A scope for debugging SMPS/DC-DC circuits - Page 1
Forum section: electronics / beginners

author: derGoldstein
author_rank: Regular Contributor
timestamp: August 26, 2017, 05:53:05 pm
content: I've been looking into a scope that I'd use in part to debug DC-DC converter circuits. The DS1054Z seems great in terms of features and cost, and is well-liked on this forum, but I was wondering if 50MHz would be sufficient.Many new DC-DC chips switch at 1MHz, and some of the compact ones switch at 1.5MHz. Some of the integrated-inductor chips go up to 3MHz, but that seems rare.When the switching frequency is around 1MHz, I've seen waveforms being inspected at 400ns/div, and then zoomed in further. Articles about what's "sufficient" for different applications differ, but I've seen suggestions anywhere between 50MHz and 400MHz when working on SMPS.The Siglent SDS1202X-E is 200MHz and reasonably priced, but for debugging DC-DC converter circuits I've read that 4 channels are recommended if I don't want to be constantly detaching/attaching probes (is this true?).Anything above 200MHz seems to spike the price significantly. I'd rather not spend over $500, but I can go up to maybe $750.Any suggestions welcome.

author: madires
author_rank: Super Contributor
timestamp: August 26, 2017, 06:02:53 pm
content: Just simple low voltage buck converters or also isolated flybacks or mains powered SMPSUs?

author: derGoldstein
author_rank: Regular Contributor
timestamp: August 26, 2017, 06:12:45 pm
content: Quote from: madires on August 26, 2017, 06:02:53 pmLow-voltage, mostly non-isolated. Nothing involving mains.

author: madires
author_rank: Super Contributor
timestamp: August 26, 2017, 06:30:17 pm
content: I was asking to figure out if you need differential probes. You might still need some for isolated converters, but you could start with the poor man's version (ch1 + inverted ch2, ground clips removed). I'd go for the DS1054Z.

author: David Hess
author_rank: Super Contributor
timestamp: August 27, 2017, 05:22:53 am
content: 50 to 100 MHz of bandwidth will be plenty for all but the most extreme switching regulator applications. Getting good signal fidelity with 200 MHz of bandwidth will require using better probing techniques anyway like short ground leads or coaxial connections.Instead of thinking in terms of bandwidth, think in terms of rise and fall time. 100 MHz is 3.5 nanoseconds. How fast are you expecting the switching power transistors and diodes to switch? How fast will the gate drivers be when loaded?The only place you are likely to see a faster signal is in the reverse recovery time of a diode and even then, it is not going to be that fast do to inductance and capacitance.

author: tautech
author_rank: Super Contributor
timestamp: August 27, 2017, 08:16:54 am
content: Quote from: derGoldstein on August 26, 2017, 05:53:05 pmBandwidth will not likely be an issue if you hack the Z to 100 MHz and depending on how you go about debugging channel count might not be either.QuoteWhich is enough for just a scope but for detailed SMPS analysis you'll need a current probe as well and expect to spend as much again.Then you can/might venture into the realm of Power Analysis where neither of these scopes offer such advanced options.Power analysis measurements need only 2 channels, one for a voltage probe and another for a current probe and the scope provides computed results on a large range of efficiency and startup parameters.

author: derGoldstein
author_rank: Regular Contributor
timestamp: August 28, 2017, 09:51:46 pm
content: Quote from: tautech on August 27, 2017, 08:16:54 amI'm not going to go that deep, if only because I don't have the time to specialize, and I'm not very good with analog.The circuits I'm trying to debug are almost always functioning to some extent. I use ICs that have relatively clear implementation datasheets, and often I can find an eval board datasheet to start me off with a particular layout. I don't want to reinvent the wheel, just reduce noise levels, and, if possible, increase efficiency.The forum posts I read that talked about higher frequencies referred to inductor self-resonance, parasitic capacitance, and artifacts that arise from ceramic caps. In order to identify and counter these noise problems some of them needed 200MHz scopes. Depending on the V peak-to-peak of these output noises, they're either just a nuisance, or can cause the output to go beyond the spec of the thing it's going to power. That's why I'm worried that I might encounter a noise problem that I won't be able to track down with slower scopes.

author: tautech
author_rank: Super Contributor
timestamp: August 28, 2017, 10:14:37 pm
content: Sounds like you can do all you need with 2 channels.I'd bring to your attention the full BW sensitivity of Siglent DSO's at high sensitivity settings....something you'll need as you'll want to use 10x probes so to add minimal capacitance from your measurements.ALL measurements affect the DUT in some way so to minimise the probing effects is desirable.I'd be confident the 1202X-E can do what you want.