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10th November 2012, 04:14 PM  #1601 
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A possible solution to getting that PSU you crave for
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10th November 2012, 05:15 PM  #1602 
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Join Date: May 2007

Is that a commonmode or differentialmode choke? Either would be inappropriate in that position. CMmode would fail because you have grounds either side of it, thus removing any small benefit from it. DMmode would fail because you can't assume that the two ripple currents/voltages will be the same.
Two separate chokes would work, but for some strange reason CMmode chokes have suddenly become popular and so get sprinkled around circuits like magic dust. 
12th November 2012, 03:07 AM  #1603  
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12th November 2012, 03:08 AM  #1604  
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12th November 2012, 08:42 AM  #1605  
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the transformer will provide as much current as the capacitors demand. which is, how much? is it limited? i think not. So it will peak. Perhaps then a circuit should delay turning on the psu until the AC crosses zero so at least that way it won't be BAM max current to the caps 

12th November 2012, 10:06 AM  #1606  
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12th November 2012, 04:45 PM  #1607  
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12th November 2012, 05:22 PM  #1608 
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My apologies. I should have made it clear that I was talking about the second choke. The first choke may do some good. Sorry for any confusion.

28th November 2012, 03:19 AM  #1609  
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There are no ideal components or conductors. Transformer windings have R, L, and C. Capacitors have R, L, and C. Wires and traces have R, L, and C. Everything does. Which ones are significant is an important question, of course. In this case, the R and L of the transformer windings, and the R of the capacitor, and the R and L of the wiring to the location's electric service's pole transformer (et al), are all significant. Also, just FYI, switching on at the AC line's zero crossing will not necessarily give the best behavior. It can depend on what the AC's phase angle was when the transformer was last switched off, i.e. in what magnetizaion state the core was left. But I think that if you controlled both switch off and switch on for the AC's phase angle (in relation to zero crossing), then you could achieve some sort of bestcase (or at least morepredictable) peak inrush behavior. I forget the details, at the moment. But that still only means you could probably avoid the occasional worstcase inrush. Of course, the normal inrush might still be too much. But there are relatively easy and cheap ways to slow it down and limit the peak values to almost any desired level. But, as AndrewT was saying, first you would need to determine if it was even necessary. If the timeduration is shortenough, many components can withstand peak values that are tens, hundreds, or even thousands of times their rated maximums. Some manufacturers' datasheets do contain that information. I had to find and use such data for a resistor in a power supply peakinrushlimiting circuit that I designed, once. I used a big MOSFET to detour the inrush current through a lowvalue resistor on its way to the capacitors, until the rail voltage got highenough. A resistor that was rated for the current level needed would have been far too large (and costly). I found that according to the manufacturer's datasheet, for the 2 ms or so that the inrushlimiting circuit would operate, the resistors I was looking at could safely dissipate over 500X their rated power, which they did, and never even got the least bit warm. If you want to worry about peaking, another probably moreimportant consideration would be the steadystate peaking, after the initial inrush is over. It's just the way this type of power supply circuit operates. The capacitors usually charge with short spikes of current. That's why a lot of people worry about "Power Factor" and use moreadvanced circuit topologies for their power supplies. Cheers, Tom Last edited by gootee; 28th November 2012 at 03:35 AM. 

28th November 2012, 05:00 AM  #1610 
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Who wants to be a betatester, for my spreadsheet? It's attached. I did it with Excel 2007 but saved it as a plain .xls file, so it might work with older versions of Excel.
It actually emulates the secondorder differential equation for a transformerrectifiercapacitor circuit, including leakage inductance and resistance of the transformer windings, and uses a nonlinear model of diode resistance vs current. It uses a fourthorder RungeKutta iteration algorithm. It automatically switches back and forth between the full secondorder equation and a firstorder one, depending on whether or not a charging pulse is occurring. So it can accurately plot the output waveforms versus time. It also includes the scalable transformer modeling from measurements. So you can change the transformer size and ratings at will, and can use any transformer you can measure. You get to enter some parameters and it calculates everything else, and plots some of it. It also has provisions for overriding the calculated capacitor ESR value. And you can enter an additional series resistance and an additional series inductance, if you want. I've attached a jpeg image of the main screen. It works really well but there are a few things I'm still not certain are correct, such as the diode modeling for different circuit topologies, and the way the transformer parameters are used. But it closely matches my LTSpice simulations for "normal" cases. It does NOT yet let you specify a ripple amplitude and pick a capacitance for you. I'm just trying to get the basic analysis correct, with this one. But it's still a very useful tool. Also, it's very slow. It evaluates the differential equation 4000 times, to produce a few tens of milliseconds of output. So it takes about 40 seconds to calculate, on my little computer. But it also provides things like min and max output voltage, rms and avg output voltage, ripple amplitude, peak diode and capacitor currents, et al. I think it should be quite useful. Note that you have to enable macros, in Excel, for the Calculate button to work. Let me know what you find wrong with its current capabilities. Cheers, Tom Last edited by gootee; 28th November 2012 at 05:06 AM. 
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