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Class D uses pulse width modulation (PWM) signals to replace the linear signals typically used in Class AB amplifiers . The PWM signal includes the audio signal as well as the PWM switching frequency and harmonics. Class D audio amplifiers are much more efficient than Class AB amplifiers because the output MOSFET can be converted from very high impedance to very low impedance, allowing operation in the active region for only a few nanoseconds. With the above technique, the loss on the output stage is extremely inefficient. In addition, the LC filter or the sensing element of the loudspeaker can store energy in each cycle and ensure that switching power is not lost in the loudspeaker.introduction
Although Class D amplifiers have been around for a while, many people still don't understand the basics of Class D amplifier operation and why they provide higher efficiency. This article explains how the Pulse Width Modulation (PWM) signal is created and shows that you are hearing the audio frequency rather than the switching frequency of the PWM waveform. This article will detail why the output PWM waveform is much more efficient than the output linear waveform. It also explains why some Class D amplifiers require an LC filter and some do not.
B> Class D Output Signal (PWM) How do I include an audio signal?
The TPA3001D1 block diagram (see Figure 1) helps explain how the PWM signal is formed. First, the analog input D class uses a preamplifier to obtain the input audio signal and ensure the differential signal. The integrator stage can then pass through the filtered audio signal for anti-aliasing and stability. The audio signal is then compared to a triangular wave to create a pulse width modulated (PWM) signal. The gate drive circuitry uses a PWM driven output FET that will create a high current PWM signal at the output.
Figure 1: Structure diagram of the TPA3001D1.
Figure 2 shows how a typical PWM signal is formed from the comparator function block in Figure 1. The audio input can be compared to a 250-kHz triangular wave. When the audio input voltage is greater than the 250-kHz triangular wave voltage, the non-inverting comparator output state is high, and when the 250-kHz triangular wave is larger than the audio signal, the non-inverting comparator output state is low. When the non-inverting comparator output is high, the inverting comparator output is low; when the non-inverting comparator output is low, the inverting comparator output is high. The average PWM non-inverted output voltage VOUT+(avg) is the busy idle multiplied by the supply voltage, and D represents the busyness, or the "on" time t(on) divided by the total period T.
VOUT+(avg) = D * Vcc (1)
D = t(on) / T (2)
The busy output of the inverting output, VOUT- and VOUT+, is 1. If the input is only half, the busyness of VOUT- and VOUT+1 is 0.5.
VOUT-(avg) = (1-D) * Vcc (3)
Figure 2: Comparator Input and PWM Output of a Typical Class D Amplifier
Both the TPA3001D1 and the TPA3002D2 use a filterless modulation scheme in the TPA2005D1. With this modulation scheme, the positive output VOUT+ is the same as a typical D-type PWM, but the negative output VOUT- is not exactly the opposite of VOUT+. In this case, there are two comparators, and the positive integrator output is compared to the triangular wave to create the VOUT+ PWM, and the integrator's negative output is compared to the triangular wave to create the VOUT-PWM. Figure 3 shows the comparator input and PWM output for a filterless modulation scheme. Here we assume that the audio signal is dc voltage because the frequency of the audio signal is much lower than the 250 kHz triangle wave. Figure 3 also shows the differential output voltage.
Figure 3: TPA3001D1 and TPA3002D2 Input Output and PWM
Figure 4 shows the TPA3001D1 PWM output with a 20 kHz audio input signal. Please note how busyness increases as the input voltage increases.
Figure 4: (Sine and PWM) Scope Diagram showing the input signal, the pre-output filter, and the post-output filter
The audio signal in the PWM waveform is much easier to find in the frequency domain. The PWM signal consists of the input frequency, the switching frequency, and the harmonics of the switching frequency plus the sideband. Figure 5 shows the amplitude vs. input frequency, PWM output, and filtered output. Figure 5 also shows how the audio signal is extracted from the PWM by low pass filtering. The filtered output has a 1 kHz sine wave frequency component, any 1 kHz harmonic that appears as distortion in the audio band, and any ripple voltage left from the switching frequency. The speaker cannot duplicate the switching frequency and its harmonics, even if the speaker can be copied, the ear can't hear it. If both the filtered and unfiltered PWM signals are sent directly to the speaker, the listener will not find the difference between the two in Figure 5.
Figure 5: Displaying the amplitude and frequency phase of the input signal, the pre-output filter, and the post-output filter
What is the efficiency of a Class D amplifier ? How to calculate efficiency?
A linear amplifier provides a constant amount of current for the desired output voltage. In a bridge-tied load (BTL) class AB amplifier, the supply current is equal to the output current. Class D amplifiers are a sampling system that provides quantitative power to the load at a given cycle. Class D amplifiers output pulse-width modulated (PWM) signals and use de-tantalum capacitors and output filter inductors or speaker inductors (for filterless modulation) as energy storage elements to provide power from the power supply to the load Quantitative power. The PWM signal is switched between the output rails of the power rails to achieve an extremely low voltage drop across the output transistors. In contrast, Class AB output FETs spend most of their time on the active area of ​​the power rail, resulting in significant power consumption and, in turn, inefficiency.
The ideal Class D amplifier is 100% efficient because its purpose is to provide the same amount of power from the power supply to the load. The ideal MOSFET for a Class D amplifier should be such that the drain-to-source resistance should be zero in the "on" rDS(on) state and the indirect-to-source resistance in the "off"-rDS(off) state should be infinite. . Unfortunately, all MOSFETs are not zero in the rDS(on) state, while the resistance is limited in the rDS(off) state. The power loss generated by rDS(on) and rDS(off) is called conduction loss. A voltage divider is formed by rDS(on), rDS(off) and output load or speaker RL. The value of rDS(off) is large enough so it can be ignored when calculating efficiency.
Equation 5 gives the equation for the computational efficiency , ie the ratio of output power to supply power. Filtering the inductor or speaker inductance (for filterless modulation) keeps the high frequency switching current low so that the current obtained here is the current in the audio band. When discussing the static loss in the following sections, we will consider the switching current loss. The current through rDS(on) is equal to the current through the load, which causes the output power to be inconsistent with Equation 5, which makes the efficiency of the conduction loss independent of the output power. Equation 7 shows the efficiency of the conduction loss effect.
EffICiency = POUT / PSUP (5)
Efficiency (CONDUCTION) = iL^2 * RL / iL^2 * (2rDS(on) + RL) (6)
Efficiency (CONDUCTION) = RL / (2rDS(on) + RL) (7)
Equation 7 can be used as a first approximation to calculate the effect of rDS(on) on efficiency. For rDS(on) 0.1 ohm and load resistance RL 4 ohm
Efficiency = POUT / PSUP = POUT / (POUT + PD1 + PD2 + PD3 ...) (9)
The bias current, gate charge, and switching current loss of the amplifier can be considered independent of the output power, because the conduction loss dominates at the maximum output power and can be accounted for as the static loss PQ. The electrostatic loss is calculated as follows: The supply current of the device with no input signal (with the filter and load to be used in production) is multiplied by the supply voltage.
PQ = IDD(q) * VCC (10)
In order to use the efficiency equation (9), the power dissipation in conduction losses must be derived from Equation 7. Solving Equations 7 and 9 yields the power PD (CONDUCTION) consumed in conduction losses. Equation 12 shows the results.
Efficiency (CONDUCTION) = RL / (2rDS(on) + RL) = POUT / (POUT + PD(CONDUCTION)) (11)
PD (CONDUCTION) = POUT * 2rDS(on) / RL (12)
The consumption loss in Equations 10 and 12 is inserted into Equation 9, and the Class D efficiency is calculated as follows:
Efficiency = POUT / POUT + (POUT * 2rDS(on) / RL) + PQ (13)
Electrostatic losses dominate at low output power levels, while conduction losses dominate at high power levels.
Class D amplifiers are much more efficient than Class AB amplifiers. Higher power means lower power consumption, which allows us to use a 12V Class D amplifier without the need for a heat sink, and a comparable Class AB amplifier is inseparable from the heat sink. With an output power of 10W, the TPA3002D2 consumes only 3.7 W at 4 ohms, while its comparable Class AB amplifier consumes up to 14 W!
Why do some Class D amplifiers require filters, while Others do not?
The development of filterless modulation schemes has greatly reduced or even eliminated the need for output filters. The filterless modulation scheme minimizes the switching current, which allows us to use a lossy inductor or even a speaker instead of an LC filter as a storage element and still ensure high efficiency of the amplifier.
The traditional Class D modulation scheme has a phase difference of 180 degrees for each of its differential outputs and changes from ground to supply voltage VCC. Therefore, the differential pre-filtered output varies between positive and negative VCC, while the filtered 50% busyness has zero voltage in the load. Note that although the average load voltage is zero (50% busy), the output current peak is still high, which causes filter losses and increases the supply current. Conventional modulation schemes require an LC filter so that higher switching currents can be recirculated in the LC filter without being consumed by the speaker.
In a filterless modulation scheme, each output is switched from ground to supply voltage. However, VOUT+ and VOUT- are now in phase with each other and have no input. In the case of a positive voltage, the busyness of VOUT+ is greater than 50%, while that of VOUT- is less than 50%. In the case of a negative voltage, the busyness of VOUT+ is less than 50%, and the VOUT- is greater than 50%. The voltage across the load is zero during most switching cycles, greatly reducing I2R losses in the filter and/or speaker. The low switching losses allow the speaker to act as a storage component while still ensuring the efficiency of the
Although the switching frequency component is not filtered out, the speaker has a high impedance at the switching frequency, so the power loss of the speaker is extremely small. The speaker still cannot replicate the switching frequency, and even if the speaker is available, the human ear cannot hear frequencies above about 20 kHz.
If the trace from the amplifier to the speaker is short, a 5V filterless Class D audio amplifier like the TPA2005D1 can be used without an output filter. The TPA2005D1 passed the FCC and CE radiation tests when the speaker line length was 10 cm or less unshielded. Wireless handheld terminals and PDAs are excellent applications for filter-free Class D. Higher voltage filterless Class D amplifiers like the TPA3001D1 and TPA3002D2 require ferrite bead filtering for all applications
A ferrite bead filter is often used if the design does not use an LC filter and should not pass the radiation standard and the frequency sensitive circuit is greater than 1 MHz. This is a good choice for circuits that must pass the FCC and CE standards, since the above two standards only test for radiation greater than 30 MHz, while ferrite bead filters are less effective at attenuating frequencies greater than 30 MHz. The performance of the LC filter is better. If you choose a ferrite bead filter, you should choose a high impedance at high frequencies and a low impedance at low frequencies.
If a low frequency (< 1 MHz) EMI sensitive circuit is present and/or leads from the amplifier to the speaker are long, an LC output filter is required.
in conclusion
A Class D audio amplifier creates a pulse width modulated PWM signal by comparing the input audio waveform to a triangular wave. Class D amplifiers output PWM through inductive components, traditional Class D uses filter inductors, and Filterless Class D uses speaker voice coils. Class D amplifiers are more efficient than Class AB amplifiers because Class D amplifiers obtain the required output power from the power supply instead of obtaining the required current from the power supply and do not consume the remaining power in the output transistors. The stereo Class AB amplifier consumes 14W at 10W from a 12V supply and a 4 ohm load, while the TPA3002D2 consumes only 3.7 W under the same conditions. The modulation scheme used by the TPA3001D1 and TPA3002D2 allows the use of a ferrite bead filter without the need for a complete LC filter.
bibliography
1. TPA2000D2 2-W Filterless Stereo Class D Audio Power Amplifier Data Sheet, Texas Instruments, March 2000, publication number: SLOS291D;
2, TPA2005D1 1.1-W mono filterless class D audio power amplifier data sheet, Texas Instruments, July 2002, publication number: SLOS369B;
3, TPA3001D1 20-W mono D-class audio power amplifier data sheet, Texas Instruments, December 2002, publication number: SLOS398;
4. TPA3002D2 9-W Stereo Class D Audio Power Amplifier Data Sheet with DC Volume Control, Texas Instruments, December 2002, publication number: SLOS402.
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