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I'm working to power a micro controller that requires something like a USB power supply - 5VDC with a normal draw of .75 - 1 amp but could possibly draw as much as 1.5 amps.

I know that with smaller voltages such as a 9V battery it would be possible to use something like a 7805 Voltage Regulator but sacman has mentioned:

I just found out that these Regulators will stop regulating somewhere between 7v and 7.5v

Since I'm not going from AC->DC I don't need a (bridge) rectifier, right? But do I still need a (step down?) transformer or for a relatively low power draw like this are there other methods? If so,

  • Which method would be the smallest?
  • Which would be the least expensive?
  • Would it be the same answer if I had a 50VDC input voltage instead of 40VDC?

I did search this site for related questions but mostly found things that were AC->DC or low->high.

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did you consider using a switching DC-DC step down regulator? I think this is a must because also if you don't mention it, a linear regulator would dissipate as much as 70W, give or take some. And that's an awful lot. Curiosity: how comes you have this 40~50Vdc? What is your application? – Vladimir Cravero Apr 20 '14 at 20:05
@VladimirCravero - yes, considered it but wasn't sure this was the definitive way to go. this is kind of an odd voltage to be coming from. so you're saying linear is one option but it is pretty inefficient / lots of waste? reason for 50/40 - two possible voltage sources one is ~40VDC and one is ~50VDC - still need to get to 5VDC either way – cwd Apr 20 '14 at 20:07
I believe linear is not an option, 70W is really too much. I don't even know if there are chips that can dissipate that much power, in any case you would need a huge heatsink that adds cost and space. Search for "dc dc step down" on rs, mouser, digikey or whatever. – Vladimir Cravero Apr 20 '14 at 20:09
The term you want is "Buck" regulator. As opposed to boost regulators which is low to high, or transformer, which is ac to dc. – Passerby Apr 20 '14 at 23:18
up vote 6 down vote accepted

The smallest method (only one power IC) would be using LM2576HV. It accepts maximum 60V as input (only the HV version).

Here is the schematic from datasheet:

enter image description here

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While it is considered cheating in some circles, you did ask about cheapest. You can buy assembled versions of Cornelius's schematic for very low cost. See, for instance, http://www.ebay.com/itm/5pcs-DC-DC-3A-Buck-Converter-Adjustable-Step-Down-Power-Supply-Module-LM2596S-/181350963662?pt=LH_DefaultDomain_0&hash=item2a395c19ce which will get you 5 modules for a total of $6.23, including free shipping. This is probably less than you'd spend buying the parts from Digikey.

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Very nice - except I would update your link to just be a general search on ebay for "LM2576HV" instead of a specific item as it will only be a matter of time before that is a broken link. Also, the link you added was for the LM2596S and I think as @Cornelius mentioned I would need the LM2576HV version in order to get up to 60V ( even the 60V one seems to have a recommended working voltage of below 50V so it may be better to get the HV for either 40V or 50V to have extra tolerance ) but you're right - this is certainly something worth buying pre-built - thank you! – cwd Apr 21 '14 at 1:00
Be sure that the module you get has properly rated input capacitors. Those ones in the link use 50v caps but i think some of them use a 42v chip but only 35v caps and try to pass the thing off as a 42v reg. You can almost always see the cap rating in the picture. – EternityForest Apr 21 '14 at 6:48

Transformers only work with alternate current, they reach saturation in DC and you get nothing (good) out of them. For DC DC conversion, the main methods are:

  • Regulators based on ballast transistors: the series transistor is controlled so that the output is as desired - the transistor achieves this by opening itself more or less, which adjusts the drop in voltage. Voltage times current gives the power dissipated. In your case, that's (40-5)*1.5 = 52.5W which is monstruous for a 7805: since its thermal resistance to ambient is 65°C/W, you would end up with a junction temperature of 52.5*65+20=3432°C!
  • Switching (regulators are also a form of DC DC converters) DC-DC converters: those are based typically on a half-bridge (see google) controlled by pulses modulated in width (PWM) to achieve in average the voltage you want by chopping the output between high and low. Requires heavy filtering, but the power dissipation is very low. That's your best option, if you can't reduce the upstream voltage.

P.S: The above LM2576 is a form of switching DCDC converter

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+1 - thank you for the nice explanation and math! – cwd Apr 21 '14 at 1:05

Have you checked out TI's search page?

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