Patent Description:
Prior art electronic amplifier circuits are often limited in the range of output voltages that they can provide by the supply voltage that they are provided with. For battery-operated devices this can be problematic when the battery voltage is not sufficiently high to deliver the desired output signals (under all circumstances). This is for example a problem in audio amplifiers which typically have output signals with peak signals that are many factors higher than the average signal power (so called high crest-factor).

To overcome this supply limitation, many amplifier circuit extensions have been proposed in the past that momentarily boost the available supply voltage beyond what is available from the external supply. Two approaches may be noted in particular: circuits that use a big storage capacitor (a 'flying capacitor') that can momentarily double the supply, such as in <CIT>, which document shows an amplifier device including an amplifier having an input for receiving an audio input signal and an output for sending an output signal to a load, wherein a boosted-rail circuit is connected to a power source and has a single boosted rail connected to the BTL amplifier, such as shown in <FIG> and further circuits that use a separate switching power stage to generate the local supply, typically consisting of a booster circuit with an inductor and capacitor as shown in <FIG>. Both variants of the prior art that was discussed above still has problems. In case of the flying capacitor, a primary problem is that it discharges during use and can only achieve as a maximum a doubling of the supply. Also, when it is heavily discharged, re-charging will give power dissipation, such as in a diode being present. In case of the switching booster, a problem is that it takes time to charge the local supply <NUM> above the external supply <NUM>. In more detail, this second problem entails the following: when a boost phase is desired, the inductor <NUM> first has to be charged by connecting it to ground <NUM> via the bottom transistor <NUM>, which has a side-effect that the local supply <NUM> drops before it is boosted. This initial charging gives a momentary drop in the output before it starts to rise above the supply, and is a manifestation of the 'right-half-plane' zero that is a well-known problem in control of switching booster stages in general. In the application for the amplifier, it means that either some headroom is needed in the control of the supply (to give some margin for undesired supply variations), or some delay is needed in the processing of the input to be able to 'look ahead' and anticipate with the switching before the boost is actually needed. Another disadvantage of the switching booster in <FIG> is that the supply current always runs through the primary inductor <NUM>, even when no boost is needed. The parasitic resistance of this inductor causes additional losses.

Incidentally <CIT> recites a system including a combination of a boost converter and a power converter coupled together in series, such that the series combination boosts an input voltage to the series combination to an output voltage greater than the input voltage such that a voltage boost provided by the series combination is greater than a voltage boost provided by the boost converter alone. The system may also include an amplifier, wherein an input of the amplifier is coupled to an output of the series combination of the boost converter and the power converter. The system is however relatively complex, and not very energy efficient. <CIT> relates to DC-DC switch-mode converters, particularly for portable electronic devices. These converters replace linear voltage regulators due to their higher efficiency, which is critical for battery-powered devices. D2 describes a DC-DC converter with an adaptive switch control system to optimize power efficiency: by dynamically adjusting the switch transition rate based on: load current demand (i.e., inductor current), supply voltage levels, device activity mode (e.g., high-power gaming vs. low-power audio playback), by reducing switch transition losses by modifying the turn-on and turn-off rates of power switches and by improving efficiency by using a programmable pre-driver that adapts switching speeds based on real-time conditions.

It is an objective of the present invention to overcome disadvantages of the prior art booster circuits, and especially electrical and audio functioning thereof, without jeopardizing functionality and advantages.

The present invention relates in a first aspect to a booster stage circuit according to claim <NUM> for a power amplifier circuit for amplifying an input signal and generating an output signal, including a circuit to generate the supply rail voltage to the power amplifier. The supply voltage for the power amplifier is lifted by a flying capacitor when the output signal exceeds the voltage that is available from an external supply (e.g. a battery). The flying capacitor is automatically recharged with high efficiency after each lifting cycle through an inductor, also increasing the voltage across the flying capacitor to above the external supply. When boosting is needed, then the capacitor <NUM> is lifted and placed in series with the supply by switch-transistor <NUM>. As such, an instantaneous doubling of the supply is available, without the charging delay present in the booster prior-art in <FIG>. When switch-transistor <NUM> closes, current will also start to build-up in the primary inductor <NUM>. This inductor current will re-charge capacitor <NUM> when the circuit returns to the normal supply by opening <NUM> and closing <NUM>. By alternating sufficiently fast between the normal supply <NUM> closed, <NUM> open and the boosted supply <NUM> open and <NUM> closed, a relatively small capacitance value <NUM> can be used while avoiding significant discharge, which is a marked advantage over the traditional flying capacitor supply doubler. On top of the above functionality, what also happens is that the average voltage on the flying capacitor increases, as a function of the duty cycle of the two switching phases. Also no degradation of power efficiency is provided as the capacitor is recharged with a high efficiency through the inductor <NUM> instead of through a dissipating diode <NUM> in the prior art of <FIG>. As with other booster circuits switching between modes can take place with variable, namely limited or full, energy transfer from the inductor to the capacitor.

So the present booster stage circuit is specifically suited for a power amplifier. Were reference is made to "ground" also the lower input voltage of a supply can be referred to. In between boost mode and base mode the switches <NUM>,<NUM> may both be open, which may be referred to as a dead zone mode, in order to prevent shorts.

In a second aspect the present invention relates to an external supply voltage power amplifier <NUM> comprising at least one booster stage circuit according to any of claims <NUM>-<NUM>, wherein the power amplifier is selected from an audio amplifier, a hearing aid amplifier, an electric motor control amplifier, a variable power supply unit, a time varying power supply.

Applications are especially those amplifiers that have a fixed supply voltage, either because they are battery powered (including automotive amplifiers) or because the supply-unit is fixed. A primary application is audio amplifiers. However, the concept is easily extended to other fields, e.g. electric motor control, variable power supply units, or any other field where time-varying power-signal are desired.

Thereby the present invention provides a solution to one or more of the above mentioned problems.

Advantages of the present description are detailed throughout the description.

The present invention relates in a first aspect to a booster stage circuit according to claim <NUM>.

In an exemplary embodiment of the present booster stage circuit the first and second switch <NUM>, <NUM> may each individually be selected from a transistor, such as an NMOS transistor, a PMOS transistor, a bipolar transistor, a FET, such as a GaN FET, an IGBT, and combinations thereof.

In an exemplary embodiment of the present booster stage circuit the capacitor <NUM> may be selected from a ceramic capacitor, a fill capacitor, an electrolytic capacitor, a non-polarized capacitor, a multilayer capacitor, with a capacitance of <NUM> pF- <NUM>µF, preferably <NUM> pF- <NUM>µF, more preferably <NUM> pF- <NUM>µF, even more preferably <NUM> nF- <NUM>µF, such as <NUM> nF- <NUM>µF. For a high power application a larger capacitance is preferred.

In an exemplary embodiment of the present booster stage circuit the inductor <NUM> may be selected from an air-core inductor, a ferro-magnetic-core inductor, a variable inductor, a choke, a solenoid, with an inductance of 1µH to <NUM>, preferably 1µH to <NUM>.

In an exemplary embodiment of the present booster stage circuit each individual switch may be adapted to operate at a switching frequency of <NUM>-<NUM>, preferably <NUM>-<NUM>, such as <NUM>-<NUM>.

In an exemplary embodiment the present booster stage circuit may further comprise a feedback loop details thereof, preferably wherein the feedback loop comprises a feedback filter.

In an exemplary embodiment the present booster stage circuit may further comprise a clock, wherein the clock is adapted to provide a clock frequency of > <NUM>, preferably <NUM>-<NUM>, more preferably <NUM>-<NUM>, such as <NUM>-<NUM>.

In an exemplary embodiment the present booster stage circuit may further comprise a rectifier parallel to the switch <NUM>,<NUM>, such as a diode.

The invention although described in detailed explanatory context may be best understood in conjunction with the accompanying examples and figures.

<FIG>, <FIG>, <FIG>, and <NUM>-<NUM> show details of booster circuits.

The figures are of an exemplary nature. Elements of the figures may be combined. In the figures:.

<FIG> shows prior art circuits that use a big storage capacitor (a 'flying capacitor') that can momentarily double the supply, as shown in <FIG> (with various embodiments conceivable).

<FIG> shows prior art circuits that use a separate switching power stage to generate the local supply, typically consisting of a booster circuit with an inductor <NUM> and capacitor <NUM>. The control and transistors for such an amplifier are for example present in Texas Instruments TAS2563.

<FIG> shows a present circuit which can be considered as a combination of the flying-capacitor supply doubler (augmenting the supply with the voltage stored on the capacitor) and a switching booster, overcoming the above drawbacks. During normal operation, in the base mode, such as when no boosting is needed, the local supply <NUM> is directly connected to the external supply <NUM>, with only a switch-transistor <NUM> in between, so there is no additional dissipation in a supply inductor. During this operation, the voltage across the flying capacitor <NUM> becomes equal to the supply <NUM> because its bottom plate is discharged to ground <NUM> via the inductor <NUM>.

When boosting is needed, then the capacitor <NUM> is lifted and placed in series with the supply by switch-transistor <NUM>. As such, an instantaneous doubling of the supply is available, without the charging delay present in the booster prior-art in <FIG>. When switch-transistor <NUM> closes, current will also start to build-up in the primary inductor <NUM>. This inductor current will re-charge capacitor <NUM> when the circuit returns to the normal supply by opening <NUM> and closing <NUM>. By alternating sufficiently fast between the normal supply <NUM> closed, <NUM> open) and the boosted supply (<NUM> open and <NUM> closed), a relatively small capacitance value <NUM> can be used while avoiding significant discharge, which is a marked advantage over the traditional flying capacitor supply doubler (prior art of <FIG>).

On top of the above functionality, what also occurs is that the average voltage on the flying capacitor increases, as a function of the duty cycle of the two switching phases, following similar relations as other switching converters. An example of the voltage and current waveforms involved are shown in <FIG>.

Various extensions of the concept are possible. First of all, the boost stage can be loaded with multiple amplifiers in parallel, with an example with two amplifiers (60a and 60b) shown in <FIG>. A similar technique can be used with a conventional booster from <FIG>, but with the conventional booster, all amplifiers will automatically use the higher supply even if their output does not require a higher supply, leading to more power consumption. In the proposed topology of <FIG>, the rail-voltage on <NUM> is at the normal supply during part of the cycle and at the boosted voltage during another. Switching schemes can be arranged such that any amplifier that only needs to produce small output voltages can switch its transistors to the rail <NUM> when that rail is at the normal supply, while those amplifiers that need higher output voltage use the rail <NUM> when it is boosted.

The signals that control the behavior of the switches in the booster <NUM> & <NUM> can be derived based on an input signal with pulse-width modulation techniques, possibly with compensation of the non-linear pulse-width to voltage relation, similar to what is for example done for a conventional boost circuit. As is customary in switching power converters, for better control over the output signals, the output voltage <NUM> and/or the current in the inductor <NUM> can be sensed and fed back to the controller <NUM>, as is shown in <FIG>. For the voltage and/or current-sensing, any of the various methods known in the field of power conversion for can be applied. The controller <NUM> itself can be an analog circuit or a digital controller. A digital controller, e.g. the one described in [<CIT>], first digitizes the sensed signals with analog to digital converters and subsequently uses digital control algorithms to create the pulse-width modulated (PWM) signals. Regardless of the method of control, once the PWM signals are created they need to be converted to the proper voltage levels to control the switches, which is usually done in a so-called Gate driver circuit <NUM>.

Another option is to cascade multiple booster stages, as shown in <FIG>. In such a cascade, the output (215a) of the first booster stage (100a) is connected to the supply input of the second booster stage (100b), which in turn creates the local supply (215b) for the amplifier. Which such a cascade, higher boosting factors are easier to achieve than with a single boosting stage. When the boost mode of the two stages is done simultaneous, then the external supply can be tripled at the onset of the boosting mode, with higher boosting factors possible once the capacitors are charged to higher values. Alternatively, when the boosting and base mode of the two stages are alternated, then the amplifier supply (215b) can get a boosted supply for a longer percentage of the time.

Another embodiment of the booster stage is shown in <FIG>. This embodiment (<NUM>) is the inverting, negative side equivalent of the supply-booster (<NUM>). Instead of lifting the supply, the flying capacitor (<NUM>) is now used to boost the low side of the amplifier supply (<NUM>). In base mode, switch (<NUM>) connects the low side of the amplifier (<NUM>) to ground (<NUM>). In boost mode, switch (<NUM>) opens and switch (<NUM>) closes, pushing the low side (<NUM>) below ground with a voltage equal to the stored voltage on capacitor (<NUM>), while simultaneous charging the capacitor (<NUM>) via inductor (<NUM>) which is connected to the external supply (<NUM>).

A combination of the high-side booster (<NUM>) and a low-side booster (<NUM>) leads to the embodiment shown in <FIG>. This embodiment enables boosting of both sides of the supply of the amplifier (<NUM>) which not only enables larger boosting factors, but also enables a more balanced behavior, in the sense that the voltages at both ends of the load (<NUM>) can change in opposing directions, also during boost modes. Such balanced or differential behavior can reduce the electromagnetic interference (EMI) generated by the amplifier.

Another amplifier embodiment that is enabled by the combination of a high-side and low-side booster from <FIG> is a single-ended amplifier, as shown in <FIG>. The single-ended amplifier (<NUM>) normally requires a positive and a negative supply voltage to be able to generate positive and negative output voltages. With the low-side booster stage (<NUM>), the negative supply voltages (<NUM>) can be created on demand. The high-side booster stage (<NUM>) also enables positive output voltages above the external supply (<NUM>). It would also be possible to omit the high-side booster (<NUM>) and connect the amplifier supply (<NUM>) directly to the external supply (<NUM>), but then the highest positive voltage on the load would be limited by the supply.

Further combinations of the various embodiments are of course also possible, such as the use of a cascade of boosters (as in <FIG>) in a symmetric configuration (as in <FIG> or <FIG>), or the use of multiple amplifiers (as in <FIG>) in a symmetric configuration.

Another option is to sense the output voltages of the amplifier as shown in <FIG> and use those signals <NUM> and <NUM> as an indirect indication of the rail voltage.

Claim 1:
Booster stage circuit (<NUM>) for a power amplifier comprising
an electrical connector (<NUM>) configured to receive an external power supply as input,
a first switch (<NUM>), the first switch having a first side and a second side,
a second switch (<NUM>), the second switch having a first side and a second side,
a capacitor (<NUM>), the capacitor having a first side and a second side, the capacitor being directly connected at the first side thereof to the first side of the first switch (<NUM>) and at the second side thereof to the first side of the second switch (<NUM>), wherein each of the first switch (<NUM>) and the second switch (<NUM>) is adapted to operate at a switching frequency of ><NUM>, wherein the second side of each of the first switch (<NUM>) and of the second switch (<NUM>) are directly connected to the electrical connector(<NUM>), and wherein in a boost mode the first switch (<NUM>) is in an open status and the second switch (<NUM>) is in a closed status, and wherein in a base mode the first switch (<NUM>) is in a closed status and the second switch (<NUM>) is in an open status,
an inductor (<NUM>) at one side directly connected to (i) the second side of the capacitor (<NUM>), and (ii) with the first side of the second switch (<NUM>) and at the other side (iii) directly connected with a ground, wherein the at least one inductor (<NUM>) has an inductance of 1µH to <NUM>,
control inputs of the first and second switch (<NUM>, <NUM>) are adapted for receiving control input from a controller for operating the first switch (<NUM>) and the second switch (<NUM>), respectively,
wherein the output voltage (<NUM>) of the booster circuit is provided at the first side of the capacitor (<NUM>), and optionally the controller for operating the first and second switch (<NUM>, <NUM>).