Can I stick a diode on this regulator's output for reverse voltage protection? - Page 1
Forum section: electronics / projects

author: Starlord
author_rank: Frequent Contributor
timestamp: November 29, 2017, 10:32:32 am
content: I'm considering using this buck regulator to supply 5V at high currents:http://www.ti.com/lit/ds/symlink/tps62135.pdfHowever, in some instances there may be 5V already on the output but the battery on the input might be below 5V, and I need to make sure its protected.But I noticed that there's a direct path through one of the regulator's internal mosfets where its body diode would allow current to flow when VSW is at a higher voltage than VIN. This is bad.My first thought was to simply place a diode on the output of the regulator. But this poses a problem. A diode's forward voltage isn't fixed and varies with the current draw, and my regulated 5V output would now be something less than 5V and no matter what value I chose to boost it by to account for the diode it would still be wrong most of the time.I couldn't see a solution for this until today when I was looking at this other boost/flyback regulator and I noticed it's schematic featured a diode between the inductor and the voltage divider on the feedback pin:http://www.ti.com/lit/ds/symlink/tps55330.pdfEureka! If I put a diode there on my preferred regulator, whatever the instantaneous forward voltage of the diode is won't matter, because the feedback pin will see the end result and adjust the output voltage back to 5V! Right?I'm pretty sure this will work, but I wanted a sanity check to make sure I haven't overlooked something important!Also, the diode's forward voltage should be as low as possible to keep the power dissipation to a minimum, correct?

author: RobK_NL
author_rank: Frequent Contributor
timestamp: November 29, 2017, 03:12:16 pm
content: Yes, that will probably work. I would put the output capacitor in front of the diode and choose something like a PMEG3050 for the diode.

author: Scrts
author_rank: Frequent Contributor
timestamp: November 29, 2017, 03:16:51 pm
content: Is it worth adding a diode prior to the regulator then?

author: Starlord
author_rank: Frequent Contributor
timestamp: November 29, 2017, 03:28:43 pm
content: Quote from: RobK_NL on November 29, 2017, 03:12:16 pmYou mean like inductor -> capacitor -> diode -> r1?Why? Do you think the diode will cause instability? I can't say I've ever seen an output cap placed before the diode on a switching regulator that has one.Also, damn! How do you people remember these part numbers? I spent over an hour this morning searching for the perfect small diode that could handle sufficient current and had a low forward voltage to keep the power dissipation to a minimum but which was cheap and available in large quantities, and that was the exact one I'd finally settled on after almost giving up and you just pull it out of thin air immediately!

author: Starlord
author_rank: Frequent Contributor
timestamp: November 29, 2017, 03:36:58 pm
content: Quote from: Scrts on November 29, 2017, 03:16:51 pmI wonder if that would make more sense.There was some reason I didn't want to do that, but I can't remember what it might have been now. Perhaps I got confused and was thinking the current draw on the input side would be higher than the output? I can't see any reason why putting it on the input side wouldn't work better and result in lower power dissipation in the diode.I guess maybe there could be a case where the battery voltage is low enough that the additional voltage drop by the diode trips a low voltage cutoff... I'm not sure if this regulator has that feature though. I'll have to look into it. But I doubt even if that was the case it would be an issue with such a low forward voltage drop. Maybe if it were like 1.2V or something it would be a concern.Ok, I looked into it. The regulator has a 3V under voltage lockout. That diode has a Vf of 360mV @ 500mA, so it doesn't look like that's a concern even if a 3.7v lithium battery is connected. It will only drop to 3V when it's dead anyway.

author: RobK_NL
author_rank: Frequent Contributor
timestamp: November 29, 2017, 03:59:05 pm
content: Quote from: Starlord on November 29, 2017, 03:28:43 pmWhat do you mean by that? If you are referring to that other regulator you linked to in your starting post: That's an entirely different topology altogether.QuoteIt'sa kinda magic.Having spent some time selecting appropriate diodes for my own converters helps a bit too.

author: Starlord
author_rank: Frequent Contributor
timestamp: November 29, 2017, 04:22:49 pm
content: Quote from: RobK_NL on November 29, 2017, 03:59:05 pmI just meant I've looked at a lot of regulator datasheets for all kinds of topologies, buck, boost, buck/boost, and I've never seen a cap before a diode. Of course you're right in that topology matters. And I just looked at a handful of TI buck regulators and I can't find any with diodes in their schematics. So perhaps I was recalling some boost regulators I've looked at in the past.

author: RobK_NL
author_rank: Frequent Contributor
timestamp: November 29, 2017, 04:39:22 pm
content: I'm too busy for my own good at the moment, so it didn't register until I was doing some dishes just now:You want to use a buck regulator to get 5V out of a 3.7V battery? How is that supposed to work?

author: Tomorokoshi
author_rank: Super Contributor
timestamp: November 29, 2017, 05:14:18 pm
content: Quote from: RobK_NL on November 29, 2017, 03:12:16 pmOne detail, depending on the loading and ambient temperature, I had trouble with a 150 deg C PMEG3050 when temperatures got elevated. It went into reverse current thermal runaway. The 175 deg C PMEG4050 variant was enough better to solve the problem.

author: Starlord
author_rank: Frequent Contributor
timestamp: November 29, 2017, 05:18:11 pm
content: Quote from: RobK_NL on November 29, 2017, 04:39:22 pmNah, I just prefer the design to be flexible. In some use cases I might connect servos that require a 5V 3A supply. But in other instances I might only need to power the microcontroller and sensors that only require 3.3V. Its in those latter cases that I would like the option to connect a smaller battery. Of course, I could add a second connection point for these smaller batteries and stick the diode on that input (which could still be exposed to 5V from a USB connection), but that creates a point of failure where someone could connect a battery to the wrong input.

author: Scrts
author_rank: Frequent Contributor
timestamp: November 29, 2017, 09:20:54 pm
content: Quote from: Starlord on November 29, 2017, 03:36:58 pmYou either use a good schottky then or a FET.

author: David Hess
author_rank: Super Contributor
timestamp: November 30, 2017, 08:48:57 pm
content: A PNP transistor can be used as a very low forward voltage drop diode at the output of the switch. This is only suitable at low voltages where the reverse base-emitter breakdown will not be exceeded.