Reverse protection of Multicomp pro DC/DC converters MP-K78xx?

Hi,

There is a partial answer that is relevant, in the datasheet:

4. Converter cannot be used for hot swap and with output in parallel;

Even with any alternative (more expensive) product that is more fully specified, it's unlikely to allow multiple sources to be connected together in this way. Usually some 'OR' circuit is needed, perhaps with diodes, or MOSFETs. In your example, perhaps the USB source could be connected to the input of the DC-DC converter instead (via a Schottky diode). It might be slightly out-of-spec since USB power isn't exactly 5.0V, and may even go out of spec with different loads too, but maybe such a cut-down method is acceptable for some applications.

Are there USB connectors that have an Earth switch in them like DC Barrel Jacks - when a USB cable is plugged in it disconnects an Earth turning an alternate power source off?

Hi Andrew,

Neat idea! I've not seen one, but if it exists, it would be a useful part. I once used a normal switch (microswitch) positioned such that the user couldn't physically insert the USB connector, without pushing the microswitch lever down first. Since 3D printers are so popular, maybe a sliding door for a USB connector and switch could be designed.

I suppose there are alternate physical approaches: need to make sure that the alternate power source becomes un-powered.

Good questions and you're right to research them.  LM78xx (and LM79xx) don't like reverse current so I always provide diode protection in case there is any over voltage/current problem downstream.  I've used these types of parts quite a lot and it's good to know that Multicomp are now doing them as they look significantly cheaper.  When using them, I've always followed the same principles in terms of protection as I have with a typical LM78 component.

These are footprint compatible with LM78XX devices so you could get a good idea of what is/isn't allowed from other data sheets for an actual LM78xx (Texas Instruments data sheets are usually comprehensive in my experience).  Also, this device is a re-branded LM alternative so find an equivalent from TracoPower or Recom and check out their data sheets (I'd bet someone else's bottom dollar that it's actually manufactured by TracoPower or Recom!!.)  

For example, here's a datasheet for a Recom LM78XX replacement.  It shows how to handle reverse current if it's powered down.    I think this answers your Q1 and, in fact, Q3 as if there is no input to it, there's no output which is pretty much the same as powered down I think??

Q4 (your second Q3) and Q2 should be covered by the diode protection circuit with the Schottky diode - as Shabaz mentioned in his response.  

Thanks for the responses. Shabaz, I have read that sequence about "hot swap and parallel" several times but did not realize that in fact suggests the answers!

Seems the answer to all questions is "it would probably fail" :-) and to prevent it, a Schottky (or a P-FET for lower drop) in series with the output should do the trick. A diode across the converter from Vout to Vin is not a good option as that would make a short from the external source to ground via this diode in case of my first Q3 situation (my bad for two threes, sorry).

I doubt using such nice-but-unheard-of USB connector would be practically useful for transparent power source switching as activating that mechanical switch would not be timely and instant enough to both prevent the two sources be active at the same time and therefore collide and at the same time not to cause power drop because of time gap between one source gets disconnected to the other getting connected. Or?

They aren't designed for instantaneous power changeover, no.  They would disconnect the Earth before connecting the power so they wouldn't both be active at the same time but there would be a time where nothing was powered.  Basically, your approach should be what Shabaz and I have mentioned - you may also need to consider common grounding as well.

Interesting, they are a really good price. I've added a few of the in-stock options into the shopping basket since they are so useful.

I saw the cross-section of one of those last night: