David Baker, Senior Staff Engineer, Class D Audio Systems, at Infineon Technologies
The previous article in this 4-part series discussed the challenges of designing low-power audio equipment and showed how Infineon’s MERUS™ multilevel class D amplifiers could be used to overcome them.
This third article looks at the figures of merit for audio amplifiers used in high-power audio applications (250+ W), like theatres, concert venues, mobile audio, and subwoofer applications. Furthermore, it discusses the features and benefits of highly integrated class D amplifier solutions from Infineon which audio designers can use to achieve them.
Figures of merit for class D amplifiers in high-power audio applications
Two of the most important figures of merit (FoM) for a high-power audio amplifier are noise and total harmonic distortion (THD).
When using a class D topology, these are caused by non-ideal circuit behavior like finite switching times, over- and undershoots, and variations in power supply voltages. Minimizing the impact of these is the responsibility of the error amplifier, which does so by applying a correction factor after comparing the audio input and output signals.
Unfortunately, the operation of a Class D amplifier is analogous to a switched-mode power supply, which can generate high levels of switching noise. Finding class D amplifier solutions that reject this switching noise can be difficult.
Highly integrated class D solution
Infineon’s MERUS™ IRS2092 class D audio amplifier driver with PWM modulator is a highly integrated device that features all the building blocks of a practical class D solution in a single package (Figure 1).
Level shifting and MOSFET switching
A class-D topology requires the input and outputs to be located in close proximity to one another. In a discrete implementation, a designer must determine the best way to isolate the noise-sensitive analog input part of the circuit from the noise produced by the switching output stage. Achieving electrical isolation between the two circuits is the most difficult challenge in an integrated solution.
Infineon’s MERUS™ IRS2092 uses a proprietary junction-isolation technique to ensure sufficient noise isolation. Once the error amplifier has processed the audio signal so that the shape of the output signal is proportional to the shape of the input, the comparator uses the analog signal to generate a PWM (pulse-width-modulation) signal. The PWM comparator in the MERUS™ IRS2092 does this with a short propagation delay, allowing greater flexibility when designing an optimized feedback loop.
Afterward, the next challenge is to transmit the PWM signal from the quiet error amplifier domain to the noisier switching stage. A high-voltage shifter level shifts the switching signal to a different floating potential, allowing the PWM signal to be accurate regardless of any voltage difference on each side (similar to the operation of an ideal differential amplifier).
Minimizing dead time maximizes amplifier linearity safely
The PWM signal from the comparator (which is referred to ground) is sent to the gate-drive stage, which down-level-shifts it to the gate-drive signal levels. These levels are referenced to the voltage potentials powering the high-side and low-side MOSFETs. At this stage, dead time is inserted between ‘on’ states to prevent the high-and low-side MOSFETs from turning on simultaneously.
Precise gate control is critical to achieving excellent audio performance. Therefore, the gate driver must have low pulse-width distortion by carefully matching the high- and low-side gate driver stages. Minimizing dead time duration is crucial for the linearity performance of the amplifier. Dead time insertion is regarded as the most critical part of the switching-stage design in a class-D amplifier, preventing shoot-through, thereby ensuring safe circuit operation. However, if incorrectly performed, it can introduce nonlinear behavior that causes signal distortion. So, trading off THD performance against an adequate safety margin is challenging for audio designers.
The MERUS™ IRS2092 has built-in guaranteed dead time control, which designers can adjust depending on the switching MOSFET selected. Compared to using an external design approach to managing dead time, integrated dead time insertion (with guaranteed duration) means designers do not have to consider the worst-case scenario for their application. The MERUS™ IRS2092 still exhibits an excellent THD performance of 0.005 percent, as shown in Figure 2.
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Figure 2. THD performance of MERUS™ IRS2092 class D audio amplifier
Protecting against overload conditions
Power dissipation in a MOSFET is proportional to the square of its load current, so protection circuits usually monitor the load current to prevent MOSFET failure if an overload condition occurs.
An external shunt resistor is commonly used for this purpose, but the choice resistor selection and noise filtering are critical to implementing this correctly. This takes time while also increasing cost and making the solution more complex. The protection circuit is also required to mitigate against the additional switching noise caused by stray inductances in the current-loop path of the power stage.
The MERUS™ IRS2092 features integrated overload protection, which is determined by the on-resistance of the MOSFET. This circuitry monitors the output current and shuts down the PWM signal if a predetermined threshold value is exceeded. In addition, the large positive temperature coefficient of the MOSFET on-resistance reduces the over-current threshold with the junction temperature, further improving amplifier safety.
External error amplifier for applications requiring additional design flexibility
Some applications demand exceptionally low noise and distortion performance and require a class D amplifier solution that enables the use of an external, high-grade audio-quality error amplifier.
Infineon’s MERUS™ IRS20957 is a high-voltage, high-speed MOSFET driver with a floating PWM input for class D audio amplifier applications. Bi-directional current sensing can detect over-current conditions during positive and negative load currents without using external shunt resistors. It also features an integrated protection control block that provides a secure protection sequence against over-current conditions, with a programmable reset timer. In addition, the amplifier’s internal dead-time generation block enables accurate gate switching and optimum dead-time setting for better audio performance, including lower THD and noise floor. The PWM input and protection logic is constructed on a floating well for convenient use in a half-bridge configuration (Figure 3).
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Figure 3. Typical application circuit for Infineon’s MERUS™ IRS20957 class D audio amplifier IC
High-power audio reference design
The IRAUDAMP9 reference design (Figure 4) is a single-channel 1.7-kW (@ 2Ω load) half-bridge Class D audio power amplifier, which demonstrates the capabilities of the MERUS™ IRS2092S Class D audio controller and uses an external gate buffer to implement protection circuitry. The design has an optimum PCB layout featuring two pairs of StrongIRFET™ IRFB4227 TO-220 MOSFETs. For a convenient setup, it provides all the required power supplies, and this single-channel design is capable of delivering higher than its rated power if fitted with a heat sink (Rth < 2° C/W).
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Figure 4. Typical application circuit for Infineon’s MERUS™ IRS20957 class D audio amplifier IC
This article discussed the critical figures of merit for high-power audio applications – low THD and noise, before presenting the features and benefits of the MERUS™ IRS2092 and IRS20957 class D amplifier solutions from Infineon, which can be used to design audio applications that require hundreds of watts up to multi-kilowatts of power including AV receivers, home theatre systems, powered speakers mobile audio, sub-woofers, and musical instrument amplifiers.
The final article in this 4-part series discusses challenges presented by mid-power audio applications (50-250 W) before discussing Infineon’s vision of the future for class D audio. Watch out for it.
For more information, please visit our MERUS™ class D audio amplifier IC website. Click here.