Replace Schottky with Mosfet

Discussion in 'General Electronics Chat' started by Vindhyachal Takniki, Mar 19, 2016.

  1. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
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    1. I have a application in which I have schottky connected at input. At input a solar panel is connected & at output battery is connected along with charge controller. Schottky serves two purposes; one is protect the circuit in case reverse connection of panel are connected & second prevents reverse flow of current from battery to solar panel known as dark current.

    2. Max current from panel is 10A, so I have connected two parallel schottky PDS1040. Datasheet of PDs1040 state max Vf drop of diode around 0.55V at 10A(wrost case Tj=-65) & 0.52V at 5A.
    Max panel voltage goes to around 21V.

    3. These two diodes gets heat up very quickly & I want to replace them with Mosfet. Now I was looking for how I can replace it with Mosfet.

    4. First option is to use ideal diode. I have checked linear LTC4358, which is for 5A. Operating voltage range is 9V-26.5V. I am thinking I will connect these two in parallel. One problem is its lower voltage range is 9V so from datasheet I didn't understand what will happen when voltage falls below 9V, will it turn off, or it may partially tun on which can increase the voltage across mosfet & heat it up. Second it very costly around $5.38 unit price.

    5. Other option I found here ( https://www.google.co.in/search?q=i...hXQwI4KHeDWAFcQ_AUIBigB#imgrc=CPkV_TNPueyWpM: )

    It uses p channel mosfet & BCM856DS. Its an doubled matched transistor.

    6. Any other circuit which I can use?
     
  2. crutschow

    Expert

    Mar 14, 2008
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    Option 4 should work.
    I'm not sure what it will do below 9V but when will you be operating at such a low voltage?

    Option 5 will work as long as the PNP's are matched as shown.

    Don't know of any other circuit that will do what you want.
     
  3. RamaD

    Active Member

    Dec 4, 2009
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    One issue with option 4 is that reverse polarity protection will not be there. Not only that, reverse voltage application will take the LT4358 also down with that as the minimum voltage is -0.3V only.
    Below 9V, MOSFET will not turn on, and body diode will come into play. Its working depends on whether it is forward biased, and how much current is flowing through it.

    Circuit required is a normal reverse polarity protection circuit, which should be turned on only when the panel voltage is higher than the battery voltage.
     
  4. N11778

    Member

    Dec 4, 2015
    39
    7
    How hot is hot ? Rule of ouch. can you stand touching the pds1040's
    if you can keep your finger on them gritting your teeth it should be ok :)
    Looks like they want them connected to copper runs on a ckt board for a heat sink.
    they should only dissipate 3 watts worst case
    do you really need them. Normally the charge controller has that protection built in.
     
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Agree with Rama. This can be as simple as two power MOSFETs, one resistor, and one zener (because of the 21 V max). The two FETs are connected in series, drain-to-drain*. 12 V zener from both gates to the input side source, 10 K resistor from both gates to GND.

    * Note - might be source to source; not at my machine, working from memory.

    ak
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Don't understand how that can work as an ideal diode and still block reverse current when the panel loses its illumination? :confused:
     
  7. Roderick Young

    Member

    Feb 22, 2015
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    Might I suggest
    : SolarMOSFET.GIF
    The circuit does not need to be fast, but low voltage drop across the MOSFET is a plus, so use one rated for a very high current. You can find MOSFETs with an on resistance of just a milliohm or so. They have huge gate charge, and are a bear to drive if you need to do switching, but in your case, that doesn't matter. Note that the transistor is installed with source at a slightly higher potential than the drain. This is backwards from many hobby switching applications, but the normal connection for synchronous rectification. The body diode shown in the schematic is actually part of the transistor, not a separate part. Virtually all high-current MOSFETs have this diode. The diode will conduct in operation, but the transistor will carry the bulk of the current.

    This is an N-channel MOSFET, because N-channels have better characteristics. You might be able to get away with minimal to no heat sinking, since the transistor will be essentially always on during the day, and always off during the night. If you're worried about the transistor being partially on, and dissipating a lot of power, right at the transition between day and night, you can try a heat sink, then remove it if the transistor seems fine. I think you will find that your solar panel's power falls off very rapidly under such conditions, and that when the gate voltage of the transistor falls low enough to put the transistor in the linear region, the panel will be putting out minimal current. But don't take my word for it, experiment. You can also choose R1 and R2 (see below) to make the transistor turn off as soon as the panel voltage goes below the expected battery voltage, but at the penalty of losing a lot of potential charging time.

    R1 and R2 are chosen in a ratio to insure that Q1's maximum Vgs will never be exceeded in normal operation. That would depend on the transistor, but a start would be R1=470k, R2=1.5M . The values can be high, because the circuit does not need to be fast. Note that this also limits the negative-going gate voltage to acceptable limits in the event that the solar panel is hooked up backwards.

    I'm going to assume that you have taken care of the details of charging the battery. Simply hooking up a panel capable of 10 amps directly to a battery is generally not a good idea.
     
  8. BR-549

    Well-Known Member

    Sep 22, 2013
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    I've done a lot of small system installs, but no designing.

    I recall the circuit that ak is referring to, but I can't recall where I saw it, or what it was called.
     
  9. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Many of Linear Tech's hot swap, electronic circuit breaker, and ideal diode app circuits use the dual-MOSFET technique to guarantee that off means OFF. I first saw it in a Siliconix (now part of Vishay?) app note in the late 60's. It was an audio muting circuit, and it may have used J-FETs. Whatever. Many years later, Unitrode was about to be sued for patent infringement over the technique. One of their apps guys mentioned it at one of their power supply design seminars, so I sent him a copy of my "first edition". The suit (or that part of it) went away.

    ak
     
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  10. crutschow

    Expert

    Mar 14, 2008
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    I don't see how that can block reverse current since the MOSFET Vgs is always forward biased by the battery, thus keeping it on for both forward and reverse currents.
     
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  11. Roderick Young

    Member

    Feb 22, 2015
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    DOH! You're right. What if we swap the positions of battery and solar panel, and move R2 between the gate and source of Q1? Need to think about it.

    EDIT: no, that won't work either. The diode would still conduct reverse current. Rats. If I was doing this, at this, at this point, I'd probably give up and use a polarized connector to insure correct polarity of the panel.
     
    Last edited: Mar 19, 2016
  12. dannyf

    Well-Known Member

    Sep 13, 2015
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    Use p CH MOSFET to break the high side or a n CH MOSFET to break the ground. Make sure the g-s sees the load. May need a zener plus resistor to protect the g-s.

    Irf has an app note on this from way back.
     
  13. RamaD

    Active Member

    Dec 4, 2009
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    @crutschow
    A N-channel MOSFET on the ground side in the attached circuit.
    View attachment buckwsaac.asc

    1. When panel is connected with reverse polarity, D1 blocks it. Will have Battery voltage + Panel voltage applied on the reverse.
    2. When panel is connected with correct polarity, D1 is reverse biased till the PV is higher than panel voltage.

    When panel voltage is higher than battery voltage, MOSFET M3 can be turned on, providing just the IR drop of the MOSFET. D1 & D2 are the body diodes of the respective MOSFETs.
    I hope everything is alright...
     
  14. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Something I deployed over 10 years ago into MIL vehicles, still going strong. Updated here to show a solar panel instead of a 1275 source. It uses an exceptional and relatively expensive FET, but we had a barrel of them left over from something else so they were free to the project. Just about any 40 V N-channel power MOSFET will work; lower Rdson = $.

    ak
    ReverseDiode-1-c.gif
     
  15. crutschow

    Expert

    Mar 14, 2008
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    That will block a reverse voltage connection but not any reverse current when the solar panel looses illumination.
     
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
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  17. MrAl

    Well-Known Member

    Jun 17, 2014
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    Hi,

    One thing i noticed about all of the circuits i have seen here so far is that none of them have any hysteresis.
    Hysteresis keeps the array connected to the battery after the MOSFET turns on.
    Without that, it will oscillate. There may be other ways to connect though.
    For example, in one circuit the diode conducting allows the array to be maybe 0.5v higher than the battery, and that means we have a 0.5v signal to detect. After the MOSFET turns on however, we may only have 0.1v and that may not be enough because the gate voltage (without some amplification) may start to turn off the device again.
    For an example of how this might work, i'll throw out some numbers just for illustration. Say if the array went up to 12v we want the MOSFET to turn on because the battery is 11.5v. That 0.5v tells us to turn on the MOSFET, or the 12v tells us that. Once the MOSFET turns on, the array voltage drops down to meet the battery voltage unless there is enough series resistance to prevent this. If not, the voltage drops back down to around 11.5v and turns the MOSFET off again.
    If we had some hysteresis built in, it would not turn off at 12v just because it turned on at 12v, it may turn off at 11v or something like that, or simply when the array voltage becomes lower than the battery voltage.
    That's just a basic run through, but you get the idea :)


    Edit:
    I just saw the last post before this one. However, even an electronic "ideal diode" could have this problem. Check to make sure.
     
    Last edited: Mar 21, 2016
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  18. crutschow

    Expert

    Mar 14, 2008
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    After some thought I realized that reverse connection protection is not possible with a series diode, since the current direction with a reverse connection of the battery is the same direction as the normal charging current direction when the panel is illuminated.
    You can however, protect against reverse voltage with a diode connected in parallel across the panel (cathode to plus panel output) with a fuse or circuit breaker in series. That way, with a reverse connection of the battery, the diode will conduct and blow the fuse.

    To prevent battery discharge when the panel is not illuminated, you could use a MOSFET circuit such as your option 5 or the circuit posted by AK in post #16.
     
  19. Vindhyachal Takniki

    Thread Starter Member

    Nov 3, 2014
    348
    6
    I want reverse polarity protection fro solar panel only & reverse current going into panel.
    Currently I have decided that I will place three PDS1040 in parallel & large coper pads with vias on it on PCB for 10A current & will see the result.

    Since ideal diode from linear is around $5.
     
  20. crutschow

    Expert

    Mar 14, 2008
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    You do understand that, as I noted in post #18, a series diode will not protect against reverse polarity.
     
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