Breadboard vs real circuit impact on test design - Page 1
Forum section: electronics / projects

author: XavierM
author_rank: Newbie
timestamp: January 24, 2021, 09:52:30 am
content: Hello everyone !I've been working on a flyback project for a bit now, using a LM3478 and it doesn't really work as it should.In summary, following the TI documents I should get something around +/-15V at the outputs @200mA, but I'm only getting about +-9V and it can't sustain a 100mA charge without dropping at 2V.Fiy I'll attach the original design. Also, for R3, instead of the 100Ohm resistor TI suggest, I had to use a 22kOhm for switching to occur ! This was quite surprising !A TI engineer suggested I'd drop Rsense to 30mOhm, but according to him "it looks like it is still in the current limit. The voltage threshold is 156mV typical. " (When only hitting around 50mV)Now I'm gonna Buy couple new chips cause I wondering in mine could be dead. But there's an other I can't shake and I'm hopping this forum could help answer : I've done the whole design on breadboard, trying to keep wires as short as possible of course, and still we're dealing with 30mHom resistors. So what is the impact of the breadboard itself ? Could the contacts and wires be of that same range of resistance and messing with my design ?Thanks in advance for your help !Xavier

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 24, 2021, 10:08:02 am
content: Could well be your construction but without either a crystal ball or picture or dowsing rods how do you expect anybody to comment

author: Ian.M
author_rank: Super Contributor
timestamp: January 24, 2021, 10:13:53 am
content: There's not a snowflake's chance in hell during a 100 year record heatwave, of that working properly on a solderless breadboard. Contact resistances are at best comparable to your sense resistor, and worst case may easily be an order of magnitude higher.

author: tggzzz
author_rank: Super Contributor
timestamp: January 24, 2021, 10:22:27 am
content: If you suspect that solderless breadboards are causing your problem, then you can do one of two things:continue fiddling until itappearsto work, then not understand why it has started workingswitch to using a decent prototyping techniquesm e.g.https://entertaininghacks.wordpress.com/2020/07/22/prototyping-circuits-easy-cheap-fast-reliable-techniques/ Basically solderless breadboards are the work of the devil that should be avoided if at all possible - because you will frequently spend more time debugging the breadboard than debugging your circuit. Fortunately there are easy alternatives that are cheaper, higher performance, more predictable and more robust. What's not to like about that?!

author: Ian.M
author_rank: Super Contributor
timestamp: January 24, 2021, 10:51:06 am
content: I generally like solderless breadboards for their convenience when quickly trying circuit ideas, or lashing up single use test jigs, but you have to be a significantly better engineer to use them successfully vs soldered prototyping, as you have to keep their limitations- stray capacitance, contact resistance, parasitic inductance, lack of a ground plane, and low safe maximum current and voltage-in mind all the time, to judge the likely effect on your circuit, and decide whether you need to build some section of it as soldered plugin modules to reduce the effect of parasitics.There are also issues with poor quality solderless breadboards with malformed mystery metal contacts, that either have an insertion force high enough to need a pile-driver to insert a component lead or rapidly go intermittent and loose contact if a gnat lands on the other end of your bench, which aren't even fit to use as a jig when soldering header pins to DIL adapters!TLDR: If you don't have enough experience to have developed a 'gut' feeling for likely problems, follow Tggzzz's advice and *DON'T* use solderless breadboards for even mildly challenging circuits.

author: XavierM
author_rank: Newbie
timestamp: January 24, 2021, 01:32:25 pm
content: Hi all !So yes, you confirm my fears !I went for solderless as it allowed me to easily swap component to fine tune things and I did use a high quality one (as I had trouble with fiddling contacts before on other projects) but the tiny value of Rsense worried me from the beginning.To be honest, until then I've dealt with mostly simpler design, logic and arduinos... So beyond intuition, actual knowledge and your precious advices are really welcome !BTW, thanks for the link tggzzz ! Will definitely look into that !Thanks for the help !Take care !

author: tooki
author_rank: Super Contributor
timestamp: January 24, 2021, 04:30:47 pm
content: Quote from: XavierM on January 24, 2021, 09:52:30 amAre you just using the datasheet, or did you simulate the design in Webench Power Designer? I’ve found that tool to be highly effective at producing good designs. The product page says it’s supported in webench:https://www.ti.com/product/LM3478(But you’ll still have to solder it up properly, this can’t work well on a breadboard.)

author: XavierM
author_rank: Newbie
timestamp: January 24, 2021, 05:30:23 pm
content: QuoteI've followed the "How to Design Flyback Converter With LM3481 Boost Controller" instruction to calculates the values for the components (although they lost me with the calculation of the transfer function and the lowpass filter they apply on the feedback loop...). So it seems that the LM3478 and LM3481 are fairly similar as the said document was registered on the LM3478 product page.Now it is my understanding that with the Webench once can only do Boost and Sepic ? And since the design in the document above (for flyback design) is quite different (regarding the feedback loop) compared to the spec sheet (which describe more traditional boost topology) I followed that document.QuoteWhat make you say that ?Current wise I should be within range from the calculation as my coil should be able to support up to 5A peak and I should hit about 2A. Also I'm aiming for a 200mA output but I'm aiming for a 100mA consumption... Also I tuned the switching frequency so it would work with my input voltages. (see graph in the attachement)@tggzzz : I red that article -> very interesting. I've been using matrix board but never would have though of the "dead bug" design. Thanks again !

author: penfold
author_rank: Frequent Contributor
timestamp: January 28, 2021, 12:29:11 am
content: Hi,I'll first apologise that I've not intently studied your problem so be as critical as you want with the following... but my biggest quarm with solderless breadboards is the stray capacitance. You can often bring stray inductances down to "low enough" and compensate for contact resistance with kelvin 'inspired' connections, maybe soldering the sense side directly to the resistor bypassing the contact resistance. But you'd be surprised how often the effects of stray capacitances can manifest themselves as all kinds of other problems.A word of advise though, I'm pretty sure there's no such thing as quick and easy prototypes, especially when you're starting out. It can be worth putting in a bit of extra time with some perf board (without any copper on) and being in 'almost' complete control of your parasitics, it can eliminate some of the time you end up investing by chasing around trying to debug or at least it doesn't add quite so many unknown variables as solder less breadboards

author: jonpaul
author_rank: Super Contributor
timestamp: January 28, 2021, 09:44:18 am
content: Hello all:I have used many breadboard techniques, the solderless clip type are fine for DC or low frequency, but never for SMPS or high frequence, very high capacitance and parasitic inductane affects everything.See the notes of Lin Tech Jim Williams on techniques.The "dead bug" on a PCB ground plane is one easy solution.solderless boards just waste time.Finally the design and layout of SMPS is an art that takes a lot of time, study and experience to get right.Suggest to Read and study, if you really want to learn.Bon ChanceJon

author: tggzzz
author_rank: Super Contributor
timestamp: January 28, 2021, 09:47:43 am
content: Quote from: XavierM on January 24, 2021, 05:30:23 pmYou are welcomeThe references are worth reading too - several times since there is a lot of useful information in them.

author: 2N3055
author_rank: Super Contributor
timestamp: January 28, 2021, 11:12:29 am
content: Quote from: jonpaul on January 28, 2021, 09:44:18 amAbsolutely well said!This should be a sticky!

author: jonpaul
author_rank: Super Contributor
timestamp: January 28, 2021, 12:25:33 pm
content: Bonjour 2N3055:Mille mercies pour votre réponses!Since 1970s I have used a pad per hole "vectorboard" with parts connected by bus wire or leads. I learned this from ex Marantz designer Sidney Smith of Marantz tube hifi 7C, 8 and 10 B fame.You can layout the PCB in your hands with this method.For SMPS and RF, use of adhesive 3M copper tape provides ground planes.Examples in photos are mostly audio breadboards, but same method works for SMPS and RF.Finally our old friends Bob PEASE and Jim Williams (RIP 2011) led the way with many fine books on analog design and breadboarding.https://www.amazon.com/Troubleshooting-Analog-Circuits-Robert-Pease/dp/0750691840https://www.amazon.com/Analog-Circuit-Design-Personalities-Engineers/dp/0750696400https://www.amazon.com/Science-Analog-Circuit-Design-Engineers/dp/0750670622Finally, IEEE Spectrum had an article on the "dead bug" breadboardinghttps://spectrum.ieee.org/geek-life/hands-on/with-the-dead-bug-method-hobbyists-can-break-through-the-highfrequency-barrierI hope that these notes will encourage the OP and others to improve there breadboarding techniques!Just the ramblings of an old retired EEJon

author: jonpaul
author_rank: Super Contributor
timestamp: January 28, 2021, 02:14:16 pm
content: Rebonjour Monsieur Xavier M;BRAVO pour votre apprentissage les techniques électronique de puissance!SVP me contactait si vous avez l'intérêt RDV quelque temps, à Paris.You can find some deeper app notes and eval boards from Liner Technology.Amenities;Jon

author: jonpaul
author_rank: Super Contributor
timestamp: January 28, 2021, 04:16:20 pm
content: To 2N3055:From the 1960s.....Thanks for the memories....Jon