JP3909280B2 - Digital audio output device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an audio signal amplification circuit based on a class D power amplification system, and more particularly to a class D power amplification system digital audio output system in which an H-type bridge circuit is switched with a digital signal converted into a digital signal.
[0002]
[Prior art]
The power amplification method has been conventionally classified into Class A, Class B, Class C, and Class D according to the driving range of the amplifying element, but in recent years, class D power amplification is also used for audio (audible frequency band). A method is used (for example, see Patent Document 1).
[0003]
Here, there is an H-type bridge circuit system as a kind of this class D power amplification system, which is a type of switching circuit called an H-bridge circuit that is turned on / off with a digital signal to obtain an amplification function. Therefore, the binary signal generating means and the H-type bridge circuit are used, the binary signal generating means converts the input analog signal into a binary digital signal, and a balanced output is generated corresponding to the digital signal. It is what has become.
[0004]
By the way, in this class D amplification system, the output of the class D amplification unit composed of an H-type bridge circuit is converted into a digital signal as a control input from the maximum value to the minimum value of the power supply voltage supplied from the DC power supply. Full swing is performed accordingly.
[0005]
Then, at the subsequent stage of the class D amplifier, the fully swept output power is converted into an analog signal by a DAC circuit (digital / analog conversion circuit), thereby efficiently converting to analog power.
[0006]
Therefore, the reference potential of the analog signal output from the class D power amplifier circuit, that is, the output potential when the input is no signal, is a voltage that is ½ of the supplied power supply voltage. The analog signal output from the power amplifier circuit drives a speaker as a load under a reference potential that is ½ of the power supply voltage.
[0007]
Here, FIG. 3 shows an example of a conventional class D amplifier circuit. An analog input signal (audio signal) supplied to the input terminal 1 is first an ADC circuit (digital-analog converter circuit) that functions as a binary signal generating means. ) 2 is converted into a digital signal (binary signal), and then supplied to the driver circuit 3.
[0008]
At this time, the driver circuit 3 functions to convert the input digital signal into a signal necessary to drive the gate of the FET (switching element) of the H-type bridge circuit 4 serving as the final-stage power switching circuit. .
[0009]
Here, the H-type bridge circuit 4 is composed of four MOS FETs arranged in an H-shape between both ends (+ end and − end) of a DC power supply, and between these drain terminals, A digital signal in which the added digital signal has substantially the same potential difference as the power supply voltage is generated as a balanced output.
[0010]
A digital signal having the same potential difference as the power supply voltage is processed by an LPF circuit (low-pass filter) 5 serving as a DAC circuit, and then supplied to a load, that is, a speaker 6.
[0011]
Here, the power transformer 9, the rectifier circuit 8, and the capacitor 7 connected to the AC (alternating current) power supply path are the above-described direct current power supplies. It is designed to be insulated.
[0012]
Then, an alternating current of a predetermined voltage output from the secondary winding of the power transformer 9 is rectified by the rectifier circuit 8 and smoothed by the capacitor 7, and then the H-bridge circuit 4 and the driver circuit 3 are supplied with the voltage V 1. A DC voltage is supplied.
[0013]
At this time, the positive side of the power supply circuit is connected to the power supply line + L, and the negative side is connected to the ground line, that is, a line that is connected to the conductive casing and kept at a common potential, and therefore becomes a load when there is no signal. The average value of the DC potential appearing at both ends of the speaker 6 is fixed to a voltage that is ½ of the voltage of the power supply line + L, and has a DC potential of the voltage V1 / 2 with respect to the housing.
[0014]
Note that the power supply circuit in the prior art shown in FIG. 3 is composed of only a power supply that generates a positive voltage with respect to the housing, and does not include a power supply that generates a negative voltage. I will decide.
[0015]
By the way, in the case of an audio amplifier (power amplifier), the load is generally a speaker, and the speaker is usually connected to the amplifier by a user. For example, there is a possibility that an electric wire connected to the speaker may be brought into contact with the housing by carelessness.
[0016]
Therefore, in the case of the prior art shown in FIG. 3, this is because the voltage V1 / 2 power supply is short-circuited and is generally called a ground fault. Problems arise in maintaining sex.
[0017]
Therefore, in order to suppress such destruction due to a ground fault, a class D power amplifier circuit shown in FIG. 4 has been conventionally proposed. Here, in this figure, blocks having the same functions as those in the prior art in FIG.
[0018]
In the prior art of FIG. 4, as shown in the figure, a power transformer 9 having a center tap T in the secondary winding is used, and the center tap T is connected to the ground line, and then the power line + L and the power source are connected. Two smoothing capacitors 7 and 8 are connected in series between the line -L, and the connection point between them is also connected to the ground line.
[0019]
In this case, in the circuit of FIG. 4, power supplies having the same positive and negative absolute values in two systems of + V1 / 2 and −V / 2 are paired with respect to the ground line at the common potential of 0. As a result, a voltage of + V1 / 2 and a voltage of -V / 2 are applied to one power supply line + L and the other power supply line -L, respectively, with a ground line having a potential of 0 as a neutral point. It will be supplied symmetrically.
[0020]
As a result, the DC potential at the output of the H-type bridge circuit 8 when there is no signal is half of the applied voltage, as in the case described above. In this case, the potential at the terminal of the speaker 6 is 0V. Thus, destruction of the amplifier and the speaker due to the ground fault can be surely prevented at least when there is no signal.
[0021]
However, in the case of the prior art shown in FIG. 4, a power transformer 9 having a center tap T is required in the power circuit, and smoothing capacitors 7 and 13 are required on the plus side and the minus side, respectively. 8, in order to drive the gates of the MOS-type FETs connected to the plus side and the minus side, unlike the driver circuit 3 in the prior art of FIG. 3, a driver circuit 11 having a complicated circuit configuration is required. For this reason, it becomes disadvantageous in terms of cost.
[0022]
In addition, the level shift circuit 10 is necessary to drive the driver circuit 11 normally, and if there is a variation in timing between the rising and falling of the power supply voltage on the plus side and the minus side, a large through current will be generated. Since there is a possibility that it will occur, there will be an increase in the cost, such as requiring a protective measure.
[0023]
Therefore, a class D power amplifier circuit shown in FIG. 5 has also been proposed. Here, the same reference numerals are given to the blocks having the same functions as those of the prior art in FIGS. 3 and 4, and a detailed description thereof will be omitted.
[0024]
Here, the signal converted into digital form by the ADC circuit 2 is supplied to the driver circuit 3 and the output switched by the H-type bridge circuit 4 is supplied to the speaker 6 via the LPF circuit 5 as described above. 3 and FIG. 4 are the same as those of FIG. 4, but in the prior art of FIG. 5, the signal digitally converted by the ADC circuit 2 is supplied to the driver 3 via the capacitor 15. First is different.
[0025]
Next, the case of FIG. 5 is the same as the case of FIG. 3 in that the power transformer 9 and the rectifier circuit 8 operate as a single power supply circuit with one smoothing capacitor 7, but the minus side is the ground line. And is connected to the other power supply line -L.
[0026]
Correspondingly, two resistors R1 and R2 having the same resistance value are connected in series between one power supply line + L and the other power supply line -L, and then between them. A connection point is connected to a ground line, and the potential at this connection point is fixed to a housing potential, that is, a common potential (also referred to as a ground potential).
[0027]
Therefore, also in the prior art of FIG. 5, the DC potential appearing at both ends of the speaker 6 that is the load is the ground potential as in the case, and as a result, the amplifier and the speaker are destroyed due to the ground fault as in the prior art of FIG. At least when there is no signal, it can be surely prevented, and the possibility of destruction can be greatly reduced even in other cases.
[0028]
[Patent Document 1]
JP-A-6-335082 [0029]
[Problems to be solved by the invention]
The above-mentioned conventional technology has a problem in safety because it is not considered to suppress cost increase and maintain performance.
[0030]
First, in the prior art shown in FIG. 5, after the output of the ADC circuit 2 is input to the driver circuit 3 via the capacitor 15, the resistor R2 is set until the common line of the driver circuit 3 reaches the common potential point. After passing, it follows a fairly complicated current path that returns to the ground potential point that is a common line of the ADC circuit 2.
[0031]
In this process, the current path of the ADC circuit 2 in which only a relatively small current flows and the current path of the H-type bridge circuit 4 in which a considerably large current flows follow the same path in the ground line.
[0032]
As a result, in the prior art of FIG. 5, various phenomena that impair the performance as a power amplifier such as an increase in noise and unnecessary radiation and deterioration in sound quality occur, and it is difficult to maintain the performance.
[0033]
On the other hand, in the conventional technique shown in FIG. 4, it is difficult to suppress the increase in cost as described above, and in the conventional technique shown in FIG. 3, it is difficult to maintain safety as described above.
[0034]
Therefore, in the prior art, it is difficult to simultaneously suppress the increase in cost and maintain the performance, which causes a problem in safety.
[0035]
An object of the present invention is to provide a digital audio output device that can easily maintain performance and maintain safety while suppressing an increase in cost.
[0036]
[Means for Solving the Problems]
The above object includes a binary signal generating means for converting an input analog signal into a binary digital signal, and an H-type bridge circuit driven by the digital signal and generating a balanced output corresponding to the digital signal. In the digital audio output device operating in the class D amplification system, an optical coupling element is provided in the transmission path of the digital signal between the output of the binary signal generating means and the input of the H-type bridge circuit, and the H-type Two resistors with the same resistance value are connected in series between the positive side and the negative side of the power supply line, and only the connection point between these two resistors is connected to the ground line. performed from a DC power supply connected is achieved in so that the.
[0037]
At this time, the optical coupling element may be provided between the output of the binary signal generating means and the input of the driver circuit on the input side of the H-type bridge circuit.
[0038]
In a class D amplification type digital power amplifier circuit, in a system equipped with a circuit that handles large power and a circuit that performs predetermined signal processing with small power, the current flowing through the large power circuit has a considerable impact on the function of the small power circuit. Give rise to problems such as degradation of performance and increase of unnecessary radiation.
[0039]
According to the present invention, means for solving these problems that are inevitably generated when high-efficiency power amplification is performed on an audio signal by digital processing, and the DC potential of the speaker terminal used directly by the user is changed to the DC potential of the housing. The same means is provided, and as a result, an audio digital power amplifier capable of minimizing a ground fault can be realized.
[0040]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, a digital audio output apparatus according to the present invention will be described in detail with reference to the illustrated embodiments. Here, first, the basic principle of the digital audio output apparatus according to the present invention will be described with reference to FIG.
[0041]
In FIG. 1, reference numeral 16 denotes a photocoupler (PC), which is also called an optical coupling element. The other circuit blocks and the connection configuration are the same as those of the prior art shown in FIG. 5. Therefore, the embodiment shown in FIG. 1 corresponds to the capacitor 15 in the prior art shown in FIG. .
[0042]
In FIG. 1, the output of the ADC circuit 2 flows through the LED on the input side of the PC photocoupler 16, drives (emits light) the LED, and then flows into the ground potential from the common terminal of the ADC circuit 2.
[0043]
On the other hand, the light generated by the LED at this time is detected by a phototransistor on the output side of the photocoupler 16, and the detected current is input to the driver circuit 3. Will flow through the line.
[0044]
According to this connection method, the ADC circuit 2 and the common line of the preceding stage, that is, the ground line, and the common line of the class D amplifier including the H-type bridge 4 can be electrically completely separated. As a result, the ADC circuit 2 and Prior to that, it is possible to eliminate the possibility that the common line current in the class D amplifier, which is an obstacle to securing the performance, will be affected.
[0045]
Further, as is apparent from FIG. 1, in this embodiment, as in the prior art of FIG. 5, since the single power supply system is provided with resistors R1 and R2, the terminals of the speaker 6 can be kept at a common potential. Therefore, safety is ensured.
[0046]
Next, an embodiment of the present invention will be described more specifically with reference to FIG. Here, also in FIG. 2, blocks having the same functions as those in FIGS. 1 and 5 are given the same numbers.
[0047]
In FIG. 2, the analog signal applied to the input terminal 1 is first converted into a digital signal by the ADC circuit 2, and from there, it is output as a digital output Q and a digital output Q￣ having the opposite logic.
[0048]
These digital outputs Q and Q￣ drive the LEDs in the photocouplers 16A and 16B, and the driven current returns to the common line of the ADC circuit 2, that is, the ground line.
[0049]
At this time, the ADC circuit 2 receives power supply from the power source constituted by the diode D2 and the smoothing capacitor C3 connected to the dedicated secondary winding provided in the power transformer 9 and the stabilized power circuit 17. Operate.
[0050]
Digital signals detected by the phototransistors in the photocouplers 16A and 16B are supplied to the driver circuit 3, and circulate as input signals in the driver circuit 3.
[0051]
Therefore, the driver circuit 3 processes this input signal and is necessary for driving the gates of the four FETs (MOS-FETs) Q1, Q2, Q3, and Q4 constituting the H-type bridge circuit 4 in the subsequent stage. To a new signal, that is, digital output Q, Q￣.
[0052]
First, one digital output Q is supplied to the gates of FET Q1 and FET Q3 of the H-type bridge circuit 4, and the other digital output Q is supplied to the gates of FET Q2 and FET Q4.
[0053]
As a result, first, when one of the digital outputs Q is enabled, the FET Q1 and FET Q3 of the H-type bridge circuit 4 are simultaneously turned on, whereby the one terminal 6A of the speaker 6 serving as a load has a power supply voltage V And the other terminal 6B is maintained at approximately 0V potential.
[0054]
Next, when the other digital output Q￣ is enabled, the FET Q4 and the FET Q2 are simultaneously turned on, this time the other terminal 6B of the speaker 6 becomes almost the power supply voltage V, and the one terminal 6A becomes almost 0V.
[0055]
At this time, an LPF circuit 5 functioning as a digital-analog conversion circuit is provided between the H-type bridge circuit 4 and the speaker 6, and the digital signal switched at a high speed by the H-type bridge circuit 4 is the LPF circuit. 5 is converted into an analog signal.
[0056]
Here, the LPF circuit 5 is a simple circuit composed of the coils L1 and L2 and the capacitors C1 and C2. However, the function as a digital-analog conversion circuit can be sufficiently obtained.
[0057]
Here, a diode D 1 and a smoothing capacitor 7 are connected to the secondary winding of the power transformer 9, thereby forming a DC power source. From this, a class D amplifying unit comprising the driver circuit 3 and the H-type bridge circuit 4 is connected. In this case, the resistors R1 and R2 having the same resistance value are connected in series between the power supply line + L and the power supply line -L. Connected to the earth line.
[0058]
Then, since the earth line has the same potential as that of the housing, that is, a common potential, one potential of the resistor R1 becomes + V1 / 2 when viewed from the earth line, and one potential of the resistor R2 is − V1 / 2.
[0059]
As a result, the potential at the time of no signal with respect to the ground of both terminals 6A and 6B of the speaker 6 becomes 0V, and the probability of occurrence of destruction due to a ground fault can be reduced.
[0060]
In the embodiment of FIG. 2, the relatively large current flowing in the class D amplification unit remains only in the class D amplification unit, and therefore does not affect the previous stage connected via the photocoupler 16. .
[0061]
Therefore, according to this embodiment, unlike the prior art described with reference to FIG. 5, there is no possibility that problems such as deterioration in performance and increase in unnecessary radiation occur.
[0062]
Here, in the case of this embodiment, these effects are obtained by the fact that the common line of the circuit is separated by the photocoupler 16. At this time, the position of the photocoupler 16 has an important meaning. That is, as shown in the figure, it is a requirement to be connected between the ADC circuit 2 and the class D amplifier.
[0063]
For example, even if a similar optical coupling element is connected to the input stage of the ADC circuit 2, it is considered that unnecessary radiation and the performance of the class D amplifier are not affected, but in this case, the signal passing through the optical coupling element is analog. It is a signal, and when this analog signal passes, it is limited by the S / N and dynamic range of the signal.
[0064]
Therefore, according to the present invention, as shown in the figure, the photocoupler 16 is connected between the ADC circuit 2 and the class D amplifier, so that these problems can be solved.
[0065]
As a means for separating the DC potential, it is conceivable to use a pulse transformer in which the primary side and the secondary side are galvanically insulated. In this case, electromagnetic induction or electrostatic induction between the primary and secondary is considered. As a result, it is important to use an optical coupling element as a digital signal coupling element as in the present invention.
[0066]
【The invention's effect】
According to the present invention, the advantages of the class D amplification method can be fully utilized with a simple configuration. As a result, a high-performance and safe digital audio output device can be easily provided at low cost.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of a digital audio output device according to the present invention.
FIG. 2 is a detailed block diagram illustrating an embodiment of a digital audio output device according to the present invention.
FIG. 3 is a block diagram showing a first example of a conventional digital audio output apparatus.
FIG. 4 is a block diagram showing a second example of a digital audio output device according to the prior art.
FIG. 5 is a block diagram showing a third example of a digital audio output device according to the prior art.
[Explanation of symbols]
1 Input terminal (for analog input signal input)
2 ADC circuit (digital-to-analog converter circuit that functions as a binary signal generator)
3 Driver circuit 4 H bridge circuit (Main part of Class D amplifier)
5 LPF circuit (low-pass filter that functions as a digital-analog converter)
6 Speakers (Class D amplifier load)
7 Capacitor (smoothing capacitor)
8 Rectifier circuit 9 Power transformer 16 Photocoupler (Optical coupling element)

Claims (2)

Class D amplification system comprising: binary signal generating means for converting an input analog signal into a binary digital signal; and an H-type bridge circuit driven by the digital signal and generating a balanced output corresponding to the digital signal In the digital audio output device that operates in
An optical coupling element is provided in the transmission path of the digital signal between the output of the binary signal generating means and the input of the H-type bridge circuit;
DC power supply to the H-type bridge circuit is
Two resistors having the same resistance value are connected in series between the positive side and the negative side of the power supply line, and only the connection point between these two resistors is connected to the ground line. digital audio output apparatus characterized by being configured to us so that. The digital audio output device according to claim 1.
The digital audio output device, wherein the optical coupling element is provided between an output of the binary signal generating means and an input of a driver circuit on an input side of the H-type bridge circuit.

Images