Switching Power Supply PCB Layout Seminar

welcome welcome everyone to to PCB West 2013 session 9 switching power supplies my name is I'm your speaker and my name is Scott Nance I'm a senior printed circuit designer at optimum design associates I've been a PCB designer in the Service Bureau industry for 30 years I'd like to point out that I'm not at electrical engineer on our power supply designers so this presentation is from my perspective and that is out of a printed circuit designer so the reason for this presentation I think is kind of simple switching power supplies in their layouts or everywhere we see them in simple designs tough designs cheap consumer products and your high-end phones we see them throughout our computer supplying the power and all throughout the computer at point of loads so that's the reason for the presentation the reason that I'm here is that we we were it was suggested by my boss that we each take an article to write and mine was switching power supplies and I invite you to look at some other articles that were written by other designers from opt-in design you can find them but design in the trenches calm and they are there covering topics like DDR timing rationale silkscreen valor NPI OD B plus plus and several others so you can see abstracts of these articles at again design in the trenches comm we see that there's tons of available information about switching power supplies for the electrical engineer volumes about magnetics and power loss but there's a there's notice a much good information for the PCB layout professional and I think this this this presentation might help clear up some of the confusions that PCB designers have when it comes time for a switching power supply we need to be able to identify it and be able to lay it out so that each layout acts and intent and is and is uh it works as the manufacturer intended and so I intend to briefly tell the history of switching power supplies we won't spend too much time there and then explain how they work and then I'm going to provide some specific layout techniques and examples some do's and don'ts and then all this is just meant to provide the lay out professional with enough information that he becomes empowered to become a better member of his design team so let's begin the agenda first of all switching power supplies what are they where they look like how do we identify them how do they work and then we'll get into the PCB layout and that'll be probably more fun but we'll get to that as soon as we can and then if we have time we have some power supply basics or review and at any time if anyone has any questions or anything is unclear please feel free to ask questions and we'll see if we can get to them in the time allotted so again switching power supply history this is going to be a short just recap on where they've been not where they're going but just a short where they've been where they have been where they came from and then we're going to go over some supply types and topologies so you can identify them as a PCB layout professional it might not be as as important to know these things because most of these decisions have been made before they ever get into layout the engineers already decided all the parameters for the switching power supply we'll get more into the meat of it when we get down into switching power supply circuitry a little bit of the history the the switching power supply histories that the principles were known in the 1930s they were used on condensers and arc welders and things of that nature IBM used it in their 7:04 mainframe and of course it was giant and not as efficient as the switches that we see today NASA used them Telstar satellite is a good example and then the famous one is the Apple 2 personal computer because the switching power supply was introduced and actually made the computer small enough and light enough that it could be used in the home so there's lots of people that want to take credit for the popularity the switching power supply Apple comes to mind rod Holt was the engineer that introduced it in the Apple 2 and he got a lot of credit which witches do but he did not invent the switching power supply he only applied it to the home computer what really should be credited with the explosion of popular switching power supplies are innovations in the semiconductor industry which are going to be the controller chips that control the switching power supply and make them efficient and the other thing was a power it was needed that a power switch was needed to be able to quickly switch high currents and the vertical metal-oxide-semiconductor power transistor support enabled this and that's that's a fab term for vertical metal-oxide-semiconductor it's the fab process for the chip and it allowed for the quick switching this was important for consumer products because at the time bipolar transistors were used for a while and they work very well for high power applications but they didn't switch yeah in the older days nowhere near switch fast enough and what happened was the switching frequency wasn't above the audible hearing range of humans so we hear things like squeals and TVs and things like that and and now the frequencies are much higher and they're way more efficient because of that so a little more history these switching power supplies used to be called switch mode power supplies motorola started enforcing their trademark so they no longer are called that they're called variations they're called they're often called switched mode switching mode or SMPS I like the universal term switcher because it applies to all of them and I'll be using that term from now from this point forward so when you think of a switching power supply though if you're shopping for a switching power supply what comes to mind is the computer main supply it's actually more than a switching power supply and I'll show you in a minute but it's it we call that a power supply unit and that's what sources the mains voltage the 110 and supplies all the voltages through the computer that you need additional regulation is happening at the controller at the graphics card in any other place that's stepping down the voltages from the main power supply and we call those regulators or point of load and this is just that's a little example of a little linear ball grid array point of load regulator so here's some examples just it's it's a showing of pictures of the vast difference and maybe maybe adding to the confusion of what is a switching power supply the computer main power supply a cellphone charger an adjustable laboratory-grade switching power supply the linear ball grid array which looks looks benign but it's really that one's quite exotic an off-the-shelf module that you could use for for applications that would work for off-the-shelf and then that's a car audio 800 watt power amplifier so here's a block diagram of the computer PSU that I was talking about as you can see the first stages of them are really preparing the voltages for the switching power supply fuse EMI filter rectification if you know about power supply after rectification it becomes a DC voltage so really the switching power supply is not really converting AC into DC it's a it's taking a DC voltage and I'll show you it's actually converting it to an AC and then back to a DC for its output voltage for the purpose of efficiency in this picture right here I have after the rectifier I have a PFC circuit and that is in some higher-end PS use and that sounds for power factor correction and there's two types as a passive and an active and if it's an active power factor corrector circuit it's actually another switching power supply in line preparing the voltage before the main power supply and your common DC voltages that you would see your standby voltage is your plus 12y plus five plus 3.3 sometimes minus 12 and minus five so this is what we're going to be foot we're going to be talking about the PSU anymore just the power supply the the switching power supply sections so by definition a switching power supply uses a power switch magnetics out filter caps and a rectifier to transfer energy from and that's from an input to an output source providing a regulated voltage and it works by rapidly turning that power switch on and off so that output voltage is calculated by what the input voltage to the switcher is and the duty cycle and the duty cycle is the proportion of time that that switch is on vs during the on state they call it saturation mode and it's an efficient state it's got negligible voltage drop across it in the off state it's cut off and it has no current going across it and so the power switch stays in these two states for four for some of the time and those are very efficient states and so during these times they dissipate very little power this is the theory behind the switching power supply and of course efficiency is usually the reason that you're using a switching power supply linear regulators are typically in the 60% and and a switch and power supplies are regularly in the 90 percents and they're never 100% but they're they can be 98 higher efficiency of course means lower power drain on the input source longer life for your batteries lower heat buildup all the things we need for our small modern-day electronic devices so compare comparing them to the predecessors which are linear regulators switchers don't require the large heavy low frequency transformer so you would have seen and maybe the Apple one before the Apple 2 they were large transformers switchers don't require these but they do require high frequency filtering and these are done with a lot smaller components so the filtering is done with an LC circuit it's going to be an inductor and a cap as opposed to a large transformer these these aren't dissipating as much heat and so we see a higher frequency a higher efficiency by doing this it also allows us to miniaturize and in conjunction with with the higher power efficiency it gives them a huge advantage over the linear regulators the disadvantage to the switcher is they can be demanding in layout even when they are laid out correctly because of the fast switching and because of the high current they they're noisy they can radiate noise and so we have to be aware of that we have to we have to be aware of where this noise is coming from there are two main types of switching power supplies there's isolated non isolated what these mean is if there's a transformer in the middle of the switching power supply and typically typically you're going to need a transformer isolated switching power supply when the voltages are higher and this is for a safety reason so anything above forty two and a half volts this is pretty much a worldwide standard but here I'm showing ul requirements require this as well again this is for safety but if you don't need it so the lower voltages ones can be extremely small and many of the power components can be on the same chip as the control circuitry so that's why we find modules that have very few external components so here's three of the common non isolated these would be the smaller lower voltages they're called buck boost and buck boost and they're and they're identified by your input and output voltage requirements if the step-down regulator is called a buck the input voltage is going to be higher than the output the boost obviously the output is going to be higher than the input and the buck boost is going to be polarity inverting so sometimes it's called polarity inverting and something not as common it's called a non isolated fly back sometimes by mistake they're called fly back but without the transformer they're not fly back you'd have to call it a non isolated fly back so here's a this is the simplest circuit this is the step-down regulator the buck converter and like the first thing we'll do is identify all the key power components the filter capacitors are identified as C in and C out the power switch here is u1 that's that's also of the function of a series pass element l1 is the magnetic element in this case an inductor and then d1 is the output rectifier in this case that's a Schottky diode trying to keep the forward drop the forward voltage drop low and then you see there's three different topologies but they're really created by just rearranging the switch the rectifier and the inductor so by these arrangements they're slightly different but what's happening is that the energy has been recovered from the magnetic element differently so we're getting a boost up in voltage with the boost and a polarity inverting by it just with rearranging the three components and then big words a synchronous versus synchronous synchronous is often called an ultra efficient switching power supply and I mentioned forward voltage drop of the rectifier in an efficient switching power supply a lot of time half of the losses or even over half the losses are attributed to that rectifier and so it's being replaced by another MOSFET sometimes confusing in layout the two the two are doing two different things but both of them have their own critical function the control lines that are controlling the two are often called top gate and bottom gate they're called top FET and bottom FET one of them this again the series pass element the other one is going to be the output rectification they're also called upper and lower sometimes but you will see these inspection they'll be called synchronous or ultra efficient and then interleaved and multi-phased interleaving is copying the series pass element along with the magnetics and what this does is it lowers the current stresses on these devices you're able to share the input in the output filter caps and by doing this you're actually able to reduce the size of the output filter cap again more more efficient and and in this case it's multi-phase you can tell by the control Land's the control lines and what that does is it actually reduces noise and increases the efficiency all at once you're going to see this particular one doing things like supplying the core voltage of a microprocessor so these are the isolated topologies these are going to be typically for the higher voltages there's there's six common ones that identify here but there's really you know they're inventing ones all the time for different applications the I'm showing some specific or some common applications but the reality is that any of these topologies will work in any application they just have different characteristics that make them more suited for a specific application the flyback is the one that I said later earlier was in the tv-tv high voltage that's typically where you see the flyback or some some cheaper computer power supplies the forward would be the higher-end computer PS use to switch forward again just for higher power you can see the power typically going up in range because each one of these topologies is better suited for that range any topology can be interleaved and so you saw that waiting up to 1,000 Watts when they're going up to 10,000 watts that's typically the full bridge that's being interleaved so and you can interleave dozens of times there's multiple switches and multiple inductors these things can look very complex but the switching principles are the same as the simple ones and so I like showing the simple schematics because what we learn here is just replicated on some of these more complex ones so the isolate topologies I'm showing the flyback and the forward these don't look too much different but what they're doing really the flyback is the is derived from the previous I showed you the buck boost the polarity inverting really all it's happening is the magnetic element is split and coupled and wrapped around forming a transformer so that's the isolation but in reality it is that's why it's sometimes called a fly back when it's non isolated the forward converter is a change from the buck converter and then all the other isolated topologies are really derivation of the forward converter more switches higher power more efficient at that power and then the last two would be the half bridge and the full bridge this wraps up the last six of the isolated topologies more and more switches more and more efficient for more and more power and I just want to put if you see an H bridge that's not a that's not an abbreviation for the half bridge that's really showing that you're using a full bridge and the H is just really how the switches look on the schematic formula H so we'll get into PCB layout any questions so far so reference layout critical paths in EMI and analog circuitry the reference layout is something that you're going to find if you have a microcontroller in a manufacturer that outputs those data a lot of times a reference layout is going to be used exactly you'll be able to copy it exactly and it will act just like the manufacturer intended to I don't get any of those I rarely ever see any data sheets at all i'm usually searching really hard for data sheets and application notes one hint is when you when you can't find them contact a manufacturer they will give you information that you can't always find online so again always reference the manufacturers datasheet now any application notes and again this is going to apply if you have a manufacturer that has a controller or a critical device in there that's going to show you how to make it work many times they're not available at all so we're going to talk about some reasons that the reference layout can't be copied very common I mean if we wouldn't have this class right here if all you had to do was copy a layout every time right and we're talking about where the changes can be made and where they should not just some quick sample reference suggested layouts they come in all forms some of them are cartoonish liking some but they're always giving you something that the manufacturer knows is needed most often without enough explanation of why some of them are just demonstration circuits that they give you that they made work for them and may or may not even apply with your application of it your layout may not look anything like that but that's what you get as far as PCB layout direction so here's some of the reasons a recommended layout cannot be implemented as is the first one is the major components are different in size and shape I think every switcher I've ever laid out has a different size inductor and a different size rectifier than what's shown in the reference layout and I think that's typically because the electrical engineer might be going through a cost reduction analysis or he might be just changing parts out so that it can be used parts that are in his company stock that's the most common one and it does change the layout quite a bit when the shape is different you can no longer maybe make return paths with the way that they were circuit functions emitted or added chemical restrictions proximity to other components all these things are going to affect if you can implement a recommended layout as is test requirements would be like ICT test points having to put vias on every signal line and the manufacturer telling you you can't find pitch parts requiring thin or copper weight if a manufacturer says this layout has to be done with 2 ounce copper but you have a fine pitch part that says you have to do this with 3/8 out copper you're going to have to plan these current paths differently and do changes to their layout you just want to make sure it works as well as they intended larger vias many times they'll tell you to put a via here and here and if you're forced because of because of company standards or because of reliability concerns to use a different size via you may not have that same availability for a placement for a via so you might be changing the layout just to get the vias in there and of course different number of PCB layers that's that's a common one so we hope I understanding how the switcher works and where the critical power paths are we'll be able to change the layout so that these things don't affect the sensitive analogs circuitry and again your company's design standards might even bring other other changes in mind here Veon pad thermal thermal reliefs footprint sizes all these things that your company may tell you you have to use you might be looking at a layout and it'll make it literally impossible to implement it so the most critical the most critical paths in a switcher layout always are the AC current loops we need to identify them so we can plan them first and like it says right there these pads take priority over all others so we're laying out the switcher for the AC current loops when we can identify those we can start laying out our switcher so again the buck converters the simple step-down regulator so that's a that's an easy one to start with the DC current loops you know input and output source they're coming from the source and charging the positive terminal of CN and then that currents being returned from the negative terminal of CN back to the source same as the load it's coming from the current is being sourced from the positive terminal of C out and being returned to the negative terminal of C out first thing you do you want to identify which where these filter caps are in your schematic and identify them at as this because these connections here need to be made at the terminals of the capacitor you want to make them with lots of vias and low impedance the AC current loops are going to be the power switch loop and that this is formed when the switch is on so the current is flowing from the positive terminal of CN and through the series pass element through the magnetic element to the positive terminal of C out and being returned from the negative terminal of C out back to the negative terminal of CN when the switch is off we're recovering energy that's being stored in the magnetic element so that current loop is a different current loop slightly it's being sourced from the from the inductor charging the positive terminal C out and being returned from the negative terminal sorry out the negative terminal of C out through their output rectifier and back to the magnetic element there's very little information showing non-isolated there's very little for the PCB designer but it's really quite simple when you when you start marking where the AC current loops are again this is a single output so it's it's quite benign looking but when they get complex which they do because they're tapping multiple voltages off of each of these transformers you still want to identify the loops and they're separated in the isolated form I'm also showing an output an opto coupler to get the feedback back to the controller because again this is for safety reasons that you would have an isolated transformer it's a higher voltage so this the output of the of the series pass element or the switch is called the switch node and it's and it's commonly called SW or SW node it's part of the forward AC current path and it carries the high amplitude voltage swings and all the switching frequencies and that that node in particular needs to be short as possible it needs to be sized so that carries the current that's that's needed for the power supply but you don't want to make it wider to compensate for a longer line and the reason is because that line and its ability to become an antenna and radiate EMI is related to its length so the idea with that node in particular is to make it as short as possible in the return path the node you really want to be aware of is the difference the difference of the two AC loops this applies on the non isolated switching power supplies the difference being because the two loops you see overlay overwrite a/c out and some manufacturers say you don't have to worry about those because they consider those DC voltages because the voltage is on at all times this is kind of front this this is not a safe way to view this because there's other things at play here we don't want to treat those as DC loops they're two independent AC loops but the difference is in particular needs to be a short common point low impedance connection at C n that is very short to the the anode of the output rectifier it's going to be a common point ground that that in switches is going to also apply to P ground and any thermal pads for your controllers and here's just a sample layout of a buck converter lay out all the power all the power components are on the same side of the board their connections are made without vias and then the return pads are made with vias without thermal relief the output rectifier is always placed very close to the to the magnetic element excuse me a return bath to see in as well that's our switch node that's made as small as possible so the AC return-path should match the forward paths as much as possible and the best way to do that is with a full ground plane on layer two right underneath your switching power supply it's pretty much universally recommended that you have a full ground plane underneath your switching power supply unless you're doing a one-layer board and then then you really have to think about this how you're going to get the return loop paths shortened and small the reason for this is that the magnetic fields when closed cancel each other out so this reduces EMI so the switch node in particular need because it carries the the switching and the high current path it needs to be protected and needs to be located in a way that it's not near any other circuitry or any other switchers this particular layout is a is a buck converter but there's something missing the rectifier it is on board this is an ultra ultra efficient synchronous rectifier so you don't see the rectifier but when the connections come out to the PC board they follow the same rules as when it's outside so that's enough that's it on the high current pass whether it was there any questions on that you're happy to move forward then so the duty cycle control is what determines the output voltage and this signal is going to carry the switching frequency it's also considered medium current and should be protected from the AC high power paths as much as possible and because it's medium current and carries the switching the switching frequency it needs to be protected it needs to be away from any sensitive analog circuitry that it could affect you spent you might spend a lot of time working on these gate lines right after you plan your AC current loops and then one form of duty control is pulse width modulation just changes the time that the switch is on and off in variation to the variation of the input voltage the area of each block is the same and this just helps provide a really stable output voltage so to make the duty cycle work correctly we need some kind of feedback I'm sorry this is a duty cycle again I'm sorry this is a gate driver as opposed to an integrated controller and these signals many times have to be routed as a pair and route it internally again this is the close this is to contain the loop make the loop as small as possible to reduce EMI and and also provide common mode noise rejection this is this is what you're going to see when you start building switchers out of discrete components as opposed to getting controllers to do it for you and to get an accurate duty cycle we need some type of feedback from the output either voltage or current so many times it's voltage and many times it's done with a voltage divider just sensing the output voltage and then it's going to feed into an analog error correcting amplifier in this case this is going to be on a controller chip this it's commonly called the FB or feedback node and that node in particular is high impedance which means it's sensitive to noise the other type of feedback might be a current feedback how much current is the is the power supply supplying at any given time dynamically and that's done through a sense resistor and a comparator that senses the voltage drop across the known resistor with that they can calculate how much current is going through that that resistor at any time you can see these aren't things that you're going to see an auto router do the neck classes are of high current yet they turn for a short period of time into the analog signals that have to be treated as a differential pair noise immunities what you're after here and the routing of that is pretty specific it's called a Kelvin connection many time you might need to ground shielded depending on what's around it this will be another example of a Kelvin connection this was the synchronous buck converter that you're going to the multi-phase synchronous buck fir you're going to see supplying your micro processors your core voltages and such and just real quick you can see in the middle there is to see ends you're to switch your two series pass elements then to rectifiers to inductors and then to sense resistors and you can see the V is coming from the middle of them and then C out coming back the analog ground plane in the middle the next slide will be the bottom side view you can see the controller picking up the Kelvin connections from the two sense resistors and then providing the duty cycle back to the series pass element so these analog signals the feedbacks that we're talking about the kelvins and specifically the voltage divider networks they're analog and they they need to be not corrupted by the high current paths and so for that reason many times you have to have an analog ground plane for them to reference so then typically you're gonna have a common point to tie that analog ground plane back to some point in the switcher see out is a common place for it but uh manufacturers will show you many many times how this how the component is laid out internally will dictate a different place for this common point they said here's another spot for it I mean this is a common point dictated by the manufacturer analog circuitry down at the bottom s ground stands for signal ground but that in this case that is an analog ground that's what s grant stands for so when you identify that in the common point from s ground to P ground you're going to know where the currents where the the high current returns are and where to stay away from the idea anything coming in the the analog signals that are coming into the analog area of this controller should optimally cross in at the common point thermals always a big issue with switchers switchers are not 100% efficient so they are losing some power to heat and because we make them so small that heat is often hard to get out this one now obviously isn't too small this is a this is an inverter for a solar panel so there's a lot of heat because it's outside in the Sun already we're trying to extract the heat and we have a heat sink on the backside in reality we would like to shorten the gate and control lines but what we have here is we want to use the low impedance DC voltages for all the heat sinks we want to use the V in the V out and ground what you don't want to use is the switch node and what often happens is switch node is actually the best mechanical way to get heat out of a switcher but that's your radiating EMI antenna that you want to reduce at all at all costs of course another way to get heat out is the airflow they're tightly packed in tall components all the time and your switcher itself is going to have tall components it's going to have a tall inductor and tall filter caps and it may be shadowing your you may be shadowing your series pass element your your switch itself is where you're trying to get the heat out of if you're using just air flow you really need to know the air direction you really might be rotating the switcher just for heat extraction another form assuming laptops is thermal conduction we make contact to the components to extract the heat in this case we have a backside conductive cooling element that contact and on top as well but many times this is mechanically done it might be done from a previous product it might be done because the mechanic gets to do it first but this is an example of pre placed components so in layout we don't like pre placed components because it gives us very little leeway on how we're going to lay it out so when you're forced to do it this way and you're forced to do the switcher to where it works well you may end up with with a placement that's far denser in certain areas than others so let's go to some common do's and dont's some layout mistakes also we need to be creative and coming up with some loot some solutions so that we don't implement mistakes just because we're being forced one direction with our layout don't a lot of times we're given stack ups that we're forced to use this is a flight Aerospace hgi stack up we're not going to change that and get this layout out this year so we have to make this work for us but as you can see the return path for our AC current loops are in layer 5 we have some high speed signals on 3 & 4 and if we didn't plan this we could be routing those signals right through our AC current loops in our switcher just being aware of this is going to force you to make sure this does not happen if you let this go this is an easy mistake to make route right through it right so what I suggest is possibly using multiple layers and having them well stitch together you can either bring the return path closer to the forward return or vice-versa the advantage of bringing the forward current back down to the return current is that you're widening the copper you do have to well stitch it together but it increases your current capability and it your ambient temperature rise is lowered don't place the voltage sense components where they're sensing this is a common mistake you'd be surprised here we have the high impedance feedback trace wrapping around right past the switch node it's going to be very difficult to get an accurate reading of what's really happening on the output of the switcher it's going to be noise induced on that we basically are making a big antenna for pickup so we want to do is place them as close as possible to that feedback node you'll see that in switches a lot the term AC ap as close as possible and then you bring the DC voltage as a traceback that one's benign and and immune to noise here's our current sense our Kelvin connections many times I showed you a layout where the Kelvin connections must be made with vias if at all possibly we've tried not to make them with vias the Kelvin connections made the same way but if vias are needed well they're sensing power connections right so those Nets by definition are already plain Nets probably in your layout and so they very easily could be shorted right to the plane and not allowing you to get an accurate reading across what's happening across our sense so we use our CAD tool to make sure that those vias do not get shorted to the plane where we don't want them to you know we there's several ways to do that I like drawing small little circle voids but I like to document this so in case there's a further revision of this layout it's some future point in time these aren't just little pieces of drawers that get floated around and affect other circuitry going to be aware of where our switching noise isn't a switcher so that it doesn't affect other circuitry we don't want it near anything sensitive and we don't want to near other circuits other switchers this case right here is a couple couple don'ts we have the to transform the two inductors next to each other and their coupling and causing this is a transformer now we have noise induced from one into the other so that's a couple don'ts right there and what you do what I do suggest doing is the first thing on a layout I like to do is have working placements of all the switchers when you do that you know where the switch nodes are you know where to face them and you know how to keep them away from anything else that's that's going to be sensitive other people may start with other circuits but I always start with the switchers first this one might be the worst mistake of all placing C out at the load a lot of the afib if you're using multiple series pass elements C out size often decreases in size so it becomes a small ceramic cap and if you don't have it identified early as C out it could easily get mistaken for a missing bypass cap at a device somewhere else on the board and what you've done here is you've you've taken away the ability to filter out the voltage ripple at the output and what happens here the simple little mistake but you will have the voltage ripple across all your your plane in between here and you will see it on all your signals on that voltage rail all your digital outputs will see that switching frequency on it so what do you do with C out you put it right next to the magnetic element that's forming the LC filter switchers do give off heat I said they are 100% efficient that power loss is output as heat so at that phase when you're first laying out your switcher and you're getting a placement you can work with plan how you're going to get the thermal out of it thermal vias in the exposed pad of the controller flooding with all your DC voltages planning airflow direction all these things thermal vias have heard I've heard them defined as something being a 14 mil or larger via of course you can use smaller vias sometimes you have to especially when they're in pad but they seem to work the best for thermal extraction of 14 mil via so obviously knowing how these current paths are and where the analog circuitry is going to allow us to lay out a switcher in the best possible way when we when we have to change how that layout is some of these issues that I've said they aren't as critical and so this gets confusing because some people don't care on certain switchers well these issues increase when you when the current goes up and when the switching frequency goes up and I guess that's what I'm saying there that each application is unique I have a power supply review course it's not really a apropo but it does explain some of the eight why we call AC voltage or AC current return I said forward AC current which sounds uh which sounds wrong because everyone thinks AC current only goes one or it goes up two different polarities alternating current alternating current can also have a square waveform but by definition it supplies a SiC cyclically varying voltage over time and what we know is that it's not a DC current because DC current is a uniform direction of flow and amount or voltage of electricity so one thing we know it's not a DC voltage and a DS and the DC voltage that switched on and off rapidly as in a switcher is a sickly varying voltage over time it's either positive or negative relative to where it was just a second ago and regulation is necessary because the input voltages are not perfect and today's processors working at sub 1 volt require really stable output regulation switches can do that if they're if they're laid out correctly that's why linear regulators are so inefficient is because all the loss goes to heat they need Headroom 60% efficient is is common for a linear regulator that means all that heat is is that all that power is lost to heat common application would be a 12 volt regulator outputting a 5 volt output if it's got 1 amp that it's outputting that's 7 volt drop across it and it's it's 7 watts of heat that you must extract from that that's a pretty common application but 7 watts is is killer in the wrong environment and this is why switching power supplies are more efficient they're turning it on and off and your output voltage is really just a voltage average the rise and fall the switching is is well for pulse width modulation the rise and fall times are going to change but typical frequencies are going to be it depends what you're doing I mean we can see you know a pulse width modulation happens at a very slow speed sometimes but kilohertz hundreds of kilohertz is for a like a high powered audio transformer but you could also get into the megahertz it's not tens or hundreds of megahertz but it is a hard fast switching signal that you do have to worry about the harmonics of the of the edges so it's less of the rise and fall time but really about the hard edges yeah that it's a the switching power supply doesn't really handle that though I mean it handles a pulse width modulation for the input voltage but there has to be an EMI filter rectification all that happens before the switches so the switcher really isn't there to take care of that now they're in the main power supply of a computer they had something called power factor correcting and that helps because it's boosting up the voltage to pre - it's like a preamp for that for the for the main power supply yeah but but switchers are typically just plugged into the wall you know they have they have filtering and prepping before they get the voltage itself I want to thank you all if there's any other questions or anything else that could help with I'm more than happy I'm going to be at the exhibition tomorrow optimum design has a tent over there please feel free to stop by and talk and and I'd love to talk about your layouts please do that alright have fun with your layouts and contact me any time thank you all thank you very much

Channel: Optimum Design Associates

Views: 58,385

Keywords: switching power supply, pcb layout, optimum design

Id: gq-0ZpcGm8E

Length: 49min 3sec (2943 seconds)

Published: Sun Oct 27 2013