GB2383907A

GB2383907A - A merged class D audio amplifier and power supply

Info

Publication number: GB2383907A
Application number: GB0200223A
Authority: GB
Inventors: Justin Noel Staines
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-01-07
Filing date: 2002-01-07
Publication date: 2003-07-09
Also published as: GB0200223D0

Abstract

This high efficiency Class D audio amplifier and power supply uses the pulse width modulation of the audio signal 16 to control the power supply 19. A preferred topology here employed is a fly-back mode where the unregulated power supply either from the rectified mains AC (primary) 4 or from an isolated secondary power supply (secondary). Here the primary AC mains 1 is switched. The PWM of the on-line switching circuit is controlled through an optical isolator 22 by the digital or analogue audio input from the source requiring amplification 16. On the secondary side of the on-line switcher, the resultant signal is then passed through a Low Pass Filter 10, 11 and 12. Any feedback information which needs to be fed back to the switching control circuitry, can be done so either from a tertiary winding 7, or if greater accuracy is required, through data passed to by circuitry in the Low Pass Filter 10 and 14. In addition the tertiary winding can provide a power supply 24 for other low current electronics or mechanics. Boost or Cuk converters may be used.

Description

CLASS D AUDIO AMPLFIER AND POWER SUPPLY This invention relates to a design for a high efficiency Class D audio amplifier and power supply. It uses the pulse width modulation of the audio signal to control the power supply.

The problem with conventional Class D audio amplifiers are that they have to be supplied power from an external regulated power source. If this power source is a linear regulated supply then up to as much as 50% of the power can be wasted. Alternatively if a switched mode power source is used although the efficiency may be higher depending on switching frequency, harmonics and filtering, another level of noise is introduced which must be filtered before this aforesaid noise is introduced into the amplification circuit.

Especially in audio applications, switched mode power supplies are usually avoided as they are usually not synchronised with the audio amplifier and can generate unwanted noise. With Class A/B audio amplifiers, switched mode power supplies maybe used, but because an analogue signal is input into the amplifier, the switched mode supply requires heavy filtering leading to very large capacitance. With Class D audio amplifiers, even though the input can be digital thus removing the DAC (digital to analogue convertor), all present Class D amplifiers aren't suited to use as either on-line or off-line switchers and lack the necessary performance or voltage requirements.

An object of this invention is to solve the problem of energy loss associated with the power supply of a Class D audio amplifier. Another object is to reduce the component count of a product with an audio amplifier (i. e. CD, MP3 or DVD player) and therefore the overall cost of this product.

One essential feature of the invention is to use the same PWM (pulse width modulated) digital signal from the audio source as the PWM signal used for the switched mode power-supply. By modulating the actual power supply for the audio amplifier there will be increased power savings. Another is to have the digital audio signal directly control the PWM of the power supply.

Preferably the switched mode power-supply is an on-line buck mode topology switcher, but may be of a different topology such as a off-line secondary switcher, or boost or cuk convertor. Also it is preferable that the audio signal is isolated from the on-line switched mode circuitry for safety using an opto-isolator or similar.

A preferred embodiment of the invention will now be described with reference to the accompanying drawing which: Figure 1 shows a schematic of the circuit diagram for an on-line convertor

As shown in Figure 1 the analogue or digital low level un-amplified audio signal 16 from a source such as a magnetic or CD/DVD pick-up is converted into a PWM (pulse width modulated) digital signal which can be amplified by traditional transistor methods (i. e.

NPN/PNP or FET's) 18,19 & 20 and by the nature of these devices power loss is only during a conducting period, so if this period is minimised by switching the signal this can significantly reduce power consumption. It does this by taking the unregulated power supply either from the rectified mains AC (primary) 4 or from an isolated secondary power supply (secondary). Here the primary AC mains 1 is switched on-line (known as on-line switching). The topology employed here is a fly-back mode. The PWM of the online switching circuit is controlled through an optical isolator 22 by the digital audio input from the source requiring amplification, of course bearing in mind there will be a phase change of the signal after it is carried through the transformer 17. Because the input signal is digital there is no DAC (digital to analogue) circuit required. The resultant switching signal is then transferred by inductive coupling 17, 7 & 8, (in this instance a high frequency transformer) which gives the necessary isolation from the mains AC.

By switching the primary AC the overall efficiency will be greater as the voltage is higher and the current is lower resulting in reduced losses when currents are lower. Also the switching frequency can be higher and the isolating transformer therefore smaller. A second embodiment is to use a off-line switching topology in the same manner as described. This however has disadvantages that the isolating transformer is larger and less efficient operating at the 50/60Hz cycle speed.

On the secondary side of the on-line switcher, the resultant signal is then passed through a Low Pass Filter 10, 11 & 12. Any feedback information which needs to be fed back to the switching control circuitry, can be done so either from a tertiary winding 7, or if greater accuracy is required, through data passed to by circuitry in the Low Pass Filter 10 & 14. In addition the tertiary winding can provide a power supply 24 for other low current electronics or mechanics. The AC audio PWM signal can simply be rectified 9 through a boost, buck or cuk style circuit to generate multiple output voltages which may be required for DSPs, CD/DVD laser, LCDs. Of course the output current of this tertiary winding will follow the transmitted power of the amplifier, so this needs to be understood when designing the balance of the whole system.

Claims

1. A Class D audio amplifier and power supply which uses the PWM (pulse width modulation) of the audio signal (whether digital or analogue) to control the PWM signal used by its switched mode power supply.

2. An audio amplifier as claimed in Claim 1 where the energy loss is reduced because there is only one switching frequency part or stage.

3. An audio amplifier as claimed in any preceding claim wherein signal degradation is decreased and audio quality increased by having no DAC (digital to analogue convertor) and having the digital version of the audio signal directly control the PWM of the power supply.

4. An audio amplifier substantially as herein described above and illustrated in the accompanying drawings.