GB2250151A

GB2250151A - Power amplifier protection circuit

Info

Publication number: GB2250151A
Application number: GB9116391A
Authority: GB
Inventors: Christopher Jack Evans; Christopher Newy
Original assignee: NAD ELECTRONICS Ltd
Current assignee: NAD ELECTRONICS Ltd
Priority date: 1990-07-30
Filing date: 1991-07-30
Publication date: 1992-05-27
Anticipated expiration: 2011-07-30
Also published as: GB9016668D0; GB2250151B; GB9116391D0

Abstract

An audio power amplifier comprises an input 1 from an audio signal source which couples a signal to be amplified to an audio power amplifier Q1-Q4 via a clipper 3. The audio power amplifier Q1-Q4 is connected, together with a protection circuit Q5, Q6, Q11 across voltage power rails and the output of the amplifier couples power through a loudspeaker load ZL. The protection circuit is also connected to the output of the amplifier 2 and to the clipper 3. The protection circuit is arranged to monitor the power amplifier output by means of current and voltage sensing circuits Q5, Q6 and to activate the clipper in order to limit the amplifier input, if the output exceeds a predetermined threshold value. In another example, the protection circuit monitors the temperature of one or more components of the power amplifier and a transistor Q11 activates the clipper 3 if the temperature exceeds a predetermined threshold value. In a further embodiment (Fig 3) the over voltage, over current and over temperature may all be used to control the clipper 3. <IMAGE>

Description

POWER AMPLIFIER PROTECTION CIRCUIT The present invention relates to power amplifier protection circuits and in particular to such circuits for protecting high-fidelity audio power amplifiers intended primarily for domestic use.

Conventional amplifiers of this type generally have a push-pull output stage, and in class G amplifiers this output stage may comprise a line of four commutating transistors which are all connected between two high voltage power rails and the inner two transistors of which are connected between two lower voltage power rails. Under normal operating conditions when the power level through the amplifier is maintained at a relatively low level, the two outer transistors are not functional. In this condition, only the two inner transistors pass current, in the manner of a class B amplifier, so that thermal dissipation is relatively low. Thus, due to the normally low power operating conditions of the amplifier, the size of components of the amplifier, particularly a heat sink and mains power transformer, can be kept relatively small, in order to produce a cost-effective design.

The only times in which the outer transistors of the amplifier output stage come into operation is when there is a peak of high power through the amplifier.

However, because of the use of amplifier components selected for music use, it is necessary to prevent the amplifier from being driven with a continuous high level substantially sinusoidal power waveform which would cause the output transistors and power transformer to rapidly exceed their safe operating temperature. Consequently, an amplifier protection circuit is required.

Conventional protection circuits for audio power amplifiers, particularly of the class G type, work by switching off the high voltage power rails, when an overload condition is detected. This has the effect of preventing commutation by the outer transistors and thus limiting the power output. However, this type of protection circuit tends to establish a cycling effect between a fault condition and a protect condition in the event of a continuous overload and also requires the use of high voltage, high current, switching devices. Furthermore, the power is only re-applied once the amplifier heat sink has cooled down which can take a relatively long time.

It is an object of the present invention to provide a protection circuit which is suitable for use with an audio power amplifier of cost-effective design, and which may be capable of allowing short-duration high power peaks of substantially sinusoidal form and transients but does not allow the amplifier to be driven continuously with a high sinusoidal power output.

Accordingly, the present invention consists in a protection circuit for a power amplifier, the circuit comprising means for monitoring the output of the power amplifier and/or the temperature of one or more components of the amplifier, and means for limiting the input of the amplifier if the monitored output or temperature exceeds a predetermined threshold value.

The monitoring means may be arranged to monitor the temperature of components consisting of a heat sink for output power devices of the power amplifier and/or of the output power devices themselves.

The input limiting means may be arranged to activate a circuit to clip the input signal of the power amplifier when the monitored output or temperature exceeds the predetermined threshold value.

In one embodiment, the output monitoring means are arranged to determine an average output over a period of time. The output monitoring means may be arranged to monitor the current signal and the voltage signal of the output and to integrate the current and voltage signals over said time period, the resultant being applied as a feedback to the amplifier input.

The predetermined threshold value is preferably set so as to correspond substantially to a maximum safe continuous output power. Alternatively or additionally, the predetermined threshold value may be set so as to limit the average power to be delivered to a loudspeaker.

The temperature monitoring means may comprise a thermal sensor for detecting excessive operating temperatures or thermal faults, occurring due to for example poor ventilation or operation of the amplifier at a high ambient temperature, the sensor being arranged to activate the input limiting means.

In another embodiment the temperature monitoring means may comprise means for generating a first voltage output indicative of the temperature of the component or components being monitored, means for generating a second output indicative of the true power dissipation of the amplifier, and means for computing the temperature of said components or components from said first and second outputs. In one example, the means for generating the second output comprise means for multiplying a voltage output and a current output of the power amplifier.

The invention will now be further described by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows schematically, in block circuit form, an audio power amplifier system incorporating a protection circuit in accordance with one embodiment of the invention; Fig. 2 shows a circuit diagram of part of the system shown in Fig. 1, including the protection circuit; and Fig. 3 shows schematically, in block circuit form, an audio power amplifier system incorporating a protection circuit in accordance with another embodiment of the invention.

Referring first to Figure 1, the audio power amplifier system comprises an input 1 from an audio signal source (not shown) which transmits a signal to be amplified to an audio power amplifier 2 via a clipper 3. The audio power amplifier 2 is connected, together with a protection circuit 4, across voltage power rails +V and -V, and the output of the amplifier transmits power through a loud speaker load ZL. The protection circuit is also connected to the output of the amplifier 2 and to the clipper 3. As will be described in more detail hereinafter, the protection circuit is arranged to monitor the current and voltage signals and to integrate them over a period of time. The protection circuit then activates the clipper in order to limit the amplifier input, if the integrated output exceeds a pre-defined limit corresponding to a maximum safe continuous output power above which an overload condition occurs.

Referring now also to Figure 2, four commutating transistors Q1 to Q4 form the push-pull output stage of the audio power amplifier, which is preferably of the class G type. All four transistors are connected across highvoltage power rails +HV and -HV and the two inner transistors Q2 and Q3 are connected across two lower voltage power rails +LV and -LV.

In the protection circuit 4, a current monitor consisting of a diode D1, and transistor Q5 t monitors the current signal of the amplifier output transmitted via the transistors Q1 to Q4. At the same time, a voltage monitor, consisting of resistors R3 and R4 and diode D2, monitors the voltage signal of the amplifier output. These two signals are integrated through resistors R5 and R6 and capacitor C1. If the amplifier is being driven to an overload condition, the base junction of transistor Q6 will be forward biassed and the voltage presented at transistor Q7 and consequently Q10 will be reduced. Transistors Q8 and Qg provide a tracking negative voltage equal to that at the emitter of transistor Q7. As these voltages both reduce, the clipper 3, consisting of diodes D3 and D4, clips the input audio signal in order to reduce its level.

Therefore, by means of a feed-back loop system, the protection circuit effectively limits the input signal in order to prevent continuous transmission of high power levels through the amplifier via transistors Q1 and Q4, whilst still allowing transients to be passed. The input signal is also limited relatively slowly, even if severe clipping is necessary.

In the event of a thermal fault, a transistor Qil is used to pull down the clipping level. In this mode, the clip point tracks the high voltage rail, via Zener diode D5 and resistors R7 and R8. A capacitor C2 filters any ripple on the supply to prevent modulation of the input signal.

Thus, in another example of the present invention, the protection circuit can be arranged to monitor the temperature of one or more components of the power amplifier, such as its heat sink or its output power devices, and to activate the clipper in order to limit the amplifier input if the monitored temperature exceeds a predetermined limit indicative, for example, of the manufacturer's maximum operating temperature. This may be achieved by use of a simple thermal sensor, such as a thermistor.

A more sophisticated embodiment for monitoring temperature is illustrated in block circuit form in Figure 3, wherein like parts are labelled with like reference numerals with respect to Figure 1. In this embodiment, a true power dissipation signal is derived by a multiplier 5 which receives instantaneous voltage V and current I signals from across the output devices of the power amplifier 2 and multiplies the signals to produce the power dissipation signal p. A voltage signal VT which is representative of the temperature of the heat sink of the power amplifier 2 is generated and is fed together with the power dissipation signal p to an analogue computer 6.By modelling the thermal characteristics of the output devices and the heat sink, the analogue computer 5 generates a signal representative of the junction temperature of the output devices and compares it with a pre-set temperature corresponding to the manufacturer's maximum operating temperature. If the measured temperature exceeds the maximum temperature, then the analogue computer 6 will send an activating signal 7 to activate the clipper 3 to limit the amplifier input.

Whilst particular embodiments of the present invention have been described, various modifications will be envisaged without departure from the scope of the invention, as defined in the appended claims. For example, although the protection circuit in the described embodiment has been used to protect a class G audio power amplifier, it may be used to protect any class of amplifier, such as a conventional class B amplifier. In addition to, or as an alternative to, setting the threshold limit to correspond to a maximum safe continuous output level, it may be set to limit the average power delivered to a loudspeaker, thus preventing loudspeaker failure. If the loudspeaker system has a passive crossover, two such protection circuits are required with each one driven from a matching filter.

Claims

1. A protection circuit for a power amplifier, the circuit comprising means for monitoring the output of the power amplifier and/or the temperature of one or more components of the amplifier and means for limiting the input of the amplifier if the monitored output or temperature exceeds a predetermined threshold value.

2. A circuit as claimed in claim 1, wherein the monitoring means are arranged to monitor the temperature of components consisting of a heat sink for output power devices of the power amplifier and/or the output power devices themselves.

3. A circuit as claimed in claim 1 or 2, wherein the input limiting means are arranged to activate a circuit to clip the input signal of the power amplifier when the monitored output or temperature exceeds the predetermined threshold value.

4. A circuit as claimed in any one of claims 1 to 3, wherein the output monitoring means are arranged to determine an average output of the power amplifier over a period of time.

5. A circuit as claimed in claim 4, wherein the output monitoring means are arranged to monitor the current signal and the voltage signal of the power amplifier output and to integrate the current and voltage signals over said time period, the resultant being applied as a feedback to the amplifier input.

6. A circuit as claimed in any preceding claim, wherein the predetermined threshold value is set so as to correspond to a maximum safe continuous output power.

7. A circuit as claimed in any preceding claim, wherein the predetermined threshold value is set so as to limit the average power to be delivered to a loudspeaker.

8. A circuit as claimed in any preceding claim, wherein the temperature monitoring means comprise a thermal sensor for detecting excessive operating temperatures or thermal faults, the sensor being arranged to activate the input limiting means.

9. A circuit as claimed in any preceding claim, wherein the temperature monitoring means comprise means for generating a first voltage output indicative of the temperature of the component or components being monitored, means for generating a second output indicative of the true power dissipation of the amplifier, and means for computing the temperature of said component or components from said first and second outputs.

10. A circuit as claimed in claim 9, wherein the means for generating the second output comprise means for multiplying a voltage output and a current output of the power amplifier.

11. A protection circuit for a power amplifier, said circuit being substantially as herein described with reference to Figures 1 and 2 or Figure 3 of the accompanying drawings.

12. A power amplifier system including a protection circuit for the power amplifier,the circuit being constructed in accordance with any preceding claim.