Fuse and reverse polarity protection, is this OK?

[TomS_]

Hi everyone.

Looking at a small piece of circuitry related to power supply to a DC-DC converter module (datasheet here http://power.murata.com/datasheet?/data/power/uls-100.pdf, which may be relevant to help understand/answer the question.) Im currently using this as the 5V supply for my prototyping as its the best thing I have at hand before I get a proper bench supply (and I work in telecoms, so I already had a 48V power supply handy to feed this thing.)

This DC-DC module has a control input which, when dragged low, enables the output. The datasheet says (on p26), however, that damage to the module may occur if you apply voltage to the control pin when no voltage to the input pins is present.

The datasheet also says on p24 that there is an on-board diode which will forward bias under reverse polarity, so an external diode might not be necessary in this case, but I suppose its not a bad idea to prevent the module itself from having to do the work by using external components?

Im looking to implement some sort of reverse polarity protection with a fuse which would ultimately prevent voltage from reaching the control pin in a reverse polarity situation.

I am thinking that if the fuse were placed on the negative side of the input, which is where the control pin is tied to, and with the reverse polarity protection diode situated as is, if you reverse the input, the excessive current will blow the fuse preventing any positive voltage from reaching the control pin. Sounds safe.

Would this be an agreeable theory? Moreover, is it acceptable to have a fuse on the negative side or should it really be on the positive side?

My alternate option would be to implement an optocoupler per the datasheet to drag the control pin down only when the input voltage is applied the correct way around. Or perhaps a small diode between the control pin and negative?

Many thanks!

Tom

[MosherIV]

Hi

What you want for reverse polarity is shown. (Sorry for the poor editing, I am in a hurry to go home, It is Fri )

[ajb]

Either of those circuits will work.  In either case, the voltage at the control pin will be sufficiently limited.  In the case of the first, it's limited to -Vf of the diode, and in the second it's limited to basically nothing since the diode blocks.

The first arrangement, where the diode clamps any reverse voltage, works well when you also want to clamp the maximum input voltage, as you can use a suitably-rated TVS diode rather than a normal rectifier to provide reverse polarity and over voltage protection in a single device. 

The second one, where the diode blocks any reverse voltage, has the advantage of not having to replace the fuse after a polarity reversal.  If you were to use a polyswitch in place of the fuse in the first arrangement you'd have a similar advantage, but sizing the diode and poly switch so that the diode doesn't get cooked by the time the polyswitch trips can be tricky.  This arrangement also has the disadvantage of a constant diode drop in series with the supply, which can result in significant power loss at high currents.

A variant of the blocking diode arrangement uses a FET in place of the diode, which can have a much lower voltage drop and hence lower dissipation.

Generally, clamping is preferable when the available fault energy is relatively low, but the fault voltage may be rather high.  Note that energy matters as much as power, because there's a time component here.  You can limit the energy even at quite high powers if you're willing to blow a fast fuse in the process, but when the protection is slow, then the time to trip becomes a major factor in selecting protection elements.  When blocking, you have to make sure that the fault voltage will not exceed the breakdown voltage of the blocking device. 

[suicidaleggroll]

I generally prefer to use a fuse in series, bidirectional TVS in parallel, followed by a Pch FET arranged for reverse polarity protection.

You get overvoltage protection with the TVS and fuse, transient protection with the TVS, reverse polarity protection with the FET, and very low forward voltage drop so it doesn't kill efficiency like a series diode would.

[TomS_]

Hi

What you want for reverse polarity is shown. (Sorry for the poor editing, I am in a hurry to go home, It is Fri )

I dont blame you.

I suppose my question re this circuit is, and taking in to consideration what the datasheet says, wouldnt this potentially allow voltage to reach the control pin in reverse polarity situations even if nothing can flow through the module? Its just the way that the datasheet is worded...

Applying an external voltage to pin 2 when no input power is applied to the converter can cause permanent damage to the converter.

Ping 2 being the control pin.

A diode on the positive side and a reverse polarity situation would effectively equal no input power to the module, but would still represent power applied to the control pin? I appreciate that there may be some specific knowledge with this module that means this cant be answered here, but thats just how I think of it. And I realise it wouldnt allow for current to flow out the other side, so if nothing can flow then nothing can blow right?? Maybe Im making a mountain out of a mole hill here.

But in such a case, might I be better off putting my fuse on the positive side, and the diode on the negative side perhaps, something like the attached? This would then also address the point ajb raised about not having to replace the fuse if reverse polarity is applied.

Thanks, and apologies if Im being really pedantic...

Tom

[suicidaleggroll]

A diode on the positive side and a reverse polarity situation would effectively equal no input power to the module, but would still represent power applied to the control pin? I appreciate that there may be some specific knowledge with this module that means this cant be answered here, but thats just how I think of it. And I realise it wouldnt allow for current to flow out the other side, so if nothing can flow then nothing can blow right?? Maybe Im making a mountain out of a mole hill here.

Remember, all voltages are relative.  There is no such thing as a "positive voltage" or a "negative voltage" or "ground".  Ground is simply the node that we decide to call "ground", a "positive voltage" is a node that has a voltage that's higher than the node we arbitrarily chose to call "ground", and a "negative voltage" is a node that has a voltage that's lower than the node we arbitrarily chose to call "ground".

A reverse polarity situation occurs when you apply more voltage at node A than you do to node B, when the circuit expects the opposite.  A series diode or Pch FET arranged for reverse polarity protection works by dropping the applied voltage across itself, instead of the circuit.  In the case of MosherIV's schematic, if you applied 0V at CON1-1 and 5V at CON1-2, you would end up with "5V" at -VIN, CTRL, and +VIN.  Since all pins on the IC would be at the same voltage, there is no reverse polarity, since there's no polarity at all.  It's only "5V" because we chose to call it "5V", the IC doesn't care.  Instead the applied 5V gets dropped across the diode, which can handle it just fine, and the rest of the circuit is isolated from CON1-1.

Sticking the diode on the return side is generally frowned upon because it raises the "ground" potential of the IC by 0.7V, which can cause problems when interfacing with other circuits that are using "0V" for their ground.  In the case of an isolated supply it doesn't really matter though.