WO2023244118A1

WO2023244118A1 - An improved audio amplifier with a transformer-less output stage and a method for operating an improved audio amplifier

Info

Publication number: WO2023244118A1
Application number: PCT/NO2023/050131
Authority: WO
Inventors: Stephen Mark Harrison
Original assignee: Zenitel Norway AS
Current assignee: Zenitel Norway AS
Priority date: 2022-06-15
Filing date: 2023-06-07
Publication date: 2023-12-21
Anticipated expiration: 2024-12-15
Also published as: NO20220678A1; NO347259B1

Abstract

The disclosure relates to an improved audio amplifier with a transformer less output stage comprising: an audio input stage (10), a self oscillating full bridge Class D amplifier (20A,20B), an isolated gate drive (2A,2B) power output stage for providing a galvanic barrier between the low voltage self oscillating circuit and the power output stage of the Class D amplifier (20A,20B), an isolated power supply (7) for powering the isolated gate drive (2A,2B) power output stage, a first amplifier feedback (8) connected via first resistor (R1) to the amplifier output, a second amplifier feedback (9), the differential compensating feedback, connected via a second resistor (R5) to the floating ground signal (7') of the isolated power supply (7) to compensate for common mode voltage applied to the power output stage. The disclosure further relates to a method for operating an improved audio amplifier.

Description

An improved audio amplifier with a transformer-less output stage and a method for operating an improved audio amplifier

Technical field

The present disclosure relates to an improved audio amplifier with a transformer- less output stage and a method for operating an improved audio amplifier. More specifically, the disclosure relates to an improved audio amplifier with a transformer-less output stage and a method for operating an improved audio amplifier as defined in the introductory parts of the independent claims.

Background art

Voice evacuation systems utilize loudspeakers usually driven at 100 V, often being powered by a 24V battery system. Thus, the 24 volt power supply must be transformed up to be able to supply 100V AC. Traditionally this is achieved by providing step up transformers arranged at the output of the amplifier stage of the system before connection to the loudspeakers. In addition to providing the required voltage transformation, the transformers provides a galvanic isolation between the amplifier stage and the output.

In EP 3223427 Al it is shown a Bridge-Tied Load output self-oscillating class D amplifier that includes a comparator receives an input signal from a signal input circuit at a second input terminal and outputs a positive-phase pulse width modulation signal and a reversephase pulse width modulation signal by comparing voltages of the two input terminal.

In US 2020/0228075 Al it is shown an audio power amplifier that includes a pair of switching devices, drive circuitry for driving the pair of switching devices to produce a signal, and an output filter to filter the signal from the pair of switching devices.

In WO 2018/211085 Al it is shown a ciass-D amplifier with an amplifier input and an amplifier output.

In WO 2022/021504 Al it is shown a composite parallel type audio digital power amplifier without dead zone distortion.

In WO 2019/110152 Al it is shown an amplifier, such as a Class D amplifier, having one or more feedback loops comprising a path from the input to the primary amplifier input.

In WO 2013/036710 Al it is shown an audio power amplifier with a four-quadrant power supply having a synchronous output rectifier that includes a planar transformer.

In US 7084799 Bl it is shown a sigma-delta modulated amplifier is disclosed that is oper-able to sum an input signal and a feedback signal. In Nielsen, D. et al, 2014 16th European Conference on Power Electronics and Applications, it is shown a Class D audio amplifier with 4th order output filter and selfoscillating fullstate hysteresis based feedback driving capacitive transducers.

In US 2003/0067348 Al it is shown a Class D switching audio amplifier incorporating four state modulation.

A problem with the solutions of the prior art is that they comprise many components, are expensive and large in physical size/volume. There is thus a need for improved audio amplifier having fewer components, occupy less volume, and with a lower life cycle cost.

Summary

It is an object of the present disclosure to mitigate, alleviate or eliminate one or more of the above-identified deficiencies and disadvantages in the prior art and solve at least the above mentioned problem. According to a first aspect there is provided an improved audio amplifier with a transformerless output stage comprising: an audio input stage, a selfoscillating full-bridge Class D amplifier, an isolated gate drive power output stage for providing a galvanic barrier between the low voltage self-oscillating circuit and the power output stage of the Class D amplifier, an isolated power supply for powering the isolated gate drive power output stage, a first amplifier feedback connected via a first resistor to the isolated gate drive output, and a second amplifier feedback, the differential compensating feedback, connected via a second resistor to the floating ground signal of the isolated power supply to compensate for common mode voltage applied to the power output stage.

An advantage with this is that the power output stage is not driven by a transformer, but the operation is comparable to the galvanic isolation provided by a transformer of prior art.

According to some embodiments, the isolated power supply is a switch mode power supply with a minimum output of +-30V.

Typically the switch mode power supply deliver a power output of 100 volts RMS, that is for example by supplying +-71 volts one either side giving a +-142 volt over the full-bridge Class D amplifier. This corresponds to 100V RMS.

According to some embodiments, the improved audio amplifier comprises a coupling resistor for providing synchronization of the two halves of the bridge. The coupling resistor ensures that the pulse-width-modulated, PWM, signal of the two halves of the bridge, of the Class D amplifier are kept in synchronisation to provide an optimal in-phase PWM signal. The audio modulation of each half of the bridge is in anti-phase.

According to some embodiments, the improved audio amplifier comprises: a common mode choke arranged at the output of the improved audio amplifier.

The common mode choke is configured to provides a considerable attenuation to common mode signals, but letting audio signals, being out of phase, through. The common mode choke also considerably reduces the currents flowing in the output filter due to the outputs switching in phase.

According to some embodiments, the output stage further comprises:

- an output low-pass filter,

- one or more audio speaker lines, and

- one or more loudspeakers.

According to some embodiments, the audio input stage further comprises:

- a DC input on the audio input stage, and the audio speaker line comprising:

- an end of line resistor.

By arranging a current sensor for sensing current through the end-of-line resistor it is possible to monitor the audio speaker line and thereby identifying loss of end-of-line resistor.

According to some embodiments, the audio speaker line comprises:

- an end of line resistor, and the isolated power supply further comprising:

- a common mode DC voltage connected to the floating ground, and

- a common mode voltage detector for monitoring the common mode voltage and detecting earth faults on the audio speaker lines.

When a small DC voltage is applied to the floating ground, this is reflected on the output, and any current flowing to earth will change the common mode voltage detected, and an earth fault may be detected.

According to a second aspect there is provided a method for operating an improved audio amplifier, the method comprising the following steps: providing an improved transformer-less audio amplifier according to the first aspect, wherein the isolated power supply is at least +-30V, and cancelling common mode voltage applied to the output by : providing a first amplifier feedback connected via a first resistor to the amplifier output, and providing a differential compensating feedback via a second resistor being connected to the ground of the isolated power supply.

Thus, a DC voltage from the common ground will cancel out the offset voltage from the isolated power supply in the amplifier topology being a differential amplifier stage.

According to some embodiments, the isolated power supply is in the range +-30V to +- 100V.

According to some embodiments, the method comprises the steps:

- connecting a low pass filter and a loudspeaker via a loudspeaker line to a common mode choke stage of the improved audio amplifier,

- the common mode choke providing a common mode filtering for reducing or substantially eliminating recirculating currents of the output filter, and

■■ providing improved Electro Magnetic Interference, EMI, immunity.

When the output from the power output stages are in phase the energy consumption of the amplifier is drastically reduced, and the EMC immunity achieved by the common mode choke is further improved by the isolation effect of the isolated gate drivers and the floating supply.

According to some embodiments, the method further comprises the steps, when a common mode DC voltage is applied to the floating ground:

- providing a voltage monitor on the common mode DC voltage, and

- detecting earth faults in the audio speaker lines by analyzing variances in the monitored common mode voltage.

According to some embodiments, the method further comprises the steps:

- providing a DC input on the audio input stage,

■ providing an end of line resistor on the audio speaker line,

- providing a current sensor for sensing current through the end-of-line resistor, and - identifying loss of end-of-line resistor by sensing current through the end-of-line resistor.

Effects and features of the second aspect are to a large extent analogous to those described above in connection with the first aspect. Embodiments mentioned in relation to the first aspect are largely compatible with the second aspect.

The present disclosure will become apparent from the detailed description given below. The detailed description and specific examples disclose preferred embodiments of the disclosure by way of illustration only. Those skilled in the art understand from guidance in the detailed description that changes and modifications may be made within the scope of the disclosure.

Hence, it is to be understood that the herein disclosed disclosure is not limited to the particular component parts of the device described or steps of the methods described since such device and method may vary. It is also to be understood that the terminology used herein is for purpose of describing particular embodiments only, and is not intended to be limiting. It should be noted that, as used in the specification and the appended claim, the articles "a”, "an", "the”, and "said" are intended to mean that there are one or more of the elements unless the context explicitly dictates otherwise. Thus, for example, reference to "a unit" or "the unit" may include several devices, and the like. Furthermore, the words "comprising", "including", "containing" and similar wordings does not exclude other elements or steps.

Terminology

The term "power output stage" is to be interpreted as any type of voltage -controlled transistor devices, such as, but not limited to: MOSFETs, IGBTs, and GaN HEMTs.

The term "isolated power supply" is to be interpreted as any power supply in which the output voltage is galvanically isolated from the input power source.

Brief descriptions of the drawings

The above objects, as well as additional objects, features and advantages of the present disclosure, will be more fully appreciated by reference to the following illustrative and nonlimiting detailed description of example embodiments of the present disclosure, when taken in conjunction with the accompanying drawings. Figure 1A shows a conceptual circuit diagram according to an embodiment of the present disclosure.

Figure IB shows the conceptual circuit diagram of figure 1A, with additional optional output stages II, and III

Figure 2 shows one half of the full bridge Class D amplifier.

Figure 3 shows the feedback topology according to an embodiment of the present disclosure.

Detailed description

The present disclosure will now be described with reference to the accompanying drawings, in which preferred example embodiments of the disclosure are shown. The disclosure may, however, be embodied in other forms and should not be construed as limited to the herein disclosed embodiments. The disclosed embodiments are provided to fully convey the scope of the disclosure to the skilled person.

Figure 1A shows a circuit diagram of a first embodiment of present disclosure. It should be understood that the different sections and stages may be: included as illustrated, be omitted in case there is no need for the stage/feature, or combined in other equivalent manners obvious to the skilled person.

The first aspect of this disclosure shows an improved audio amplifier with a transformerless output stage comprising: an audio input stage 10, a self-oscillating full-bridge Class D amplifier 20A,20B, an isolated gate drive 2A,2B power output stage for providing a galvanic barrier between the low voltage self-oscillating circuit and the power output stage of the Class D amplifier 20A,20B, an isolated power supply 7 for powering the isolated gate drive 2A,2B power output stage, a first amplifier feedback 8 connected via a first resistor R1 to the isolated gate drive 2A,2B output, and a second amplifier feedback 9, the differential compensating feedback, connected via a second resistor R5 to the floating ground signal 7' of the isolated power supply 7 to compensate for common mode voltage applied to the power output stage.

The circuit provides a low voltage side and a high voltage side, notified by the dotted line 100 through both the isolated floating supply 7 and the isolated gate drivers 2A, 2B. The circuit shows a bridge amplifier having two power stages 20A, 20B providing for the audio signal in antiphase, and the PWM signal in phase, wherein the topology of each amplifier is a self-oscillating class D amplifier.

The phase shift around the loop ensures the circuit oscillates at a certain frequency.

The signal input 10 is the modulation signal inputted to the comparator stage 2A, 2B varying the duty cycle providing pulse width modulation, PWM. The PWM signal is filtered in the output stage 4, 11A, 11B to provide the amplified audio output signal.

The industry standard uses backup battery voltage of 24 Volts, and transformers to step up the output voltage to 100V which is the standard audio distribution voltage. The transformers needed for this stage requires large space, and it has a high cost. Thus, utilizing existing techniques is undesirable on many levels.

Present discloser provides the amplifier without such output stage transformer, but rather providing the high voltage by using an isolated floating supply 7.

The advantage with this is that the power output stage is not driven by a transformer, and still it is, due to a high impedance of the feedback resistors, a defensive layer between the high voltage output stage and the low voltage stages. The feedback resistors provide a DC path at a very high impedance. The first amplifier feedback and the second amplifier feedback acts as differentiating compensating feedback enabling the signal to float and emulating a transformer like behavior.

So, as in a transformer coupled output stage, a large potential can be applied to the output stage, or even a short to ground, without affecting the circuit operation.

The same is provided with the improved audio amplifier according to present disclosure wherein the amplifier stage is operating like a differential amplifier as shown in figure 3, and the feedbacks are amplified differentially but cancels out common mode. Using the signal from the floating common ground 7' as the second feedback 9 to the second node of the amplifier stage, provides the ability to design the output stage as pseudo-floating in that it is possible to impose a common mode DC voltage 5 on the floating ground 7', and the circuit will keep working. It is even possible to short any of the outputs to ground and the circuit will keep working. Thus, acting very much like a transformer output stage, without the drawbacks of large size, and expensive components. In one embodiment the isolated power supply 7 is a switch mode power supply with a minimum output of +-30V.

The present technique is advantageously used for voice evacuation systems. Such voice evacuation systems uses 100 volts audio powered from 24 volts batteries. The key problem is to use a 24 Volt power supply and get 100 volts AC from it. This has traditionally been done by doing the amplifying at 24 volts and then using a step up transformer to 100 volts between the amplifier and to the speaker.

However, as shown in present disclosure the topology provides for typical voice evacuation systems wherein all that is needed is plus and minus 71 volts on the one side, plus and minus 71 volts on the other side of the bridge, which gives plus and minus 142 volts on the differential output stage, which results in 100 volts RMS.

Typically, any voltage level may be provided by the floating supply 7, but any voltage below 30V do not make any sense if the system is powered by 24 volts.

However, if the system is used in lower voltage systems with considerably smaller basic supply voltage, it is within the present disclosure to use isolated power supply 7 with lower than +-30V.

The bridge 20A,20B may comprises a coupling resistor Rc for providing synchronization of the two halves 20A,20B of the bridge.

The coupling resistor Rc is provided to make sure that both amplifiers are switching at the same frequency, synchronized, and to avoid ending up with a beat frequency on the output. The synchronization brings both of the PWM waveforms in phase. The audio modulation is anti-phase.

A common mode choke 4 may be arranged at the output of the improved audio amplifier.

So the PWM waveforms are in the same phase, and the common mode choke provides a significant attenuation to the common mode signals without providing any attenuation to the audio because the audio is out of phase.

The common mode choke will reduce, or substantially eliminate the current flowing in the output filter because power outputs are switching in phase, which again means that the energy consumption of the amplifier is considerably reduced. The goal of reducing the energy consumption of the amplifier is thus achieved by having the power outputs in phase.

The common mode choke further provides a good EMC immunity by providing a large common mode inductance between the amplifier output stage and the speaker wiring/cabling.

In a further embodiment the output stage may further comprise:

- an output low-pass filter

- one or more audio speaker line 13, and

■ one or more loudspeaker 12, and optionally a capacitor 14’ in series when end of line resistor is provided for end of line resistor monitoring.

And, further the audio input stage may further comprise:

- a DC input 6 on the audio input stage 10, and the audio speaker line 13 comprising:

- an end of line resistor 14.

Then, to monitor the speaker lines 13, it may be provided an end of line resistor 14, and by applying a DC bias on the input, it may then be possible to monitor this end of line resistor 14. The advantage of this circuit, even though it's isolated and behaves like a transformer, is that it’s DC coupled throughout.

A further embodiment of present disclosure provides audio speaker line monitoring by providing a current sensor 15 for sensing current through the end-of-line resistor 14 and for identifying loss of end-of-line resistor 14.

The DC offset puts a DC voltage on the output and thereby providing for a separate current sensing circuit 15, which then measures the current through that end of line resistor. Losing the end of line resistor may then be detected.

Multiple loudspeakers with individual end of line resistors may be connected in parallel at the audio output stage. The current sensor 15 may be set up to monitor all, a set of, or a single end-of-line resistors. Even, more current sensors 15 may be provided for monitoring individual or groups of end-of-line resistor(s). In figure IB there is illustrated optional one or more additional audio output stages II, and III in addition to a first output stage I. Several loudspeakers may be arranged in a parallel coupling to the output stage of the audio amplifier. Typical embodiment of the audio amplifier of present disclosure may distribute and output (alarm-) audio messages in building complexes such as, but not exclusively: office buildings, school buildings, industrial plants and other. Each audio output stage as illustrated here may then be corresponding to loudspeakers arranged in separate floors, for example, I: ground floor parking lot, II: first floor public area reception rooms, and III: the second floor housing the offices of the employees. The current sensor is here used to identify whether any of the output stages has been corrupted, e.g. by a shortage or loss of end-of-line resistor 14. When an error is detected: each output stage, I, II, and III, may be switched (not shown) out/in to determine which output stage is faulty. In a further embodiment there could be arranged multiple current sensors 15 (not shown) corresponding to measuring individual output stages, thus, providing a more efficient detection regime for short circuits/end-of-line resistor presence detection.

A further embodiment of present disclosure provides audio speaker line monitoring by providing a common mode DC voltage 5 connected to the floating ground 7', a common mode voltage detector 5' for monitoring the common mode voltage, and thus detecting earth faults on the audio speaker lines 13.

When a small common mode voltage is applied on the output, the voltage may be monitored by the common mode voltage detector 5', and any current flowing to earth can be detected as an earth fault.

The second aspect of this disclosure shows a method for operating an improved audio amplifier, the method comprising the following steps: providing an improved transformer-less audio amplifier according to present disclosure, wherein the isolated power supply 7 is at least +-30V, and cancelling common mode voltage applied to the output by : providing a first amplifier feedback 8 connected via a first resistor R1 to the amplifier output, and providing a differential compensating feedback 9 via the second resistor R5 being connected to the ground 7' of the isolated power supply 7.

Criterion for feedback common mode cancellation is given by the layout of figure 1A by:

Then it is possible to select differential compensating feedback resistor, R5, by:

In one example of component values selection it is important that R1 and R5 are high values to achieve high common mode impedance. Typical Values can then be chosen:

Rl~3OOk

R2=15k

R3~-10k

R4=600k

R5=2OOk

Rs-lk

Rc-lOk

The isolated power supply 7 may be provided to supply voltage in the range +-30V to +-100V.

The method may further comprising the steps:

- connecting a low pass filter 11A,11B and a loudspeaker 12 via a loudspeaker line 13 to a common mode choke 4 stage of the improved audio amplifier,

- the common mode choke 4 providing a common mode filtering for reducing or substantially eliminating recirculating currents of the output filter, and

- providing improved Electro Magnetic Interference, EMI, immunity.

The method may further comprising the steps:

- providing a voltage monitor 5' on the common mode DC voltage 5, and

- detecting earth faults in the audio speaker lines 13 by analyzing variances in the monitored common mode voltage 5.

The method may further comprising the steps:

- providing a DC input 6 on the audio input stage 10, - providing an end of line resistor 14 on the audio speaker line 13,

- providing a current sensor 12 for sensing current through the end-of-line resistor 14, and

- identifying loss of end-of-line resistor by sensing current through the end-of-line resistor 14.

The person skilled in the art realizes that the present disclosure is not limited to the preferred embodiments described above. The person skilled in the art further realizes that modifications and variations are possible within the scope of the appended claims. Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed disclosure, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. An improved audio amplifier with a transformer-less output stage comprising: an audio input stage (10), a self-oscillating full-bridge Class D amplifier (20A, 20B), an isolated gate drive (2A, 2B) power output stage for providing a galvanic barrier between a low voltage self-oscillating circuit and a power output stage of the Class D amplifier (20A, 20B), an isolated power supply (7) for powering the isolated gate drive (2A, 2B) power output stage, a first amplifier feedback (8) connected via a first resistor (Rl) to the isolated gate drive (2A, 2B) output, and a second amplifier feedback (9), the differential compensating feedback, connected via a second resistor (R5) to a floating ground signal (7') of the isolated power supply (7) to compensate for common mode voltage applied to the power output stage.

2. The improved audio amplifier according to claim 1, wherein the isolated power supply (7) is a switch mode power supply with a minimum output of +/-30V.

3. The improved audio amplifier according to claim 1 or 2, wherein the bridge (20A, 20B) comprising a coupling resistor (Rc) for providing synchronization of the two halves (20A, 20B) of the bridge.

4. The improved audio amplifier according to claim 3, further comprising: a common mode choke (4) arranged at the output of the improved audio amplifier.

5. The improved audio amplifier according to claim 4, wherein the output stage further comprising:

- an output low-pass filter (11A, 11B),

- one or more audio speaker line (13), and

- one or more loudspeaker (12).

6. The improved audio amplifier according to claim 5, wherein the audio input stage further comprising:

- a DC input (6) on the audio input stage (10), and the audio speaker line (13) comprising: an end of line resistor (14).

7. The improved audio amplifier according to claim 6, further comprising: audio speaker line monitoring by providing a current sensor (15) for sensing current through the end-of-line resistor (14) and for identifying loss of end-of-line resistor (14).

8. The improved audio amplifier according to any one of claim 5 to 7, wherein the audio speaker line (13) comprising:

- an end of line resistor (14), and the isolated power supply (7) further comprising:

- a common mode DC voltage (5) connected to the floating ground (7'), and

- a common mode voltage detector (5') for monitoring the common mode voltage and detecting earth faults on the audio speaker lines (13).

9. A method for operating an improved audio amplifier, the method comprising the following steps:

- providing an improved transformer-less audio amplifier according to any one of claim 1 - 8, wherein the isolated power supply (7) is at least +/-30V, and

- cancelling common mode voltage applied to the output by :

- providing a first amplifier feedback (8) connected vi a first resistor (Rl) to the amplifier output, and

- providing a differential compensating feedback (9) via a second resistor (R5) being connected to the ground (7') of the isolated power supply (7).

10.The method according to claim 9, wherein the isolated power supply (7) is in the range +/-30V to +/- 100V.

11.The method according to claim 9 or 10, further comprising the steps:

- connecting a low pass filter (11A, 11B) and a loudspeaker (12) via a loudspeaker line (13) to a common mode choke (4) stage of the improved audio amplifier,

- the common mode choke (4) providing a common mode filtering for reducing or substantially eliminating recirculating currents of the output filter, and

- providing improved Electro Magnetic Interference, EMI, immunity.

12.The method according to claim 11, wherein the method further comprising the steps, when a common mode DC voltage (5) is applied to the floating ground (7'):

- providing a voltage monitor (5') on the common mode DC voltage (5), and

- detecting earth faults in the audio speaker lines (13) by analyzing variances in the monitored common mode voltage (5).

13.The method according to any one of claim 10 to 12, wherein the method further comprising the steps:

- providing a DC input (6) on the audio input stage (10), - providing an end of line resistor (14) on the audio speaker line (13),

- providing a current sensor (12) for sensing current through the end-of-line resistor (14), and

- identifying loss of end-of-line resistor by sensing current through the end-of-line resistor (14).