Output Filter Components Breaking Class D Amplifier Chip

Question

Hi I am designing a class D amplifier PCB using the TPA3122D2 amplifier chip. I am planning to base my circuit on the schematic from the evaluation board:

I bought components of the same values given in the schematic and tried building out the circuit on a breadboard. However I have run into a problem which I have isolated to the output filter circuit, shown here:

Before connecting the LOUT output pin the output filter I checked the LOUT pin with a scope and saw the high frequency modulated audio signal I would expect at a class D output. At this point the output circuit looked like this:

I then tried connecting the output to the output filter with no speaker attached, at which point the chip began overheating and the signal at the LOUT pin disappeared. After this the chip is fried, and overheats whenever it is given power. I was able to repeat this several times. At the point where the chip breaks the output circuit looks like this:

I also tried connecting the output filter to a fresh chip without the capacitor to ground, at which point the chip does not break:

At this point I am fairly certain the problem in my selection of either the L_FILTER or C_FILTER component, highlighted below:

The EVM board uses the following components:

Inductor: A7503AY-220M Datasheet (Not Commercially Available)

Capacitor: Capacitor, metal poly, 0.68µF, 63V, B32529C684J

When building out the circuit I used:

Inductor: DR0608-223L

I chose this inductor because it had the same DC current rating, and nearly the same maximum DC resistance (78mOhms vs 97mOhms).

Capacitor: 50V 0.68uf Ceramic

I used this capacitor because I had it on hand and was hoping to use SMT ceramic caps where possible for simplicity of fabrication.

In Conclusion:

Is there a necessary rating I am missing in one of these two components that is causing the amplifier chip to break in this way? What rating should I be looking at? How can I ensure I am selecting the proper components to make the circuit work?

I would like to understand this, because I don't want to use the components from the EVM. The inductor is not available on Mouser or DigiKey, and I would like to find a capacitor with a smaller footprint for my eventual PCB.

Couple Disclaimers:

I rechecked the circuit several times and am certain is built correctly on the breadboard. I am also aware that breadboards are non ideal for high frequency circuits, but I am only looking for proof of concept, not audiophile quality.

I have also acquired the EVM for this component and tested that the component works when given the proper circuitry.

I used a line level sine wave for testing the audio input. This problem occurs whether or not I give the amplifier any input.

Thanks!

Answer 1

CLass "D" amps like SMPS cannot be tossed into a breadboard to make work. Mutual coupling of wires from coil current and 10nH/cm can make a big difference to work or fail.

I experimented with their design to see why and in theory it should not oscillate, but in your layout it does.

Making another tank circuit seems to help.

But we need more details on your design differences.

You can simulate here

But while you are learning these tools and effects of anti-resonant currents, it is wise to have an adjustable DC current limiter for bench testing to save on fried parts.

This intersection of RLC is consistent with the simulations

Answer 2

I am posting this as an answer, because I do not yet have the necessary mojo to add a comment. And yes, I realize this is a very old post. But, I have run into the very same problem (though rather than frying chips, mine is going into shutdown with a distinctive tick-tick-tick sound) and this is one of the few web resources I've found that actually identifies the problem.

My solution to the problem: remove the EMI filter.

It works and is documented by TI as a plausible way to reduce cost in this PDF: https://www.ti.com/lit/an/slyt198/slyt198.pdf

The key takeaways from the document are: quiescent current will increase, and it is advisable to keep the speaker leads short and use a driver with high inductance. If you're building something like powered speakers, your leads will be short. If you're building a guitar or bass amp with a big driver, your inductance will be high.

I have configured a bridge-tied mono amp and all of the components you have in your schematic have been removed, except for the 0.22u boot-strap capacitor. It's sitting on a breadboard driving an old 3.5" 8-Ohm Radio Shack full-range special and working fine.

Save the headache, skip the filter.

Answer 3

I know. It's too late. I also tried class D amps. What i understand is the filter inductor is not capable of passing that much current though it without core saturation. when you add 0.68uF capacitor after inductor, huge current is passed through the LC network because most of the supply voltage (high frequency component) appear across inductor. inductor current starts rising at a rate proportional to the supply voltage. If the switching frequency or inductance is not high enough the inductor current may reach the core saturation current and its permeability disappears causing a short circuit frying the MOSFETs. even if there is no core saturation the inductor may become too hot if it is of poor quality in terms of core loss or DC resistance. You have to either buy a good quality inductor or wind (preferably bifiliar or trifiliar) one with low loss toroid core. In some cases the LC filter is tuned to the switching frequency or its harmonics idle current will be higher. The inclusion of 0.68uF capacitor actually cancels out some of the inductive reactance causing an increase in idle current. you can verify it by measuring the voltage across inductor/capacitor. you will see higher voltage than the supply voltage across them.

The capacitor you have chosen is MLCC. They are highly non linear. Their capacitance vary with applied voltage. You need to choose a non ceramic film capacitor for better linearity.