JP4305275B2 - Overmodulation prevention circuit in PWM modulation - Google Patents

Description

  The present invention relates to an overmodulation prevention circuit useful for a PWM (Pulse Width Modulation) modulator used in a digital audio amplifier, a motor driver, or the like.

For example, a PWM modulator used in a digital audio amplifier has a circuit configuration equivalent to that of the PWM modulator 1 shown in FIG. 5, and the level V of the (analog) audio signal 2 that is a modulated wave. (including so-called sawtooth wave.) and _a, a triangular wave is the carrier 3 and a level _{V B} of comparison operations in the comparator Comp3, and outputs a PWM output signal 4 which is PWM-modulated (level _{V C).}

6 and 7 show a level V _B of a triangular wave 3 (for example, a triangular wave of 400 KHz with ± 1.0 V amplitude) that is a carrier wave of the PWM modulator 1 and a gently curved (analog) audio signal 2 that is a modulated wave. FIG. 6 is a waveform diagram comparing the level V _A of FIG. 1 and the level V _C of the PWM output signal 4.

In the conventional pulse modulator 1, as can be seen from FIG. 6, when the maximum amplitude of the audio signal 2 is always smaller than the maximum amplitude of the triangular wave 3, the modulation is performed without any problem. However, as shown in FIG. If the amplitude is as larger than the maximum amplitude of the triangular wave 3 (beyond modulation of 100%) called overmodulation mode, and the level V _C of the PWM output signal 4 is stuck to the high voltage side or low voltage side A period occurs when the pulse disappears.

  In the digital audio amplifier or the like, when the overmodulation mode occurs in the PWM modulator, the power MOSFET of the output stage and the coil of the low-pass filter are heated.

  Further, as shown in FIG. 7, when the maximum amplitude of the triangular wave (including sawtooth wave) 3 and the amplitude at the time of the comparison operation of the audio signal 2 are substantially the same magnitude, the modulation pulse is shown as shown in a circled portion. May become a thin pulse waveform that is not output in a normal short waveform (so-called whisker pulse), but if the waveform of the modulation pulse is broken in this way, the audio characteristics (distortion rate, etc.) deteriorate. So it becomes a problem.

  Furthermore, depending on the response time at the output stage, the collapsed pulse waveform (whisker pulse) may not be output to the switching of the power MOSFET and may be in the above-described overmodulation mode.

  Therefore, the maximum modulation degree of the PWM modulation in the current PWM modulator is always within 90% or less in order to avoid the over-modulation mode (not exceeding the maximum amplitude of the triangular wave 3). In many cases, a limiter is provided to limit the amplitude of the audio signal 2 that is a modulated wave input to the comparator Comp3.

  As an overmodulation prevention circuit in a general audio device, the following [Patent Document 1] describes an overmodulation prevention circuit that is installed in an FM recording system such as a video tape recorder and prevents overmodulation of an audio signal to be recorded. The invention is described. This amplifies the audio signal, charges the capacitor by the detection output of the audio signal, and gives a logarithmic compression characteristic to the audio signal according to the charging voltage, and the noise reduction circuit that has passed through the noise reduction circuit A quick charge circuit that detects the level of the audio signal and rapidly charges the capacitor according to the level is provided.

  [Patent Document 2] below discloses an invention of a limiter circuit for limiting the signal amplitude of an audio signal of an FM recording / reproducing system VTR. This is to limit the amplitude when recording audio signals of FM recording / playback VTRs, expand the amplitude during playback, expand and contract the signal amplitude as necessary, and substantially increase the dynamic range regardless of the recording and playback mode. The limiter circuit selects the signal path of the feedback circuit for the amplifier according to the input signal selected from the plurality of input signals, and selects the signal path to the feedback circuit. On the other hand, a voltage limiter is added.

JP-A-2-44570

Japanese Patent Laid-Open No. 62-232206

  For digital audio amplifiers and motor drivers, the maximum modulation degree in the PWM modulator is preferably as high as possible as close to 100% from the viewpoint of performance and efficiency. In order to prevent overmodulation mode due to external factors such as noise, it is necessary to suppress the maximum allowable modulation degree to some extent, and it is possible to set only 90% at most. It was.

  If the maximum modulation degree in the PWM modulator is determined stably and reliably and can be set to a value close to 100% higher than the conventional one, a high-performance digital audio amplifier or motor driver will be realized. .

  In this regard, the techniques of the overmodulation prevention circuit and limiter circuit in the above-mentioned known documents relate to FM modulation, not to PWM modulation, and suggest technical applicability to prevent overmodulation in PWM modulation. There is nothing.

  After all, as a means for preventing overmodulation in PWM modulation at present, there is only a general-purpose technique for limiting the amplitude of the modulated wave such as the audio signal 2 before PWM modulation, and the maximum modulation degree is at most about 90%. Stays.

  The present invention has been made in view of the above circumstances, and prevents the occurrence of an overmodulation mode and the generation of whisker pulses in PWM modulation used in digital audio amplifiers, motor drivers, and the like, and an arbitrary maximum modulation degree. An object of the present invention is to provide an overmodulation prevention circuit that can be stably set to a value as close to 100% as possible, for example, exceeding 90%.

The present invention
(1) A level V _B of the triangular wave 3 which is a carrier wave in the PWM modulator 1 and a high voltage side voltage level V _H corresponding to the maximum allowable modulation degree on the high voltage side of the modulated wave 2 in the PWM modulator 1 The high-voltage side maximum modulation degree pulse signal V _D that is synchronized with the triangular wave 3 in comparison with the first comparator Comp1 is output, and the level V _{B of the} triangular wave 3 and the low voltage side of the modulated wave 2 are output. maximum for outputting the acceptable and low voltage-side voltage level V _L, corresponding to the maximum modulation factor is, the low-voltage side maximum modulation index pulse signal V _E for comparing in the second comparator Comp2 synchronized with the triangular wave 3 Modulation degree pulse signal generation means 12;
A PWM modulator PWM output signal 4 after PWM modulation by 1, type and the high voltage side maximum modulation index pulse signal V _D, the AND circuit 13 for outputting a pulse signal V _F of the logical product of them, the output signal V _F of the aND circuit 13, the enter the low-voltage side maximum modulation index pulse signal V _E, the OR circuit 15 which outputs a pulse signal V _G of the logical OR of the PWM output signal 14 And the maximum modulation degree pulse signal injection means 12 comprising: an overmodulation prevention circuit 20 in PWM modulation, which solves the above-mentioned problem.

Since the over-modulation prevention circuit in PWM modulation according to the present invention is configured as described above,
(1) While having a very simple circuit configuration, it is possible to completely prevent an overmodulation mode in PWM modulation.
(2) In addition to preventing heat generation of an output stage driver such as a digital audio amplifier or a motor driver using a PWM modulator, the life of the output device can be extended.
(3) Since the pulse width of the pulse signal to be injected can be reliably set to an arbitrary value, the maximum modulation degree can be freely set. For example, the maximum modulation degree can be set to a value close to 100%, such as 95%.
(4) By controlling the high voltage side voltage level and the low voltage side voltage level corresponding to the allowable maximum modulation degree as variable, it is possible to freely set the maximum modulation degree of the PWM modulator.
(5) Since the maximum modulation degree is stabilized, generation of whiskers can be prevented.

  The best mode of the overmodulation preventing circuit in PWM modulation according to the present invention will be described with reference to the drawings.

  FIG. 1 is a conceptual diagram for explaining the outline of an overmodulation prevention circuit according to the present invention. FIG. 2 is a diagram for explaining the maximum modulation degree pulse signal generating means of the overmodulation preventing circuit according to the present invention. FIG. 3 is an overall circuit diagram of an overmodulation preventing circuit according to the present invention added to an equivalent circuit diagram of a conventional PWM modulator. FIG. 4 is a timing chart (simulation) of signal waveforms at each node of the overmodulation preventing circuit according to the present invention.

  The idea of the present invention is that the PWM output signal 4 after PWM modulation of the conventional PWM modulator 1 to which a modulated wave without amplitude limitation is input is periodically and desired only in the overmodulation mode by some means. If the pulse signal is injected at the timing, even if the PWM output signal 4 after PWM modulation is in the overmodulation mode, the overmodulation mode is surely canceled in the PWM output signal into which the pulse signal has been injected. .

The overmodulation preventing circuit 20 of the present invention in FIG. 1 realizes the above idea, and the maximum allowable modulation degree (for example, 95) on the high voltage side and low voltage side of the modulation wave 2 in the conventional PWM modulator 1. maximum modulation index pulse signal generating the respective PWM output signal of%) corresponding high voltage side maximum modulation index pulse signal V _D and the low-voltage side maximum modulation index pulse signal V _E is generated in synchronism with the carrier 3 to be used for PWM modulation a means 11, a high voltage side and low voltage side of said high voltage side maximum modulation index pulse signal V _D and the low-voltage side maximum modulation index pulse signal V _E and the PWM modulator according to 1 PWM modulation after PWM output signal 4 And a maximum modulation degree pulse signal injection means 12 for injecting only in the overmodulation part.

Here, the maximum modulation degree pulse signal generating means 11 is a level V _B of the triangular wave 3 which is a carrier wave in the conventional PWM modulator 1 and a modulated wave in the PWM modulator 1, as shown in FIG. a high voltage-side voltage level V _H corresponding to the maximum allowable degree of modulation of the high-voltage side of the audio signal 2 (e.g. 95%), high that compared with the first comparator Comp1 synchronized with the triangular wave 3 The voltage side maximum modulation degree pulse signal V _D is output, and also corresponds to the level V _{B of the} triangular wave 3 and the allowable maximum modulation degree (for example, 95%) on the low voltage side of the audio signal 2 as the modulation wave. a low-voltage side voltage level V _L, has a configuration for outputting a low-voltage side maximum modulation index pulse signal V _E which is synchronized with the triangular wave 3 compared with the second comparator Comp2.

And Thus, a pulse of each pulse width is the minimum width the state that is a maximum modulation index allowed subsequent of the high voltage side maximum modulation index pulse signal V _D and the low-voltage side maximum modulation index pulse signal V _E It will be configured.

FIG. 3 shows the maximum PWM output signal 4 (level V _C ) after PWM modulation of the conventional equivalent PWM modulator 1 to which a modulated wave (audio signal) 2 without amplitude limitation is input. A modulation degree pulse signal generation means 11 and a specific maximum modulation degree pulse signal injection means 12 (consisting of a logical product circuit and a logical sum circuit) for the PWM output signal 4 after PWM modulation. An overall circuit having a configuration to which a modulation preventing circuit 20 is added is shown.

The maximum modulation degree pulse signal injection means 12 in FIG. 3 includes the PWM output signal 4 (level V _C ) after PWM modulation and the high voltage side output from the first comparator Comp1 in the maximum modulation degree pulse signal generation means 11. An AND circuit 13 that inputs a maximum modulation degree pulse signal V _D and outputs a pulse signal V _F of the logical product thereof, an output signal V _F of the AND circuit 13, and the maximum modulation degree pulse signal generation a low voltage side maximum modulation index pulse signal V _E output from the second comparator Comp2 in unit 11, the OR circuit 15 to output as a PWM output signal 14 a pulse signal V _G of the logical OR to input The circuit configuration is extremely simple.

As can be seen from the voltage timing chart (simulation) at each node of the circuit configuration shown in FIG. 4, the conventional PWM output signal 4 (level V _C ) has an overmodulation mode. overmodulation mode by the pulse signal V _{G (PWM} output signal 14 which prevents over-modulation) of high-voltage-side maximum modulation index pulse signal V _D and the low-voltage side maximum modulation index pulse signal V _E injected pulse injection means 12 It's gone. In the partial non-overmodulation, it appears to the high voltage side maximum modulation index pulse signal V _D and the low-voltage side maximum modulation index pulse signal V _E is not masked by the circuit logically injected to the original PWM output signal .

Further, beard pulse which has been conventionally generated are erased superimposed on the high voltage side maximum modulation index pulse signal V _D or the low-voltage side maximum modulation index pulse signal V _E is injected at the same timing.

As is apparent from the above, in the PWM modulator to which the overmodulation preventing circuit 20 of the present invention is added, the high voltage side voltage level V _H and the low voltage side voltage level V _L in the maximum modulation degree pulse signal generating means 11 are arbitrarily set. By setting to, the maximum modulation degree of PWM modulation can be set freely and stably.

For example, when the level V _B of the triangular wave 3 as the carrier wave of the PWM modulator is 400 KHz with an amplitude of ± 1.0 V and the maximum modulation degree is desired to be set to 95%, the level V _{B of the} triangular wave 3 is 2.5 μs in period. its 2.5% 62.5ns is a pulse width of the high voltage side maximum modulation index pulse signal _{V D} and the low-voltage side maximum modulation index pulse signal _{V E.} Therefore, the high voltage side voltage level V _H and the low voltage side voltage level V _L may be set to be constant (about ± 0.975 V) according to this. Of course, the allowable maximum modulation degree is not limited to 95%, but can be set to a value as close to 100% as possible.

In the overmodulation preventing circuit 20 in FIG. 3, the high voltage side maximum modulation degree pulse signal V to be injected is controlled by variably controlling the voltages V _H and V _L with an external control circuit (such as a microcomputer) in the overmodulation preventing circuit 20 in FIG. can be changed freely according to the pulse width _D and the low-voltage side maximum modulation index pulse signal V _E on the situation, it can be seen that the maximum modulation index of the PWM modulator can be easily and precisely controlled.

  The combination of the logical product circuit 13 and the logical sum circuit 15 of the maximum modulation degree pulse signal injection means 12 of the overmodulation prevention circuit 20 in FIG. 3 is the simplest, but of course, other logical circuit configurations are also possible. It is feasible.

Further, the in maximum modulation index pulse signal generating means 11, but to generate a high voltage side maximum modulation index pulse signal V _D and the low-voltage side maximum modulation index pulse signal V _E from the triangular wave 3 is the carrier, other means As described above, it can also be generated from a rectangular pulse signal (not shown) which is a basis for generating the triangular wave 3 in the PWM modulator 1.

  Furthermore, the idea of the overmodulation prevention circuit of the present invention that injects a pulse signal having a maximum modulation degree into a PWM output signal after modulation by a conventional PWM modulator can be applied to PDM (pulse continuous modulation).

It is a conceptual diagram for demonstrating the outline | summary of the overmodulation prevention circuit which concerns on this invention. It is a figure for demonstrating the maximum modulation degree pulse signal generation means of the overmodulation prevention circuit which concerns on this invention. FIG. 6 is an overall circuit diagram of an overmodulation prevention circuit according to the present invention added to an equivalent circuit diagram of a conventional PWM modulator. 4 is a timing chart (simulation) of signal waveforms at each node of the overmodulation prevention circuit according to the present invention. It is the circuit diagram which showed the conventional PWM modulator equivalent. FIG. 7 is a waveform diagram comparing a level V _B of a triangular wave that is a carrier wave of a conventional PWM modulator, a level V _A of an (analog) audio signal that is a modulated wave, and a level V _C of a PWM output signal. And level V _B of the triangular wave when the conventional overmodulation mode PWM modulator, a level V _A of the audio signal is a waveform diagram of comparing the level V _C of the PWM output signal.

Explanation of symbols

2 Minimum width pulse signal generation means
3 Minimum width pulse injection means
4 AND circuit
5 OR circuit 20 Overmodulation prevention circuit Comp1 1st comparator Comp2 2nd comparator Comp3 comparator P1 Minimum width pulse on the upper side P2 Minimum width pulse on the lower side
V _A (analog) audio signal
A triangular wave (including a so-called sawtooth wave) that is a V _B carrier wave.
V _C PWM output signal
V _D upper side minimum width pulse signal
_VE lower side minimum width pulse signal
V _H maximum level voltage
_VL minimum level voltage
The output signal of the V _F AND circuit
Pulse signal V _G logical sum (without overmodulation PWM output signal)

Claims (1)

The first comparator compares the level of the triangular wave that is a carrier wave in the PWM modulator and the high voltage side voltage level corresponding to the maximum allowable modulation degree on the high voltage side of the modulated wave in the PWM modulator. A high voltage side maximum modulation degree pulse signal synchronized with the triangular wave is output, and a second voltage level of the triangular wave and a low voltage side voltage level corresponding to an allowable maximum modulation degree on the low voltage side of the modulation wave are obtained. A maximum modulation degree pulse signal generating means for outputting a low voltage side maximum modulation degree pulse signal synchronized with the triangular wave by the comparator of
An AND circuit that inputs a PWM output signal after PWM modulation by the PWM modulator and the high-voltage-side maximum modulation degree pulse signal and outputs a pulse signal of the logical product thereof, and an output signal of the AND circuit And a logical sum circuit that inputs the low voltage side maximum modulation degree pulse signal and outputs a pulse signal of these logical sums as a PWM output signal, maximum modulation degree pulse signal injection means,
An overmodulation prevention circuit in PWM modulation, comprising:

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