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Limiting In-rush Current in MOSFET Power Switches
Control the voltage slew rate of a MOSFET high side switch to limit in-rush current into downstream capacitors.

Here are some electronic soft-start power switch circuits for controlling the slew rate of a MOSFET switch. Why is that important? Rapidly turning ON power to some devices can compromise their lifetime and reliability owing to large in-rush currents into their input capacitors. In particular, some devices place large tantalum capacitors on their inputs to filter ripple from the input voltage, or to provide switching currents to downstream switch mode power supplies. Suddenly providing voltage to those tantalum capacitors can result in in-rush currents of hundreds or thousands of amperes for a short time, greatly exceeding their in-rush or surge current capability, and sometimes even resulting in a small explosion! Tantalum capacitors are particularly prone to fireworks. A slew-rate-limited soft-start switch solves this problem.

The in-rush load current problem is solved by using a MOSFET power switch circuit that slew-rate limits its output. Instead of allowing the switch's output voltage to bounce up to the full supply voltage almost instantaneously, if its rate of increase is limited the maximum current into any downstream capacitors is also limited. In fact, the current is proportional to the rate of change (slew rate) of the switched power rail voltage, as,

I = C dV/dt

where C is the downstream capacitance and V is the switched voltage.

The MOSFET based soft start switch circuit of Figure 1 modifies a simpler latching switch circuit with the addition of a diode, resistor and capacitor to slew-rate limit the voltage rise at its output, the MOSFET's drain, resulting in surge-current-limited power switching:

Circuit schematic of an in-rush current limited ON/OFF toggle MOSFET high-side switch
Fig. 1  Inrush current limiter circuit schematic. A latching push button ON/OFF power switch uses a MOSFET high side switch with slew rate limited output voltage to prevent excessive capacitor in-rush current.

The jumper can be permanently wired or set to control the default behavior of the circuit, whether it automatically turns ON (when the 0.1 μF capacitor is connected to the input power) or stays OFF (when the 0.1 μF capacitor is connected to ground), when power is first applied.

The circuit conveniently uses two MOSFETs packaged together, the IRF7319.

The addition of the 100 KΩ resistor and 0.01μF capacitor at the gate of the high-side MOSFET converts it into an integrating amplifier whose output rises linearly in response to a step change in its input, limiting the in-rush current to downstream capacitors. The values shown limit the slew rate of the voltage rise at the output to about 2V/millisecond. For a downstream capacitance of 1000μF, by Eqn. 1 the in-rush current is limited to 2A. Once the switch is fully ON, it is no longer current limited, and supplies the full load current.

You can understand the circuit by considering the following equivalent circuit:

Equivalent circuit models the high-side MOSFET switch as an integrating amplifier
Fig. 2  An equivalent circuit models the high-side MOSFET switch as an integrating amplifier.

In the above soft start circuit, in the OFF state the switch is in the V+ position and the output is low (the MOSFET is OFF). Turning on the MOSFET is initiated by switching the switch to the ground position. Initially the MOSFET remains OFF as the feedback capacitor, Cf, discharges through the input resistor, Rgate, towards ground. The dead time continues until the voltage at the negative input of the amplifier (the MOSFET gate) reaches the gate's threshold voltage, shown as VGS in the schematic. At that point the MOSFET starts turning ON, entering its region of linear operation. It's operation holds the voltage at the negative input nearly constant1) as the output voltage rises linearly. The rate of output voltage rise is,

dV/dt = VGS/(RgateCf)

The voltage continues its linear rise until the MOSFET is fully turned on, and the output voltage saturates at the input voltage value. Then, the gate voltage continues its drop toward ground.

The output current into the capacitive load is limited, and equal to,

I = Cload dV/dt = (Cload/Cf)/(VGS/Rgate)

Knowing the load capacitance, you can choose Rgate and Cf to limit the in-rush current to any value desired.

The additional diode in the circuit assures that despite the modifications to the gate circuitry the circuit remains OFF when power is first applied, if the Auto-ON jumper is configured for that. The Auto-ON jumper connects the capacitor to ground to automatically turn ON the switch on power-up; if the jumper connects the capacitor to the input voltage the switch is held OFF on power-up. If you permanently wire the circuit for automatic turn on when power is applied, the gate diode is not needed.

To learn more about in-rush current protection, read this excellent Motorola application note:

You can apply the principles of this soft-start circuit to many other dual MOSFETs, including HEXFET, OptiMOS, PowerTrench, SIPMOS, STMOS, STripFET, and TrenchFET, from manufacturers Alpha & Omega Semiconductor Inc, Diodes Inc, Fairchild Semiconductor, Infineon Technologies, International Rectifier, NXP Semiconductors, ON Semiconductor, Renesas Electronics America, Rohm Semiconductor, STMicroelectronics, Toshiba, and Vishay Siliconix. For example, any of these N-channel / P-Channel dual-MOSFETs should work:

AO4611 AO4612 AO4613 AO4614 AO4616 AO4618 AO4619 AO4620 AO4627 AO4629 AO6601 AO6602 AOD603 AOD607 AOD609 AON3611 AON3613 AON4605 AON7611 AUIRF7343 BSL308 DMC3018 DMC3021 DMC3025 DMC3028 DMC3035 DMC3036 DMC4028 DMC4040 DMC4050 DMG4511 DMG6601 DMG6602 FDC6333 FDC6432 FDD8424 FDD8426 FDS4501 FDS4559 FDS4897 FDS8333 FDS8858 FDS8928 FDS8958 FDS8960 FDS8962 IRF7309 IRF7319 IRF7343 IRF7379 IRF7389 MMDF2C0 MMDF2C03 MP6M11 MP6M12 NDS8852 NDS8858 NDS8958 NDS9952 NTGD4167 NTMC1300R2 QS8M12 QS8M13 SH8M2 SH8M24 SH8M24TB1 SH8M2TB1 SH8M3 SH8M3TB1 SH8M4TB1 SH8M5 SH8M5TB1 SI3590 SI4501 SI4505 SI4532 SI4539 SI4542 SI4559 SI4561 SI4563 SI4564 SI4565 SI4567 SI4569 SI4599 SI5504BD SI5504D SI5511D SI6544BD SI7501DN SI7530DP SP8M10 SP8M10 SP8M10FU6 SP8M2 SP8M2FU6 SP8M3 SP8M3 SP8M3FU6 SP8M4 SP8M4FU6 SP8M5 SP8M5 SP8M5FU6 SP8M6 SP8M6 SP8M6FU6 SP8M7 SP8M7 SP8M7FU6 SP8M8 SP8M8 SP8M8FU6 SP8M9 SP8M9 SP8M9FU6 STS4C3F60L STS7C4F30L STS8C5H30L TPC8403 TPC8405 TPC8406 TPCF8402 TPCP8402 TT8M2 uPA2590T1H UPA2790GR UPA2791GR VEC2616 ZXMC3A16 ZXMC3A16DN8 ZXMC3A17DN8 ZXMC3A18DN8 ZXMC4559DN8 ZXMC4A16DN8

Schematic files: TINA-TI Schematic file

Just how "constant" depends on the MOSFET's transconductance
This page is about: In-rush Current Limited High Side MOSFET Switch, Soft-start Power Bus Switch Circuit – Use a slew-rate limited MOSFET high-side switch as an inrush current limiter to limit the maximum current into downstream capacitors. This soft-start circuit prevents excessive surge currents in power supply capacitors or wherever you need a slew rate limited load switch. MOSFET power switch, limit input surge current, limiting inrush current