I have the following circuit for providing regulated 5V from a 12V power supply: Power input through diode

I have several hundreds of PCB with this circuit, and they work fine. However recently I have had a few of these PCBs stop functioning. I have tracked the problem to diode D1.

When applying 12V to the input I have measured 1.8V on the cathode of the diode, and no appreciable input current (the power supply shows 0.000W output power), so I have discarded a short-circuit down the line as a possibility.

Now comes the interesting part. I tried slowly increasing the voltage to see if there would be any change on the output current. To my surprise, the diode started conducting at around 16V, and when I measured the voltage drop across the diode it was back to its normal 0.7v to 0.8V. After this the PCB was functioning normally again, and I was not able to reproduce the behavior. As matter of fact, now when applying a 12V input, it works as intended.

I have seen this behavior on at least 3 or 4 PCBs out several hundreds identical PCBs.

I am assuming this is a defective diode, but it is the first time I observe this mode of failure and I would like to get more information about it.

EDIT: I would like to add that I made sure that the connector and cable were not at fault here. Also the measurements are on the pins of the diode itself. Furthermore, when increasing the voltage on the power supply, I made sure I was not touching any of the cables or PCB because I suspected that could be one of the problems, but it is not apparently.

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    \$\begingroup\$ are you sure the diode isn't soldered in the wrong way around? \$\endgroup\$ – Marcus Müller Mar 21 '17 at 14:50
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    \$\begingroup\$ Yap, first thing I checked. Also as I mentioned, the same diode started conducting again after increasing the voltage, and the forward voltage returned to the normal 0.7V. \$\endgroup\$ – Endika Mar 21 '17 at 14:53
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    \$\begingroup\$ Yes, nor have I. Have you measured the voltage directly across the diode? \$\endgroup\$ – Trevor_G Mar 21 '17 at 15:19
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    \$\begingroup\$ If substitute Diode is rated for <30A pk and ESR of 10uF cap and voltage source is < 0.4 Ohms (possible) then it will exceed Diode rating on turn on, resulting in exceeding absolute max rating. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 21 '17 at 16:20
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    \$\begingroup\$ @Trevor Don't forget to write that up in an answer! \$\endgroup\$ – W5VO Mar 21 '17 at 17:46

I suspect it has something to do with the initial forward voltage when you first apply power, which, because of the caps, will be the full supply voltage.

Although the S1D-13-F should be able to handle the full supply forward voltage and inrush current to the caps, "should" is the operative word here though.

You stated "I have had a few of these PCBs stop functioning" which suggests they were working for some time and then failed. That suggests something is getting stressed, and the peak forward voltage and current would be by prime suspect. If it IS overstressing it, then failure mode could be weird.

Moving the diode to the right of C1 would solve that issue. Easy enough rework to the cap connection too.. Though it does leave the cap open to being reversed, it at least limits the threat.

Ultimately, if it is a real and present danger of reverse connection, a better method is to use a different technique, such as a up diode between ground and 12V and a polyfuse. That method gets rid of that unnecessary diode voltage drop in the line too.

BTW. I would have wired that connector with the centre ground and 12V on both pins 1 and 3. That way it will not matter if it gets plugged in backwards.


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