This article compares traditional moving coil speakers, analyzes the characteristics of new ceramic piezoelectric speakers and the requirements for the required audio power amplifier, compares and analyzes various commonly used boost structures and audio power amplifiers, and tests a variety of boost The structure and audio power amplifier cooperate with the relevant data and use effect of the ceramic piezoelectric speaker, considering from various aspects such as sound pressure, efficiency, total harmonic distortion and so on. It is concluded that using the Boost boost structure and driving a new type of ceramic piezoelectric speaker with a Class D audio power amplifier is the most compromised solution.
1 Development trend of portable products
The development trend of portable products is shown in Figure 1.
With the development of portable consumer electronics, people are increasingly demanding small and thin portable electronic devices. Ceramic piezoelectric speakers are gradually being used by many portable consumer electronic products because of their ultra-light, ultra-thin, efficient, and no need for large sound cavity Favored. The development of portable consumer products in the direction of ultra-thin, light and small, how to achieve a slim profile, and to extend the use of a single rechargeable battery has become a major design consideration for various consumer products. Such system requirements place thinner, smaller, and more power-saving requirements on individual electronic components.
2 Basic characteristics of ceramic piezoelectric speakers
Compared with dynamic coil speakers, the diaphragm of a piezoelectric speaker is driven by the piezoelectric material bonded to it to bend, so the shape of the diaphragm is almost unlimited, and the diaphragm or paper cone of the dynamic coil speaker It is usually round or oval, which often limits the product's shape design. All moving coil speakers must have a magnet to drive the voice coil, which increases the overall height and weight of the speaker, but the ceramic piezoelectric speaker does not need a magnet to drive, so that a thin shape can be achieved to reduce the terminal The height of the product.
Faced with compact mobile phones and thinner and thinner computers, moving-coil speakers have become a limiting factor for manufacturers to produce ultra-thin products. Ceramic piezoelectric speakers can provide highly competitive sound pressure levels (SPL) in ultra-thin, compact packages, and have great potential to replace traditional moving coil speakers. The main differences between ceramic piezoelectric and dynamic coil speakers are shown in Table 1.
Amplifier circuits that drive ceramic piezoelectric speakers have different output drive requirements than traditional moving coil speakers. The structure of the ceramic piezoelectric speaker requires the amplifier to drive a large capacitive load and output more current at a higher frequency while maintaining a high output voltage. The efficiency of traditional moving coil speakers is easy to calculate. The audio coil winding can be approximated as a fixed resistance in series with a large inductance. If the speaker resistance is known, Ohm's law can be used to calculate the load power: P = I2R, or P = VI. Most of the power of the speaker is converted into the heat of the coil. Since ceramic piezoelectric speakers have capacitive characteristics, the heat generated when consuming power is not high.
Figure 2 is a waveform diagram of a ceramic piezoelectric speaker at 1 kHz, 90 dB (measured distance 10 cm) sound pressure, the voltage is 8 V, the current is 15.6 mA, the phase difference between the voltage and current is 79.2 ° The power consumption is 8 & TImes; 15.6 & TImes; cos79.2 ° = 23mW. It is equivalent to 18% of the power consumption of a dynamic speaker with a diameter of 20 mm at a sound pressure of 90 dB (measured distance of 10 cm).
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