At 01:10 PM 2/13/2003 -0500, you wrote:
>Quoting Chris Loiacono <chris@MAIL2ASI.COM>:
>
> > Of course any MOSFET can be used as a high or low side switch, but some
> > devices can be used bidirectionally, others not so well. Now let's see if I
> > can remember which is which...
>
>What is the definition of a high/low switch... informally...

A high side switch switches the supply rail so that the load can be grounded.

The is especially desirable in automotive applications where one side of
load is typically grounded. You need a pair (one high, one low) to make a
half-bridge.

It's trivial (use a P-channel MOSFET) when the voltage is Vdd, but for
higher voltages (12V or 300V or 600V), you run into problems - for example,
there are no common P-channel MOSFETs rated for >500VDC AFAIK. Also IGBTs,
which come into their own at high voltages are only available in "N" type.
To use N-channel MOSFETs (say) as a high side switch you need a drive voltage
that is above the +ve rail.

An H-bridge is two half bridges, a 3-phase bridge is 3 half-bridges.

                A            B                C


             High side                      +V
          +V                                 |  Half bridge
           |   _/                            o
            -o/  o--                         '\  High
                   |                           \
                  .-.       +V               o  \
                  | |        |               |
             Load | |       .-.              .
                  '-'   Load| |              +---------+
                   |        | |              '     Out
                   |        '-'              |
                  ===        |               o
                  GND        |               '\   Low
                             o                 \
                             '\              o  \
                  Low side     \             |
                             o  \            |
                             |              ===
                             |              GND
                            ===
                            GND


(view with fixed pitch font only)

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
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