GB2275384A - Controlling amplifier power supply in dependence on signal level - Google Patents

Abstract

The invention relates to a main power supply circuit including a rectifier and a loading capacitor providing an operating voltage for a power amplifier fed by a signal. It is an object to reduce the size and heat dissipation of said power amplifier. There is inserted between the output of said rectifier 5 and said loading capacitor 7 a thyristor 6 being fired by a gate pulse G the time position of which within main voltage period is controlled in dependence from the amplitude of said signal (L, R). Especially for an audio output power amplifier within an audio or television broadcast receiver. An alternative embodiment uses a switched mode power supply instead of the thyristor. <IMAGE>

Description

Main power supply circuit The invention relates to the concept in the control of a main power supply circuit in accordance with the introductory part of claim 1 and 8.

A power amplifier for the audio signal within an audio or television broadcast receiver has to handle a relative high power varying between about 0,5 and 40 W or 120 W for audio applications according to the instantaneous loudness of the music signal. The size of a power amplifier is usually constrained by the sizes of heatsinks and transformers.

It is an object of the present invention to reduce the heat dissipation due to power losses and thus the size of the power amplifier and also to reduce the size of the transformer used within such a power amplifier or the power supply circuit providing the operating voltage for said amplifier.

Heatsinks are used to transfer heat from the attached device, in this case: the power transistors, to a cooling environment. The temperature of said heats ink must maintain within the specified range. Heatsink size varies proportionally with heat dissipation from the device which in turn is due to power losses.

In a conventional power amplifier the operating voltage is almost constant, subjected only to load regulation of transformer. When music signal amplitude is low, the difference in supply voltage to output level contributes to a large voltage drop across said device in form of a power transistor. This contributes to the power losses, thus power dissipations.

In the present invention, the supply voltage varies in accordance with the input music signal with a small residual voltage of about 3V at all music amplitude variation. Power dissipation is largely reduced, thus enables one to use a smaller heatsink.

To realise this, input music signal is delayed to compensate for the processing time taken to vary the supply line with said signal. The amount of delay is dependent on the method used to design the controlled main power supply.

Method 1 (Thyristor Control): The invention relates to a main power supply circuit including a rectifier having its input connected to main terminals and having its output connected to a loading capacitor for providing an operating voltage for a power amplifier fed by a signal with substantially varying amplitude.

According to the invention there is inserted between the output of said rectifier and said loading capacitor a thyristor being fired during period of main voltage by a gate pulse the time position of which within said period being controlled by a control voltage in dependence from the amplitude of said signal.

In a preferred embodiment of the invention said rectifier is connected via a first thyristor to a first loading capacitor providing a positive operating voltage and via an opposite polarized second thyristor to a second loading capacitor providing a negative operating voltage. For deriving said control voltage, said signal is fed via a firing-angle controller, said controller having a further input fed by the main voltage and an output for deriving said gate pulse. Preferably said input signal is fed to an envelop detector the output of which being used for providing said control voltage. -Especially the output of said detector is fed to a peak and hold circuit the output of which providing said control voltage.

According to another embodiment of the invention a voltage to time position conversion is provided, having a first input fed by said control voltage, a second input fed by main voltage and an output providing said gate pulse.

In order to compensate for a delay introduced by controlled power supply delay means are provided between input terminals for said signal and input of said power amplifier. The delay caused by said circuits for providing said control voltage and said gate pulse lies in the range of some microseconds and therefore is negligible as compared to delay (big) caused by power supply. It can be shown by mathematic calculation and by measurements that power dissipation of power amplifier may be reduced by about 50% ~ 80 80% depending on output power.

In order that the invention may more readily be understood, a description is now given by way of example only reference being made to the accompanying drawing showing circuits and diagrams for illustrating the invention. Within the drawing Fig.1 shows in a simplified manner a circuit according to the invention, Fig.2 a modified portion of the circuit according to Fig.

1, Fig.3 waveforms for illustrating the function of the circuit according to Fig. 2, Fig.4 a block diagram showing a modification of the circuit according to Fig. 1, Fig.5 a block diagram showing a circuit for providing said gate pulses, Fig.6 waveforms showing selection of delay time (applicable to method 2), Fig.7 shows in a simplified manner the invention using switching mode method, Fig.8 waveforms for illustrating the function of the circuit according to Fig. 7 and Fig.9 a block diagram of the integration of the invention into integrated circuit for method 1 and 2.

In Fig. 1 audio signal L of the left stereo channel or R of the right stereo channel is fed from terminal 1 via a delay circuit 19 to the input of a power amplifier 2 the output of which being connected to loudspeaker 3. Main voltage AC is fed from termi nals 4 to rectifier 5 the output of which being connected via thyristor 6 to operating voltage input terminal of power amplifier 2, thus being fed with operating voltage V+ provided at loading capacitor 7.

Further there is provided a circuit 8 generating gate pulse G for firing thyristor 6. One input of circuit 8 is supplied with main voltage AC from terminals 4. Signal L or R, whichever higher, is fed to an envelop detector 9 providing a control voltage VC according to envelop of signal L or R. VC is fed to a second input of circuit 8.

During half period of main voltage AC one gate pulse G appears at gate of thyristor 6. Time position of said pulse G with respect to zero crossing point of main voltage is derived in dependence from VC and thereby in dependence from envelop amplitude of signal L or R. If signal L or R has a low amplitude, that means that low power is produced, then the time position of gate pulse G is near zero crossing point of main voltage AC so that less power is transmitted to loading capacitor 7 and operating voltage V+ is decreased. If loudness and therefore amplitude of L or R is increased, thereby increased the amplitude of VC, then gate pulse G is shifted in time to higher value of main voltage AC that means in direction to maximum of sine wave AC. The result is that more power is transmitted to loading capacitor 7 and V+ is increased as desired for higher loudness.

Fig. 2 shows a circuit operating according to Fig. 1. A first thyristor 6a is fired by gate pulse G1 and providing positive voltage V+ at loading capacitor 7a. V+ is fed to resistor 2a indicating load represented by amplifier 2. Additionally a second thyristor 6b fired by gate pulse G2 is provided for generating a negative operating voltage V- at loading capacitor 7b for load 2b. Such symmetric operating voltages V+ and V- are often needed for power amplifiers for stereo signals.

Fig. 3 shows waveforms occurring in Fig. 2, time shifting of gate pulses G1, G2 being indicated by arrows 10.

Fig. 4 shows a block diagram of the complete circuit. Input signals LIN and RIN are fed to firing-angle unit 11 and delay unit 19. Unit 19 provides output signals LOUT and ROUT for power amplifier 2 feeding loudspeakers 3a and 3b. Unit 11 provides gate pulses G1, G2 and being supplied with main voltage AC.

Amplifier 2 generally is a 2 channel power amplifier unit. Main voltage AC and gate pulses G1, G2 are fed to power supply unit 12 working according to principles of Fig. 2,3. Unit 12 supplies operating voltages V+, V- as well as ground lead for amplifier 2. As illustrated in Fig. 1-3 amplitude of V+ and V- is controlled in dependence from envelop amplitude of input signals LIN and RIN as described above and indicated by waveform 13.

Fig. 5 shows at circuit for generating gate pulses G1, G2. Input signals LIN and RIN are fed via amplifiers 14a and 14b to envelop detectors 15a and 15b, the outputs of which are fed to a peak and hold circuit 16 for providing control voltage VC according to Fig. 1-3 which is fed to voltage to firing angle circuit 17. On the other hand main voltage AC is fed to a sample and reset circuit 18 the output of which being connected to a second input of circuit 17. The envelop of the music signals LIN and RIN is extracted by detectors 15a, 15b. The peak and hold circuit 16 is used to detect the peak amplitude between the left and right channel at an interval of 8,33 ms corresponding to 120 Hz. Sample and reset circuit 18 is then used to convert the peak level to firing pulses with angle corresponding to the peak level.Note that the sampling circuit will sample at zero crossing at every 8,33 ms. Delay circuits 19a and 19b within the signal path each having a delay of 20 ms (2 cycles of 120Hz + a) are necessary to ensure that the capacitors (refer to envelop detector's charging capacitor) have ample time to reach required level W (Figure 6). Delay used has to be multiple of 8,33 ms due to this reason. For 120VAC and 60Hz, said delay has to be multiple of 8,33 ms and multiple of 10 ms for 220VAC and 50Hz. a is included to account for delay for processing circuitry.

Method 2 (Switching Mode): The main controlled power supply in this method is a switching mode power supply instead of the said thyristor-controlled power supply.

This embodiment of the invention relates to a main power supply circuit including a rectifier having its input connected to main terminals and having its output connected to a loading capacitor for providing an operating voltage for a power amplifier fed by a signal with substantially varying amplitude. According to this embodiment of the invention there is inserted between the output of said rectifier and said loading capacitor a switch mode power supply being switched by switching pulses with width or frequency being controlled by a control voltage in dependence from the amplitude of said signal.

In a preferred form of this embodiment said rectifier is connected via a switch mode power supply to a first loading capacitor providing a positive operating voltage and via the said power supply to a second loading capacitor providing a negative operating voltage. For deriving said control voltage said input signal is fed via a voltage-to-frequency converter or a voltage-to-pulse-width converter to output said switching pulses which will be frequency-modulated or pulse width modulated respectively.

In order to compensate for both the delay introduced by controlled power supply and the delay caused by said circuits for providing said control-voltage and said gate pulse which is some microseconds (this delay cannot be neglected as the delay caused by controlled power supply is small), delay means are provided between input terminals for said signal and input of said power amplifier. Said embodiment of the invention will now be described in connection with Fig. 7-9.

In Fig. 7 main voltage AC is fed from terminals 4 to rectifier 5 the output Vdc of which being connected via switch mode power supply to operating voltage input terminal of power amplifier 2, thus being fed with operating voltage V+ provided at loading capacitor 7. In a preferred embodiment said switched mode power supply 20 has one input supplied by the main voltage AC converted to Vdc from terminal 4. The second input is fed by circuit 21 or 22 which can be either a frequency-modulated signal 23 or a pulse-width modulated signal 24. Further there is provided a circuit 21 or 22 generating switching pulses 23 or 24 for switch mode power supply circuit 20. Signal L or R, whichever higher, is fed to an envelop detector 9 providing a control voltage VC according to envelop of signal L or R. VC is fed to the input of circuit 21 or 22.For the voltage-to-pulse width converter pulse width increases with higher control voltage VC. As for voltageto-frequency converter pulse frequency increases with higher control voltage VC.

Note that the delay introduced by circuit 19 will correspond to the period used for switching the power supply. It has to be a multiple of said period. For example, if the switching frequency is 10 kHz, the delay can be 0,1 ms, 0,2 ms... in multiple of 0,1 ms. Fig. 6 shows that two cycles are enough to charge to required voltage W.

The invention also includes to customise a controller for both methods in order to achieve to first objective, to reduce power amplifier size. The customized integrated circuit is connected first input from terminal 1, the audio signals L of the left stereo channel and R of the right stereo channel. It can have internal or external RAM for delay function and output a delay LOUT and ROUT audio signals to amplifier 2. The delay can set by parallel or serial ports control. Also output is another set of lines, providing pulses, to be connected to the thyristors (6) [method 1] or to the converter (21 or 22) [method 2]. The pulses generated will be as a method required. This is shown in Fig.9.

Claims (10)

Claims

1. Main power supply circuit including a rectifier (5) having its input connected to main terminal (4) and having its out put connected to a loading capacitor (7) for providing an op erating voltage (V) for a power amplifier (2) fed by a signal (L,R) with substantially varying amplitude, characterised in that there is inserted between the output of said rectifier (5) and said loading capacitor (7) a thyristor (6) being fi red during period of main voltage by a gate pulse (G1, G2) the time position of which within said period being control led by a control voltage (VC) in dependence from the envelop amplitude of said signal (L, R).

2. Circuit according to claim 1, characterised in that said rec tifier (5) is connected via a first thyristor (6a) to a first loading capacitor (7a) providing a positive operating voltage (V+) and via an opposite polarized second thyristor (6b) to a second loading capacitor (7b) providing a negative operating voltage (V-).

3. Circuit according to claim 1, characterised in that said sig nal is fed via a firing-angle controller (11), said control ler having a further input fed by the main voltage (AC) and an output for deriving said gate pulse (G1, G2).

4. Circuit according to claim 1, characterised in that said sig nal (L, R) is fed to an envelop-detector (15) the output of which being used for providing said control voltage (Vs).

5. Circuit according to claim 4, characterised in that the out put of said detector (15) is fed to a peak and hold circuit (16) the output of which providing said control voltage (VC).

6. Circuit according to claim 5, characterised in that a voltage to time position conversion is provided, having a first input fed by said control voltage (VC), a second input fed by main voltage (AC) and an output providing said gate pulse (G1, G2).

7. Circuit according to claim 5, characterised in that between input terminal for said signal (LIN, RIN) and input of said power amplifier (2) delay means (19) are provided for compen sating delay produced by the power supply.

8. Main power supply circuit including a rectifier (5) having its input connected to main terminals (4) and having its output connected to a loading capacitor (7) for providing an operating voltage for a power amplifier (2) fed by a signal with substantially varying amplitude, characterised in that there is inserted between the output of said rectifier (5) and said loading capacitor (7) a switch mode power supply unit (20) being switched by switching pulses with width or frequency being controlled by a control voltage (VC) in dependence from the amplitude of said signal (L,R).

9. Circuit according to claim 8, characterised in that said rectifier (5) is connected via a switch mode power supply unit (20) to a first loading capacitor (7a) providing a positive operating voltage and via said power supply (20) to a second loading capacitor (7b) providing a negative operating voltage.

10. Circuit according to claim 8, characterised in that in deriving said control voltage (VC) said input signal is fed via a voltage-to-frequency converter (21) or a voltage-to pulse width converter (22) to the output said switching pulses which will be frequency-modulated (23) or width modulated (24) respectively.