Class D amplifiers use PWM modulation to improve EMI performance

Class D amplifiers attract the attention of design engineers with their high efficiency, which has been widely used in various battery-powered electronic devices. However, as long as Class D amplifiers are used in the system, designers may have to compromise on the following aspects, including EMI interference, high complexity, and high cost due to the need for more external components. .

For EMI problems, this is due to the inherent characteristics of the Class D amplifier's switching characteristics. For different applications, the demand for EMI performance may be different. This is a very critical indicator in many applications. Device providers have been looking for solutions to this problem. At the 14th International IC Symposium & Exhibition Shenzhen Station Spring Show, IC giant Maxim exhibited a new class of Class D amplifiers with ultra-low EMI interference.

The reason why this series of devices can achieve excellent EMI performance is that we have adopted three patented technologies, namely special modulation topology scheme, spread spectrum modulation technology and active radiation limiting technology, the company's multimedia business department. Introduced by Roderick Hogan, Business Development Manager for Audio Products.

In the traditional class D amplifier, PWM modulation technology is used. This technology is known for its high efficiency, but the problem is its inherent high-speed switching characteristics, which generate a lot of EMI interference, even with very sophisticated filters. In addition to these disturbances, sufficient EMI performance cannot be met, and another problem is poor sound quality.

To improve this problem, Maxim has adopted a new modulation topology scheme that eliminates the need for a filter with a large inductor, called the filterless modulation technique, which the company's MAX9700 series is. . Its modulation topology is shown in Figure 1.

Figure 1: Modulation topology of the MAX9700 filterless Class D amplifier.

According to Roderick Hogan, in this modulation structure, the comparator reference is sawtooth and compared to the audio input. The topology uses a double-half-bridge symmetrical structure, each with its own comparator modulator and amplifier. This design allows the two outputs to have the shortest on-time at the same time. This ensures a very low power consumption, especially due to the adoption of the double-half bridge, using the same chip consistency, achieving a perfect symmetric differential structure, so that the common mode interference signal is better offset, completely eliminating the external The LC ground filter reduces EMI interference while ensuring low power consumption and high sound quality.

Spread spectrum modulation

In Maxim's new Class D amplifier, another patented technology, spread spectrum modulation, is also used. In this technique, the switching frequency of the amplifier is randomly spread-modulated. Spreading does not affect the audio information, but extends the audio energy over a wider spectrum, rather than focusing on the switching frequency and its harmonics as before expansion. By spreading, the high frequency energy at the output is reduced, which greatly improves EMI performance. “Actually, the spreading factor does not need to be too high. According to our experiments, ±6% is the best value,” said Roderick Hogan. Figures 2 and 3 show the actual effect comparison before and after spreading. It can be seen from the figure that by spreading, the 3rd harmonic after spreading is already reduced to the base noise.

Figure 2: Switching frequency and its harmonic distribution characteristics before spreading.

Figure 3: Switching frequency and its harmonic distribution characteristics after spread spectrum.

In Maxim's Class D audio amplifiers, in addition to the above two patented technologies, active noise limiting techniques are also employed. Active radiation limiting means that the minimum pulse width of the amplifier is set by the active radiation limiting circuit. Combined with the cross-switching, rise/fall time and clock frequency control, the power spectrum generated during operation can be limited to one designation. The output power level is below. The goal is to reduce the spectrum to a level that allows the device to meet radiated limits without any external filtering and long external speaker connections.

In fact, the intention to use active radiation limits is also around reducing EMI interference. Because when the output power is high, even a few inches of speaker wiring, it acts like an antenna, radiating high energy, which seriously threatens EMI performance. At this time, simply changing the clock frequency is not very effective, but it is necessary to change the PWM waveform of the amplifier itself. That's why Maxim's Class D amplifiers (like the MAX9705) don't use pulsed waves but use sawtooth waves, Roderick Hogan said.

Based on the special technology of our patents above, we guarantee the unique EMI performance of our Class D amplifiers, allowing lead lengths of up to 20 inches, and engineers have great flexibility in the design process. However, Roderick Hogan also reminded that the designer still needs to pay attention to the cable length selection of the speaker in the system design, because in some systems, the EMI radiation caused by the long cable is not to be underestimated.

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