Product How-To: 350W + 350W Class D power amp in the size of an iPod
Introduction
In the previous era of audio amplifiers, performance was a trade-off for efficiency, size and thus cost. By using an output power device in a switching state, Class D topology brings the benefits of digital device evolution to audio power amplification.
The latest silicon technology with finer device geometry enables lower power loss and improves switching speed at the same time. Consequently, the newer Class D technology enables less heat, and better audio performance to enable a better amplifier. To demonstrate these benefits, this article illustrates a design with fully integrated control IC with application tailored power MOSFET, all in the size of an iPod.
Class AB: the founder of high efficient power amplification
Single-ended Class A was the dominant circuit topology used in audio signal amplification since Lee De Forest invented the triode in 1906. This simple circuit configuration offers good linearity, but with the device always conducting current the topology is very inefficient.
The single-ended Class A amplifier has 25% maximum efficiency at maximum output power. The most problematic characteristic of Class A in terms of efficiency was the large amount of idling current that kept high power consumption in the output power device even at no output power.
The double-ended Class B amplifier was invented to solve the idling power consumption deficiency by introducing a concept of switching two output devices to carry only half the audio cycles per device. Due to the elimination of idling current, the efficiency of Class B can reach 78.5% at maximum output power and no power consumption at idling.
A typical Class AB amplifier applies a small idling current (in the range of mA) through both devices. While similar in efficiency to Class B, Class AB eliminates crossover distortion by adding a conduction angle overwrap. Hence, Class AB became the dominant power amplification topology.
The efficiency of linear amplification is determined by its load condition and bus voltage, not from device parameters such as current gain hfe or transition frequency fT. The power loss is induced by the product of voltage across the power device and current flowing through the device. Because the topology is in the linear operation region, power losses are high and efficiency is typically less than 25%. Class AB audio amplifiers require large thermal management systems because more power is dissipated as heat than delivered to the speakers.
There are topologies with dynamic supply voltage modulation designed to obtain higher efficiency than Class AB, such as Class H and Class G. These approaches sacrifice audio performance and rely on switching power supply topologies.
Class D: the topology with no efficiency limitation
Class D topology introduces the pulse width modulation (PWM) concept, essentially eliminating linear mode operation. An ideal switching device would generate no power loss in either state used in this topology, therefore, delivering no power dissipation and offering 100% efficiency. The gain in the Class D switching stage is proportional to the duty cycle of the MOSFET, thus it can be controlled with great linearity
In Class D, the input audio signal is converted into a series of pulses whose instantaneous average value is proportional to the input signal. This binary signal controls the power MOSFET to create an amplified version of the PWM. An LC passive low pass filter removes high frequency carrier signal components and recovers amplified audio signal. It is interesting to realize that once the audio signal is converted to a PWM signal, the rest of signal path can be digital logics, making circuit integration and level shifting easier.
Figure 1: Class AB vs. Class D topology comparison
Class D topology is perfect in theory. It is free from non-linearity, meaning there is zero distortion. It is also 100% efficient, meaning there is no power dissipation. The greatest benefit from the text book perfection of Class D is that the degradation of performance comes from the figure of merit in the power device. For this reason, Class D keeps evolving with power MOSFET technology advancements.
MOSFET figure of merit revolution
Class D was invented decades ago when there were no power devices to achieve commercially viable performance figures. Sony introduced the first Class D audio amplifier product, the TA-N88 in 1979. It used a power JFET called Vertical FET (V-FET) switching at 500kHz. The FET was not easy to handle as it was a normally-on device. The large input capacitances necessitated a large gate driver stage with To-220 BJTs with heatsink. Yet it demonstrated what Class D can bring to audio amplification.
With today’s state-of-the-art MOSFET technology, a practical power MOSFET can achieve well above 90% power efficiency in the Class D output stage. One simple way to see how the device is close to the ideal power switching device is to take a look at an R*Qg figure of merit (FOM). On silicon technology, on resistance (RDS(ON)) and gate charge (Qg) oppose each other. In other words, the lower the RDS(ON), the higher the gate charge will be, hence slower the switching.
To achieve the highest efficiency in Class D amplifiers, conduction loss from RDS(ON) and switching loss from switching speed dictated by Qg need to be optimized. Figure 2 illustrates how an optimal die size for a given output power is chosen. The minimal power loss is at an optimal die size. As the device technology advances, the total power loss at the optimal point reduces and the die size gets smaller, allowing for better performance and smaller system size
Figure 2: Power MOSFET die size optimization
Improvement in RDS(ON) and Qg FOM indicates the advancements in power MOSFET technology. For example, let’s take a look at the two 100V rated devices that are a couple of generations a part. The IRF540 planer structure from the 1980s has 66m ohms with 55nC of Qg. The latest trench structure MOSFET IRF6665 is a 53m ohms device with 8.4nC gate charge. The FOMs are 3,300 and 445 respectively, showing the latest MOSFET has greater than 7 times better FOM. The newer MOSFET requires much lower gate drive effort to achieve the switching speed, consequently lower switching power loss.
Integrated Class D driver
To form a practical Class D amplifier, there are four essential functions; gate driver, level shifting, deadtime generation and under-voltage lockout protection (UVLO). Each of these functions is complex and involves a mixture of analog and logic circuitries. The high device counts encourages the integration of all four functions into a single IC.
In order to enjoy the benefits of state-of-the-art power MOSFETs, precise control of the gate drive signal is crucial. Stable deadtime control and level shifting to control the MOSFET according to the PWM signal from the modulator section are vital. Deadtime is a major source of non-linearity in Class D amplifiers. Insertion of deadtime increases distortion as it modulates the gain in the Class D power stage as it reduces the duty cycle of each MOSFET.
While motor drive inverters require 500ns to 1us deadtime range, a high-performance Class D amplifier has a stringent deadtime requirement of one tenth of these values. The amount of deadtime is primarily dictated by the switching speed of the MOSFET but in reality a design uses 2 to 3 times the amount of deadtime to deal with propagation variability in the gate driver stage. MOSFETs with better FOA require a smaller deadtime amount from the faster switching speed, therefore, contributing to better linearity.
Robust protection features are as important as the Class D loop controls. Gate driver ICs should offer under-voltage lock-out (UVLO) protection to protect the MOSFETs from operation in the linear mode that can damage them. Other protection features include thermal shutdown and over-current protection.
The nature of the power dissipation source in Class D in an over-loaded condition result from conduction loss from RDS(ON). Monitoring a voltage across the MOSFET while it is in an on state is a good way to sense over-loading conditions. This scheme does not take any additional power shunt device that degrades efficiency and requires a large footprint. A bonus feature from the RDS(ON) based current sensing is that sensitivity of load current detection increases with MOSFET temperature, making the over current protection much more robust. Design Example
Figure 3 is an example of a 2 channel 370W+370W Class D amplifier. This design features the IRF6775M DirectFET® from International Rectifier rated at 150V, 47m ohms, 25nC in a surface mountable package that enables small footprint while optimizing PCB layout.
Figure 3: Gate driver features (IRS2052M)
Although power dissipation of a Class D amplifier is significantly smaller compared to its conventional Class AB counterpart, thermal design is still not a negligible part of the effort. Unlike Class AB, efficiency is a function of the MOSFET temperature. The higher die temperature increases RDS(ON) , thus conduction loss in the MOSFET. On the other hand, a good portion of the switching loss comes from reverse recovery charge Qrr in the body diode which has a positive temperature coefficient. Higher junction temperature increases overshoot/undershoot due to the higher Qrr.
The IRS2052M integrates all four essential functional blocks along with clock oscillator and thermal protection. A clean THD+Noise vs. output power waveform in Figure 4 indicates successful noise management in the controller IC.
Figure 4: THD+N vs. Power
Thanks to the low Qg in the MOSFET making gate drive easier, the control IC can also deal with a noise sensitive analog section on the same die. In addition, the low Qg in the MOSFET allows low gate drive power so that all four gate driver stages fit in a small MLP package.
Figure 5: Picture of IRAUDAMP10
The resulting design example is a 4" x 2.8" board space that offers a complete design in the size of an iPod at the efficiency of 90% while providing comparable audio performance to an equivalent output power Class AB amplifier. The latest Class D design brings higher audio quality with superior power efficiency at lower system cost which makes this topology achieve ever close to the ideal amplifier.
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