Mosfet providing polarity protection without diode voltage drop

For a P channel mosfet, whenever the Gate voltage is lower than the (Source Voltage – Gate Threshold voltage) the MOSFET conducts. If the gate voltage is higher than this, it does not conduct. The greater the voltage difference from the Source the more the MOSFET can conduct.

Used in this way the P channel mosfet diode protects against incorrect polarity at the input but is also bypassed by the mosfet when voltage with the correct polarity is present.

Take care when using mosfets in this configuration to ensure that higher input voltages will not exceed the gate voltage.

Mosfet to stop back powering from motors

A simple solution if you need to avoid diode voltage drop is to use the ‘Mosfet True Switch / Bi-Directional Switch With P-Channel MOSFETs’ shown here.

Also consider how you will dissipate a voltage that becomes too high from a motor behind the protected side, to ensure the mosfet max voltage specifications are not exceeded.

We benefit hugely from resources on the web so we decided we should try and give back some of our knowledge and resources to the community by opening up many of our company’s internal notes and libraries through mini sites like this. We hope you find the site helpful.
Please feel free to comment if you can add help to this page or point out issues and solutions you have found, but please note that we do not provide support on this site. If you need help with a problem please use one of the many online forums.


Your email address will not be published.