I used to work on a bike with an old-fashioned generator that
had a segmented commutator and put out dc.  That means that a 
voltage regulator has to have a rectifier in it, to block 
the battery from discharging into the armature when you 
turn the bike off (they used a relay in the old days).
I built a solid state regulator for the bike using a 
shottky rectifier, but now I'm thinking about ways to use 
a low-resistance mosfet in the high side so it doesn't 
require a heat sink.  The suggested current limit for this 
generator is ten amps. A ten-mOhm mosfet would only make 
one watt of heat.  The MBR3035PT shottky created five 
and a half watts at ten amps.  So now I"m trying to figure 
out how to control a mosfet in the high side as 
a blocking diode and have it turn on and off, like a 
syncronous rectifier.   
  The circuit will have a comparator that controls the 
field current, so I figured I might use a comparator for 
controlling the power mosfet as well.  Since I'll have a 
charge pump for driving the mosfet gate, I can run the 
comparator off it.  This means I can use an ordinary 
comparator to sense the voltage across the high-side mosfet.  
Here's  an illustration of the basic topology I envision



                         ,-----------,
                         |           |
                      ||-+           |
                      ||<           |
charge pump>---+-R1-+-||-+           |
   ^           |    |    |           |
   |           |    |    |           |
   |           |    |    |           |
   |           |    |    |           |
generator      |    |    |           |
armature--+----|----|----'           |
          |    |    |                |
          R2   |    '------,         |
          |    |           |         |
          |    +-R4-,      |         |
          |    |    |      |         |
          +----|-||-+      |         |
          | |\ |    |      |         |
          '-|-\|    |      |         |
            |  \    |      c         |
       LM393|   >---+-----b          |
            |  /           e         |
          ,-|+/|           |         |
          | |/ |          gnd        |
          |   gnd                    |
          '--------------R3----------+
                                     |
                                     | +
                                   -----
                                    ---
                                   -----
                                    ---
                                   -----
                                    ---
                                     | -
                                     |
                                    gnd

The only problem is that the comparator may chatter
at low charging currents.  For example, if the comparator
has an offset voltage of -5mV and the mosfet has 5mOhm 
Rds(on), then at currents less than an amp it will
oscillate.  I put the negative feedback capacitor there 
to alleviate it as much as possible.  And though it's 
unlikely the generator will have to put out very low 
charging currents for any length of time,
I still would like to find a way around the oscillation.  
Any suggestions?