Hey all,
I started playing around with LTSpice, and setup a LT3080 voltage regulator with an N-Channel Mosfet pass transistor to increase current handling capabilities. It drives a resistor whose resistance is a function of the voltage drop accros it, making it a 3A current load. I use a similar resistor setup to draw a constant 1ma from the LT3080 as it does not regulate without it. The LT3080 set current is regulated by a transistor whose resistance is stepped up from 0M2 to 3M5 in 20K steps.
I have attached the LTspice schematic so y'all can take a look at my masterpiece
Now when I was playing around, I noticed that depending on what pass transistor I chose, the current/resistor to get the output voltage to increase changes. Because I don't have much experience with transistors (Still learning...) I am unable to explain this behaviour.
What property of the mosfet dictates this behaviour? The effective resistance? The maximum voltage?
Attached are two examples of the output voltage right after the pass transistor vs the Resistance of the resistor connecting the set pin of the regulator to ground. Notice how with the IRF6644 the voltage starts rising at around 440K, while the circuit using the IRLR2908 already increases in output voltage at around 200k...
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Some more random questions:
Is using a mosfet pass transistor the proper way of getting 0-30v 3A out of this circuit/chip? If so, how much heatsinking would I need to dissipate the large amount of energy?
Would putting mutiple of the same transistor in parrallel be a good way of spreading the heat? Are there any others?
Thank you so much for your help!