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Old 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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Old 10th November 2012, 05:15 PM   #1602
DF96 is offline DF96  England
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Is that a common-mode or differential-mode choke? Either would be inappropriate in that position. CM-mode would fail because you have grounds either side of it, thus removing any small benefit from it. DM-mode 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 CM-mode chokes have suddenly become popular and so get sprinkled around circuits like magic dust.
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Old 12th November 2012, 03:07 AM   #1603
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Quote:
Originally Posted by DF96 View Post
Is that a common-mode or differential-mode choke? Either would be inappropriate in that position. CM-mode would fail because you have grounds either side of it, thus removing any small benefit from it. DM-mode 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 CM-mode chokes have suddenly become popular and so get sprinkled around circuits like magic dust.
Thanks for making it easy for me, by asking a question and answering it in the same thread. Now that's team work
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Old 12th November 2012, 03:08 AM   #1604
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Originally Posted by AndrewT View Post
Some good advice and good observations mixed in with complete coddswallop.
1. mandatory?
3. other rectifiers?
4. slow down the main reservoirs transient response?
. We do not want this part of the PSU responding to transients?
6. Use decoupling caps to ensure that the transient slows down?
8. Use a filter between main reservoir and decoupling?

Does anyone agree with "every" suggestion of OnAudio?
Did the scope and your ears tell you any different ?
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Old 12th November 2012, 08:42 AM   #1605
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Originally Posted by AndrewT View Post
but, such a device does not by itself solve the peak current demand problem.
The emf driving the current and the capacitor demanding the current in response to the emf will try to charge the capacitor in one quarter cycle. The SS relay timed to close at zero crossing will still suffer an enormous current pulse.

One must consider the components in the charging circuit and decide what the one shot peak current can be to allow all components to have a long lifetime. That peak current may be limited by some slow charging circuit.

If one decides that a slow charging circuit is required, it can be as simple as an added resistance in the charging circuit.
My reply was only about this

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
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Old 12th November 2012, 10:06 AM   #1606
DF96 is offline DF96  England
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Quote:
Originally Posted by OnAudio
Thanks for making it easy for me, by asking a question and answering it in the same thread. Now that's team work
I didn't answer my own question. I just pointed out why either answer would be wrong. Of course it could be that the apparent coupled chokes in your PSU circuit are not coupled at all but merely drawn badly. Separate chokes in the two lines would work, but one of them would be unnecessary.
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Old 12th November 2012, 04:45 PM   #1607
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Originally Posted by DF96 View Post
I didn't answer my own question. I just pointed out why either answer would be wrong. Of course it could be that the apparent coupled chokes in your PSU circuit are not coupled at all but merely drawn badly. Separate chokes in the two lines would work, but one of them would be unnecessary.
Despite the abuse of the choke symbol, they are all necessary. The conceptual model is correct in relation to the theory behind it as well as supporting field studies. The first set of chokes make up the line filter. In most PSU the chokes between the caps are replaced by a resistor or a pi filter, the purpose is the same.
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Old 12th November 2012, 05:22 PM   #1608
DF96 is offline DF96  England
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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.
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Old 28th November 2012, 03:19 AM   #1609
gootee is offline gootee  United States
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Quote:
Originally Posted by costis_n View Post
My reply was only about this

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
Of course it's limited. Otherwise the peak would be to infinity, and infinitesimally-short in duration. (Well, if it were truly unlimited it would also last for all time. I was assuming that the capacitor's capacitance was not infinite.)

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 best-case (or at least more-predictable) peak inrush behavior. I forget the details, at the moment. But that still only means you could probably avoid the occasional worst-case 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 time-duration is short-enough, 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 peak-inrush-limiting circuit that I designed, once. I used a big MOSFET to detour the inrush current through a low-value resistor on its way to the capacitors, until the rail voltage got high-enough. 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 inrush-limiting 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 more-important consideration would be the steady-state 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 more-advanced circuit topologies for their power supplies.

Cheers,

Tom

Last edited by gootee; 28th November 2012 at 03:35 AM.
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Old 28th November 2012, 05:00 AM   #1610
gootee is offline gootee  United States
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Who wants to be a beta-tester, 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 second-order differential equation for a transformer-rectifier-capacitor circuit, including leakage inductance and resistance of the transformer windings, and uses a non-linear model of diode resistance vs current. It uses a fourth-order Runge-Kutta iteration algorithm.

It automatically switches back and forth between the full second-order equation and a first-order 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 LT-Spice 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
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Last edited by gootee; 28th November 2012 at 05:06 AM.
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