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I came across a DCDC converter JCK3012D15, that takes a 9-18VDC in and gives out +-15V @ 1A (pretty handy).

I have some questions:

  1. What is the advantage of using a power MOSFET at the input over going instead directly in with the DC?

  2. According to the JCK datasheet it can accept a maximum current in of 2.8A. Would a 2.5A fuse (Fuse1) be ok/safe? Will there be an in-rush current to consider when selecting the proper fuse? Also for fuse 2 and 3?

  3. Do the ouputs of the JCK need to be regulated?

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  • \$\begingroup\$ Can you edit to add a link to the datasheet? \$\endgroup\$
    – Transistor
    Commented Mar 6, 2022 at 9:23
  • \$\begingroup\$ Just to add - 2. There should be specification stated for in rush current on the fuse used 3. The selected part will output a regulated +/-15V. Is this voltage within your targeted load requirements? \$\endgroup\$
    – kakiitek
    Commented Mar 6, 2022 at 13:04
  • \$\begingroup\$ 1. On top of what @mkeith said, it’s fine as reverse polarity protection until you supply low enough voltage on the input that the MOSFET enters linear region. If your DC/DC isn’t shut down due to UVLO, it may burn. \$\endgroup\$
    – winny
    Commented Mar 6, 2022 at 13:57

2 Answers 2

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  1. D1 and Q1 form reverse polarity protection. D1 on it's own would cause a 0.7 V drop in supply voltage and a 0.7 W/A power loss. If the polarity is correct then Q1 will be turned hard-on, it's resistance will drop to about 20 mΩ or so giving a voltage drop of 20 mV/A and a power loss of 20 mW at 1 A.
  2. The datasheet should explain the current inrush. 2.5 A slow-blow would probably be OK but may be a bit on the edge.
  3. Again, the datasheet specification will tell you the output ripple and noise.
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1. What is the advantage of using a Power Mosfet at the input over going instead directly in with the DC? That transistor is for input reverse polarity protection. If you apply 12V backwards, the FET will stay off and block the current.

2. According to the JCK datasheet it can accept a max current in of 2.8A, would a 2.5A fuse (Fuse1) be ok/safe? Well, it is hard to say. I would guess that the fuse might not blow very fast at 3 A. But the connector probably won't heat up very fast at only a slight over-current. So it may be OK. Maybe a 2 A fuse would be more prudent.

3. Will there be an in-rush current to consider when selecting the proper fuse? Also for fuse 2 and 3? Inrush probably won't be a problem for fuse 1 because the big cap is in front of the fuse. Inrush does not go through the fuse. For fuses 2 and 3 it depends on what is attached to CN1. If the load shows high inrush, then maybe it could be a problem.

4. Do the outputs of the JCK need to be regulated? Honestly I don't really understand the question. The jack goes straight to a regulator already. I don't think it makes sense to put another regulator between the jack and the existing regulator.

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  • \$\begingroup\$ Would it be possible to turn the mosfet on slower to reduce inrush current? \$\endgroup\$
    – Lenne
    Commented Mar 6, 2022 at 19:10
  • \$\begingroup\$ @Lenne not really, no. The way the MOSFET is oriented, it can never block forward inrush current, even when it is off, because the body diode will conduct. When the MOSFET is off, the body diode will be reverse biased, so will not conduct. \$\endgroup\$
    – user57037
    Commented Mar 6, 2022 at 21:35

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