This article refers to the address: http://
Digital amplifiers improve audio quality and system performance.Class D amplifiers have evolved tremendously over the past few generations, and system designers have greatly improved the system's durability and improved its audio quality. In fact, for most applications, the benefits of using these amplifiers far outweigh their shortcomings.
In a conventional Class D amplifier, the controller converts the analog or digital audio signal to a PWM signal prior to amplification by a power MOSFET that is integrated into the power back end device. These amplifiers are very efficient, use a small heat sink or no heat sink at all, and reduce the power output requirements. However, compared with the traditional A/B class amplifiers, they also have inherent cost, performance and EMI problems. Solving these problems is a new trend in the development of Class D amplifiers.
Reduce EMI
Since the birth of the Class D amplifier, a large amount of radiated EMI due to its own rail-to-rail power switch characteristics has plagued system designers, which will make the device unable to pass FCC and CISPR certification.
In a class D modulator, the digital audio signal is converted to a PWM signal by comparing the audio signal to a high frequency fixed frequency signal and modulating the result on a carrier of a fixed frequency. The resulting signal is a fixed carrier frequency of variable pulse width (usually at a few hundred kHz), and then these PWM signals are amplified by a high voltage power MOSFET, and the amplified PWM signal is then passed through a low pass filter to remove the carrier frequency and restore the original Baseband audio signal.
Although this topology is very effective, it also causes some undesired consequences, such as a large amount of radiated EMI. Since the modulator uses a fixed frequency carrier, multiple harmonic radiation of the base carrier will be generated. Moreover, due to the switching characteristics of the PWM signal itself, overshoot/undershoot and ringing will produce a fixed ratio of high frequency (range of 10 to 100 MHz) radiated EMI. In order to suppress radiated EMI, the trend of the latest generation of PWM modulators is to adopt spread spectrum modulation technology.
Spread spectrum modulation techniques are used to spread the spectral energy of the switching PWM signal over a larger bandwidth without changing the content of the original audio. An effective way to improve the high radiated EMI of conventional modulators is to change the two edges of the PWM switching signal, as shown in Figure 1. The signal is centered at the carrier frequency, but neither edge is repeated cycle by cycle. This not only maintains a fixed carrier frequency, but also because the edge does not hop at a fixed rate, the radiant energy at the carrier frequency is greatly reduced.
Improve audio quality
Compared with the excellent A/B class amplifiers, the audio performance of the class D amplifier is very poor, not only the distortion is large, but also the dynamic range is narrow. Therefore, designers of current Class D amplifiers must improve their performance. By integrating high-performance sample rate converter (SRC) and delta-sigma processing, the next-generation solution provides greater distortion (THD+N) and a dynamic range of more than 100dB.
Currently, one noise source for Class D amplifiers is the jitter of the audio sample clock. The clock is usually generated by the SOC (MPEG decoder and DSP, etc.), and even small jitter can quickly affect the performance of a conventional Class D amplifier because the audio clock is associated with the output clock of the modulator.
One way to solve this problem is to use SRC technology. Because the SRC uses a locally stable clock source to synchronize the digital audio clock, such as a quartz crystal oscillator, the output jitter of the modulator is virtually independent of other audio clocks. Another advantage of the SRC is that the output switching ratio is fixed regardless of how the sampling rate of the input audio fluctuates, unlike a PLL-based modulator. When the audio input source changes or the input clock is missing, the SRC also improves the system's durability by eliminating audible noise.
Similar to the technology used in today's high-end DACs, the audio quality of Class D amplifiers has also been improved by integrating high-order delta-sigma processing. Modulators based on delta-sigma technology employ internal feedback that reduces modulation errors. By reducing the sampling error, the modulator can improve output distortion for better sound quality.
Reduce system cost
In order to pursue the lower cost of Class D amplifiers, designers use a half-bridge amplification topology in the power amplifier stage to reduce complexity and reduce material costs. Because the half-bridge output is typically half that of a full bridge, the number of power MOSFETs and external filter components is reduced by half. This also increases the number of unit power channels per second. However, the half-bridge amplifier also requires a DC blocking capacitor at the output and is extremely sensitive to noise on the mains supply.
At startup, the DC blocking capacitor must be charged to the bias point (half the voltage of the high voltage supply rail). If the output signal does not rise from ground potential to the bias point, a large "click" sound (boot shock) is generated in the speaker. The new Class D amplifier uses a pre-charge capacitor to keep the speaker silent during startup.
One way to keep the speaker from shock when charging the DC blocking capacitor is to use digital voltage boosting, which is to slowly increase the PWM duty cycle from non-switching to 50%. This will not produce a large "beep" in the speaker, but the speaker is not silent due to the large amount of transient current generated by the MOSFET switching.
Another way to keep the speaker from shock when charging the DC blocking capacitor is to simulate voltage boosting. During this type of voltage boost, a current source charges the capacitor to the bias point. Once the voltage across the capacitor reaches the bias point, the current source turns off.
Power feedback
Since the half bridge is a single-ended topology, there is no common mode rejection in the differential full bridge topology. In a full-bridge amplifier, since the differential output of the amplifier is powered from the same voltage source, the noise on the common voltage source will cancel out at the output. In a half-bridge topology, any AC ripple noise on the amplifier's power supply is directly coupled to the output. Since the half-bridge topology is sensitive to power supply noise, it is often necessary to provide a power supply rejection feedback (PSR) circuit for noise reduction.
Analog Class D amplifiers have many inherent PSR characteristics, while full digital Class D amplifiers do not. In current digital PSR schemes, an external ADC is typically used to monitor the amplifier's power supply.
Feedback and noise cancellation processing is performed in the digital domain of the modulator. Some manufacturers only use this feedback method to compensate for the effects of AC noise coupled into the PWM output from the supply rails that degrade system performance. Other manufacturers also use it to compensate for changes in the DC supply voltage (voltage drop) due to load changes, such as the fast inrush current required by the woofer (subwoofer), or the voltage fluctuations in the supply line. The benefits of PSR feedback in AC and DC devices have been extended to full-bridge amplifiers and improve the isolation between channels in today's multichannel home theater amplifiers, effectively offsetting crosstalk and line voltage changes before they reach the output.
Home Appliance Heating Elements
Dryer Heating Element,Oven Heating Element,Home Appliance Heating Elements
Jiangsu Shazi Electric Equipment Co., Ltd. , http://www.cnhotpoint.com