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What part/circuit is needed to control in rush and standby/operating currents?

Pardon the likely lack of detailed info, I'm trying to get ideas to help a colleague out and I'm in an area where I admittedly have very little design experience.

The basic problem that he is trying to solve is managing in rush current at device power up and then current spikes when transitioning from standby to active mode. The parameters that I know about are:

Input Voltage = 24V (DC) Peak in-rush current (measured) = 65 A for ~100 us Standby current = 80 mA (measured) Operating current 2 A (measured)

To limit the in-rush current at power on he's looking at a thermistor which seems to limit the peak in-rush current nicely but then it presents too high of a resistance when switching from standby to operating mode.

What would be needed then to go along with the thermistor is something that would turn on some time after the initial in-rush current has subsided that could handle the 2A change in demand when going from standby to operating mode.

A reed relay in parallel with the thermistor looks like it might do the trick if there is one rated for ~3-4A (~50-100% margin guessing on the 2A) and a turn on current around 20-40mA (~25-50% of the measured standby).

Is this approach viable? If so, then are there any suggestions on particular parts? Since I'm far from expert in this area I'm sure there are better solutions to pursue. Any suggestions in that regard?

Thanks in advance.

KJ

Reply to
KJ
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A very simple circuit to accomplish your goals would employ an N MOSFET in the negative rail or a P MOSFET in the positive rail. The gate signal for either type of FET would ramp the gate voltage up over a few milliseconds by means of a resistor and capacitor. To complete the picture, add a diode to discharge the timing capacitor when power is removed. Should be less expensive than a relay and use almost no bias power. Paul Mathews

Reply to
Paul Mathews

Paul,

I had considered that arrangement as well, but since the FET gate is an R/C delay of the FET drain voltage it seemed to me like if the external 24V power brick voltage blipped up a bit it would create a negative gate to source voltage which would increase the source to drain resistance which would then drop the drain voltage (and therefore the gate voltage) which would make the gate to source voltage even more negative than it started at. This would continue and the feedback would cause the FET to shut down. If that happened, the FET would only come back alive by the conduction through the thermistor and the whole thing would be in some form of oscillation. Or am I just completely missing something?

KJ

Reply to
KJ

Have you looked into hot swap controller chips? They sometimes (usually) have a soft start circuit. One I've used is the LTC4211, it won't do for your voltage but take a look at the idea.

Reply to
a7yvm109gf5d1

Power mosfet + Isense resistor + op amp

Chose high side or low side for limiting.

Loads of fun to design..especially high side design..

As a bonus, the circuit can act like an electronic fuse if a time out circuit is added too..

D from BC

Reply to
D from BC

Hot swap controllers are designed specifically for this issue. You could design your own, but why when there are plenty of single chip (well, with a few support components) out there?

The large initial inrush is because the output (to the unit) is not being ramped (soft start); it's charging up caps that are either large or have low esr or both. You don't have to be a hot-swappable unit to use a hot swap controller.

The best offerings are from Linear Tech and maybe TI. Analog devices does some as well IIRC. For a few bucks you get what you need.

Cheers

PeteS

Reply to
PeteS

You don't need a thermistor...just the FET in series. On power up, the RC delays the gate voltage and the FET transconductance increases slowly. On power down, a diode in parallel with the R quickly discharges the C across the gate. Problems can arise if the gate voltage stays too low too long, since power dissipation in the FET is then high. The fancy hot-swap chips can take care of that kind of condition. Paul Mathews

Reply to
Paul Mathews

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