Understanding Rds(on)

[fido-fido]

Hi All,

I'm apologize for such basic question here; am I right that Rds(on) is the resistance which needs to be overcome in order to turn on...lets say a gate? Can I put it in this way?

Thanks in advance.

 

[dfullmer]

Hi,

The Rds referes to resistance "drain to source". Once the device is fully turned on by applying gate voltage, the amount of resistance through the device is Rds. This value can be used to determine the wattage disipation the device will need to expend for a given current flow. Rds does not refer to what it takes to turn on the device.

Hope that helps

Regards

dfullmer

 

[fido-fido]

Hi dfullmer,

Just to get this right, lets take a diode as an example, does it mean that it's the voltage difference between the "drain" and "source" when the voltage line path is turned on?

Thanks in advance...

 

[Emanuel_hr]

Not quite.
Think of your fet transistor like a variable resistor witch is controlled by Vgs. If Vgs is smaller than the threadshot voltage than this resistor has a high value, thus very little current flows and the device is turned off. If you increase Vgs over that threadshot voltage than this resistance becomes small, and we say that the device is turned on. This is Rds(on): the amount of resistance between drain and source when the transistor in on.

 

[dfullmer]

Hi fido-fido,

Emanuel_hr is correct. Fets are normaly used as simple switch. They are either on or off. Not really used in a linear fashion like a transistor. So the resistance as Emanuel_hr mentioned will be high Meg ohms from drain to source when the device is off and low resitance when the device is on ohms or even milli-ohms. When a transitor is in full saturation the wattage disipated by the device can be given by the current times the Vsat or saturation voltage. For the fet you can do the same but you can also assume the Rds on in ohms and calculate the wattage by current (squared) times Rds on. Just as you might for a fixed resistor.

Hope that helps.
Good luck.
dfullmer