AC Ripple Smoothing - Page 1
Forum section: electronics / projects

author: itdontgo
author_rank: Regular Contributor
timestamp: December 08, 2014, 11:36:14 am
content: A 470uF cap is fine for our application to smooth the rectified DC out of a bridge rectifier ready for the input of a buck convertor.What I want though is a way to dispose of this part. It's too big. We have space in x and y directions but you cannot get an electrolytic or even tantalum cap with a low enough profile to suit our needs. A supercap would be great but as far as I know you're not going to find a 40-50v SMD low profile device.I'm not always up on the latest thinking and live in hope there is a magic technique to fill the period of time when the single phase AC drops down to 0v on the high voltage side of a SMPS. The frequency is such that some MLCCs would be fine for the input caps to the buck convertor but I imagine stopping and starting the SMPS every half cycle is not going to work.We could put the bridge rectifier outside the system on a separate board and make it DC only but this is ugly.Commence your criticisms of my optimistic idea...

author: itdontgo
author_rank: Regular Contributor
timestamp: December 08, 2014, 11:43:40 am
content: Now I've thought about it this is a daft question. It's kind of impossible. The answer is make it DC only and supply an external rectifier or live with the current design which has a bank of small SMD electrolytics.

author: Andy Watson
author_rank: Super Contributor
timestamp: December 08, 2014, 11:55:54 am
content: Look at "power factor correction". Single chip and transistor solutions are available to make more use of the unsmoothed waveform. Although, I suspect it won't save much space.Edit: This is the sort of thing that I was thinking about:http://www.ti.com/product/UCC28051/toolssoftwareYou might also look LED drivers.

author: gildasd
author_rank: Frequent Contributor
timestamp: December 08, 2014, 01:19:28 pm
content: I have a similar problem with an el cheapo power supply but for a different usage:At 3V it has a 200mV triangular wave that only gets bigger...I tried 100uF, 200uF, 470uF and 4.7KuF on the output.The result was at best 160mV. Small but just sufficient to cause a slight flicker in blicky led set-ups with BC547's.Annoying.So I'm also looking at a mod(But when I took a look a that TI reference design...I could not help noticing a major design gripe: the 3 electrolytic caps "touching" the mofstets...The intern must have put this together!)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2014, 02:40:26 pm
content: Care to provide some more particulars?You can always put caps in parallel (or series) to change the aspect ratio, but the overall volume isn't going to be any better.It's theoretically possible that C0G ceramic capacitors could be made with thin enough dielectric, enough layers, and in a large enough package, to be a little bit better than electrolytics (at least if their parameters scale proportionally), but other than that, no, there is no such thing as a "fast" capacitor (fast enough to handle AC ripple -- supercaps are dense, but don't go fast enough for that) with much if any better energy density than electrolytic right now. Certainly not for the price!PFC allows you to do different things with AC line current, output voltage, harmonics and such, but still doesn't escape the physical necessity of energy storage. An extreme PFC might use multiple stages to maximize 1st stage 2 x line ripple (100/120Hz), while being able to supply smooth output power. But now you have the problem that the capacitors must be even faster than common electrolytics, and all those converters aren't free for space either.Tim

author: richard.cs
author_rank: Super Contributor
timestamp: December 08, 2014, 08:45:29 pm
content: If you're that constrained vertically that you can't put several paralleled electrolytics on their sides then as others have said it might be possible to do it in ceramic. Cost and total volume will almost certainly be higher but if you have enough space in X and Y it's possible. From memory we did 500 uF @50V in about 40mm x 40mm x 4 mm at work but don't quote me on that. Remember too that you won't get rated capacitance at max rated voltage with the kind of ceramics you'll be needing.We then had problems with other areas of the circuit due to having that much capacitance with naff-all ESR.

author: IconicPCB
author_rank: Super Contributor
timestamp: December 08, 2014, 09:21:17 pm
content: How much current does Your powersupply need to source?

author: itdontgo
author_rank: Regular Contributor
timestamp: December 09, 2014, 01:36:51 pm
content: Quote from: richard.cs on December 08, 2014, 08:45:29 pmI may look at that. It's for another version of this:It is the electrolytics that make it so thick (9mm). We could be down to 7mm or even 6mm which doesn't sound like much but it looks quite different when it's that thin. It's very tempting to go DC only and provide a bridge rectifier if necessary but that creates problems with having to produce another board etc... It's GSM so it must have a strong PSU. I suppose having low ESR would cause current loop/ground bounce problems as the input caps would be spread over a large area although there is an easy way around that.http://blog.adventcontrols.co.uk/2014/12/07/first-production-gold-g2-gsm-intercom/

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 09, 2014, 03:36:03 pm
content: Why does that get 50V AC?I should think such a device can be powered by USB, or if not entirely, then with a DC adapter.Tim

author: richard.cs
author_rank: Super Contributor
timestamp: December 09, 2014, 05:03:04 pm
content: I would be tempted to make it DC-input only unless something else about the application demands it. If you must then it looks like you could get enough capacitance on that board in ceramic, you can get 50V 10 uF in 1210 @2.5mm height but then it's 50 of them at about 9p and 12 mm2each. I suppose not too bad.Any bigger than 10u at that voltage starts to get very expensive and tend to be stacked packages anyway.

author: Kjelt
author_rank: Super Contributor
timestamp: December 09, 2014, 07:18:16 pm
content: you could make holes in the pcb and mount the capacitor on the other side, or use leaded electrolytics on their side lying in the pcb , any of those will safe you at least 1,6mm pcb thickness but probably increase the manufacturing cost and failure rate

author: itdontgo
author_rank: Regular Contributor
timestamp: December 10, 2014, 12:01:08 pm
content: It's so it can be powered from an electric gate opener and they usually have an auxiliary 24VAC output.It's not 50V AC it's 24VAC which in the UK ends up at 28VAC as the transformers are wound for 220VAC but our mains is more like 250VAC (250/220 * 24 ~27.3). Then when it has been full wave rectified through the bridge we're looking at 28*1.414 = 39.6VDC - ~1.5V for the diode drop ~38VDC That's why you need 50V caps.50 x 0.09p = £4.50 which means not a chance!One thing the last poster suggested may be the solution. If you look down the bottom their are two cutouts for the cable and microphone. We could have another one machined out for an axial capacitor by the power supply if I could find a thin enough one. It would have to be 5mm though which isn't that realistic. I'm still thinking DC only for the next version.

author: itdontgo
author_rank: Regular Contributor
timestamp: December 10, 2014, 12:05:10 pm
content: Quote from: T3sl4co1l on December 09, 2014, 03:36:03 pmGSM requires 2A at 4V! The only practical way of powering a GSM unit is using a 12V or more power supply and having a SMPS on board to drop the voltage near the device. The duty cycle is ~ 600us/4000us on/off.Or a battery right next to the amplifier chip as in a phone!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 12, 2014, 09:49:46 am
content: 2A at 4V, but at a 15% duty cycle is only 300mA average!Holding on for 600us is a far cry from 10ms!For a 5% dropout (4 down to 3.8V), that's 6mF. Pricey, but quite available in a few pieces of 10V tantalum or 6.3V aluminum polymer types.You still have to maintain 0.3A over >10ms, but if you allow the supply to drop out pretty generously (i.e., using a wide input buck to drop the min. 30VDC down to 4V regulated), you need more than 4V * 0.3A * 10ms = 12mJ, so you can afford E = 0.5 * C * (Vmax^2 - Vmin^2) = 27uF or more for all your input needs -- readily available in aluminum polymer. (You probably wouldn't want to use regular electrolytic here, because they aren't good for much ripple.) Probably, 47 to 100uF at 50V (or enough totaling the same) would work very nicely here. Not sure if you can get those in the same low profile chip format the low voltage types are available in, but even if you have to bend over a through-hole radial into a slot, it shouldn't be bad at all.If the buck converter is fast enough to supply those transients, you don't need much local bypass (the 6mF), and you won't need any extra at the input.Tim

author: ruffy91
author_rank: Regular Contributor
timestamp: December 12, 2014, 10:15:26 am
content: A 47uF input capacitor should be enough so that the input voltage to the buck converter will never be below 12V.If you design your buck regulator to work down to 12V you have solved your problem.Most buck converters should have a PSRR (power supply rejection ratio) of over 60dB at 100Hz (your input voltage ripple).So for 20V ripple at the input ripple at the output is 20mV more.50V, 47uF electrolytics are readily available in 5mm diameter cans. So with a cutout in the board you should reach your 6mm goal.