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I want to add an NMOS just before the supply negative side and thereby controlling its slew rate so as to create a soft start.

I've seen many examples but mostly with high side PMOS (high Rds on) or NMOS (additional circuitry to drive in high side). I doubt there must be some reason.

Can we use a low side N-channel MOSFET in a DC-DC converter for limiting inrush current? Are there any drawbacks?

Pro Backup
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Akash
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  • Yes this can be done. – Andy aka Apr 07 '16 at 14:28
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    I am not totally clear on where you want to put the NMOS. Maybe a sketch schematic would help. But do show the input and output filter caps and inductor. One thing to watch out for any time you use NMOS to open a ground path, is that it may be accidentally bypassed by earth ground connections to external devices. This may be a reason why some people avoid doing it under some conditions. – user57037 Apr 07 '16 at 15:57
  • It's been done, for example here is a nice thread about it (includes schematics & layouts) - you may need to adjust for your voltages / currents, but the N-channel mosfet principle is the same, and it has a few details you may be interested in (slow turn-on resistor, zener diode to clamp gate voltage, options for an on/off switch, etc.) - P.S. - I am posting this as a comment instead of an answer as it's basically only a link, if someone wants to summarize it into an answer & attribute properly I would not be opposed. – user2813274 Mar 28 '17 at 02:03

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This can definitely be done. However, like any low side control, you should only do this if the return of your load is isolated from the ground of the source.

In many electrical and electronic systems, exposed conductors (like shields or chassis) are at ground potential. Thus it's not safe to have two systems (your power supply and load, for example) with different "ground" potentials, because they could easily be shorted together. This, combined with the convention of positive supplies (i.e. systems where ground is the lowest potential) probably explain why it is easier to find high-side inrush control solutions.

user49628
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You can find both ways of achieving this. You may also use a series resistor with input cap that you will bypass using a fet/thyristor/triac. It was mentioned your "ground" won't be exactly "ground" anymore but it's close enough that it usually won't matter.

If you google for inrush current limiting you will find many examples. Motorola AN1542 shows a simple method of high/low side limiting with mosfet using gate capacitance to do something useful.

Note that on AC/DC circuit the current limiting resistor is subject to a lot of power. It may be only for an instant but a regular 1W resistor would die horribly.

Barleyman
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  • If it's only an instant, where an instant is less than a few milliseconds, then you should refer to the datasheet. E.g. even little 0.2 W MELF resistors can cope with an instantaneous rate of 10 W for up to 1 ms. Ref: Pulse Load Handling for Fixed Linear Resistors. – Andrew Morton Jul 01 '17 at 20:20
  • 10W? Not nearly enough. For 230VAC you're dealing with around 335V peak so with a 10R resistor that'd work out to 11kilowatts. I'm not going to dig out application notes this late night but typically resistors designed for this particular job are used. If you ramp up the resistance to, say, 1k, you'd need 47ms to get 2/3 charge to that 47uF input cap, at comfy 112 watts.. Depends on your power level and cap size of course. – Barleyman Jul 02 '17 at 03:42
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Using another mosfet just for soft-start is probably not economical, but would most likely be possible.

On Linear technology switching regulators / controllers, there is usually a soft start pin that you can hang a capacitor on. This limits the current that the IC will allow the circuit to draw from the supply, ramping it up slowly. This is the preferred way to do it, as a capacitor in mass production costs less than a penny, and mosfets can be in the dollar range.

Mike Barber
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