Reverse Protection Diode

[krich]

I'm still pretty new to electronics, but I'm learning a lot as I go (usually from forums like these). I have a question regarding what type of Diode to use as a "reverse protection diode" in a very simple DC-DC 5V power supply (78L05 based, and a couple decoupling caps).

My goal is to protect the board from someone hooking up the DC power source backwards. Clearly this is the job for a diode in series on the positive connection. I've done this before and have used just a 1N4001 for the job, but my current project is all SMD parts, so...I'd like to use something I have in my surface mount parts cabinet. All I have in SMD for diodes is Zeners (not good), or Schottkys (STPS140, to be exact).

My question is...is there any reason that a Schottky won't work in this case where I would normally use a general purpose diode? In electronics, I've found that the devil is in the details, so I worry I'm missing something here. The STPS140's have If=1A, Vr=40V, and Vf=0.5, all are well over what I anticipate will be needed in this circuit.

Thanks for any input.

Ken.

 

[dknguyen]

Clearly this is the job for a diode in series on the positive connection. I've done this before and have used just a 1N4001 for the job, but my current project is all SMD parts, so...I'd like to use something I have in my surface mount parts cabinet. All I have in SMD for diodes is Zeners (not good), or Schottkys (STPS140, to be exact).

Not necessarily...there are two methods of using a diode for reverse polarity protection:

1. Stick the diode in series with the rest of your circuit so that current can only flow into your circuit one way.

Advantages: Straightforward, no "resetting" required. Comes into effect "instantly".
Disadvantages: Introduces a voltage drop, diode must handle current draw of circuit (sucks when the circuit draws a lot of current). You CANNOT use a schottky diode since it's reverse leakage current is too much and may still damage your circuit in reverse polarity.

2. Stick the diode in anti-parallel with your circuit and a fuse in series with both the circuit AND the anti-parallel diode. During reverse polarity the diode will produce a short-circuit across the circuit clamping it to a low negative voltage (equal to the forward voltage drop of the diode) until the fuse burns out, thus disconnecting the circuit.

Advantages: Introduces no forward voltage drop. THe diode only needs to be able to handle a short current pulse about double the current draw of the circuit so it can be much smaller (no need for the continuous rating). THis is because it only has to be able to conduct enough current to burn out the fuse which is sized for the current draw of the circuit and most fuses burn out very quickly at double the rated current. After the fuse burns out, all current into the circuit will stop.
Disadvantages: The time taken for the circuit to take protect is longer than the other one since the diode must first turn on to conduct the short circuit current through the fuse. THe fuse will then burn out and need to be replaced. You can use a schottky diode in this situation that would increase the speed. In this case, the small reverse leakage current during normal operation doesn't do any harm since it does not go into your circuit. It does introduce a small negative voltage drop (due to the forward voltage drop of the diode) across your circuit while the short-circuit current is trying to burn out the fuse. Your circuit has to be able to survive this. Agan, a schottky diode will help because it has a low forward voltage drop.

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So the simple answer to your question, exactly as you worded it, is YES, in general, there is a reason you cannot use a schottky diode. If your circuit can tolerate the higher reverse leakage current of a schottky during reverse polarity condition it's good (and the reduced forward voltage drop of the schottky during normal operation also increases efficiency over other diodes). But in general, you cannot.

 

[dknguyen]

Found a graphic to better describe the two methods, especially the second not-so-obvious one. THe fuse for (a) is not involved in the reverse polarity protection.

 

[krich]

You CANNOT use a schottky diode since it's reverse leakage current is too much and may still damage your circuit in reverse polarity.

Ah yes. I see that the reverse current is 1A in the datasheet. That will never do. The parts in the circuit are designed for a max current of 100mA.

Excellent explanation of the two options, Dknguyen. I can see where option 2 is probably the best when considering the parts I have. I'm pretty sure my circuit is able to tolerate the negative voltage drop you are suggesting in option 2. The downside is that I do not have a fuse on the board, rather I have it on the battery leads. As long as I (or anyone else) uses a fused battery, everything is fine. But I can see a case where the board will be hooked up in a lab setting to a simple wall wart (i.e. no fuse). The benefit of Option 1 is that it will be effective even if there is no fuse.

I can tolerate the forward voltage drop of a GP diode, so maybe I should just keep digging in my parts cabinet, or try to salvage one off one of the many commercial circuit boards I have laying around. Maybe I'll find a little SMD fuse that I can use! Then I'll use option 2, which seems to me to be a much better solution. Why burn up 1V for protection when you don't need to.

Thanks!

Ken.

 

[dknguyen]

Ah yes. I see that the reverse current is 1A in the datasheet. That will never do. The parts in the circuit are designed for a max current of 100mA.

1A reverse current? Don't you mean more like 1mA?

The parts in the circuit are designed for a max current of 100mA.

It's hard to tell exactly how much reverse current at reverse voltage a part will tolerate since the tolerance for reverse polarity current through a component is almost never given. It's not the same as the regular current rating (and obviously much much less).

 

[krich]

Yeah, my bad. I read the wrong figure. Ir=12uA to 2mA depending on temperature (25C to 100C).

I found a MELF packaged diode that (I think) is a Vishay-LL4150 or similar. It's got a 100nA reverse current, so that's more like it. It's orange with a black band. Judging from here: (**broken link removed**), that's a GP diode...I guess I could try testing the reverse current just to see what I get. My meter goes down to uA measurements, so I should be able to do that.

Thanks again!

Ken.