JP2009118447A - Switching amplifier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching amplifier which puts a driver transistor reliably in an ON state to reliably put a switching output circuit in an ON state. <P>SOLUTION: A capacitor C1 is connected in parallel to a photovoltaic coupler 31 which generates a constant voltage, so that a constant voltage, which causes the transistor Q2 of the driver 11 to be in the ON state, is constantly held by the capacitor C1, regardless of high and low levels of a PWM signal. Accordingly, the transistor Q2 can be absolutely put in an ON operation in response to the PWM signal. As a result, because the MOSFET 15 can reliably be put in the ON state in response to the PWM signal, the output waveform of the switching amplifier 10 can be a waveform obtained by exactly amplifying an input signal, distortion and noise can be prevented, and furthermore instability in oscillation can be prevented. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a switching amplifier, and more particularly, to a switching amplifier that can output an accurate output signal from a switching output circuit.

  FIG. 4 is a schematic circuit diagram showing a main part of a conventional switching amplifier 50. The switching amplifier 50 is supplied with a PWM signal from a preceding PWM circuit (not shown) and includes a driver 11 that outputs a drive signal and MOSFETs 15 and 16. The MOSFETs 15 and 16 are alternately turned on and off in response to the drive signal from the driver 11. And a switching output circuit 12 for outputting an output signal. For the driver 11, only the positive side portion for driving the positive side MOSFET 15 is described, and the negative side portion for driving the negative side MOSFET 16 is the same as that of the positive side, and thus description thereof is omitted.

  The driver 11 includes transistors Q1 to Q4. When the PWM signal is at a high level, the transistors Q1 and Q2 are turned on, Q3 and Q4 are turned off, and a high-level drive signal is supplied to the MOSFET 15. Turn on the. On the other hand, when the PWM signal is at the low level, the transistors Q1 and Q2 are turned off, and the transistors Q3 and Q4 are turned on to supply a low-level drive signal to the MOSFET 15 so that the MOSFET 15 is turned off.

  A capacitor C51 for charging a voltage for turning on the transistor Q2 is connected between the emitter of the transistor Q2 and the emitter of the transistor Q4. When the MOSFET 15 is in the off state and the MOSFET 16 is in the on state, the voltage at the output terminal of the switching output circuit 12 to which one end of the capacitor C51 is connected becomes the negative power supply voltage −VCC. It is charged by the voltage from the regulator 51 that generates a constant voltage by the power supply voltage -VCC. On the other hand, when the MOSFET 15 is in the on state and the MOSFET 16 is in the off state, the voltage at the output terminal of the switching output circuit 12 becomes the positive power supply voltage + VCC, so that the capacitor C51 is not charged by the regulator 51. At this time, the voltage of the capacitor C51 is discharged by supplying a bias current to turn on the transistor Q2. That is, the capacitor C51 is repeatedly charged and discharged in accordance with the OFF state and the ON state of the MOSFET 15.

  Here, in order to reduce ripple noise included in the power supply voltage, a large value is selected for the capacitance of the capacitor C51. However, as described above, the capacitor C51 is repeatedly charged and discharged within one cycle of the PWM signal. Therefore, when the capacitor C51 is charged, if the capacitance of the capacitor C51 is large, the time constant increases, and the capacitor C51 has a transistor. It takes time to charge the voltage for turning on Q2. In particular, immediately after switching from the power-off state to the power-on state, the capacitor C51 is not charged at all, so the voltage for turning on the transistor Q2 is sufficiently charged while the MOSFET 16 is on. I can't. As a result, the transistor Q2 is not completely turned on during the period in which the PWM signal is at a high level, and a high level drive signal is not sufficiently supplied from the driver 11 to the MOSFET 15. Therefore, the switching output circuit 12 cannot accurately output the positive power supply voltage + VCC while the PWM signal is at a high level, and as a result, a waveform different from the input signal is output from the switching amplifier 50. There is a problem that distortion and noise are generated from the speaker as a load.

  FIG. 5A is a simulation result showing an output waveform of the switching output circuit 12 in the switching amplifier 50 of FIG. The horizontal axis indicates time, and the vertical axis indicates amplitude. The upper waveform shows the waveform of the L channel, and the lower waveform shows the waveform of the R channel (however, the switching amplifier of FIG. 4 is a circuit diagram for one channel). According to the switching amplifier of FIG. 4, it can be seen that an accurate output waveform is not output immediately after the power is turned on (left side portion).

  Even if it is not immediately after the power is turned on, the same problem occurs when the input signal has a low frequency and a positive amplitude value. 5B is a waveform diagram for explaining this problem. FIG. 5A shows an output waveform of the switching amplifier 50, and FIG. 5B shows a charging voltage of the capacitor C51. As shown in FIG. 5B (a), when the input signal has a low frequency and the amplitude value is positive, the high level period of the output signal of the switching amplifier 50 is long and the low level period is short. Since the charging voltage of the capacitor C51 is discharged during the high level period and charged during the low level period, if the high level period is too long, the charging voltage of the capacitor C51 turns on the transistor Q2 as shown in FIG. 5B (b). It will be lower than the voltage to make the state. Therefore, as shown in FIG. 5B (a), the falling edge of the output signal of the switching amplifier 50 does not become steep and does not become an accurate output signal, leading to the generation of distortion and noise.

  Further, when the MOSFET 15 is in the ON state, as shown by an arrow in FIG. 4, the bias current is applied from the positive power supply voltage + VCC (and / or the capacitor C51) to the load via the transistor Q2, resistors R2, R5, and the MOSFET 15. It flows to a certain speaker. As a result, there is a problem that noise from the speaker is generated by the voltage due to the bias current. This noise can be reduced to some extent by connecting a resistor to the speaker, but it is not possible to completely eliminate the bias current flowing through the speaker. Further, a loop is formed from the regulator 51 back to the regulator 51 through the transistor Q2 and the MOSFETs 15 and 16, and the regulator 51 is connected to a power supply circuit having large components such as a capacitor and a coil (not shown). The area of the loop becomes very large, and the noise radiated from the loop increases.

Special table 2006-522541 JP 2003-264435 A JP-A-2004-72276 JP-A-2004-328413

  The present invention has been made to solve the above-described conventional problems, and an object of the present invention is to provide a switching amplifier that reliably turns on a transistor of a driver and reliably turns on a switching output circuit. is there.

  According to a preferred embodiment of the present invention, a switching amplifier is supplied with a voltage supply circuit that supplies an operating voltage, a pulse modulation signal that repeats a high level and a low level, and an operation voltage from the voltage supply circuit. In response to the driving means for outputting a driving signal that repeats a high level and a low level, and a switching output circuit that is turned on or off in response to the driving signal from the driving means. The driving means is turned off when the first transistor is turned on in response to the pulse modulated signal, and the first transistor turned on or turned off in response to the pulse modulated signal, And a second transistor that turns on when the first transistor is off. The voltage supply circuit includes a capacitor connected between the first electrode of the first transistor and the first electrode of the second transistor, and the first supply voltage regardless of the high level and the low level of the pulse modulation signal. Constant voltage generating means for constantly holding a constant voltage capable of turning on the transistor in the capacitor.

  In a preferred embodiment, the constant voltage generating means includes a photovoltaic coupler including an LED that emits light when a current flows and a plurality of photodiodes that generate voltage at both ends when the LED emits light. The capacitor is connected in parallel with the plurality of photodiodes.

  In a preferred embodiment, when the switching output circuit is turned on by the drive signal, the switching amplifier starts from one end of the capacitor through the first transistor and the other end of the switching output circuit. The bias current flows through the first electrode.

  Since the constant voltage generating means always keeps a constant voltage for turning on the first transistor of the driving means in the capacitor regardless of the high level and low level of the pulse modulation signal, the first transistor according to the pulse modulation signal. Can be reliably turned on. As a result, the switching output circuit can be reliably turned on according to the pulse modulation signal, so that the output waveform of the switching amplifier can be a waveform obtained by accurately amplifying the input signal, and distortion and noise are generated. It is possible to prevent the oscillation from becoming unstable.

  In addition, by realizing the constant voltage generating means with a photovoltaic coupler, a voltage is generated in the photodiode by the current flowing in the LED, and as a result, the capacitor is charged with a voltage. Can always hold a constant voltage.

  In addition, when the switching output circuit is turned on, the bias current does not flow to the load connected to the switching amplifier, but flows from one end of the capacitor to the other end. Therefore, noise can be prevented from occurring.

  Hereinafter, preferred embodiments of the present invention will be specifically described with reference to the drawings. However, the present invention is not limited to these embodiments. FIG. 1 is a block diagram showing a schematic configuration of a typical switching amplifier 10. The switching amplifier 10 includes a pulse width modulation circuit 20, a driver 11, a switching output circuit 12, and an LPF (Low Pass Filter) 13. The switching amplifier 10 further includes a negative feedback circuit 14 as necessary.

  The pulse width modulation circuit 20 performs pulse width modulation on an input signal (for example, an audio signal) to generate a first PWM signal PWM1 and a second PWM signal PWM2. The first PWM signal and the second PWM signal are signals that repeat a high level and a low level, and when one is a high level signal, the other is a low level signal.

  The driver 11 receives the first PWM signal and the second PWM signal, and outputs drive signals DRV1 and DRV2 for driving the switch elements of the switching output circuit 12. As will be described later, the driver 11 is supplied with an operating voltage from a voltage supply circuit (not shown in FIG. 1), and outputs a drive signal that repeats a high level and a low level.

  The switching output circuit 12 is connected between a first power source (for example, positive power source + VCC) and a second power source (for example, negative power source -VCC), and in response to a drive signal, the positive power source voltage + VCC or The negative power supply voltage -VCC is selectively output. The switching output circuit 12 includes switch elements (for example, MOSFETs) 15 and 16 that are turned on or off in response to a drive signal.

  The LPF 13 is connected between the output terminal of the switching output circuit 12 and the output terminal of the switching amplifier 10, removes high frequency components from the output signal of the switching output circuit 12, and outputs it to a load such as a speaker. The LPF 13 includes, for example, a coil 17 and a capacitor 18. The negative feedback circuit 14 is connected between the output terminal of the switching output circuit 12 and the inverting input of the pulse width modulation circuit 20, and reduces the distortion component of the signal included in the output signal of the switching output circuit 12.

  FIG. 2 is a schematic circuit diagram illustrating a main part of the switching amplifier 10 according to a preferred embodiment of the present invention. In FIG. 2, the switching output circuit 12, the driver 11, and the voltage supply circuit 30 are described, and the negative side portion of the driver 11 for driving the negative-side MOSFET 16 is used for driving the positive-side MOSFET 15. The description is omitted because it is the same as the positive portion of the driver 11.

  Driver 11 includes npn-type transistor Q1, pnp-type transistor Q2, pnp-type transistor Q3 and npn-type transistor Q4 that are turned on and off by the first PWM signal, resistors R1 to R5, and diode D1.

  The connection configuration of the driver 11 will be described. In the transistor Q1, the first PWM signal is supplied to the base, the collector is connected to the base of the transistor Q2, and the emitter of the transistor Q2 and the voltage supply circuit 30 are connected via the resistor R1. (The one end of the capacitor C1). The emitter of the transistor Q1 is connected to the collector of the transistor Q2, and is connected to the gate of the MOSFET 15 through the resistor R2 and a parallel connection of the resistor R5 and the diode D1.

  The transistor Q3 is supplied with the first PWM signal at the base, the collector is connected to the base of the transistor Q4, and the output of the emitter of the transistor Q4 and the voltage supply circuit 30 (the other end of the capacitor C1) via the resistor R3. And the output terminal of the switching output circuit 12. The emitter of the transistor Q3 is connected to the collector of the transistor Q4, and is connected to the gate of the MOSFET 15 through the resistor R4 and a parallel connection of the resistor R5 and the diode D1.

  The voltage supply circuit 30 is a circuit that receives a positive power supply voltage and a negative power supply voltage and supplies an operating voltage for operating each transistor of the driver 11. The voltage supply circuit 30 always supplies a constant voltage capable of turning on the transistor Q2 to the driver 11 regardless of the high / low level of the PWM signal (that is, the on / off state of the MOSFETs 15 and 16). The voltage supply circuit 30 includes a capacitor C1 and constant voltage generation means 31 that always holds a voltage capable of turning on the transistor Q2 in the capacitor C1. Preferably, the constant voltage generating means is constituted by a photocoupler. More preferably, the constant voltage generating means 31 is constituted by a photovoltaic coupler.

  The photovoltaic coupler 31 has a function as a floating power source, and includes an LED 31a and a plurality of photodiodes 31b. A positive power supply voltage + VCC (+25 V) is supplied to the anode of the LED 31 a, a negative power supply voltage −VCC (−25 V) is supplied to the cathode, a current flows through the LED 31 a, and the LED 31 a emits light. A voltage is generated at both ends. The photovol coupler is commercially available from Toshiba Semiconductor Corporation under the product number TLP3914.

  The capacitor C1 is connected in parallel to both ends of the plurality of phototransistors 31b of the photovoltaic coupler 31, and is always charged with the same voltage as the voltage generated in the plurality of phototransistors 31b. Supply as the bias voltage of Q2. Therefore, by supplying a constant current to the LED 31a, a constant voltage can be generated in the photodiode 31b, and as a result, the capacitor C1 can hold the constant voltage.

  Preferably, the voltage supplied to both ends of the LED 31a of the photovoltaic coupler 31 is supplied from a positive and negative power supply voltage (that is, a large voltage is supplied), and the resistance value of a resistor (not shown) connected in series to the anode of the LED 31a is increased. Set. Thereby, the error of the electric current which flows into LED31a based on the ripple component etc. of a power supply can be reduced. As a result, the voltage across the photodiode 31b, that is, the charging voltage of the capacitor C1, can be held at a constant voltage very well. Alternatively, a constant current circuit may be connected to the LED 31a of the photovoltaic coupler 31, and a constant current may be passed from the constant current circuit to the LED 31a to hold the constant voltage in the capacitor C1.

  The operation and action of the switching amplifier 10 having the above configuration will be described. FIG. 3A is a waveform diagram showing the operation of the switching amplifier 10, wherein (a) shows the output waveform of the switching amplifier 10 (switching output circuit 12), and (b) shows the charging voltage of the capacitor C1. In particular, (a) shows a case where the input signal has a low frequency and a positive amplitude value. In the voltage supply circuit 30, positive and negative power supply voltages are supplied to both ends of the LED 31a of the photovoltaic coupler 31, and the LED 31a emits light when a current flows. As a result, a voltage is generated across the plurality of photodiodes 31b, and the capacitor C1 is charged to the voltage across the photodiodes 31b. The charging voltage of the capacitor C1 is charged through the photovoltaic coupler 31, and is always held at a constant voltage (for example, 9 V) that enables the transistor Q2 to be turned on (see FIG. 3A (b)). In other words, unlike the conventional case, the charging voltage of the capacitor C1 is always held at a constant voltage regardless of the on / off state of the MOSFET 15 of the switching output circuit 12. That is, even immediately after the power supply voltage is turned on, even if the input signal has a low frequency and a positive amplitude value, the charging voltage of the capacitor C1 is always held at a constant voltage.

  In the driver 11, as described above, the charging voltage of the capacitor C <b> 1 is supplied as a bias voltage, and the first PWM signal is supplied to output the drive signal DRV <b> 1 to the MOSFET 15. That is, when the first PWM signal is at a high level, the transistors Q1 and Q2 are turned on and the transistors Q3 and Q4 are turned off. As a result, the driver 11 supplies a high level drive signal to the gate of the MOSFET 15. The MOSFET 15 is turned on in response to a high level drive signal (at this time, the MOSFET 16 is turned off), and the switching output circuit 12 outputs a high level voltage (positive power supply voltage + VCC).

  Here, as shown in FIG. 3A (b), since the capacitor C1 is always charged with a voltage capable of turning on the transistor Q2 as described above, when the first PWM signal is at the high level, the transistors Q1, Q2 Can be reliably (fully) turned on, and the MOSFET 15 can be reliably (fully) turned on. As a result, when the first PWM signal is at a high level, the switching output circuit 12 can accurately output the positive power supply voltage + VCC. Therefore, it is possible to prevent noise from being reproduced from the speaker as a load. That is, even immediately after the power supply voltage is turned on or even when the input signal has a low frequency and a positive amplitude value, the charging voltage of the capacitor C1 is always held at a constant voltage. ), An accurate output signal (a waveform having a steep falling edge) can be output.

  Further, the path of the bias current when the first PWM signal is at the high level flows in order from one end of the capacitor C1 to the transistor Q2, the resistors R2, R5, the MOSFET 15, and the other end of the capacitor C1, as shown by the arrow in FIG. . Therefore, unlike the conventional case, the bias current does not flow to the speaker as a load. As a result, it is possible to reliably prevent noise from being generated from the speaker due to the voltage due to the bias current. The photodiode 31b is insulated from the LED 31a connected to a power supply circuit (not shown), and the path (loop) through which this bias current flows does not include the power supply circuit. Therefore, since the area of the loop can be made very small, the noise generated by the loop can be reduced extremely well.

  On the other hand, when the first PWM signal is at a low level, the transistors Q1 and Q2 are turned off, and the transistors Q3 and Q4 are turned on. As a result, the driver 11 supplies a low level drive signal to the gate of the MOSFET 15. The MOSFET 15 is turned off in response to the low level drive signal (at this time, the negative side MOSFET 16 is in the on state), and the switching output circuit 12 applies the low level voltage (negative side power supply voltage −VCC). Output. Explaining the current path at this time, current flows from the MOSFET 15 to the diode D1, the resistor R4, and the transistor Q4, and the parasitic capacitance voltage of the MOSFET 15 is discharged.

  FIG. 3B is a simulation result showing an output waveform of the switching output circuit 12 in the switching amplifier 10 of FIG. The horizontal axis indicates time, and the vertical axis indicates amplitude. According to this example, it can be seen that an accurate PWM waveform is always output.

  As described above, in this embodiment, a constant voltage for turning on the transistor Q2 of the driver 11 is always held in the capacitor C1, regardless of the on / off state of the MOSFET 15. Therefore, when the first PWM signal is at a high level, the driver 11 reliably supplies a high-level drive signal to the MOSFET 15 so that the MOSFET 15 can be reliably turned on. Further, the capacitor C1 does not repeat charging and discharging during one cycle of the PWM signal as in the prior art, so that a capacitor having a large capacity can be used without considering the time constant, and the ripple component is reduced. It can be removed sufficiently.

  As mentioned above, although preferable embodiment of this invention was described, this invention is not limited to these embodiment. In FIG. 2, the negative side portion of the driver 11 is not shown, but a similar circuit can be applied to the negative side portion. Further, a circuit configuration corresponding to a single power source (single power source), that is, a circuit configuration not including a negative side portion may be used. Further, the transistor of the driver 12 is not limited to the circuit configuration of FIG. 2 and may be any transistor that can output a drive signal to the gate of the MOSFET 15 in accordance with the PWM signal. That is, the driver 12 only needs to include at least the transistors Q2 and Q4. Moreover, the voltage which sends an electric current to LED31a is not limited to a power supply voltage. In addition, any appropriate photocoupler other than the photovoltaic coupler may be employed. Further, the pulse modulation signal may be, for example, a PFM signal other than the PWM signal.

  The present invention can be particularly suitably employed as an audio amplifier, for example.

It is a schematic block diagram which shows a typical switching amplifier. It is a schematic circuit diagram which shows the principal part of the switching amplifier by preferable embodiment of this invention. It is a wave form diagram which shows operation | movement of the switching amplifier of FIG. It is a simulation result which shows the output waveform of the switching output circuit of the switching amplifier of FIG. It is a schematic circuit diagram which shows the principal part of the conventional switching amplifier. It is a simulation result which shows the output waveform of the switching output circuit of the switching amplifier of FIG. FIG. 5 is a waveform diagram showing an operation of the switching amplifier of FIG. 4.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Switching amplifier 11 Driver 12 Switching output circuit 30 Voltage supply circuit 31 Constant voltage generation means, photovol coupler

Claims (3)

A voltage supply circuit for supplying an operating voltage;
Drive means for supplying a pulse modulation signal that repeats a high level and a low level and an operating voltage from the voltage supply circuit and outputting a drive signal that repeats a high level and a low level in response to the pulse modulation signal;
A switching output circuit that is turned on or off in response to a drive signal from the drive means;
A first transistor that is turned on or off in response to the pulse modulated signal; and a driver that is turned off when the first transistor is turned on in response to the pulse modulated signal; A second transistor that turns on when the first transistor is off;
The voltage supply circuit includes a capacitor connected between the first electrode of the first transistor and the first electrode of the second transistor, and the first supply voltage regardless of a high level and a low level of the pulse modulation signal. And a constant voltage generating means for causing the capacitor to always hold a constant voltage capable of turning on the transistor.   The constant voltage generating means includes a photovoltaic coupler including an LED that emits light when a current flows and a plurality of photodiodes that generate voltages at both ends when the LED emits light, and the plurality of photodiodes; The switching amplifier according to claim 1, wherein the capacitor is connected in parallel.   When the switching output circuit is turned on by the drive signal, a bias current flows from one end of the capacitor to the other end of the capacitor via the first transistor and the switching output circuit. The switching amplifier according to claim 1 or 2.