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? I can't connect them t=
o
> GND, because then the current will be shunted to GND when the MOSFET is o=
n,
> instead of going through the two MOSFETs.
>  =20
Do you want to turn on the 'switch' and allow charge or discharge, or do=20
you want to turn 1 FET on to allow charge or turn the other one on to=20
allow discharge?

If you tie the sources together, then the one with the drain to the=20
battery needs a low gate voltage to turn on, and the one to the=20
circuitry needs a high gate voltage to turn on.  Or you could just use 1=20
FET from the battery - to ground, if you want to turn charge AND=20
discharge on/off together.

Kerry

--=20
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