This article refers to the address: http://
Class D amplifiers are key components of modern consumer TV and audio products that enable small size and high power audio output, but building a high efficiency Class D amplifier solution is not straightforward. In particular, the design of the power switching stage is a big challenge for audio design engineers who are not familiar with power electronics design.Off-the-shelf Class D amplifier chips can solve these power design challenges, but they also limit the ability to optimize other amplifier performance, and design engineers will not be able to differentiate their products by enhancing performance.
The ideal situation is to provide audio design engineers with a one-stop solution for power electronics, allowing them to focus on their expertise in areas such as feedback paths and output filters.
Class D amplifier design flexibility
To help understand how Class D amplifier solutions provide these off-the-shelf functionality and design flexibility, Figure 1 shows the functional block diagram of a generic Class D amplifier. The input audio signal is compared to a high frequency sawtooth wave to produce a pulse width modulated square wave representative of the input signal. The sawtooth frequency is set to be much higher than the highest audio signal, typically around 400 kHz, to reduce distortion and simplify the output filter design.
| Figure 1: Functional block diagram of a generic Class D amplifier. |
The pulsed-modulated audio signal is then used to drive the amplifier output stage, which may be a full-bridge MOSFET array or a half-bridge MOSFET array. The choice of output topology depends on system requirements including cost, output power, power supply design, and signal characteristics. For example, a half-bridge output stage requires both positive and negative voltage rails, while a full-bridge output stage circuit can operate from a single supply and can produce greater output power at a given switching rate.
In either case, the output MOSFET characteristics are optimized for Class D audio amplifiers to maximize efficiency and ensure low total harmonic distortion plus noise (THD+N) and EMI. This requires low on-resistance to achieve high power density in the final product while also requiring optimized gate charging and body diode reverse recovery characteristics for fast, efficient switching.
The amplified audio signal is contained in the square wave signal output from the MOSFET bridge. Therefore, a low-pass filter is required to filter out the frequency outside the audio, and a pure audio signal is recovered to drive the speaker.
Switch stage circuit design challenges
For design engineers who are not familiar with the electronic design principles of switching power supplies, the Class D amplifier power switching stage design that generates gate drive signals for MOSFET bridges is the biggest challenge they face. For superior audio performance, precise gate control is necessary, which requires small pulse width distortion and good matching between the high-end and low-side drive signals to maintain good linearity.
The insertion of dead time is a particularly daunting challenge. In order to damage the output MOSFET for the shoot-through current, the dead time must be inserted, but the insertion dead time can introduce nonlinearity into the amplifier characteristics, so design engineers often fail to achieve a satisfactory balance between audio fidelity and safety margin. Other protection features for gate drivers and MOSFETs, such as over-temperature protection and over-voltage protection, will also present more complex challenges, requiring design engineers to have extensive switching power electronics design techniques.
It is very difficult to make a design perfect in the face of so many challenges, because any defect can cause catastrophic failures that are difficult to analyze and correct.
To help design engineers overcome these difficulties and quickly deliver successful Class D amplifier products, International Rectifier (IR) has introduced the complete MOSFET gate driver IRS2092 with built-in protection. The device also integrates an error amplifier and PWM comparator, allowing designers to quickly implement audio solutions based on Class D amplifiers.
There are other important amplifier features that are closely related to the power switch stage circuit design, including the elimination of clicks during startup and shutdown. The IRS2092 also integrates these features internally, further reducing design overhead and device count. The solution addresses the power electronics design challenges associated with Class D amplifiers and provides design engineers with the assurance that professional audio technology can further improve product performance.
The IRS2092 is also designed to be flexible enough for design engineers to determine output filter characteristics and to select feedback points in many locations to achieve optimal audio performance and amplifiers at cost and device count. stability.
Differentiated design of class D amplifier
Customized feedback loop
The feedback loop design is an important feature to achieve differentiation of Class D amplifiers. In some cases, an open-loop configuration can provide satisfactory performance, but the timing errors inherent in the amplifier increase distortion and noise. Negative feedback can be used to solve this problem well. The easiest way is to feed back a portion of the switching signal to the input of the error amplifier and pre-process it with a passive RC low-pass filter. Many D-class amplifier chips on the market use this feedback method. However, design engineers may wish to further optimize the amplifier's distortion performance by taking feedback signals closer to the output to reduce load dependence. For example, the feedback point may be at the farthest point of the audio filter output, ie just in front of the speaker. Some design engineers may further improve performance by including speaker electronics in the feedback loop because the effects of these devices can be considered part of the amplifier's output filter. Still other designs may use a combination of two feedback signals from the switching node and the filter output.
The IRS2092 gate driver chip allows design engineers to freely choose the feedback points that are considered optimal. With sensible feedback and compensation for stability, design engineers can achieve high-fidelity sound quality levels of harmonic distortion and noise (THD+N) performance.
2. Optimize output filter performance
Output filters have a large impact on overall efficiency, reliability, and audio performance. A simple LC filter is common, with a cutoff frequency just above the audio band and 4014 per 10 octaves of carrier rejection. In many applications, a simple filter can provide good enough performance by carefully controlling the impedance of the speaker. On the other hand, experienced design engineers may wish to use more complex filters (with higher order response characteristics, better notch characteristics or special carrier suppression methods) to further improve distortion and noise performance.
Since parameters such as the regulated switching frequency can be freely adjusted, the IRS2092 allows engineers to optimize the output filter design and select devices that provide the desired characteristics, such as high linearity and low DC resistance. An important benefit of programmable switching frequency is that it allows for a balance between management bandwidth and switching frequency. For example, if you use a subwoofer amplifier with a bandwidth of 200 Hz, the ability to set a lower switching frequency would be beneficial.
3. Maximize the damping coefficient
Since the design of the feedback and filter circuits affects the output impedance of the amplifier, the design engineer also needs to control the design of these circuits to achieve a sufficiently high damping coefficient. The damping factor is an important quality factor, especially for car audio systems with low load impedance. Design engineers must ensure that the speaker is strictly controlled to achieve acceptable performance in the car's environment. Careful control of the amplifier output and feedback design can achieve a damping factor greater than 100, which is usually sufficient for most audio amplifiers.
4. Scalability
An integrated chip solution with four basic Class D amplifier building blocks, such as the IRS2092, has the added benefit of being scalable to meet higher output power or additional channel requirements. It quickly expands the amplifier by defining an appropriate level of external MOSFET, re-optimizing dead time and overload protection thresholds. By retaining the same basic design or even the same PCB design, this rapid expansion capability can reduce time-to-market and reduce design costs as end products evolve. In addition, multi-channel systems can be quickly implemented through multiple iterations of the driver-MOSFET combination. For example, cost-effective and compact 6.1 or 7.2 channel surround sound devices can be configured quickly and easily.
Class D Amplifier Reference Design
Class D audio amplifier chips not only solve design challenges, accelerate project completion, but also effectively reduce the number of components. To help design engineers take advantage of these advantages, International Rectifier has introduced a reference design that configures the IRS2092 gate driver chip and two IRF6645 DirectFET MOSFETs into a half-bridge configuration (Figure 2).
| Figure 2: Comparison of the design board (left) based on the A/B class amplifier and the reference design of the IR class D amplifier (right). |
With this reference design, the design engineer can build a 120W switching amplifier per channel on a 61cm 2 PCB area that is less than 16% of the reference design area of ​​a 120W Class A/B amplifier. Since the Class D amplifier does not require a heat sink during normal operation, the total volume reduction is also considerable. This complete Class D amplifier solution occupies only 109 cm 3 of space and is less than 6% of the A/B class amplifier.
In addition, this reference design can achieve a low THD+N performance of 0.005% while outputting 60W of power on a 4Ω load, with residual noise of only 170μV and a damping coefficient of 170 at 1kHz. With this functional base platform, design engineers are free to customize feedback and output filtering designs to provide optimal audio performance and overall cost for a specific application.
Rotary Dryer,Briquette Belt Dryer,Mesh Belt Dryer
Radiant Burners,Wood Pellet MillCo., Ltd. , http://www.coal-gasifiers.com