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A Simple TPA3250 BTL Board
A Simple TPA3250 BTL Board
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Old 24th July 2020, 04:39 PM   #1
matt_garman is offline matt_garman  United States
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A Simple TPA3250 BTL Board
Default A Simple TPA3250 BTL Board

I recently assembled a 3e-audio tpa3251 amplifier. I was really happy with the result, and thought it would be nice to have a similar amp board built around the tpa3250. I don't need a lot of power (20-30 watts is more than enough), and the tpa3250's power pad down design allows the PCB to be used as a heatsink, allowing for a smaller board. 3e-audio appears to have made a tpa3250 board at one point, but it is no longer available.

I haven't made time for diyAudio projects in the last year or so, and combined with the desire to have a tpa3250 board, I thought I'd take a stab at designing my own. The first PCB I ever designed was for the tpa3118 chip; I called it the Hot Doug, as an homage to DUG's tpa3116/8 board. (I built an amp around DUG's boards over five years ago, and it's been in continuous operation in our living room ever since.)

Also noteworthy is the Gmarsh-designed Wiener Pro prototype based on the tpa3250. He doesn't appear to be active on diyAudio any more, unfortunately.

So here I present the "Hotter Doug" tpa3250 board. This is by far the most complex board I've designed; in the interest of trying to minimize problems, it is essentially a slightly simplified version of the TI TPA3250D2EVM (TI's own evaluation module/reference design). Design notes:
  • Fairly compact: 92x73 (mm).
  • Component references exactly same as TI EVM.
  • I originally wanted to use the "deliciously overkill" VER2923 inductors, but decided instead to use the Coilcraft ua801x series for cost and space savings.
  • Uses only 805 or bigger SMD components. This is my personal threshold for what is comfortable to solder by hand. (Also allows for more caps to be C0G.)
  • Strictly BTL (stereo), does away with all EVM jumpers except SE/DIFF input.
  • Omits the EVM's supervisor IC (for managing the RESET pin) in favor of continuous 3.3v on that pin (copied the idea from the tpa3220evm-micro).
  • Nearly continuous bottom layer groundplane, stitched to top layer groundplane, to make whole PCB act as a heatsink for the tpa chip.

My main goal with this version is just to have something that works. Unless I made a copy error, the schematic should be valid, since it's a direct clone of the TI EVM. Component density is high (for me anyway), so my biggest concern is soldering errors. I plan to use my frying pan/skillet method. Assuming everything works, some ideas for future revisions:
  • Take measurements, investigate post-filter feedback.
  • See if there's a simpler/smaller/cheaper way to derive the 12v and 3.3v power rails. The EVM uses a lot of board space to do this (LM5010 SMPS for PVCC to 15v, LM2940 LDO for 15v to 12v, TLV1117 LDO for 12v to 3.3v).
  • Look for a simpler/smaller/cheaper way to do the input buffer (that allows single-ended or differential input, and supplies the necessary differential input to the TPA chip).

But that's getting ahead of myself! At this point, I think I'm good on the layout, other than minor tweaks---unless somebody points out potential issues that I'm not aware of! I'm slowly working on a BOM and Mouser project. After getting some feedback, and finishing the BOM, I'll post the Kicad files as well as the bill-of-materials.

Thanks for looking and any feedback!
Attached Images
File Type: jpg pcb_top_solid.jpg (276.0 KB, 258 views)
File Type: jpg pcb_bottom_solid.jpg (405.8 KB, 257 views)
File Type: jpg pcb_top_3d_render.jpg (370.3 KB, 254 views)
File Type: jpg pcb_top_3d_raytrace.jpg (356.0 KB, 254 views)
File Type: jpg pcb_bottom_3d_render.jpg (134.0 KB, 270 views)
File Type: jpg pcb_bottom_3d_raytrace.jpg (162.6 KB, 34 views)
Attached Files
File Type: pdf tpa3250_btl_amp_v1.pdf (111.1 KB, 18 views)
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Old 24th July 2020, 05:16 PM   #2
drMordor is offline drMordor  Russian Federation
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A Simple TPA3250 BTL Board
Good job!
I did it differently, separate powerline - GVDD, AVDD, and opamps.
I do not see PFFB parts on PCB.

Last edited by drMordor; 24th July 2020 at 05:24 PM.
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Old 24th July 2020, 05:30 PM   #3
matt_garman is offline matt_garman  United States
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A Simple TPA3250 BTL Board
Quote:
Originally Posted by drMordor View Post
Good job!
I did it differently, separate powerline - GVDD, AVDD, and opamps.
I do not see PFFB parts on PCB.
Thanks! Did you post your implementation?

Having the other power rails proved somewhere else (i.e., off-board) was something I thought about. But I wanted a simple "plug and play" board, where I just have to supply the main power, and the board will take care of the rest. Perhaps a simple future enhancement is to have terminals for user-provided power rails, and a jumper or something to disable the on-board regulators.

There are no PFFB parts - that's a "maybe" feature for a future revision. I want to keep it as simple as possible this time around.
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Old 24th July 2020, 05:57 PM   #4
drMordor is offline drMordor  Russian Federation
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A Simple TPA3250 BTL Board
TPA3255 - all about DIY, Discussion, Design etc.
My implementation
TPA3255 - all about DIY, Discussion, Design etc.
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Old 26th October 2020, 05:57 PM   #5
matt_garman is offline matt_garman  United States
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A Simple TPA3250 BTL Board
It's taken me a few months, but I finally have a board up and running!

Before sending the board off to be fabbed, I changed my mind and broke it up into three separate boards: (1) actual tpa3250 amp board, (2) power supply, and (3) op-amp front-end.

My first attempt at the power supply board was a failure, because I used the wrong footprint for the LM5010. I fixed it, opting for the crazy-small WSON-10 package. When I saw how small that chip was, with pins that are barely visible to the naked eye, I thought, "no way I'm going to be able to solder this!" My way of doing surface mount soldering is to use a frying pan on my cooking range --- clearly not a precision operation!

But to my pleasant surprise, it actually worked!

That gave me the confidence to populate the actual amp board. That was fairly straightforward. Though, again I was concerned about solder quality of the tpa3250 chip; it has lots of pins that are tiny and very close together. After I soldered that board, I inspected for solder bridges on the amp IC. I did in fact see one or two, and used my regular soldering pen with some solder wick braid to remove the bridges. I did a final check using my phone, taking zoomed-in pictures of the chip pins, making sure the solder job looked reasonable.

After that, I used my DMM in resistance mode to spot check the board for obvious problems, e.g. shorts to ground.

I didn't find any obvious problems, so I went ahead and did an actual test... success! At this point, I haven't built the op-amp front end, and used a DAC with differential output as a direct source for the amp. I didn't hear any crackling or obvious distortion; I also used a Kill-a-Watt mains power meter to get an idea of the total power draw. It showed about five watts at idle and very low listening levels. So overall, I think it's mostly correct. However, a couple observations might indicate that something isn't quite right:
  • I didn't measure the actual temperature of the tpa3250 chip, but put my finger on it. Even at idle/low playback volume, it was surprisingly hot. I could only comfortably keep my finger on it for a few seconds. I seem to recall the older tpa311x chips staying pretty cool to the touch in similar circumstances. When I cranked the music up a bit, the chip was too hot to touch for more than a second. And even in this "cranked" state, the AC power meter only showed about 13 watts being drawn, so clearly no where near this chip's rated output power. (Maybe I doomed myself by naming the board "Hotter Doug" --- certainly lives up to the name!)
  • Also when the playback was cranked, I did see the clipping indicator LED light up. Interestingly, it lit up in sync with the music's rhythm!

I was using a pair of 8-ohm Overnight Sensations speakers for testing. These should be a pretty easy load for any amplifier.

I can post Gerbers, BOM, KiCad files if there's any interest. Though I intend to make some revisions. At a minimum, I want to get away from screw terminals and use something like JST connectors. I also want to build up the op-amp front-end. After further testing, I'll likely discover other issues that need to be resolved.
Attached Images
File Type: jpg tpa3250_psu_v1_render.jpg (241.5 KB, 31 views)
File Type: jpg tpa3250_amp_no_front_end_v1_render.jpg (529.2 KB, 34 views)
File Type: jpg hotter_doug_live_test_20201025.jpg (677.9 KB, 38 views)
File Type: jpg tpa3250_psu_20201025.jpg (293.0 KB, 37 views)
File Type: jpg tpa3250_amp_board_20201025.jpg (425.0 KB, 43 views)
File Type: jpg tpa3250_ic_pin_solder_detail_1_20201025.jpg (530.9 KB, 43 views)
File Type: jpg tpa3250_ic_pin_solder_detail_2_20201025.jpg (372.0 KB, 39 views)
Attached Files
File Type: pdf tpa3250_psu_v1_schematic.pdf (36.4 KB, 5 views)
File Type: pdf tpa3250_amp_no_front_end_v1_schematic.pdf (77.8 KB, 11 views)
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Old 26th October 2020, 06:17 PM   #6
joensd is offline joensd  Germany
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Good job on tackling the layout yourself.
I rather bought an amp in a lazy moment.
But maybe I'll take your kicad files and go from there.
I'd love to have a compact 4-channel amp.

As it seems you could easily use a big thermal pad and mount the bottom side to a solid Aluminium plate or similar.
That works quite well when the surface is that big and the pad is not too thick. Just do not tighten the screws too much!
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Old 26th October 2020, 07:46 PM   #7
drMordor is offline drMordor  Russian Federation
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A Simple TPA3250 BTL Board
Quote:
Also when the playback was cranked, I did see the clipping indicator LED light up. Interestingly, it lit up in sync with the music's rhythm!
This is Overtemperature Warning
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Sorry, my english bad))) But I'm learning!
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Old 27th October 2020, 01:25 AM   #8
matt_garman is offline matt_garman  United States
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A Simple TPA3250 BTL Board
Quote:
Originally Posted by drMordor View Post
This is Overtemperature Warning
OK, well that seems consistent with how hot it felt to the touch.

Do you have any thoughts on what is likely causing it to get so hot? I feel like an implementation error is unlikely, since it actually works.

My suspicion is that the power pad on the bottom of the IC didn't get a good solder to the PCB. And/or the ground pins don't have a good solder connection to the PCB.

Another thought is that I damaged the chip during the "frying" procedure.

But if the power pad and/or ground pins are the problem, then joensd's suggestion (a pad to thermally mate the PCB to a metal chassis), probably won't help, right? Since, if this reasoning is correct, then the chip isn't correctly transferring heat to the PCB.

I wasn't sure how much solder paste to put on the exposed ground of the PCB (that is intended to mate to the amp IC). I was afraid of using too much and having it run out and create solder bridges on the actual pins. Now I'm wondering if I used too little?

Dunno, just thinking out loud. Open to suggestions!

Thanks!
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Old 27th October 2020, 08:06 AM   #9
joensd is offline joensd  Germany
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Quote:
Do you have any thoughts on what is likely causing it to get so hot? I feel like an implementation error is unlikely, since it actually works.

My suspicion is that the power pad on the bottom of the IC didn't get a good solder to the PCB. And/or the ground pins don't have a good solder connection to the PCB.
You guessed it, one of the two!

Layout/implementation/GND routing can still cause the chip to oscillate and generate excessive heat (although it works).
Chances are also that the pad is "cold" soldered to the PCB and there is a huge thermal resistance in that spot. Did you use solder paste?

Can you visually make out if the connection of the thermal pad is good or not?
How hot does the chip get when it idles? (no input signal)
Does it work properly when you play music with little volume?

In the forelast picture on the top right I see a big blob of solder, right?
The cap doesn´t look shorted but watch out for solder pellets or similar and inspect the whole board.

Last edited by joensd; 27th October 2020 at 08:09 AM.
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Old 27th October 2020, 08:15 AM   #10
joensd is offline joensd  Germany
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is there a short between the IC pins?
in a more closer look I also see lots of solder pellets under the IC
which is somewaht expected when using the "pan-fry-method" ;-)
Attached Images
File Type: jpg tpa3250_ic_pin_solder_detail_1_20201025_edit.jpg (553.2 KB, 52 views)
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