Protection devices for home cordless phone

Discussion in 'The Projects Forum' started by P75052, Jan 22, 2013.

  1. P75052

    Thread Starter New Member

    Aug 31, 2012
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    Hi, I am looking for protection schemes that are used at the terminals of home cordless phone to protect the handset/base from ESD or power glitch when the handset is mated to the base for charging.
    I have a project that consists of a handheld device with Li-Ion battery, and a base that has two terminals to charge the handset - very similar to a home cordless phone. When I put the handset to the base, I saw spike on power rails that sometimes crash the CPUs on both handset and base units. The power used between the two units is regulated 5VDC, and I have a TVS on 5V line, but it does not seem to help. I also added a 6.3V zener across the terminals, that does not help either.
    Any help would be appreciated.
    Thanks,
    P75052
     
  2. wayneh

    Expert

    Sep 9, 2010
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    3,031
    You mean a spike in voltage above the regulated 5V? Or a spike down? What else is in the handset load besides the battery?
     
  3. P75052

    Thread Starter New Member

    Aug 31, 2012
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    The spikes are on all sub voltages - 3.3V, 1.8V and 1.2V with both positive +7V above and negative -6V below the normal voltage. In the handset there are two micro controller with associate circuitry.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,093
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    I think you need to give a schematic of your regulation circuitry. One of the power supply experts around here will likely have some good ideas for preventing those spikes.
     
  5. P75052

    Thread Starter New Member

    Aug 31, 2012
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    I will try to create a simplified schematic, due to the proprietary information. But attached is the high level overview of the system. What I observed is even with a floating 24" wire lead that I carry around touches the ground of the base is enough to generate spikes on the rails on the base. I incline to say it is ESD because it does not happen all the time; only when I pick up the handheld, carry it around and put back to base, then it happen. I can sit there, remove it from base, and then put it back and no spikes observed.
     
  6. wayneh

    Expert

    Sep 9, 2010
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    3,031
    My cats suffer a similar problem this time of year. ;)
     
  7. P75052

    Thread Starter New Member

    Aug 31, 2012
    4
    0
    Wayne,
    Do you know of what they normally use at the terminal of a handheld device to protect it? All TVS vendors say "Use ours" but I don't see any proof the device works. I saw a design that has inductors in series with the terminals; unfortunately, due to the layout I can not break the trace to insert the inductors (5V and Gnd are directly connected to plane or embedded).
     
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