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I need to kinda soft-start an array of fans, all connected to one reference PWM. The problem is our fans at top speed can roughly draw 2.5 times as much power as our DC-source can handle (on-board custom PCG for an inverter not easily replaceable). And of course, they also draw a lot of power during start-up.

We got 4-wire fans, so I plan to use a PWM to control their speed, this works fine after start-up, but during the start-up the fans will draw a lot more power even when setting the duty-cycle to 10%. Are there any tricks I can use? Does the PWM frequency affect the start-up? Due to small differences between the fans (6 in total) they also won’t start synchronous, this affects changes in the duty cycle too, their speed will vary for a couple of seconds (also shows in power consumption).

Fan details:

PWM 10-100% (Max for our dc-source is around 55%)

Fan-PWM-Frequency: 1-30kHz

So basically what is better in such a case a high or low frequency PWM? Also have you any recommendations at how fast I should change between duty-cycle levels?

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  • \$\begingroup\$ maybe use an inductor in series with the supply lines. Inductors smooth the current. I'm not sure though, just a guess. and beginner. \$\endgroup\$ Dec 8 '20 at 12:51
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    \$\begingroup\$ Power half via one relay; power the other half via another. Start the two relays a few seconds apart, before increasing the single PWM to full power. \$\endgroup\$ Dec 8 '20 at 18:37
  • \$\begingroup\$ Do you have the ability to put a large capacitor on the output of your power supply? The capacitor could temporarily supply some extra current during the startup phase. \$\endgroup\$
    – user4574
    Dec 8 '20 at 18:49
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    \$\begingroup\$ So based upon your comment to SolarMike's Answer, there is not much you can do. One control driving 6 loads. Can you adjust voltage of the fans? Answer: No. Back to one control driving 6 loads. I'm also sure you've tried different software settings before coming here. No PCB delay will be significant to have any impact. \$\endgroup\$ Dec 8 '20 at 20:35
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Start each fan 1 second apart or 1/2 second - depends what’s needed.

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  • \$\begingroup\$ Sadly I can't do this so easily, because I don't have access to each fan. I'm limited to 1 pin for PWM (all fans are connected to this). Worst case our hardware needs to add delays on the PCB directly. \$\endgroup\$
    – Darki
    Dec 8 '20 at 13:08
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You could try using an NTC thermistor to limit the inrush current

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