Looking for some help in designing Boost converters - Page 1
Forum section: electronics / projects

author: 2nOrderEDO
author_rank: Regular Contributor
timestamp: March 13, 2013, 03:09:05 pm
content: Hi!I'm viewing all the Dave's VLogs from the very first one to the last one. Yesterday, I was seeing EEVBLOG #139, in which Dave tries to find a one cell DC-DC boost converter IC with more or less the next requirements:-Minimum input voltage: 0.8 V-Output voltage: 5V-Output current: 500mAFree link to the EEVBLOG #139:http://youtu.be/-V_p1GBH4pkAt the end of the video Dave saids that he has three options: Use a very expensive DC-DC IC that he found, parallel some cheaper DC-DC IC, or design and build it's own boost converter. I was curious about all this stuff and I said to myself: "Let's try to build such a boost converter". So this is my simulation schematic (better resolution -> see attached img.):As you can see in the graph below the circuit, the control loop is very good. The "black-box" named VCPWM is just a linear voltage controlled PWM running at 10 KHz. The control voltage can go from 0V to 5V (0% to 100% duty).The problem that I have found is that my circuit cannot deliver very much power, even with (I think it is) a overrated inductor. If I change the Load Resistor (RL) to 300 Ohms, the output voltage falls dramatically.I revised a lot of technical papers, tutorials an application notes from manufacturers and third people, but I don't see how to solve the problem while maintaining my constraints. I think asking in the forum is a good way to go because I can learn quickly with your help.Let me know what you think! Thanks in advance!PD1: I know the thing you gonna say about the feedback resistors, I did it this way because is just a simulation xD.PD2: Sorry, my English is no perfect, make me know if there are some spelling mistakes or whatever ^^

author: baoshi
author_rank: Regular Contributor
timestamp: March 13, 2013, 04:12:22 pm
content: Hi,I'm not pro in this field, just some suggestion:You can download TI's power stage designer from herehttp://www.ti.com/tool/powerstage-designerThis will calculator required inductor value for you. It seems 47uH is on the small side, especially when the load is light.Bob

author: darko31
author_rank: Regular Contributor
timestamp: March 13, 2013, 06:40:48 pm
content: You should probably use higher frequency, something like 50kHz or even 100kHz. It will allow you to use smaller inductors and maybe even take care of your power problem.

author: Harvs
author_rank: Super Contributor
timestamp: March 14, 2013, 02:30:00 am
content: Firstly you need to limit the duty cycle of the PWM. A boot converter works by storing energy in the magnetic field of the inductor during the "on" period, then transferring it via the diode during the off period. Therefore you need an off period. The smaller the off period, the higher the current needs to build up to prior to satisfying the load (which there's lots of practicality limits for.)

author: IanM
author_rank: Newbie
timestamp: March 14, 2013, 02:49:33 am
content: The problem with your power may be your source. If everything is perfect, your source has to be able to supply 3.125 amps at 0.8V (unless I don't understand what is going on).