V G wrote:
> On Thu, Sep 1, 2011 at 11:25 PM, Kerry Wentworth <
> kwentworth@skunkworksnh.com> wrote:
>
>  =20
>> FETs conduct equally in either direction when on, but you have left out
>> a lot of details.
>>
>>    =20
>
> Do they also block in both directions when off? Or do they only block wit=
h
> the inherent diode, blocking drain to source?
>
>
>  =20
>> Is the "high side" going to go negative?  How far negative?
>> How much current do you expect to conduct?
>>
>>    =20
>
> No negative voltages are expected with respect to ground. All positive.
>
> I'm going to conduct a maximum of 2A in either direction. I'll choose the
> MOSFETs accordingly. The "solid state relay" is going to be used to contr=
ol
> the charging/discharging of a battery pack and will need to conduct in bo=
th
> directions and it's relative position between "high side" and "low side"
> will therefore dynamically change based on whether the battery is chargin=
g
> or discharging.
>
>
>  =20
>> Some FETs, particularly high current ones, have a diode between source
>> and drain, so although they conduct equally in both directions, the
>> don't block equally in both directions.
>>
>>    =20
>
> So I guess I'll need to use two MOSFETs back to back. The problem with th=
at
> is, what do I connect the common "source" pins to?=20
You can just connect them to each other. When off the two mosfets will=20
act as a pair of diodes back to back. When on both mosfets will conduct.

Only gotcha is you have to make sure that your gate voltages are=20
high/low enough to provide the desired behaviour. This means that for=20
N-chanell mosfets your ON voltage needs to be higher than the highest=20
voltage you will be switching and your off voltage needs to be arround=20
the lowest voltage you will be switching.
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.