Transistorgrab

Diodes are often used to realize inverse-polarity protection. For circuits with high input currents the forward voltage of the diode can lead to a very high power dissipation of the inverse-polarity protection.

Diodes have a forward voltage that is not to be disregarded. Silicon diodes have a forward voltage of typically 0.6..0.7 Volts. At high currents this voltage can raise up to more than 1 Volt. Schottky diodes are better than silicon diodes but for high currents there are not many choices and also not very cheap ones.

The high forward voltage of diodes leads to a considerable heat dissipation.
Lets take a 1N5400 for example. This diode has a forward current of 3 Amperes.
The diode can handle this current up to 75°C (167 °F) ambient temperature. The forward voltage at 3 Amperes is 1.2 Volts. The power dissipation on the diode is therefore 3.6 Watts. The thermal resistance R_θja of the diode is 20K/W. That means the temperature of the diode raises by 72°C. The maximum allowed junction temperature of the diode is 150°C (302 °F).

An alternative for diodes are MOSFET-transistors. MOSFET have a low on-state resistance (R_DS-on). For inverse-polarity protection P-MOSFET are a perfect choice because they can be inserted into the positive power supply line.
To be able to handle the same forward current as the above mentioned diode we choose a P-MOSFET RSS060P05. This has a forward current of 6 Amperes and can block up to 45 V. Its on-state resistance is maximum 53 mΩ. At 3 A the dissipated power equals (3A)² × 0,053 Ω = 0,477 W. The voltage drop equals only 3A × 0,053 Ω = 0,159 V. The temperature raise of the MOSFET (R_θja = 62,5 K/W) is then 29.8 °C.
That means the curcuit will dissipate more than 3 W less power when using a MOSFET and not a diode. Additionally the MOSFET (three times more thermal resistance than the diode) will warm up only about 30 °C where the diode will warm up more than 70 °C.

inverse-polarity protection with P-MOSFET

The circuit above is much more complicated than a simple diode. But MOSFET can handle much more current. A MOSFET that can handle more than 10 Amperes is very easy to get. The Gate-Source junction of the MOSFET is very sensitive to high voltages. Thats why there is a protection with a Z-diode for the Gate.
Additionally you can see that the MOSFET is used “inverse”. That is important for the desired use of the MOSFET as a inverse-polarity protection. The figure shows the parasitic body diode that is connected in forward direction.
That means even if the MOSFET is not “open” and the voltage source has the right polarity there can flow a current to the protected circuit. By this “trick” there can flow a current even at low voltages. But keep in mind that the body diode can withstand only small currents.
The MOSFET needs a Gate-Source voltage of about 3 Volt to turn on. Tat means the gate voltage must be 3 Volts lower than the source voltage. That means that this curcuit is only usable for circuits tha run at more than 3 V.
When the voltage between Source and Gate is high enough the MOSFET turns on and the body diode is released. To protect the body diode at low voltages against high currents one can connect a external diode parallel to the body diode.

In the case of inverse polarity of the voltage source the body diode is connected reverse and will block the flow of current. Additionally the MOSFET will not turn on becuase its Gate is more positive than its Source.

A MOSFET is of course more expensive than a diode. A 1N5400 is available for 0,05 EUR. The MOSFET RSS060P05 costs almost 1,00 EUR, it is about 20 times more expensive. The difference depends on the choosen transistor but you can see the direction.

Despite this disadvantages of the costs and more complicated circuit there may be cases where the MOSFET preferable to the diode.
In applications running on batteries the lower dissipation power can lead to a cheaper battery or longer operation times.
In applications that operate in high temperature environments it can be that the ciruit using a diode will not be possible to realize.