For any of hte heavy hitters out there, I'd like a recomendation of literature on switch mode power supply design. I've gone over the basics like buck converters, boost converters, preventing overshoot and basic feedback control. But I'm trying to understand the design of a real, working design in front of me, and I'm going to need information that is more advanced than you typically find on the web but not as high level as a thesis on the subject. In my experience, this means I need to turn to printed material. But I'm rather concerned with dropping sixty dollars or more on an outdated, or poorly written book. THe web sites out there list far too many at a time to make any comparisons on as well. So are there any veterans here that know of a good book?
I learned about SMPSes from a friend (Christina Mahoney). I have found some links about power supplies (I had to learn about this from Christina before I understood all the terms, though!). http://dalsemi.com/appnotes10.cfm/ac_pk/20#134 http://repairfaq.org/sam/smpsfaq.htm Just be aware that line-powered switchers (and even some DC powered switchers) can hold dangerous voltages for a long time after they're unplugged! The resulting shock can cause injury either directly, or by triggering an involuntary reaction. My friend Mary was repairing a PC that doesn't boot up (it was an extra small desktop PC that uses a laptop CPU and GPU). I was doing my own work, when all of a sudden I heard a very loud thump. It turns out she got a shock from the high voltage capacitors, causing her to jerk her arm back violently. It was quite serious, too, as she actually broke her arm! Bottom line: Always discharge the high voltage capacitors with a christmas tree bulb before servicing or you may get hurt! P.S. The fault with the PC is an open startup resistor. Thus, the SMPS doesn't start and the capacitors retain almost all of their charge for a very long time.
I'll look at the first link, and I've read the second one already. I actually am looking for somthing more in depth than this link. I now understand the basics. But none of the material I've read explains some of the 'extra' components I'm seeing in the supply I'm trying to fix now. It's problem isn't anything obvious. All the semiconductors seem OK, there are no burned components but I looked for and checked the startup resistors with out finding anything wrong with them. So since the supply is built from a single layer PCB, I decided to start tracing and recording the circuit so I and plug it into my variac and do some live troubleshoting at lower voltages. But even before I get to the first switching transistor, I've seen some odd looking RC networds that I think are to prevent high freq leakage back into the the mains, and there are some odd transformers that I don't have the slightest idea what they are for. (these aren't flybacks, they are toroidal, maybe 1:1 or other close ratio) I'm acutely aware of the capacitors, I went through some basic electronics in my last few years of high school (basic meaning plain vanilla serias/parallel circuts up to very basic transistor biasing and boolean algebra). It was enough to be a basic technician. Currently, I'm on my last semester of my BS in electrial engineering. Too bad I took all microprocessor and DSP electives rather than any in power electronics So I'm looking for somthing to bridge the gap between basic theory and PhD thesis papers.
The various coils, capacitors, MOVs, and other components before the high voltage rectifier are just to limit the inrush current (so the rectifier doesn't burn out) and to cut the power in the event of an internal short circuit. In many high quality PSUs, there is also a boost converter between the rectifier and high voltage capacitors to allow the PSU to work with a wider range of input voltages (you can tell, because there will be a rather large toroid inductor right next to the large capacitors). The boost converter actually converts the rectified 100-250v from the power line to a semi-regulated 380v or so (wait a minute! I thought the purpose of a computer PSU is to reduce voltage, so it seems like a step backwards. However, anyone who does advanced SMPS design will quickly learn that capacitors storing the same amount of energy can be physically smaller (a real consideration for SFF computers!), and design of the flyback converter itself is often easier). The flyback transformer is the large transformer near the middle of the board (in most PSUs, anyways). Along with the transformer, a small capacitor (a small capacitor, an inductor, and two diodes, to be more accurate) creates a resonant tank circuit, from which energy is added selectively by means of a power transistor (usually a MOSFET, though I've seen IGBTs used in some high quality PSUs), and a secondary rectifier (usually Schottky diode, although many PSUs use MOSFETs) can tap off this power so it can be used. Then, the 12v from the flyback converter is stepped down to 5v and 3.3v by buck converters. As you can imagine, controlling the whole unit is not an easy task. Special ASICs (usually a microcontroller with some SMPS control hardware) take care of the control, and in the case of a PSU with multiple switchers, try to lock the timings of the converters to reduce capacitor current (to reduce ESR losses, and this works by trying to get the output period of one switcher and the input period of another switcher (that gets its power from the first switcher) to land on each other, reducing net capacitor current). PSU design is not an easy task (though ICs have simplified it a lot), and large flyback converters tend to be one of the hardest circuits to design.
To be fair, a christmas tree bulb isn't really going to stand up to anything higher than 15V let alone rectified mains voltage Maybe a wirewound resistor would be better suited to the job.
I have actually done this trick (which I learned from Christina) and it works. After all, christmas tree bulbs are just about perfect because the resistance varies with voltage (~200 ohms cold, ~3k hot). A wirewound resistor would work, but it doesn't give any visual feedback whether it discharged the capacitors or not.
Thanks for the info, I had learned aout some of what you mentioned but not all of it. The basic topologies are familar like a buck and boost converter, as well as the switcher, diode and coil. Belive it or not, I'm probally fine on the feedback control system (yea, now I know what the COntrol Systems course I took is good for! ). I guess I'll start trying to figure the circuit out, and if I run into problems, I'll ask here.
