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Old 3rd March 2013, 12:58 PM   #21
MarianB is offline MarianB  Romania
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Quote:
Originally Posted by AndrewT View Post
I like the diodes solution.
What about the power dissipation on the diodes? 60W waste is not something to be ignored, they would need serious heatsinking and as one knows the voltage drop on the diode depends on the load current, for the moment we do not know if those 20A are a constant draw or just a max vallue, and it is important cus for low load current the voltage drop will not be sufficient tyo ensure you do not exeed those 21V...Anyway untill it is not sayd what cind of load needs to be powered up by this we can only make assumptions...
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Old 3rd March 2013, 02:28 PM   #22
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I saw some high current diodes, they don't seem to have constant voltage drop. at low currents, they are dropping less, so at lower current levels, the drop is less, making the voltage exceed 22v which is a problem.
the current could exceed 20a also, but with an appropriate duty cycle.
i am also looking at alternative chips that can get rid of the 22v limitation. then i will be able to use 24 straight away.

Last edited by doors666; 3rd March 2013 at 02:34 PM.
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Old 3rd March 2013, 02:49 PM   #23
MarianB is offline MarianB  Romania
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That's exactly my point, no silicon diode will have constant voltage drop, look at the datasheet respective graph...

PS: What is that chip anyway? cus i doubt i can get any real help if you are so secretive, you cannot ask someone to do your job blindly...
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Old 3rd March 2013, 03:36 PM   #24
MarianB is offline MarianB  Romania
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If looking for simple and easy fix than linear regs should be the solution, so here are 2 sugestions of mine, first one uses zenner diodes, it is simple but has a rather poor load regulation cus it depends on the transistors Vbe, but you get the advantage of simplicity and stability, the second ine uses the TL431 shunt regulator, it has a much better load regulation but some compensation might be needed, seeing how 431 is an EA... both of them are overcurrent and shortcircuit protected, the fuse assures that the latter situation will last only a few seconds, if it does not blow up when needed than you could either move it in front of the regulator or lower it's current rating. Be aware that the power devices needs a rather big heatsink.

PS: If switching solutions are what you prefere than i can draw up a basic buck regulator witch can then be upgraded/changed/modified as needed.
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