Jelly Bean 3A npn transistor - Page 1
Forum section: electronics / beginners

author: mainul004
author_rank: Newbie
timestamp: November 09, 2014, 09:39:09 am
content: I am a newbie (literally, registered few minutes ago) here. But, I have been watching videos (almost finished all videos by Dave) and reading posts from the forum for some time.I am designing a power supply outputting 5V 3A from 12V-25V source as a part of a project. I have watched EEVblog #110 () and chose to use MC34063 as the controller IC in buck mode. But, MC34063 can output upto 1.5A current only. It can be extended by using an external switch (http://www.ti.com/lit/ds/symlink/mc34063a.pdf,page 8, figure 9a) to provide a higher current path, 3A in this case.My question is, what can be a good jelly bean 3A npn tansistor for this project? Please note, I am considering jelly bean due to two aspects: competitive price and availability.Alternatively, if you know how I can solve the problem with MOSFET, please reply as well. I would like to know the circuit to bias the MOSFET, and a jelly bean MOSFET suggestion.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2014, 10:06:17 am
content: Please don't use MC34063... the world deserves more dignity...You'll save more cost using an integrated switch controller, just on the inductor and capacitor size alone.Example:http://www.ti.com/product/TPS54240Tim

author: mainul004
author_rank: Newbie
timestamp: November 09, 2014, 10:39:45 am
content: Hi T3sl4co1l,Thanks for the reply. I understand, MC34063 is an ancient IC. But, still trying to understand why this is not economical choice (power conversion efficiency is not a major consideration for the project). Your idea is good but as I am trying to avoid design on proprietary IC, can you please suggest a jelly bean integrated switch controller?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 09, 2014, 10:29:47 pm
content: Quote from: mainul004 on November 09, 2014, 10:39:45 amIf you require "jellybean", one of those SimpleSwitcher (or other brand name) devices would do as well (e.g. LM2576). LM2576 for example shows stock at Digikey from TI, On Semi and Micrel. But don't expect the performance (efficiency, regulation) to be much better than the 34063...If you're still heart-set on the 3A transistor, TIP31 would seem to be the contemporary device to choose. It's slow, so don't forget the heatsink. The whole thing will be slow and hot, so that shouldn't be a problem. Like I said, you will spend more on the capacitors and inductor(s) than the silicon.If you'd rather avoid ICs at all, there are discrete solutions too.Tim

author: kxenos
author_rank: Frequent Contributor
timestamp: November 10, 2014, 12:08:32 am
content: Quote from: T3sl4co1l on November 09, 2014, 10:29:47 pmlike a chick I used to have. At least this thing will hopefully do something right

author: mainul004
author_rank: Newbie
timestamp: November 10, 2014, 02:58:13 am
content: Quote from: T3sl4co1l on November 09, 2014, 10:29:47 pmLets forget jelly bean and go to the basic question. What can be the best solution for a buck converter producing output of 5V 3A from 12V-25V source? The design is mostly constrained by price.

author: mainul004
author_rank: Newbie
timestamp: November 10, 2014, 02:59:54 am
content: Quote from: kxenos on November 10, 2014, 12:08:32 amListening to the feedback, I am not so sure.

author: kxenos
author_rank: Frequent Contributor
timestamp: November 10, 2014, 03:32:10 am
content: Check out this tool. Maybe you get some ideashttp://webench.ti.com/webench5/power/webench5.cgi?lang_chosen=en_US&VinMin=12&VinMax=25&O1V=5&O1I=3&op_TA=30

author: planet12
author_rank: Regular Contributor
timestamp: November 10, 2014, 04:19:01 am
content: If you want 3A out of a buck converter like this, your switch actually needs to be handle more than 3A - as 3A is the _average_, not the peak, which can be as high as twice the average if it's running in discontinuous mode.If you're set on the MC34063 (old, not the greatest, but whatever - I have a stack of them myself because they're _useful_), you might want to consider an external P-channel MOSFET switch:http://www.jiggerjuice.info/electronics/projects/power/buck-diode-pmos.htmlA properly-driven PFET in this use case can probably get away without a heatsink - but you're almost certainly going to need one if you use the basic PNP circuit from the datasheet - a slow transistor is going to take a good while to come out of saturation, losing power as heat while it does so (however, if you want to go this way, look up what a "baker clamp" and use one). Even with the clamp, it's still going to get hot, as the voltage drop in saturation at >= 3A is going to be moderately high.

author: mainul004
author_rank: Newbie
timestamp: November 12, 2014, 06:05:46 am
content: Thanks to you guys for the responses. We have decided to drop this SMPS for now as it was an optional feature.