App note: Limiting inrush current


Aimtec’s app note on inrush current on power converters and their solution. Link here

Inrush currents can be problematic in circuits that utilize overload protection devices such as fuses and circuit breakers. The selection of overcurrent protection devices is made more complicated when high inrush currents are present. False overload conditions can trigger unwanted protection events.

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  1. Well that’s a bit of a lame summary.

    The audio-amplifier crowd have a habit of using large toroidal transformers that suffer huge inrush (both magnetisation current and charging up a huge filter C). They tend to use a high power resistor shorted by a relay that closes after 100ms or so. Very soft start, with the only long-term power loss being the relay coil.

    NTCs provide no protection for warm starts, and they’re pretty inefficient anyway because they have to stay warm! If you have a power bump – the typical 500ms dropout that happens when the distribution network changes over to a backup feed – then you’ll find out why warm-start matters for things like toroids. You also have to be sure that the control system has fast dropout, i.e. is capable of detecting all power losses longer than about 2 or 3 cycles.

    Before you know it, there’s a microcontroller in your design performing power sequencing…

    1. Agree, Aimtec’s app note should have “with passive devices” appended.
      Rather short for an app note, a bit lazy there.
      Well, who faces issues with inrush current these days? The audio-amp crowd like you say, maybe the radio folks, who else? Most of us doing low power or battery power never need to deal with inrush…

      1. It also matters for anyone who is powering devices (even small ones) over long cables, because cable L + input C = 2*Vin spikes that can blow up the downstream power supplies. So overcurrent is not the only problem: inrush can cause overvoltage.

        PFC is the ultimate inrush limiter!

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