LM317 HV PSU

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Just wondering if anyone has built this and how much heat sink the IRF820 will need. Thanks.
 

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I don't have a specific application in mind. I was thinking about laying out a pcb for it and was wondering if I should put the IRF underneath the pcb, to bolt to a bigger heatsink, or if a smaller on board type, like the one attached, would work.
 

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In this circuit there will be about 8V over the LM317 (12V zener - Vgs for enhancement mosfet).

Power dissipated on IRF820 is
(Vin-Vout-8)*I

example:
Vin 350
Vout 300
Current 100mA

(350-300-8)*0.1 = 3.8W

Power dissipated on Lm317: 8V * 0.1A = 0.8W

missed the 33 ohm resistor. At 100mA thiw will dissipate 0.33W, and the LM317 about 0,5W.
 
Hi dotneck335

why do you think that?

what I dont like much about the 317 in this application is that a 317 requires 5mA, some say 10mA load, to regulate well. At 300V output that is 1.5W (5mA), a potentiometer that can take that is quite big, expensive and probably not very reliable. There is a recent thread on the LT3080 - I would probably use that device if I were to build one of these regulators.
 
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