JPS61287307A - Switching power amplifier circuit - Google Patents

Abstract

PURPOSE:To relax the required characteristic of a low-pass filter (LPF) by extracting a component of a stop band of the LPF restoring a signal power from a PWM waveform and impressing the component to a signal superimposing means connected to the output of the LPF via an amplifier so as to cancel the component of the output stop band of the LPF. CONSTITUTION:The low-pass filter consists of L and C and restores a fluctuation signal waveform from a PWM waveform and gives a power to a resistor RL. Only a high frequency component only is extracted from a voltage across the capacitor C by a high-pass filter 5 and power-amplified by a wide band AC amplifier 6 and the result is inputted to a primary winding of a transformer T. The secondary winding of the transformer T is inserted between the low-pass filter output and the load RL in the polarity cancelling the high frequency ripple voltage of the low-pass filter output thereby suppressing the noise having an undesired PWM frequency. Since the high-pass filter 5 need not to pass power and has only to divide the output of the low-pass filter and processes a low level signal, a high characteristic is realized easily. Further, the transformer T treats a high frequency component only and the amplitude is small, then the small shape can be realized.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a switching power amplifier circuit using pulse width modulation.

(Prior art) As a conventional technology of this kind, Japanese Patent Application Laid-Open No. 58-107071
(switching circuit), and JP-A-56-35511 (
A conventional example is explained with reference to Fig. 2. In Fig. 0, Q is a field effect transistor.

1 is a positive power supply terminal, 2 is a pulse width modulation (hereinafter referred to as PWM) signal input terminal, 3 is supplied with power from terminal 4, and the gate of Q (G
) and source (S).

Q is driven by a PWM signal and changes to 0N depending on its pulse width.
- Performs OFF operation.

Coil L g L z HL 3 and capacitor c1
．． c2. c.

constitutes a low-pass filter, and if the low-pass filter blocks the PWM repetition frequency, the load RL has a PWM ON duty of the voltage of the power supply terminal 1.
This means that a voltage twice as high as that of the current is supplied. If the low-pass filter passes the frequency of the modulation input signal, the ON duty of pulse width modulation will change according to the amplitude of the modulation signal, so the voltage supplied to the load RL will also change according to the modulation signal. Become. Therefore, if the switching is ideal, the voltage of the load RL can be changed without loss. Fo is a diode for high-speed switching, and O of Q
During the FF period, a flywheel operation is performed that satisfies the continuity of the current in the Boyle L and returns the energy stored in the coil to the power source. C4 is a power/source decoupling capacitor.

In an application where the above circuit is used as a modulator for a series modulation type radio broadcaster, RL is a modulated power amplifier and R
In order not to add spurious related to the PWM repetition frequency to L, use a 10KHz low-pass filter.
It is required to have characteristics such as passing audio signals up to 80 dB flatly without loss and suppressing the PWM repetition frequency, its harmonics, their sidebands, etc. by 80 dB or more.

(Problems to be Solved by the Invention) However, in the above conventional example, the repetition frequency of PWM is 100 KHz due to the current state of the switching elements.
Because it cannot be higher than the front and back, the characteristics required of a low-pass filter are particularly sophisticated, especially for a power filter, and the characteristics of a low-pass filter tend to change as the shape increases and load impedance changes. In addition, there were problems such as an increase in the delay time of the low-pass filter.

The present invention aims to improve the above-mentioned drawbacks of the prior art.The purpose of this invention is to relax the required characteristics of a low-pass filter, reduce the number of stages of the low-pass filter, reduce the size of the filter, and improve its characteristics against load fluctuations. An object of the present invention is to provide a PWM switching amplifier circuit that can sufficiently suppress noise or spurious related to PWM signals while reducing deterioration and shortening signal delay time.

(Means for Solving the Problems) The features of the present invention for achieving the above object include a switching means for performing pulse width modulation according to a modulation signal, and a switching means for restoring the modulation signal from the pulse width modulation waveform of the output thereof. a switching power amplifier circuit comprising: a signal superimposition means inserted between the low-pass filter and the load; a bypass filter connected to the output of the low-pass filter; - amplifying the output of the bypass filter; and an amplifier for supplying the signal to the signal superimposing means.

(Function) In the above configuration, the bypass filter extracts a component in the stopband of the low-pass filter, applies it to the signal superimposition means connected to the output of the low-pass filter via an amplifier, and then extracts the component in the stopband of the low-pass filter. components are canceled out. Therefore, even if the noise related to the PWM repetition frequency is not sufficiently reduced by the low-pass filter, the noise will be canceled out by the output of the bypass filter, and therefore the characteristics required for the low-pass filter are much more relaxed than in the past. Become.

(Example) FIG. 1 shows an embodiment of this invention, and shows 1 to 4. Q.

F D + RL r C4 is the same as the description of the conventional example. L and C are low-pass filters that restore the modulated signal waveform from the PWM waveform and provide power to RL, which is the same as the explanation of the conventional example, but in this example, two elements L and C, that is, a second-order configuration It is. Here, only high-frequency components of the voltage C are extracted by a bypass filter 5, power amplified by a wideband AC amplifier 6, and input to the primary winding of the transformer T. The secondary winding of the transformer T is inserted between the low-pass filter output and the load RL with a polarity that cancels the high-frequency ripple voltage of the low-pass filter output.

In the application of this embodiment as a modulator for a radio broadcasting machine using the serial modulation method, the maximum frequency of the modulation signal is 10 KHz,
If the PWM repetition frequency is 100KHz, a second-order Butterworth-type low-pass filter will have a frequency of 0 at 10KHz.
．． If the attenuation is 5 dB or less, the amount of rejection at 90 KHz is 3
I can't even get 0dB. Therefore, from the voltage of this low-pass filter output, the audio signal frequency 0 to 0 is determined by the bypass filter.
Blocks 10KHz and amplifies ripple only in the high range,
By superimposing it on the low-pass filter output using a transformer T at a level and phase that exactly cancels out the ripple, unnecessary PWM repetition frequency noise can be suppressed. Here, the bypass filter 5 does not need to pass power.

Since it is sufficient to divide the output voltage of the low-pass filter and handle low-level signals, it is easy to achieve advanced characteristics, and active filters can also be used.

Furthermore, the transformer T handles only high-frequency components of 10 KHz or higher, and its amplitude is small, so it can be realized with a small shape. Also, regarding the output required for the power amplifier 6, IK
Calculations for the W radio broadcasting device are as follows.

The load R, is a modulated power amplifier whose output is Po,
If its efficiency is η, the input to the modulated power amplifier P! teeth.

If p , =P O η, Po=1000(W)−η=0.85, then P! '', 1180 (W).

Here, suppose that the amplifier 6 required to provide 100% modulation is
It is well known that the output of It's not an exact calculation, but 100
% level, and the required output P^ of the power amplifier 6 is 0.65 (W), which shows that even if an amplifier with sufficient margin is provided, it is not a problem compared to the power to be handled.

Furthermore, although the input of the bypass filter is coupled from the output of the low-pass filter, the same effect can be obtained even if the input is coupled from the load voltage and the ripple is compressed by feedback operation.

(Effects of the Invention) As described above, according to the present invention, the required characteristics of a low-pass filter that restores signal power from a PWM waveform can be relaxed, and its order can be reduced.

Not only can the shape be made smaller, but the amount of delay can also be reduced.

In radio broadcasting applications, the impedance of the low-pass filter output becomes mismatched due to impedance fluctuations in the antenna system and the narrow band nature of impedance matching.However, because the low-pass filter has a simple configuration, there are few characteristic fluctuations, making it easy to use as a filter. Since the characteristics depend on the bypass filter inserted in the cancellation circuit, whose load is always matched, variations in the modulation characteristics can be made extremely small. Also, since the amount of modulation delay can be reduced, AM
This is particularly useful for broadcasting equipment used for stereo broadcasting or synchronized broadcasting. Furthermore, in the application of a switching regulator circuit, a power supply with fast output response and low switching noise can be realized.

[Brief explanation of the drawing]

FIG. 1 is a circuit showing an embodiment of the present invention, and FIG. 2 is a circuit showing a conventional example. 1.4-m-power supply terminal, 2-m-modulation signal input terminal, 3-m-pulse width modulator, RL--one load, Q-m-
Field effect transistor, 5-11 bypass filter, 6-- amplifier, T--
-Trans. C, C1~C4---capacitor, L, L1~L,---coil, FD-m-diode.

Claims (3)

[Claims] (1) A switching power amplifier circuit having a switching means that performs pulse width modulation according to a modulation signal, and a low-pass filter that restores a modulation signal from the pulse-width modulation waveform of its output and applies it to a load, comprising: (a) the low-pass filter; A signal superimposing means inserted between the loads; (b) a bypass filter connected to the output of the low-pass filter; and (c) an amplifier that amplifies the output of the bypass filter and supplies it to the signal superimposing means. A switching power amplifier circuit characterized by: (2) The switching power amplifier circuit according to claim 1, wherein the input of the bypass filter is connected to a connection point between the low-pass filter and the signal superimposing means. (3) The switching power amplifier circuit according to claim 1, wherein the input of the bypass filter is connected to a connection point between the signal superimposing means and the load.