50v in 24V out linear regulators?? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 07, 2016, 11:08:04 am
content: Heat issues aside is there a linear voltage regulator that will take 50V DC in and give 24V DC out at about 0.5 amps? Thanks.

author: JPortici
author_rank: Super Contributor
timestamp: June 07, 2016, 11:17:51 am
content: Sure. your typical lm317. but why waste half the power in heat?

author: Kjelt
author_rank: Super Contributor
timestamp: June 07, 2016, 11:20:47 am
content: Not the typical LM317 but it's High Voltage version: LM317(A)HVT can go to 57V.

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 07, 2016, 11:47:29 am
content: A DC to .DC converter would perhaps do but I cannot see any that handle more than 35v in.Thanks for the replies so far

author: JPortici
author_rank: Super Contributor
timestamp: June 07, 2016, 11:51:05 am
content: you can roll your own! there are about 100 buck controller/converters on TI's catalog that handle more than 50V on the inputhttp://webench.ti.com/wb5/PQS.jsp?&isPqsHome=true&expCriteria=true&vinRadio=1a&userVin=50&userVout1=24&userIout1=.5&&efPT=1000064&cfPT=1000662&cfPT=1000661&cfPT=1001647&cfPT=1000400&cfPT=1000527&cfPT=1002030&cfPT=1000720&cfPT=1000689&cfPT=1000404&cfPT=1000100&cfPT=1000690&cfPT=1000394&cfPT=1001992&cfPT=1000685&cfPT=1000462&familyId=64&showAdditionalParameters=no&compare=yes&download=yes&sort=yes&customize=yes&paramResults=yes&paramCriteria=no&familyTree=no&military=&baSystem=yes&sortOption=&sortMode=&pageId=64&templateId=2&navigationId=11354&family=analog&military=&contentType=4&undefined&uiTemplateId=PQS_RESULTS_T&callingTemplateId=POWER_HOME_T

author: madires
author_rank: Super Contributor
timestamp: June 07, 2016, 11:54:06 am
content: There are high voltage versions of some buck converters, like the LM2575HV (57V).

author: grumpydoc
author_rank: Super Contributor
timestamp: June 07, 2016, 11:56:57 am
content: If you want off-the-shelf try looking for 48V input, as that is standard for telecoms equipment.Eghttp://www.powerstream.com/dcdc-48V.htm

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 07, 2016, 12:18:51 pm
content: Quote from: Chris Wilson on June 07, 2016, 11:47:29 amYou can't have been looking very hard - there are plenty of 48v converters out there.

author: Kjelt
author_rank: Super Contributor
timestamp: June 07, 2016, 12:34:33 pm
content: DC-DC converters are NOT linear as you requested in your openingspost.But what is the application, if it is not audio or rf sensitive opamp stuff why not just use a smps like a dc-dc converter? They can be get usually in the 36V upto 72V input as mike said, dozens available.Just an example from Meanwell

author: mariush
author_rank: Super Contributor
timestamp: June 07, 2016, 01:50:14 pm
content: Quote from: Chris Wilson on June 07, 2016, 11:08:04 am(50v - 24v) x 0.5 = 13 watts of power. Most linear regulators are internally capped to a maximum of 15 watts of thermal dissipation, you're awfully close to the max and may have issues if your circuit suddenly uses more than 0.5A for brief periods of time.You say heat issues aside... but dissipating 13 watts is still a big deal.If you insist on linear regulators, there's few who will take 50 volts in (look for high voltage versions of chips or for automotive). Alternatively, you could always slap a bunch of diodes in series and each diode will drop 0.7-1v and this way, you'd drop the voltage down to a maximum that's supported by a wider range of linear regulators. A lot of regulators support up to 40v, so it may be cheaper to put about 10-12 diodes on a tiny pcb to spread the heat and maybe a zener diode to protect a linear regulator to make it see only 39v or so at most.

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 07, 2016, 02:47:54 pm
content: Thanks for the further repliesI have looked at the circuit, a Skanti solid state 250W RF power amp that is without its sister stuff, so I need to supply 48V at a lot of amps (done, a server SMPS off Ebay that will be more than adequate), from which i can also get 5V for which I have built a small buck converter now, using one of the HV devices mentioned, and it works. I feed 10V from it it into a linear regulator as it is for the PA transistor bias, and I wanted it to be as clean as possible, I *think* what i have done will help there? So I will build a 24V out version, but I am stuck on where to get and what type of choke it needs. the data sheet for the device shows a table and a graph, and I think it means I need either a 680uH or a 1mH device. Quite what other requirements the choke needs I am unsure, as well as where to get it in the UK. If anyone has the inclination and patience to look at the data sheet and tell me just what parameters the choke needs for say a max of 0.5 amps at 24V from a 48V input I'd be very obliged.The link to the data sheet ishttp://www.gatesgarth.com/data.pdfas it's too big to attach here. The ready made ones seem to all be in China, or overly large and expensive, and I wanted get this running ASAP. Thanks again everyone.

author: rstofer
author_rank: Super Contributor
timestamp: June 07, 2016, 03:10:18 pm
content: Dave Jones did a video on designing a switch mode power supply:https://www.youtube.com/watch?v=qGp82xhybs4https://www.youtube.com/watch?v=qGp82xhybs4His process mirrors very closely the calculations shown in the datasheet you linked. Perhaps it will be a help.

author: Zero999
author_rank: Super Contributor
timestamp: June 07, 2016, 03:11:13 pm
content: Quote from: Kjelt on June 07, 2016, 11:20:47 amThe ordinary LM317 can take 50V in and output 24V, as it's a floating regulator and only sees the difference between the input and output. The trouble is you lose the short circuit protection and you need to be careful that any capacitors charging on the output don't trip the short circuit protection, causing the input-output differential to exceed the rated 40V.

author: JPortici
author_rank: Super Contributor
timestamp: June 07, 2016, 04:43:35 pm
content: Keep in mind thisAs the regulator will not clean the ripple that much(for example, lm317: from about 10kHz the ripple rejection drop. at 200kHz it is only 30 dB. As jim williams points out in this video a ferrite bead before the regulator filters will be more effective in cleaning the output than the regulator)

author: Chris Wilson
author_rank: Super Contributor
timestamp: June 07, 2016, 11:18:37 pm
content: Quote from: JPortici on June 07, 2016, 04:43:35 pmVery VERY useful. That presentation has put e straight on how I should alter the regulator input and output, many thanks.

author: JPortici
author_rank: Super Contributor
timestamp: June 08, 2016, 03:42:50 am
content: also in the datasheet of the lm317 the ripple rejection drops again down to 30 db or so if Vi-Vo is greater than 20V. switching + post regulation is what you want to use

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 08, 2016, 05:13:28 am
content: Great video, thanks @JPorticiThis is a great point that everyone should note well...Quote

author: Kjelt
author_rank: Super Contributor
timestamp: June 08, 2016, 09:16:27 am
content: Nice video thanks!Can some explain a bit more, what is the formula to calculate the beads properties, if I look at the parameters there are (with example values):Impedance @ 100MHz 49ohmDC Current Rating 5ADC Resistance Max 600µohmSo the DC current and DC resistance are pretty clear, if your regulator should supply 1A the bead should (easily) be able to handle that, and the voltage drop across the bead is simply Ohms law, but how can i calculate from the impedance at @100MHz how much effect it will have at 20kHz or 5MHz ? Or is it a bit of trial and error ?

author: JPortici
author_rank: Super Contributor
timestamp: June 08, 2016, 10:05:58 am
content: An impedance graph is usually on the datasheet

author: grifftech
author_rank: Frequent Contributor
timestamp: June 08, 2016, 10:27:48 pm
content: I would try 2 in series, one from 50 volts to 36 volts and the other from 36 volts to 24 volts.