From 40V-100Vdc to 8V-12Vdc to 5Vdc

[Vindhyachal.takniki]

1. I have a voltage range of 40V-100Vdc & have to make 5vdc from it and power a microcontroller with it. Max current is 700ma.
2. Since have to measure analog so I want to first make a voltage of around 8-12Vdc then use a linear regulator like LM7805 to make 5Vdc.
3. I need module only. SO I checked for dc-dc buck. they are very costly >$100.
4. For lost cost option I see some people use a resistor & zener first to drop the voltage but I didnt find this good solution.
5. Ground of 40V-100Vdc should be same as of 5vdc & system should be non-isolated.
Any low cost solution for this.  I want module based system in this.

[lilshawn]

problem with using an LDO regulator on a voltage that high is you have to drive it so far down you waste a lot of energy.

you could do a 7824 which has a VIN of 40 volts max and use the 24v out to drive a 7805 not ideal, but it would work.

what about one of these?

http://www.ebay.ca/itm/Buck-Step-down-LM2596-Power-Converter-Module-DC-4-0-40-to-1-3-37V-LED-Voltmeter-/161476280982?hash=item2598bc8296:g:-kcAAOSwyDxXhIGH

[james_s]

You'll need the HV version for 40V input.

http://www.ebay.ca/itm/LM2596HVS-LM2596HV-DC-DC-Adjustable-Step-Down-Buck-Converter-Power-Supply-Module-/262136732980?

I've used a bunch of these for various stuff, they work great.

[Vindhyachal.takniki]

One I found here is:
http://www.ti.com/tool/lm5008eval
current is 300mA only.

[samofab]

perhaps this helps:
https://github.com/samo4/LM5017-step-down-converter

max current is 650mA, I think

[Ian.M]

If the chopper transistor of a simple buck converter fails it *WILL* take out your load.
If its got to tolerate 100V in, you'll need something with significantly over a 100V max rating.

I'd look as single chip flyback SMPSU controllers with integrated chopper transistors.  As long as the minimum UVLO threshold is under 40V and you derate the output power according to the ratio of the actual input voltage to the normal application supply voltage, it should work out OK.   It will require a custom wound transformer though, with windings for the 5v supply and whatever you need for the analog stuff.   

[Kjelt]

$100 ? I think you should look better, a quick search at tme revealed already two suitable candidates for $20 (see picture).
I don't know what your target price is or how many you need ?

Iam.M has a good point though if the converter fails your load is toast, so add a simple crowbar with fuse in series.

[Vindhyachal.takniki]

1. Source of DC: Either a 20S LFP battery 50V-72V or 20S Li-Ion battery 60-84V.
10V margin I want to keep on both sides. Since battery is used in automotive application.
2. Is this margin ok for automotive application , it is used in E-auto or E-bike.
3. For me price above $50 is high.
4. In zener based, I have to use very high wattage resistor, since current required is 500mA, that why I dont want to use it.
5. I found these ok:
http://www.digikey.com/product-detail/en/texas-instruments/LM5017EVAL-NOPB/296-37050-ND/3640503
http://www.digikey.com/product-detail/en/xp-power/RDD08110S12/1470-3083-5-ND/5726803 

6. Can I use Xp-power by shorting both the grounds, since I dont want this in isolated system, both 84V ground & MCU ground should be same.
7. Which one should I prefer for E-auto or E-bike system where transient due to motor occur

[samofab]

LM5017 is a great chip and fits perfectly with your requirements. Costs about 3.5 EUR in single quantity..
The passives add a few EUR and you can have a board for a few EUR, too (see my post above for the PCB layout - I can upload gerbers if you want, too).
Also, TI has a nice samples program (works if you use company email).

[Vindhyachal.takniki]

Thanks for your response.