115vdc into 12vdc out regulated power supply design - Page 1
Forum section: electronics / projects

author: streeterk
author_rank: Unknown
timestamp: June 30, 2015, 09:35:01 am
content: I am kind of new to electronics design and an self taught, with the help of all the wonderful people on the internet of course.Thank you for all your time and effort to teach people like me.I'm working on a design to step down voltage from a 115vdc solar array for control circuitry requiring ~ 12vdc.I am having trouble finding an off the shelf regulator that can handle this so I have come up with this circuit.It looks good in spice, but It seems too good to be true, so I was hoping someone with more knowledge that me would evaluate it before I try and blow myself up.Thanks in advance,Kurt

author: nitro2k01
author_rank: Frequent Contributor
timestamp: June 30, 2015, 10:03:07 am
content: Consider trying a regular switching PSU brick intended for mains use. They should generally run quite happily on 115 VDC as they're typically rectified inside to produce DC for the primary switching circuitry anyway. They won't go much lower, though, since the low voltage cutoff will kick in and disable the PSU. Ie you might get dropouts when it's cloudy, perhaps.

author: amyk
author_rank: Super Contributor
timestamp: June 30, 2015, 11:17:19 am
content: The simpler adapters, the ones based on discretes, will run down to ~30-40V input.

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2015, 11:18:14 am
content: The question is: how much current do you need to draw?If it's just 4.8mA then a linear regulator is not a bad idea, since it will only need to dissipate about 500mW.As far as using mains power supply is concerned: yes that will work but it needs to be a switched mode, designed for work in Japan where the mains voltage is 100VAC.

author: langwadt
author_rank: Super Contributor
timestamp: June 30, 2015, 11:46:50 am
content: Quote from: Hero999 on June 30, 2015, 11:18:14 amit'll probably work, but remember 100VAC is ~140V when rectified so 115VDC is on the low side

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2015, 01:20:20 pm
content: If it's designed to work off 100VAC, it should work down to 90VAC and there will be the valley in ripple on the filter capacitors, so it should work at lower DC voltages than the peak AC voltage.Quote from: streeterk on June 30, 2015, 09:35:01 amHave you tested the circuit at different loads, input voltages and temperatures? You'll find it won't remain at 12V, if the conditions change.

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2015, 01:43:22 pm
content: The TL431 with some transistors to boost the current and voltage may work.EDIT: Whoops, I put 15V, rather than 115V.

author: streeterk
author_rank: Unknown
timestamp: June 30, 2015, 06:47:23 pm
content: Thank you for all your input.I'm sorry I did not provide more information from the start. It was very late when I posted, and I was a bit groggy, but I should know better.The power supply needs to provide about 12vdc, it could swing a volt or two without causing any problems as it is basically just supplying a time delayed relay board, but I need somewhere on the order of 150mA max for the load.The solar array powers a pump that is designed to operate at 30vdc to 300vdc directly off the array so the input voltage can vary widely.I measured the open circuit voltage of the array at 115vdc, so I am considering that my upper limit. Since I don’t need to control the pump if it’s not operating, I consider 30vdc my lower limit.I am considering adding a circuit to disable the regulator if the input voltage drops below 16vdc or so once I get the basic regulator working.I tested my circuit in spice for stepped input voltages and stepped load resistors and it appeared to perform well. I did not consider heat dissipation and, now that you mention it, that could be the catch that I suspected was lurking about.The circuit may be all wrong from the start. I got the idea from a mistake I made in my first concept to build a switch mode pre regulator from a NE555 and a mosfet having the zener regulate the voltage for the NE555 and the output from the NE555 driving the gate of the mosfet.I mocked up the circuit and tested it but it did not work as expected. I could not get the output voltage to exceed the output voltage of the NE555 and I needed the supply the NE555 with 16v to get the output voltage in the range I needed.I thought this was too close to the upper limit for the NE555 and the circuit was not functioning as expected, so I better try and figure out what was going on.After going over the circuit awhile I discovered that I had set up the circuit so that the load was affecting the voltage difference between the source and gate since I had the load on the source side of the mosfet.While I was considering how this “feedback” affected the circuit it occurred to me that I could use this feedback to control the mosfet directly, so I abandoned the NE555 altogether, hence the existing circuit.I have included a screenshot of the simulation results.

author: tszaboo
author_rank: Super Contributor
timestamp: June 30, 2015, 07:36:00 pm
content: Quote from: streeterk on June 30, 2015, 06:47:23 pm150mA at 110V is 16W. So unless you are ready to start heatsinking, I would not recomment using the linear regulator you draw.Read this:http://www.ti.com/lit/an/slyt077/slyt077.pdfTI appnote. So basically what is different, it has a winding to the DC-DC so after it started, the controller drains it power supply from the transformer (which is switched by itsel), not the main input. Kinda self powering after "bootstrapping" the circuit, which is the keyword you should search for for more information.

author: Zero999
author_rank: Super Contributor
timestamp: June 30, 2015, 10:21:50 pm
content: Perhaps a modified Black regulator circuit?http://www.romanblack.com/smps/smps.htm

author: BennVenn
author_rank: Regular Contributor
timestamp: June 30, 2015, 11:05:21 pm
content: http://secure.oatleyelectronics.com//product_info.php?cPath=21&products_id=1040Designed for an Open circuit voltage of around 95v but I'm sure with a few mods it'll run quite fine at 115.