Calculating Current for NTE2380 (Mosfet)

[xray]

I have put together a motor driver for a linear actuator, 12V 4 amps 10% duty pulse width less than 300 Usec. The circuit is a variable duty 15V 555 timer driving the gate of an NTE2380 (Mosfet transistor). I have been trying to use the datasheets and measured values to calculate the drain to source current with no succes. I am in the saturation region, VDS > VGS - Vt therfore I am using iD = .5*Kn'*W/L*(VGS - Vt)^2,. Sedra Smith 4th edition. My question, The datasheets, NTE 2380, give a value for transconductance, A/V, but the coeffecient for the equation is A/V^2, which is Kn'. Any sugestions to get me in the ballpark from paper to circuit would be appreciated.

 

[cbarn24050]

The easiest way it to look at the data sheet. You should find a graph showing Id vs Vds for various Vgs values. Normaly in a switching circuit Id is controlled by the external circuit and not by the fets characteristics.

 

[bogeyman]

Due to wide variations of FET characteristics you are unlikly to get very accurate results using any form of calculated value.

Not keeping the FET saturated will cause an increase in Vds that will help the FET to get HOT, possibly so hot that the gray smoke they put in them when they make them gets out and it won't work any more.

Keeping the FET saturated under all possible conditions calls for as large a gate voltage as is practical to be applied to the FET.

Do you want to limit the current to 4A? or is this just the specified operating current of the actuator when running?

I do not know what the actuator type is or its current characteristics are, but if it is a DC motor and gearbox/screw affair the current drawn from the 12v supply will be speed dependant and almost certainly much higher when the motor is starting or loaded such that it is running slowly.

In addition to the usual motor start current you probably will find that the self inductance and capacitance of the actuator windings will play merry hell with the expected values of dynamic current during the on period of the PWM controler.

Use a big fat zenner across the FET (Vz < Vgs max)or a fast diode accros the load to protect the FET from the inevitable back EMF at switch off (this will occur at the end of each pulse).

If you require an accurate pulse of current it is almost certain that a feedback controled current source will be needed.
To measure the actuator current I suggest that you include a low value sensing resistor (say 10 milli ohm)in the FET drain would provide an output voltage of 10mV per amp which could be applied to a scope or other measuring device.

 

[OperaHouse]

There are a lot of cookbook designs for the 555, but i have a general belief that any 555 design can't be good. There are a number of switching regulator chips out there that can do this function with less parts, add protection and be more stable. An example is the TL494 that has been around almost as long as the 555. You get an internal reference as well as a second input for current sense.

 

[xray]

Thanks for the good comments. I just needed to read further. When given the transconductance Gm V/A the equation reduces to Ids = Gm*(VGs - Vt), where Gm is Kn'W/L*(VGs - Vt) A/V. This does get me in the ballpark for Ids. Thank you for taking the time.