LM7805 alternative (Heat problems) - Page 1
Forum section: electronics / projects

author: ianscott
author_rank: Contributor
timestamp: March 28, 2015, 02:54:48 am
content: Hey guysI have recently done a project using a DPAK LM7805 and because of the voltage drop the heat sink is approximately 100 degrees Celsius. But the strange thing is that only some of the 15 boards I have made get to those up to those temps. the rest sit at around 30-40 degrees Celsius.The input voltage is 12V (mainly running of a 3s Lipo) and the max current (from the 5v side) is around 350maJust wondering if there is a alternative to the LM7805, the reason I chose it is because its low board surface area. I have found some replacements like DC-DC converters but they are around $15 each.So small and cheap is what I'm going forThanks!

author: georges80
author_rank: Frequent Contributor
timestamp: March 28, 2015, 03:12:42 am
content: Well, you a dropping about 7V at 350mA so that's over 2W of dissipation in the regulator.There's several small/compact DC:DC converters that are in the $3 - $4 range (single unit pricing) on digikey that would take care of your 5V supply issues.Seems a bad choice to me to be using a linear regulator for 5V... you are burning more power in the regulator than you are in the circuit it is powering...cheers,george.

author: codeboy2k
author_rank: Super Contributor
timestamp: March 28, 2015, 03:25:34 am
content: Yeah it's about 2.5W.Did you pick the DPAK or the D2PAK ? The D2PAK is my preferred choice, it's much betterThe DPAK has a 90 deg C / Watt Junction-Ambient, whereas the D2PAK is about 65 deg C/WSo if you can dissipate 1W or so into the board, then the other watt is going into the air. So you can easily expect it to rise to the 100's Celcius.If you can't replace it with a SMPS, then I'd go for a small DPAK heatsink that you can glue on.The best case for DPAK and D2PAK heatsinks is if you put a large copper area under the IC, then the heatsink can go over and around the chip and contact the copper board area directly, since the DPAK and D2PAK have metal pads on the bottom that are attached to the die.If you can't do that, a small glue-on finned heatsink can be applied to the IC case. It's better than nothing but not as effective as putting a heatsink directly to the copper board area.

author: David_AVD
author_rank: Super Contributor
timestamp: March 28, 2015, 04:18:20 am
content: Quote from: ianscott on March 28, 2015, 02:54:48 amIs it possible that the ones that are getting hot are oscillating? Maybe check your capacitor choice / placement.

author: digsys
author_rank: Unknown
timestamp: March 28, 2015, 05:02:03 am
content: Or if you really can't use a small DC-DC converter, why not look at washing of ~6V at the input with a diode(s)/zeners or power resistor and spread the heat.

author: voja
author_rank: Contributor
timestamp: March 28, 2015, 06:07:35 am
content: There is no linear regulator alternative, it will always dissipate I*(Ui-Uo). I regulary use switching regulator LM2674-5.0 and there is no noticeable heat dissipation. It requires two 100 uF electrolytic capacitors and one 100 uH coil, but it is still smaller and much cooler than one heatsink.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 28, 2015, 06:22:23 am
content: Quote from: ianscott on March 28, 2015, 02:54:48 amhuh?! i just bought 3A dcdc for $0.70 a pop in 10qty...http://www.ebay.com.my/itm/171637521472?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649its either what i bought is immitation or the quality is crap thereof or your source is from suck arse company or the quality and longevity is so great... otherwise, if linear type is so critical at the end, feed the lipo to dcdc and then to the linear reg. linear reg will burden much-much less set at say 1-3V drop.

author: Niklas
author_rank: Frequent Contributor
timestamp: March 28, 2015, 08:45:04 am
content: Are all the 7805s from the same manufacturer? I have seen some TL431 zeners in SOT23 packages that had different thermal impedance depending on the manufacturer. 200K/W vs 380K/W

author: w2aew
author_rank: Super Contributor
timestamp: March 28, 2015, 12:56:03 pm
content: Quote from: David_AVD on March 28, 2015, 04:18:20 amIt is also possible that the ones that are getting hotter could be insufficiently soldered on the large tab under the part. Very difficult to inspect for. This would dramatically increase the thermal resistance.

author: wraper
author_rank: Unknown
timestamp: March 28, 2015, 01:10:42 pm
content: Quote from: ianscott on March 28, 2015, 02:54:48 amThis suggests that the issue is not in the LM7805. Either those boards have increased current consumption because of some other fault, or you have not decoupled LM7805 properly and they are oscillating.

author: Jeroen3
author_rank: Super Contributor
timestamp: March 28, 2015, 01:17:40 pm
content: Get thesehttp://www.recom-international.com/pdf/Innoline/R-78xx-0.5.pdfThey're very robust, and we have yet to encounter a failed one. (>2k products)

author: wraper
author_rank: Unknown
timestamp: March 28, 2015, 01:38:12 pm
content: Quote from: Mechatrommer on March 28, 2015, 06:22:23 amTry to run them at close to the 3A current, I have serious concerns if it will survive even 1A current. But seriously, usually they are crap and actual IC inside is not what is written on top of it. Often even switching frequency is completely different from what genuine IC would do.

author: Zero999
author_rank: Super Contributor
timestamp: March 28, 2015, 01:49:49 pm
content: Quote from: Mechatrommer on March 28, 2015, 06:22:23 amThere's no brand name and it's highly unlikely it'll meet all the specifications.You're better off buying a branded product. The muRata series has the advantage of being pin compatible with the LM7805 package.http://power.murata.com/data/power/dms-78xxsr.pdf

author: TimFox
author_rank: Super Contributor
timestamp: March 28, 2015, 04:40:01 pm
content: Quote from: wraper on March 28, 2015, 01:10:42 pmTo avoid oscillation with the 7805, it is imperative to have a capacitor close to the unit between "input" and "common". The original data sheets specified 0.33 uF, and I always used 0.47 uF X7R ceramic as close as practicible to the 7805. A capacitor at the output may be useful, but not to prevent oscillation. Without the input capacitor, oscillation at frequencies around 100 kHz or higher was common.

author: LabSpokane
author_rank: Super Contributor
timestamp: March 28, 2015, 04:47:45 pm
content: Quote from: Jeroen3 on March 28, 2015, 01:17:40 pm+1 for the Recom for a 1/2 amp solution. Murata make a 1.5A version that is also economical and performs well.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 28, 2015, 07:44:47 pm
content: Quote from: Hero999 on March 28, 2015, 01:49:49 pmthanks for the link, the module looks similar, and with detail specs i can make comparison when the immitation arrived. cross my finger on this...