How to increase the current capability of a switching power supply? - Page 1
Forum section: electronics / projects

author: hazuki
author_rank: Contributor
timestamp: November 11, 2014, 09:10:57 pm
content: Hi all. I need a +12V to +24V boost converter in my next project. I am looking through reference material for the LM2585 switching regulator, and have come across this schematic:Vout is specified at 0.6A maximum. I am needing 2.5A. What is the limitation in this circuit? Is it inductor size? Does anything overheat in this circuit over .6A or is it just that regulation cannot be maintained? How would I go about figuring out how to increase the current capability?Thanks

author: halexa
author_rank: Regular Contributor
timestamp: November 11, 2014, 09:47:09 pm
content: Hi,Look at this application report from TI "Basic Calculation of a Boost Converter's Power Stage".http://www.ti.com/lit/an/slva372c/slva372c.pdfBut I doubt that the LM2585 can give you 2.5A as an output current.

author: planet12
author_rank: Regular Contributor
timestamp: November 11, 2014, 09:54:42 pm
content: The limit for this particular circuit is the current handling capacity and power dissipation ability of the internal switch in the LM2585. Due to the design of these chips, they're simple to use, but you can't really add an external switch to them to boost their capacity.An output of 2.5A @ 24V is 60W, so you're getting into non-trivial boost converter sizing here. If you manage to get 80% efficiency, you're talking input currents of 6.25A @ 12V - ie. 75W, with 15W lost in the converter, so you probably want to aim for more efficient than that, which will normally mean synchronous rectification etc. Good, modern converters can reach 95% or so efficiency.Head over tohttp://www.ti.com/and have a look at their Webbench software - you can plug in your input voltage range, desired output voltage and current, and it will give you a number of design options which you can then refine.

author: hazuki
author_rank: Contributor
timestamp: November 11, 2014, 11:12:51 pm
content: Wow. The TI document and the Webench software are both great! Thank you. I'll have a read!

author: nctnico
author_rank: Super Contributor
timestamp: November 12, 2014, 12:47:00 am
content: Is second the recommendation of using a switcher with synchronous rectification. It takes a lot of problems with cooling away.

author: David Hess
author_rank: Super Contributor
timestamp: November 12, 2014, 02:38:03 am
content: Adding an external power switch to raise the current would not be difficult or a tapped inductor could be used or both.Quote from: planet12 on November 11, 2014, 09:54:42 pmA synchronous rectifier is only needed for efficiency if the output voltage is low which is not the case here. With a 24 volt output, the 1.2 volt drop from a silicon rectifier will only degrade the efficiency by about 5 percent. A schottky rectifier would halve that.

author: oldway
author_rank: Super Contributor
timestamp: November 12, 2014, 05:40:26 am
content: Quote from: hazuki on November 11, 2014, 09:10:57 pmFour of them in parallel

author: void_error
author_rank: Frequent Contributor
timestamp: November 12, 2014, 12:10:38 pm
content: Take a look at theTL494. You will also need an external power switch (BJT or MOSFET) and maybe a MOSFET driver chip.The TL494 is versatile, dirt cheap and is readily available. There's also anapplication notefor it.You can find all the math for a boost converterhere. The best way to go is to put all the formulas into a spreadsheet and let it spit the numbers out.