How much do you know about Class D power amplifiers?

                                     How much do you know about Class D power amplifiers?

The basic structure of class D power amplifier can be divided into three parts, a. modulator; b. amplifier; c. low-pass filter.

Class D power amplifiers are more difficult to understand than Classes A, B, and C, because they need to be modulated, and they look like PWM waves with different duty cycles. The waveform looks nothing like our audio analog waveform.

Fundamental:

The simple understanding is: the audio signal is compared with the triangular wave high-frequency carrier through a comparator to obtain a PWM wave (modulator part) with a different duty cycle, and then the obtained PWM signal passes through the amplifier to drive the MOS tube-to-tube (amplifier), and finally passes through filter input to the speaker. The modulated PWM contains audio components, and then the high-frequency carrier is filtered out by a low-pass filter to restore the original signal.

2. How to measure the output power of class D power amplifier

Significance of power measurement: To prevent the actual power of the speaker from exceeding its

nominal maximum value, causing broken sound and accelerated speaker damage, some derating will be done in general use.

From the basic working principle of the above-mentioned class D power amplifier, it can be known that the modulated PWM input to the speaker contains audio components, and the high-frequency carrier needs to be filtered out by a low-pass filter to restore the original signal. As shown below,

Test steps: First, for the convenience of measurement, it is necessary to input a 1kHz audio signal to the class D power amplifier, and then use a low-pass filter to restore the signal at the output of the speaker (4W/4Ω). After testing, the waveforms of points B and C are respectively

3. How to solve the problem of encountering POP noise:

Causes of POP noise

POP noise is caused by the rapid charging caused by the voltage difference between the two ends of the DC blocking capacitor on the audio path when it is powered on. Generally, it is adjusted on the input RC LPF of the Audio PA, and it is improved by reducing the C capacitance in series, but The reduction of the capacitance will cause the attenuation of the low-frequency components, causing the sound quality to deteriorate and the sense of hearing to be hard.

a. Measure 1

The enable pin, when powered on, ensures that the power amplifier is not working, and the hardware solves it with pull-up and pull-down resistors. Turn on the power amplifier after ensuring that there is a signal in the timing control, and turn off the power amplifier first and then stop the signal;

b. Measure 2

The sound fades in and out: the volume is lowered when it is turned on, and then gradually increased, and the volume is gradually lowered when it is turned off, and then turned off;

4. Efficiency comparison between class D amplifier and class AB amplifier:

a. Because the amplifier works in a low output power state most of the time when playing language or music, the efficiency of class D amplifiers is 2.5 to 3 times higher than that of class AB amplifiers on average;

b. This high-efficiency feature determines that Class D amplifiers are especially suitable for use in portable devices (extending battery life, reducing volume without radiators) and super-power devices (reducing power consumption);

The above are some problems I encountered in the process of using Class D power amplifiers and some useful things I found. I wrote a rough outline and more details. I hope that all netizens can explore, research, and discover by themselves. In addition, more research can be done on the LC filter circuit design during the use of Class D power amplifiers, and the comparison between Class D power amplifiers and other types of power amplifiers.