Overload protection circuit

Abstract

This invention refers to an overload protection circuit for an oscillatory circuit driving an electro-acoustic converter and more specifically concerns a circuit for automatically inhibiting the oscillatory circuit in the presence of an overload. The overload protection circuit monitors the collector-emitter voltage signal and the base-emitter voltage signal for providing an overlap signal responsive to the interval when both mentioned voltage signals are in their high condition. Such a condition is indicative of the power dissipated in a switching transistor of the oscillatory circuit. The overlap signal is integrated and the integrated signal is compared to a predetermined voltage level. When the integrated signal exceeds the predetermined voltage level, the oscillatory circuit is inhibited. SUMMARY OF THE INVENTION This invention broadly relates to a overload protection circuit for an oscillatory circuit driving an electro-acoustic converter and more specifically concerns a circuit for automatically turning off the oscillatory circuit in the presence of an overload. Quite specifically, the overload protection circuit disclosed hereafter monitors the voltage condition at the base electrode of a power transistor, such voltage being indicative of the current flowing in the collector electrode, and the collector electrode voltage of the switching transistor used to drive the converter. In the event that both voltages are simultaneously in the high condition, that is, the transistor is dissipating excessive power, for a period of time in excess of a predetermined amount, the oscillatory circuit is switched off. One problem encountered in operating oscillatory circuits or power supplies for providing energy to an electro-acoustic converter is the wide range of load apparent at the converter. The oscillatory circuit must provide the power without a failure of the switching transistor which drives an output transformer for delivering the power to the converter. The switching transistor has a maximum allowable power dissipation inherent in its design and manufacture. The maximum allowable power dissipation is proportional to the product of the collector-emitter voltage (Vce), hereinafter referred to as the collector voltage, and the collector current (Ic). The provision of an overload protection circuit in an oscillatory circuit allows more power to be delivered to the converter than with a conventional oscillatory circuit. The oscillatory circuit, specifically the switching transistor, is operated closer to the maximum allowable power dissipation condition, since in the event of an overload condition the protection circuit will turn off the oscillatory circuit. In prior oscillatory circuits, the switching transistor has been operated at a condition well below the maximum permissible power dissipation responsive to varying load conditions. The present invention, therefore, provides an improvement over heretofore known oscillatory circuits. In prior overload protection circuits, the collector voltage and collector current are monitored and the oscillatory circuit is turned off when the collector current exceeds a predetermined value at the time when the collector-emitter voltage is at its high condition. These prior current monitoring circuits require current sensing transformers and the like which are generally costly and large. In an ideal oscillatory circuit, the collector voltage changes from the condition of zero volts, when the transistor is on and the collector current is flowing from the direct current power supply voltage, to the condition when the transistor is off and no collector current flows. In the ideal situation no power is dissipated in the transistor since power is the product of the collector voltage and the collector current in the transistor. In an actual circuit or when the oscillatory circuit is overloaded, the transistor is not completely turned on and the collector-emitter voltage is, therefore, not zero when a collector current is flowing. The power dissipated during this condition will, in a relatively short period of time, cause transistor failure. The present invention relies upon the fact that the collector current will flow only when the base electrode of the transistor has a positive voltage with respect to the emitter electrode, hereinafter referred to as the base voltage. Therefore, if the base voltage and collector voltage are simultaneously in their high conditions, there is collector current at a time when the collector-emitter voltage is high and the transistor is dissipating heat. In the present invention, the two electrode voltages, collector and base, are monitored with references to the emitter electrode voltage for providing a signal indicative of the overlap of the waveforms. The overlap, which occurs when both junction voltages are in the high voltage condition, is indicative of the power dissipation in the switching transistor. The overlap signal is integrated and the integrated value is compared to a predetermined level. If the integrated signal exceeds the maximum allowable power dissipation level represented by the predetermined level, a signal is produced for causing the oscillatory circuit to be turned off. Additional means are provided for overriding the protection circuit during the initial start up of the oscillatory circuit and for ensuring that the power supply is maintained off in the absence of a turn on signal. Another undesired mode in an oscillatory circuit for driving an electro-acoustic converter occurs when the frequency of the converter changes under the influence of an overload. This condition causes the applied high frequency signal to the converter to be at an incorrect frequency. An integrator circuit in the overload protection circuit will provide a signal of increased amplitude when the converter shifts to a higher frequency. The signal is processed as an overload indication which causes the oscillatory circuit to be turned off. A principal object, therefore, of the present invention is the provision of an overload protection circuit for use in an oscillatory circuit used for driving an electro-acoustic converter, the protection circuit turning off the oscillatory circuit in the event of an overload condition. Another object of the invention is the provision of an overload protection circuit for turning off an oscillatory circuit when the oscillatory circuit is operating at a non-desired high frequency due to an overload condition. Another salient object of the invention is the provision of an overload protection circuit for preventing the failure of a switching transistor forming an essential part of an oscillatory circuit. Further and still other objects of the present invention will become more clearly apparent from the following specification when taken in conjunction with the accompanying drawings.

Dependencies are inferred from the claim text.