Zeta Converter Design Questions + Aluminium Polymer Capacitors as Filters? - Page 1
Forum section: electronics / projects

author: sean0118
author_rank: Frequent Contributor
timestamp: February 12, 2015, 01:49:29 pm
content: Hi everyone,I'm building a battery charger that's partly based on this application note:AN1467.On their output they have three unpolarised 10uF caps and two polarised 47uF caps, but the actual type of capacitors are not listed.The calculations I have for my design show a maximum output current ripple of 1.7A RMS. So, for the 47uF capacitors I went with the 25VPanasonic 25SVPF47Maluminium polymer capacitors. They have a rated ripple current of 2.8A and with two in parallel it's sort of overkill.Now, what I'm not sure about is the 10uF ones. Why didn't they use 0.1uF or 1uF as high frequency bypass? Would these have to be ceramic or film, or would a 10uF aluminium polymer work just as well?

author: DanielS
author_rank: Frequent Contributor
timestamp: February 12, 2015, 07:02:11 pm
content: If the type is unspecified beyond non-polarized, it is usually ceramics.If you use MLCCs, keep in mind that X7R/X5R caps lose capacity with DC bias, especially in small footprints and low voltage ratings. So you want to use the largest cost-effective package you can accommodate and bump the voltage rating a few notches above whatever you will be working with.

author: SArepairman
author_rank: Frequent Contributor
timestamp: February 12, 2015, 07:02:23 pm
content: I only ever used sanyo oscon aluminum polymer capacitors as per linear technologies appnote 84 on measuring low voltage noise on linear regulators. I like how the package looks, its like a pimped out electrolytic.I think that any low ESR/ESL capacitor is appropriate for the HF bypass, if you can find a voltage/capacitance chart for a ceramic (use X7R) 10uF (to make sure your capacitance does not die down due to bias voltage) then it would be your cheapest solution most likely.The only thing you might have a problem with is maybe micro phonic noise, if you choose a ceramic, however, for a battery charger, I hardly think this matters.If you happen to have a bunch of film/tantalum caps @ 10uF @ the junk bin then they are also fine to use.the ESR of those oscons is already pretty low though, I would be interested to see measurements on the 10uF capacitors before/after, if you have the noise measurement capability. Seems way over kill for a battery charger. I thought yoou can pretty much feed a battery garbage.I believe that basically there is some filtering "rule of thumb" popularized by a book that says you should have a high ESR capacitor (sometimes made artificially high with something like a carbon composite (not film) resistor + 1/10th the size low ESR capacitor.I heard this on the forum first (but this was with regards to an extra filter inductor on op-amp rails), then I found it in the AD797 ultra low noise op amp app notes (they however do not add a inductor, but the numbers were essentially the same as the numbers I got testing an LC(with extra ESR) filter in order to kill resonance.. In this case there was a "golden ratio" between the ESR of the inductor and the ESR of the high value "dampening" capacitor.*keep in mind this was for a op-amp that draws maybe a few mili amps and was serving a super quiet system.Can you post oscope pics before and after whichever additional capacitors you choose? load/no load.this topic comes up alot on the forum, it would be nice to have a reference (I never diddled around with a SMPSU)**it seems that electrolytic capacitors, the larger they are, the lower ESR they have, IDK if this is true for aluminum polymer capacitors though. It's worth investigating. You can also try adding low inductance resistance to your "fast" capacitor to see if it increases noise.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 13, 2015, 12:48:37 am
content: Aluminum polymer capacitors are, in all respects, essentially film capacitors scaled down for lower voltages and higher capacitances. The ESR*C product, and energy density, are very comparable to typical polyester or polypropylene caps for DC link / supply bypass applications.As such, the main hazard is having too little ESR, so that the supply dynamics are underdamped and it exhibits much more overshoot, or outright oscillation, than it's supposed to. Even in passive networks, the low ESR tends to reinforce parasitic resonances, as it acts like a nearly ideal AC short to ground.If the regulator is compatible with large ceramics as bulk filtering, it'll probably be fine.Tim

author: sean0118
author_rank: Frequent Contributor
timestamp: February 14, 2015, 06:30:31 am
content: Thanks for the replies.Quote from: SArepairman on February 12, 2015, 07:02:23 pmNo, but I do have some 82nF and 100nF low loss 1kV rated polypropylene film capacitors. I think they would suppress noise quite well, but I'll get some 10uF ceramics too and do a comparison like you suggested.Quote from: T3sl4co1l on February 13, 2015, 12:48:37 amDo you guys think I could put a 0-ohm SMD resistor in series with each capacitor, so I can add actual resistance later if I need to?Edit: One more question, is there a typical ripple current rating for MLCCs? I've seen some that say they have "low self-heating and high ripple resistance due to low ESR", but none that actually quote a rating in amps.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 15, 2015, 01:51:55 am
content: Ripple current can be estimated very well if you have some data handy. TDK, Kemet and others have database utilities that you can find (maybe) data on their parts. Such as impedance vs. frequency, DC bias, temperature shift and so on. Current ratings are often listed directly.Otherwise, you can estimate or find losses by ESR/DF. Coming up with a heat dissipation rating isn't as simple, but it's often on the low side, like 1/8W for a 1206. Dielectric ceramic isn't as conductive as the alumina base used for chip resistors.As long as the voltage is kept small, all ceramic caps handle a pretty good bit of current. 1206 1nF C0Gs will do an ampere or two, even at voltage (100s of V at ~1MHz!). X7R (and other type 2 ceramics) don't handle voltage so well, and high voltage swings (more than half the rating, say) see dramatic changes in capacitance; for example, using an R+C damper (with a too-small type 2 capacitor) on a switching node results in a linearly rather than exponentially decaying envelope of the ringing.Tim

author: splin
author_rank: Frequent Contributor
timestamp: February 15, 2015, 06:12:37 pm
content: Quote from: sean0118 on February 14, 2015, 06:30:31 amIt's odd isn't it that so few MLCC datasheets have anything to say about ESR or ripple current. Perhaps it's because it isn't a big issue in most applications, and ceramics weren't commonly used for high ripple current applications such as SMPS output filters. MLCCs specifically designed, or at least marketed for such situations are better specified.There is this:http://product.tdk.com/capacitor/mlcc/en/faq/pdf/12_ripple_current_for_mlccs.pdfMurata also provide some comprehensive data on their MLCCs, unlike many who at best may only publish some charts for two or three example parts. See:http://www.murata.com/en-eu/support/faqs/products/capacitor/mlcc/char/0001Folllow the 'Characteristics Data' link to find the data for specific parts; many (but not all) have ESR and ripple current graphs. For example:http://psearch.murata.com/capacitor/product/GRM21BD71A226ME44%23.html

author: sean0118
author_rank: Frequent Contributor
timestamp: May 10, 2015, 07:23:29 am
content: Hey everyone,I'm still working on this battery charger, the basic design and component selection is nearly done.One problem I'm having though is designing the MOSFET driving circuitry. I could copy Microchips solution from pg.2-3 of the application note I linked to in my original post.But to be honest I don't understand how they apply the MCP14628 driver IC. I get that it could be damaged by the negative voltage present in the Zeta topology, but I don't understand how the extra protection circuitry added (Fig. 5 of app note) operates.Can anyone explain how they protect the MCP14628 from the negative voltages? Or does anyone have another solution for driving the MOSFETs? I'm aiming to switch at 500kHz so will really require >0.5A driving current.Also, while I'm at it, what the hell is U2.3 at the top of the schematic (pg.4)? Is it the power supply for the Op-amp rails? Is C22 the bypass capacitor for it?Thanks guysEdit: In case it's of interest IRFB3806PBF are the MOSFETs I'm using.Edit2: After having a closer look at the MCP6V02 opamp datasheet it looks like they can be powered from just the 5V rail. But the question of what U2.3 is still remains...

author: sean0118
author_rank: Frequent Contributor
timestamp: May 15, 2015, 02:49:04 pm
content: Just to clarify my question a bit more.I completely understand the operation of the standard bootstrap driver (as explained in thisapp note).But the Zeta converter has the addition of capacitor C1 between the 'SW' node and 'PHASE'. So if the bootstrap causes the high side driver to float at 'Phase' + 'Vcc' how is the required MOSFET Vgs threshold reached? It would need to be floating at 'SW' + Vcc?I'm assuming capacitor CBLK is the solution here. But I can't quite see the mechanism in which it works, apart from CBLK charging through D1 as QH begins to conduct.I'm sure it comes down to basic capacitor circuit theory, I know it's going to be awhen finally get it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 15, 2015, 09:51:14 pm
content: Cblk-D1-R1 serve as DC restorer. It's a crummy way to do it, but if you can ensure some conditions (it never stays on for more than a fraction of t = R1 * Cblk), it works out. In this case, the bootstrap gate drive is referenced to the bottom side of the switch coupling cap, but since it's cap coupled anyway, it doesn't matter which side it's connected to (other reasons, such as driver common mode range, may limit this).Tim

author: sean0118
author_rank: Frequent Contributor
timestamp: May 16, 2015, 02:41:23 am
content: Ah, ok I see, the signal at the MOSFETs gate is shifted up and will never fall below the potential at the diodes anode? And it can't stay on for more than a fraction of the time constant otherwise the signal will stop resembling a square wave as Cblk discharges?R1 is really just there to make sure the MOSFET gate discharges completely during the off time right? In the app note Microchip use a 1k resistor, that seems kind of low to me, wouldn't a 10k allow the MOSFET gate capacitance to charge slightly quicker? Then I could also reduce the value of Cblk so the time constant was equivalent.Thanks for your help Tim!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 16, 2015, 03:34:03 am
content: Yup!R1 slowly pulls down on the "top" of the square wave, so it keeps it smacking into the diode on the downward swing and keeping Cblk charged. The biggest problem is everything is in a random state before you start, then wham it goes on, and you hope it's in the right state for that first half cycle. (Starting with everything zero, it should be OK though.)Cblk needs to be many times Ciss to be able to deliver the correct voltage, so it's desirable to have a somewhat small R1 to keep it charged in the right direction.Tim

author: mazurov
author_rank: Frequent Contributor
timestamp: May 16, 2015, 04:46:27 am
content: I've built several copies of this circuit with variants(mainly code). Don's sweat the caps: plain ceramics and junk box grade alu work just fine. Zeta won't give you much ripple anyway and the AN circuit is intended to charge 10A SLA.

author: sean0118
author_rank: Frequent Contributor
timestamp: May 17, 2015, 01:13:54 pm
content: Thanks again Tim, that does make sense now.Quote from: mazurov on May 16, 2015, 04:46:27 amOk good to know, thanks. By the way, did you use the same MOSFETs as the app note? I searched the part number from the schematic but didn't get any hits. I've already chosen my MOSFETs but I wouldn't mind comparing my choice to theirs. It seems theirs must be logic level because they drive them with 5V?

author: ashish.agni05
author_rank: Newbie
timestamp: October 16, 2017, 05:40:20 pm
content: I am starting with this project.Though I haven't looked at code yet, still, does this project have digital compensator for Zeta Converter included in the software ?

author: ashish.agni05
author_rank: Newbie
timestamp: October 16, 2017, 06:02:31 pm
content: I think I have figured out in the application note: pid(vout,vref) and pid(iout,iref)

author: crossflow
author_rank: Newbie
timestamp: March 30, 2018, 08:02:23 am
content: Quote from: T3sl4co1l on May 15, 2015, 09:51:14 pmHi Tim, please can you suggest a less crummy way to do it?I'm looking for a more elegant alternative that allow pushing up performance, even at the cost of increasing driving losses.Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 30, 2018, 09:10:15 am
content: Old thread....I don't know why MCP14628 doesn't do a proper bootstrap driver, but dual LS/HS drivers are very common.Tim