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Posted on 17 July 2019

Class D Audio Amplifier Matches Class AB With Higher Efficiency

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Class D audio amplifiers are now geared up to replace class AB

An integrated high voltage audio driver facilitates the design and construction of a high power class D audio amplifier on a single layer printed circuit board (PCB) that outperforms the traditional linear class AB solution. Additionally, the high voltage class D driver implements a self-oscillating PWM topology that offers adequate power supply rejection ratio to achieve high performance with unregulated power supply.

Although, many high performance medium and high power audio applications prefer to use class AB designs for linearity, advances in semiconductor process technologies and circuit techniques are making class D solutions attractive for a variety of high power low noise audio applications. As a result, class D audio amplifiers are proliferating in the world of high fidelity sound equipments with the ability to handle hundreds of Watts of power with higher efficiency and linearity. And simultaneously attaining total harmonic distortion (THD) that is far below 0.05 percent.

By Jun Honda, Manuel Rodriguez and Wenduo Liu, International Rectifier, El Segundo, Calif.

 

However, designing and building a high power class D audio amplifier that matches the performance of a traditional linear class AB amplifier with fewer components and a single sided printed circuit board (PCB) design is not a simple task. Especially, when using traditional through-hole components that contribute to switching noise due to increased stray inductances and resistances.

To address these challenges and assist the designer in the design and development of a high power class D audio amplifier using through-hole components on a single sided PCB board, International Rectifier has developed an integrated high voltage class D audio driver circuit with features and functions tailored for achieving higher noise immunity. In addition, power MOSFETs have been specifically tailored for this application. Called digital audio MOSFETs, its die size and parameters have been optimized to obtain the best performance from a class D audio amplifier, while concurrently reducing the size and the cost of the solution 1.

Structurally, to achieve low on-state resistance per silicon area, each digital audio MOSFET consists of two power MOSFET switches connected in half-bridge configuration. In addition, its gate charge Qg, body diode reverse recovery Qrr and internal gate resistance RG(int), as well as packaging, are all optimized to enhance class D audio amplifier performance in areas like efficiency, THD and EMI. Because the parameters will vary with die size, the designer must select the most appropriate MOSFET for the desired output power. To simplify the MOSFET selection process, a table has been created that identifies four different digital audio MOSFETs with different set of combinations of these parameters along with drain-source breakdown voltage (BVDSS) for different output power levels. These include IRFI4024H-117P, IRFI4212H-117P, IRFI4019H-117P and IRFI4020H- 117P as shown in Table 1.

Optimized digital audio MOSFETs with typical key paremeters

Hence, when the integrated high voltage driver is combined with the most favorable power MOSFETs, the class D audio amplifier design is capable of delivering up to 500 W audio power on a single layer PCB with performance that is comparable to class AB solution but with higher efficiency.

Integrated Audio Driver

Designed to handle both half- and full-bridge topologies, the new audio driver IRS2092 integrates on-chip four key functions required for high performance class D audio amplifier design 2. These include error amplifier, PWM comparator, gate driver and robust protection circuitry, as shown in Figure 1. The built-in protection circuitry simplifies the complex task of overload protection with self-reset control and under-voltage lockout protection (UVLO). In addition, the chip delivers programmable preset dead-time for improved THD.

Error amplifier, PWM comparator, gate driver and protection circuitry on class D audio driver chip

While the error amplifier minimizes the noise generated by the switching amplifier and the fluctuations of the power supply, the gate driver also plays an important role in achieving high audio performance. Besides maintaining a close match between high- and low-side gate driver stages, it also controls the dead-time to prevent simultaneous “on” states in high- and low-side MOSFETs. By allowing designers to program and preset dead-time as per the MOSFETs selected, the integrated driver IC prevents any shoot-through and ensures safe operation of the amplifier. Since the dead-time is set based on the voltage applied to the DT pin of the IC, the audio driver circuit makes this selection easy and reliable by employing only two external resistors connected to pin DT, as shown in Figure 2.

For improved total harmonic distortion (THD), the dead-time is preset with resistors R1 and R2

In reality, the error amplifier compares the output audio signal with the input, and then filters the output using an external LC lowpass filter. Also, to ensure that the output does not contribute any noise to the input due to close proximity of the two stages on-chip, the input and output circuits are isolated using proprietary high voltage stateof- the-art junction isolation technology.

Capable of handling switching frequencies up to 800 kHz, the driver’s analog PWM modulator allows self oscillating PWM modulation for better performance and robust design. In essence, a self oscillating PWM modulator is an analog version of a second order sigma-delta modulation having a class D switching stage inside the loop. The benefit is that all the error in the audible frequency range is shifted to the inaudible upper frequency range by the nature of its operation.

Regarding protection, both low- and high-side MOSFETs are protected from overload condition via current sensing across the on-resistance of the MOSFETs. For that, the voltage across drain-to-source during the on-state of the MOSFET is measured. When this voltage gets higher than the threshold, IRS2092 turns off the MOSFET. While the voltage setting on the OCSET pin programs the threshold for lowside current sensing, the high-side setting is accomplished by CSH and VS pins. However, the threshold for CSH is internally set at 1.2 V. An external resistive divider is used to program this threshold. In addition, an external reverse blocking diode is employed to block the high voltage feeding the CSH pin. Consequently, the forward voltage drop of 0.6 V across the blocking diode is the minimum threshold for the high-side MOSFET.

To further take full advantage of the high noise immunity of the driver chip for attaining optimum audio performance, the class D circuit board is designed to minimize trace impedances and curb noise coupling between analog and switching sections. Accordingly, the PCB design ensures that the analog signal ground is separated from the switching stage, as well as the power ground.

Comparing The Performance

Combining the integrated audio driver IRS2092 with the appropriate optimized digital audio power MOSFETs, a class D audio amplifier reference design has been developed using the half-bridge system shown in Figure 3. Because this design is scalable, the output power can be scaled from 25 W per channel to 250 W per channel by simply changing the output MOSFETs with appropriate voltage ratings. To further assist the user in picking the right MOSFET for the desired output and load impedance, Table 2 has been produced that identifies the optimized MOSFETs for a specific output power level.

Simplified half-bridge system level class D audio amplifier circuit with scalable output power to 500 W.

Scalling the output from 25W to 500W using a variety of optimized digital audio MOSFETs

To demonstrate its capability, the THD plus noise (THD+N) and efficiency performance of the scalable class D reference design is compared with that of a class AB design using similar test conditions. Thus, measurements conducted with sinusoidal signal frequency of 1 kHz at 1 V rms and 4 Ohms load impedance shows that the THD+N for class D remains far below 0.03% over a wide range of output power. The switching frequency used here is 400 kHz and the supply voltage is ±35 V. Utilizing the same test parameters, the class D performance is then compared with class AB, which in this case is a brand name amplifier. The test results are displayed in Figure 4. From this comparative figure, it is seen that for output power levels between 50 and 100 W, the THD+N performance for class D is comparable with class AB. Below 50 W, class D outperforms class AB, while above about 150 W the performance for both the amplifiers drops rapidly due to clipping.

Comparing THD+N performance of class D amplifier with class AB for a wide output power range

Although, the above measurements were conducted at 1 kHz, it was observed that the distortion performance of the class D audio amplifier design is equally good over the entire audio band. As demonstrated in Figure 5, THD+N versus frequency test indicates that the distortion is low (<0.03%) and remains consistent over the entire audio range of 20 Hz to 20 kHz, even as the output power is increased from 10 W per channel to 50 W per channel with 4 ohms load impedance.

This test indicates that the THD+N performance for the class D remains low

Similar test conducted for noise indicates that the noise floor for class D remains below -80 dBv over the entire audio range (Figure 6).

The noise floor for class D amplifier remains below -80 dBv over the entire audio range

Regarding efficiency, the performance for class D audio amplifier is high because the combined power (conduction + switching) losses have been minimized using the optimized digital MOSFETs and the driver circuit. Hence, the measured performance for the reference design in figure 3 tailored for a 50 W per channel stereo amplifier driving 4 Ohm load is about 90%. This performance is consistent even as the output power is scaled to 250 W per channel as illustrated in Figure 7.

As illustrated, the class D amplifier’s efficiency consistently remains near 90 percent

Similarly, employing the brand name amplifier, class AB performance was measured under similar test conditions and presented in Figure 8.

From this figure, it is observed that class AB efficiency is high (about 65 percent) around 150 W output power

From figure 8, it is observed that class AB efficiency is high (about 65%) around 150 W output power, but much lower than class D efficiency. Plus, unlike class D, the class AB efficiency drops significantly as the output power drops. As can been seen from figure 8 that class AB efficiency at 100 W is slightly over 50% and further drops to around 35% at 50 W. And this downward trend continues almost linearly, declining to 20% efficiency around 20 W output. By compari- son, class D accomplishes dramatic improvements in efficiency performance, especially at lower power levels. For instance, it maintains efficiency over 80% all the way down to 40 W as illustrated in figure 7. The same figure shows that between 20 W and 40 W output, class D design offers >70% efficiency. In other words, class D’s efficiency performance is far superior to class AB, especially at power levels that are practically used.

Also, the power supply rejection ratio (PSRR) measured for the class D amplifier was -65 dB at 1 kHz signal. This high PSRR is attributed to the self-oscillating frequency of the driver. And it enables the class D amplifier to offer high performance even with unregulated power supplies.

Conclusion

Combining the attributes of a dedicated high voltage audio driver circuit with optimized digital audio power MOSFETs, a high power class D stereo amplifier is built whose output is scalable. And its performance is comparable to a class AB amplifier, but with improved efficiency. In addition, it can deliver high performance with unregulated power supplies. That means, high power class D audio amplifiers are now geared up to replace class AB amplifiers in high performance audio equipments.

 

References:

1. “Choosing Right MOSFET Parameters For Class D Audio Power Amplifiers”, By Jun Honda, International Rectifier, El Segundo, Calif.
2. “High Voltage Class D Audio Driver Realizes 500 W on a Single Layer Board”, By Manuel Rodriguez and Jun Honda, International Rectifier Corp., El Segundo, Calif.
3. Class D Audio Power Amplifier Reference Design IRAUDAMP7D.

 

About The Authors

Jun Honda is a senior staff engineer in International Rectifier’s Consumer- IC design center in El Segundo, Calif. Jun has more than 15 years of experience in designing consumer, as well as professional audio circuits, including class D audio amplifiers.

Manuel Rodriguez is a senior systems engineer for Class D Audio at International Rectifier’s headquarters in El Segundo, Calif. Manuel has over 20 years of experience in designing audio power amplifiers and switching mode power supplies for major audio manufacturers, where he designed car and home audio amplifier products for leading brands. Manuel began consulting for IR in June 2005 before joining the company as an employee in 2007.

Wenduo Liu is a systems applications engineer in International Rectifier’s Emerging Technology Group in El Segundo, Calif. Wenduo has more than 10 years of experience in power electronics. His research focuses on integrated high density power modules, including class D audio amplifiers, iPOWIR and GaN based power devices.

 

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2 Responses

  1. avatar mk says:

    Class D has finally come of age!  After having Memphis Audio SA4.50(s) which utilizes the IRS2093M chipset I can say this Class D amplifier has an almost "tube" like sound quality.  Unfortunately the SA4.50 improperly implemented the chip by crippling it with an input threshold.  At low volume the amplfier turns off despite having the 12 volt trigger on.  This unfortunate fatal flaw forced me to remove the Memphis amps but I continue to search for any other automotive 4-ch amplifier which uses this chipset.   If anyone reading this comment is aware of any other car audio amp using the IRS2093M please let me know.

    mk

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    • avatar mk says:

      Btw, the 2093 is a 4-ch version of the 2092 but as I understand it, the performance is the same. 

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