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更大限度減小放大器尺寸和降低熱負荷的汽車音頻設計注意事項

2019-12-06 15:33:22 來源:德州儀器
These higher-channel-count systems in an infotainment system may integrate:
這些信息娛樂系統中的高通道數系統可能集成:

  • A center speaker.
    中置音箱。
  • Separate tweeter/midrange/woofer speakers.
    獨立高頻音箱/中音音箱/低音音箱。
  • Instrument cluster chimes or warning tones.
    儀表盤警報聲或提示音。
  • Additional speakers to communicate information, such as warning drivers to take control of steering or braking if the vehicle is operating in a semi-autonomous driving mode.
    其他可以傳遞信息的音箱,如警告駕駛員在車輛處于半自動駕駛模式時控制方向盤或剎車。
Some higher-end car models may actually have as many as 20 speakers. The speakers in these audio systems are driven by an external amplifier typically placed near the trunk of the car. These audio systems also incorporate more advanced sound algorithms such as active noise cancellation to deliver a more personalized audio experience.

一些高端汽車模型實際上可能有多達20個音箱。這些音頻系統中的音箱由通常安裝在汽車后備箱附近的外部放大器驅動。這些音頻系統還包括更先進的聲音算法,如可以提供更加個性化音響體驗的主動降噪技術。

With each subsequent model year, automakers are adding more and more electronics. Coupled with the need to drive six to eight speakers directly from the infotainment system, space behind the dashboard is now at an all-time premium. Therefore, it’s becoming a priority for audio hardware designers to develop smaller automotive audio amplifier solutions with lower heat dissipation. In this paper, I’ll describe four factors that drive overall audio amplifier size:

在隨后的每一個車型年,汽車制造商正在增加越來越多的電子器件。再加上需要直接從信息娛樂系統驅動6到8個音箱,儀表盤后面的空間現在是前所未有的出色。因此,音頻硬件的設計師應首先開發散熱更低的小型汽車音頻放大器解決方案。在本文中,我將描述驅動整體音頻放大器的四個因素:

  • Efficiency/thermal performance.
    效率/熱性能。
  • Switching frequency.
    開關頻率。
  • Inductor size.
    電感器尺寸。
  • Package design.
    包裝設計。
Efficiency/thermal performance
效率/熱性能


Designers have traditionally designed car radios using Class- AB linear audio amplifiers. Class-AB linear amplification is drastically less efficient than the newer but well-established Class-D switching technology. Figure 1 highlights the difference.
傳統上,設計人員使用AB類線性音頻放大器來設計汽車收音機。AB類線性放大技術遠沒有新的、但相當成熟的D類交換技術高效。圖1突出了其差別。


Class-AB efficiency loss leads directly to additional internal heat generation, which then requires dissipation outside the audio amplifier. The need for a larger heat sink in Class-AB designs also exacerbates the challenge to continuously reduce the overall automotive audio amplifier system solution size.

AB類效率損失直接導致內部產生額外熱量,然后需要在音頻放大器外部進行散熱。由于AB類設計需要更大的散熱器,這也使持續減小整個汽車音頻放大器系統解決方案尺寸變得更加困難。

Class-D amplifiers can achieve the same output power but dissipate significantly less heat, enabling designers to use a much smaller and less complex heat sink to transport dissipated power to the ambient environment.

D類放大器能夠獲得相同的輸出功率,但散熱顯著減少,這使得設計人員能夠使用更小、更簡單的散熱器將散熱量傳輸到周圍環境中。

Switching frequency
開關頻率


The number of electronics mounted behind the dashboard in a relatively tight space increases the possibility that circuits can emit interfering signals in close proximity. Ultimately, modern radios and audio amplifiers must provide better immunity from electromagnetic interference (EMI) in the AM band to meet these challenges.

安裝在儀表盤后面相對狹小空間內的電子器件數量增加了電路可以近距離發射干擾信號的可能性。更重要的是,現代收音機和音頻放大器必須在AM波段中提供更好的抗電磁干擾性能(EMI),以應對這些挑戰。

In the U.S., AM radio stations broadcast in the 535-kHz to 1705-kHz frequency band. Existing Class-D audio amplifier designs typically operate with a fundamental switching frequency in the 400-kHz to 500-kHz range. These lowers-witching- frequency Class-D amplifier designs create harmonics that occur directly within the AM band, as shown in Figure 2.

美國AM廣播電臺的波段范圍為535-kHz至1705-kHz。現有的D類音頻放大器設計通常在400 kHz至500 kHz范圍內以基本開關頻率運行。這些低開關頻率D類放大器設計直接在AM波段內產生諧波,如圖2所示。


The harmonics create interfering signals that reduce the sensitivity of the AM receiver, thereby hindering AM radio station reception. Implementing an AM avoidance scheme on Class-D amplifier designs mitigates the effects of these harmonics.

諧波會產生降低AM接收器靈敏度的干擾信號,從而妨礙AM廣播電臺接收。在D類放大器設計上運用AM避免技術能夠減輕這些諧波的影響。

Class-D audio amplifiers require reconstruction filters to convert the pulse-width modulation (PWM) signal from the amplifier output into the desired analog audio signal. These output filters are made with inductors (L) and capacitors (C) (as shown in Figure 3) for a typical bridge-tied load (BTL) amplifier circuit, and help minimize EMI from the highspeed switching transients on the output stages of Class-D amplifiers.

D類音頻放大器需要重建濾波器將放大器輸出的脈沖寬度調制(PWM)信號轉換為所需的模擬音頻信號。這些輸出濾波器由電感器(L)和電容器(C)構成(如圖3所示),用于典型的橋接負載(BTL)放大器電路,且能夠更大限度地減少D類放大器輸出級上的高速開關瞬態電磁干擾。

Automotive Class-D audio amplifiers that operate at a 2.1-MHz switching frequency provide significant margin above the AM band, as shown in Figure 4. This design is free of any lower-frequency spikes that would interfere with the AM band, thus eliminating the need for an AM avoidance scheme.
 
在2.1-MHz開關頻率下運行的汽車D類音頻放大器在AM波段上方提供顯著裕度,如圖4所示。此設計不存在任何會干擾AM波段的低頻尖峰,因而不再需要AM避免技術。
As an additional benefit, a 2.1-MHz switching frequency enables a lower inductance value for the output filter due to the inherent reduction in ripple current. A lower inductance for an equivalent current rating leads to a smaller inductor, reducing printed circuit board (PCB) area and subsequently the EMI footprint.
 
另一個好處是,由于紋波電流的內在減少,2.1-MHz開關頻率可以使輸出濾波器的電感值更低。等效額定電流的低電感會導致電感較小,從而減少印刷電路板(PCB)面積,且隨后減少EMI占用面積。
Inductor size
電感器尺寸

For Class-D automotive audio amplifiers, the value of the inductor required in the LC filter to ensure the proper PWM demodulation filter characteristic depends on the switching frequency. As shown in Figure 5, a 400-kHz automotive audio amplifier typically uses either a 10-μH or 8.2-μH inductor value, while a 2.1-MHz higher-switching-frequency amplifier design can take advantage of a much smaller and lighter-weight inductor in the range of 3.3 μH to 3.6 μH (assuming that each amplifier provides the same output power).

對于D類汽車音頻放大器,LC濾波器所需的電感值(用以確保合適PWM解調濾波器特性)取決于開關頻率。如圖5所示,400-kHz汽車音頻放大器通常使用10-μH或8.2-μH電感值,而2.1-MHz高開關頻率放大器設計可以利用3.3μH至3.6μH范圍內更小更輕的電感器(假設每個放大器提供相同的輸出功率)。

As I mentioned earlier, a typical car radio design has at least four channels to drive two front speakers and two rear speakers. This simple configuration requires eight inductors for a Class-D automotive audio amplifier, since each channel requires two inductors, as shown earlier in Figure 3. Thus, the size of each inductor is multiplied by 8, which is a significant contribution to overall PCB size and design weight. As a general reference, the transition from 8.2-μH inductors to 3.3-μH inductors can save over 85% in inductor space on the PCB and over 85% in weight.

正如之前提到的,典型汽車收音機設計至少有4個通道來驅動2個前端揚聲器和2個后端揚聲器。這種簡單的配置需要8個電感器以用于D類汽車音頻放大器,因為每個通道需要2個電感器,如圖3所示。因此,每個電感器的尺寸乘以8,對整體PCB尺寸和設計重量有著重要的影響。一般來說,從8.2-μH電感器轉換到3.3-μH電感器可以節省電路板上85%以上的電感器空間和減小85%以上的重量。

Package design
包裝設計


Another audio amplifier consideration that can greatly contribute to the overall system solution size of an automobile’s infotainment system is the design of the amplifier package.

另一個能夠大大減小汽車信息娛樂系統中整體系統解決方案尺寸的音頻放大器注意事項是放大器包裝的設計。

A square-shaped package design has inputs on the bottom of the package and two audio outputs with LC filters orthogonally placed on either side of the amplifier. As you can see in Figure 6, this type of package design greatly contributes to the overall PCB footprint.
正方形包裝設計在包裝底部有輸入,還有兩個音頻輸出,且LC濾波器正交放置在放大器的一側。如圖6所示,這種類型的包裝設計大大增加了整個PCB的占用面積。

 
A better option is a rectangular package that has a “flowthrough” audio signal design. Figure 7 illustrates how the analog input signals come into the amplifier on one side of the chip; amplification of the audio signal takes place on the opposite side of the amplifier, where the signals are then delivered into external output filters.
 
帶有“流式”音頻信號設計的正方形包裝是更好的選擇。圖7說明了模擬輸入信號如何進入芯片一側的放大器;音頻信號的放大發生在放大器的另一側,信號隨后被傳送到外部輸出濾波器中。

The TPA6304-Q1 audio amplifier uses a 2.1-MHz highswitching- frequency Class-D amplifier technology that features TI Burr-Brown™ technology. By combining 3.3-μH metal alloy inductors and a flow-through package design, the TPA6304-Q1 delivers a four-channel automotive Class-D amplifier solution size that measures only 17 mm by 16 mm. See Figure 8.

TPA6304-Q1音頻放大器使用具有TI Burr-Brown™技術的2.1-MHz高開關頻率D類放大器技術。TPA6304-Q1通過結合3.3-μH金屬合金電感器和直流包裝設計,可以提供一個尺寸只有17 mm x 16 mm的4通道汽車D類放大器解決方案。見圖8。

 
The TPA6304-Q1, including all of the passive electronic components for the full system solution implementation, is even smaller than the traditional Class-AB amplifier by itself, as shown in Figure 9.
 
TPA6304-Q1(包括用于整體系統解決方案的所有無源電子元件)比傳統的AB類放大器還要小,如圖9所示。


Conclusion
結論

The more electronics added to a car, the more the overall heat signature increases in an already tightly confined space behind the dashboard. Thus, the challenge for automotive audio hardware designers is to implement smaller and smaller audio solutions with lower and lower heat dissipation. Audio amplifier efficiency will only become more important in the future of infotainment system design.

汽車上安裝的電子器件越多,儀表盤后狹小空間內的整體熱量就越來越高。因此,汽車音頻硬件設計人員面臨的挑戰是實現更小、散熱更低的音頻解決方案。音頻放大器的效率只會在未來的信息娛樂系統設計中變得更加重要。
The TPA6304-Q1 makes replacing a Class-AB automotive audio amplifier easy. Its 2.1-MHz switching frequency and tiny system solution size allow you to achieve Class-D efficiency at a Class-AB system cost.
TPA6304-Q1可輕易取代AB類汽車音頻放大器。TPA6304-Q1的2.1-MHz開關頻率和小型系統解決方案尺寸可以讓你以AB類系統成本實現D類效率。


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