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Old 14th April 2019, 08:29 PM   #21
woofertester is offline woofertester  United States
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Here is a follow up question concerning % of SOA used as a bias point for PL amps.

I own a gorgeous Aleph30. Per the schematic, the source resistors are 0.47 ohm with 0.25V across the resistor giving 0.531A per transistor.

At 24.75v across each transistor, the power dissipation per transistor is roughly 13.16W.

The Harris IRF244 datasheet claims 125W dissipation.

So, the % of maximum dissipation used in circuit is about 10% for each transistor.

Is that the intent to use around 10% of the nominal datasheet dissipation for each output device? My guess is that 10% keeps the device far enough away from the problematic SOA area.
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Old 15th April 2019, 12:01 PM   #22
indra1 is offline indra1  Indonesia
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Quote:
Originally Posted by Zen Mod View Post
though, someone found some worthy of implementation
Ixys has a line of big die devices categorized "extended FBSOA" designed for linear operation such as the IXTK22N100L. Other big dies outside the line exhibit some "electro thermal instability" within the SOA on linear mode. Similar problem could be the reason why Bob Cordell avoids trench mosfets for amplifier use while Hugh Dean and Paul Bysouth recommend gate to drain RC shunts to prevent oscillation on high gm Fairchild mosfets.
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Old 15th April 2019, 04:32 PM   #23
woofertester is offline woofertester  United States
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Quote:
Originally Posted by indra1 View Post
Ixys has a line of big die devices categorized "extended FBSOA" designed for linear operation such as the IXTK22N100L. Other big dies outside the line exhibit some "electro thermal instability" within the SOA on linear mode. Similar problem could be the reason why Bob Cordell avoids trench mosfets for amplifier use while Hugh Dean and Paul Bysouth recommend gate to drain RC shunts to prevent oscillation on high gm Fairchild mosfets.
Thanks for this info. It looks like this series has part numbers ending in L2.
Here is one from that series. The 25 deg C dissipation is 800-ish watts and the 75 deg C dissipation is 400-ish watts. What I am thinking at this point is to acquire some parts and lash up a standard Aleph at 1A as Nelson suggested and run it hard for a long time. Then slowly increase the current and see how the amp performs at higher current.

Is there a torture test for the Aleph that would tend to bring on a failure sooner than other tests?
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File Type: jpg IXTN200N10L2_SOA_Curves.jpg (120.8 KB, 127 views)
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Old 15th April 2019, 05:02 PM   #24
woofertester is offline woofertester  United States
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Default SOA Datasheet Test

For the part just above, IXTN200N10, here is the datasheet spec for SOA test. 500W, 5A at 100V for 5 seconds.

For a "monster current" Aleph, I would aim for 20V, 5A and 50 deg C heatsink temperature. That would be well inside the datasheet 5-second SOA test.

Is there a link to a spreadsheet for a standard Aleph for bias current and power output at various load impedances?
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Old 15th April 2019, 05:02 PM   #25
woofertester is offline woofertester  United States
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SOA spec
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Old 15th April 2019, 05:36 PM   #26
chede is offline chede  Germany
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Default Aleph Power Spreadsheet

Hi woofertester,

here is the Aleph power spreadsheet :

Aleph 3 how is power 4 ohm

Posts #3 and #4


Best regards,
Claas
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Old 15th April 2019, 06:42 PM   #27
woofertester is offline woofertester  United States
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Originally Posted by chede View Post
Hi woofertester,

here is the Aleph power spreadsheet :

Aleph 3 how is power 4 ohm

Posts #3 and #4


Best regards,
Claas
Got it. Thanks. It is interesting to try different rail voltages and bias settings.
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Old 16th April 2019, 06:33 PM   #28
Nelson Pass is offline Nelson Pass  United States
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Quote:
Originally Posted by indra1 View Post
Ixys has a line of big die devices categorized "extended FBSOA" designed for linear operation such as the IXTK22N100L.
We go back to post 19:

In the Ixsys, they have solved it as follows:

"Every transistor cell is designed with a ballast resistor at the source
to limit its current."
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Old 16th April 2019, 07:16 PM   #29
indra1 is offline indra1  Indonesia
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Yes sir, I was posting for the benefit of those who did not read the Ixys article.
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Old 16th April 2019, 08:27 PM   #30
woofertester is offline woofertester  United States
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Quote:
Originally Posted by Nelson Pass View Post
We go back to post 19:

In the Ixsys, they have solved it as follows:

"Every transistor cell is designed with a ballast resistor at the source
to limit its current."

If the IXYS devices are an array of parallel cells with each cell having a source resistor, the device is similar to the array of devices each with a source resistor in the large PL amplifiers. Is this a reasonable analogy to draw?

For the IXTN200N10L2 in particular, the RDSon is 11 milli-ohms indicating that there are so many cells in parallel, that the effective source resistance, while not zero, is vanishingly small.

The IRF244 in my Aleph30 has an RDSon of 0.24 ohms typical. Would it be fair to compare the IXTN200N10L2 to a quantity of the IRF244 in parallel?
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