Power distribution

[Prince Gautam]

Hello,
I am trying to step down 75-80v dc to 5v dc to drive uC which is Atmega328.[typical current to drive is 100mA as it has other sensor connected]
i have tried all possible Ic's as buck convertor but not all support this wide range input. i found some switching regulator Ic's but either they are too expensive or they are not available.
So i thought of using 7805 voltage regulator[SMD-DPACK] with zener diode and NPN transistor to drop input voltage to about 18v-15v so the 7805 can do its job.
I tried all the maths i knew [including some explained in the video and discussed on the blog] but no satisfying value could be obtained for me to try this ckt practically on breadboard.
Can any one guide me through this problem, if there is all ready any commonly available IC that does such step down or with the zener diode calculation.
Thank you for your time.

[fourtytwo42]

Your idea is fine BUT 100mA will pass through your NPN transistor dropping 80-15=65V so it will dissipate 6.5Watts and so you will need a heatsink, if you dont want that keep hunting for a switcher, there has to be something out there

[Ian.M]

Your Google-fu is lacking.
Googling: hv buck
found several leads, then 5 minutes with LT's parametric search found http://www.linear.com/product/LTC3638 which looks suitable - up to 150V in, and up to 250mA out.   Its not that cheap, but heatsinking costs money and with 6.5W to dissipate, the linear regulator will probably have a total cost in the same ballpark.

I expect other manufacturers have alternatives, and there are probably some that are better, cheaper or in packages that are easier to prototype with.   It would also be worth looking at really cheap Chinese USB charger designs.  The complete units typically retail for under $2, so you should be able to source the parts, and modified magnetics for 80V-100V DC bus for well under that in quantity..

[dmills]

Forget the typical current, what is worst case?

I am not seeing any particular shortage of 75V, half amp class bucks from the usual suspects (TI, Linear, Maxim if you really must), but if they are unsuitable for some reason then 60V is a very common upper limit for a buck converter (Telecoms loves its 48V rails) so I would probably do the zenner thing to get there or there about, then use a small buck converter.

if you assume say 1W worst case (200mA @5V) then with say 50V at the buck input you are looking at ~20mA on the input side, if we design to drop say 25V across a zenner then the zenner would be dissipating 0.5W.

Note that there is a small trap here, in that the series zenner will dissipate MORE power as the input voltage drops, because the buck will consume progressively more current.

24V, 1.5W zenner in series with the input and say a LMR16006YDDCT in a datasheet circuit would be my instinctive approach, make sure your minimum load pulls sufficient current thru the zenner at maximum input voltage to keep the voltage drop sufficient.

Regards, Dan.

[Prince Gautam]

thanks for your reply.I tried the IC u suggested LTC3638 is not available.
 

[dmills]

Mouser has well over a hundred, and I would be surprised if Digikey didn't have stock also.

Not a particularly cheap part, but the LT stuff is usually totally solid, and only you can decide how the cost/efficiency tradeoff works for your application.

Regards, Dan.

[prasimix]

Quick search (one that I set faster then wrote down this message) on TI.com returns the following: LM5008, LM5009, LM5018, LM5019. All are switchers, below 2 USD and easily obtainable.