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INA226 Reverse Polarity Protection

Other Parts Discussed in Thread: INA226, INA300, INA282, INA210

Hi,

I'm designing a device where I need to measure power and the INA226 is a great device for this task but I want to know if the VBUS pin is protected against reverse polarity or if there is an easy way to protect it from it like putting a high enough resistro in the VBUS pin to limit the current for any protection diode in that input.

In case of not having such a thing, I'm thinking of using a comparator in the inputs to activate a MOSFET to stop the flow of current if the polarity is reversed. I can't use a diode since it would just take too much power from the load.

Does anyone has any ideas that could help me ? 

Thank you.

  • Alex,

    As far as I know, the VBUS pin is subject to the same rules as the VIN+ and VIN- pins; that it can only go between -0.3V and +40V before damage can occur.  I'll confirm that statement but let's assume that's correct for now.

    The first question I have is "how much reverse polarity?"  If you're swinging to -40V, then this is going to be a problem.  The VBUS pin has an 830kOhm input impedance already, so I'm not sure series resistance will help.  Again, I'll confirm that.

    Let's talk more specifics.  What supply voltage are you going to run at?  What shunt resistor value and load current do you expect?  What kind of swing do you expect?  Is the VBUS pin required for this application?  I'm not sure I fully appreciate the concept of your design - could you explain it to me a little more or send a schematic or block diagram for me to review?  If you don't want to share it here publicly, just email me directly at bridgmon_jason@ti.com. 

    When you say you want to use a comparator to shut it down, you made me think about our INA300 - which is a high/low side, 0 to 36V common mode, bidirectional current shunt monitor with integrated comparator.  Like I said, I don't totally understand your application but I thought I would throw that out there as a potential solution you might not be aware of.

    Please let me know some more of the details and I'll be happy to help.  I'll post here again when I've confirmed the operating range of the VBUS pin.

    Thank you!

    Jason Bridgmon

  • Hello Jason,

    Thank you for your quick response. The idea is that the INA will be part of a diagnostic device that measures power for some other devices we have. So this device can be connected to other devices and the plugs used can be connected backwards so in this case we have to protect the INA from reverse polarity.

    Supply voltage will be 3.3V. The voltage in VBUS can go up to 35V so it could go from -35V to 35V we don't expect it to measure when is connected backwards but we don't want it to fail in that case. The shunt is 20 mOhms and we expect a maximum of to 5 amps.

    We've thought of dectecting the reverse voltage and immediately cutting the current path or adding schottky diodes to the inputs to clamp the voltage but that will affect the measurements.

    How absolute is that -0.3V limit? I'm asking because regular diodes with bigger drop can have less leakage than schottky.

    Do you need anything else? If you want I can send you a simple schematic or block diagram.

    Thank you.

    Alex Méndez.

  • Alex,

      Alex,


    That -0.3V limit is very firm.  Beyond that, damage could easily occur.

    My colleague and I drew up a couple circuits that might work for protecting your devices.  One is a diode solution that plays off the 3.3V supply (assuming the 3.3V supply is on when the board is plugged in backwards or otherwise.

    The other is a BJT solution that turns on when the Vbus is positive and off when negative.

    Now of course, without knowing the particulars of your design, we can't guarantee anything, so I would love to see a schematic and block diagram to further assist.

    Thank you!

    Jason Bridgmon

  • Hi Jason,

    Thank you for all the support. I like the BJT solution but it will introduce a relative big error to the measurement.  I have another possible solution which would be the same as your first with the diode connected to ground to clamp negative voltages. The problem is that it is dependent on the diode's forward voltage. Also I would have to protect the differential current inputs and the schottky diodes have too much leakage and it affects measurement. I will have to see about that.

    I have to measure voltage form 0V to 32V and current from 0 to 5A since it will be a diagnostic device.

  • Hi again,

    Could you give us feedback on the circuit proposed in the last reply? 

    Could it also be used for the differential inputs?

    What else do you need to know about our application? It is basically a power meter for diagnostic. We just need to protect it against overlvoltage and reverse polarity.

    Thank you.

  • Alex,

    This is the most straightforward solution but the diode clamp is at -0.7V (or around there, depending on current) and the device can't go below -0.3V, which is why we suggested the solutions we did.  I'll ask around for more circuit ideas and let you know what I find out.

    I expect the diode clamp could be used for the differential inputs as well, though it still suffers from the -0.7V problem.


    Jason Bridgmon

  • Hi Jason,

    Thank you. Using a germanium diode isn't good enough? Something like this: http://www.fairchildsemi.com/ds/RB/RB751S40.pdf

    I would need to look for a more suited diode.

    The other solution I was thinking of was to have a comparator connected to both inputs driving a MOSFET to cut the path in case of reverse polarity.

    Anyway, I can't think of anything better to protect the INA226 without affecting the measurements.

    I would greatly appreciate any other possible solution. Once tested I will share the results with you.

  • Alex,

    The germanium diode is close, for sure, but it cuts off at -0.37V with 1mA of current, and from the "Forward Voltage vs Temperature" plot, it looks like -0.3V is at about 600uA.  If you don't think the current will make the diode exceed -0.3V, then you could probably use it.

    The issue is that this is a process dependent voltage and exceeding it may cause damage, even with just a transient spike in some cases.  The last thing we want you to have are field failures.

    Regarding the idea with the MOSFET, are you thinking of that in place of the BJT in the circuit we suggested, or diode-connected?  One person here suggested that maybe a CMOS FET might have a forward voltage higher than the -0.3V limit.  I haven't looked too hard yet but that might be a good place to start.

    Is there no way to "key" the connectors to avoid plugging them in backwards?  I know this is more of a hardware solution but if it's not too late in the production phase, it might not be a bad idea.  Even just blocking an unused pin socket in a female connector and clipping the appropriate pin on the mating male connector would do it, post-assembly, if that's an option.

    Please do share your results with me, I am curious to know what works well.

    Regards,

    Jason Bridgmon

  • Hi Jason,

    Yes I thought of that but since the current will be limited by the resistor I add in series I could make it less than 600uA since the input bias current is 10uA, right?

    I was thinking of using the MOSFET to cut the path from the source to the load and the ground of my circuit so no current would flow.

    I will have to take a look at the CMOS FET option. 

    I also thought of adding a bridge rectifier made with low RDs MOSFETs to the inputs and sensing the input controlling the rectifier. So in case of reverse polarity I would reverse it with the MOSFETs. What do you think? In that case I could also measure negative voltage and negative current.

    We can't key the connectors we have to use banana plugs. This device will be kind of like a handheld multimeter so we have to prevent any misuse from the operator.

    Regards.

  • Hi Alex,

    Jason asked me to take a look at this.  This simulates well as a first start.  Some resistance should be added in series with each gate (to reduce current inrush).  Also, GND and V+ need to always be on and the highest and lowest potential on the board.  As an example: if Vbus is at 40V, then the V+ needs to be at 40V also. 

    I might be able to configure this to be powered from Vbus and GND by inserting diode pairs at each gate to ensure highest/lowest potential on the bulk.

    Let me know if this will work for you or if it needs power from Vbus and GND.

    Regards,

    Jamieson Wardall

    Sensing Applications

  • Hi Jamieson,

    Thank you for the information. I will keep that in mind. V+ and GND have to be in the highest and lowest potential respectively.

    Your setup is what I was thinking on using, but the MOSFET would have to be controlled by external circuitry since the source Vy could go lower than the VGS threshold of the MOSFETs but also they would have to be logic level MOSFETs or I would have to use a charge pump since the device will be battery powered.

    In addition the best MOSFETs I could find for this can't handle the power dissipation needed in some cases and having two or more MOSFETs for each path would be too many.

    We might need to adjust the requirements or find any way to use key connectors.

    We can only think of a solution using a relay which we could use to cut the path in case the polarity was inverted. The problem is that it would take about 10ms to cut the current path. We would use an SPDT relay to only consume when it has to protect. 

    Do you think that 10ms having the germanium clamping diodes could be a good enough solution?

  • Hi Alex,

    It will depend on how much current enters the pin.  Since the current into the pin is unknown and the pin voltage is violated, I wouldn't recommend it.

    I think key connectors will be the safest and most reliable.

    Regards,

    Jamieson

  • Hi,

    Sorry for taking so long, I forgot about the forum because I've been very busy. 

    We have solved the problem using a MOSFET with a comparator in the ground path that connects our circuit from the test circuit. So if the comparator senses a reverse voltage the MOSFET is shut off by the comparator and grounds are split.

    So far in our first prototype worked well. We are waiting to get boards back to test it further. 

    Regards,

    Alex

  • It's a long time there was no activity on this topic.

    I have the same design problem what was discussed I try to brush it a bit.

    I am developing a Battery Tester unit and got to the same problem to solve.

    My solution plan for VBUS is a differential opamp input amplifier where the input/output is easier to clamp.

    I plan to use of a decoupling relay at the input controlled by the processor. Based on a simple polarity converter the processor decides to switch the relay or not.

    The backdraws of this solution is the need of simmetrical supply and you have to divide and recalculate the VBUS value according to the max output swing of the opamp.

    And the advantage is the possibility of the 4 wire measurement.

    I have no idea to protect from negative transients the IN+- current sensor inputs.

    There is a very nice TVS solution for INA282 here: www.ti.com/.../tidr413.pdf

    But INA282 has -14V +80V common mode input, and differential +-5V what is much comfortable from this point.

    And there are a suggested TVS solution for INA210 family in the datasheet  where the common mode input is similar -0.3 to +26V.

    I thought this can be used for the INA226 too.

    But studiing the SMAJ, SMBJ unidirectional devices I see the Forward voltage is min 3,5V, using a simple diode was discussed above.

    Therefore this protection in the INA210 datasheet looks not suitable for our purpose.

    Those sources are fine for positive transient solutions for VBUS and IN+, IN- at 40V, but there is no solution for the -0.3V limit.

    It means the INA226 (and INA210-215) could be damaged, when negative transients are present in the circuit. 

    Is there any safe solution?

    Thanks, Csaba

  • Csaba,

    The INA226 is a digital output part, and the others you mention are analog. Is your end goal an analog output or digital output?

    Regarding the -0.3V input limit, it is process defined and exceeding it will cause damage. I believe the MOSFET protection circuit will work for small currents and with particular MOSFETs, but you have to be careful.

    Using something with a wider range, biasing the input common mode up, or keying the hardware are all good solutions for protecting against reverse polarity hookups.

    Regarding transients, the INA210-215 family has the A and B device designators. The B devices are more robust when it comes to transient protection and avoid most latching problems with fast transients. Inserting filters and voltage limiting zeners can protect against artifacts on the inputs as well, so long as the filter resistance is <10 ohms on each sense line (for most current shunt INAs).

    I hope this helps,