Did I purchase the right components for power electronics PCB prototyping? - Page 1
Forum section: electronics / beginners

author: engineheat
author_rank: Frequent Contributor
timestamp: September 04, 2017, 04:03:16 pm
content: I'm interested in designing a boost converter using the TPS61021A chip from TI.Data sheet:http://www.ti.com/lit/ds/symlink/tps61021a.pdfOriginally, I wanted to build a Manhattan type dead bug prototype circuit by following the example circuit on page 12. I bought components off Mouser by going by the guidelines given in page 13-14.For the inductor, I bought the exact Coilcraft inductor specified in the datasheet so there is no problem. But when it comes to capacitors and resistors, I calculated the values I need, but there are different types/power rating/mount type, and I don't think I bought the most suitable ones. Here are the ones I bought, along with labels corresponding to the components in the circuit on page 12:A few issues:1) R2: Notice how big R2 is. I believe this is because I bought one with too high of a power rating. This is definitely not suitable.2) R1: This looks like a typical resistor. Notice how it says "metal film" while R2 says "metal oxide." For my application, does it matter whattype of resistor to use?3) Capacitors: I believe I got X7R ceramic caps for all. Do you see any problems with the capacitors? There are no problems with the values. Ialso believe I took into account of capacitance derating under dc bias voltage. Notice the high voltage value for the capacitors on the bag.Anyway, those were purchased a month ago. Then I realized it's hard to make a dead bug style circuit since the IC chip in question is of the WSON package which is way too small for me to work with. I plan to make a PCB prototype. Are there places where I can send in the components and an Eagle file and they can prototype cheaply and quickly?Thanks

author: KuroZ
author_rank: Contributor
timestamp: September 04, 2017, 09:18:53 pm
content: Quote from: engineheat on September 04, 2017, 04:03:16 pmI'll assume you bought a 383K resistor for R1 to meet your specific voltage needs, around 3.83V.R2 is a 3W resistor. It's way overrated for your use, but the circuit should work normally using it. This resistor is used as a feedback to the switching IC, and it's pretty easy to have and idea of the power dissipated.If your output was 5V (higher than 3.3V or 3.8V, this will give you some margin), the DC current flowing through R1 and R2 would be 5V/(R1+R2), which is around 10µA.Now you should be able to calculate the power for each resistor as P = R*i²:R1 = 383k * (10µA)² = 38.3µWR2 = 100k * (10µA)² = 10.0µWThat means you could even be using very small SMD resistors which are rated 1/16W.Quote from: engineheat on September 04, 2017, 04:03:16 pmThere should be no problem with these common types of resistor.Quote from: engineheat on September 04, 2017, 04:03:16 pmI wouldn't say that the 22µF capacitor has a high voltage value at 6.3V, but it should work.About C3, the datasheet recommends a 50kHz fFFZ.fFFZ= 1/(2pi * C3 * R1) = 1/(2pi * 10pf * 383k) = 41.56kHzMaybe you should've got a 8pF capacitor.Quote from: engineheat on September 04, 2017, 04:03:16 pmYes, there are.Seeedstudio should be able to do PCBA, so you can have a look with them. If you are going through this route, I suggest using 0805 or 0603 resistors for R1 and R2.

author: engineheat
author_rank: Frequent Contributor
timestamp: September 06, 2017, 04:34:56 pm
content: Quote from: KuroZ on September 04, 2017, 09:18:53 pmThanks a lot for the calculations. It really helps me to double check which is what I'll do. Then, I will reorder components for PCB prototyping. I'll probably fly it by you before I place an order this time.

author: engineheat
author_rank: Frequent Contributor
timestamp: September 06, 2017, 04:40:30 pm
content: Quote from: evb149 on September 04, 2017, 09:36:20 pmThanks. I never realized there are different four digit numbers corresponding to the package size. I understand the benefit for using the most commonly "in stock" size.So I should use 1210, 1206 or 0805 sizes for the capacitors?KuroZ recommended 0805 or 0603 for resistors. I'm going to look into this more.

author: KuroZ
author_rank: Contributor
timestamp: September 06, 2017, 06:02:12 pm
content: Quote from: engineheat on September 06, 2017, 04:40:30 pmI tend to stick with a given size for both resistors and capacitors on a project, with (almost) everything being 0805 or 0603. You may find some components harder or more expensive to get in smaller sizes, like 10µF or more capacitors. If that's the case, I get these components in a bigger package.Any of these packages is fine, but I find 0805 and 0603 to be the most common. If you think these may be too small for probing the circuit or repair by hand later, you may want to go with 1210 or 1206, as they are bigger.PS: Be careful with these numbers, they may be in metric or imperial size. I'm talking about the imperial size, as I find it to be the most common. 0603 metric is very different from 0603 imperial. Have a look athttps://en.wikipedia.org/wiki/Surface-mount_technology#Rectangular_passive_components

author: lem_ix
author_rank: Regular Contributor
timestamp: September 06, 2017, 08:03:32 pm
content: Besides the size/power rating, the metal film resistor is a 1% tolerance vs 5% on the other one. Since these resistors are used to set the voltage you may prefer 1% tolerance. Since this is a 2 Mhz converter you definitely want a tightly packed smd layout. Nice example in the datasheet. Probably easier to find caps in a larger size.

author: engineheat
author_rank: Frequent Contributor
timestamp: September 22, 2017, 05:12:44 pm
content: I decided to do a PCB prototype using PCBexpress. They can also assemble it for me.I have never used Eagle and right now is the school year and I'm busy. Anyone here interested in preparing me a Gerber file for me based on the layout given in the datasheet? I am willing to pay. PM me if interested.

author: engineheat
author_rank: Frequent Contributor
timestamp: September 22, 2017, 05:16:34 pm
content: Quote from: KuroZ on September 04, 2017, 09:18:53 pmCan I pay to have you prepare a Gerber file for me to get it prototyped? I have never used Eagle or any other PCB software before. Moreover, this circuit seems to require experience in getting it right (the distance between components, etc...). Thanks

author: KuroZ
author_rank: Contributor
timestamp: September 22, 2017, 08:08:48 pm
content: Quote from: engineheat on September 22, 2017, 05:16:34 pmSorry, I don't want to.It kinds of interest me, though, that you said you were interested in designing a boost converter but don't want to design the board. Any reason for that? Do you need the final design for something else or just wanted to design the boost converter?If the first, is there any reason you can't use a off-the-shelf boost converter? If the second, I really think designing the board is part of designing the circuit itself.

author: Teledog
author_rank: Regular Contributor
timestamp: September 23, 2017, 02:36:40 am
content: I can appreciate the study/work you've done on this, but for less than 99 cents;https://www.ebay.ca/sch/i.html?_odkw=boost+converter+1A&_osacat=0&_from=R40&_trksid=p2045573.m570.l1313.TR0.TRC0.H0.Xboost+converter+2A.TRS0&_nkw=boost+converter+2A&_sacat=0Sometimes it's just not worth it..been there..done that...

author: engineheat
author_rank: Frequent Contributor
timestamp: September 23, 2017, 08:06:57 pm
content: Quote from: KuroZ on September 22, 2017, 08:08:48 pmThe boost converter is used for something else. It's part of a larger system. The output voltage will be used to drive a motor controller. There will be a microcontroller too. I don't want to use off the shelf boost converter because : A) I want a circuit I understand 100%, B) I want a customized single PCB eventually that incorporates all functions. C) It's hard to find one at the right price/size that gives me the spec I need. I spent a lot of time finding this particular TI chip that does what I need as far as input/output voltage and output current go.I want to quickly find out how that particular chip work. If that chip wasn't so small, I would've built a dead bug circuit myself over the summer.I'd love learn Eagle myself and design/learn this. It's just that I'm still in school and I have lots of job interviews coming up. I won't get around to it for awhile. More importantly, I worry my lack of experience will result in a subpar design. For example, how wide/thin should the leads be so it won't go up in flames?I guess I will try to design the PCB in Eagle myself eventually and post it up here to get it briefly looked over before I send it out to get prototyped...Thanks