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Class D Switching Power Amplifiers and Power D/A conversion

class d amplifier current output
class d amplifier current output
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Old 5th February 2017, 02:17 PM   #31
Pafi is offline Pafi  Hungary
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Join Date: May 2004
Location: Budapest
Quote:
Originally Posted by franticDIY View Post
So it is better to feedback the voltage/current before the filter or almost after the inductor. Is it right?
I don't know what you mean. Voltage and current feedback are 2 different quantities, for both there are different options, but voltage is on a node, while current is on a branch so they can't be defined the same way. "Almost after..." ? :-)

For voltage the most important point of feedback is definitely on the output of LC filter. Voltage of switching node is not important, basically it is determined directly by PWM signal. But for current you have 2 options: current of inductor or current of load. Both has its purpose.

I can't tell which is the truely best combination (linear combination) of the feedback for the 3 state variables and the filter parameters, the optimalization for so many parameters in head is beyond my capabilities. :-) And the optimisation goal is not defined really well. All of my really successful projects started with well defined goals. They almost automatically lead to the best solution, or at least cuts off most of the wrong ways. But defining the goals properly takes 5-20% of the project efforts. (This is why in many cases the goals are adjusted to the achievements afterwards. This is how a suboptimal development goes.)

Quote:
Today i noticed that generating the pwm signal with my fpga (open loop) at the output of the class d amplifier there is a dc offset greater than 1Volt (mean measure with oscilloscope).
How could it is possible?

In the FPGA an NCO generates a 10 bit discrete sinusoid (-511,+511). All the samples are scaled in order to compare it to triangle wave (-125,+124). Dead times are added on resulting pwm.
Offset of what signal? Differential output voltage of the bridge? Or common mode voltage? Or you don't have full bridge yet? What is your topology currently?

How much dead time, and exactly how? If you insert it asymmetrically, then it will cause offset naturally.

Quote:
Seems that the dc offset is more significative for duty cycle greater than 50%.

Have you any idea? Things could be better with an IC and closing the loop?
Let me know.
Of course offset will be lower in closed loop. I don't know what kind of IC you think of.
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Old 4th February 2018, 07:55 PM   #32
kees52 is offline kees52  Netherlands
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Join Date: May 2009
Location: Sprang-capelle Holland.
Quote:
Originally Posted by franticDIY View Post
Why PID?

Another question: do you know any IC (pwm controller) that could be used for my application?
Thanks.
PID? wel just read this paper, and use capacitor current feedback, but also need a voltage feedback also to get good results, as for what we know now afcourse.. PID is used here to rule out the two low pass poles of output filter. A good reading for learning new things.

https://pure.tue.nl/ws/files/46824541/573760-1.pdf

I like currrent feedback because of the bandwidth and low tim, dislike the offset problems and need a servo, for class d things are different.



regards.
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Old 8th February 2018, 06:17 PM   #33
franticDIY is offline franticDIY  Italy
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Join Date: May 2014
Quote:
Originally Posted by kees52 View Post
PID? wel just read this paper, and use capacitor current feedback, but also need a voltage feedback also to get good results, as for what we know now afcourse.. PID is used here to rule out the two low pass poles of output filter. A good reading for learning new things.

https://pure.tue.nl/ws/files/46824541/573760-1.pdf

I like currrent feedback because of the bandwidth and low tim, dislike the offset problems and need a servo, for class d things are different.



regards.
Thank you. The paper seems very interesting.
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Old 8th February 2018, 08:17 PM   #34
kees52 is offline kees52  Netherlands
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Join Date: May 2009
Location: Sprang-capelle Holland.
Quote:
Originally Posted by franticDIY View Post
Thank you. The paper seems very interesting.
Your welcome.
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