Reverse Polarity Protection - Page 1
Forum section: electronics / beginners

author: jrozner
author_rank: Contributor
timestamp: June 06, 2017, 07:57:22 pm
content: I've been reading about implementing reverse polarity protection and want to add some for a battery powered project I'm working on. Everything I've seen for common solutions (diode, schottky diode, p/n channel mosfet) discusses that diodes are typically the cheapest but have a large voltage drop, schottky diodes are a bit more expensive but have a lesser drop, and p or n mosfets have the lowest voltage drop but cost the most. All of the solutions I've seen are based around larger voltages, powering things like light bulbs, and are powering stuff that either needs lesser or equal voltage to the power source. In my case I'm using a CR123A battery (~3v) and I want the circuit to run between 3-3.3v for powering the microcontroller and everything else. My concern is that any voltage drop is going to be an issue off the bat because at best case the battery is only going to deliver 3v max. Is one of those solutions better than the other in this circumstance and would I want to use something like a boost converter after it to bring the voltage back up after the drop or is there a better solution for this that I should be looking at?

author: danadak
author_rank: Super Contributor
timestamp: June 06, 2017, 08:12:21 pm
content: The MOSFET best approach, but your LV environment is a challenge to drive it into saturation.Although you can get sub 1 mOhm MOSFETS at a gate drive of 2.5 VA low Vcesat bipolar might work -http://www.onsemi.com/pub/Collateral/NSS20300MR6T1G-D.PDFA boost compounds because it has its own efficiency issues. Stated another way losses.Regards, Dana.

author: stevelup
author_rank: Regular Contributor
timestamp: June 08, 2017, 11:41:05 am
content: We put what we colloquially refer to as an 'idiot diode'acrossthe input of our devices.

author: mikerj
author_rank: Super Contributor
timestamp: June 08, 2017, 12:18:28 pm
content: Quote from: stevelup on June 08, 2017, 11:41:05 amWhich works well provided there is some kind of overcurrent protection as well, e.g. polyfuse.

author: Naluto
author_rank: Newbie
timestamp: June 08, 2017, 02:18:34 pm
content: Quote from: danadak on June 06, 2017, 08:12:21 pmHello,I saw a simple reverse polarity protection circuit that contained of a P-Mosfet that was simply put in the power line from the voltage source to the load with the gate connected to ground via a resistor.Like this for example: (See page 7 Figure 5)http://www.infineon.com/dgdl/Reverse-Batery-Protection-Rev2.pdf?fileId=db3a304412b407950112b41887722615Let's say I want to protect a circuit that get's powered with 9V in that configuration.The Drain-Source Voltage of the P-Mosfet would have to be above these 9V so it doesn't get destroyed with the reverse polarity.For example one with VDS=-20V.Now that Mosfet has a maximum VGSof +-8V. That means, if I would connect the Gate straight to ground it would get destroyed from the -9V I would have there on normal operation.The question I have is:Is the Zener Diode necessary? What would happen if you build that circuit without the Zener diode?And how does one dimension either case? (with and without the zener)The zener voltage should be below the maximum VGS. So if Vgs is +-8V, something like a 6.8VBZX79C6V8Zener?But with a Zener diode there, wouldn't I always conduct current towards ground if I have more than Vr over the Zener?Do you calculate with the ZZK(and a voltage below the min Vz of 6.4V) and just have the IZKflow towards ground?Would appreciate some help.

author: ali6x944
author_rank: Frequent Contributor
timestamp: June 08, 2017, 07:11:25 pm
content: Quote from: danadak on June 06, 2017, 08:12:21 pmI agree with Dana completely on the points brought up, and I want to ask a question about the current draw of ur circuit, at high collector currents the base-on voltage and the collector-emitter saturation voltage increase with a drop in DC current gain, this may put a lot of constraints on u due to the tight operating voltage, away to lower this effect bipolar transistors is to use NPN instead of PNP because they usually offer higher current gain and and operating speeds and lower cost in comparison to PNP transistors.