POWER-SUPPLY CIRCUIT

Abstract: In order to maintain the voltage across a first capacitor (C₁) for energizing a motor (M) substantially constant over a wide range of mains voltages, this capacitor (C₁) is charged during the rising edges of the applied mains voltage in that above a specific input voltage (U min ) a first transistor (T₁) is turned on via a second capacitor (C₂) and a first resistor (R₂). When the mains voltabe rectified by a diode (D₂) exceeds a specific value (U₂) defined by a first zener diode (Z₂) a second transistor (T₂) is turned on and, consequently, the first trans­ istor (T₁) is turned off. When the first transistor (T₁) is cut off the first capacitor (C₁) is discharged through the motor (M). The second capacitor (C₂) is then discharged via a second zener diode (Z₁) and prevents the first transistor (T₁) from being turned on.

Description: The invention relates to a power-supply circuit for energizing a load, comprising between two input terminals for the application of an input voltage, a rectifier, a first capacitor having terminals for the connection of the load, a first transistor switch having a control electrode, and a second transistor switch for turning off the first transistor switch when the rectified input voltage exceeds a specific value. Such a circuit is suitable for energizing a load with different input voltages. Such a circuit is particularly suitable for use in a dryshaver, in which the circuit is utilized for energizing the motor. This enables the shaver to be used with different mains voltages available in various countries without the need of adaptation or switching over. Such a circuit is known from United States Patent Specification 4,001,668. In this circuit the first capacitor is maintained at an average constant voltage in that both during the rising edge and the falling edge of the rectified input voltage the first capacitor is recharged to the value of the rectified input voltage defined by the second transistor switch. A drawback of this circuit is that it generates a comparatively large amount of spurious radiation. This is because during the falling edge of the rectified input voltage the first transistor switch is turned on at the value of the input voltage defined by the second transistor switch, whilst at this instant the voltage across the first capacitor is lower than this value. As a result of this, the first capacitor is charged with a comparatively large charging current, so that the voltage across this capacitor increases stepwise to the value of the rectified input voltage defined by the second transistor switch. Therefore, it is the object of the invention to provide a power-supply circuit which produces practically no spurious radiation. According to the invention a power-supply circuit of the type defined in the opening paragraph is characterized in that the control electrode of the first transistor switch is coupled to one input terminal by a second capacitor and to the other input terminal by at least one diode. During the rising edges of the input voltage the first transistor switch is turned on by the charging current of the second capacitor, whilst during the falling edges the second capacitor discharges through the diode so that the first transistor switch is kept in the non-conductive state. As a result of this, the first capacitor is charged only during the rising edges of the rectified input voltage. The voltage across the first capacitor then follows the gradually increase of the input voltage, which precludes the occurrence of spurious radiation. Moreover, as compared with the known circuit, the second capacitor reduces the dissipation in the control circuit of the first transistor switch. A first embodiment of the invention is characterized in that the circuit comprises first protection means for turning off the first transistor switch when a specific current through the load is exceeded. For example, in the event of a short-circuit of the load the first protection means turn off the first transistor switch in order to prevent said transistor switch from being damaged by an excessive current. These first protection means may be characterized further in that they comprise a resistor which is arranged in series with the load and which is coupled to the control electrode of the second transistor switch by at least a first reference-voltage element. A second embodiment of the invention is characterized in that the circuit comprises second protection means for turning off the first transistor switch when a specific current through the first capacitor is exceeded. The second protection means prevent the first transistor switch from being damaged by an excessive charging current when the power-supply circuit is switched on in the case of a large instantaneous value of the rectified input voltage and in the case of voltage transients. In a further embodiment these second protection means may be characterized further in that they comprise a resistor which is arranged in series with the first transistor switch, and which is coupled to the control electrode of the second transistor switch by at least one diode. A third embodiment of the invention is characterized in that the circuit comprises a correction device for increasing the value of the rectified input voltage above which the second transistor switch turns off the first transistor switch when the load current increases. By means of the correction device the first capacitor is charged to a voltage which increases as the load current increases in order to ensure that the average voltage accross this capacitor and hence across the load increases. In a further embodiment, if the control electrode of the second transistor switch is coupled to an output of the rectifier by a series arrangement of at least one zener diode and a resistor, the correction device may comprise a transistor whose base and emitter are connected to the terminals of a resistor arranged in series with the load and whose collector is connected to the junction point between the zener diode and the resistor in the series arrangement between the control electrode of the second transistor switch and the output of the rectifier. A further embodiment may be characterized in that the correction device comprises limiting means for limiting the correction provided by the correction circuit above a specific load current. Embodiments of the invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which Fig. 1 shows the basis diagram of a power-supply circuit in accordance with the invention, Fig. 2 shows some voltage characteristics to explain the operation of the circuit shown in Fig. 1, Fig. 3 shows a first embodiment of the invention, Fig. 4 shows a second embodiment of the invention, and Fig. 5 shows a third embodiment of the invention. Fig. 1 shows the basic diagram of a power-supply circuit in accordance with the invention. Between the two terminals 2 and 3 for the application of the mains voltage the circuit comprises a series arrangement of a resistor R₁, a rectifier, which in the present example comprises a diode D₁, a first capacitor C₁ having terminals 4 and 5 for connecting a load which in the present example comprises a motor of, for example, a dry-shaver, and a first transistor switch which in the present example comprises a single transistor T₁. By means of the series arrangement of a resistor R₂ and the capacitor C₂ the control electrode of this transistor T₁ is connected to the anode of the rectifier D₁. The junction point between the resistor R₂ and the capacitor C₂ is connected to the input terminal 3 by a zener diode Z₁. The circuit further comprises a second transistor switch, which in the present example comprises a transistor T₂ whose collector-emitter path is arranged in parallel with the base-emitter junction of the transistor T₁ and whose control electrode is connected to the cathode of the rectifier D₁ by a series arrangement of a zener diode Z₂ and a resistor R₃. The operation of the circuit will now be explained with reference to Fig. 2, which illustrates how the voltages on some points in the circuit vary. The sinusoidal mains voltage V i applied between the input terminals 2 and 3 is shown in broken lines in Fig. 2. During the rising edge in the time interval 0-t₃ of this main voltage the capacitor C₂ is charged, the charging current flowing from the input terminal 2 to the input terminal 3 via the resistor R₁, the capacitor C₂, the resistor R₂ and the base-emitter junction of the transistor T₁. The base current of the transistor T₁ is then limited by the resistor R₂ and by the zener diode Z₁, which is turned on above a specific value of the voltage across this resistor and directly drains the charging current to the input terminal 3. Initially there is no current in the collector line of the transistor T₁, because the rectifier diode D₁ is cut off as a result of the voltage still present across the capacitor C₁. In this time interval the capacitor C₁ is discharged through the load M. At the instant t₁ the instantaneous value of the mains voltage becomes higher than the voltage U min across the capacitor C₁, so that the rectifier diode D₁ is turned on. As a result of this, the capacitor C₁ is charged, the charging current flowing from the input terminal 2 to the input terminal 3 via the resistor R₁, the diode D₁, the capacitor C₁ and the collector emitter path of the transistor T₁. The charging current is then limited by the resistor R₁. The voltage across the capacitor C₁ now increases in conformity with the increase of the mains voltage V i . At the instant t₂ when the rectified input voltage has an instantaneous value U₂ the zener diode Z₂ and the transistor T₂ are driven into conduction. This causes the transistor T₁ to be cut off, so that the capacitor C₁ is not charged any further and the maximum voltage across this capacitor is consequently equal to the voltage U₂. The transistor T₂ remains conductive until at the instant t₄ the instantaneous value of the rectified input voltage becomes smaller than the voltage U₂. In the time interval t₂-t₃ the capacitor C₂ is charged to the peak value of the input voltage, the charging current flowing through the collector-emitter path of the transistor T₂. During the falling edge of the mains voltage in the time interval t₃-t₆ the capacitor C₂ is discharged completely via the zener diode Z₁ which now operates as a diode, thus preventing the transistor T₁ from being turned on. The transistor T₁ is turned on again if during the next rising edge the instantaneous value of the mains voltage exceeds the capacitor voltage U₁. When the load is constant the average value Uc₁ of the voltage across the capacitor C₁ is thus maintained constant in the above manner. The operation of the circuit is then independent of the mains-voltage amplitude over a wide range. Fig. 3 shows a first embodiment of the invention, identical parts bearing the same reference numerals as in Fig. 1. In this embodiment a resistor R₄ is arranged in series with the load M, the junction point between this resistor R₄ and the load M being connected to the control electrode of the transistor T₂ via the series arrangement of a zener diode Z₃ and a resistor R₅. These additional elements constitute the first protection means which turn off the power-supply circuit in the event of, for example, a short-circuit of the motor or blocking of the motor. Above a specific value of the current through the load and hence above a specific value of the voltage across the resistor R₄ the zener diode Z₃ is turned on, so that the transistor T₂ is turned on and consequently the transistor T₁ is turned off. The resistor R₅ then limits the base current of the transistor T₂. During normal operation of the circuit the protection means have another advantage. If the transistor T₂ turns off the transistor T₁, the collector voltage of the latter increases. This increase is transferred to the base of the transistor T₂ via the resistor R₄, the zener diode Z₃ and the resistor R₅, resulting in a positive-feedback effect which causes the transistor T₁ to be turned off very rapidly. In this embodiment a negative voltage-dependent resistor R₆ is arranged between the input terminals 2 and 3 to limit the input voltage to, for example, 600 V. Fig. 4 shows a second embodiment of the invention, in which identical parts bear the same reference numerals as in Fig. 3. In this embodiment a resistor R₈ is arranged in the emitter line of the transistor T₁ and the emitter of the transistor T₁ is connected to the base of the transistor T₂ via the series arrangement of a resistor R₉ and a diode D₂. These elements constitute the second protection means which turn off the power-supply circuit in the event of an excessive charging current through the transistor T₁. If the power-supply circuit is switched on for example as the instant at which the mains voltage has its maximum value, the transistor T₁ is turned on rapidly via the capacitor C₂. At this instant the capacitor C₁ is still without charge, so that a very large charging current flows through the transistor T₁, which current is limited only by the resistor R₁. The transistor T₁ is now protected in that above a specific current the voltage across the resistor R₉ becomes so high that the transistor T₂ is turned on via the resistor R₉ and the diode D₂ and, consequently, the transistor T₁ is cut off. The resistor R₉ then limits the base current of the transistor T₂. Fig. 5 shows a third embodiment of the invention, in which identical parts bear the same reference numerals as in Fig. 4. As the load of the motor M increases the capacitor C₁ is discharged to an increasing extent, so that the average voltage across this capacitor decreases. This causes the speed of the motor to decrease. In order to maintain the speed of the motor as constant as possible at increasing load, the average voltage across the capacitor C₁ should also increase at increasing load. For this purpose the power-supply circuit comprises a correction circuit comprising a transistor T₃ whose base-emitter junction, in series with a resistor R₁₀, is arranged in parallel with the resistor R₄ and whose collector is connected to the cathode of the rectifier diode D₁ via a resistor R₁₁. The series arrangement of the zener diode Z₂ and the resistor R₃ is connected to the collector of the transistor T₃. The correction circuit operates as follows. As the motor current increases and consequently the voltage across the resistor R₄ increases, the transistor T₃ is driven further into conduction, causing the voltage across the resistor R₁₁ to increase further. As a result of this, the voltage across the series arrangement of the resistor R₄, the zener diode Z₂ and the base-emitter junction of the transistor T₂ decreases, so that this zener diode Z₂ and the transistor T₂ are turned on at an increasingly higher instantaneous value of the input voltage. Consequently, the transistor T₁ is turned off at an increasing instantaneous value of the input voltage, so that the capacitor C₁ is charged to a voltage which increases as the load current increases. A zener diode Z₄ arranged in parallel with the resistor R₁₁ limits the voltage across the resistor R₁₁ above a specific load current. This ensures that the voltage across the capacitor C₁ cannot become higher than the maximum permissible motor voltage at increasing load current. The first and the second transistor switch may comprise Darlington transistors or other compound transistors. Instead of bipolar transistors, field-effect transistor may be used, in which case base, collector and emitter should read gate, source and drain respectively. Further, the zener diode Z₁ may be replaced by one or more series-connected diodes. The first and the second protection means may also be constructed in another manner and, in particular, the current-sensing resistors for measuring the load current and the charging current may be arranged at other locations in the circuit. Further, the transistor in the correction circuit may be replaced by any other amplifier circuit. Finally, it is to be noted that the rectifier may alternatively comprise a full-wave rectifier or a bridge rectifier.

Claims: 1. A power-supply circuit for energizing a load, comprising between two input terminals (2,3) for the application of an input voltage, a rectifier (D1), a first capacitor (C1) having terminals (4,5) for the connection of the load, a first transistor switch (T1) having a control electrode, and a second transistor switch (T2) for turning off the first transistor switch (T1) when the rectified input voltage exceeds a specifica value characterized in that the control electrode of the first transistor switch (T1) is coupled to one input terminal (2) by a second capacitor (C2) and to the other input terminal (3) by at least one diode (Z1). 2. A power-supply circuit as claimed in Claim 1, characterized in that the diode is a zener diode (Z1). 3. A power-supply circuit as claimed in Claim 1 or 2, characterized in that a resistor (R2) is arranged in series with the second capacitor (C2). 4. A power-supply circuit as claimed in Claim 1, 2 or 3, characterized in that the circuit comprises first protection means for turning off the first transistor switch (T1) when a specific current through the load is exceeded. 5. A power-supply circuit as claimed in Claim 4, characterized in that the first protection means comprise a resistor (R4) which is arranged in series with the load and which is coupled to the control electrode of the second transistor switch (T2) by at least a first reference-voltage element (Z3). 6. A power-supply circuit as claimed in Claim 5, characterized in that a resitor (R5) is arranged in series with the first reference-voltage element (Z2). 7. A power-supply circuit as claimed in Claim 5 or 6, characterized in that the second reference-voltage element comprises a zener diode (Z2). 8. A power-supply circuit as claimed in any one of the preceding Claims, characterized in that the circuit comprises second protection means for turning off the first transistor switch (T1) when a specific current through the first capacitor (C1) is exceeded. 9. A power-supply circuit as claimed in Claim 8, characterized in that the second protection means comprise a resistor (R8) which is arranged in series with the first transistor switch (T1) and which is coupled to the control electrode of the second transistor switch (T2) by at least one diode (D2). 10. A power-supply circuit as claimed in Claim 9, characterized in that a resistor (R9) is arranged in series with the diode (D2). 11. A power-supply circuit as claimed in any one of the preceding Claims, characterized in that the circuit comprises a correction device for increasing the value of the rectified input voltage above which the second transistor switch (T2) turns off the first transistor switch (T1) when the load current increases. 12. A power-supply circuit as claimed in Claim 11, characterized in that the control electrode of the second transistor switch (T2) is coupled to an output of the rectifier (D1) by a series arrangement of at least one zener diode (Z2) and a resistor (R11) and in that the correction device comprises a transistor (T3) whose base and emitter are connected to the terminals of a resistor (R4) arranged in series with the load and whose collector is connected to the junction point between the zener diode (Z2) and the resistor (R11) in the series arrangement between the control electrode of the second transistor switch (T2) and the output of the rectifier (D1). 13. A power-supply circuit as claimed in Claim 11 or 12, characterized in that the correction device comprises limiting means (Z4) for limiting the correction provided by the correction circuit above a specific load current. 14. A power-supply circuit as claimed in Claim 13, when apendant to Claim 12, characterized in that a zener diode (Z4) is arranged between the collector of the transistor (T3) and the output of the rectifier (D1). 15. A dry-shaving apparatus comprising a motor, characterized in that said apparatus comprises a power-supply circuit as claimed in any one of the preceding Claims, the load being constituted by the motor of the apparatus.

Inventors: PASMA, ANDRIES CORNELIS
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H02M3
IPC codes: H02P7, H02M1, H02M7