On Sun, Sep 4, 2011 at 7:18 PM, RussellMc wrote: > > Sources can float as shown in your diagram. > When FETs are on then both drains and both sources are at essentially > the same potential. > As long as gate drive is high enough above this common potential when > on to ensure that FETs are fully turned on then the circuit will work. > > Turn off gate drive voltage needs to be low enough for Vgs to be sure > to be below Vth for both FETs. > > In some cases notionally the circuit may not "start" as the center > sources are floating but in practice this always seems to work well. > No doubt this is fully explicable when all parameters are considered > :-). > > Usually this back to back arrangement is most economical but you may > wish to check if a separate P Channel and N Channel FET with each > turned on as required would be better in your case. This applies only > of Vin and Vout always have same relative potential. If Vin and Vout > can reverse relative polarity you either need back to back FETs as > above or a reverse diode in series with two separate P & N Channel > FETs. Thanks for the reply. To counteract the "starting" issue, can I put a 10k (or whatever) resistor from the common source to ground in order to initially reference the floating source to ground? Time to simulate in LTSPICE, although it does tend to simulate /too/ perfectly. --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .