Hi I would like to know the reason why some intances where
a diode was put across the drain and source of the Power MOSFET
Can somebody explain it to me further.. Thanks.

 

Hi,

Where the device is switching power through an inductive load, there will be a time at device turn-off when the field in the load will collapse and drive a current back through the mosfet. The diode shunts the current ant prevents damage to the mosfet.

 

If a coil is the load a coil can store power so you need a way to direct that stored power around the mosfet. If you don't the mosfet will die. If you look at relay circuits they will also have a diode in them.

 

Frequently the mosfets are connected in half or in full bridge. And frequently they drive one motor in PWM, through a LC filter. In the time OFF of the mosfet one produces negative voltages, picks, with very big values that will exceed the reverse Mosfet voltage and on this manner the mosfet would be destroyed if the diode is not placed in parallel to G-S of the mosfet. With the diode these picks are short-circuited.

 

..... parallel to G-S of the mosfet. With ....

Dragan733,

Did you mean to say D-S which is drain to source rather than G-S which I would take to mean gate to source?

hgmjr

 

Dragan733,

Did you mean to say D-S which is drain to source rather than G-S which I would take to mean gate to source?

hgmjr

Yes, yes. I had to say: D-S

 

Yes, yes. I had to say: D-S

Sorry to ask about diode function as well.What is the general application of diode beside using as a rectifier?

 

Sorry to ask about diode function as well.What is the general application of diode beside using as a rectifier?

In a signal with two polarities, through the diode always passes the signal with one polarity, + or - dependently of the orientation of the diode to the input signal.

 

In a signal with two polarities, through the diode always passes the signal with one polarity, + or - dependently of the orientation of the diode to the input signal.

Sorry I actually wanted to know the general function of diode.Example to protect the relay coil during power switching off.

 

Sorry I actually wanted to know the general function of diode.Example to protect the relay coil during power switching off.

You don't protect the relay coil with the diode. You protect the transistor, C - E, during power switching OFF, when one appears negative voltages to the transistor collector . As the inductance of the relay is bigger, and the resistance of the coil relay is smaller, these negative voltages are bigger.

The diode will be placed in parallel with the relay coil, with the cathode to + supply voltage.

 

You don't protect the relay coil with the diode. You protect the transistor, C - E, during power switching OFF, when one appears negative voltages to the transistor collector . As the inductance of the relay is bigger, and the resistance of the coil relay is smaller, these negative voltages are bigger.

The diode will be placed in parallel with the relay coil, with the cathode to + supply voltage.

Thanks for the info.

Can you kindly let me know two to three more of this kind of example so that I can know more about electronic.Thanks.

 

You don't protect the relay coil with the diode. You protect the transistor, C - E, during power switching OFF, when one appears negative voltages to the transistor collector . As the inductance of the relay is bigger, and the resistance of the coil relay is smaller, these negative voltages are bigger.

The diode will be placed in parallel with the relay coil, with the cathode to + supply voltage.

BTW, the transitor in this kind of circuit is a switch or a amplifier or something else?

 

Thanks for the info.

Can you kindly let me know two to three more of this kind of example so that I can know more about electronic.Thanks.

http://www.wam.umd.edu/~toh/ElectroSim/relay.html

http://www.kpsec.freeuk.com/trancirc.htm

 

BTW, the transitor in this kind of circuit is a switch or a amplifier or something else?

Switch. You will read more in the links above mentioned

 

Switch. You will read more in the links above mentioned

Thanks for the useful and informative link,its already added to my favorites.