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Old 1st October 2012, 10:21 AM   #1351
DF96 is offline DF96  England
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Quote:
Originally Posted by gootee
Attached are the data points and a plot of the square wave distorion vs VA Rating.
You should add the transformer effective secondary resistance to your table, as this is the crucial parameter. VA rating by itself only tells you whether the transformer will get too hot. Two transformers with the same VA could have different resistances.
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Old 1st October 2012, 10:27 AM   #1352
AJT is offline AJT  Philippines
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secondary resistance reffered to primary = Rsec + [Rpri(Es/Ep)^2]
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Old 1st October 2012, 11:37 AM   #1353
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Quote:
Originally Posted by Tony View Post
secondary resistance reffered to primary = Rsec + [Rpri(Es/Ep)^2]
oops. thats primary resistance referred to the secondary.

as long as the transformer magnetising current is well behaved (basically sinusoidal) and fairly small, then reflecting the primary series impedance to the secondary will be a good approximation.

If, however, the transformer pushes close to (or into) saturation, then the mag current will be extremely "peaky" and the voltage drop across the primary series impedance will be a lot higher than expected.

I believe the technical term for this type of transformer, in audio applications, is "lousy".

and note the use of the term "series impedance" - leakage inductance is usually the dominant parameter in transformer regulation.

look at the transformer parameters Tom has been using. The normalised impedances are:
Rp = 2.8%, Lp_leak = 9.4%
Rs = 5.2%, Ls_leak = 9.5%

for both the primary and secondary circuits, leakage inductance dominates. at rated current the voltage drop across each impedance is its Per-Unit impedance times the rated voltage. the resistive and reactive volt-drops add vectorially.

so the primary has sqrt(2.8^2 + 9.4^2) = 9.8% total volt-drop - the primary resistance is completely swamped by the primary leakage inductance. yeah Rp dissipates watts, but Lp_leak does the damage to voltage regulation

and the secondary has sqrt(5.2^2 + 9.5^2) = 10.8% total volt-drop - the secondary resistance is again swamped by the secondary leakage inductance.

it gets even worse, because the rectifier-capacitor filter draws non-sinusoidal current with a high crest factor - IOW lots of odd harmonics. resistance goes up with the square root of frequency due to skin effect, but inductive impedance is directly proportional to frequency - so yet again the leakage inductance swamps the resistance.

thats quite a few parameters. but using Tonys suggestion and making the above assumption of a non-saturating transformer, we should specify the transformers secondary-referred leakage inductance and resistance in addition to turns ratio & VA.

And Andrew is quite right - I would expect the transformer resistances to vary widely between different devices. In addition I would expect even larger variations in leakage inductance - which depends entirely on the actual transformer construction.

Years ago I learned the hard way to very carefully specify wound components. we had always designed our own SMPS magnetics, but the Ac line choke designs were farmed out to a magnetics manufacturer - we specified dimensions, inductance, operating current and that was about it. All fine and dandy, until our manufacturer burned to the ground. The new place met our "specs" but as we'd left off all the important information (core sizes, no. of turns, peak flux density that sort of thing) the resultant parts were useless.

Last edited by Terry Given; 1st October 2012 at 11:46 AM.
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Old 1st October 2012, 12:07 PM   #1354
DF96 is offline DF96  England
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With significant inductance it could be a bit more complicated than just a voltage drop. The inductance will try to keep current flowing so the charging pulse could be lengthened on the trailing side, although slowed down on the leading edge.
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Old 1st October 2012, 12:27 PM   #1355
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DF96,
yeah it does. and it spreads out the conduction angle. this drops the crest factor quite a bit - instead of a CF around 5..7, it'll be more like 3..4. Power Factor gets better too. Schaeffers analysis shows this in detail, complete with pretty pictures (hint hint)

when designing AC line chokes for off-line rectifier-capacitor filters, there is a critical inductance below which there isnt much useful effect. its about 3.5% - which is a bit below the typical leakage inductance range. its helpful enough that it occurred to me at the beginning of this thread that the transformers should be designed with L_leak = 5% (which is a lot better than Toms transformer) but no smaller.
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Old 1st October 2012, 09:03 PM   #1356
gootee is offline gootee  United States
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Terry,

I did get a whole new transformer model, baed on AndrewT's measurements of a 250 VA secondary, which I am now using instead of the original model. I will try to remember to post the new model parameters after I get home. Or you could look back at what AndrewT posted.

The new model seems to have much better regulation.

Tom

Last edited by gootee; 1st October 2012 at 09:06 PM.
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Old 1st October 2012, 09:15 PM   #1357
gootee is offline gootee  United States
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Quote:
Originally Posted by danielwritesbac View Post
Thanks Tom! There's some really disturbing news when it comes time to pay for the right size transformer. The chart is totally plausible though. I've seen a few real life situations where there's just not enough bass until buying the much higher current transformer. So, it would have been less expensive to get the transformer big enough on the first try.
Daniel,

Don't worry too much, yet. Changing the capacitance totaly changes the picture. But changing the transformer output voltge changes it even more.

I purposely chose a power level and capacitance combination that made sweeping the VA rating cause significant changes, with the given 28 VRMS transformer output voltage.

With a little more excess transformer output voltage, and any large-enough capacitance, the circuit becomes much less sensitive to everything else, especially the VA rating.

Having a high-enough PSU transformer VRMS Out is extremely important, for use with a power amplifier.

Tom
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Old 3rd October 2012, 07:10 PM   #1358
gootee is offline gootee  United States
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Default Minimum Reservoir Capacitance Calculator (xls)

Attached is a beta-test version of an Excel spreadsheet that calculates the minimum required reservoir capacitance, given the line frequency, desired maximum RMS output power, transformer RMS output voltage, nominal load resistance, and the minimum voltage across the amplifier between the power supply and the load.

It uses the calculations shown in post 1339, which were first outlined in post 1246.

Those posts are at Power Supply Resevoir Size and Power Supply Resevoir Size .

I did not insert any error-checking. If an impossible combination of transformer output voltage, nominal load resistance, and desired maximum output power is entered, then the calculated capacitance values will be negative.

Total rectifier voltage drop is currently "hard coded" as 1.4 Volts. But it could easily be changed.

Transformer VA rating is not yet taken into account.

Cheers,

Tom
Attached Files
File Type: zip Linear_PSU_Calc_for_Audio_Amp.zip (5.9 KB, 16 views)

Last edited by gootee; 3rd October 2012 at 07:25 PM.
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Old 4th October 2012, 12:05 AM   #1359
gootee is offline gootee  United States
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In my version of the spreadsheet, I changed the rectifier voltage drop from 1.4 to 1.7 Volts, since that's closer to what it is, in the simulations.

I did a series of runs (see attached plot), with Pout=50W and transformer Vrms=28V, automatically stepping the capacitance (2500-4500 uF by 500uF) and the VA rating of the transformer (100-500 VA by 100 VA, and plotted the "minimum voltage difference between the power rail and the high side of the load" (over 0.2 second) vs the reservoir capacitance and the VA rating. The minimum difference voltage (i.e. the voltage range where the amplifier resides) must stay above about 3 Volts, to avoid distorting the output with the power rail voltage ripple.

The plot verifies that the spreadsheet very closely predicted the minimum acceptable reservoir capacitance found by simulation, for the 50W / 8 Ohms / 28 Vrms case. In this case, the calculation appears to have been equivalent to using a 400 VA transformer.

The spreadsheet, if accurate-enough, could be used to create a table and/or plots that could show, for example, the regions of the [Pout, Vrms] plane where the capacitance values would be "reasonable".

I am also hoping to find out how to estimate the effect of the VA rating on the required capacitance, using sets of simulation runs like the ones that were used to create the attached plot.

Edit: I added a plot with some of the raw time-domain data from the LT-Spice runs, covering about 5% of the total time. For each run, the data for each entire run was automatically examined by the .MEASURE scripts, and the one minimum Vrail-Vload point was found, for each run, and the voltage difference was calculated and saved, then used for the first plot.

Cheers,

Tom
Attached Images
File Type: jpg MINDIFF_vs_C_50W_100VA-500VA.jpg (96.7 KB, 127 views)
File Type: jpg 50W_100VA-500VA.jpg (142.1 KB, 114 views)

Last edited by gootee; 4th October 2012 at 12:23 AM.
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Old 4th October 2012, 12:10 AM   #1360
fas42 is offline fas42  Australia
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Nice going, Tom ...

Frank
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