I only need 4.5w ,vo=15.io=.3A, if the input voltage go up o 70v,how can i fix this problem, and can you please sketch your circuitHow about using LM317HV regulators.
They have a maximum differential rating of 60V which is okay at a 15V output but is marginal under short-circuit conditions.
Also their nominal output current limit is 0.3A at a 45V differential so that could also be marginal.
To solve those problems you could use two LM317 HV in series, with the first one set for about a 35V output.
That solves the voltage problem and the current limit at that lower voltage is well above your desired 0.3A.
Note that the regulators need to be on a good heatsink as they have to dissipate 13.5W total under worst-case conditions.
thanks so much, can I get the LT spice for lm317Edited to add current limit---
My proposed scheme won't work with a 70V supply so I changed it to use a N-MOSFET source-follower preregulator in front of an LM317 to reduce the LM317 input voltage below its maximum rating.
The 33V zener causes the MOSFET source follower output to be about 28V at the LM317 IN terminal, well below the maximum LM317 rating of 40V.
The N-MOSFET can be any ≥100V, MOSFET in a TO-220 or similar power MOSFET case so it can be mounted on a heat sink.
It must be able to carry at least 0.5A at 70V as shown in the safe-area graph in its data sheet.
Thanks so much
The maximum dissipation at 70V input and a 15V, 0.3A output will be 14W for the MOSFET and 4W for the LM317 so they both must be mounted on an adequate heatsink.
The MOSFET's dissipation is 28W with the output shorted so,if you want to protect for a continuous short at the output you will need a heatsink that will dissipate 28W.
If you mount them both on the same heatsink you will need to use isolation washers for the LM317.
Q1 sets the current limit at slightly over 0.4A for an R4 value of 1.6Ω (I_Limit≈0.64/R4) .
The LTspice simulation of the circuit is shown below.
The voltages and output current are shown for a load resistance varying from 100.1Ω to 0.1Ω.
View attachment 118564
Thanks so much, did you create this model for LM317?Attached are the files I use.
Put the .asy file in C:\Program Files (x86)\LTC\LTspiceIV\lib\sym\PowerProducts.
Put the .sub file in C:\Program Files (x86)\LTC\LTspiceIV\lib\sub
It should then show up in Power Products after closing and reopening LTspice.
No, I found it on an LTspice forum, not sure which one.Thanks so much, did you create this model for LM317?
google pre-regulators. they are essentially transistors whose regulation output floats with the output of the combined regulator.how can i fix this problem,
The circuit I posted uses a fixed voltage pre-regulator which for a fixed output voltage as this has, would be the same as a floating preregulator.google pre-regulators. they are essentially transistors whose regulation output floats with the output of the combined regulator............
Thanks so much crutschow ,can you please help me in this threadThe circuit I posted uses a fixed voltage pre-regulator which for a fixed output voltage as this has, would be the same as a floating preregulator.