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According to the specifications of this Mean Well LRS-150F-12, PSU (12 V, 12.5 A) its output power Pout is 150 W.

I want to estimate the input rated power Pin at rated output.

Efficiency is given as 87.5%. So this makes Pin = 100 * Pout / 87.5

So Pin = 171.4 W

If I calculate Pin by using Pin = Vin × Iin = 2.8 A × 115 V = 322 W.

So which Pin is correct here and why do they not match?

Does that mean power factor is 171.4 / 322 ?

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1 Answer 1

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The AC input current will be worst case while charging up the high-voltage capacitors inside the PSU when power is first connected.

In general maximum input current in normal operation will occur when powered at minimum input voltage. On "universal" power supplies which typically operate between 100 V and 250 V AC the maximum current will be when fed at 100 V.

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  • \$\begingroup\$ Im sorry but I didnt get what you mean. So how can we estimate Pin for rated Pout(150W)? \$\endgroup\$
    – user1999
    Jul 28, 2021 at 9:18
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    \$\begingroup\$ You can't other than use the manufacturer's efficiency specification (as you did in your first calculation) since that's all the information you've got. Otherwise you've got to measure it. \$\endgroup\$
    – Transistor
    Jul 28, 2021 at 9:22
  • \$\begingroup\$ So can we say Pin = I_in_rms_measured x 115V. But why dont they show the typical I_in_rms_measured for rated power? The reason Im asking is: If I want to size a slow blow fuse for the line(power entry module switch). I need to know the rated input current. In this case it is 2.8 A(for 115 V). So if I calculate by 1,5 * 2.8 = 4.2A. Should the fuse be like 5A? \$\endgroup\$
    – user1999
    Jul 28, 2021 at 9:28
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    \$\begingroup\$ It seems that your question is really about fuse selection and that's not what you asked about. The datasheet says 2.8 A at 115 V so that's a good value to work from. Littlefuse's Fuseology Selection Guide may be of interest. \$\endgroup\$
    – Transistor
    Jul 28, 2021 at 9:39
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    \$\begingroup\$ Actually, 2.8 A is not the worst case current when power is first connected. The inrush current is given in the datasheet as 60 A (typ). Yup, sixty amperes. A slow-blow fuse will be needed. \$\endgroup\$ Jul 28, 2021 at 12:45

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