Is it possible to have a high side MOSFET switch that will allow any direction of current? I was trying to design one but without the success. Or should I use a relay?
Fun fact: MOSFETs (JFETs too!) will conduct in either direction when Vgs is above threshold.
The issue with MOSFETs (3-terminal ones, with the body tied to source) is the parasitic body diode formed by the FET structure and body-source tie also conducts when the drain-source voltage is in the reverse direction, even if the FET gate is off.
More about that pesky body diode here: How should I understand the intrinsic body diode inside a MOSFET?
In other words:
- FET 'on': conducts in both directions through drain-source channel with Rds(on) resistance
- FET 'off': blocks forward drain-source, conducts reverse through body diode with one Vf drop.
Note that when 'on', the FET drain-source channel shorts out the body diode, so the current flows through the normal drain-source path. As the parasitic diode’s forward threshold isn’t met, there’s practically no diode current at all.
The workaround is to tie two FETs in series, back-to-back, source-to-source or drain-to-drain. This allows each FET to block the other's parasitic body diode when 'off'.
BONUS: A simulation showing high-side switches built with n-FETs in three different ways: 4-terminal, 3-terminal with body diode, and back-to-back 3-terminal.