Audio direct from power supply

Abstract: A novel circuit that generates direct audio from a power supply that uses no power amplifier and that can provide full isolation to any impedance load. It has no power amplifier output stage and has a zero signal-to-noise ratio when there is no signal input because the circuit is designed with MOSFETS that do not conduct unless there is a signal input.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates in general to audio amplifiers and in particular a method and apparatus for obtaining off-line direct audio from a power supply without the use of a relatively high-power amplifier circuit. 2. DESCRIPTION OF RELATED ART Audio circuits are well known in the prior art and generally utilize an audio amplifying circuit to drive the audio speaker. The audio amplifier is required to a be power audio stage. Such power amplifier circuits are illustrated in U.S. Pat. Nos. 4,409,559 and 4,651,112. A typical class AB audio amplifier has a power supply and the audio amplifier. The cost of these two units together is significant in the manufacture of an audio output device. There are many switching power amplifiers that attempt to avoid the requirement of an audio power amplifier in the circuit. Such circuits are shown in U.S. Pat. Nos. 4,763,080, 4,517,522, 4,047,120, and 4,453,131. Class D amplifiers are always switching at fifty percent duty cycle even when no signal is applied. This causes an inherent noise that increases the signal-to-noise ratio when no signal is applied. It would be advantageous to have a highly efficient effective audio output circuit deriving its energy from a power supply and avoiding the requirement of having an expensive audio amplifier as the final stage. U.S. Pat. No. 4,573,018 describes a switching amplifier system using a power supply voltage of first and second polarity. A full-wave rectifier is coupled to a terminal that receives an audio input signal and rectifies the audio input signal. A pulse-width modulator is coupled to the full-wave rectifier and isolated from the power supply voltage and modulates the rectified audio input signal. A transformer is coupled to the modulator and has an input winding that receives the rectified and modulated audio input signal. An output winding of the transformer is connected at each end to a full-wave rectifier detector, the rectified outputs of which are supplied to averaging filters. The outputs of the averaging filters are combined in an electronic switch. SUMMARY OF THE INVENTION The present invention relates an off-line direct audio circuit utilizing the power supply and not requiring a power amplifier output stage. Because all power is delivered directly to the audio speaker from a switching transformer that provides a modulated signal obtaining variable voltages, there is no traditional audio stage like AB or Class D amplifiers. In the present invention, the audio input signal is applied to an error amplifier with a traditional feedback circuit. The input signal is full-wave rectified after the error amplifier. A pulse-width modulator circuit modulates the signal from the full-wave rectifier and that signal is coupled to a high-powered switching circuit. The high-powered switching circuit comprises a gate-drive unit isolated from the pulse-width modulator with a transformer and an output switching power transformer that transforms modulated high voltage/low current on the input to a modulated low voltage/high current on the output. Because the audio signal has been pulse-width modulated, the signal polarity at the output of the switching power transformer must be determined. Therefore, a zero-crossing detector is utilized to determine the zero crossings of the input audio signal. Those signals are coupled to first and second switches that gate the proper signal polarity outputs of the switching power transformer. The outputs are then filtered with a low-pass filter to remove the modulation signal and generate the audio output signal having sufficient amplitude to drive the speaker. A phase-lag compensation from the feedback circuit is coupled to the input signal and the error amplifier to prevent oscillations of the circuit. This circuit needs no power audio stage, is extremely efficient due to high voltage on the switching gates, and has a cost about one-half with respect to traditional Class AB power amplifier circuits. Further, when no audio signal is applied, there are no pulses from the pulse-width modulator and thus the signal-to-noise ratio is zero because there is no idle current that flows. Thus, it is an object of the present invention to provide a direct audio output signal from a power supply. It is another object of the present invention to pulse-width modulate an input audio signal and to use the pulse-width modulation to drive isolated gates that have a high voltage applied thereto and low current and then utilize an output transformer for switching the high voltage/low current pulse-modulated voltage to a low voltage/high current signal on the output of the transformer that can be properly detected with a switching circuit and then demodulated to provide the audio output signal for driving the speaker. It is still another object of the present invention to use MOSFETS for the gates utilizing the high voltage and low current inasmuch as the MOSFET devices are more efficient because lower current represents lower losses due to the internal resistance of the MOSFETS. Further, it is an object of the present invention to provide power MOSFETS that do not switch when there is no input audio signal thus having a zero signal-to-noise ratio when no signal is applied. It is still another object of the present invention to provide a direct audio from the power supply using an off-line system wherein the power supply is connected to AC line or the AC mains. Thus, the present invention relates to a method of obtaining off-line direct audio from the power supply comprising the steps of receiving an input audio signal, modulating a power supply voltage with the input audio signal, the power supply voltage being sufficient to drive an audio speaker, coupling the modulated power supply voltage to a filter circuit to recover the audio signal and driving the audio speaker with the recovered audio signal without further amplification. The invention also relates to apparatus for obtaining off-line direct audio directly from a power supply comprising a terminal for receiving an audio input signal, a voltage source having sufficient voltage of first and second polarity to drive the audio output speaker, a modulator for modulating the voltage from the voltage source with the audio input signals on the terminal, and a filter circuit for recovering the audio signal and providing a driving voltage sufficient to cause the audio signals to be reproduced by the audio output speaker. BRIEF DESCRIPTION OF THE DRAWINGS These and other features of the present invention will be more fully disclosed when taken in conjunction with the following Detailed Description of the Preferred Embodiment(s) in which like numerals represent like elements and in which: FIG. 1 is a block diagram of the novel audio circuit; FIG. 2 is a block diagram of the power supply forming a part of the novel circuit of FIG. 1; FIG. 3 is a waveform diagram illustrating the various waveforms at particular points in the circuit of FIG. 1; and FIG. 4 is a graph illustrating the cost of a typical prior art 100-watt AB audio amplifier as compared with the cost of the present circuit where direct audio is obtained directly from the power supply. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) FIG. 1 illustrates the novel audio drive circuit 10 in block diagram form. The audio input signal at terminal 12 (FIG. 3a) is coupled through the novel circuit 10 to drive the audio speaker 14 (FIG. 3i). The signal passes through an error amplifier 16 into a full-wave rectifier 18 and a zero-crossing detector 20. From the full-wave rectifier the input signal shown in FIG. 3(a) is rectified to that waveform shown in FIG. 3(b). The signal from the full-wave rectifier 18 is coupled to a pulse-width modulator 22 that generates the output shown in FIG. 3(d) to the input winding of transformer 24. Although a pulse-width modulator 22 is described in the preferred embodiment, other types of modulation such as frequency-modulation and phase modulation may be used. Transformer 24 serves to isolate the power MOSFET gates 30 and 32 from the pulse-width modulator circuit 22. The positive and negative voltages from the power supply (shown in FIG. 2) on terminals 34 and 36 are coupled to terminals 34 and 36 shown in FIG. 1 as an input to the MOSFETS 30 and 32. The MOSFETS 30 and 32 are triggered by the pulse-width modulated signals from modulator 22 through transformer windings 26 and 28. Because there is a high voltage coupled to the power terminals 34, 36 of the MOSFETS, there is a high voltage and low current through the MOSFETS. This is advantageous since the MOSFETS are used as high-frequency switches and the internal resistance when they are conducting can be analoged to the contact resistance of any switch. Since the power losses are dependent of the RMS current passing through the MOSFETS and the internal ON resistance, the losses caused by the MOSFET internal resistance in the ON state are minimal because of the low current. Since the positive 160 volts is coupled to the MOSFET 30 and negative 160 volts is connected to the MOSFET 32, MOSFET 30 gates the positive-going pulse-width modulated signals to the input winding 38 of switching power transformer 40. In like manner, MOSFET 32 switches the negative-going pulse-width modulated pulses to the input winding 38 of switching power transformer 40. Thus the gates 30 and 32 are alternately switched ON by the first and second polarity pulse-width modulated signals.* Power transformer 40 transfers the energy to the filter demodulator formed by inductors 62 and 66 and capacitors 64 and 68. Since there are positive and negative-going pulse-width modulated signals being applied to transformer 40, the output must be gated so that at the proper time the positive pulses are transferred to the filter demodulator and at the proper time the negative pulses are transferred to the filter demodulator. In order to do that, the zero-crossing detector 20 produces signals on line 21 that are coupled through buffer 23 to switches 54 and 60. These zero-crossing detector signals gate either switch 54 or switch 60 to enable the proper signals to be taken off the center tap 44 of secondary winding 42 of the power transformer 40.* See the wave shapes 3(e) and 3(f). The filter demodulator comprises inductors 62 and 66 and capacitors 64 and 68. When switch 60 is conducting, the positive pulses are demodulated and shown as the signal in FIG. 3(g). When switch 54 is ON, the negative pulses are demodulated and are shown as the signal in FIG. 3(h). The composite signal that is applied to the speaker 14 is shown in FIG. 3(i). FIG. 2 illustrates in block diagram the power supply that is coupled to the MOSFETS 30 and 32 and transformer winding 38 of the switching power transformer 40. The line voltage at 76 is coupled to an electromagnetic interference filter 78 and then to a line bridge rectifier 80. The outputs produced are +160 volts on terminal 34, -160 volts on terminal 36, and the center tap at terminal 39. There are several advantages to the novel off-line direct audio from power supply circuit. First, no power audio stage is needed. Further, a better efficiency is obtained due to the high voltage on the AC power line 76. Third, the cost of the completed audio speaker driver circuit is about one-half the traditional Class AB amplifier circuit. Note, in FIG. 4, that the cost for 100-watt audio amplifier for an AB bipolar system is approximately $26. About $14 of that is for the amplifier and approximately $12 is for the power supply. Note that the novel circuit for obtaining direct audio from the power supply costs approximately $14. Thus, there is a great advantage to utilizing the present circuit to generate the audio signals for driving the audio speaker. Further, when there is no audio signal applied, there is no output from the pulse-width modulator 22 and thus the MOSFETS 30 and 32 are not conducting. Therefore the signal-to-noise ratio is zero because there is no current flowing through the MOSFETS 30 and 32 when there is no signal input. This is not the case in the traditional amplifiers when there is noise generated by the amplifiers even though no signal is applied because they are still conducting without a signal input. Thus, there has been disclosed a novel circuit for a power supply that is connected directly to the AC line or AC main and that generates direct audio from the power supply to drive an audio speaker. All power is delivered directly from a switching transformer where the modulated audio signal is recovered to provide an output that drives the speaker after being demodulated. There is no traditional audio stage such as in conventional Class AB or Class D audio amplifiers. Further, by utilizing an isolating transformer and a switching power transformer, the current through MOSFETS is lower and is determined by the turns ratio of the switching transformer. MOSFET devices are more efficient because lower current represents lower losses due to the ON resistance of the devices. Thus, where high voltages are involved, there are low currents and the switching power transformer because of its turns ratio generates an output that has lower voltage and higher current. Finally, because the power MOSFETS conduct only when there is a signal input, the signal-to-noise ratio is zero when there is no signal because there is no current passing through the MOSFETS and, thus, no output current.

Claims: A method of obtaining off-line direct audio from a power supply, comprising: receiving an input audio signal (12), modulating said input audio signal (22) to provide a modulated input audio signal; and coupling said modulated input audio signal to the primary winding of an isolating transformer (24), and characterised by: coupling a first gate (30) between one end of a first secondary winding (26) of the isolating transformer and a first power supply voltage (34), coupling a second gate (32) between one end of a second secondary winding (28) of the isolating transformer and a second power supply voltage (34) such that the first and second gates are alternately switched on because of the polarity of said modulated input audio signal; detecting zero crossings (20) of said audio signal and generating corresponding signals (21), coupling the output voltage of each of said first and second gates to the input winding (38) of a switching power transformer (40), the switching power transformer producing a reduced voltage and increased current based on a centre-tapped output winding (42), gating (50-60) the output audio signals from the centre tap of the output winding of said switching power transformer with said detected zero crossing signals of said input audio signal to provide pulse-width-modulated signals having sufficient voltage amplitude to drive an audio output speaker without further amplification to a filter circuit to recover the audio signal; and driving an audio speaker (14) with said recovered audio signal without further amplification. The method of claim 1 wherein the step of modulating said input audio signal further comprises : full-wave rectifying said input audio signal (18); and converting said full-wave rectified input audio signal into a pulse-width modulated audio signal (22). The method of claim 2 wherein the step of coupling the output voltage of each of said first and second gates to the input winding of said switching power transformer further comprises: coupling the first gate between said power supply voltage source and the first secondary winding for passing the pulse-width modulated audio signals of a first polarity that are applied to the primary winding of said isolating transformer; coupling the second gate between said power supply voltage source and the second secondary winding for passing the pulse-width modulated audio signals of a second polarity opposite to the first polarity of the pulse-width modulated signals that are applied to the primary winding of said isolating transformer; and coupling the output voltage of each of said first and second gates that is modulated with said first and second polarity pulse-width modulated signals to the input winding of the switching power transformer. The method of claim 3 wherein the step of gating the audio output of the switching power transformer further comprises: a first switch coupled to the center-tapped output winding of said switching power transformer for gating said pulse-width modulated audio signals of the first polarity from the centre-tap of the output winding to said filter circuit when activated by said detected zero crossing signals; and a second switch coupled to the centre-tapped output winding of said switching power transformer for gating pulse-width modulated audio signals of the opposite polarity from the centre tap to the filter circuit when activated by said detected zero crossing signals. The method of claim 1 wherein the step of coupling the output voltage of each of said first and second gates further comprises : coupling an output of each one of said first and second gates to the input winding of said switching power transformer having an input winding for receiving said increased output voltage and an output winding generating an output voltage lower than the voltage on the input winding; coupling a first switch to said center-tapped output winding for passing pulse-width modulated signals of a first polarity; coupling a second oppositely polled switch to said center-tapped output winding for passing pulse-width modulated signals of a second polarity; and coupling said input audio signal zero crossing detector to said first and second switches to cause said switches to alternately conduct and transfer all power directly from said power transformer to said filter circuit for driving said audio speaker without further amplification. Apparatus for obtaining off-line direct audio directly from a power supply comprising: a terminal (12) for receiving an audio input signal; a power supply voltage (34, 36) having sufficient voltage of first and second polarity to drive an audio output speaker (14), a full-wave rectifier (18) coupled to the terminal for rectifying said audio input signal; a modulator (22) coupled to said full-wave rectifier and isolated from said power supply voltage for modulating said rectified audio input signal; and an isolating transformer (24) coupled to said modulator and having an input winding for receiving said rectified and modulated audio input signal; characterized in that: a first gate (30) is coupled between a first output winding (26) of the isolating transformer and said power supply voltage (34), for switching said first polarity power supply voltage to an output based on the rectified and modulated audio input signal at the input winding of the isolating transformer; a second gate (32) is coupled between a second output winding (28) of the isolating transformer and said power supply voltage (36), for switching said second polarity power supply voltage to said output based on the rectified and modulated audio input signal at the input winding of the isolating transformer; a switching power transformer (40) having an input winding (38) and a center-tapped output winding (42), the input winding being coupled to said output of said first and second gates; and a filter circuit (62-68) coupled to the center tap (44) of the center-tapped output winding (42) of said switching power transformer, for recovering said audio signal and providing a driving voltage sufficient to cause said audio signals to be reproduced by said audio output speaker (14), a first switch (54) coupled to a first end of said center-tapped output winding for coupling only positive voltages from said switching power transformer to the center tap (42), and a second switch (60) coupled to a second end of said center-tapped output winding for coupling only negative voltages from said switching power transformer to the center tap (42). The apparatus of claim 6 wherein said modulator is a pulse-width modulator. The apparatus of claim 7 wherein said switching power transformer comprises: a power transformer having a first voltage input winding and a center-tapped, step-down voltage output winding, the voltage input winding having first and second ends, and the center-tapped, step-down voltage output winding having first and second ends and the center tap. The apparatus of claim 8 further including: first and second diodes coupled between said first end of said center-tapped output winding and said first and second switches, respectively; said first diode being connected to said first switch and said second diode being connected to said second switch in opposite polarity to said first diode; third and fourth diodes coupled between said second end of said center-tapped output winding and said first and second switches, respectively; said third diode being connected to said first switch in parallel with said first diode and said fourth diode being connected to said second switch in parallel with said second diode. The apparatus of claim 6 further including a phase-lag compensation circuit being coupled to said filter circuit for preventing oscillation of said apparatus.

Inventors: RODRIGUEZ MANUEL D, RODRIGUEZ, MANUEL D.
Applicants: HARMAN INT IND, HARMAN INTERNATIONAL INDUSTRIES, INCORPORATED
Application year: 1999
Main classification: H03F3
IPC codes: H03F1, H03F3