10V @10A with output load disconnect

Discussion in 'The Projects Forum' started by mishra87, Aug 18, 2016.

  1. mishra87

    Thread Starter Member

    Jan 17, 2016
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    Hi all,

    I need to develop dc dc converter rated 5V@10A with output load disconnect. Means there should be shut down function which is enable by microcontroller and it disconnect the output when it is not in use to avoid the drainage of battery.

    Input is Li battery 3.7V.

    Could anybody suggest me a chip or EVM for testing and evaluation.

    Thanks in Advance !!!
     
  2. jpanhalt

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    Jan 18, 2008
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  3. mishra87

    Thread Starter Member

    Jan 17, 2016
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    I haven't test with load . let me check tommorrow and update you. we have work OFF today.

    I have posted the problem in TI community and reply was this IC TPS61088 does not have output load disconnect so the output won't get zero at any how.

    I was told it will deliever 5@5A while it written in datasheet 10A switch current .

    Is there any way to achieve the objectives . We have to work with multiple solutions in order to save time.

    I have tested attached one but it does not work.

    Thanks for your reply !!!

    20160817_222815_800x453.jpg
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Again...10V @ 10A or 5V @ 10A ?
    And Do you have a 3.7V, 30Amp li-on battery considering a 10V@10A converter ?
    And you can use a 20Amp DC Relay to switch off output
     
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  5. mishra87

    Thread Starter Member

    Jan 17, 2016
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    My application is 5V@10A.
     
  6. R!f@@

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    And do you have the 3.7V li-on cell which can supply above 13Amps
     
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  7. jpanhalt

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    Since you have a microcontroller on the enable pin, you could also control a logic-level power mosfet or even a small mosfet like a 2n7002 and a relay. Because you are sourcing current and may want your device and battery ground/common to be the same , I would probably go with a P-channel power mosfet. If you go the relay route, a simple N-channel will work.

    John

    Edit: Sorry Mishra, I did not see the schematic you attached to post # 3. That is basically what I was suggesting, except you might be able to get by with just the power mosfet (top one), unless you microcontroller is not 5V tolerant.
     
    Last edited: Aug 18, 2016
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  8. Papabravo

    Expert

    Feb 24, 2006
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    Your problem is unlikely to be the chip, but rather the inductor. A boost converter will need an inductor that can handle twice the maximum input current. That, and the low ESR capacitors you will require. That is where you should focus your design efforts. I would also recommend specifying a PCB with 3 oz. copper for this application.
     
  9. mishra87

    Thread Starter Member

    Jan 17, 2016
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    Is that any modifications I need to do in the attached schematic of post#3 because that din work.
    I do not know the reason why it did not work.

    Please share schematic according to your suggestions.

    We are operating microcontroller at 3V so my logic output will be 3V.

    Thanks
     
    Last edited: Aug 18, 2016
  10. mishra87

    Thread Starter Member

    Jan 17, 2016
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    We have size constraint so we can not go ahead high Amps rated relay.
     
  11. mishra87

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    Jan 17, 2016
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    Hi,

    i have tested with 10K and 20E resistor respectiviley .
    But i din see any fall in voltage it same as described above .
    EN=1, 5V
    EN=0, Vin-Vf

    So it does not work.
    Lets think about other options..!!!
     
  12. jpanhalt

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    Jan 18, 2008
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    Try controlling that P-mosfet with a switch directly. Be sure to observe the need for a common ground. Have the boost converter enabled.

    John
     
  13. mishra87

    Thread Starter Member

    Jan 17, 2016
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    Hi John,

    I have P-CH Mosfet namely STT7P2UH7.

    Now my connections are -

    1. 3V to DC-DC converter Input from Power Source.
    2. I have 5V output of DC-DC Boost converter
    3. P-CH MOS source is connected to 5V
    4. Gate is connected to a switch and 3V
    5. From Drain I am monitoring output w.r.t to GND.
    6. GND of DC-DC and power source is common and DC_DC Vin_GND and Vout_GND are common.

    Now result is it does not work . While floating the switch at gate end.
     
  14. jpanhalt

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    If the source is at 5 V (be sure the boost converter is enabled), the gate must be approximately 5 volts relative to ground to turn it off. Vgs threshold is about -1V, as I recall. The resistor you show should keep the mosfet off.

    Just connect the gate to ground to see if it turns on. I would add a small resistor (100 Ω) in that connection, just in case the connections are wrong and you create a short.

    John
     
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  15. mishra87

    Thread Starter Member

    Jan 17, 2016
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    Thank you so much once again John...!!

    I tried with attached schematic but it did not work.
    Could you please what else i need to add in this schematic to get it worked.
     
  16. jpanhalt

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    Jan 18, 2008
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    Here is what you did:
    upload_2016-8-20_5-54-56.png

    Here is what I suggested doing:

    upload_2016-8-20_6-4-40.png

    1) Why did you remove the pull-up resistor from the gate to the +5V?
    2) I suggested grounding the gate. Your connection doesn't do that. Did someone else tell you not to?
    3) When you say "doesn't work," that could mean a lot of things. Does it mean there is no voltage, the voltage stays on, it generates smoke, etc.? Please describe what it does or doesn't do.

    John
     
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  17. mishra87

    Thread Starter Member

    Jan 17, 2016
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    Well ,

    Please consider my test circuit for below explanations ----

    There were no smoke or something like that and what I wanted to tell you that I was getting always getting 5V output irrespective of changing the gate signal .

    Now how I was changing the gate signal-
    1. You can see there +3V which is the input of DC DC converter . To turn OFF the PMOS, The same voltage I was connecting to Gate of Mos and I was expecting 0V at output but I was getting 5V.

    2. Now to turn ON the PMOS i was connecting 0V i.e. Gnd to Gate of PMOS and result was 5V this time as well.

    Now let me discuss about your suggestions, as per your circuit-

    1. If I short Gate to Gnd , i theoretically agree that it will OFF the MOS and there's no output but then now how do turn ON the MOS because to Turn OF/OFF I will use controller signal.

    1. In post #14 you suggested
    " Just connect the gate to ground to see if it turns on. I would add a small resistor (100 Ω) in that connection, just in case the connections are wrong and you create a short."
    I do not understand this..

    Is there possible to turn OFF DC DC converter by using Single MOS...!!!

    Hope this help in further discussion..
     
  18. jpanhalt

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    You do not seem to understand how that mosfet (or any enhanced mosfet) works. Speicifically for a P-channel mosfet see this illustration:

    upload_2016-8-20_6-47-40.png
    Source: http://www.electronics-tutorials.ws/transistor/tran_7.html

    Now, please try the design I suggested. In other words, +3V relative to ground WILL NOT turn off that mosfet, if its source is at +5V relative to ground.

    John
     
  19. mishra87

    Thread Starter Member

    Jan 17, 2016
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    So I need a BJT to make it works..
    I think you wanted to say...
    Alone PMOS can not do the job..!!

    Please find the attached schematic and it has tested by me and worked. This is what you wanted.

    But cons of this circuit is extra power consumption of battery in run mode .


    This is the reason I wanted to do it by PMOS.

    Please consider me as a novice and reading articles and applications notes can not do the job for me..because I hardly understand 50%.

    If PMOS can do a job please upload your fresh schematic so it's easier to understand to me..

    Thanks..!!
     
  20. jpanhalt

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    Jan 18, 2008
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    It is indeed frustrating when you claim a 50% understanding, but you don't take advice. Yes, a BJT can switch the gate, but so can a SPST switch. Try the switch with a resistor from gate to source as I suggested and see if you can get that to work. Then you are free to enhance the switch on the gate anyway you want. BTW, what prompted you to change the gate-source resistor to 200 Ω? Do you understand that connecting the gate to ground will result in a 50-mA 25 mA current or 0.5W 0.125W dissipated in that resistor? Be sure to use a resistor that will handle that power.

    Going step-by-step is not "reading articles and application notes;" it is a common method to troubleshoot a problem. In the past, you have complained about people at AAC not trying to help you. Please try to learn from that.

    John

    Corrected math. Incorrectly used 10V for boost voltage.
     
    Last edited: Aug 20, 2016
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