Rehash of stepping down 57 volts to 12v, POE related - Page 1
Forum section: electronics / projects

author: lk
author_rank: Regular Contributor
timestamp: November 27, 2012, 11:25:53 am
content: Greetings,I'm still thinking of how to exploit the power part of POE, having been pushed in the right direction on how to make the switch provide the power, I'm still stuck with how to deal with the 57 volt that a the POE PD can potentially receive. I guess that I will need 12v at 500mA output. My primary driver is cost, and the fact that I only have 70 x 70 mm room in the box I want to mount this in, and there has to be room for both the POE related circuity and 2 ethernet jacks and some sort of 2.1 mm power out jack, I have not finalized how I want to do that.I have looked at the SIMPLE SWITCHER ™ HV parts, and they are form the a technical standpoint, but they are fairly expensive from ~3.5 USD and up no including shipping in small volume (lm2576hv, lm2575hv and so on), and i just feel there is some other way that is cheaper to do it.These are the ways I thought up, all have more or less failed ?1. A string of diodes that could drop the voltage from 57 volt to something that is manageable by the MC34063, this just got silly, as I would need 20 diodes in series to get to less than 40 volts, and this would be all over the place with the changes in current ?2. A zener shunt regulator that could take the voltage down to 39~ volt, and then I could pass the remaining voltage through the MC34063, this was almost perfect apart from the 6.5 watts that I have to dissipate in the series transistor ?3. “Roman Blacks SMPS”http://www.romanblack.com/smps/smps.htm] http://www.romanblack.com/smps/smps.htm , problem here is that I have trouble finding transistors that can handle the current, and the voltage. If forced to handle 57 volt I was looking at Vce of 45 volts or more, for some of the transistors. Q1 if i remember correctly.I'm thinking that some sort of discrete solution in line with Roman Blacks is what I need, just one that is able to handle a little more input voltage, OK twice as much ?And I refuse to give up and inject my own power-lk

author: kripton2035
author_rank: Super Contributor
timestamp: November 27, 2012, 11:39:23 am
content: here is a schematic from texas instruments, supplying 5v @2.5A or 3.3Vyou can adapt it to 12v

author: kripton2035
author_rank: Super Contributor
timestamp: November 27, 2012, 11:41:57 am
content: another schematic from olimex with a zener to drop the voltage to 4v

author: lk
author_rank: Regular Contributor
timestamp: November 27, 2012, 12:38:32 pm
content: Kripton2035, thanks for your replies.the tps40061PWP is a 5$ part so I'm not much better of than with the simple switchers, apart from the fact that the simple switchers, look to be easier to work withBut thank you for taking time to dig it up for me, the poe initialization part looks interesting.and for the other one, to me it looks like all the current that is going to the 3 volt rail has to go through the zener, is that not upwards of 0.5 watt dissipated per milliamp going through it? i might have missed something, and usually i have no idea what I'm talking about-lk

author: Simon
author_rank: Unknown
timestamp: November 27, 2012, 12:39:28 pm
content: otherwise make a "descrete" smps with parts that can handle the voltage, a comparator driven circuit with mosfet switch, inductor, diode and capacitor will make a decent pre regulator, if you know it is constant 57V in you can do a fixed pulsewidth generator (555) running a mosfet switch etc.

author: nctnico
author_rank: Super Contributor
timestamp: November 27, 2012, 04:40:02 pm
content: You really need a PoE switcher chip like the LM5070 for use with PoE. Without a special burden resistor and the initialisation protocol a PoE switch will not output any power. There are readily made transformers for use with the LM5070 so all you need is to take the LM5070 reference design and recalculate the output voltage and use a 12V transformer.

author: gxti
author_rank: Frequent Contributor
timestamp: November 28, 2012, 12:33:28 am
content: There are parts that do just the negotiation along with some other housekeeping functions, and leave it up to you to make something out of the power you get. NCP1090 is a good example, and is about $1.60 each. But an integrated controller is probably going to be your best value, and you really want a transformer to isolate the line from your circuit even if you don't care about compliance.

author: lk
author_rank: Regular Contributor
timestamp: November 28, 2012, 07:39:12 am
content: gxti: the NCP1090 was just what i was looking for for that part of the circuitry, and looks like budgetary price is only 0.45$, but i guess that is when you need 4990 units more than me, but anyway i will see if i can couple ncp1090 with my own homemade switching regulator, and see how much magic blue smoke i get from that-lk

author: Simon
author_rank: Unknown
timestamp: November 28, 2012, 07:53:23 am
content: Quote from: lk on November 28, 2012, 07:39:12 amwhich country are you in ?

author: lk
author_rank: Regular Contributor
timestamp: November 28, 2012, 10:19:19 am
content: seems like i forgot some settings on my profile, but anyway the answer is Denmark-lk

author: Simon
author_rank: Unknown
timestamp: November 28, 2012, 10:40:17 am
content: How about these:http://uk.farnell.com/jsp/search/browse.jsp;jsessionid=QQ4EGOZ4CSEEGCQLCIPJN4Q?N=0&Ntk=gensearch&Ntt=NCP1090&Ntx=mode+matchallpartial&exposeLevel2Refinement=true&suggestions=false&ref=globalsearch&_requestid=435953? I can get them for you if you want (farnell price+vat+postage)

author: mickpah
author_rank: Regular Contributor
timestamp: November 28, 2012, 10:56:26 am
content: have a look at these -http://www.semiconductorstore.com/cart/pc/viewprd.asp?idProduct=13987$8.40 in low quantity , other models also available for higher power etc.Have used these with the freetronics etherten in fact freetronics sell them already on a board with a pretty blue led addedhttp://www.freetronics.com/collections/ethernet/products/power-over-ethernet-regulator-8023af#.ULXtrazql8E

author: lk
author_rank: Regular Contributor
timestamp: November 28, 2012, 11:13:04 am
content: Simon: thank you for you offer to ship the parts, i found them at digikey.dk, and will get them from there, have enough stuff to qualify for free shippingmickpah: those look great, but i don't mind the challenge of making this work, and remember i need to be able to fit them in a small junction style box and i need the ethernet connectors and power output as well.-lk