desired Vdipmax = 0.25 V.
Step 1: Convert Cgate to farads: 2000 pF = 2e-9 F.
Step 2: Convert C1 to farads: 0.1 µF = 1e-7 F.
Step 3: Total capacitance Cpar = 1e-7 + 2e-9 = 1.02e-7 F.
Step 4: Stretch factor = 1 / 0.25 = 4.
Step 5: TC = 4 × 50 µs = 200 µs = 0.0002 s.
Step 6: Rgate = 0.0002 / 1.02e-7 = about 1961 Ω.
How to Use This Calculator
Enter the MOSFET gate capacitance in pF from the datasheet.
Enter optional C1 soft-start capacitance in µF. Use 0 if no external capacitor is added.
Enter the un-mitigated voltage dip duration in microseconds.
Enter original voltage dip magnitude and desired maximum voltage dip.
Click Calculate to get R3 gate resistance and the soft-start time constant.
About This Calculator
Design a MOSFET load switch soft-start network to reduce inrush current.
The CalcBoy Inrush Current Limiting Load Switch Calculator estimates gate resistance and time constant from gate capacitance, optional C1, voltage dip duration and desired dip reduction.
When a load switch turns on quickly, downstream capacitors can draw a large inrush current. This sudden current can pull down the input rail, reset microcontrollers, trip protection circuits, stress connectors or create EMI problems. A MOSFET soft-start network slows the gate transition so the load capacitor charges more gradually.
This calculator is useful for USB power switches, battery-powered circuits, hot-plug modules, sensor boards, DC distribution rails, MOSFET high-side switches and power sequencing circuits. By entering the un-mitigated voltage dip and the desired maximum dip, it estimates how much the turn-on event needs to be stretched. Then it calculates the gate resistance from the total timing capacitance.
Use this result as a starting point. Real behavior depends on MOSFET threshold voltage, gate charge, load capacitance, source impedance, PCB layout, output capacitor ESR, current limit behavior and power dissipation during startup. Always verify the final circuit with an oscilloscope and check MOSFET safe operating area during turn-on.
Best UseMOSFET load switch soft-start gate resistor estimates.
Supported InputsGate capacitance, C1, voltage dip duration and voltage dip target.
Helpful ForUSB rails, DC modules, battery boards, hot-plug circuits and power sequencing.
Design ReminderConfirm startup waveform and MOSFET heating on real hardware.
Tip: If the MOSFET gets hot during startup, the soft-start is too slow or the load capacitance is too large. Check safe operating area, not only resistance value.
Frequently Asked Questions
What is inrush current?
Inrush current is the high startup current that flows when capacitors, motors or loads are first connected to a power rail.
How does a gate resistor reduce inrush current?
A gate resistor slows MOSFET turn-on, which limits how quickly the load capacitor charges and reduces voltage dip.
What is C1 used for?
C1 adds extra timing capacitance to the MOSFET gate network, increasing the soft-start time constant.
What if I do not use C1?
Enter 0 for C1. The calculator then uses only MOSFET gate capacitance for the timing calculation.
Can this replace oscilloscope testing?
No. It gives a design estimate. Final voltage dip, gate ramp and MOSFET stress should be measured on hardware.
Why is MOSFET safe operating area important?
During soft-start, the MOSFET may operate in its linear region and dissipate heat, so safe operating area must be checked.
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