24
\$\begingroup\$

Does a MOSFET allow current flow in reverse direction (i.e.; from source to drain)?

I made a Google search, but couldn't find a clear statement about this matter. I have found this similar question, but it is about detecting current direction from the schematic symbol of a MOSFET. And under the same question, there is this answer which states that MOSFETs have no intrinsic polarity, thus they can conduct in both directions. However, that answer has no up/down votes or comments, so I can't make sure of it.

I need a clear answer on this. Does a MOSFET conduct in both directions?

enter image description here

\$\endgroup\$
  • \$\begingroup\$ For what it's worth, BJT actually does conduct in either direction. It's just pretty lousy in reverse i.e. current gain is in single digits. That's because the semiconductor elements are hardly symmetrical. \$\endgroup\$ – Barleyman May 14 '19 at 10:20
26
\$\begingroup\$

Yes it does conduct in either direction.

Due to the body diode, most discrete MOSFETs cannot block in the reverse direction, but the channel will conduct in either direction when the gate is biased "on".

If you want to conduct and block in both directions you need two MOSFETs in series.

MOSFETs used as near-perfect rectifiers are usually used in the reverse direction for conduction (so they can block in the other direction).

Edit: Your schematic here: enter image description here

Illustrates one example of switching AC with two MOSFETs (one of which will be conducting in reverse at any given time when the switches are on).

Another example is here, from the LT4351 datasheet:

enter image description here

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Using two MOSFETs like this? You can attach this image to your answer if it is correct. \$\endgroup\$ – hkBattousai Apr 6 '14 at 9:48
  • \$\begingroup\$ Yes, that's right. Effectively back-to-back. They're easier to drive if the common can be grounded as you've shown. See the LT4351 datasheet for another example. \$\endgroup\$ – Spehro Pefhany Apr 6 '14 at 9:53
3
\$\begingroup\$

In case anyone else stumbles across this thread, the TLP175A (and plenty of others) is a single component which directly implements the circuit suggested in the above answers. That particular IC features optocoupler isolation and is even suggested use as a relay replacement.TLP175A internal schematic demonstrating use of back-to-back MOSFETs

| improve this answer | |
\$\endgroup\$
  • \$\begingroup\$ Great idea, and it really separates left side from right one. Unfortunately most of them are either high voltage (like enough to support EU 240V) or high current (to power e.g. 100W light bulb) but not both :( \$\endgroup\$ – Dominik Szymański Jul 11 '19 at 16:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.