Stabilized DC power supply device

[0002] 1. Field of the Invention

[0003] The present invention relates to a stabilized DC (direct current) power supply device that supplies a stabilized DC voltage to a load.

[0004] 2. Description of the Prior Art

[0005] A conventional stabilized DC power supply device as shown in FIG. 4 is described hereinafter as an example. As shown in FIG. 4, a base, an emitter, and a collector of an NPN transistor 1 are connected to an output terminal of an operational amplifier 2, an output terminal O, and a power supply voltage V_CCrespectively. At the same time, the emitter of the transistor 1 is connected to ground via resistors 3 and 4, each of which has a large resistance and is connected in series so as to detect an output voltage V_OUT(a voltage supplied to a load 100) at the output terminal O. A DC voltage Vref is applied to a noninverting input terminal (positive input terminal) of the operational amplifier 2. A node between the resistors 3 and 4 is connected to an inverting input terminal (negative input terminal) of the operational amplifier 2.

[0006] In the circuit configuration as described above, a current I_OUTis supplied from the power supply voltage V_CCvia the transistor 1 to the load 100 that is connected to the output terminal O. It can be assumed that the current I_OUTpassing through the load 100 is nearly equal to an emitter current I_Epassing through the transistor 1 because the resistors 3 and 4 have a large resistance. Therefore, if it is assumed that the load 100 has an impedance of Z, then the output voltage V_OUTis given by

[0007] V_OUT≈I_E×Z

[0008] The emitter current I_Eof the transistor 1 is controlled by a control circuit 10 comprising the transistor 1, operational amplifier 2, resistors 3 and 4, and connections arranged therein so that a voltage measured at the node between the resistors 3 and 4 becomes equal to the DC voltage Vref. As a result, the output voltage V_OUTis stabilized at a constant voltage even if the power supply voltage V_CCfluctuates in DC voltage or at an extremely low frequency.

[0009] Here, it is possible that the power supply voltage V_CCfluctuates at a frequency ranging from approximately 10 Hz to 10 KHz due to noise or the like. However, the aforementioned conventional stabilized DC power supply device has a problem in which the output voltage V_OUTfluctuates in phase with the power supply voltage V_CCwhen the power supply voltage undulates at the aforementioned frequency. This is caused by the control circuit 10 not capable of controlling the emitter current I_Eof the transistor 1 in response to the frequency, thereby vibrating the emitter current I_Eaccordingly. Consequently, if a ripple voltage arises in the power supply voltage V_CC, a resulting ripple voltage also arises in the output voltage that can not be cancelled satisfactorily.

[0010] An object of the present invention is to provide a stabilized DC power supply device having a high capability of removing a ripple voltage from an output voltage even if the ripple voltage arises in a power supply voltage.

[0011] To achieve the above object, according to one aspect of the present invention, a stabilized DC power supply device that maintains an output voltage thereof at a constant level despite a fluctuation arising in a power supply voltage fed thereto comprises a control circuit that compares the output voltage with a reference voltage so as to output a control signal by which the output voltage is maintained at a predetermined level, and a ripple cancellation circuit that amplifies an undulating component included in the power supply voltage at a predetermined gain and that feeds out a resulting signal as an undulation compensation signal for correcting the output voltage, wherein the undulation compensation signal for correcting the output voltage is superimposed on the reference voltage so as to prevent an undulation arising in the power supply voltage from appearing in the output voltage.

[0012] In this structure, the reference voltage of the control circuit that controls the output voltage is automatically compensated so that the output voltage can be maintained at a constant level if the power supply voltage fluctuates or even if the power supply voltage undulates at a higher frequency.

[0013] This and other objects and features of the present invention will become clear from the following description, taken in conjunction with the preferred embodiments with reference to the accompanying drawings in which:

[0014] FIG. 1 is a circuit diagram showing a stabilized DC power supply device of a first embodiment of the invention;

[0015] FIG. 2 is a circuit diagram showing a configuration of a ripple cancellation circuit shown in FIG. 1;

[0016] FIG. 3 is a circuit diagram showing a stabilized DC power supply device of a second embodiment of the invention; and

[0017] FIG. 4 is a circuit diagram showing a conventional stabilized DC power supply device.

[0018] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a circuit diagram showing a stabilized DC power supply device of a first embodiment of the invention. Such identical components as are found also in FIG. 4 showing a conventional stabilized DC power supply device are identified with the same reference numerals, and overlapping descriptions will not be repeated.

[0019] In the stabilized DC power supply device of the first embodiment, an output voltage V_OUTof a control circuit 10 is maintained at a predetermined level when a fluctuation or an undulation that a conventional stabilized DC power supply device is not capable of correcting. To achieve this, resistors 3 and 4 are connected in series between the output voltage V_OUTand ground. A voltage obtained by dividing the output voltage V_OUTat a node between the resistors 3 and 4 is fed to an inverting input terminal (negative terminal) of an operational amplifier 2. A reference voltage Vref is fed to a noninverting input terminal (positive terminal) of the operational amplifier 2.

[0020] Because of this arrangement, if the voltage fed to the inverting input terminal, i.e. the voltage obtained through dividing the output voltage V_OUT, becomes lower than the reference voltage Vref that is fed to the noninverting input terminal, the operational amplifier 2 feeds out a control voltage to a base of a transistor 1 from an output terminal thereof. The control voltage thus fed to the base induces an increased flow of an emitter current I_Ethrough the transistor 1 and raises the output voltage V_OUT. On the other hand, when the voltage obtained through dividing the output voltage V_OUTbecomes higher than the reference voltage Vref, the control circuit functions in the opposite way.

[0021] However, as described before, if the power supply voltage V_CCundulates at a frequency ranging from approximately 10 Hz to 10 KHz, then the control circuit can not respond to the frequency and, as a result, undulations arise as a ripple voltage in the output voltage. Therefore it is necessary to effectively control the control circuit 10 so that the ripple voltage appearing in the power supply voltage V_CCdoes not appear in the output voltage V_OUT. This is achieved by changing the reference voltage that is fed to the noninverting input terminal of the control circuit 10 in the direction opposite to the ripple voltage.

[0022] FIG. 2 is a circuit diagram showing a configuration example of a ripple cancellation circuit 5 shown in FIG. 1. Resistors 51 and 52 are connected in series between the power supply voltage V_CCand ground. A noninverting input terminal (positive terminal) of an operational amplifier 53 is connected to a DC voltage Vref2 and an inverting input terminal (negative terminal) thereof is connected to a node between the resistors 51 and 52 via a resistor 54. An output side of the operational amplifier 53 is connected to the inverting input terminal thereof via a resistor 55 and is also connected to one end of a capacitor 21 that does not permit the DC current to pass therethrough. Another end of the capacitor 21 is connected to the noninverting input terminal of the operational amplifier 2 of the control circuit 10.

[0023] In this configuration, if appropriate resistances are given to the resistors 51, 52, 54, and 55 respectively, it is possible to feed to the noninverting input terminal of the operational amplifier 2 as an undulation compensation signal, a voltage obtained through superimposing the reference voltage Vref on an AC (alternating current) component extracted from the power supply voltage after inverting and amplifying the power supply voltage.

[0024] Therefore, the reference voltage fed to the noninverting input terminal of the operational amplifier 2 changes according to the ripple voltage arising in the power supply voltage V_CCso as to cancel the ripple voltage. As a result, the output voltage V_OUTdoes not undulates even if the power supply voltage V_CCundulates at a reasonably high frequency, thereby contributing to removing the ripple voltage appearing otherwise in the output voltage V_OUT.

[0025] FIG. 3 is a circuit diagram showing a stabilized DC power supply device of a second embodiment of the invention. The control circuit 10 has a different configuration from the one shown in the first embodiment. More specifically, the noninverting input terminal of the operational amplifier 2 is connected to the node between the resistors 3 and 4. The inverting input terminal is connected to a node between the reference voltage Vref and an output side of the ripple cancellation circuit 5.

[0026] As described before, the inverting input terminal receives a resulting signal obtained through superimposing the reference voltage Vref on an AC component signal that serves to compensate the ripple voltage. At the same time, in lieu of the NPN transistor 1, it is also possible to use a PNP transistor 6 having a base thereof connected to an output side of the operational amplifier 2, an emitter thereof connected to the power supply voltage V_CC, and a collector thereof connected to a node between an output terminal O and the resistor 3.

[0027] As described above, according to the stabilized DC power supply device of this invention, the output voltage thereof can be maintained at a predetermined level if ordinary fluctuations or even if undulations at a higher frequency arise in the power supply voltage because the undulation compensation signal is superimposed on the reference voltage.