Worried about switching from Class AB to Class D? No need to worry

Aguilera

Worried about switching from Class AB to Class D? No need to worry [Copy link]

Have you ever thought about doing something big and meaningful but worried that it would be hard to do? Then, when you finally get the courage to do it, you look back and are surprised at how easy it was.
I recently talked to automotive audio design engineers about switching from traditional Class AB amplifiers to Class D amplifiers for car radio solutions, and they had the same concerns. Now let's talk about the two main issues I hear most often: the impact on printed circuit board (PCB) size and potential electromagnetic interference (EMI) issues.
Problem 1: Class D amplifiers take up more PCB space
Conventional Class D audio amplifiers use a switching frequency of about 400 kHz, requiring an 8.2-H or 10-H inductor to achieve the desired audio effect.
TI's TPA6304-Q1 Class D amplifier uses a 2.1-MHz switching frequency. The reduction in ripple current means that a smaller and lighter 3.3-H inductor can be used, as shown in Figure 1.

Figure 1: Inductor size vs. switching frequency
The TPA6304-Q1 uses TI’s latest mixed-signal manufacturing technology, and when used with a 3.3-μH inductor, the total size of the entire 4-channel amplifier solution (including all required passive components) shrinks to 272 mm2, as shown in Figure 2.

Figure 2: TPA6304-Q1 4-channel Class D amplifier
To this point, the entire TPA6304-Q1 solution shown in Figure 3 is smaller than a traditional Class AB amplifier.

Figure 3: Size comparison of TPA6304-Q1 Class D amplifier solution vs. Class AB amplifier
Issue 2: Class D amplifiers cause electromagnetic compatibility (EMC) issues
By nature, Class D audio amplifiers turn their outputs on and off on every clock cycle, while Class AB amplifiers do not. But this does not mean that Class D amplifiers cause electromagnetic compatibility (EMC) issues that cannot be addressed.
I would like to specifically review several ways that the TPA6304-Q1 amplifier design mitigates EMC issues:
· The TPA6304-Q1 amplifier design is highly optimized to handle overall EMC behavior. In addition, the 3.3-H inductor described earlier is part of the inductor-capacitor (LC) filter that helps minimize electromagnetic compatibility of high-speed switching transients on the Class D amplifier output stage.
· As shown in Figure 4, conventional Class D amplifiers that switch in the 400-kHz range generate harmonics that are directly within the AM band. These harmonics create interfering signals that desensitize AM receivers, thereby preventing AM broadcast station reception. Therefore, some type of EMI avoidance technique must be implemented on these 400-kHz Class D amplifier designs to mitigate the effects of these AM band harmonics.

Figure 4: Typical 400-kHz Class D amplifier harmonics
By operating at a higher 2.1-MHz switching frequency, the TPA6304-Q1 no longer needs to implement EMI avoidance techniques for the AM band because the TPA6304-Q1 provides significant margin above the AM band. This design also does not have any low-frequency spikes that would interfere with the AM band, as shown in Figure 5.

Figure 5: The high switching frequency of the TPA6304-Q1 is above the AM band
To prevent EMI issues with some PCB layout designs, the TPA6304-Q1 has implemented a proprietary spread spectrum technology developed by Kilby Labs. Figure 6 illustrates how this feature helps spread narrowband energy sources over a larger band, thereby reducing peak energy.

Figure 6: Spread spectrum technology background
ConclusionThe
TPA6304-Q1 2.1-MHz high switching frequency automotive Class D audio amplifier meets industry requirements for next-generation car radios and external amplifiers. In addition to reducing the thermal load in the system, the design of this amplifier also addresses the PCB size and EMC issues that are of great concern when converting from Class AB amplifiers to Class D amplifiers.