mosfet voltage follower

[technogeek]

I'm sure you've all seen this, I know it's used commonly to create reference voltages. What you do is take the output of a zener diode and feed it into a mosfet voltage follower to boost the output current without the zener overheating.

(See attached diagram)

My question is, how do you calculate all of the parameters of this circuit?

I assume the MOSFET doesn't take in any current.. so the zener and dropping resistor can be low wattage, and use minimal current. However, how do you size the fet, and do you want to pick one over another (for instance one with a lower Vgs threshold?)

For instance, if I wanted to take a 48 volt power supply and turn it into a 24v "regulated" power supply @ 4mA, what are the steps to doing this? How do I find the proper mosfet? How do I calculate the power dissipated in it (I^2*Ron?) For such a simple circuit you'd think google would have a handy calculator built in.

 

[Phasor]

How do I calculate the power dissipated in it (I^2*Ron?)

Would not V*I be more appropriate, seeing as the MOSFET is not fully 'on'? Anyhow, at 4mA, you're looking at roughly (48-24)V*0.004A = 96mW = bugger all power. Breakdown voltage is going to be the most important parameter of your MOSFET.

 

[Roff]

If you only need 4mA, an NPN emitter follower would be more appropriate. It would give you a more predictable output voltage and better regulation.

 

[Hero999]

The source will always be the threashold voltage at the current being drawm below the gate voltage.

Whay don't you use a propper regulator IC like the LM317 or LM338?

 

[technogeek]

it's a learning exercise mostly

 

[audioguru]

The current in a Mosfet at the threshold voltage is nearly nothing, 250uA.
The threshold voltage is a range of voltages.
So you cannot predict the output voltage of a source-follower.