High Frequency SMPS...what are drawbacks? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: June 21, 2023, 11:30:40 am
content: Hi,We are doing an SMPS for a person at the moment….unconnected with that, but necessary as an add-on for his job, is that we must also produce a “high frequency SMPS” (10MHz +) for driving LEDs.I apologise in advance, but the spec is very open, ….all we have to do is in some way demonstrate the cost and efficiency and size advantages of a high frequency SMPS….by making some kind of HF SMPS that drives LEDs.Anyway, I take it offline LED drivers are out of the question at these frequencies?....since ferrite is impossible due to core losses, and an air core transformer, with 3500VAC isolation is going to be spewing RF all over the place?As discussed, the spec is very open…..but the customer has mentioned that he likes the following advantages of HF SMPS…..1…..Use of PCB printed coils instead of “normal” inductors.2…The use of much smaller energy storage components, ie inductors and capacitors.3….The use of GaN transistors, for high switching frequency.4…Customer thinks some of his products don’t need a radiated emissions EMC test pass anyway…..so if there are extra RF emissions, then “so what”.5…customer thinks there can be cost savings. (eg due to not needing “bought” inductors., but just PCB inductors)..Anyway, I am thinking that skin effect issues could well make any solution bigger and more expensive? Also, the only capacitors possible are ceramics or “air dielectric” ones, but even ceramics have SRF of eg 5MHz for a typical 1uF, 0805, X7R.I have read multiple web articles, but all are gushingly worshipping of 10MHz plus SMPS’s, without detailing any of the drawbacks.I am suspicious that there are pretty good reasons why 10MHz+ SMPS can never realistically be cheaper than todays 50-1MHz SMPS?Also, there is the situation of ferrite-core-less SMPS’s suffering issues of coupling if they are placed near conductive objects?Also, there is the issue of the tolerance on ceramic capacitors which make up resonant SMPS elements?Also, many modern RF Amplifiers use switching techniques anyway (Class E?)…they are almost an SMPS in their nature, and these are not known for their cheapness….so why is it that their application in SMPS is going to be cheap? (cheaper than what exists now).So can you think of a good example LED driver SMPS that we can put forward for this?

author: Faringdon
author_rank: Super Contributor
timestamp: June 21, 2023, 04:47:29 pm
content: The following article details a diabolically bad 10MHz SMPS.......are there any articles on the web that detail a sucessful 10MHz + SMPS?....ie, one that could sometime become a realistic commercial product?Paper:"A 10 MHz GaNFET Based Isolated High Step-Down DC-DC Converter: Design and Magnetics Investigation "https://backend.orbit.dtu.dk/ws/portalfiles/portal/171273244/08664603.pdfAyk, its good to know where your bridges are before you cross them…..so that you can actually cross them.The paper details a 20W, 200-300v to 0-28v buck, at 10MHz.I think even the authors would confess that what they have produced is at best “uncompetitive”, and more realistically, a very poor offering indeed.…..showing that HF SMPS is not yet feasible……what the paper produces is neither cheaper, smaller, or more efficient, than a “standard” ~100kHz solution.Here are the problems of the 10MHz converter of the kindly provided paper……._ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _*..efficiency is only 85.3% maximum.*…inductor & transformer ferrite pieces are custom made and made of fair-rite 67, which may be in short supply compared to good old 3F3, N87, 3C96, etc etc.*…the shown design has a fixed frequency of 10MHz, and cannot handle any variation of vin and vout outside of the fixed range.*…The design of the resonant L and C needs use of a calibrated Agilent 4294A Impedance Analyzer. (many thousands of dollars)*..A expensive Maxwell 3D simulator is needed to develop the magnetics (it’s a finite element modelling simulator and is very expensive).*..As page 6 shows, foil windings are necessary…these are going to be custom made , at great expense.*…As the paper confesses……QUOTE>> There is limited data available on the design of high frequency (HF) and very high frequency (VHF) power magnetics. Power magnetics have high flux drive. For most of the materials, large signal loss data are not available at above a few MHz. <<UNQUOTE…..So its going to be very difficult, in any HF SMPS design, to select the magnetic material…..becuase there is no data available for showing its characteristics…………and there is no data regarding the tolerance of its characteristics….and the temperature drift etc…so any design of any HF SMPS is going to be a “hope it works” thing, with a likely high field failure rate.*..The paper is honest enough to confess that high frequency magnetics materials are simply not available at realistic costs for operation at 10MHz plus…….QUOTE>> With the emergence of GaN and SiC devices, there has been a significant advancement in semiconductor device switching speed, but magnetics has become a primary limitation constraining miniaturization. By increasing the switching frequency of the converter, the absolute value of capacitance and inductance can be reduced but the actual size reduction at very high frequencies depends on the allowable loss power density. Appropriate core material and winding structure have to be selected for these high frequencies to reduce the loss and realize the achievable miniaturization. Emerging thin-film magnetic materials are a good choice for frequencies greater than 10 MHz. These materials are typically alloys with Fe, Co and Ni. But these are not commercially available at economical costs<<UNQUOTE*..The paper is honest enough to confess that the only way they could drive the primary and secondary FETs at 10MHz was to use a costly Virtex-5 FPGA development board !!.....An FPGA needed just to act as the switching frequency oscillator….this alone makes the entire thing totally un-commercially-viable.*… The paper is honest enough to confess that the only way they could provide the high side drive was to use batteries!..again, this alone makes the entire thing totally un-commercially-viable.*.. The paper (pg 9) is honest enough to confess that the expensive simulator that they used to calculate the magnetics parameters was very innacurate compared to what they actually measured with the precision impedance analyser…and that this was due to the under-estimate of parasitic winding capacitance.*.. The paper (pg 10) is honest enough to confess that the only way that they could get a GaN FET with low enough gate capacitances was to use a 650V part , which has a high Rdson of 220milliohm….and with all the circulating currents you get in resonant converters, this is going to hit efficiency.*..The paper is honest enough (pg 10) to confess that they need a biopolar gate drive supply in order to ensure proper switching of the GaN FETs at 10MHz.*…The paper is honest enough to confess that they can only manage output power variation by doing dead time adjustment…this comes with extra complexity and cost….not only that, but with high dead time, the cct current RMS values go up.*..the 5v max gate drive voltage of the GaN FETs could be breached by ringing in the gate drive…because at this high switching frequency, damping in the gate drive wont be permitted…..so we will likely see dead GaN FETs at some point._ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _So this paper details an SMPS which provides for a specification...and does it in a diaboilcally bad fashion.....even a junior EE could do it massively better using the standard approach of today with switching frequencies around say 100kHz.Every other paper one reads about HF SMPS examples is just as diabolically bad as this......has anyone even heard of any even remotely sensible SMPS being done at 10MHz plus?

author: SeanB
author_rank: Super Contributor
timestamp: June 21, 2023, 05:24:05 pm
content: Losses are likely to be high, and your core materials will start to be expensive, in that you will be well past using cheap ferrites, into the more esoteric, and expensive, high power high frequency RF parts, which also have loss. However filtering is easier, in that a 100n capacitor is likely to be all you need for filtering, and that can be a much more stable ceramic with a decent RF rating, but still will be expensive. Yes you can get by with using PCB coils, but your PCB material will be a lot more expensive, likely using a planar transformer with some flex PCB stacks as windings, and then a top and bottom ferrite cup as former, soldered to the main board and in a cutout in the board, possibly with the output rectifiers also as part of the flex PCB as well. Drive not really a problem, you basically will be designing a class C FM transmitter, using a few parallel MOSFET devices, and that is well enough done to be common these days, but drive will be pretty high power, to charge and discharge all the gate capacitance, gate drive might very well be 30% of the total power use.

author: Gyro
author_rank: Super Contributor
timestamp: June 21, 2023, 06:51:00 pm
content: Why on earth would you need a 10MHz SMPS for an LED driver?Reallyflicker sensitive customer?

author: Faringdon
author_rank: Super Contributor
timestamp: June 21, 2023, 07:05:52 pm
content: Thanks, ive combed the web seeking a ~20W LED driver (DCDC) that uses say just a 4 layer PCB with printed coils, and a cheap ferrite, but nothing.The advantage of the HF is said to be the small size that you can get....and the relief in cost in using printed PCB inductors and transformers...but that maybe is not so?So why, ultimately, is it, that we can have high power switching amplifiers for HF RF transmission, but we cant have HF SMPS?

author: jonpaul
author_rank: Super Contributor
timestamp: June 22, 2023, 06:40:42 am
content: ARGGH!!!FTTS.... will you ever retire ? Are you a bot or Chat GPT cleverly configured to ask seemingly sincere but uselss time wasting questions?"We are" ...pure imagination there is no "we" just FTTS.High frequency SMPS/planar PCB coils/GaN.....Modern designs will optimize the switch topology, devic choices and magentic design according to the required priorities, cost, compliance, availability etc. There are many commercial SMPS at the speeds you mention, but very few if any LED ballasts.main benefit og th very high speed SMPS is small size. As most use soft switching or are resonant, the EMI is easier to comply than lower few supplies.LED driver.....optimized decades ago for lowest cost ...thousands of types are are avail as commodity parts ...most are same topology.Do some ACTUAL work and dont pose useless questions....Humbly suggest that you1/ read the many app notes, papers, texts on the topics2/ Research LED drivers and HF SMPS on the market.3/ BUY some and take apart/de[pot/reverse engineer the device/Finally you may see the real problems...Another 5 min down the FTTS toilet bowl of time....HAVE A GREAT DAYj

author: Faringdon
author_rank: Super Contributor
timestamp: June 22, 2023, 07:53:11 am
content: Thanks JonPaul,But this post is not about doing the best led driver...this post is about doing "a" LED driver that runs at multiple MHz switching frequency....as i put now in bold italic in the top post, we simply have to do it, because we are told to do so.But i wonder if the topic of multi MHz SMPS is actually a bogus one?.....there are no example papers of it being done even slightly well.What is the highest frequency for a sucessful SMPS, say DCDC 24vin to 5 vout at say 20w?Is it 1MHz, 2MHz, 3MHz, 5MHz?....at what frequency does it start getting ridiculously bad....i'd say above 2MHz, but what are your thoughts?

author: ArdWar
author_rank: Frequent Contributor
timestamp: June 22, 2023, 08:07:40 am
content: Quote from: Faringdon on June 22, 2023, 07:53:11 am50MHz...

author: jonpaul
author_rank: Super Contributor
timestamp: June 22, 2023, 09:10:31 am
content: FTTS see modified resp.Your title does not reflect what you ask in the last post.The object in real LED drve is NOT small size nor other issues, priority is cost, then soecs then compliance.Most run at 20..100 kHz as a sweet spot for passives, magentics, parts count.j

author: tggzzz
author_rank: Super Contributor
timestamp: June 22, 2023, 09:58:18 am
content: Quote from: Faringdon on June 21, 2023, 11:30:40 amThat doesn't ring true, because:"...necessary add-on for his job..." and "we must". Either it is necessary and therefore not an add-on, or it is an add-on and therefore not necessary. Which is it?exactly why is >10MHz a requirement? What is the benefit?you have been designing SMPSs for a long time. Surely you are more competent in this topic than other people hereyou have a history of using the word "we", when all indications are that "I" is correctthere have been previous observations that your questions and topics are, shall we say, oblique. This continues with the existing topic title "High Frequency SMPS...what are drawbacks?" not matching the contentfirst you used the moniker treez, then ocset, then faringdon. Why so many monikers?I'm open toanswers(not responses) to the above, and corrections wheredemonstrablywrong.

author: JohnG
author_rank: Frequent Contributor
timestamp: June 22, 2023, 01:23:20 pm
content: Been there, done that. Lots of people have tried. If you go down this road, you will learn a lot, but odds of making a successful product are very slim.Does it make sense? Well, that depends on the specs. Intel has given it a shot, just google FIVR. Also, take a look at Airfuel wireless power (not commercially successful, but it works), and there is a wealth of information if you dig in. It's probably the best practical information out there.There had better be a really good reason for operating at such high frequencies. If they want it because they have fallen for the hype, better make it a "cost plus" or "best effort" contract. If they want it because it operates in a high magnetic field and they can't use magnetic cores, or they want a huge isolation gap, those might be real reasons.Just my free opinion.John

author: Faringdon
author_rank: Super Contributor
timestamp: June 24, 2023, 11:45:43 am
content: Thanks , i didnt find anything when i googled "Intel FIVR".The highest frequency commecrially available SMPS i can find is the MPM8310, which is a pretty measly 7W, 6Vin, DCDC module at 3.5MHzMPM8310https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/document_type/Datasheet/lang/en/sku/MPM3810GQB/document_id/2083/..about $1-$23mm x 2.5mm x 0.9mm...but you have to add Cin, Cout and R(FB)Efficiency 91% maxAs discussed above, the problem for >5MHz is the lack of "datasheet info" available for the ferrites that need to be used. And Ayk, if you dont use ferrites, and instead use air core, then you are spewing radiated emissions all over the place.The problem for >3MHz SMPS, is that you need an FPGA to provide the controllable oscillator thats needed to provide the switching frequency.These are expensive if they provide a tightly toleranced switching frequency.....and for resonant converters, the f(sw) needs to be tight toleranced, or else malfunction results.Also, if you need controllable dead time, or variable frequency to control a resonant converter, then the cost goes up more.Ayk, you cannot do a good resonant converter with simple ON/OFF control.....unless it has really limited line/load regulation.Resonant converters, with their high Q resonant circuits, dont take kindly to being fast ON/OFF'd.Above 2MHz and really, even at low voltage, you need a resonant converter, and not hard switched, not even quasi resonant will do the job.

author: JohnG
author_rank: Frequent Contributor
timestamp: June 24, 2023, 05:44:26 pm
content: https://edc.intel.com/content/www/us/en/design/ipla/software-development-platforms/servers/platforms/intel-pentium-silver-and-intel-celeron-processors-datasheet-volume-1-of-2/005/fully-integrated-voltage-regulator-fivr/I don't know if they are using it commercially still, but they did for at least a year or two.Indeed, there are not many commercial examples. It's not just because the commercial folks are lazy or behind the times.John

author: Faringdon
author_rank: Super Contributor
timestamp: June 25, 2023, 10:01:13 am
content: Thanks @JohnG,..though , ayk, the datasheet of the Intel FIVR SMPS doesnt actually say what switching frequency it is at.......(i suppose they dont want to give away too much)But the fact remains, that above 5MHz, there are literally no examples on the inernet , of commercially viable SMPS....Not anywhere.To be honest, above about 50W, and i reckon there's nothing commercially viable above 2MHz....And even those >2MHz are likely to only be <24Vdc on Vin and Vout.Vicor modules switch at 1MHz, and they are the only examples of commercially viable, HF SMPS....though really, in this post, the question is concerning >2MHz SMPS.

author: jonpaul
author_rank: Super Contributor
timestamp: June 26, 2023, 06:22:45 am
content: FTTS: SMPS Mfg seldom specify SMPS switch fréq as its not relevant to the user and may change.Very fine Vicor modules use fine patents of Mr. Vinciarelli for energy return low loss snubber, from 1980s.https://patents.justia.com/inventor/patrizio-vinciarelliAs most commodity SMPS are designed NOT around a particular freq but for the customers specs, PFC and EMI compliance, cost, size, the entry point to the design is NOT "high frequency" but those other requirements. The optimum topology, switch devices, controller and magnetics are PART OF THE DESIGN ENGINEERS JOB.The freq is often driven by cost/availability of magnetics, EMI, footprint, profile. It is SELDOM optimal to work at the very high speeds you desire, but it CAN and HAS been done, just not in commodity commercial SMPS.Another 10 min down the FTTS toilet bowl of time....jPS: Just ONE example of early HF design:In 1992 we designed 500 kHz SMPS and EBU, with available FETs, magnetics and controllers.100,000s of our Metcal soldering stations 500 kHz SP-200s (also OK industries, and others) designs are in service even today.Some other Metcal soldering iron types use a 10 MHz supply for the Curie pt tip constant current.

author: Faringdon
author_rank: Super Contributor
timestamp: June 26, 2023, 08:52:55 am
content: Thanks, and the benefit of 10MHz+ SMPS, being that a magnetic core is not needed at all.?.....and air core transformer is ok?...this is nonsense?A magnetic core is needed for transformers in SMPS so that they can get the pri:sec coupling.And air core inductors at HF?.......again too much spewing out of mag field?

author: JohnG
author_rank: Frequent Contributor
timestamp: June 26, 2023, 01:04:34 pm
content: First, FIVR runs at 80 and 140 MHz, and they are buck converters. If you didn't find that in your search, you are not looking hard enough. It's strictly low voltage, a few volts or less, on chip.For higher power, voltage, current, etc.:Loosely speaking, you need to get to several 10s of MHz to see a magnetics power density benefit going to air core. There have been some fairly comprehensive studies on this by David Perreault, Charlie Sullivan, and their students. To first order, air core gets better as sqrt(f), but magnetic cores don't, so there's a crossover where air core starts to look better. BUT...Such high frequencies drive you to soft-switched topologies, with all the associated challenges, including higher RMS magnetic currents per unit. Control at such speeds becomes a timing challenge, adding to the challenge of adequately controlling soft-switching converters. Gate drive power grows large. Those are the big problems, and there are dozens of little problems for each big one.GaN, and to a lesser extent SiC, go a long ways to make this doable, but there's a lot of open questions and it remains to be seen whether or not VHF power conversion is viable for anything beyond niche applications.If you still want to pursue this path, I gave you some clues where to start. Here's a few more: Nathan Sokal, Steve Cripps, Andrei Grebennikov, Scott Kee (Caltech), Juan Rivas (Stanford). Look at their work and dig into the references in their work, so you can find more clues yourself (there are many more). The first three are industry consultants.Note that this list of names displays my own personal and technical bias, and there are a lot more people who've worked on this that I haven't given (my apologies to them). If you are willing to spend some months full-time of study getting grounded in the basics, and if you have some background in VHF band electronics, then maybe you can start having a real discussion about it.John

author: f4eru
author_rank: Super Contributor
timestamp: June 26, 2023, 07:59:34 pm
content: Quote from: ArdWar on June 22, 2023, 08:07:40 am37.895 MHz

author: Faringdon
author_rank: Super Contributor
timestamp: June 27, 2023, 06:44:14 am
content: Thanks, the above Intel FIVR spec sheet gives details on page 50 of "switching frequencies up to 1MHz"...there is no mention of any higher switching frequency than that being used.It would be interesting to see the details further, about this FIVR SMPS, but there is nowhere on the web where they detail it.

author: jonpaul
author_rank: Super Contributor
timestamp: June 27, 2023, 06:59:03 am
content: FTTS: Usual nonsense here "... benefit of 10MHz.. magnetic core is not needed... for transformers in SMPS so that they can get the pri:sec coupling.And air core inductors at HF?.......again too much spewing out of mag field?"Examine any high end PC motherboard PSU, most use very effective HF/multiphase/transformer/inductor for the CPU and other regs.Re HF history, see Nikola TESLA (NOT the car!) Charles Steinmentz for basics of AC/HF transformers and inductors.Dr Nathan SOLAL was a great engineer and close friend since 1980s, inventor Class E topology.Sadly Nat never monetized his inventions and passed away years ago. There is a book he wrote with Dr REDL, another great power engineer:Dynamic Analysis of Switching-Mode DC/DC ConvertersJon