How do you smooth the ripple on a constant current buck converter? - Page 1
Forum section: electronics / beginners

author: nbritton
author_rank: Frequent Contributor
timestamp: September 26, 2015, 09:16:46 pm
content: I'm thinking about getting three $13 adjustable buck converter modules and assembling/re-wiring them into a project box so I can use them as a cheap triple output power supply. I would like to wire in a fan, re-wire the LEDs and POTs so they can be used in a project box, and presumably improve the DC output ripple and noise on them. The buck converters have a constant current mode.How do you smooth the ripple on a constant current device that is being used in a current limiting mode? Normally I would just put a large value capacitor on the output, but in this instance I want to do current limiting, so that seems counter productive.Here are the buck converters I'm planning to use:http://www.amazon.com/gp/product/B00GZG7X9Y

author: dom0
author_rank: Super Contributor
timestamp: September 27, 2015, 01:17:29 am
content: voltage source ... parallel capcurrent source ... series inductor

author: ez24
author_rank: Super Contributor
timestamp: September 27, 2015, 02:07:56 am
content: According to Art of Electronics, a linear regulatorie caps and coils do not work well on buck regulators (vs linear regulators)And no matter what you do, it will never be as good as a linear ps.So you set up your buck about 3 volts higher then drop the 3 volts with a linear regulator such as a LM317 (this is the best non-exotic solution I have found so far)Another alternativehttp://www.aliexpress.com/item/DC-15A-Buck-Converter-Step-Down-Module-4-32V-Power-Supply-Output-1-2V-32V-New/32470077459.html?spm=2114.01020208.3.79.oOp3pm&ws_ab_test=201556_2,201527_2_71_72_73_74_75,201409_2this has a large heat sink and coil and "space" - your pick does not. You cannot monitor the heat on the ones you picked. So you will not know if you are burning them up.and meter like thesehttp://www.aliexpress.com/item/DC-0-100V-10A-Dual-Red-LED-Digital-Voltmeter-Ammeter-Panel-Amp-Volt-Gauge-42065/1746376273.html?spm=2114.01020208.3.158.vb7NNX&ws_ab_test=201556_2,201527_2_71_72_73_74_75,201409_2I am NOT recommending the sellers, I would look around especially for wiring diagrams, The little meters are tricky to wire up and some vendors show a diagram.It takes a while to find a diagram and they come in different colors.Again your last pick and these do not make for frequent change of voltages. I bought 2 good quality pots to replace mine with for the one I am using as a variable supply.

author: nbritton
author_rank: Frequent Contributor
timestamp: September 27, 2015, 03:18:11 am
content: How would I stick a variable LM317 onto the output of a variable buck converter? You would want only one knob to control the voltage. It sounds like it would be hard to use one pot that controlled both the buck and LM317 in perfect synchrony.I like your idea of using a separate volt/amp meter, that might make it easier to mount three of them into a single project box. I was thinking of re-wiring them with panel mounted pots and leds. Also I intend to put a fan in the project box.

author: nbritton
author_rank: Frequent Contributor
timestamp: September 27, 2015, 03:31:02 am
content: Quote from: dom0 on September 27, 2015, 01:17:29 amWith an inductor what happens when the PSU circuit is turned off, doesn't the waveform collapse in the coil send a voltage spike down the line? What about the output wire wrapped around a toroidal ferrite bead? Is it the same as a regular inductor?

author: ez24
author_rank: Super Contributor
timestamp: September 27, 2015, 04:09:39 am
content: Quote--> no not on a variable supply, I forgot about thatQuote--> Not alone. You asked how to reduce ripple - put a LM317 at the end of a buck to reduce ripple. Caps and inductors do not work. Search around for Chap 9 sample of Art of Electronics about this. Chap 9 is free.I started (sorry lost it) a topic on this and it can get really crazy trying to balance ripple rejection with voltage drops so I suggest sticking with a 317.A link to the chapter is somewhere here:https://www.eevblog.com/forum/beginners/electronics-primers-course-material-and-books/You lose about 3 volts per supply so for 5 v you set the buck for aprox 8 volts. You could set up a by-pass switch around the 317 and only use it if the ripple is too much for you(of course you would have to adjust the buck)The major problem is the current limit of a stand alone 317 which is about 1.5 ampsSo at 1.5 amps and 3 volts you use up 4.5 watts and probably your ps is 90 watts. And if you want to go to the breadboard limit of 2 amps you have to heat sink the 317 (what a drag)Again the 317 is for ripple reduction, if you don't need reduction (and the truth is you probably don't) you do not need the 317.Do you have a scope to measure the ripple with? If not, for about $60, a DSO112 will work fine. (don't tell anyone that I said this)

author: nbritton
author_rank: Frequent Contributor
timestamp: September 27, 2015, 04:18:29 am
content: The LM317 is only rated for 1.5A. How about the LM350T? That one is only 34 cents more on mouser and is rated for 3A. Do you know off hand what size resister you would use to create the 3V differential? I'm assuming you would connect Vout from the buck into the adjust pin on the LM350. However, I'm not sure how to figure out the fixed resistance that is necessary to create a constant 3V drop on the Vout pin on the LM350.

author: nbritton
author_rank: Frequent Contributor
timestamp: September 27, 2015, 04:32:24 am
content: Quote from: ez24 on September 27, 2015, 04:09:39 am2 MHz @ 5 MSa/s LOL! I have an Analog Discovery coming in the mail on Tuesday, it has a 25 MHz @ 100 MSa/s scope built into it. If that doesn't work I can go digging in storage for my old analog 60 MHz Textronix 2213A.What prompted me to start this thread was Dave's informative video on how to measure ripple and noise:

author: ez24
author_rank: Super Contributor
timestamp: September 27, 2015, 07:11:08 am
content: Quote from: nbritton on September 27, 2015, 04:32:24 am

author: nbritton
author_rank: Frequent Contributor
timestamp: September 27, 2015, 09:20:38 am
content: I ran it in a SPICE simulator and as far as I can tell you would connect Vout from the buck to Vin on the LM317. Then you would short the Vout pin on the LM317 to the adjust pin, and leave the adjust pin floating. This would provide the greatest possible efficiency for the linear regulator. The Texas Instruments datasheet says to connect a 10 uF cap between the adjust pin and ground for improved ripple rejection.See the attached screenshot.

author: nbritton
author_rank: Frequent Contributor
timestamp: September 27, 2015, 10:38:55 am
content: Quote from: ez24 on September 27, 2015, 07:11:08 amYou can use the heating element on an electric stove as a resistive load. The internet says the large elements average 27 ohms and the small elements average 45 ohms. 18 volts @ 27 ohm = 12 watts. Typically a stove has four elements, so you can hook these up in whatever serial or parallel configuration you want. Two large elements wired in parallel should produce about 14 ohms of resistance, and at 18V that would approximately be 23W and 1.29A. The great thing about them is the large ones can handle 2000W and the little ones can handle 1000W.

author: nbritton
author_rank: Frequent Contributor
timestamp: September 27, 2015, 11:05:56 am
content: Quote from: ez24 on September 27, 2015, 07:11:08 amYeah. The bundle I got was awesome, I saved $218.54!Analog Discovery: $159 (normally $279)Analog Parts Kit: $Free (normally $64.99)NI Multisim: $9.95 (normally $40)Project Box: $3.50 (normally $7)BNC Adapter Board: $19.99BNC Scope Probes: $19.99Mini Gragger hooks: $14.99Student cost: $227.42, Normal cost: $445.96To get the same deal that I got all you need to have is a .edu student email address. If you're not taking a class that requires this device simply write into the verification box "none". Below is an example of the verification form... that's all you need to do.School/university: Your Schools NameCustomer's email: Your .edu email address.Professor/instructor: NoneProfessor/instructor email: NoneDepartment/Class Website: NoneInstitution Type: Two Year CollegeUser Type: Undergraduate/StudentProduct Use: ResearchDepartment/Area: Electrical/Computer Engineering

author: ez24
author_rank: Super Contributor
timestamp: September 27, 2015, 06:25:30 pm
content: Quote from: nbritton on September 27, 2015, 11:05:56 amNice. Did they send an email confirmation code to the .edu account?If you do not know, Dave has a video on this.

author: nbritton
author_rank: Frequent Contributor
timestamp: October 05, 2015, 04:49:49 am
content: Quote from: ez24 on September 27, 2015, 02:07:56 amI tried putting an LM317T on the output of a homemade buck converter (+5V square wave through a coil and capacitor) and it had no appreciable difference on the output of the waveform. Whatever ripple was on the input was mirrored on the output.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 05, 2015, 12:11:07 pm
content: You can use a CLC filter for constant voltage, or an LCL filter for constant current. Note that, in either case, the impedance isn't very low or high at the cutoff frequency. This connects with a theorem: a filter must operate into a characteristic resistance, at some point. At frequencies very far removed from the cutoff frequency, the impedance can be arbitrarily high (CC) or low (CV), so you must define the rate at which you require the output to appear "CC" or "CV", as the case may be.Thus, if you require a dynamic output impedance of at least 1kohms (a pretty reasonable "CC" output for power application) at 60Hz, then you can't have a cutoff frequency lower than e.g. 60kHz for a 1 ohm (characteristic impedance) filter. Which will have poor attenuation until perhaps 200kHz or more, so you can see, if you need stringent demands on the impedance, even at very low frequencies, you can easily reach the limitations between switching speed (ripple frequency), cutoff frequency (and filter order, and ripple attenuation), design impedance, overshoot, and so on.For a general purpose power supply, it may be desirable to allow both CC and CV, rather than restricting oneself to one or the other characteristic. Which implies that you'd like to have a CLC for CV effect, but follow it with L for CC effect, but maybe a bit less L so it's not just an extension of the same CLC filter. But then again, followed by a C, of still less value, for better CV, and so on; the result is the lumped equivalent of a tapered transmission line, where the cutoff frequency rises along the length of the filter, and the impedance (being the sum effect of all those characteristic cutoff frequencies) is approximately a constant resistance at high frequencies: neither inductive (~CC) nor capacitive (~CV). That resistance would simply be the best choice of resistance given the operating range: or simply, Vmax / Imax. The range of cutoff frequencies would depend on how much you can tolerate between cutoff and ripple frequencies, and how much attenuation you need; such a plan could give flatter impedance (but neither CC nor CV characteristic, but only a compromise inbetween) to higher frequencies, and better attenuation with the least total amount of L and C.That would be quite complicated to design and tune, so I would simply suggest handling it in such a way that its CV-ness is managed. When starting a load in CC mode, for example an LED, start with the output set to zero volts, or disabled or unpowered. Then connect the load, then apply power. The output voltage will rise slowly, until it goes into current limit.If you truly need faster response time, a custom design may be required (a fast, responsive switcher into a fairly large inductor, maybe with some LCL to help take the edge off?), or at least a linear regulator (which is potentially limited only by the drain or collector output capacitance of the pass device).Tim