Over Voltage and Reverse Polarity Protection - Page 1
Forum section: electronics / projects

author: pallav.aggarwal@gmail.com
author_rank: Contributor
timestamp: July 11, 2016, 10:51:35 am
content: Hello Friends,I am building an equipment which can take power supply input from 0.5V to 120V DC and it consumes 10 -16 Amps and i need to implement over voltage and reverse polarity protection.Some ways i could think of are:1. MOVs(for over voltage) & a reverse diode(for reverse polarity) and a series fuse on the input terminal but it will involve fuse replacement which is not an ideal situation user would want.2. A circuit(powered by different power supply) that detects the voltage/polarity and takes a decision to turn PMOSFET (using some Mosfet driver)/Relay on or not.3. Power Diode Bridge removes the possibility of reverse condition but then i want it to be working till 0.5V. Also a separate over voltage detection and cut off mechanism.I am thinking more towards option 2.Can any one please help with some suggestions on how to implement an over voltage protection (protecting till 150V DC) and also protects from reverse condition.Thank you,Regards/Pallav

author: danadak
author_rank: Super Contributor
timestamp: July 11, 2016, 12:49:22 pm
content: SCR as a crowbar to blow a fuse.....http://www.linear.com/solutions/1648Google "scr crowbar circuits", lots of hits.Regards, Dana.

author: pallav.aggarwal@gmail.com
author_rank: Contributor
timestamp: July 11, 2016, 01:02:50 pm
content: Thank you very much for your input and help.But i am thinking in a direction, where circuit will cut off the equipment from power supply , instead of blowing a fuse.I even considered if i could use :http://www.linear.com/product/LTC4366,but that will not work @ 0.5V and reverse protection is not available. Otherwise its a clever device.

author: aandrew
author_rank: Frequent Contributor
timestamp: July 11, 2016, 01:12:36 pm
content: I've not built one for 150VDC before, but I've been a fan of this mosfet/zener protection circuit.Q5 protects against reverse voltage. The body diode LOOKS like it's causing trouble but when input voltage is reverse-biased the diode is also reverse-biased.The rest of the circuit is an overvoltage cutoff. The zener and bias resistor need to be chosen with Q6's Vgs(th) in mind; you want to set it so that Q6 turns on sufficiently when you approach your "too high" input voltage level. R99 holds Q4 on in the default state.Q4 and Q5 need to be able to handle your rated current; Q6 can be something (much) smaller. For 150VDC I think you might have some trouble finding mosfets with sufficiently high gate voltage ratings.

author: bktemp
author_rank: Super Contributor
timestamp: July 11, 2016, 01:21:03 pm
content: Quote from: pallav.aggarwal@gmail.com on July 11, 2016, 01:02:50 pmFigure 8 on page 20 adds reverse protection.If you connect Rss to a negative voltage, the circuit should work at 0.5V.

author: pallav.aggarwal@gmail.com
author_rank: Contributor
timestamp: July 11, 2016, 02:21:53 pm
content: In LTC4366 datasheet figure 8(page 20), could not understand the rev polarity protection circuit.example @ 100V DC in rev, R6, D4 and R5 will conduct and R5 will see approx 62V, how will MMBT3904 bear 62V VBE? Am i getting it wrong, sorry for stupid question

author: bktemp
author_rank: Super Contributor
timestamp: July 11, 2016, 02:41:44 pm
content: Quote from: pallav.aggarwal@gmail.com on July 11, 2016, 02:21:53 pmThere won't be 62V on VBE, because the BE diode will be forward biased. The transistor will start conducting, shorting gate and source of the mosfet, turning the mosfet off. Therefore the mosfet will block the reverse voltage.

author: Donziboy2
author_rank: Contributor
timestamp: July 12, 2016, 10:11:10 am
content: Quote from: aandrew on July 11, 2016, 01:12:36 pmInteresting, a zener on the gates with limiting resistors for both would protect them from higher voltages.Just an FYI, most protection circuits will have the drawback of dropping some voltage.One thing I played with on another project was measuring the input voltage and using a contactor/relay to close or open based on the voltage.My reason for not using it was mainly cost and the amount of power needed exceeding my designs available reserves.D4 protects against reverse polarity.D3 protects the opamp from high voltage.R31 limits current to both D3 and D4.R28 and R29 set the minimum voltage.The circuit needs an npn or nmos to drive instead of driving it with the LM324. I dont have a schematic with the full circuit I used anymore.

author: Donziboy2
author_rank: Contributor
timestamp: July 12, 2016, 10:59:02 am
content: Here is a respin of aandrew's design using the parts I picked for my CC Dummy Load system.One issue I see is oscillation in some zones when there is inductance on the input side. Needs looking into before implementing. Maybe someone smarter then me will chime in.As you can see for this example, the Vload is 0v when Vinput is above 100V or below 0V.

author: bktemp
author_rank: Super Contributor
timestamp: July 12, 2016, 11:17:13 am
content: One problem with this design: It has a finite gain at the transition point. If the voltage rises slowly, it may operate in linear mode and dissipate a lot of power (100V, 10-16A = >1kW).But you can solve it probably by adding a small hysteresis. This should also suppress the oscillations. Adding a resistor between the output and the gate of M7 should provide some hysteresis.I wouldn't use P-channel mosfets here, because at 10A even mosfets with 50mOhm RDson will dissipate at least 10W. N-channel mosfets typically have a lower RDson, but you need a highside driver if switching the negative rail is not possible.

author: Donziboy2
author_rank: Contributor
timestamp: July 12, 2016, 12:27:31 pm
content: That makes a big differenceAdded a large load resistor and cap to make sure its stable. Seems to be happy. Before it was going nuts on restart.This is definitely something to try before making pcb's lol. And it would need some heat sinking. Assume 1W per A and your looking at 16W x 2.Between using the Relay approach or Pmos it really comes down to what you want/need and how it fits into your product.The relay will need logic voltage for the opamp/comparator along with a good amount of current from logic to power the coil. The relays can get expensive if your not careful.The Pmos will cause a Vdrop and will require heatsinking. Assume for safety that you need 30-35W of combined cooling for the 2 large Pmos.Either case you should plan for a fuse. Fuses aren't to protect the product they are to protect the customer against fire. Which protects you against lawsuits lol.

author: aandrew
author_rank: Frequent Contributor
timestamp: July 12, 2016, 01:39:40 pm
content: Quote from: bktemp on July 12, 2016, 11:17:13 amAbsolutely correct; the usual application for this (for me) is to protect against the wrong wall wart being used, although I have also used this in automotive applications to protect against load dump conditions. Neither of these conditions "wander" but I agree that the transition zone is quite "soft" and absolutely depends on the specific mosfet part number.QuoteThis is a very slick way to add some hysteresis and prevent oscillation.

author: aandrew
author_rank: Frequent Contributor
timestamp: July 12, 2016, 02:02:00 pm
content: Quote from: Donziboy2 on July 12, 2016, 12:27:31 pmPerhaps it's because my bloodstream is not yet sufficiently caffeinated, but at 70mOhm, the RSJ250P10 would drop 70mV at 1A. Where are you getting 1W/A dissipation? The part is good for 25A continuous, which means it would be dissipating 25*25*.07 or 43W at full load. It's not case-to-ambient, but its "channel to ambient" rating is 2.5C/W so it'd be almost 110C above ambient. At 1A though would it not be rising 70mW*2.5C/W or slightly less than 0.2C per FET?Interestingly enough, its pulse rating graph on page 5 of the datasheet says 58C/W. I'm not sure how to interpret that.Something that I had forgotten to address in my circuit was protecting against excessive gate voltage. The particular FETs you chose don't need it (integrated protection diode) but I really like how you've neatly solved that with the 12V zeners and 10k resistors. One of those "duh" moments for me for sure.QuoteAlways fuse. Shit happens, and it's a lot nicer to have the fuse give up the ghost rather than a component sit there and simmer, glowing red hot until flame erupts. You'd think the semiconductor would just give up (and oftentimes they do) but it's not something you want to rely on. Use the fuse. Love the fuse.

author: bktemp
author_rank: Super Contributor
timestamp: July 12, 2016, 02:46:40 pm
content: I have tried to improve the circuit a little bit, because the simple circuit needs at least 4V to operate. The required input supply range down to 0.5V makes it a bit hard.Adding a negative bias voltage (let's call it auxillary gate voltage) helps turning the mosfets on even for low input voltages.But the auxillary gate voltage makes the reverse polarity protection difficult. R2 sets the hysteresis, D2+D3 set the overvoltage protection voltage, R3 limits the base current in case of overvoltage. R5 senses the input voltage and cuts the gate voltage if a negative voltage is sensed at the input. This works fine if you apply a negative before the mosfet is turned on. But if the mosfet is already on, in and source will be shorted, so it will keep conducting because Q1 sees no negative voltage. D4 shorts the negative voltage, increasing the voltage drop across the mosfet until it reaches 0.7V and Q1 switches everything off.For a true reverse polarity protection probably some more accurate polarity sensing is necessary.If cost is crucial I would use this circuit, otherwise I would use something more advanced like LTC4366 + N-channel fets for lower voltage drop.

author: dario200
author_rank: Newbie
timestamp: August 20, 2020, 09:14:58 pm
content: I know this is an old post, but I'm trying to find a protection circuit for an automotive application (using step-down switching regulator from 12V down to 5V @ 1A Max). I was wondering if the circuit with the two P-channel MOSFETs to protect for reverse polarity and overvoltage could be reduced to only one MOSFET instead of using two. This way the MOSFET should open whenever there is an overvoltage (for me it would be around 30V) to protect against load dump situations, and it should open as well whenever the voltage is reversed.Additionaly, I was thinking in replacing the zener and the small FET that controls the larger MOSFET's gate with a TL431, to provide a faster response in overvoltages situations. What do you think?Good work BTW!

author: rednoka
author_rank: Contributor
timestamp: October 11, 2021, 08:08:37 pm
content: Sorry but you need 2 mosfets because of body diodes of the mosfets. If u use one you can handle either reverse polarity OR overvoltage. You need back-to-back 2 mosfets for both.