Engineer's Reference Manual (II): Class D Amplifier Design Tips

2. Class D amplifier heat dissipation considerations

　　Abstract: Class D amplifiers have higher efficiency and better thermal performance than Class AB amplifiers. However, when using Class D amplifiers, it is still necessary to carefully consider their heat dissipation. This application note analyzes the thermal performance of Class D amplifiers and illustrates the principles that should be followed in good design through several common examples.

　　Continuous sine waves and music

　　When evaluating Class D amplifier performance in the lab, a continuous sine wave is often used as the signal source. Although it is convenient to use a sine wave for measurement, such a measurement results in a worst-case thermal load for the amplifier. If a Class D amplifier is driven with a continuous sine wave close to the maximum output power, the amplifier will often go into thermal shutdown.

　　Common audio sources, including music and speech, often have RMS values ​​that are much lower than their peak output power. Typically, the ratio of peak to RMS power (i.e., crest factor) for speech is 12dB, while the crest factor for music is 18dB to 20dB. Figure 1 shows the waveforms of an audio signal and a sine wave in the time domain, with the results of measuring their RMS values ​​using an oscilloscope. Although the peak value of the audio signal is slightly higher than that of the sine wave, its RMS value is about half that of the sine wave. Similarly, the audio signal may have sudden changes, but as the measurement results show, its average value is still much lower than that of the sine wave. Although the audio signal may have a peak value similar to that of a sine wave, the thermal effect exhibited by the Class D amplifier is much lower than that of a sine wave. Therefore, when measuring the thermal performance of a system, it is best to use an actual audio signal rather than a sine wave as the signal source. If only a sine wave is available, the resulting thermal performance will be worse than that of the actual system.

　　Figure 1. The RMS value of a sine wave is higher than the RMS value of an audio signal, which means that a Class D amplifier will heat up more when tested with a sine wave.

　　PCB heat dissipation considerations

　　In the industry standard TQFN package, the exposed pad is the primary path for heat dissipation from the IC. For packages with an exposed pad on the bottom, the PCB and its copper are the primary heat dissipation channel for the Class D amplifier. As shown in Figure 2, the best way to mount a Class D amplifier to a common PCB is to follow these guidelines: Solder the exposed pad to a large copper block. Place as much copper as possible between the copper block and adjacent Class D amplifier pins and other components that are at the same potential. In this case, the copper is connected to the upper and lower right sides of the thermal pad (see Figure 2). The copper trace should be as wide as possible, as this will affect the overall thermal performance of the system.

　　Figure 2. The exposed pad is the primary heat dissipation path for Class D amplifiers in TQFN or TQFP packages.

　　The copper block connected to the exposed pad should be connected to other copper blocks on the back of the PCB with multiple vias. The copper block should have as large an area as possible while meeting the requirements of the system signal routing.

　　Try to widen all the connections to the device, which will help improve the heat dissipation performance of the system. Although the pins of the IC are not the main heat dissipation channel, there will still be a small amount of heat in actual application. In the PCB shown in Figure 3, wide connections are used to connect the output of the Class D amplifier to the two inductors on the right side of the figure. In this case, the copper core winding of the inductor can also provide an additional heat dissipation channel for the D amplifier. Although the improvement in overall thermal performance is less than 10%, such an improvement will bring two completely different results to the system - even if the system has ideal heat dissipation or severe heat generation.

　　Figure 3. The wide trace on the right side of the Class D amplifier helps conduct heat.

　　Auxiliary cooling

　　When Class D amplifiers are operated at higher ambient temperatures, adding an external heat sink can improve the thermal performance of the PCB. The heat sink must have the lowest thermal resistance possible to optimize heat dissipation. With an exposed pad on the bottom, the bottom of the PCB is often the thermal path with the lowest thermal resistance. The top of the IC is not the primary heat dissipation path for the device, so it is not cost-effective to install a heat sink there. Figure 4 shows a PCB surface mount heat sink (218 series, provided by Wakefield Engineering). This heat sink is soldered to the PCB and is an ideal choice for size, cost, assembly ease, and thermal performance.

　　Figure 4. When a Class D amplifier is operated at higher ambient temperatures, an SMT heat sink such as the one shown may be required (Image courtesy of Wakefield Engineering).

　　Thermal Calculation

　　The die temperature of a Class D amplifier can be estimated using some basic calculations. In this example, the temperature is calculated based on the following conditions:

　　TAM = +40°C

　　POUT = 16W

　　Efficiency (η) = 87%

　　ΘJA = 21°C/W

　　First, calculate the power dissipation of the Class D amplifier:

　　Then, the die temperature TC is calculated by the power consumption, and the formula is as follows:

　　Based on these data, it can be inferred that the device has relatively ideal performance when operating. Because the system rarely operates exactly at the ideal ambient temperature of +25°C, a reasonable estimate should be made based on the actual ambient temperature of the system.

　　Load impedance

　　The on-resistance of the MOSFET output stage of a Class D amplifier affects its efficiency and peak current capability. Reducing the peak current of the load reduces the I²R losses of the MOSFET, which improves efficiency. To reduce the peak current, the speaker with the highest impedance should be selected under the conditions of ensuring the output power, the voltage swing of the Class D amplifier, and the supply voltage limit, as shown in Figure 5. In this example, it is assumed that the output current of the Class D amplifier is 2A, and the supply voltage range is 5V to 24V. When the supply voltage is greater than or equal to 8V, the load current of 4Ω will reach 2A, and the corresponding maximum continuous output power is 8W. If the output power of 8W meets the requirements, a 12Ω speaker and a 15V supply voltage can be considered. At this time, the peak current is limited to 1.25A, and the corresponding maximum continuous output power is 9.4W. In addition, the efficiency of the 12Ω load is 10% to 15% higher than that of the 4Ω load, which reduces power consumption. The actual efficiency improvement varies with different Class D amplifiers. Although most speakers use 4Ω or 8Ω impedance, speakers with other impedances can also be used for more efficient heat dissipation.

　　Figure 5. Selecting the optimum impedance and supply voltage maximizes output power.

　　Another thing to note is the change in load impedance within the audio bandwidth. A speaker is a complex electromechanical system with multiple resonant elements. In other words, an 8Ω speaker presents an 8Ω impedance only within a very narrow frequency band. Over most of the audio bandwidth, the impedance will be greater than its nominal value, as shown in Figure 6. Over most of the audio bandwidth, the impedance of this speaker will be much greater than its nominal value of 8Ω. However, the presence of the tweeter and crossover network will reduce the impedance value. Therefore, the total impedance of the system must be considered to ensure adequate current drive capability and heat dissipation performance.

　　Figure 6. The impedance of an 8Ω, 13cm loudspeaker changes dramatically with frequency.

　　in conclusion

　　Class D amplifiers are much more efficient than Class AB amplifiers. Although this efficiency advantage reduces the requirements for heat dissipation design when designing a system, system heat dissipation cannot be completely ignored. However, using Class D amplifiers can make audio system design simpler if good design principles are followed and reasonable design goals are set.