Class D audio amplifier Application Note with Ferroxcube gapped toroid output filter Class D audio amplifier with Ferroxcube gapped toroid output filter The concept of a Class D amplifier has input signal shape but with larger bridge configuration. Each topology been around for a long time, however amplitude. And audio amplifier is spe- has pros and cons. In brief, a half bridge only fairly recently have they become cially design for reproducing audio fre- is potentially simpler,while a full bridge commonly used in consumer applica- quencies. is better in audio performance.The full tions. Due to improvements in the bridge topology requires two half- speed, power capacity and efficiency of Amplifier circuits are classified as A, B, bridge amplifiers, and thus more com- modern semiconductor devices, appli- AB and C for analog designs, and class ponents. cations using Class D amplifiers have D and E for switching devices. For the become affordable for the common analog classes, each type defines which A Class D amplifier works in the same person.The mainly benefit of this kind proportion of the input signal cycle is way as a PWM power supply, except of amplifier is the efficiency, the theo- used to switch on the amplifying device: that the reference signal is the audio retical maximum efficiency of a class D Class A 100% wave instead of the accurate voltage design is 100%, and over 90% is Class AB Between 50% and 100% reference. achieve in practice. Class B 50% Class C less than 50% Let’s start with an assumption that the Other benefits of these amplifiers are input signal is a standard audio line level the reduction in consumption, their The letter D used to designate the class signal.This audio line level signal is sinu- smaller size and the lower weight. D amplifier, is simply the next letter soidal with a frequency ranging from 20 Their advantages are obvious in low after the C, and does not stand for dig- Hz to 20Khz. This signal is compared power battery operated personal ital. Class D and E are sometimes mis- with a high frequency triangle or saw- audio players and laptop computer. takenly defined as digital because the tooth waveform to create the PWM However they are also progressively output waveform superficially resem- signal.The input signal is converted to a displacing more traditional linear bles a pulse train of digital symbols. sequence of pulses whose averaged designs in mainstream applications value is directly proportional to the such as home entertainment systems, amplitude of the signal at that time.The automotive sound systems, and pro- Class D amplifiers frequency of the pulses is typically ten fessional installations where high qual- or more times the highest frequency of ity audio is important. A class D amplifier is basically a interest in the input signal. switching amplifier or pulse width modulator (PWM from now on) This PWM signal is then used to drive Audio Amplifiers amplifier. In this kind of amplifier all the power stage, creating the amplified power devices are operated in on/off digital signal, and finally a low pass filter An electronic amplifier is a device for mode, reducing the power losses in is applied to the signal to filter out the increasing the current, voltage or the output devices significantly. PWM carrier frequency and retrieve power of a signal. It does this, by tak- the sinusoidal audio signal. The result- ing power from a power supply and Class D amplifiers can be categorised ing filtered signal is then an amplified controlling the output to match the into two topologies; half bridge and full replica of the input. Figure 1: Block diagram of a Class D audio amplifier CLASS D Audio Power cy by –20dB per decade.The switching Amplifier with GAPPED frequency of the amplifier can influ- FERRITES ence the choice of the filter order, the higher the fs, the lower the order One of the most important parts in a required to achieve a given attenua- class D, audio power amplifier is the tion within a specified passband. This output filter.The overall efficiency, reli- would seem to dictate the use of the ability and audio performance depends highest switching frequency possible. on it. A reference circuit has been The trade-off is that increasing fs modified in order to substitute an increases the switching losses and the inductor by a gapped ferrite.This com- EMI, thus decreasing the efficiency of ponent is designed to behave with the the amplifier. Figure 2: Example of the transfer function of a low pass filter over normalized frequency (f/fc). same performance as the first one. Several order filters are shown. Output Filter Design The overall efficiency, reliability and audio performance depend on the quality and linearity of the output fil- ter.The main goal of the output filter is attenuation of the high frequency switching component of the class D amplifier while preserving the signals in the audio band. The order of the filter determines how many poles exist at the same fre- quency, with each order increasing the attenuation above the cut-off frequen- 0 0.2 0.3 0.4 0.5 0.6 0.7 0.8 1 2.0 3.0 4.0 5.0 6.0 7.0 8.0 10 A second order LC filter is the com- sated for by means of proper feedback Magnetic core mon approach, as it is lossless and has network design. Some manufacturers of the output filter a –40dB/decade slope, allowing for a simply leave it that way, so the reasonable rejection of the carrier if response is strongly dependent on the The linearity of the filter versus fre- the parameters of the filter and the load. Surely a non-desirable situation. quency and output power strongly switching frequency itself are properly influences the performance of the designed. More information about designing the amplifier and quality of the audio sig- output filter as a function of the cut off nal.Therefore, care has to be taken in frequency and load impedance is pre- order to select the optimal magnetic sented in annex 2. core at a reasonable cost. The output inductor withstands the Iron powder cores are the cheapest whole output current without satura- solution, but the switching frequency tion, as well as keeps the energy for (several hundred kilohertz) is too high the off cycle, as in any non-isolated to make them suitable for this applica- switching converter (Class-D half tion. Figure 3: Second order low pass filter structu- bridge design is in fact analogous to a re. buck converter, its reference voltage Powder alloy cores are widely used, being the audio signal). mainly because they are very popular as output chokes in DC/DC convert- The first thing to do is to design the The ideal inductor (in terms of linear- ers and SMPS.The drawback is that the transfer function for the filter. Usually, ity) is an air-core one, but the size and saturation curve is not flat, but with a a Butterworth or similar frequency number of turns required for typical constant slope drop.To overcome this response is chosen, with a cut-off fre- Class-D operation makes it impracti- problem the designer has to over quency slightly above the audio band cal, so a core has to be used in order specify the product in the sense that (30-60KHz). Have in mind that one of to reduce turns count and also pro- only a small region of the saturation the design parameters is the termina- vide a confined magnetic field that curve is used. Losses at high frequen- tion load, that is, the speaker imped- reduces radiated EMI. It can be use cies are also a thread, implying the use ance. Usually, a typical 4 or 8 ohm either powder cores or ferrite cores. of low permeability materials wound resistor is assumed, but that would Ferrite cores, must have a “gap” where with some 30 to 50 turns. produce variations in the measured energy is stored.Wire size should also frequency response in presence of dif- be carefully chosen to keep DC and ferent speakers.That must be compen- AC losses low (requiring thick wire). µ Effective Permeability Effective Figure 4: Saturation curve: ferrites have lower saturation, but flat response, while powder cores show high saturation with a continuous slope. Figure 5: International Rectifier IRAUDAMP1 demo board. Features 500W + 500W peak stereo power, efficiency 93% at 1 kHz and 100 watts.Voltage supply ±50 Volts (allowed ±25 to ±60). Compact size: 100 x 140 x 38 mm. Ferrite cores are the third option. They have the lowest losses at high frequencies, and though the saturation is lower than in powder cores, the curve is completely flat. This curve allows the designer to optimize the core volume and gap size to the required energy to store.The result is very low distortion with a low turns count (10 to 25) and, finally, at the most affordable cost. A practical Class D audio amplifier has been evaluated to measure the influ- ence of the magnetic core on the dis- tortion of the audio signal.The amplifi- In the original design, the output filter only difference between them is the er is driven by an International of each channel is made with an AVX magnetic material of the inductor and Rectifier IR2011S device, delivering up capacitor (BF074E0474J with 0.47uF) the winding count. to 500 + 500 watts peak. and a Micrometals inductor core (NPT0104, core T106-2). It has an The measuring equipment used to test inductance value of 18 µH and it is the amplifier performance is Audio The reference design wound with 37 turns of AWG18 mag- Precision System Two.