Protecting 5V power source from back-feed with higher voltage - Page 1
Forum section: electronics / projects

author: SeaMaster
author_rank: Contributor
timestamp: August 20, 2021, 04:03:04 pm
content: I have a project where the our PCBA is providing 5V power to external sensor on a connector. My concern is if 12V or more is connected to the 5V connector. This will destroy the 5V PS as the current will definitely exceed the sinking current of the 5V PS and and once that happens the uController will be exposed to the back fed voltage. One way is to put a diod, but his is not acceptable solution as the sensor requires 5V not 4.X. Any ideas how I can accomplish that?Thank you in advance![ Specified attachment is not available ]

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2021, 06:34:03 pm
content: Use a crowbar. An SCR & a 5.6V zener diode, will short circuit the supply and blow the fuse, if the voltage is too high.https://www.electronics-notes.com/articles/analogue_circuits/thyristor-scr-triac/overvoltage-protection-crowbar-circuit.phphttps://en.wikipedia.org/wiki/Crowbar_(circuit)

author: Ian.M
author_rank: Super Contributor
timestamp: August 20, 2021, 06:49:51 pm
content: @SeaMaster,Your circuit can't work as drawn. Hint: you can never turn the sensor power off as it will be fed through the MOSFET body diode.

author: bdunham7
author_rank: Super Contributor
timestamp: August 20, 2021, 07:04:31 pm
content: As pointed out, your MOSFET is backwards. You could leave it that way, add a second MOSFET in series with the drain and source reversed, connect the gates together and then put a 5.6 or 6.2V zener from the output to the gates. Assuming your sensor power control circuit can sink 20mA or so, if there is overvoltage the zener will conduct and drive the gates high and the MOSFETs both off. No blown fuse and the whole system would just recover automatically once you connect it correctly.

author: ConKbot
author_rank: Super Contributor
timestamp: August 20, 2021, 11:29:19 pm
content: If you're area constrained, some "eFuse"s can replace the dual FET and PTC combo, and provide you with reverse current blocking. Just as an example, the TI TPS25947 seems like it may fit if the 0.5A minimum overcurrent limitation fits. TPS2640 if you want the overcurrent limit to be able to be set down to 0.1A etc... Other manufacturers make them too, and who knows what is actually available anymore.Bear in mind that not all of them provide reverse current protection. Some have dual mosfets internally to prevent reverse powering when off, but in your case, as soon as the micro commands it to turn on, it'll connect the 5V rail to the 15V on the output, so always-on reverse current blocking is needed.A discrete solution (as mentioned above) may end up cheaper, since you dont actually need the ideal-diode function, just for the FETs to turn/stay off with an over-voltage on the output. Along with having the chance of being able to source alternate parts if youre not buying all you may need, before the board even goes out to make sure you have stock with a leadtime under a yearAnd yeah, this is a good thing to protect from. Ive had a board with a processor and a bunch of widgets on it, that connected to another board with a connector which interfaced to the outside world. It had a 5V power on the connector to power more widgets that could get plugged in, but also a 12v battery pin. Unfortunately, the designer of the interface board was a little lax with deciding the pinout of the connector facing the world. The den of howler monkeys that is the software team managed to jam the connector in weird, bend a pin, short the battery+12v to the 5V, overvolt the e-fuse on the 5V line (6.5v max, meant for normal USB) which failed short as mosfets do, raised the 5v rail to 12v, blow out every dc/dc converter on the 5V rail (which are also 6-8V max, so their high-side FETs fail short also) which then presents 12v to a variety of 3.3-1.8v devices. And here I was thinking I was idiot-proofing it by putting an efuse on the 5V output, figuring the software team would manage to short it to ground at some point during development. But instead they found the single method of attack that could bypass that, and roast the entire board.With this kind of escalation, eventually I'll have HDMI and USB3 ports hardened against 240VAC.

author: SeaMaster
author_rank: Contributor
timestamp: August 21, 2021, 10:20:06 am
content: Thank you all for the comments.I decided to go with the following option:the downside is that I realized two PTC fuses are needed.One PTC to protect the 5V DC-DC converter if. the triac gets shorted by feedback voltage,and another one to protect the triac from overcurrent from the external source.I'm wondering if there is a simpler solution? Keep in mind that isolated DC-DC converter Im using to create the 5V line has short protection for max of 1 secondThe DC-DC Part# is PCSA1-S5-S5-S and here is link to the data sheethttps://www.cui.com/product/resource/pcsa1-s.pdf

author: mariush
author_rank: Super Contributor
timestamp: August 21, 2021, 11:23:17 am
content: You could also just use a very low drop linear regulator, to give the sensor 5v - 0.1..0.2v dropout voltage in the linear regulator.For example:Toshiba TAR5S50TE85LF :https://www.digikey.com/en/products/detail/toshiba-semiconductor-and-storage/TAR5S50TE85LF/6198755max 15v input, max 200mA output, 0.2v drop at 50mATI TPS76050 :https://www.digikey.com/en/products/detail/texas-instruments/TPS76050DBVR/13529200max 16v input, max 50mA output, 15mV drop at 1mA , 90mV at 10mA , 180mV at 50mA

author: Zero999
author_rank: Super Contributor
timestamp: August 21, 2021, 01:07:24 pm
content: Quote from: mariush on August 21, 2021, 11:23:17 amUnfortunately neither of those parts fit the bill.The TAR5S50TE85LF is not rated for the output voltage to exceed the input by 0.2V and the TPS76050 has a diode connected between the input and output, which will protect it, but still fry anything connected to the input, if 12V is connected to the output.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 21, 2021, 04:45:12 pm
content: What about raise the 5V rail to something like 5.2V and add a Schottky?

author: Ian.M
author_rank: Super Contributor
timestamp: August 21, 2021, 05:35:36 pm
content: You can get ideal diode controller ICs rated for over 30V, that work down to 4V, and low Vgs threshold 30V MOSFETs. Some of the ideal diode controllers can control a back to back pair of MOSFETs to also provide current limiting and shutdown, replacing much of your original circuit. If there is a risk of idiots applying mains to the sensor supply output pins and you need it to be able to withstand that, put a fast HRC fuse in line with the sensor supply terminal, and a beefy crowbar circuit set at say 28V from between the fuse and the ideal diode controller to ground, with a beefy antiparallel diode across the crowbar circuit to handle the negative half cycles. OTOH it may be preferable to simply have a pair of green LEDs across the 5V output in series with fusible resistors If both fusible resistors go open you know its been grossly abused so can reject the warranty claim and tell the customer that its chargeable because their installer has been an idiot!

author: bdunham7
author_rank: Super Contributor
timestamp: August 21, 2021, 05:40:03 pm
content: How much current does the sensor draw and how precise does its 5V supply have to be?Also, how much currentcanyour 5V PSU sink? Perhaps just one PTC by itself, or with a single 5.6V zener, would be good enough?

author: mon2
author_rank: Frequent Contributor
timestamp: August 22, 2021, 08:54:08 pm
content: https://www.ti.com/lit/ds/symlink/tps25947.pdf?ts=1629662947760

author: SeaMaster
author_rank: Contributor
timestamp: August 23, 2021, 09:47:54 pm
content: I must use isolated power, thus I use the 5V to 5V isolated DC-DC converter (PCSA1-S5-S5-S), therefore I can't use mon2 recommendation ofhttps://www.ti.com/lit/ds/symlink/tps25947.pdf?ts=1629662947760but thank you, interesting part to keep in mind for future projectsI have some concerns about. the speed of the PTC fuses but for now I'm going with the attached schematic unless there is strong objections?