switching regulator with mcu
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From: John Devereux on 21 Oct 2009 12:06 dagmargoodboat(a)yahoo.com writes: > On Oct 21, 8:52 am, dagmargoodb...(a)yahoo.com wrote: > >> Yes, I've seen SEPIC inductors sold by one of the major mfrs as such; >> can't recall who. > > Here's one: > http://www.coilcraft.com/lpr4012.cfm > > Price? Dunno. Neither do Digikey, Mouser, Farnell or findchips.com :( -- John Devereux
From: John Devereux on 21 Oct 2009 12:13 Joerg <invalid(a)invalid.invalid> writes: > John Devereux wrote: >> Joerg <invalid(a)invalid.invalid> writes: > > [...] > >>> Another option would be to use a CD40106 or something similar as a >>> Schmitt oscillator, with its VCC capped/zenered around 6-8V. This can >>> drive a little FET, a simple logic level device like a 2N7002 as long >>> as doesn't cost much. Pipe Vref out of your MCU (hoping it has that >>> ...) and use a cheap opamp to pull the Schmitt oscillator input "to >>> the side" when the target voltage is reached. That reduces the duty >>> cycle as much as needed to maintain regulation, pretty much like the >>> throttle on a gasoline-powered generator. If the target voltage >>> doesn't have to be very precise you could also use a NPN plus zener >>> for that, without a reference source. Probably a TL431-type device >>> would work as well and those are quite cheap, in the penny range. >> >> Hi Jeorg, >> >> Are you talking about a SEPIC/flyback arrangement here? I.e., 2N7001 >> drives a transformer? How do you stop the opamp railing during startup >> or a load step? The schmitt osc would stop and .. how does it go >> ... Phut! :) >> > > If you use an opamp a simple trick to avoid this is a diode. That way > the enchilada can pull the Schmitt to one side but cannot push it to > the other. If you are really brazen use a comparator with O/C -> no > diode, saves 1-2 cents. If you use a transistor or TL431 the problem > goes away on its own, sans diode. Of course, neat. Finally the famed Jeorgian discrete SMPS is starting to take shape :) > Now, set the max duty cycle to > whatever worst case demand you calculated, at the minimum expected > input voltage, plus 20% margin or so. >> Also of course by the time he's actually bought a SEPIC transformer it >> could all approach his 1.8 Euros. Or are there some super-cheap ones >> now? >> > > Yes, but only when you buy in Asia. Anyone know how to do this for smaller quantities (<~1k, say?). I've got plenty of Asian suppliers trying to get our PCB business, but nobody trying to sell me inductors :(. > For some reason you can't get them much under a buck inside the US > even though I could imagine those are also made in China. Coilcraft is > typically the best deal for coupled SEPIC coils if it has to be > domestic. > > Using two individual inductors gets you around the problem. -- John Devereux
From: Joerg on 21 Oct 2009 12:49 John Devereux wrote: > Joerg <invalid(a)invalid.invalid> writes: > >> John Devereux wrote: >>> Joerg <invalid(a)invalid.invalid> writes: >> [...] >> >>>> Another option would be to use a CD40106 or something similar as a >>>> Schmitt oscillator, with its VCC capped/zenered around 6-8V. This can >>>> drive a little FET, a simple logic level device like a 2N7002 as long >>>> as doesn't cost much. Pipe Vref out of your MCU (hoping it has that >>>> ...) and use a cheap opamp to pull the Schmitt oscillator input "to >>>> the side" when the target voltage is reached. That reduces the duty >>>> cycle as much as needed to maintain regulation, pretty much like the >>>> throttle on a gasoline-powered generator. If the target voltage >>>> doesn't have to be very precise you could also use a NPN plus zener >>>> for that, without a reference source. Probably a TL431-type device >>>> would work as well and those are quite cheap, in the penny range. >>> Hi Jeorg, >>> >>> Are you talking about a SEPIC/flyback arrangement here? I.e., 2N7001 >>> drives a transformer? How do you stop the opamp railing during startup >>> or a load step? The schmitt osc would stop and .. how does it go >>> ... Phut! :) >>> >> If you use an opamp a simple trick to avoid this is a diode. That way >> the enchilada can pull the Schmitt to one side but cannot push it to >> the other. If you are really brazen use a comparator with O/C -> no >> diode, saves 1-2 cents. If you use a transistor or TL431 the problem >> goes away on its own, sans diode. > > Of course, neat. Finally the famed Jeorgian discrete SMPS is starting to > take shape :) > My first one took shape about 15 years ago, tons of them in use by consumers right now, still coming off the conveyor belt (in China). I don't know if it will outlast the VW Beetle in production but it might. It has already survived the client company's president :-( Back then I learned a valuable lesson: Inductor prices in the western world must be taken with a grain of salt. A custom mfg place in Taiwan outbid a catalog (!) part from a western manufacturer, big time. And we even got an EMI-savvy toroid for our money versus the cheap open cores from mainstream distributors. Of course it does help not to have a middleman in the game. >> Now, set the max duty cycle to >> whatever worst case demand you calculated, at the minimum expected >> input voltage, plus 20% margin or so. > >>> Also of course by the time he's actually bought a SEPIC transformer it >>> could all approach his 1.8 Euros. Or are there some super-cheap ones >>> now? >>> >> Yes, but only when you buy in Asia. > > Anyone know how to do this for smaller quantities (<~1k, say?). I've got > plenty of Asian suppliers trying to get our PCB business, but nobody > trying to sell me inductors :(. > <1k? No, I don't think they'd even talk to you. However, Asia is all about one thing: Connections. Ask the PCB stuffing places what sort of deals they can get locally. They might be able to swing it, with a supplier they already buy lots of other stuff from. If you have reputable places there that do <1k qty board runs with decent quality let me (and others here) know. I bet lots of us are interested. [...] -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: Joerg on 21 Oct 2009 12:52 John Devereux wrote: > dagmargoodboat(a)yahoo.com writes: > >> On Oct 21, 8:52 am, dagmargoodb...(a)yahoo.com wrote: >> >>> Yes, I've seen SEPIC inductors sold by one of the major mfrs as such; >>> can't recall who. >> Here's one: >> http://www.coilcraft.com/lpr4012.cfm >> >> Price? Dunno. > > Neither do Digikey, Mouser, Farnell or findchips.com :( > With Coilcraft you have to become a "member". Sign up and then you get instant pricing access on their site. If you don't want to then request a quote, comes within minutes via email. Their pricing is actually quite favorable compared to other domestic suppliers, maybe because they do a lot of automotive. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: John Devereux on 21 Oct 2009 13:15
Joerg <invalid(a)invalid.invalid> writes: > John Devereux wrote: >> Joerg <invalid(a)invalid.invalid> writes: >> >>> John Devereux wrote: [...] >> Of course, neat. Finally the famed Jeorgian discrete SMPS is starting to >> take shape :) >> > > My first one took shape about 15 years ago, tons of them in use by > consumers right now, still coming off the conveyor belt (in China). I > don't know if it will outlast the VW Beetle in production but it > might. It has already survived the client company's president :-( > > Back then I learned a valuable lesson: Inductor prices in the western > world must be taken with a grain of salt. A custom mfg place in Taiwan > outbid a catalog (!) part from a western manufacturer, big time. And > we even got an EMI-savvy toroid for our money versus the cheap open > cores from mainstream distributors. Of course it does help not to have > a middleman in the game. > > [...] >> Anyone know how to do this for smaller quantities (<~1k, say?). I've got >> plenty of Asian suppliers trying to get our PCB business, but nobody >> trying to sell me inductors :(. >> > > <1k? No, I don't think they'd even talk to you. I should have said "single reel" since there are typically more than that on a reel. I guess we could probably make a "strategic" buy of ~5k if needed. If they're *really* cheap 10k! :) > However, Asia is all about one thing: Connections. Ask the PCB > stuffing places what sort of deals they can get locally. They might be > able to swing it, with a supplier they already buy lots of other stuff > from. > > If you have reputable places there that do <1k qty board runs with > decent quality let me (and others here) know. I bet lots of us are > interested. Sorry, I meant bare PCB not stuffed. They advertise stuffing too - down to any quantity - but they said once they buy non-free issued parts from Digikey... :) I just thought it strange that I can get small qtys of PCBs at 1/2 the price of local suppliers, but nobody does the same for parts. (Which ought to be much easier to supply). > [...] -- John Devereux |