Popular Post JohnSwenson Posted January 18, 2019 Popular Post Share Posted January 18, 2019 Here is some (maybe unwelcome) details on switching supplies etc. First off there are two very different types of "switching things": The ones that plug into a wall (ie the input is AC mains, 120/230V AC, 50/60Hz), and ones you find on a board which take DC in (say 12V) and output DC (usually lower voltage). These behave quite differently and I think it is important to not ascribe the same properties to both. The AC line input ones I'm going to call "SMPS", the other I'm going to call "switching DC/DC converter", you can also call this a switching regulator. An SMPS takes the raw AC line and switches it, breaks it up into tiny time slices, since this is slicing the AC line, it the amplitude of these slices is still going up and down at line frequency. The frequency of the slices is somewhere between 30KHz and 100KHz. A lot of old ones worked at 30KHz but modern ones seem to like 60KHz. On the other side of the transformer there is a diode bridge, and caps and some circuit that measures the voltage.The output of the voltage sensor is fed back to the circuit that slices the line voltage which modulates the slices so that on the other side you get pretty flat DC. So you don't need the giant capacitor banks to smooth out the line AC, as the line voltage goes up, less power is sent to the transformer, and as the line goes lower more power goes to the transformer. So why even bother doing all this? Primarily is the reduction in size, weight and cost of the transformer, and also the vast reduction in capacitors after the diode bridge. Line transformers have to be large heavy and expensive to work at 50/60 Hz. The transformers that work at the high switching frequencies are much smaller, cheaper and lighter weight. The switching DC/DC converter takes DC in, slices it up and feeds an inductor with the pulses (note: NOT a transformer). These frequently run in the 400KHz to 1MHz range. Again a sensor looks at the output voltage and adjusts the slices to get the output voltage required. Again why do this? Primarily because switching DC/DC converters conserve POWER and linear regulators conserve CURRENT. So waht does this mean? Lets look at a linear regulator, lets say the load is pulling 1 amp, this means the input current is also 1A. If you have a large voltage across the regulator you have to dissipate a lot of power. If your input is 12V and your output is 1V at 1A, the input is 12V at 1A, so 12W in and 1W out, that means the regulator has to burn up 11W!! The switching regulator is constant POWER, the input power is the same as the output power, thus for above the input is only 1W instead of 12W, the current is 1/12 W. Non of these circuits are perfect so there is SOME power lost in the switching regulator, usually not very much. The "noise" that comes out of an LPS and an SMPS are very different. An LPS with a decent regulator has broadband noise usually measured in micro-volts (uV), the best ones are around 1uV, the worst can get up to 60uV. If you look at this on a scope you usually see a flat line, you have to increase the gain dramatically to see this noise. An SMPS (and DC/DC converter) have ripple at the switching frequency, usually measured in milli-volts (mV), this is much easier to see on a scope, but is very hard to see using normal settings. Say you have a scope set to measure 12V, you will see a flat line, 20mV won't even show up on that line. You have to go to AC coupling and turn the gain up. Not as much as with a LPS, but still a lot more than a normal setting. In most circumstances either of these noise levels is not going to make any difference, since almost everything you are powering with an external box, will have their own regulators, THOSE are what really matter. Now there are two other aspects of a power supply that DO seem to have an affect on what is powered, they are a lot harder to understand than noise, and nobody ever puts measurements for these in a spec sheet so there is no way to compare. These two things are output impedance and leakage current. The output impedance is what the voltage does when a change in load current happens. This is particularly important for quick changes in load current. Slow gradual changes can easily be handled by the PS, but quick changes cause the output voltage to change, frequently it will then slowly recover to the original value. This is most important for devices like computers which have large, quick changes in current draw. In many cases the regulators in the device can't handle this voltage change either and it winds up getting to the circuitry. There is no generalization about this with LPS VS SMPS. Both types have some that have very low output impedance (this is good) and some have high output impedance. Since nobody measures this or puts it in spec sheets there is no way to know. The only correlation seems to be that the specialty, expensive LPS tend to have very low output impedance. I have done a bunch of measuring of PS and have found some SMPS that beat a lot of the less expensive LPS. But you can't tell which they are, they don't look any different, they don't cost more, they just are better. BTW some of the SMPS that are touted as being low noise have horrible output impedance. The other property is leakage current. I'm not going to go into details on this, I have written tons of posts on this. For this there is a BIG difference between SMPS and LPS. The both have leakage, but the type that comes from an SMPS seems to be much more damaging to digital audio than what you get out of an LPS. The SMPS DO vary a fair amount, again hard to tell which are the lower ones. The largest amount of leakage I have ever measured is from an SMPS that talks about how low its noise is. A switching DC/DC converter does not generate leakage, it does not STOP leakage, but it does not generate its own. If a switching DC/DC converter is fed from an SMPS, whatever that SMPS produces is what will be on the output of the converter. One thing about SMPS that is talked about a lot is noise from the SMPS being "backwashed" into the AC mains. This used to happen many years ago, but modern SMPS don't seem to have this issue at all. Because this is something people can understand I think this gets the credit when I think in most cases the real culprit is the leakage current. So what does all this mean for a computer? The primary effect from powering a computer from an LPS seems to be the reduction in leakage current, NOT ripple on the output of the supply. There are fairly inexpensive ways to deal with leakage current, so in most cases using LPS ATX supply is mostly wasting money. If you have a computer directly driving a USB DAC there are some waysto prevent most of the leakage from getting through. But in my opinion a better way is to use a renderer powered from an LPS. In this case all you have to do is prevent the leakage coming over the Ethernet connection from getting onto the USB connection. Fortunately this is very easy to achieve and quite inexpensive. So the upshot is that you can achieve most of what advantage there is in using an ATX LPS in other ways for a LOT less money. John S. the_doc735, richard_crl032, jabbr and 4 others 5 2 Link to comment
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