From: ehsjr@verizon.net   
      
   On 12/6/2020 9:36 PM, Rick C wrote:   
   > Working on a gadget that uses a 12 volt motor (10 amp peak) and misc   
   electronics running on lower voltages.  It is powered from an external PSU   
   like a laptop supply, but 12 volts.  There is also a sealed lead acid battery   
   for backup when the power    
   fails.   
   >   
   > This design uses pairs of pFETs to route the power from the 12 volts input   
   when present, or the battery when the line is not present.  There is also a   
   means of shutting down all power.   
   >   
   > Initially when designing this there was a problem with shoot through from   
   the battery at a slightly higher voltage than the line power initially.  Then   
   I added the capacitors present on the motor circuit which helped.  But that   
   made large surge    
   currents in the capacitors when the power is shut on or off.   
   >   
   > I'm hoping someone can make some suggestions.  I've run out of ideas on how   
   the time the turn on of one switch and turn off of the other while also   
   bringing up the power slowly to reduce the inrush of the capacitors.   
   >   
   > Here is the LTspice schematics.  I didn't use anything not in the LTspice   
   library.   
   >   
   > Thanks,   
   >   
   > Rick   
      
      
      
   Put a resistor in series between the power input and the   
   capacitors to limit the current.  Sense the voltage on the   
   caps - feed it to a comparator to drive a relay or mosfet   
   (your choice) that shorts out the resistor when the voltage   
   rises to whatever value you choose.  You can use the same   
   approach to disconnect the load until nominal voltage is   
   reached, if necessary.   
      
   Ed   
      
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