SMPS design help - Page 1
Forum section: electronics / projects

author: thatguyyouknow
author_rank: Contributor
timestamp: October 20, 2016, 11:18:10 am
content: I want to make a SMPS two output voltages, a 230v output and a 70v output.The 230v output would have a max expected current of 100ma and the 70v would have a max expected current of 4 amps or more.These will taken from a 115v ac mains.My main priorities are low noise in the audio range and minimal EMI, small size, low cost.I'm a beginner to SMPS and I have some questions.The first one being, what are the reasonsnotto just go straight for a 4MHz smps? Other then efficiency losses which I don't care about too much it seems like it would be the cheapest option and any noise will be way above the frequency range I care about. The problem would be EMI I'd assume but how big of a problem is this?

author: diyaudio
author_rank: Frequent Contributor
timestamp: October 20, 2016, 12:14:00 pm
content: Quote from: thatguyyouknow on October 20, 2016, 11:18:10 amLearner you say. Don't play around with SMPS 115VAC (162VDC) it will kill you!!first try isolated lower voltages like 30V to 12V then progress to work with higher voltages. +1 for asking for advice.

author: thatguyyouknow
author_rank: Contributor
timestamp: October 20, 2016, 12:34:10 pm
content: I regularly build and use circuits that require as much as 1200v, usually around 300v. I'm trying to build a relatively low voltage supply that not hulking big, heavy, and expensive.No worries about high voltage here, I need to know where to start and hope to god I don't have to spend 3 years designing it.I've read that high voltage high frequency switching is nigh impossible so maybe 4MHz isn't the best start.What frequency would be best for minimal EMI troubles and low cost?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2016, 12:46:32 pm
content: Under 150kHz, assuming you're in America.Tim

author: thatguyyouknow
author_rank: Contributor
timestamp: October 20, 2016, 12:50:39 pm
content: Yeah I'm in the USA. I'll arbitrarily choose 90khz.What sort of transformer do I use for this application? How do I calculate power rating vs frequency? Would something like this work for my 115v to 230v AC needs?http://www.mouser.com/ProductDetail/Vacuumschmelze/T60403-F4185-X046/?qs=1gGcJ6gR7Y0ORpDU5ZhC1w%3d%3d

author: diyaudio
author_rank: Frequent Contributor
timestamp: October 20, 2016, 01:46:00 pm
content: Quote from: thatguyyouknow on October 20, 2016, 12:34:10 pmA != BQuote

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 20, 2016, 01:54:20 pm
content: Quote from: thatguyyouknow on October 20, 2016, 12:50:39 pmI mainly design switchers for a living and while I won't claim it is rocket science, there is quite a bit of deep knowledge required to get these things to work. I strongly recommend getting some development or evaluation boards if you want to save some time and money (and by "some" I mean "years" of time and "thousands" of dollars in burnt components and board redesigns). There's no substitute for a solid understanding of the theory and practice, though, and that doesn't come about quickly.Coincidentally, I am right now working on a series of rapid prototyping kits for switchmode power supplies aimed squarely at busy product designers that just want to get something working quickly. I'll be posting a thread about the first one in the series - a 4W, very wide range input (80-425VDC), dual-output "housekeeping" power supply.As for your application, I would go with a two-switch forward running at 100kHz. The reasons for this recommendation are: it's an appropriate choice for the output power; it's damn near indestructible as long as the gate drive is good and duty cycle is limited to 50% max; energy stored in the leakage inductance is recycled back to the input capacitor so there is no problem with spikes blowing up your switches (high leakage does reduce the transfer of power from primary to secondary, however, so should generally be minimized); the switching frequency is just right for the power range without being too demanding of layout.If the offline flyback rapid prototyping kit sells well I will do the same for the two-switch forward and boost PFC converters next. But that won't help you right now, so if you are on a tight schedule you'll probably want to see what sort of evaluation boards are around that come close to your desired power level (no way will you find either - much less both - of your desired output voltages; usually smps evaluation boards target common voltages like 5V, 12V, 24V and 48V).

author: chris_leyson
author_rank: Super Contributor
timestamp: October 20, 2016, 02:02:32 pm
content: Get yourself a copy Switched Mode Power Supply Design by Kieth Billings or Billing and Pressman for a later edition. Will teach you a lot.

author: thatguyyouknow
author_rank: Contributor
timestamp: October 20, 2016, 02:51:02 pm
content: QuoteYay, a specialist. I usually only deal in simple analog, this is my first endeavor into more arcane electronic and I really don't have the countless hours of time to learn the theory of SMPS.Could you give me an example of one of these boards?Also you mention a two switch forward. Isn't forward a buck type? How am I going to buck 115vac into 230v dc?

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 20, 2016, 03:08:05 pm
content: Quote from: thatguyyouknow on October 20, 2016, 02:51:02 pmCome on, now... you aren't seriously asking me to Google That For You, are you??? ON Semi, ST Micro, TI, NXP, etc., all have two-switch forward evaluation boards available. I, personally, haven't bought an SMPS eval. board in years.Quote from: thatguyyouknow on October 20, 2016, 02:51:02 pmYou seem to be forgetting there is a transformer involved, but yes, the forward converter is derived from the buck. In fact, it is a buck with a transformer in between the switch and the freewheeling diode/choke.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 23, 2016, 09:48:28 pm
content: Quote from: thatguyyouknow on October 20, 2016, 11:18:10 amThese goals are not compatible with each other.Quote from: thatguyyouknow on October 20, 2016, 11:18:10 amYou are just not going to build something like this as a beginner without spending "countless hours to learn the theory of SMPS".Read this, it's a good introduction to designing a SMPSJim Williams knew what he was doing back in 1987.Switching Regulators For Poets (PDF) - Linear Technologyhttp://cds.linear.com/docs/en/application-note/an25fa.pdf