[HELP]ferrite chip as a SMPS secondary filter - Page 1
Forum section: electronics / projects

author: aruna1
author_rank: Regular Contributor
timestamp: June 09, 2012, 05:11:39 pm
content: Hi allI'm building a 2v to 5v boost converter using MAX856 IC. I have built the smps and it is working as it should, now i want to add a output filter to the boost converter.my output current is about 200mASo my question is , is it possible to use a ferrite chip as the output filter(not the main inductor) of the boost converterI found these on ebay(http://www.ebay.com/itm/ISI-1206-Chip-Ferrite-EMI-SMD-BEAD-LCB1206-601-Qty-100-/160409326313?pt=LH_DefaultDomain_0&hash=item25592412e9)Or do i have to wind a small toroid for this?

author: T4P
author_rank: Super Contributor
timestamp: June 09, 2012, 06:48:54 pm
content: Why not a SMD inductor/TH inductor?

author: free_electron
author_rank: Super Contributor
timestamp: June 09, 2012, 07:26:13 pm
content: Ferrite beads are used to block emi from slipping ont wires. Doesnt do anything for ripple filtering... You need a choke, not a ferrite bead.

author: aruna1
author_rank: Regular Contributor
timestamp: June 10, 2012, 01:31:20 am
content: Quote from: DaveXRQ on June 09, 2012, 06:48:54 pmwhat type of smd inductor can i use for this?will chip inductors like these will do the job? can they handle currents above 200mA?or do i have to use inductor like thisi want to use the smalleset inductor possible and thats why I'm looking for smd part

author: free_electron
author_rank: Super Contributor
timestamp: June 10, 2012, 02:04:02 am
content: There are ferrite beads that handle way over 1 ampere. Problem with ferrite beads is that they are specced to be x ohms @ 100MHz... Again, ferrite beads are to block hf noise from propagating. It doesnt do anything.You need to look at the produced spectrum of your regulator and then come up with an l-c solution.You can't just shotgun things in there. Besides , who says there is a problem to begin with ?

author: aruna1
author_rank: Regular Contributor
timestamp: June 10, 2012, 02:15:27 am
content: Quote from: free_electron on June 10, 2012, 02:04:02 amI'm not sure whther we are on the right track.let me explain my needI have a boost converter which produce 5v(200mA) from 2v. its output has about 200mv ripple. currently I'm using a 15uH toroid inductor as a output filter to reduce this ripple to about 30mV. Circuit is built on a vero board and I'm designing the PCB for it. For the PCB I would like to use small components as much as possible. Since my 15uH toroid inductor is large (toroid size), I wold like to find a small size replacement.For that I need surface mount solution.Since we cant use ferrite chips for this I have to use 15uH chip inductors or small smd inductors. I would love to use a chip inductor if it is possible as they are small and available cheaply in ebay. But question is whether it can handle 200mA current.if it cant handle 200mA then I have to go to another alternative other than a toroid.for this I need some advices and guidence.thank you

author: free_electron
author_rank: Super Contributor
timestamp: June 10, 2012, 05:13:07 am
content: If you have 200mV ripple then trying to filter that is the wrong approach ! There is something wrong with the design, and that needs fixing !What do you have as output cap ? Whats you layout look like ? What diode do you use ? What input cap ? And most of all what inductor ?

author: codeboy2k
author_rank: Super Contributor
timestamp: June 10, 2012, 05:20:33 am
content: Maybe you've already gone down this path, but I'll suggest it anyways:Can you get a lower ESR output cap, or parallel several smaller valued caps that add up to the total capacitance you need for your desired ripple output?Assuming you are switching 500Khz < Fsw< 1Mhz , unless my calculations are incorrect (?) I figure you only need a standard value 15uF output capacitance or less, and you can get that in a small chip capacitor, either tantalum or ceramics. Beware of X7R and X5R/Y5V ceramics, they can lose 60-80% of their nominal capacitance at their rated voltage, so you don't want to run them anywhere near their rated voltage.Are my calculations off? (done with Smath Studio)Quote from: free_electron on June 10, 2012, 05:13:07 amThat's what I was thinking too. Since the no-load ripple is a function of the inductor current through the output cap, too high ripple would seem to point to something wrong in the design. The calculations I did were for loaded ripple. With a 200mv no-load ripple, probably the output cap ESR is way too high.

author: aruna1
author_rank: Regular Contributor
timestamp: June 10, 2012, 06:43:58 am
content: hii'm so sorry that i have left some infoactually 200mv ripple is when load is about 170mA.i was able to drop it to 30mV (loaded) by adding 15uH filter inductor.i didnt do any calculations on this and circuit is made on a vero board

author: Psi
author_rank: Super Contributor
timestamp: June 10, 2012, 08:06:30 am
content: What is the output capacitor you're using?It must be a special type (low esr)

author: aruna1
author_rank: Regular Contributor
timestamp: June 10, 2012, 08:49:02 am
content: hii use 100uf tantulm with 100mohm esr and a 0.1uf ceramic capacitor (in parallel) as output capacitor

author: nctnico
author_rank: Super Contributor
timestamp: June 10, 2012, 09:08:36 am
content: I'd add a 10uf ceramic capacitor to the output if Maxim allows it. Read the datasheet carefully. Maybe a different chip will do much better. I've used chips that work at 500kHz which only need a tiny external inductor and two small ceramic capacitors (input and output) to work. You could start looking on TI's website.

author: free_electron
author_rank: Super Contributor
timestamp: June 10, 2012, 06:44:33 pm
content: 100millohm esr is to much. Throw the craptalium away. And dont overdo the total capacitance ! People think they can just add capacitance.. Not with switchers ! It changes the switching behavior and performance.I would put 3 or 4 10 uf ceramics in parallel. That way you divide esr by 4 as well.Now, on vero board ? That means your connections are not right. You need to make a proper pcb , using correct layout, if you want this thing to run right. You can't breadboard this.