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TPA3255 Reference Design Class D Amp GB
TPA3255 Reference Design Class D Amp GB
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Old 1st August 2019, 07:11 AM   #1
xrk971 is offline xrk971  United States
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TPA3255 Reference Design Class D Amp GB
Default TPA3255 Reference Design Class D Amp GB

Edit Aug 17, 2019: latest 3d render with improvements (listed here) included:
Click the image to open in full size.

Latest bottomside of PCB (note extensive use of groundplane and large short triangular traces for power input and bulk caps all connected to Vdd pins):
Click the image to open in full size.

***** ORIGIBNAL THREAD START *****
3D render of TPA3255 Reference Design with components populated (top)
Click the image to open in full size.

First some history... When I first got started in DIYA projects back in 2012, there were no cheap premade Texas Instruments TPAxxxx Class D amps available. Back then, I had to make my own, and I did it with a 25w TPA3118D2 (closely related to the super popular TPA3116D2 in the Juggernaut thread inspired by my experiments in P2P SMT Class D amps).

My first DIY amp ever, a dead-bug flip-chip P2P TPA3118D2 Class D amp on foam core with aluminum foil ground plane:
Click the image to open in full size.

The TPA3118 sounded fantastic and eventually, inexpensive variants started popping up on Aliexpress and eBay, culminating in the ubiquitous $12 TPA3116D2 50w Class D amp. I used the TPA3116 for many years in many speakers, often driven by a miniDSP for 2-way to 4-way active speakers. Somewhere along the way, I switched to Class A amps, and you know the rest of the story - culiminating in the popular Alpha series amps and finally, the ABBB.

So what brings me to go back to my Texas Instruments Class D roots? Well, there has been a lot of development by TI over the past 6-7 years and I need some more power to drive some PA speakers and subwoofers. I have a current subwoofer project that has 4 drivers, each driven by its own amp channel and DSP. So I needed a way to make circa 150w to 200w per channel at 8ohms. The TPA3255 is perhaps the best-in-class Class D amp to do this job. One thing that really piqued my interest is that the TPA3255's datasheet says that it only has 85uVrms of noise with no input signal. That is a super quiet background!

So I have been working with JPS64 and Jhofland to develop a new Class D amp based on the TI Reference design, but with some tweaks to make it even better. The design philosophy is to keep the amp more general to allow the user to select SE, BTL, or PBTL operating modes rather than force the user to the typical BTL-only offerings available on eBay/Aliexpress. The TPA3255 when operated in SE mode provides for 4 channels of output, and the audio quallity is very high as the harmonic distortion vs frequency is flat rather than rising (as is the case for BTL). It is also a good thing to keep the possibility of balanced input for the amp to enable a nice quiet drive for folks with high end DACs or preamps. We also added some nice features to make the amp easier to use and more reliable. Like Molex Minfit Jr connectors for quick and reliable connections rather than screw terminal blocks. An NTC ground lifted chassis ground Faston spade is also provided, as well as a header block to allow master/slave operation and also carrier frequency selection. Basically, all optional features are kept to allow the user the widest latitude of how they want to apply the amp.

We are also mounting the main TPA3255 chip on the bottom side so that its thermal pad faces down towards the mounting surface. In this way, the board just needs to be mounted to a metal chassis with the usual standoffs and a small thermal conductor block (aluminum or copper) can be used to mount this amp inside a sealed (non-ventilated) chassis, yet still permit adequate cooling. It eliminates the need for a separate heatsink element. Of course, the amp uses only the best symmetric star-grounding-centric layout methododologies that you have come to know and love from JPS64. Don't forget the thousands of via-stitched traces for extra low impedance current paths. Finally, the crown-jewel of the design is use of premium high current CoilCraft flat copper wire shielded inductors. Those of you familiar with souped-up TPA3116's will reognize these as the ultimate inductors for Class D amps. They look cool too.

The design ended up at a generous 120mm x 124mm and will be 2.0mm thick with 2oz copper traces and ENIG finish. Typical JPS64 via stitching now covered with solder mask (tented vias) to reduce solder bridges. An on-board DC-DC converter provides the necessary 12v auxilary voltage for operation. Maximum power supply voltages is 51vdc single-rail. Typical 48vdc SMPS are perfect for powering this amp. If you already have a 400VA trafo that can make 48v to 51vdc, it can work very well too.

PCB top:
Click the image to open in full size.

PCB bottom:
Click the image to open in full size.

3d render showing bottom side with TPA3255's thermal pad exposed:
Click the image to open in full size.

Schematic is in the PDF file below.

If you are interested, please add your name, number of boards, and country to ship to below.

A huge thanks to JPS64 for his amazing layout of this amp, and also thanks to jhofland for helpful advice on the schematics.

I expect the pricing to be $32 ea and the usual shipping of $5 for CONUS, $10 for CAN, and $15 for everywhere else for up to 4 boards. Additional shipping charges for larger quantities of boards. I still need to order the verification board and build and test to make sure it works before the production boards can be ordered.

GB Interest List example:

JaneDoe - 2 boards - USA
Attached Images
File Type: jpg TPA3255-3d-Top-PCBonly.jpg (241.0 KB, 1101 views)
File Type: jpg TPA3255-3d-Bot-PCBonly.jpg (166.9 KB, 1087 views)
File Type: jpg TPA3255-3d-Top.jpg (216.5 KB, 1518 views)
File Type: jpg TPA3255-3d-Bot.jpg (151.2 KB, 1092 views)
Attached Files
File Type: pdf TPA3255_Class_D_SCH_001.pdf (64.8 KB, 97 views)
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Last edited by xrk971; 17th August 2019 at 07:13 PM.
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Old 1st August 2019, 07:33 AM   #2
vacuphile is offline vacuphile  Netherlands
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I am interested. Does it implement PFFB? And do you have measurements of a completed board?
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Last edited by vacuphile; 1st August 2019 at 07:36 AM.
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Old 1st August 2019, 07:42 AM   #3
xrk971 is offline xrk971  United States
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TPA3255 Reference Design Class D Amp GB
I still have to build and test it. It currently does not have the option for post filter feedback (PFFB). There is a marginal reduction in THD vs frequency with PFFB - is this something that you think will make this amp sound better?
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Old 1st August 2019, 08:50 AM   #4
jimk04 is offline jimk04  United Kingdom
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Will it come SMD populated? If so I'm interested.
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Old 1st August 2019, 09:20 AM   #5
vacuphile is offline vacuphile  Netherlands
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In the sense that it will be less sensitive to load impedance, yes. The reduction in THD per se, not sure. At the same time, PFFB is what Bruno did with the Ncore and they are great. To have this option would be a reason for me to want these boards. It is just a couple of passive components anyways.
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Old 1st August 2019, 09:50 AM   #6
doctormord is offline doctormord  Germany
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I find this layout a bit "suboptimal" in terms of power supply bulk capacitor connection. I also suspect some THD penalties due to inductor cross coupling when mounted like this. (Beside that, the power connector is labeled misleading, as the top pins are "negative")


Measurements will show.

Last edited by doctormord; 1st August 2019 at 09:52 AM.
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Old 1st August 2019, 01:11 PM   #7
xrk971 is offline xrk971  United States
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TPA3255 Reference Design Class D Amp GB
DrMord,
Thanks for your feedback. Regarding the bulk caps, I looked at the layout and do not see any possible way to get the pins closer to the caps!? Those traces go from the big double layer stitched via power bus to a trace going about as directly as possible to the power supply input pins. Please share how you would do it to improve?
Thanks,
X
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Old 1st August 2019, 03:14 PM   #8
doctormord is offline doctormord  Germany
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As the return path is also frequency depended both +/- should be together as close as possible to minimize loop area. (Things that work for slow signals like Class-A Audio may not work in switching applications. Also star-ground might make things worse than better.

I'd suggest routing the power via the bulk caps so the current path goes from connector to bulk cap to mlcc to power pins. And make sure every GND+Supply pin on the 3255's (where the 4 MLCCs are) is connected equal to this.

Fundamentals:
http://www.ti.com/lit/an/scaa082a/scaa082a.pdf
A look at “Common” mistakes - ppt video online download
https://www.edn.com/design/pc-board/...e-relationship


I lastely made it like so:


TPA3255 Reference Design Class D Amp GB-shunt-png

The split before the bulk caps is usually not needed, back in the days we wanted to see if this will have any effect on cross-talk/channel imbalance.
Attached Images
File Type: png Shunt.png (72.5 KB, 1005 views)

Last edited by doctormord; 1st August 2019 at 03:31 PM.
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Old 1st August 2019, 08:25 PM   #9
xrk971 is offline xrk971  United States
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TPA3255 Reference Design Class D Amp GB
Quote:
I'd suggest routing the power via the bulk caps so the current path goes from connector to bulk cap to mlcc to power pins. And make sure every GND+Supply pin on the 3255's (where the 4 MLCCs are) is connected equal to this.
Dr Mord,
The bulk caps are reservoir caps, they can be connected to the same fat low impedance PVDD trace and off to the side as a branch. They do not need to come before between the PSU pins and MLCC caps, which are the ones doing the filtering and are indeed mounted right on the pins (just on opposite side of double layer sandwich and connected by the via stitching). We have essentially the same routing and I would argue that distance from the PSU connector is even shorter on our layout rather than a long traverse from the back. I am still quite happy with the star ground in the physical centroid of the board. The low level analog ground plane is off to the front edge where it avoids interaction with the large currents near the central star ground. JPS64 has a lot of experience designing high speed digital (30Ghz), mixed analog and digital, plus power circuits. I am pretty confidant that we are in good hands with his design. Thanks for your comments though.

Here is our path from PSU pins for PVDD to the IC inlet pins - note that the 4 MLCC SMT filter caps are essentially in same place as on yours. Bulk reservoir caps are off on a short low impedance branch to the side and as close as possible.

TPA3255 Reference Design Class D Amp GB-ad54facf-03a9-40b7-b3b8-645ce7eb7fdc-jpeg
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File Type: jpeg AD54FACF-03A9-40B7-B3B8-645CE7EB7FDC.jpeg (498.1 KB, 921 views)
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Last edited by xrk971; 1st August 2019 at 08:44 PM.
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Old 1st August 2019, 10:25 PM   #10
doctormord is offline doctormord  Germany
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I didn't sayed these MLCC doing anything else than filtering. They're there to reduce EMI and suppress ringing.



But yeah, measurements will tell.

Last edited by doctormord; 1st August 2019 at 10:29 PM.
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