POWER-SUPPLY ARRANGEMENT

Abstract: A flyback converter comprises the series arrangement of a primary winding (n₁), a transistor switch (S₁) and a resistor (R₁) between two input terminals (1, 2) for receiving an input voltage. The series arrangement of a secondary winding (n₂) and a diode (D₁) is arranged between the output terminals (3, 4) for connecting a load (M). The transistor switch (S₁) is controlled by pulses from a pulse-width modulator (10), a sawtooth voltage generated by an oscillator (20) being applied to a first input (11) and a first feedback voltage being applied to a second input (12) via a control amplifier (40), which feedback voltage is proportional to the difference between a referen­ ce voltage from a reference source (30) and a fraction of the output voltage, and a second feedback voltage equal to the voltage across the resistor (R₁) being applied to said second input via a capacitor (C₁).

Description: The invention relates to a power-supply arrangement comprising the series arrangement of at least a primary winding of a transformer, a transistor switch and a first resistor between a first and a second input terminal for receiving an input voltage, the series arrangement of at least a secondary winding of the transformer and a diode between a first and a second output terminal for connecting a load, a pulse-width modulator for supplying control pulses to the transistor switch, an oscillator for supplying a sawtooth signal to a first input of the pulse-width modulator, and a control amplifier for supplying to a second input of the pulse-width modulator a first feedback signal which is a measure of the difference between a reference signal and a signal which is proportional to an output signal of the power-supply arrangement. Such a power-supply arrangement is suitable for general purposes and is particularly suitable for use in a rechargeable shaver for charging the batteries and/or powering the motor. Such a power-supply arrangement is known from the article "Steuer- und Regelschaltung TDA 1060 für Schaltnetzteile" in Technische Informationen für die Industrie, April 1977. In this known arrangement the output voltage of the arrangement is maintained substantially constant in that the difference between a reference voltage and a fraction of the output voltage is amplified and is fed back to a pulse-width modulator which compares this voltage with a sawtooth voltage to control the width of the control pulses for the transistor switch. The output voltage of the power-supply arrangement depends on the input voltage. When the input voltage increases at constant pulse width the maximum value of the current through the primary winding increases, so that the energy stored in the transformer also increases. In order to minimise variations of the output voltage as a result of input-voltage variations the arrangement employs feed-forward control, a voltage proportional to the input voltage being applied to the oscillator in such a way that the rise time of the sawtooth voltage decreases as the input voltage increases. When the input voltage increases this results in a decreasing width of the control pulses for controlling the transistor switch. However, this control method has the drawback that only allowance is made for variations of the input voltage, which variations directly influence the width of the control pulses. Therefore, the output-voltage variations are still comparatively large, so that the control range of the feedback loop must be comparatively large and, consequently, the components used therein must have a comparatively large bandwidth. Moreover, these comparatively large variations complicate stabilisation of the feedback loop. Therefore, it is the object of the invention to provide a power-supply arrangement whose output-signal variations are comparatively small. According to the invention a power-supply arrangement of the type defined in the opening paragraph is characterized in that the power-supply arrangement comprises means for additively combining and supplying to the second input of the pulse-width modulator a second feedback signal which is a measure of the voltage across the first resistor. Applying the voltage across the first resistor to the second input of the pulse-width modulator provides direct compensation for the effects of input-voltage variations and other variations on the current through the primary winding and hence on the energy content of the transformer. This reduces the output-voltage variations of the power supply arrangement, so that the circuit arrangement can have a smaller control range and can be constructed by means of components of comparatively small bandwidth and the first feedback loop can be stabilised more simply. An embodiment of a power-supply arrangement in accordance with the invention may be characterized in that said means comprise a capacitor having a first terminal coupled to the second input of the pulse-width modulator and a second terminal to one end of the first resistor. This embodiment has the advantage that, if for stabilising the first feedback loop a capacitor is connected to the output of the amplifier to integrate the output voltage of this amplifier, this capacitor may be combined with the capacitor for applying the second feedback signal. This embodiment may be characterized further in that the oscillator is adapted to supply a negative-ramp sawtooth signal, the end of the first resistor which is connected to the second input terminal is connected to a point at earth potential, and the second terminal of the capacitor is coupled to that end of the first resistor which is not connected to the second input terminal. Alternatively, this embodiment may be characterized further in that the oscillator is adapted to supply a positive-ramp sawtooth signal, the end of the first resistor which is not connected to the second input terminal is connected to a point at earth potential, and the second terminal of the capacitor is coupled to that end of the first resistor which is connected to the second input terminal. In accordance with a further embodiment the power-supply arrangement in accordance with the invention may be characterized in that the load comprises a rechargeable battery and the output signal of the power-supply arrangement is a current for charging the battery, and in that a motor can be arranged in parallel with the load, the output signal of the power-supply arrangement in the parallel-connected condition being constituted by a voltage for energising the motor. In this case the power-supply arrangement may be characterized further in that the arrangement comprises a first control amplifier for supplying a feedback signal which is a measure of the difference between a reference signal and a signal which is proportional to the average charging current of the battery and which appears on the second input of the pulse width modulator if the load is constituted by the battery, and a second control amplifier for supplying a feedback signal which is a measure of the difference between a reference signal and a signal which is proportional to the motor voltage and which appears on the second input of the pulse-width modulator if the load is constituted by the battery in parallel with the motor. 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 a first embodiment of a power-supply arrangement in accordance with the invention, Fig. 2 shows some voltage characteristics to illustrate the operation of the arrangement of Fig. 1, Fig. 3 shows a second embodiment of a power-supply arrangement in accordance with the invention, Fig. 4 shows some voltage characteristics to illustrate the operation of the arrangement shown in Fig. 3, Fig. 5 shows a third embodiment of a power-supply arrangement in accordance with the invention, and Fig. 6 shows a fourth embodiment of a power-.supply arrangement in accordance with the invention. Fig 1 shows a first embodiment of a power-supply arrangement in accordance with the invention. The arrangement comprises two input terminals 1 and 2 for receiving an input voltage, which may be a direct voltage or a rectified alternating voltage, In the present embodiment the input terminal 2 is connected to earth. A series arrangement of the primary winding n₁ of a transformer, a transistor switch S₁, which in the present case is constituted by a transistor T₁, and a resistor R₁ is arranged between the input terminals 1 and 2. The secondary winding n₂ is arranged in series with a diode D₁ between two output terminals 3 and 4 the output terminal 4 being connected to earth. The diode D₁ is poled in such a way that the power-supply arrangement forms a flyback converter. Between the terminals 3 and 4 a load is arranged, which load in the present case is constituted by a motor but which instead may be constituted by any other load. The base of the transistor T₁ is coupled to the output 13 of a pulse-width modulator 10, which has a first input 11 connected to an output 21 of an oscillator 20 and a second input 12 coupled to the output of a control amplifier 40 and, via a capacitor C₁, to that end of the resistor R₁ which is not connected to the earth terminal 2. The non-inverting input of the control amplifier 40 is connected to the output 31 of a reference-voltage source 30 and the inverting input is connected to the output terminal 3 of the arrangement by an attenuator 50. The operation of the arrangement will be explained with reference to Fig. 1. On its output the pulse-width modulator 10 delivers a control pulse by means of which the transistor T₁ is driven into conduction. The current through the primary winding n₁ and hence through the resistor R₁ increases as a linear function during the forward interval, the rise time being directly proportional to the input voltage applied between the terminals 1 and 2. When the transistor T₁ is turned off the polarity of the voltage across the secondary n₂ is reversed, so that the diode D₁ is turned on. The energy which is stored in the transformer T r during the forward interval is delivered to the motor M in the form of a charging current in the flyback interval. This current decreases as a substantially linear function of time until the transistor T₁ is again turned on by a subsequent control pulse. Variations of the load constituted by the motor will also give rise to variations of the voltage across the motor. In order to maintain this voltage as constant as possible the width of the control pulses for controlling the transistor T₁ is controlled depending on the difference between a reference voltage and a voltage which is proportional to the voltage across the motor. For this purpose a fraction of the output voltage is applied to the inverting input of the control amplifier 40 by means of an attenuator 50, which may be for example a simple voltage divider, the gain of said amplifier being fixed at a specific required value. A reference voltage is applied to the non-inverting input of said control amplifier. The amplified difference between these two voltages is applied to the second input 12 of the pulse-width modulator as a first feedback voltage. This input 12 is connected to earth by the series arrangement of the capacitor C₁ and the resistor R₁. This series arrangement together with the output impedance of the amplifier 40 or together with an impedance arranged in series with the output constitutes a low-pass filter which removes high-frequency variations from the first feedback voltage. This improves the stability of the overall negative-feedback loop. The voltage appearing across the resistor R₁ during the forward interval is applied to the input 12 via the capacitor C₁ as a second feed-back voltage. In the pulse-width modulator 10 the sum of the first feedback voltage and the second feed-back voltage is compared with the saw-tooth voltage from the oscillator 20, which sawtooth voltage is applied to the first input 11 of the modulator. The sawtooth voltage has a negative ramp and varies as indicated by the reference numeral I in Fig. 2a. In this Figure the first feedback voltage bears the reference numeral II and the second feed-back voltage bears the reference numeral III. At the instant t₁ the sawtooth voltage I exceeds the first feedback voltage II and a pulse as shown in Fig. 2b appears on the output 13 of the pulsewidth modulator 10. This causes the transistor T₁ to be turned on. The resulting linearly increasing voltage across the resistor R₁ is added to the first feedback voltage via the capacitor C₁ to form the second feed-back voltage III. At the instant t₂ the sum of these voltages exceeds the sawtooth voltage and the transistor T₁ is turned off. The current through the primary winding n₁ is then substantially directly proportional to the input voltage applied to the input terminals 1 and 2, so that the voltage across the resistor R₁ is also substantially directly proportional to the input voltage. By applying this voltage to the input 12 of the pulse-width modulator 10 direct compensation is obtained for the effect of a varying input voltage on the primary current and hence on the energy content of the transformer by adapting the pulse width of the control pulses. Fig. 2 illustrates the effect of a higher input voltage on the pulsewidth for the second sawtooth. This counteracts not only the effect of variations of the input voltage on the output voltage across the motor M but also the influence of variations in the other components of the arrangement, such as variations in rise time of the sawtooth voltage of the oscillator 20 or variations in gain of the control amplifier 40. The effects of these variations on the primary current and hence on the energy content of the transformer are compensated for directly by the second feed-back control in the forward interval without the variations first giving rise to variations in output voltage which are not counteracted via the overall negative-feed-back loop until the subsequent forward intervals. As a result of this the output-voltage variations are substantially smaller than in the known arrangement. This enables the control range of the overall-feedback loop to be reduced, so that the components used in this feedback loop, such as the control amplifier 40, may have a smaller bandwidth. Moreover, as the variations in the first-feedback voltage are smaller, the capacitor C₁ may have a smaller value. Since the variations in the maximum value of the primary current are smaller the transformer may also be dimensioned more tightly. This enables the arrangement to be constructed by means of simpler and hence cheaper components. Fig. 3 shows a second embodiment of a power-supply arrangement in accordance with the invention in which identical parts bear the same reference numerals as in Fig. 1. In this embodiment that end of the resistor R₁ which is remote from the input terminal 2 is connected to earth. The capacitor C₁ is now connected to the terminal 2. In the present embodiment the oscillator 20 supplies a positive-ramp sawtooth voltage. The operation of the arrangement will be explained with reference to Fig. 4. In this Figure the sawtooth voltage bears the numeral I and the first feedback voltage, which is applied to the input 12 of the pulse-width modulator 10, bears the numeral II. At the beginning of a sawtooth a control pulse appears on the output 13 of the modulator 10, by means of which pulse the transistor T₁ is turned on. This results in an increasingly negative voltage III across the resistor R₁, which is added to the first feedback voltage II via the capacitor C₁. At the instant at which the voltage on the input 12 becomes smaller than the voltage on the input 11 the control pulse is terminated and the transistor T₁ is turned off. The rise time of the voltage across the resistor R₁ decreases as the input voltage increases, as a result of which the pulse width is reduced, as is illustrated for the second sawtooth in Fig. 4. Further, the arrangement operates in the same way as that shown in Fig. 1. Fig. 5 shows a third embodiment of an arrangement in accordance with the invention, identical parts bearing the same reference numerals as in Fig. 1. Between the output terminals 3 and 4 of the power-supply arrangement a battery 6 is connected. This battery 6 comprises, for example, two nickel-cadmium cells arranged in series. During the forward intervals the power-supply arrangement supplies the charging current to the battery 6. In order to preclude damaging of the battery this charging current must have a specific constant average value. A resistor R₂ across which a voltage proportional to the charging current appears is connected in series with the seconeary winding n₂. Via an integrator 56 and an attenuator 55 this voltage is applied to the inverting input of a second control amplifier 60, whose non-inverting input receives the reference voltage from the voltage source 30. The output of the control amplifier 60 is coupled to the input 12 of the pulse-width modulator by a switch S₃. The output voltage of the control amplifier 60 is integrated by the capacitor C₁ and is subsequently applied to the input 12 of the pulse-width modulator 10 as a first feedback voltage. The voltage across the resistor R₁ is also applied to this input 12 via the capacitor C₁ as a second feedback voltage. In response to these voltages the modulator 10 controls the width of the control pulses for the transistor T₁ in such a way that the average charging current is substantially equal to a desired value. By means of a switch S₂ a motor M can be connected in parallel with the battery 6. When the switch S₂ is closed the arrangement delivers the supply voltage for the motor M. Simultaneously with the closure of the switch S₂ the input 12 of the pulse-width modulator 10 is coupled to the output of the first control amplifier by a switch S₃. The voltage across the motor M is then maintained substantially constant in the same way as is described with reference to Fig. 1. Obviously, it is also possible to construct the arrangement of Fig. 5 with the emitter of the transistor T₁ connected to earth, as is shown in Fig. 3. Fig. 6 shows a fourth embodiment of a power-supply arrangement in accordance with the invention, identical parts bearing the same reference numerals as in Fig. 1. In this embodiment the transistor switch S₁ comprises the series arrangement of a high-voltage transistor T₂, which can be turned off comparatively slowly, and a low-voltage transistor T₁, which can be turned off comparatively fast. A capacitor C₂ is arranged between the base of the transistor T₂ and the emitter of the transistor T₁. Further, the basis is coupled to earth by a Zener diode Z₁ and, via the series arrangement of a switch S₄ and a resistor R₃, to that end of the secondary switch n₂ which is positive during the forward interval. It is to be noted that the resistor R₃ may be replaced by a current source. The switch S₄ is opened and closed by means of pulses which are derived from the pulse-width modulator 10 and which have the same width as the control pulses for the transistor T₁. Upon termination of a control pulse the transistor T₁ is turned off and the switch S₄ is opened. The charge stored in the collector-base capacitance of the transistor T₂ and the space charge then flow into the capacitor C₂, the voltage across this capacitor being limited by the zener diode Z₁. This ensures that the transistor T₂ is turned off rapidly. Upon the start of a subsequent pulse the base current for the transistor T₂ is supplied by the capacitor C₂, which is discharged via the base-emitter of the transistor T₂ and the collector-emitter of the transistor T₁. This ensures that the transistor T₂ is turned on rapidly. The base current for the transistor T₂ is derived from the positive voltage across the secondary n₂ via the resistor R₃ and the switch S₄, which is now closed. Further, the arrangement operates in the same way as that shown in Fig. 1. The invention is not limited to the embodiments shown. Within the scope of the claims many variants are conceivable to those skilled in the art. For example, instead of by means of a capacitor the voltage across the resistor R₁ may be applied to the second input of the pulse-width modulator in any other way. In the embodiments described herein the power-supply arrangement is each time constructed as a flyback converter. However, by reversing the polarity of the diode D₁ the arrangement may also be constructed as a forward converter. Moreover, the transistor switch S₁ may be constructed in a manner other than shown. The attenuators shown in the embodiments are not necessary in all cases and may then be dispensed with. Furhtermore, the load of the power-supply arrangement may be constituted by any other load. If the load is constituted by a motor a capacitor may be arranged in parallel with the motor in the embodiments shown.

Claims: A power-supply arrangement comprising the series arrangement of at least a primary winding of a transformer, a transistor switch (S₁) and a first resistor (R₁) between a first and a second input terminal for receiving an input voltage, the series arrangement of at least a secondary winding of the transformer and a diode between a first and a second output terminal for connecting a load, a pulse-width modulator (10) for supplying control pulses to the transistor switch, an oscillator (20) supplying a sawtooth signal to a first input (11) of the pulse-width modulator, and a control amplifier (40) for supplying to a second input (12) of the pulse-width modulator a first feedback signal which is a measure of the difference between a reference signal and a signal which is proportional to an output signal of the power supply arrangement, characterized in that the power-supply arrangement comprises means for additively combining and supplying to the second input of the pulse-width modulator a second feedback signal which is a measure of the voltage across the first resistor. A power-supply arrangement as claimed in claim 1, characterized in that said means comprise a capacitor (C₁) having a first terminal coupled to the second input of the pulse-width modulator and a second terminal to one end of the first resistor. A power supply arrangement as claimed in Claim 2, characterized in that the oscillator is adapted to supply a negative-ramp sawtooth signal, the end of the first resistor which is connected to the second input terminal is connected to a point at earth potential, and the second terminal of the capacitor is coupled to that end of the first resistor which is not connected to the second input terminal. A power-supply arrangement as claimed in Claim 2, characterized in that the oscillator is adapted to supply a positive-ramp sawtooth signal, the end of the first resistor which is not connected to the second input terminal is connected to a point at earth potential, and the second terminal of the capacitor is coupled to that end of the first resistor which is connected to the second input terminal. A power-supply arrangement as claimed in Claim 1, 2, 3 or 4, characterized in that the load comprises a motor and the output signal of the power-supply arrangement is constituted by the voltage for energizing the motor. A power-supply arrangement as claimed in Claim 1, 2, 3 or 4, characterized in that the load is constituted by a rechargeable battery and the output signal of the power-supply arrangement is constituted by a current for charging the battery. A power-supply arrangement as claimed in claim 6, characterized in that the power-supply arrangement comprises first connecting means (S₂) for selectively connecting and disconnecting a motor in parallel with the battery and in that in the parallel-connected condition the output signal of the power-supply arrangement is constituted by a voltage for energizing the motor. A power-supply arrangement as claimed in claim 7, characterized in that the second feedback signal is a measure of the difference between the reference signal and a signal which is proportional to the average charging current of the battery and that the power-supply arrangement further comprises a further control amplifier (40) for supplying a third feedback signal which is a measure of the difference between the reference signal and a signal which is proportional to the voltage for energizing the motor, and second connecting means (S₃) operable in synchronism with the first connecting means (S₂) for selectively coupling to the second input of the pulse-width modulator the third feedback signal when the first connecting means is connecting the motor in parallel with the battery and the second feedback signal when the first connecting means is disconnecting the motor from the battery. A power supply-arrangement as claimed in any one of the preceding claims in any one of the preceding claims, characterized in that the load is constituted by a dry-shaving apparatus.

Inventors: RAUSCH, FREDERIK
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H02M3
IPC codes: H02M3, H02M7, H02J7