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Old 25th December 2006, 08:21 PM   #1
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Default Chips blow up when powering the amp.

hi,

a couple of years ago i was building a 6w+6w amp using tpa1517, and it kept burning the chip every time i'd turn it on.
after blowing a couple of them i changed the power switch from "after the PS" to "after transformer, before diodes". it stopped burning, i never knew exactly why this happened.
see figures 1 and 2.


this new era finds me building a small sub woofer plate amp using TDA2040 and a switching power supply (the one HP printers use, 16v@650mA + 32v@950mA).
apparently i made the same mistake at my first desing. as a result of this, the chip is history (short circuit between output and Vs).
this time, as we are talking about a SMPS, i don't know if the same change i did before will fix it. any way, it would be quite a mess trying to place an interruptor between transformer and diodes inside the SMPS.
the first design (the one burning the chip) is fig. 3.
will interrupting the mains save me ?see fig. 4.
or should i replace the 220uF for a 1000uF or 2200uF instead?
i don't know. you tell me...

i'd appreciate a lot your helping me with a solution, just a piece of advice, or an explanation of this burning chip problem.

thanks a lot in advance!

Click the image to open in full size.


PS: the amp circuit is ok. it's the one found in the TDA2040 datasheet as a typical application for single supply and i've seen it working well with a linear PS interrupted "after transformer, before diodes", and with another one interrupted at the mains.
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Old 25th December 2006, 08:50 PM   #2
teemuk is offline teemuk  Finland
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I don't know about SMPS but I guess the reason why your chips blew in the first scenario was the sudden voltage and current introduced to them: The potential will shift very fast from 0 to whatever and I guess the chips just could not take the transient. If you place the switch before the rectifier it will introduce a delay since the supply voltage will have to rise ("smoothly") from 0 to whatever as the reservoir capacitors charge.

In my opinion it's always best (safest, less power consuming) to just switch off the mains as in figure 4.
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Old 25th December 2006, 08:58 PM   #3
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thanks a lot!

i thought something like that was the problem, but then another question arises in my ignorant brain.

as SMPS oscillation is way faster than 50Hz, won't tension go up fast anyway? maybe not fast enough to kill the chip. maybe frequency has nothing to do and the time required to fill the caps is related to the energy required. so. it's related to the power delivered by the transformer (amplitude of voltage and current delivering capability, not frequency).

thanks a lot indeed.
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Old 25th December 2006, 09:21 PM   #4
Duo is offline Duo  Canada
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in your original 6W amp, with the linear power supply, how close was the power supply voltage to the voltage limit of the chips?

I'm thinking of the fact that linear supplies, when powered up and unloaded, will reach a much higher voltage in some cases than when loaded. When you throw that output switch, this high voltage appears at the chip until the chip discharges the capacitor down to a normal level. If the chip was rated for less than this higher voltage, it could have fried.

The above is unlikely, but then again, it's seemingly unusual for circuits like yours to fry the way yours did.
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Old 25th December 2006, 09:35 PM   #5
lineup is offline lineup  Sweden
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I suppose
it is your TDA2040 chip that is blowing up.

You would imagine to draw PCB for such a simple
but not bad amplifier as the good old TDA2040
wouldn't be difficult - not be at all critical with Layout.
But I have read several TDA2040 projects at websites (Search)
as well as read posts from Chip Amps builders,
saying you have to be careful when layout boards for TDA2040.


You should study this PDF ( I guess you have it ).
Why do you think they have bothered
to show you layout suggestion - for EVERY CIRCUIT in that PDF???
... you do not often see this is datasheets ...
My Guess it is more important with layout, than usual.


Especially you should notice:
- How the ground is drawn.
- How the connections +V, 0V and LSP-GND and Input-GND are done.
- In what order components are joined to Ground plane and rails.
- How supply bypass capacitors are put
- These Caps are in my attached diagram: C3(220uF), C5(100nF)
- Also R6 + C6 can be of great importance. To stop oscillation burn outs


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File Type: png tda2040-singlesupply-layout.png (46.7 KB, 334 views)
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Old 25th December 2006, 11:53 PM   #6
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actually i'm using that same PCB.

thanks.
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Old 26th December 2006, 12:13 AM   #7
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Default Sizzle..

Yes the switching should be at the mains on the primary side....I would bet you are indeed over the limit on the B+ supply voltage, there is really no reason for it to blow with a proven design with the switching as described.
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Old 26th December 2006, 12:45 AM   #8
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maximun ratings are 22V and 40V, for TPA1517 and TDA2040.
i'm feeding them with 14V and 32V respectively. i don't know if that's too close.

thanks from "good airs", in the neighbourhood of "slaughterhouses".
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Old 26th December 2006, 01:01 AM   #9
Zen Mod is offline Zen Mod  Serbia
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use circ No.4 and try putting shunt resistor from smps output to gnd

calculate it for some 3 to 5 W ......so-say-270/5W will do the job
ya know that PC power supply is fully and safely operational only with some nice and predictive load across 5V output?
I think that same case is here
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Old 26th December 2006, 01:17 AM   #10
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this PS has a 32V/950mA and a 16V/650mA output, would it be the same placing in one or another outputs? or must it be connected to the output that's burning the chip?

i ask this because i'll be using the 32V output of the PS close to its limit when playing loud, but the 16V will be feeding only a filter preamp with negligible power consumption.
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