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Old 6th November 2012, 07:57 PM   #1
Preamp is offline Preamp  Germany
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Default Transformer voltage droop

Being interested in how different transformer sizes might affect amplifier performance, I did some simulations in LTspice. I was mainly looking for the difference in secondary wire gauge and the resulting output resistance. Thus such things as regulation or saturation are not considered here, simply the different secondary copper resistances as stated in the datasheets were used. The load is steady and purely resistive. Output power calculation is already taking into account the typical 67% efficiency of a class AB amplifier.
The second chart shows the influence of the filter cap's ESR, like 49mR for a single one or 4mR for 10 smaller ones in parallel.

This is actually pretty basic stuff, nonetheless it's quite interesting to look at...
Attached Images
File Type: gif toroid_droop_ripple.gif (28.6 KB, 206 views)
File Type: gif toroid_esr.gif (22.4 KB, 204 views)
File Type: gif schematic.gif (30.6 KB, 204 views)
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Old 6th November 2012, 08:47 PM   #2
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Hey this is very interesting stuff, and closely related to another thread that has been going on in this forum:
Power Supply Resevoir Size

I recently developed an Excel spreadsheet to model this kind of power supply, connected to a class-AB amplifier that is driving a resistive load. Here is a link to the spreadsheet:
http://audio.claub.net/software/PS_d...ons_VER2.2.xls
There are similarities with your results, but I did not include ESR of the caps.

I would like to expand the spreadsheet to include CRC type capacitor banks, and include the cap ESR, but it would need to be done with equations instead of simulation. Would this be straightforward to formulate into an equation? Could our results be combined?

-Charlie
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Old 7th November 2012, 11:33 AM   #3
AndrewT is offline AndrewT  Scotland
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Are you using Rser to model secondary resistance?

Then a range from 0r06 to 0r85 covers an enormous range of VA in transformers. I'd guess from 2kVA to 100VA.

Hardly what one would select as a possible range of usable transformers for a specific power amplifier.

I think your Rload is too simple. The amplifier actually draws a half sinusoid in each power rail, when delivering significant power to the amplifier load, assuming of course that Iq has become small in comparison. This may throw up some differences in the way the model predicts actual behaviour.

I also suspect the Rser is too simple for what you're modeling.
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Last edited by AndrewT; 7th November 2012 at 11:40 AM.
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Old 7th November 2012, 12:26 PM   #4
Preamp is offline Preamp  Germany
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Charlie, your spreadsheet doesn't work for me since I only have OpenOffice .
ESR doesn't make that much of a difference, unless it's much too high. But since Low-ESR caps are common these days, that should not be a problem.

Andrew,
yes, I used Rser, as can be seen from the schematic.
The VA range is actually 80, 160 and 500 for each transformer with its two secondaries. The data is taken from actual datasheets.

I have a big toroid lying around with 3x24V @ 9A each and I'm considering to use it for something like a P3A or MyRef, which could easily be built with 160VA for 'home use'. Granted, 80VA is too weak, but for smaller amps you might consider to double that up to 160VA...

Of course this is only a simple simulation, leaving out the intrinsic behaviour of the toroid itself, which should make things even worse. I stated that above. But nonetheless does the difference in secondary winding resistance make a difference in output voltage under load, which is shown here.

Quote:
I think your Rload is too simple. The amplifier actually draws a half sinusoid in each power rail, when delivering significant power to the amplifier load, assuming of course that Iq has become small in comparison. This may throw up some differences in the way the model predicts actual behaviour.
I did not quite get what you mean here. A real amplifier will behave different than a resistor, that's for sure, but a constant resistive load should be something like the worst case. The amplifier load on the psu will vary depending on the music signal, even into a purely resistive load (think of white noise vs. 50Hz sinus). In the simulation the load is constant.
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Old 7th November 2012, 01:15 PM   #5
AndrewT is offline AndrewT  Scotland
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Too simplified in my opinion.

Losses are proportional to I^2R and a constant current Rload underestimates this significantly.
0r06 is in the sim
0r08 is still too low for a 500VA
I measured 0r09 for a 1kVA transformer recently. I have never measured as low as 0r06 in any of my secondaries.
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Last edited by AndrewT; 7th November 2012 at 01:24 PM.
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Old 7th November 2012, 03:33 PM   #6
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Quote:
Originally Posted by Preamp View Post
Charlie, your spreadsheet doesn't work for me since I only have OpenOffice
Uh, did you even try it? I can open it just fine with OO 3.3 Calc...

The only thing I saw that was not right was the labels in the plots.

-Charlie
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Old 7th November 2012, 04:32 PM   #7
Preamp is offline Preamp  Germany
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Quote:
Originally Posted by AndrewT View Post
Losses are proportional to I^2R and a constant current Rload underestimates this significantly.
What kind of losses? And why should the constant current underestimate this? What kind of load should I use instead?

Quote:
Originally Posted by AndrewT View Post
0r06 is in the sim
0r08 is still too low for a 500VA
I measured 0r09 for a 1kVA transformer recently. I have never measured as low as 0r06 in any of my secondaries.
I attached the datasheet. I do not own such a trafo so I can't prove if that's true.

Quote:
Originally Posted by CharlieLaub View Post
Uh, did you even try it? I can open it just fine with OO 3.3 Calc...
Yes, I did. With Calc 3.4. I cannot see any output, changing the input fields doesn't do anything.
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File Type: pdf C500_500VAEconomySeries.pdf (141.9 KB, 14 views)
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Old 7th November 2012, 04:59 PM   #8
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Quote:
Originally Posted by Preamp View Post
Yes, I did. With Calc 3.4. I cannot see any output, changing the input fields doesn't do anything.
You had me worried for a moment, so I just updated to 3.4.1 and re-opened the same xls file and... it works just like in version 3.3... So, I am not sure what problem you are experiencing.

I can change all of the input fields and see all the output plots change as a result...

When did you last update your OO version?

-Charlie
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Old 8th November 2012, 04:29 AM   #9
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Rather a simple model for the transformer, use a transformer equivalent circuit and plug in the appropriate values. Impedance from leakage inductance is usually of the same order as the DC resistance. Measure a real transformer, apply a DC current to the secondary and measure the voltage developed across it. Use a multimeter on the primary, unless it is a large transformer otherwise use the technique mentioned. Power up the transformer, measure the input and output voltage, apply a resistive load then measure the load current and voltage. The voltage sag / load current will give the transformer output impedance.
The leakage inductance can be inferred by subtracting the output referred resistance from the output impedance. This is a vector subtraction.

To calculate the output referred resistance, divide the primary DC resistance by the square of the turns ratio and add this to the secondary DC resistance.

As for the load current, it will rarely be worse than a half sinusoid of vcc -5V across either a 6 or 3 ohm load depending on the use of 8 or 4 ohm speakers. I am too tired to derive the maths for this, any power supply design reference will cover half wave supplies, a half wave load uses the same equations.
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Old 8th November 2012, 05:07 AM   #10
AJT is offline AJT  Philippines
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nothing beats actual live testing...the built up amp with the psu, a dummy load and signal source and testing for voltage droop at full power....

in my 250wpc super leach, unloaded rails are +/-85volts...both channels putting out 45vac, the dc rails drop to about +-75volts....

the EI power traffo used a 2inch center leg stacked to 3.5inches for a core rating of roughly 1500voltamperes....
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