SMPS chip but not for 240V? - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 03:49:13 am
content: I have tried to learn about buck converters, with the goal to learn how you convert from 240V to 5V but I keep running into chips for low voltage except a few for 600V-1000V. Even Davids "jelly bean" MC34063 is for max 40 voltage. Where do the 240V go in, is 40V fine, I seem not to understand how it all can be.Hope thay uoy please will help? :-)

author: james_s
author_rank: Super Contributor
timestamp: September 04, 2019, 03:57:08 am
content: While high voltage buck converters do exist, that topology is not particularly common for such applications. There are lots of power conversion ICs out there intended for converting mains voltage down to low DC voltages.

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 05:39:20 am
content: Quote from: james_s on September 04, 2019, 03:57:08 amThanks, but is that the way a standard SMPS do it?

author: oPossum
author_rank: Super Contributor
timestamp: September 04, 2019, 05:42:44 am
content: Typically flyback or forward converter.

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 06:30:39 am
content: Quote from: oPossum on September 04, 2019, 05:42:44 amI have hard of these terms but do not understand how it all works, how does Dave's jelly bean at max 40V handle 240V?

author: brabus
author_rank: Frequent Contributor
timestamp: September 04, 2019, 06:39:01 am
content: You usually have an external MOSFET or a BJT doing the hard lifting and dealing with the high voltages and currents. The controller only provides the driving commands for them.

author: oPossum
author_rank: Super Contributor
timestamp: September 04, 2019, 06:42:08 am
content: There are offline flyback controllers with integrated switch for low power applications.A random example...https://www.onsemi.com/pub/Collateral/FSL137H-D.pdfUsing a MC34063 with an external switch may be possible, but there are may other better choices.

author: magic
author_rank: Super Contributor
timestamp: September 04, 2019, 06:42:19 am
content: In addition, they usually use an auxiliary transformer winding which produces 5~20V for the SMPS chip.Startup is provided by a pullup resistor which slowly charges the controller's power supply capacitors when the PSU is connected to mains. This is possible because the controller draws little power before it becomes active.

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 06:46:43 am
content: Quote from: brabus on September 04, 2019, 06:39:01 amAm I guessing right in that the Mosfet is better because of less voltage drop?Quote from: oPossum on September 04, 2019, 06:42:08 amI'll take a good look at that! :-)Quote from: magic on September 04, 2019, 06:42:19 amDo you mean a transformer before the SMPS circuit?

author: MosherIV
author_rank: Super Contributor
timestamp: September 04, 2019, 06:47:11 am
content: Hi.Typically mains voltage smps rectify the mains AC into a high voltage cap.This HV DC is pulsed into an isolation transformer at high freq.The freq controls the output voltage/current.The output V needs low ESR caps to smooth the output of the isolation transformer.The output voltage is monitored and fed back via an opto isolator.This feedback is used to determine the freq to pulse the isolation transformer.This is all controlled by a ASIC which is powered striaght from the HV DC.I think (but I am not sure) this is the 'forward converter' topology.

author: oPossum
author_rank: Super Contributor
timestamp: September 04, 2019, 06:51:36 am
content: If it has one switch it is probably flyback. If it has two switches (or a multiple of 2) it is probably forward.Switch refers to whatever is doing the switching - BJT, MOSFET, IGBT

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 06:53:00 am
content: Quote from: MosherIV on September 04, 2019, 06:47:11 amYes thats sounds logical, and like the attached drawing. But In my book it looks like that 240V is connected to the chip?

author: oPossum
author_rank: Super Contributor
timestamp: September 04, 2019, 06:53:26 am
content: Quote from: FriedMule on September 04, 2019, 06:46:43 amSee figure 1 in the FSL137 data sheet.

author: oPossum
author_rank: Super Contributor
timestamp: September 04, 2019, 06:56:17 am
content: Quote from: FriedMule on September 04, 2019, 06:53:00 amThat is for a bootstrap circuit that gets the chip running. Operational power comes from the transformer winding.

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 07:04:49 am
content: Quote from: oPossum on September 04, 2019, 06:53:26 amSo the 240V does not go undisturbed down to the HV-pin at the chip?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 04, 2019, 08:37:57 am
content: Quote from: FriedMule on September 04, 2019, 06:53:00 amThat is rare - the most common arrangement is a resistive dropper from HV to get it started, then use a winding on the transformer for normal running.There are some pure buck converters designed for mains voltages, look at Power Integrations' LinkSwitch series.The reason that simple buck converters aren't common for mains is that mains PSUs usually need isolation, so a flyback with a transformer makes the most sense.

author: tunk
author_rank: Super Contributor
timestamp: September 04, 2019, 10:13:41 am
content: I guess it's a bit beside the point, but the 240V is an average andafter rectification and smoothing it's more like 340V (240*2^0.5).You could also take a look at some of DiodeGoneWild's youtube videos.

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 10:26:17 am
content: Oh, that makes a lot of sense. What are the difference between the two schematics? Safety, efficiency, pros / cons?

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 05:59:54 pm
content: Bump:I am not asking for a scientific explanation, just a simple one, since one has a lot more components then the other but both should buck converters. :-)

author: james_s
author_rank: Super Contributor
timestamp: September 04, 2019, 06:24:51 pm
content: Buck converters do not provide isolation between line and load. Typically if you are going to go to the effort of making a off-line SMPS you may as well go with an isolated topology.

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 09:25:08 pm
content: Quote from: james_s on September 04, 2019, 06:24:51 pmGreat advice, Both of these do look like isolated, but are one better the the other?

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2019, 10:33:09 pm
content: Quote from: james_s on September 04, 2019, 03:57:08 amQuote from: oPossum on September 04, 2019, 05:42:44 amThe reason for this, beyond galvanic isolation from input to output, is that the transformer adjusts the ratio of voltage to current in the power switch to a more advantageous value. If a buck converter is used, then the power switch has to handle the maximum output current and the maximum input voltage making it significantly more expensive and lowering efficiency as well. The transformer in a flyback or forward converter considerably reduces the current which the power switch must handle.

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 04, 2019, 11:33:42 pm
content: Quote from: David Hess on September 04, 2019, 10:33:09 pmYes I see the logic in that and you do also, as written earlier" get added safety". What I am out to achieve is a 240V to 5V isolated smps. Since I have about no idea on what to do, I hope that some of you could point me to a schematic that is universally okay. I know that higher frequency is better but do also demand a lot more skills in designing the circuit board.

author: MosherIV
author_rank: Super Contributor
timestamp: September 05, 2019, 06:54:53 am
content: There is no one circuit that is universally ok, it depends on the parameters of the circuit.Easiest is to find app notes for particular devices.Building your ownIt is highly dangerous - there is rectified mains, HV DC, with all the power of your national grid behind it !It is very difficult to find the isolation transformers. They are not easily available off the shelf.If you are doing this for learning, be very careful

author: BravoV
author_rank: Super Contributor
timestamp: September 05, 2019, 07:08:20 am
content: Just because you could do & probably understand "buck converter" from say 12 V down to 5 V, it doesn't mean you can still to stick with buck converter ONLY to do mains 240V AC down to 5 V DC.Go learn and study further other topologies as pointed by many posters above, that is notsmartsafe method of practicing electronics, especially related to mains voltage.

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 05, 2019, 08:12:31 am
content: Quote from: BravoV on September 05, 2019, 07:08:20 amQuote from: MosherIV on September 05, 2019, 06:54:53 amI am glad for your warning, but the mains side of the project am I not unsure about, it is the part I know most about and have done most work on, i.e. at worksites and home, I am not an electrician but can do most installations. I am also planning on using a variac for a minimum of safety.I keep reading, look at videos and know that there is a lot to learn. All I have previously done is bould "old" school power supplies.Do you have any advices, beside YouTuber DiodeGoneWild on who to wach, where to read and so on?

author: james_s
author_rank: Super Contributor
timestamp: September 05, 2019, 02:22:38 pm
content: You are in way over your head, I don't intend to sound rude however if you had the knowledge to do this safely and successfully you would not be asking many of these questions. SMPS design is a rather advanced topic and you will need to be proficient at all the underlying concepts. If you don't get everything just right, they have a tendency to blow up instantly when power is applied, or if they work they may be spewing copious amounts of RF hash and knocking out radio or TV reception. You are asking someone to explain to you how to fly a 787 when you have never flown a Cessna or drive a Ferrari when you have never driven a basic passenger car.Start reading every application note and textbook you can find on the subject, study existing designs, eventually you will get there but it's not easy.

author: soldar
author_rank: Super Contributor
timestamp: September 05, 2019, 05:08:56 pm
content: Quote from: james_s on September 05, 2019, 02:22:38 pm+1I have repaired dozens of this type of SMPS and have a pretty good understanding but I would not dream of building one. I would still need to study and learn a lot more. A lot. This is a very specialized field and no one is going to be designing and building this in a few hours.

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 05, 2019, 05:24:01 pm
content: Quote from: soldar on September 05, 2019, 05:08:56 pmQuote from: james_s on September 05, 2019, 02:22:38 pmSorry that I was not clear. What I meant was that the safety and handling mains, is what I know most about. I can understand that you could compare me to knowing the sea and how to navigate it safe, but what I am trying to do is to sail around the world in a DIY atomic u-boad:-)My error in thinking that I could build a SMPS was that Dave, in a video is saying that it is almost the same, that the one is not much harder then the other. And since I know how to make a decent linear power supply, what was then holding me back? :-)

author: soldar
author_rank: Super Contributor
timestamp: September 06, 2019, 08:01:40 am
content: Quote from: FriedMule on September 04, 2019, 09:25:08 pmThey are basically the same thing. One is a conceptual explanation of how the circuit works and the other is an actual application. The first one omits many minor things not needed to understand the working of the circuit.You need to read, study and understand the document in its entirety. You are asking questions which are answered in there. Reading manufacturers' application notes is a very good way of learning.You need to read up about the different topologies and what they are good for. Nobody can give you here the basic knowledge needed to design SMPS. That requires hundreds of hours of study and learning.For a start:http://ww1.microchip.com/downloads/en/appnotes/01114a.pdfBTW, it saysQuote"As the name implies"? What is the meaning of "buck" in this context? And why does it imply it con only produce lower voltage?Another:https://www.ti.com/seclit/ug/slyu036/slyu036.pdf

author: FriedMule
author_rank: Frequent Contributor
timestamp: September 06, 2019, 02:06:48 pm
content: Quote from: soldar on September 06, 2019, 08:01:40 amThank you a lot, great links and helpful comment, I will do as you and other suggests. :-) I can also start by learning ad trying out low voltage bucks (12V to 5V) and so on, if I destroys some components on the way, no harm:-)