Voltage step-down - Page 1
Forum section: electronics / beginners

author: Flávio V
author_rank: Regular Contributor
timestamp: September 13, 2012, 08:30:42 pm
content: I would like to have a answer to a small problem than i have.I need to have a input voltage in the lm2576HV of 60-63V but the input voltage from the capacitors is 68-72V..the maximum input current is 3-3.5A which makes impossible to use a zener in series.(5W digikey)I seriously need help with that because i want to finish my "high efficiency linear power supply"(or is it considered switched type?)To be shorter the problem is to step-down voltage from 68-72V to 60-63V at 3-3.5A currents..

author: Rerouter
author_rank: Super Contributor
timestamp: September 13, 2012, 08:41:48 pm
content: it is a switchmode, as for a soltution and you chip has an absolute maximum input of 60V, so its time to look for another regulator in my book,

author: Flunze
author_rank: Regular Contributor
timestamp: September 13, 2012, 08:44:22 pm
content: Very simple! Stepping down the voltage can be easily done by putting high Uf and high current in series.A normal diode has about .7V Uf, so don't use them. Use something, you can put on a heatsink, because the heat will be Uf*Current.There are Diodes, that have Uf over 1 Volt, so at 3 amps, you will draw 3Watts of heat. 10 of them in series? What does it do? 10V Voltage drop - but? Well, 30Watts of heat. You get it. It is VERY massive, but will work and is very cheap, if you just need a fixed step down. I use this, to regulate 90 Volts, because a Voltage Regulator for that Output Voltage and a Current of about 6A would cost many hundred bucks, if they would even exist.

author: Flávio V
author_rank: Regular Contributor
timestamp: September 13, 2012, 09:47:13 pm
content: Quote from: Rerouter on September 13, 2012, 08:41:48 pmI am not totally sure if it is really...because i am going to use a mains transformer,but the regulation is switched so it makes some confusion to me...and maybe it is a rare method of power supply creation....and also there aren't through hole regulators for those input voltages...Quote from: Flunze on September 13, 2012, 08:44:22 pmAfter a quick check i saw one than seems pretty good(2.2V drop at 3A,1.6V at 1A,0.8V at no load(it worries me a bit))http://media.digikey.com/pdf/Data%20Sheets/Fairchild%20PDFs/FFP08S60SN.pdfI have some heatsinks here so i will not need to buy more,and because of the difference in the voltage drop i am thinking in making a select-able series diode number...so i will get 10 of those and will use some 1n4007s than i have here for low current draw or idle use,or i could use a 330ohm resistor(210mA) in parallel with the circuit to keep each diode to drop 1.1V each making only 8 diodes needed...

author: Alana
author_rank: Frequent Contributor
timestamp: September 13, 2012, 09:54:49 pm
content: Last time i had such thing i got rid of few turns of secondary winding in mains transformer.

author: Flunze
author_rank: Regular Contributor
timestamp: September 14, 2012, 07:01:22 am
content: Alana is right - it is realy a waste of energy, to regulate Voltages down at such very high currents. 10 Volts ad 3 Amps is 30 Watts - you can get quite some light with that...

author: poorchava
author_rank: Super Contributor
timestamp: September 14, 2012, 09:19:08 am
content: Other way is to make a crude preregulator with some beefy BJT in heatsink friendly package (like TO-3 or TO-247) with a zener+resistor driving the base (something like this:http://en.wikipedia.org/wiki/File:Series_reg.gif). May be cheaper than using z bazillion of diodes, but you still can't cheat laws of physics and this power will dissipate somewhere. You also need to pay attention not to go outside of BJT's SOAR, as this will likely cause C-E short and 90V into the switchmode regulator and a cascade failure of whole system. On the other hand you can use a power mosfet because they don't suffer from thermal runaway (channel resistance usually increases with temperature).Or you can use some kind of converter chip with external mos switch. I think there are such controllers that will endure high voltage for a short period just to start up and then draw the power from their own output.

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 14, 2012, 10:54:47 am
content: Get a LM5576?

author: Flávio V
author_rank: Regular Contributor
timestamp: September 14, 2012, 11:44:59 am
content: Quote from: poorchava on September 14, 2012, 09:19:08 amMay you explain the meaning of the transistor's "SOAR"?Quote from: PA0PBZ on September 14, 2012, 10:54:47 amI found that one but it is only available in a SMD tech which drives me a bit away...(harder to cool down)

author: ptricks
author_rank: Frequent Contributor
timestamp: September 15, 2012, 01:15:39 pm
content: To lower the voltage to something your supply can regulate I would use a second transformer on the primary, or remove some windings from the primary . A 1:1 transformer is fairly cheap and removing some of the secondary windings will lower the output voltage and you are not going to lose much power doing it this way, it is a lot less wasteful than using resistors or diodes and converting the power to heat.Has Dave ever done a video on transformers and rewinding them ? Really useful skill.