原创 [视频]开关电源及其PCB Layout,不看后悔,超过30年经验的老外大牛的分享

如今随着智能手机、平板电脑、可穿戴设备的发展，电子系统中有越来越多的电源轨和电源解决方案设计，且电源负载电流范围从几个毫安（待机状态）到100安以上（用于ASIC电路）。工程师必须选择针对目标应用的合适解决方案并满足规定的性能要求:高效率，紧凑的PCB空间、准确的输出调节、快速瞬态响应、低BOM成本等。对于系统设计师来说，由于当中的许多人并没有很强的电源技术背景和经验，电源管理设计正成为一项一日频繁和棘手的工作。

该视频为 PCB West 2013 研讨会上，来至 Optimum Design Associates 的工程师 Scott Nance的精彩视频演讲。

字幕君临时抽调到打码组去了，木有人码字，演讲内容拷贝在下边，自己对着看了， \(•◡•)/

Welcome. Welcome everyone to PCB West 2013, Session 9, Switching Power Supplies. I’m your speaker. 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 an electrical engineer nor a power supply designer. So this presentation is from my perspective, and that is that of a printed circuit designer. The reason for this presentation, I think, is simple. Switching power supplies and their layouts are everywhere. We see them in simple designs, tough designs, cheap consumer products, in your high-end phones. We see them throughout our computer supplying the power, and all throughout the computer at point-of-loads.

That’s the reason for the presentation. The reason that I’m here is that it was suggested by my boss, that we each take an article to write and mine was switching power supplies. I invite you to look at some other articles that were written by other designers from Optimum Design. You can find them at designinthetrenches.com, and they are covering topics like DDR timing, rational silk screen, Valor NPI, ODB++, and several others. You can see abstracts of these articles at designinthetrenches.com.

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 not as much good information for the PCB layout professional. I think this presentation might help clear up some of the confusions that PCB designers have when it comes time for us switching power supply. We need to be able to identify it, and be able to lay it out so that each layout acts and it works as the manufacturer intended. 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. I’m going to provide some specific layout techniques and examples, some dos and don’ts. All this is just meant to provide the layout professional with enough information that he becomes empowered to become a better member of his design team.

Let’s begin. The agenda, first of all, switching power supplies; what are they, what do they look like, how do we identify them, how do they work? And then, we’ll get into the PCB layout and that will be probably more fun. We’ll get to that as soon as we can. And then, if we have time we have some power supply basics review. At any time, if anyone has any questions or anything’s 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 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 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 in the switching power supply circuitry.

A little bit of the history. 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 704 mainframe and, of course, it was giant and not as efficient as the switchers that we see today. NASA used them. Telstar satellite is a good example. And then, the famous one is the Apple II 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.

There’s lots of people that want to take credit for the popularity of the switching power supply. Apple comes to mind. Rod Holt was the engineer that introduced it in the Apple II. He got a lot of credit, which is due, 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 popularity of 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.

The other thing was, a power switch was needed to be able to quickly switch high currents, and the vertical metal oxide semiconductor power transistor enabled this. 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 worked very well for high power applications, but they didn’t switch – in the older days – nowhere near switch fast enough. What happened was the switching frequency wasn’t above the audible hearing range of humans, so we heard things like squeals in TVs and things like that. 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 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 this point forward.

So when you think of a switching power supply– 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 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, and any other place that’s stepping down the voltages from the main power supply. And we call those regulators or point-of-load. That’s a little example of a little linear, ball grid array point-of-load regulator.

So here’s some examples. It’s just showing the pictures of the vast difference, and maybe adding to the confusion of what is a switching power supply. The computer main power supply, a cell phone charger, an adjustable laboratory grade switching power supply, the linear ball grid array, which looks benign but really that one is quite exotic. An off-the-shelf module that you could use for applications that would work for off-the-shelf. That’s car audio 800 watt power amplifier.

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. A fused EMI filter rectification. If you know about power supplies, after rectification it becomes a DC voltage. The switching power supply is not really converting AC into DC. 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, after the rectifier I have a PFC circuit – and that is in some higher end PSUs – and that stands for power factor correction. There’s two types. There’s a passive and an active. 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. Your common DC voltages that you would see, your standby voltages, your plus 12, your plus 5, plus 3.3, sometimes minus 12 and minus 5.

We’re not going to be talking about the PSU any more, just the switching power supply sections. By definition a switching power supply uses a power switch, magnetics, filter caps, and a rectifier to transfer energy, and that’s from an input to an output source providing a regulated voltage. It works by rapidly turning that power switch on and off. That output voltage is calculated by what the input voltage to the switcher is and the duty cycle.

The duty cycle is the proportion of time that that switches on versus off. During the on stage – they call it saturation mode – it’s an efficient stage– it’s negligible voltage drop across it. In the off-state it’s cut off, and it has no current going across it. So the power switch stays in these two states 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’s usually the reason that you’re using a switching power supply. Linear regulators are typically in the 60%, and switching power supplies are regularly in the 90%, and they’re never 100%, but 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 comparing them to the predecessors which are linear regulators, switchers don’t require the large, heavy, low frequency transformers that you would have seen in maybe the Apple I. Before the Apple II, 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. The filtering is done with an LC circuit. It’s going to be with a conductor and a cap as opposed to a large transformer. These aren’t dissipating as much heat and so we see a higher efficiency by doing this. It also allows us to miniaturize and in conjunction with a higher power efficiency, it gives them a huge advantage over the linear regulators.

The disadvantage of 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’re noisy. They can radiate noise and so we have to be aware of that. We have to be aware of where this noise is coming from.

There are two main types of switching power supplies. there’s isolated and non-isolated. What these mean is if there’s a transformer in the middle of the switching power supply. 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 42.5 volts– this is pretty much a world wide 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. That’s why we find modules that have very few external components.

Here is three of the common non-isolateds. These would be the smaller lower voltages. They’re called buck, boost, and buck-boost, and they are identified by your input and output voltage requirements. 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. Sometimes it’s called polarity inverting, and – not as common – it’s called a non-isolated flyback. Sometimes, by mistake, they’re called a flyback but without the transformer they’re not flyback. You would have to call it a non-isolated flyback.

This is the simplest circuit. This is the step down regulator, the buck converter. The first thing we will do is identify all the key power components. The filter capacitors are identified as Cin and Cout. The power switch here is U-1. That’s also the function of a series pass element. L-1 is the magnetic element – in this case an inductor. And then D-1 is the output rectifier, and in this case that’s a Shockley diode, trying to keep the forward voltage drop low.

Then you see there’s three different topologies, but they’re really created by just rearranging the switch, the rectifier, and the inductor. By these arrangements, they’re slightly different but what’s happening is that the energy’s being recovered from a magnetic element differently. We’re getting a boost up in voltage with the boost, and a polarity inverting by just rearranging the three components.

And then big word asynchronous 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 the time half of the losses or even over half the losses are attributed to that rectifier. It’s being replaced by another MOSFET. Sometimes confusing in layout, the two are doing two different things but both of them had their own critical function. The control lines that are controlling the two are often called top gate and bottom gate. They’re called top-fed and bottom-fed. One of them again the series pass element, the other one’s going to be the output rectification. They’re also called upper and lower sometimes. But you will see these inspections. 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 and the output filter caps, and by doing this you’re actually able to reduce the size of the output filter cap. Again, more efficient and in this case it’s multi-phased, you can tell by the control lines. What that does is, it actually reduces noise and increases the efficiency all at once. You are going to see this particular one doing things like supplying the core voltage of a microprocessor.

These are the isolated topologies. These are going to be typically for the higher voltages. There’s six common ones that identified here but they’re inventing ones all the time for different applications. 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 earlier was in the TV high voltage. That’s typically where you see the flyback or some cheaper computer power supplies. The forward would be the higher end computer PSUs. Two switch forwards, 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. You saw they weigh up to 1,000 watts. When they’re going up to 10,000 watts, that’s typically the full bridge that’s being interleaved. 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. I like showing the simple schematics, because what we learn here is just replicated on some of these more complex ones.

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