what switching frequencies have you ran into before in SMPS? - Page 1
Forum section: electronics / projects

author: SArepairman
author_rank: Frequent Contributor
timestamp: March 06, 2015, 07:19:05 am
content: What is the highest switching frequency SMPS you have run across? (that is not some on-chip internal regulator (like inside of a CPU) running at absurdly fast state of the art frequencies as wes mentioned in another thread (about the fastest SMPSU).

author: AndyC_772
author_rank: Super Contributor
timestamp: March 06, 2015, 07:32:09 am
content: Depends what level of integration you'd regard as 'on-chip'.I've designed a lot of Enpirion (now Altera) regulators into products; many of them switch at around 5 MHz.

author: SArepairman
author_rank: Frequent Contributor
timestamp: March 06, 2015, 07:46:01 am
content: well, I am trying to determine what frequency range oscilloscope current probe to get that would be reasonable.I already have a HP HV fully differential HF probe, but for some reason I still want a clip on current probe.And my differential probe is pushed near the limits with its HV attenuator @ mains > 120VRMS rectified.A clip on one could be useful for 240Vrms rectified pulse forms, so it seems like a worthy addition to the lab for a (maybe) modest price. Or just getting the ability to measure 2 current wave forms at the same time (i got REALLY luck with buying my differential HP probe + attenuator).On a PCB level, both would require either soldering in a loop or a shunt resistorand by not on chip, I mean that the SMPSing is happening in a way that a probe could actually help diagnose an issue (I recall reading another thread where people were saying that like some processors have built in SMPSU that use silicon DIE level inductors/capacitors and operate at like, 100's of MHz (I think it was mainly aimed at the portable electronics market. Obviously I can't even probe that signal or fix anything (donations welcome to prove me wrong).I guess I should say, whats the highest frequency SMPSU that was like, probable by either soldering in a wire loop.I guess this thing has other uses too (antennas maybe?), but I am just trying to figure out on whats reasonable for me to keep an eye on ebay.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 06, 2015, 08:22:53 am
content: Hmm, I've seen, not really anything below 30kHz I think, for audible reasons, and a bit over 1MHz at the top end. They're getting faster and faster these days. The fastest, most powerful switching circuit I've worked on is a few hundred VA at up to 2MHz.Even with the advance of technology, those lower frequencies remain relevant -- you simply can't find anything cheaper than 30 year old materials and devices. And you have the advantage of missing the FCC Part 15 LF cutoff at 150kHz -- it's much easier to deal with harmonics near 298kHz, than the entire fundamental at 151kHz.Even with GaN transistors and advanced materials (ceramic capacitors, powder composite inductors), I think I'd be surprised if much power (>100W?) will ever be transformed much beyond 10MHz or so. As a mainstream, high efficiency switching converter, that is. It's not that any characteristics of the circuit change in a fundamental way up there, it's just that it becomes increasingly difficult to handle the power in finite sized traces (thus, limited by the resistivity of copper -- and don't forget skin effect!), while still keeping the layout compact enough not to be absolutely ruined by its stray inductance. Of course, power amplifiers of arbitrary scale (usually little beyond 1kW per device, but that's for a variety of other reasons) can be made at any frequency (into the GHz), but these are comparatively low efficiency beasts, which would, at best, make very poor approximations of switching converters.Tim

author: AndyC_772
author_rank: Super Contributor
timestamp: March 06, 2015, 08:49:07 am
content: I'd have thought the limitation was simply that mains voltages require physically large transistors so as not to break down, and that means high gate capacitance, which in turn means significant energy is required to charge and discharge the gate every time the FET switches.The 'sweet spot' ends up being at a much lower frequency than it does for low voltage converters.

author: Phaedrus
author_rank: Frequent Contributor
timestamp: March 06, 2015, 08:16:16 pm
content: In the ATX world we mostly see 80-200KHz in the more traditional half bridge / forward units, with 250-800KHz in the fancier ZVS or LLC resonant designs, often with a variable frequency mode for sub-200W operation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2015, 01:19:17 am
content: Quote from: AndyC_772 on March 06, 2015, 08:49:07 amThis is close, but also like I said, not the be-all-end-all, of course.The main thing with capacitance is it scales, so it's not as big a deal as you might think.It used to be that high voltage transistors had a very unfavorable scaling, that Rds(on) went up as the square of rated voltage, while keeping the die size constant. So for a given die size, a proportionally higher voltage rating would handle current as the inverse square, and therefore power handling goes as inversely proportional. So you need a proportionally larger device for the same power level, and it gets very prohibitive to operate at very high voltages (examples: flyback or push-pull topology on 240VAC, let alone 400-480VAC industrial supplies).Since the introduction of super-junction transistors (what, within a decade I think?), the scaling has been brought down to proportional, so HV transistors, watt for watt, perform as well as conventional types.What does remain is the reality of impedance and scaling. It's not so hard to make a 1MHz switcher at, say, 30V 3A, because the average impedance is around 10 ohms, trace lengths are small and capacitances are modest. The resonant impedance Z = sqrt(L/C) isn't hard to match. Typical devices might be around 5nF (depending on what voltage you're measuring it at, of course), and although you could try building it with strays as low as 5nH (Z = sqrt(5n/5n) = 1 ohm), more would probably be desirable to reduce switching loss. The time constant sqrt(LC) = 5ns is a sufficiently small fraction of the period (1000ns) that good, crisp switching can be had.Whereas, a 1MHz, 100W switcher following an active PFC stage has to deal with 400VDC and 0.25A, an impedance up around 1600 ohms. You need very low capacitance to do this -- to achieve the same conditions as the low voltage converter, you'd need less than 125pF and 0.2uH. The former is not too bad, really, considering it's only a 1 or 2 amp transistor; and the latter is practically trivial (0.2uH is many inches of twisted pair, for example).But you see the sweet spot actually depends on power. If this were a 500W or 1000W converter, the impedance would be down around a nice, cozy 100 ohms, and the L and C figures are pretty easy to handle. Doing a very low power converter, like a 5W phone charger, at the same supply voltage, is harder -- in the 10s of kohms, so it's very sensitive to capacitance.Doing a very high power converter has the inverse problem, with achieving low enough stray inductance; over 10kW, you probably wouldn't be able to do it with single transistors, and would instead require multiple modules and power combining networks. (This is exactly what commercial radio transmitters do, to realize 50kW at 100MHz and up -- without resorting to vacuum tubes, that is.)Ultimately, the problem comes down to: how far is the inverter impedance (sqrt(Lstray / Cj)) from the impedance of free space, or of typical transmission lines in the circuit (since trace geometry generally results in impedances less than the impedance of free space, and dielectrics reduce that further). The lengths of the transmission lines (because, ultimately, all traces and pads on a PCB, all wires through space -- everything is transmission lines), and their impedances, define the limits of the circuit. If they're too different to implement a switching circuit, you have to resort to LC (or equivalent) tuning, and reduced efficiency. In other words, RF black magic.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 07, 2015, 03:45:25 am
content: A digital audio amplifier is probably going to be the highest you're likely to see in consumer products. Some run at a few MHz.

author: Neilm
author_rank: Super Contributor
timestamp: March 07, 2015, 05:42:20 pm
content: The lowest I have ever seen was a high voltage supply. Due to the inter-winding capacitance, the converter could not be run much higher than 25kHz

author: rx8pilot
author_rank: Super Contributor
timestamp: March 07, 2015, 06:05:00 pm
content: Are you trying to understand what scope you may need to design and/or troubleshoot an SMPS?