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Audio amp supply clipping circuit

Gary Lynch
Expert 1310 points
Other Parts Discussed in Thread: TPA3110D2

We are using a TPA3110D2 in an application where the supply voltage can vary over a wide range, as high as 40 V, which exceeds the absolute max for the amplifier (30 V).

To that end we placed a clipping circuit (below) before the amplifier to prevent it from seeing more than about 23 V.

I am finding out rather late in the design cycle that this clipper is subject to rather extreme oscillations (40 dBuV) in the 60 kHz range.  These are most noticeable at the point where the zener diode first turns on, but it continues to oscillate even at higher voltages when I would have thought the transistor was full on. We tried adding capacitance between gate and source and across the diode but they make no difference whatsoever.

The oscillation has no effect on circuit operation, but causes us to fail an emissions test--a variant of CISPR 25.

We have tested the circuit shown, completely removed from the application and found the oscillation changes shape but remains a problem over a wide range of load currents.

Has anyone else had to limit the supply voltage for this part and if so, what approach did you take?

  • 0
    TI__Guru**** 189399 points

    Hi, Gary,

    We have not worked on any circuit like this here, but from a previous life, you might quell the oscillations by putting a series resistor between the zener diode and the gate of the mosfet. I also would expect adding capacitance in parallel with the zener diode would improve the oscillation.

    I would try a 10 ohm resistor and place it here:

    -d2

  • 0 in reply to Don Dapkus
    Expert 1310 points

    Thanks, Dan, for the quick reply.

    The 10 ohm resistor by itself didn't do much, and a 0.1 uF cap (which we had previously tried--alone) made a slight difference.  When we changed the cap to 1.0 uF we saw about a 10 dB drop in the disturbance. We are not out of the woods yet but making progress.

    Tomorrow we are going to try increasing the resistance. I will follow up.

    If you get the chance, can you tell me what difference that resistor makes?

  • 0 in reply to Gary Lynch
    TI__Guru**** 189399 points

    Hi, Gary,

    I was trying to find a reference to this, but failed. It's been a loooong time, but basically, you set up some kind of circuit that is likely to oscillate without the gate resistor. We used to have people put zener diodes from gate-source to protect the gate from overvoltage, but these nearly always led to parasitic oscillation. The 10 ohm resistor provides enough damping to kill the oscillations.

    -d2