Audio amplifiers are certainly the most interesting topic for the broadest range of engineers and enthusiasts. There is a vast number of different designs available, based on many different technologies. There are still amplifier designs using vacuum tubes because of that special “something” they have. Indeed, because of their popularity, terms such as the “warmth,” “presence,” “clarity” can be heard very often in the world of audio amplifiers. However, we’ll leave those to enthusiasts and deal with the facts in terms of THD, SNR, and similar. More precisely, we’ll focus on so-called constant-voltage or high-voltage sound distribution systems. High-voltage (HV) sound distribution systems are commonly used for public announcements (public address, PA) in hospitals, train and bus stations, airports, and similar places where sound distribution over long distances is required.
High-voltage sound distribution systems
A typical audio amplifier contains a low impedance output stage that can drive a load of 4 to 8 Ω. Depending on its rated power, the current through the load can reach high values, which requires the use of lower wire gauge numbers (thicker wires with less resistance). This is not a problem for shorter distances, but when covering larger areas, increased cost of the audio installation becomes an issue. Besides, lengthy cabling installations can cause significant power losses when used with low voltage (LV) output amplifiers due to increased impedance (Figure 1).
On the other hand, specialized HV sound distribution systems are designed to deliver 70 to 100 V at their output, depending on the region in which they are used: 100-volt HV systems are standard in Europe, while in the US and Canada, the output voltage is rated to 70.7 V. To avoid losses, HV sound distribution systems use the same method used in electrical power transmission: a higher voltage allows a lower current rating for the same amount of power. This allows higher wire gauge (AWG) numbers to be used, thus reducing the overall installation costs. Adding more speakers does not affect the output voltage of the amplifier, as it acts as a constant voltage source. Because of that, HV sound distribution systems are also known as constant-voltage sound distribution systems, and they are not that sensitive to the increased impedance of the installation cabling.
HV systems are not perfect; they have some drawbacks, as well. The main problem is their complexity: most HV speakers require step-down impedance-matching transformers, while some amplifier designs may include step-up transformers at their output stage. Besides increasing the costs, transformers may limit the frequency response, add ringing distortion to the audio signal, and cause additional power losses. However, with the advancement of the semiconductors production technology and emerging of the Class D amplifier topology, most of these problems have been significantly reduced or eliminated. Specialized single-chip solutions such as the IRS2452AM, an integrated two-channel Class D power amplifier from Infineon, allow for highly efficient and cost-effective HV sound distribution system solutions, eliminating the need for a step-up transformer at the output (Figure 2).
Efficient HV sound distribution system design with the Infineon IRS2452AM IC
The IRS2452AM IC from Infineon is a dual-channel Class D audio amplifier IC, rated at 400 V. It features an open-access front-end structure, allowing a lot of flexibility in choosing the PWM modulating topology. It supports both single-ended (unbalanced) and differential (balanced) input signals. Instead of power switching MOSFETs on the output stage, the IRS2452AM IC integrates only the half-bridge gate MOSFET drivers with automatic dead-time insertion. This allows displacing the heat source away from the IC, as well as greater design flexibility: the circuit designer is free to choose the most appropriate power switching MOSFETs for a specific application.
The analog inputs and low-voltage logic components are isolated from the main power supply, allowing high supply voltages between ±80 V and ±180 V to be used. Combined with high-voltage switching MOSFETs such as the IPP60R120C7 for 500 W solutions (or IPP60R180C7 for 250 W solutions), the IRS2452AM is the perfect choice for an efficient transformer-less 2ch HV sound distribution amplifier. Infineon offers the IRAUDAMP evaluation board with the reference design that is capable of delivering up to 500 W (100 VRMS) to a 20 Ω load, which is a substantial amount of power for many different applications. Some other specifications of the reference design very well demonstrate the qualities and the efficiency of the amplifier circuit built around the IRS2452AM IC:
|Idling supply current||±80 mA||No input signal, ±148.3 V|
|THD+N||0.01 %||@ 1 kHz, 100 W, 20 Ω|
|Residual noise||280 mVRMS||IHF-A weighted, AES-17 filter|
|Signal to noise ratio||110 dB|
You can find more information on the IRAUDAMP23 reference design on this LINK.
Infineon IRS2452AM key features
The IRS2452AM features many integrated functions, simplifying the design, and reducing the BOM cost. Some of its key features include lossless over-current protection, as well as under-voltage protection and over-temperature. The IC also integrates a high-precision dead-time insertion block with selectable timings for improved linearity and lower Total Harmonic Distortion (THD). Due to its ability to be powered directly by ±200 V (max), it has plenty of headroom to drive 100 V audio systems without having to use the BTL mode.
In order to compensate for the limited MOSFET transition time, a short dead-time interval must be inserted before each ON state, preventing MOSFETS from being conductive at the same time, thus causing large shoot-through currents. Dead-time is also one of the most determining factors for the nonlinearity and THD performance of the amplifier, and it has to be selected very carefully. The IRS2452AM IC integrates a high-precision dead-time insertion block which offers several different timing presets, within the range from 45 to 105 ns. Unlike an external dead-time circuit design that would require expensive low-tolerance components, the integrated dead-time block uses a simple voltage divider for preset selection, reducing the design complexity and associated costs.
Over-current protection (OCP) monitors load conditions and shuts down switching operation if the load current exceeds the preset threshold level. The IRS2452AM IC does not require dedicated sensing resistors; it utilizes the RDSON resistance of the switching MOSFETs instead, introducing no additional power losses and simplifying the design.
Under-voltage protection (UVP) ensures that the switching MOSFETs are not in the partial ON state, by monitoring both the low-side and high-side gate bias supplies. If gate bias supplies become lower than the UVLO threshold, the UVP block turns switching MOSFETs OFF, protecting them from overheating in such situations.
Over-temperature protection (OTP) block prevents overheating of the switching components. Since the IRS2452AM IC uses external switching MOSFETs, the temperature is monitored via the external PTC resistor. The PTC resistor can be connected between the OTP and COM pins and mounted on the MOSFETs heatsink. The OTP pin sources 0.6 mA through the PTC; if the voltage on the OTP pin (referenced to COM pin voltage) exceeds 2.8 V due to increased PTC resistance, the OTP gets activated.
The CSD pin controls the operational mode of the IRS2452AM. This pin can be used to shut down the IC, to set it in local oscillation mode, and to set it to normal mode. A voltage level applied to this pin determines the operating mode of the IC. The local oscillation mode ensures that the switching circuitry is working and that the respective capacitors are fully charged before the gate driver starts switching the external MOSFETs, ensuring a pop-less startup.
Some other features of the IRS2452AM IC include Bridge Tied Load (BTL) mode, as well as the external clock synchronization with in-phase or out-of-phase channels clocking.
PCB layout design and useful design references
When designing a Class D power amplifier, one of the biggest challenges is to prevent switching noise from interfering with the audio signal. The PCB designer must ensure that the low signal-to-noise ratio (SNR) of the IC is preserved in the PCB design, and that noisy currents remain within their own domains. The IRAUDAMP23 reference design demonstrates how to set a low-noise PCB layout strategy properly. Some additional notes about the PCB layout and GND planes partitioning can be found in the Infineon’s Class D audio IC IRS2452AM functional description application note. It also contains a lot of other in-depth explanations, such as the calculations for the second-order LP reconstruction filter at the output. You can download the IRS2452AM application note from the following LINK.
Class D amplifiers offer numerous advantages over the traditional Class AB amplifier topology. The IRS2452AM Class D integrated amplifier features inherently high efficiency, excellent bandwidth, more compact design, and allows reducing BOM costs significantly, thanks to its many built-in features. With its ability to stand up to 400 V (± 200 V dual PSU), it enables the development of a directly driven HV sound distribution system, eliminating the need for a bulky and expensive transformer at the output stage. If you are developing a high-performance, cost-effective HV sound distribution system, make sure you consider all the benefits that Infineon’s IRS2452AM Class D Integrated Amplifier has to offer.