circuit design problem - Page 1
Forum section: electronics / beginners

author: manjunath
author_rank: Contributor
timestamp: July 16, 2014, 03:11:36 pm
content: Hi all ,I have conducted test to obtain -5V using NCP3170 as explained in a linkhttp://www.onsemi.com/pub_link/Collateral/DN05004-D.PDF(Figure 1)In case 1: with input voltage 5V and without any output load I was able to get -5V .In case 2: with 5V, and later test with 8V input voltage, if i place a load of 15 ohm or 100 ohm across the output I'm not getting the -5V, and the chip does not turn on switching. Instead I'm getting +2V or 0V. Changing the chip twice times yields the same result.The circuit that i have tested is in attachment.Please suggest potential causes and solutions to this problem.

author: mij59
author_rank: Frequent Contributor
timestamp: July 16, 2014, 03:35:35 pm
content: Please post a photo of the actual circuit.What do you use for the inductor L1 ?

author: manjunath
author_rank: Contributor
timestamp: July 16, 2014, 03:47:19 pm
content: Attachment has actual circuit.

author: mij59
author_rank: Frequent Contributor
timestamp: July 16, 2014, 03:56:27 pm
content: Quote from: manjunath on July 16, 2014, 03:47:19 pmI can see only a circuit diagram, where is the photo of the device you build ?

author: manjunath
author_rank: Contributor
timestamp: July 17, 2014, 08:26:43 am
content: hi mij59,my actual circuit is exactly same as the circuit in the attachment but additionally i have connected 15ohm resistance across 22uF at the output side.

author: rf-loop
author_rank: Super Contributor
timestamp: July 17, 2014, 10:05:39 am
content: Quote from: manjunath on July 16, 2014, 03:47:19 pmNo, it is not. Your real world circuit is (more or less) far away what diagram show.Diagram show ideal components and zero reactance wiring. It is possible only in school books.In real world things are much much more complex and what more you know they are always more complex.Quote from: manjunath on July 17, 2014, 08:26:43 amAs mij59 told, it need good pictures about real circuit.Also it need know exactlywith datasheetyour inductor.Layout of circuit is critical also with this kind of 0.5MHz (version A) (or 1MHz version B)) switching circuit. It is very easy to fall in many traps in these SMPS circuits. They looks simple in schematics but schematic is NOT your circuit exactly. There is prasitic components (example inductances but also capacitances) and they may be in big role when there is fast rising and falling edges. One example is that some capacitors ESL need be more like nH (1-20) range.and if we talk this kind of ESL all can know that circuit wiring is also critical. (for undestanding what is one nH. It is example 2mm long straight copper wire what diameter is 0.3 mm, it have ~1nH inductance in this freq range)Circuit physical layout need be well designed before it can work good. In many cases these things are hardly under estimated when talk simple SMPS circuits because they look simple in circuit diaram. One 5cm copper wire in critical place may "destroy" whole circuit with its 60nH inductance.These are difficult things also for well educated professionals if design / do well working SMPS circuits.Your inductor is one critical component. Read carefully herewww.onsemi.com/pub/Collateral/NCP3170-D.PDFall about inductor selection and its details.Also there is some examples about circuit layouts.This is one reason why asked real photograph from real circuit/mechanic hardware exactly

author: manjunath
author_rank: Contributor
timestamp: July 17, 2014, 04:42:53 pm
content: this attachment has the circuit that i have made.

author: Precipice
author_rank: Frequent Contributor
timestamp: July 17, 2014, 04:54:58 pm
content: Did you make it on breadboard? On Veroboard? On a 4-layer PCB? (And yes - what exact inductor did you use?)Just because you wired some pins together - that isn't enough information. You can easily build something that matches that schematic, that will not work. Not even a little bit!We need a photograph of what you built, or we can't help you - it's that simple.The fact that you don't understand this, makes me a bit suspicious about what you've built. Even those of us who have been building PSUs for years, often get someone else to have a look at a design for us.