SWITCHING REGULATOR TYPE POWER SUPPLY CIRCUIT

Abstract: A switching regulator type power supply circuit comprises a converter transformer (3), a blocking oscillation circuit comprising a transistor (TR4) having a collector connected to a rectifying output of an AC power supply through the input winding (N1) of the transformer (3) and an emitter and a base connected to a positive feedback winding (N3) of the transformer, and a switching control circuit (4, 5) connected between the base of the transistor (TR4) and a fluctuation detecting winding (N4) and the positive feedback winding (N3) of the converter transformer (3). Still another switching transistor (TR6) is connected between the base of the switching transistor (TR4) and the positive feedback winding (N3) and a proportional current supply circuit (L1, D12) comprising a coil (L1) and a Zener diode (D12) is connected to the emitter and the base, the base having a resistor (R21) for bias connected thereto. As a result, when a fluctuation of the input voltage applied to the input winding (N1) or a fluctuation of a load connected to the output winding (N2) occurs, a base current in accordance with the fluctuation of the collector current is supplied to transistor for blocking oscillation.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a switching regulator type power supply circuit and, more particularly, to a switching regulator type power supply circuit using a blocking oscillation circuit. Description of the Prior Art A switching regulator type power supply circuit is classified into various types and one of them is a type in which blocking oscillation is performed by a switching transistor and a converter transformer. One of power supply circuits of this type as shown in Fig. 1 is disclosed in Japanese Patent Laying-Open No. 2570/1984. The power supply circuit shown in Fig. 1 comprises an input rectifying portion 1, a blocking oscillation portion 2, a converter transformer 3, a fluctuation detecting portion 4, a control portion 5, an output rectifying portion 6 and a feedback current control circuit 7, and only a portion of relation to the present invention is described hereinafter. The conventional example shown in Fig. 1 performs a blocking oscillation, when a switching transistor TR4 is on, by letting a positive feedback current If flow from the side of a terminal e of a feedback winding N3 of the converter transformer 3 to the base of the transistor TR4 through a path shown in the figure. The feedback current control circuit 7 comprising a constant-current circuit CA in the current path is provided to cause the positive feedback current If to be a constant current in a steady state. More detailed description is made in this respect. Since a Zener diode D10 and resistors R16 - R18 are selected such that a transistor TR5 in the circuit 7 may be turned on by the voltage between the terminals c and e of the feedback winding N3 while the switching transistor TR4 is on and the resistor R12 is set sufficiently larger than the resistor R17, the above mentioned positive feedback current If flows mainly through the transistor TR5, and which current becomes a constant current due to If = (V Z - V BE )/R17 where V BE is the voltage between the base and the emitter of the transistor TR5 and V Z is the Zener voltage of the Zener diode D10. The above positive feedback current If is caused to be a constant current for the following reason. Assuming that the feedback current control circuit 7 is not a constant-current circuit but a constant-impedance, the positive feedback current If in this case is proportional to the voltage between the terminals c and e of the feedback winding N3 and that voltage is proportional to an input voltage applied to an input winding N1. As a result, the more the input voltage applied to the input winding N1 is decreased, the more the current If is decreased. Meanwhile, referring to Fig. 1, when the input voltage applied to the input winding N1 is decreased or a load connected to an output winding N2 is increased, an "on period" of the switching transistor TR4 is controlled to be long, so that a collector current Ii of the TR4 is increased at that time. Therefore, in that case, the base positive feedback current If large enough to allow a flow of the collector current Ii of a big peak value can not be supplied and the switching transistor TR4 becomes a lack of driving, with the result that a predetermined DC output voltage can not be provided from the output winding N2. Thus, it is intended to avoid such defect by causing the positive feedback current If to be a constant current. However, in a conventional circuit shown in Fig. 1, if the current value has been set to be a relatively high value so as to supply enough positive feedback current If when the input voltage applied to the input winding N1 is low or the load connected to the output winding N2 is increased, the switching transistor TR4 becomes an overdrive state when the input voltage applied to the input winding N1 is high or the load connected to the output winding N2 is increased, with the result that a stabilized control range becomes small due to any increase in power loss of the switching transistor TR4. In addition, when the input voltage applied to the input winding N1 is high, the voltage of the feedback winding is also high as mentioned above, so that power consumption in the transistor TR5 is also increased. Meanwhile, although the positive feedback current If is always kept at a constant value while the switching transistor TR4 is on, the collector current Ii is increased linearly as time passes while the switching transistor TR4 is on and, as a result, the above current If can not be a value providing an optimum drive always during the above mentioned on period. Therefore, there is also another disadvantage that power loss of the switching transistor TR4 is originally high without regard to the state of the input voltage applied to the input winding N1 and the load connected to the output winding N2. A circuit according to the preamble of claim 1 and which is substantially equivalent to the circuit as described with respect to fig. 1 is disclosed in US-A-4 488 210. In this circuit also a constant current circuit is utilized leading to the abovementioed drawbacks. It is therefore the object of the present invention to provide a switching regulator type power supply circuit in which the switching transistor means is operated in a more improved operation state without regard to an input voltage fluctuation applied to input winding of the converted transformer means and a fluctuation of a load connected to the output winding, so that power loss of the switching transistor means is decreased and a more stable operation with less power loss is implemented. This object is achieved by a power supply circuit having the features of claim 1; the further claims are related to advantageous developments of the invention. Briefly, the present invention comprises blocking oscillation means comprising a first switching transistor means having a common electrode, an output electrode and a control electrode, coupled to DC power supply means in series through input winding of converter transformer means and connected to positive feedback winding of the converter transformer means of the control electrode of the first switching transistor means for performing a blocking oscillating operation; positive feedback current control means coupled to the control electrode of the first switching transistor and the positive feedback winding and responsive to a voltage generated in the positive feedback winding for applying a positive feedback current to the control electrode of the first transistor means; switching control means coupled to detecting winding of the converter transformer means and responsive to the fluctuation voltage outputted from the detecting winding and responsive to the fluctuation of the output voltage of the output winding of the converter transformer means coupled to rectify circuit means for rectifying and smoothing the output voltage for controlling the duration of the on period of the first switching transistor means, wherein the positive feedback current control means comprises bias means coupled to the positive feedback winding and responsive to the voltage generated in the positive feedback winding for providing a bias voltage, a second switching transistor means having a common electrode, an output electrode and a control electrode responsive to the bias voltage for being turned on at the same time when the first switching transistor means is turned on, proportional current supply means for applying a positive feedback current increasing approximately in proportion to the lapse of time during the on period of the first switching transistor means to the control electrode of the first switching transistor means. Since the present invention comprises the converter transformer means, the blocking oscillation means including the first switching transistor means, the positive feedback current control means and the switching control means, the duration of the on period of the first switching transistor means is controlled appropriately by the switching control means at the time of occurrence of a fluctuation of the input voltage applied to the input winding or a fluctuation of the load connected to the output winding through the rectifying circuit means, and also a peak value of the positive feedback current applied to the first switching transistor means by the positive feedback current control means is increased or decreased because the positive feedback current control means provides the positive feedback current increasing as a function of the lapse of time during the on period of the first switching transistor means to the first switching transistor means. As a result, undesirable power loss of the first switching transistor means can be prevented because an operation of the first switching transistor means is stabilized and can be put in an operationally efficient state without regard to such fluctuations. According to the present invention, the operation of the first switching transistor means is more stabilized and can be put in an operationally efficient state, so that undesirable power loss of the first switching transistor means can be reduced. Consequently, an advantage is brought about that a switching regulator type power supply circuit employing a stabilized blocking oscillating operation with less power loss, can be provided. These objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a circuit diagram showing one example of a conventional switching regulator type power supply circuit using a blocking oscillating operation. Fig. 2 is a circuit diagram showing one example of a switching regulator type power supply circuit using a blocking oscillation circuit in accordance with the present invention. Fig. 3 is a timing chart showing a change of a signal of each portion shown in Fig. 2. Fig. 4 is a circuit diagram showing another example of a switching regulator type power supply circuit using a blocking oscillation circuit in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 shows one example of a power supply circuit of the present invention, which is described with the same reference characters allotted to those corresponding portions used in Fig. 1. Referring to Fig. 2, the blocking oscillation portion includes a series connection of the input winding N1 of the converter transformer 3 and the switching transistor TR4 interposed between the output lines L1 and L2 of the input rectifying portion 1 and is connected to a positive feedback current control circuit 8 between the feedback winding N3 of the above transformer and the base of the switching transistor TR4. The fluctuation detecting portion 4 is also included, which detects, by means of a transistor TR1 for detecting a fluctuation and a Zener diode D5, a fluctuation of a DC voltage between lines L3 and L4, provided by rectifying and smoothing, with a diode D6 and a capacitor C3, a voltage generated between both ends h and c of the detecting winding N4 during the off period of the switching transistor TR4. The control portion 5 includes a resistor R19 and a capacitor C9 connected each other at a junction K constituting an integrated circuit (IC), connected between terminals c and e of the feedback winding N3, and the control transistor TR2 connected between the base of the switching transistor TR4 and the terminal c of the feedback winding N3, that is a reference line L4, the junction K and a node point G of an output side of the fluctuation detecting portion 4 being connected to the base of the transistor TR2. The output rectifying portion 6 rectifies and smooths, with a diode D9 and a capacitor C8, a voltage generated on the output winding N2 of the converter transformer 3 when the switching transistor TR4 is off. The above mentioned positive feedback current control circuit 8 comprises a diode D11, a capacitor C10 and a resistor R20 connected between the terminal e of the feedback winding N3 and the base of the switching transistor TR4 as shown in Fig. 2, a series connection of an inductor L1 and a PNP type transistor TR6 connected between a junction J and the base of the switching transistor TR4, and a series connection of a Zener diode D12 and a resistor R21 connected each other at a junction H between the junction J and the terminal c of the feedback winding N3, the junction H being connected to the base of the transistor TR6. The present invention is characterized by such positive feedback current control circuit 8. Now an operation of the embodiment is described. First, a basic operation of a switching regulator type power supply circuit is described and then an operation of the positive feedback current control circuit 8 is described. (I) Basic operation of a power supply circuit At the time of turning on of a power supply switch SW in the input rectifying portion 1, the switching transistor TR4 is triggered and is turned on by a current Is flowing from the line L1 of the input rectifying portion 1 through a starting resistor R2 and the positive feedback current If is supplied from the feedback winding N3 to the base of the above transistor TR4 through the positive feedback current control circuit 8 after conducting of the switching transistor TR4, so that a blocking oscillation is performed. The switching transistor TR4 is controlled in the manner to be described in the following in a steady state in which such blocking oscillation is performed. Since a rectangular wave voltage shown in Fig. 3 is generated between both ends e and c of the feedback winding N3 in a steady state, the rectangular wave voltage is integrated by the integrating circuit (IC) and a triangular wave voltage shown in Fig. 3 (b) is generated at the junction K. On the other hand, the node point G between resistors R7 and R5 connected between the collector of the fluctuation detecting transistor TR1 and the line L4 is at a positive potential corresponding to the voltage between both ends of a capacitor C3 for getting a detected voltage with respect to the line L4. Then, it follows that a potential on a node point M changes as shown in Fig. 3 (c), because the previous triangular wave voltage on the node point M is superimposed onto the positive potential. Therefore, when the potential of the point M is higher than the reference voltage of the line L4 by more than a threshold voltage Vbe between the base and emitter of the control transistor TR2, the transistor TR2 is turned on. Then, it follows that the positive feedback current If from the feedback winding N3 is bypassed by the control transistor TR2, so that the switching transistor TR4 is turned off. Then, the switching transistor TR4 becomes on state by a normal blocking oscillating operation after a predetermined period of time from the turn off. Thus, the switching transistor TR4 repeats the on and off operation hereafter and, as a result, a voltage generated on the output winding N2 is rectified and smoothed by means of the diode D9 and the capacitor C8 to provide a DC output voltage. When the DC output voltage fluctuates, the DC voltage from the fluctuation detecting winding N4 by means of the diode D6 and the capacitor C3 also changes accordingly and a potential at the point G also changes and, as a result, a turn-off timing of the switching transistor TR4, that is, the length of the on period can be controlled such that the DC output voltage is increased, the length of the on period is decreased corresponding to a potential change of the point G and then the DC output voltage is stabilized. (II) An operation of the positive feedback current control circuit 8 Meanwhile, since the transistor TR6 in the positive feedback current control circuit 8 is biased by means of the resistor 21 and the Zener diode D12 so as to be turned on by a voltage (Fig. 3(a)) generated between both ends e and c of the feedback winding N3 while the switching transistor TR4 is on, it follows that the current If (Fig. 3(d)) flows into the base of the switching transistor TR4 through that transistor TR6. At that time, that current If is represented by If = 1 L (V Z - V BE ) · t in view of and it follows that the If is a function of time t where V Z is a Zener diode voltage of the Zener diode D12, V BE is a voltage between the base and the emitter of the transistor TR6 and L is an inductance value of the inductor L1. That is, the current If increases linearly as time goes on since V Z and V BE are constant values. Although a current of the sum of that current If and a current divided into the resistor R20 flows into the base of the switching transistor TR4 as a positive feedback current, the current divided into the resistor R20 is negligible as compared with the current If because a value of the above resistor R20 is set large enough as compared with the impedance value of the inductor L1. Therefore, the positive feedback current of the switching transistor TR4 becomes the above mentioned current If substantially and that the positive feedback current If is increased during the on period of the switching transistor TR4, as shown in Fig. 3(d). As a result, even if the on period is reduced like a dashed line in Fig. 3(d) due to, for example, an increase of the input voltage or a decrease of the load and a peak value of the collector current Ii (Fig. 3(e)) of the switching transistor TR4 is decreased, the switching transistor TR4 never becomes a overdrive state because a peak value of the positive feedback current If is decreased accordingly. Conversely, when the on period of the TR4 is increased due to a decrease of the input voltage or an increase of the load, a peak value of the positive feedback current If is increased in proportion to the on period of the TR4 and, as a result, an underdrive state of the switching transistor TR4 can be prevented. Since the above mentioned positive feedback current If varies with the collector current Ii of the switching transistor TR4, that current If can be set to be in an optimum drive state for the collector current Ii at each moment over approximate all period of the on period. In addition, Fig. 3(f) shows a voltage between the collector and the emitter of the switching transistor TR4 and Fig. 3(g) shows a power loss of that transistor TR4, a dashed line showing the conventional one. The diode D11, capacitor C10 and the resistor R20 in the positive feedback current control circuit 8 become necessitated immediately after the power supply switch SW is turned on. This means that an enough voltage to turn on the Zener diode D12 is not generated on the feedback winding N3 immediately after that turning on and, therefore, a current flowing through the capacitor C10 and the diode D11 is applied to the base of the switching transistor TR4 through the resistor R20 because the transistor TR6 is off. Fig. 4 shows another embodiment of the present invention in which the transistor TR6 in the positive feedback current control circuit 8 is of a PNP type. Since an operation is the same as that in Fig. 2, its description is omitted and the same reference characters are allotted to the same or corresponding portions. The present invention is not limited to the above mentioned embodiment, insofar as the present invention comprises a switching power supply circuit of a blocking oscillation type irrespective of a type of turn-off control for the switching transistor TR4 and, therefore, the present invention is applicable to even a case when a control circuit for turning off comprises the conventional or still other structure. As described in the foregoing, the present invention comprises the blocking oscillation means including a series connection of the input winding of the converter transformer means and the first switching transistor means connected to the DC power supply means for a blocking oscillating operation; the positive feedback current control means for receiving a voltage generated in the positive feedback winding of the converter transformer means and for applying to the control electrode of the first switching transistor means a positive feedback current increasing as a function of the lapse of time during the on period of the first switching transistor means; and switching control means responsive to a voltage provided from detecting winding for detecting a fluctuation of the output voltage of the output winding of the converter transformer means due to a fluctuation of a load for controlling the length of the on period of the first switching transistor means. As a result, when a fluctuation of the load connected to the output winding through the rectifying circuit means and a fluctuation of the input voltage applied to the input winding appear, the length of the on period of the first switching transistor is controlled by the switching control means and positive feedback current increasing as a function of the lapse of time during the on period of the first switching transistor means is applied to the control electrode of the first switching transistor means. Therefore, it is possible to perform an operation of the first switching transistor means in a stable and efficient manner with less power loss to fluctuations of the load and the input voltage and, as a result, a switching regulator type power supply circuit using a stabilized and efficient blocking oscillation is provided.

Claims: A switching regulator type power supply circuit comprising: DC power supply means (1); converter transformer means (3) comprising an input winding (N1), a positive feedback winding (N3) and an output winding (N2) electromagnetically coupled to said input winding and a detecting winding (N4) electromagnetically coupled to said input winding (N1) and said output winding (N2); rectifying circuit means (6) coupled to said output winding (N2) for rectifying and smoothing the output voltage from said output winding (N2); blocking oscillation means (2) comprising first switching transistor means (TR4) having a common electrode, an output electrode and a control electrode and being coupled to said DC power supply means (1) in series through said input winding (N1) and connected to said positive feedback winding (N3) at the control electrode of said first switching transistor means (TR4) for performing a blocking oscillating operation; positive feedback current control means (8) coupled to the control electrode of said first switching transistor means (TR4) and said positive feedback winding (N3) and responsive to a voltage generated in said positive feedback winding (N3) for applying a positive feedback current during the on period of said first switching transistor means (TR4) to the control electrode of said first switching transistor means (TR4); switching control means (4, 5) coupled to said detecting winding (N4) and responsive to the fluctuation voltage responsive to a fluctuation of the output voltage of said output winding (N2) outputted from said detecting winding (N4) for controlling the duration of the on period of said first switching transistor means (TR4); said positive feedback current control means (8) comprising bias means (R21) coupled to said positive feedback winding (N3) and responsive to a voltage generated in said positive feedback winding (N3) for providing a bias voltage; and second switching transistor means (TR6) having a common electrode, an output electrode and a control electrode receiving a bias voltage and being turned on at the time when said first switching transistor means (TR4) is turned on in response to said bias voltage, characterized by proportional current supply means (L1, D12) for applying a positive feedback current increasing approximately in proportion to the lapse of time to the control electrode of said first switching transistor means (TR4) during the on time of said first switching transistor means (TR4). A switching regulator type power supply circuit in accordance with claim 1 , wherein said positive feedback current control means (8) further comprises: Trigger current supply means (R2, R20, C10, D11) for applying a necessary trigger current for said first switching transistor means (TR4) to be turned on to said first switching transistor means (TR4) immediately after said DC power supply means (1) is turned on. A switching regulator type power supply circuit in accordance with claim 1, wherein the output electrode of said first switching transistor means (TR4) is connected to the other terminal of said input winding (N1) having one terminal connected to one terminal of said DC power supply means (1), and the common electrode of said first switching transistor means (TR4) is connected to one terminal of said prositive feedback winding (N3) and the other terminal of said DC power supply means (1), said positive feedback winding (N3) having one terminal and the other terminal connected to said positive feedback current control means (8). A switching regulator type power supply circuit in accordance with claim 1, wherein the common electrode of said second switching transistor means (TR6) is connected to said proportional current supply means (L1, D12), the control electrode of said second switching transistor means (TR6) is connected to said bias means (R21), and the output electrode of said second switching transistor means (TR6) is connected to the control electrode of said first switching transistor means (TR4). A switching regulator type power supply circuit in accordance with claim 1 , wherein said second switching transistor means (TR6) comprises a PNP type transistor, the output electrode of said second switching transistor means (TR6) being connected to the control electrode of said first switching transistor means (TR4); said proportional current supply means (L1, D12) comprises first inductor means (L1) connected to the common electrode of said second switching transistor means (TR6) and first Zener diode means (D12) connected between the control electrode of said second switching transistor means (TR6) and said first inductor means (L1) such that a breakdown current flows in the direction toward the control electrode of said second switching transistor means (TR6), said first inductor means (L1) being connected so as to receive a current from the other terminal of said positive feedback winding (N3); and said bias means (R21) comprises first resistor means (R21) connected between the control electrode of said second switching transistor means (TR6) and one terminal of said positive feedback winding (N3). A switching regulator type power supply circuit in accordance with claim 1, wherein said second switching transistor means (TR6) comprises an NPN type transistor; said proportional current supply means (L1, R12) comprises second inductor means (L1) connected between the common electrode of said second switching transistor means (TR6) and the control electrode of said first switching transistor means (TR4) and second Zener diode means (D12) connected between the control electrode of said first switching transistor means (TR4) and the control electrode of said second switching transistor means (TR6) such that a breakdown current flows in the direction toward the control electrode of said first switching transistor means (TR4); said bias means (R21) comprises second resistor means (R21) connected between the control electrode and the output electrode of said second switching transistor means (TR6); and the output electrode of said second switching transistor means (TR6) is connected so as to receive a current from the other terminal of said positive feedback winding means (N3). A switching regulator type power supply circuit in accordance with claim 2, wherein said trigger current supply means (R2, R20, C10, D11) comprises third resistor means (R2) connected between one terminal of said DC power supply means (1) and the control electrode of said first switching transistor means (TR4). A switching regulator type power supply circuit in accordance with claim 2, wherein said trigger current supply means (R2, R20, C10, D11) comprises a series connection of fourth resistor means (R20) and a parallel connection of capacitance means (C10) and diode means (D11), connected between the control electrode of said first switching transistor means (TR4) and the other terminal of said positive feedback winding (N3) so as to allow a current to flow from the other terminal of said positive feedback winding (N3) to the control electrode of said first switching transistor means (TR4), the diode means (D11) being connected in such direction that a current is applied from said positive feedback winding (N3) to the control electrode of said first switching transistor means (TR4). A switching regulator type power supply circuit in accordance with claim 1, wherein said switching control means (4, 5) comprises: Integrating circuit means (IC) coupled to said positive feedback winding (N3) for integrating a voltage generated in said positive feedback winding (N3) for outputting an integrated voltage; fluctuation voltage output circuit means (4) coupled to said detecting winding (N4) for converting a voltage generated in said detecting winding (N4) to a fluctuation voltage of an appropriate DC voltage value; and third switching transistor means (TR2) coupled to said integrating circuit means (IC) and said fluctuation voltage output circuit means (4) and responsive to a sum voltage of the integrated voltage and the DC fluctuation voltage for being turned on for turning said first switching transistor means (TR4) off.

Inventors: ASAI, SHUNJI
Applicants: SANYO ELECTRIC CO., LTD.
Application year: 1987
Main classification: H02M3
IPC codes: H02M3