Logically preventing backflow of current - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 09, 2025, 02:43:21 pm
content: I was playing around with this PMOS reverse protection circuit shown here in LTspice when I noticed a flaw with it. I thought that it was important to note, so I wanted to detail what I've found here. Perhaps it might be useful for others in a similar situation. The video shows the load as a current source (I assume that this load would be resistive). However, what if the load was a capacitor instead? Then, if the main voltage source was turned off, you have a charged capacitor on the source side, which can lead to a current backflow condition. I know that this is a simulation and real power supplies may not turn off as fast as this source, but I found this to be a curious side effect of this circuit when not properly designed.I've included some pictures to show what I've found. In the first two images, 01 and 02, the PMOS resistors were chosen to give a gate-source voltage of -1 volt. For reference's sake, the PMOS gives a gate threshold voltage rating of -1 to -2.5 volts. When the source turns off quickly, the PMOS also turns off quickly with slight sudden backflow of ~180 mA. Now, in the next two pictures, 03 and 04, the gate-source voltage was increased to -1.8 volts by adjusting R1. Now, when the source turns off, the backflow current is shown to be 7 amps. That's bad.You can add some small gate capacitors to account for the sudden change of voltage to mitigate this situation, but aside from that, it all boils down to the gate-source voltage. You have to keep it within the limits of the PMOS, else you get this backflow situation. This places an inherent upper limit on the input voltage, because if the input voltage is too high, the gate-source voltage would be over the threshold leading to backflow current when the source turns off. Interestingly enough, I was able to turn on the PMOS even when the gate-source voltage was below the rated threshold. If you look at the fifth picture, I lowered the input voltage to 5 volts, and the PMOS still conducted even though the gate-source voltage was way below the minimum. I can't explain this last part, but I found my findings to be interesting and wanted to bring it up.

author: jwet
author_rank: Frequent Contributor
timestamp: October 09, 2025, 06:09:57 pm
content: These PMOS reverse polarity protectors are well known for this. They really don't work well in automotive apps for this and some other reasons. In automotive, there is a standard test (ISO 16750), where the input is cycled rapidly, this can happen in low battery start conditions (that chatter you get just before you call AAA). In this condition, the output cap is rapidly charged and discharged and can heat up and fail. These PMOS circuits work in some situations but NMOS FET's with a boost pump, etc are preferred for anything very sophisticated.These seemingly innocent looking circuits hide little devils in the details. That's why we good engineers make the big bucks!

author: BadeBhaiya
author_rank: Regular Contributor
timestamp: October 09, 2025, 06:31:27 pm
content: This reverse polarity protection circuit is usually used right at the power connectors in a circuit, to protect from a erroneous reverse plugging of the connector. Its not a 1:1 replacement for a diode as a reverse polarity protection. Some care is indeed required.And yes, you're right about the Vgs situation as well, it will operate but only within a voltage range. Its a good enough circuit for selectively choosing power sources on a battery powered circuit or something similar.

author: Zero999
author_rank: Super Contributor
timestamp: October 09, 2025, 07:01:44 pm
content: The circuit is working, because it will block the power if the voltage source is reversed.Note that it is not an ideal diode circuit. Current will go in the reverse direction, if the voltages at both the input and output nodes are positive.It's important to note that reverse polarity protection is not the same as a diode. It's similar to how overcurrent protection is not always the same as a constant current source.

author: Smokey
author_rank: Super Contributor
timestamp: October 09, 2025, 08:45:13 pm
content: LM74502Automotive rated. NMOS charge pump. Either basic ideal diode or full High side switch configurations. Inrush limiting. Over and undervoltage protection. Fast switching variant. Small package.https://www.digikey.com/en/products/detail/texas-instruments/LM74502QDDFRQ1/16553864

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 10, 2025, 02:24:34 pm
content: Quote from: BadeBhaiya on October 09, 2025, 06:31:27 pmDesigning for trade-offs is difficult. So many solutions, both good and bad, but there's always a cost.

author: PGPG
author_rank: Frequent Contributor
timestamp: October 10, 2025, 03:10:35 pm
content: Quote from: LoveLaika on October 09, 2025, 02:43:21 pmIt was exactly my conclusion when I sow this circuit for the first time so I assumed that it can be used to protect against reverse battery insertion and probably to nothing more.When protecting supply input from reverse polarity I don't think of fast switching supply off but about coming during normal operation a negative surge pulse. I assume 1kV 50us through 40 ohm resistor.People in EMC lab told me that (DC supplied) tested devices much more often have problems with negative surge than with positive.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 10, 2025, 07:26:40 pm
content: In your experience, if the sim shows a reverse backflow current of ~400 mA, would that be tolerable for most power supplies? Certainly, it's better to have no backflow at all, but what if it can't be helped?In my sim, I'm toying with the idea of that PMOS protection with an appropriate zener. A zener between the gate and source would help keep the aforementioned PMOS within the threshold range, so when the power supply shuts off suddenly, there's little backflow. Plus, with this, you can use higher voltages.On the other hand, there's this ideal diode I'm toying with. In simulation, it certainly prevents backflow well with even less backflow than the PMOS, and it's easy enough to modify for reverse-voltage rotection. The input voltage and current is limited though with this IC, and there seems to be a larger voltage drop (~1 volt) as a result of it.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 10, 2025, 07:55:03 pm
content: I hate to say it, but can some supplies tolerate backflow, or is this something to totally avoid at all costs? A zener diode between gate and source would help control things, but there's no zener that's low enough. Using a 1.8V Zener diode in simulation, I'm getting a backflow of ~400 mA. That seems relatively small.

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2025, 04:25:17 pm
content: Quote from: LoveLaika on October 10, 2025, 07:55:03 pmA zener diode will not prevent backflow. All it will do is limit the gate voltage.Not all power supplies are damaged by backflow. The output voltage will just rise in most cases and nothing will be damaged, so long as the voltage ratings of everything in the circuit aren't exceeded.

author: ejeffrey
author_rank: Super Contributor
timestamp: October 11, 2025, 04:58:18 pm
content: Yes if you want something that offers more complete protection it's better to do it with a special purpose controller rather than a simple passively biased pfet switch. There are power monitors or eFuse devices that can monitor for over voltage, under voltage, over current, or reverse current and trigger a charge pump N channel fet switch to disconnect the power. It's more complex than a simple pfet switch like this but protects against a lot more.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 15, 2025, 07:41:43 pm
content: I think that what I'm also facing in my sims are (quite possibly) unrealistic rise/fall times and how it affects the circuit. LTspice's startup parameter starts DC voltages cold, but the rise time remains 20 us regardless of the voltage value. Is that realistic, or is a typical turn-on time much slower? Fiddling around with LTspice's example of the LTC4357 with a high capacitive load indicates a big difference in reverse current protection depending on how fast the input voltage turns on and off. Despite the IC claiming a gate turn-off time of 500 ns, it doesn't turn off if the rise/fall time is too small.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 15, 2025, 11:07:16 pm
content: Quote from: Zero999 on October 11, 2025, 04:25:17 pmSorry, I should explain. If the PMOS gate-source voltage is too high, the PMOS won't shut off in time to prevent reverse backflow of current. A resistive divider can only set the voltage when fixed, but using a Zener diode between gate and source will help maintain correct voltage for a wider variety of voltages. I found that a zener close to the low end/within the range of the threshold voltage helps.Really, I keep looking for a wide variety of solutions, but they all have their own limitations. Either they don't work at the simulated turn-off speed or they are limited in input voltage.

author: Zero999
author_rank: Super Contributor
timestamp: October 17, 2025 01:13:43 pm
content: Quote from: LoveLaika on October 15, 2025, 11:07:16 pmHave you tried putting a Schottky diode across the output of the reverse polarity protection circuit? There still will be a current spike, but it will clamp the voltage to well under 1V.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: October 17, 2025 10:37:36 pm
content: Still got a massive reverse backflow of current for ~20 us. Right now, the only solution that seems to be the most convenient are these ideal diode ICs. They have a 500-nanosecond response time, much faster than what I'm getting with the PMOS by itself. Plus, thinking about the whole solution helped me be realistic of what the max input voltage should be.Running the LTC4358 in a sim, I'm getting nanosecond current spikes at a smaller amplitude, though all this is really dependent on the fall time of my power source. I'm honestly not sure what's a realistic rise/fall time for them.

author: MrAl
author_rank: Super Contributor
timestamp: October 17, 2025 11:06:10 pm
content: Quote from: LoveLaika on October 09, 2025, 02:43:21 pmHello there,There are many times when this sort of thing happens. It means that we always have to be aware of the power source and the load in case the load can supply power to the input source. It's actually very common to see this.The most striking example is probably a battery charger, but this phenomenon is not limited to that application by any means, and can be found in a lot of circuits that we might not suspect of having this problem, it's just that it may not show up right away.A more subtle example is with an inductor. A diode is used to absorb the back emf when some switch is turned off. The diode is connected from the inductor to the power source positive rail. If the inductor kicks back enough it can overpower the positive rail thus causing the voltage to rise say from 5v to some higher voltage like 8v, which can blow components out right away.The trick there is to always make sure the power source can absorb any extraneous current flows that could happen at some time or another. Sometimes an extra resistor from the rail to ground does the trick because it absorbs the extra energy. The costs for that of course is lower efficiency.You can also note that a lot of linear positive voltage regulators can output current, but they cannot sink current. That's the problem with a lot of circuits. Anything that powers the positive rail will easily force the voltage up higher than it should be, which means it's no longer regulated. It's only when the circuit can absorb the extra energy that it works ok because the linear regulator cannot do that unless it is a special design.I mentioned the battery charger because if the charging circuit is turned off, the battery can power the circuit which can cause problems. It has to be checked for that and bench tested. The circuit also has to have reverse polarity protection.

author: Zero999
author_rank: Super Contributor
timestamp: «Reply #16 on:Todayat 10:01:07 am »
content: Quote from: LoveLaika onYesterdayat 10:37:36 pmWhy is it a problem? Have you built any real circuits or just simulated? You'll find it won't be as bad in real life, because parasistics (inducatance and resistance) will limit the current and it's too shorter duration to cause any damage.Also note that this is a reverse polarity circuit, not an ideal diode. All it does is prevent the output voltage going negative. It does not prevent reverse current flow, when the output voltage is greater than the input. Again, in real life, a capacitor on the output is not going to damage the power suppy when it's turned off, assuming said power supply is correctly designed.Your simulation is most likely wrong because the power supply's output doesn't go into a low impedance short circuit, when it is switched off. It will typically look like a moderate to high value resistor.