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Class-D with TPA3118 - An Update after all these Years
Class-D with TPA3118 - An Update after all these Years
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Old 28th April 2021, 10:58 PM   #81
bucks bunny is offline bucks bunny  Germany
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For comparison I measured the 2-layer sample
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File Type: png 1_THD1kHz_TPA3118_2021_03_03_24Vdc_5R0.png (44.4 KB, 49 views)
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Old 28th April 2021, 11:00 PM   #82
bucks bunny is offline bucks bunny  Germany
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Surprise, surprise! The 2-layer PCB is slightly better in the region around 0.5V output voltage. So inspected the 4 layer and discovered that I had forgotten to solder the thermal pad of TPA3118. After soldering I got this result.
Now the 4-layer PCB comes close to the to the 2-layer version.
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File Type: png 3_THD1kHz_TPA3118_2021_04_03b_24Vdc_5R0_pad_soldered.png (44.3 KB, 49 views)

Last edited by bucks bunny; 28th April 2021 at 11:14 PM.
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Old 28th April 2021, 11:11 PM   #83
bucks bunny is offline bucks bunny  Germany
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So what are the conclusions?
Moving from 2- to 4-layer PCB did not bring any significant difference concerning THD of TPA3118. Obviously the 2-layer PCB is "good enough" for TPA3118.
As found earlier, THD does not increase with dummy load indicating there is no room of improving the output inductors here.
Inside the interesting output range of 0.15~15V THD is below 0.01% - not bad at all.
At least moving to 4-layers eased pcb-design considerably and helped reduce size to 50x100mm.
I guess em-compliance improved as well but I do not have access to appropriate measurement setup, i.e. an emc-lab.

Last edited by bucks bunny; 28th April 2021 at 11:17 PM.
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Old 29th April 2021, 11:08 AM   #84
bucks bunny is offline bucks bunny  Germany
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These results outperform TI datasheet specs with THD being more close to 0.02% at 1kHz, 24V, 4Ohms. The main difference are output filter inductors and so I did a quick and dirty survey with several inductors found in the stash.
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Old 29th April 2021, 11:11 AM   #85
bucks bunny is offline bucks bunny  Germany
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All inductors have a value of 10uH. THD vs output voltage at 1kHz, 24Vdc and 5Ohm dummy load are shown here

Last edited by bucks bunny; 29th April 2021 at 11:24 AM.
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Old 29th April 2021, 11:23 AM   #86
bucks bunny is offline bucks bunny  Germany
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While No1 &2 are close on par, the smaller cores show increasing distortion. This is to be expected as the smaller cores are driven into higher magnetic flux density than the bigger ones resulting in degraded linearity. Which confirms the experience that "size matters".
Another interesting result are the measured idle currents of the amp fitted with different inductors.
The smaller inductors cause higher idle currents than the bigger ones due to higher magnetic losses. This correlates to inductor related distortion.
So comparing inductors by simple measuring the amps idle current gives a quick overview for pre-selection.
Hint: As the measurement of total idle current includes the +-12V flybuck converter some 3~4mA are to be subtracted from the results.
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File Type: png idle_current.png (19.3 KB, 10 views)

Last edited by bucks bunny; 29th April 2021 at 11:35 AM.
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