Battery latch when power supply removed.

Discussion in 'Power Electronics' started by emoney123, Jul 10, 2018.

  1. emoney123

    Thread Starter Active Member

    Jun 4, 2012
    32
    0
    I have an application that can be powered via battery or power supply, however, the battery is always connected (but not turned on). My design issue is that I would like to be able to power this via power supply (which automatically turns the circuit on) but when i remove the power supply my circuit stays powered via battery. I would like this to totally turn off once removing the power supply. Could anyone help me figure out a way to design this?
    Thanks.
     
  2. crutschow

    Expert

    Mar 14, 2008
    20,058
    5,649
    I'm confused.
    What is "this" that you want to "totally turn off"? :confused:

    What is the battery voltage?
     
  3. avayan

    Member

    Oct 30, 2015
    32
    6
    Also, what is the point of the battery if it is not going to do anything once the power supply is removed? If it is just an error on the description, then you can solve this by using a diode. When the power supply is present, then the diode is reverse biased and the battery does not source current. When the power supply disappears, then the battery conducts.

    The only problem is the loss on the diode. You can also do a pass through switch with a high side MOSFET and then use a comparator circuitry to decide when to enable the MOSFET. This one is trickier to make, but the losses are considerably less.
     
  4. emoney123

    Thread Starter Active Member

    Jun 4, 2012
    32
    0
    The battery is always present and will be used in most cases, however, there is a "bench power" line where the power supply can be connected so that final testing can be performed w/o draining battery power. I have a diode on both the battery power line and the bench power line, but when turning off bench power... they battery power kicks and keeps the unit "ON" and really, I would like for the unit to turn off when removing bench power.
     
  5. avayan

    Member

    Oct 30, 2015
    32
    6
    You are going to need a high side switch in between the battery and the application, and some logic to define when the battery is allowed and when it isn't. What you can do is place a voltage divider at the power supply side and generate a digital signal which you can call your POWER_GOOD signal. Whenever this signal is present, the system is "ON". Then when the power supply is disabled, the POWER_GOOD becomes de-asserted and you can use it to disable the switch in between the battery and the application. What I am still failing to see is when will the battery will be allowed to power the application, but I imagine you know how to answer that question. Good luck!
     
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