SWITCHING REGULATOR

Abstract: A switching regulator comprises a switching section which is switched ON and OFF at a given frequency, this frequency being a duty cycle, for driving a transformer, and a voltage control section for regulating DC current of the switching section or DC voltage at the output of the transformer. The switching section serves as transformer for performing transformation of DC input voltage. On the other hand, the voltage control section controls current flowing through the switching section for stabilizing the DC input voltage at the switching section. In the preferred construction, the switching section and the voltage control section are connected in series.

Description: S P E C I F I C A T I O N SWITCHING REGULATOR BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to a switching regulator for controlling a transformer for stabilizing or regulating the DC power output of the latter. More specifically, the invention relates to a switching regulator with simplified circuit structure, which has satisfactory DC power regulating performance. Description of the Background Art A conventional switching regulator generally comprises a rectification circuit and a capacitor. The rectification rectifies AC power from a commercial power source into DC power. The capacitor is designed to smooth the rectified DC current. The DC power is supplied to a primary winding of a transformer. A switching element, such as a field effect transistor (FET) , is connected in series with the primary winding for converting DC power into high frequency power by switching ON and OFF at high frequency. Secondary windings of the transformer are respectively connected to a rectifier and a smoothing capacitor to obtain DC power at both sides of respective capacitors. An additional secondary winding is provided to supply a drive signal in a form of a pulse-width modulated signal via a pulse-width modulator. With this pulse-width modulated drive signal, the conductivity of the switching FET can be adjusted. The transformer in the conventional switching regulator is designed not only to operatre as a transformer but also as a DC reactor. Such transformers can operate stably with satisfactory characteristics. However, in order to provide both the transformer function and the DC reactor function in a single transformer, the circuit constant has to be determined accurately for obtaining the desired characteristics and performance. The circuit constant is in practice determined by trial and error. Therefore, development and design of this type of switching regulator requires a relatively long time, and which results in high production cost. On the other hand, it is essential with conventional switching regulators to use relatively stable DC or AC power source, such as a commercial power source. This being because, when DC voltage at the smoothing capacitor fluctuates significantly, desired performance cannot be achieved. In the worst case, excessive voltage or excessive current will cause breakage of circuit elements. In recent years, the need increase for switching regulators which can operate with satisfactorily performance even under substantial DC voltage fluctuation. For example, when the switching regulator is used as control power source for an inverter circuit, a switching regulator, that can operate stably even in substantially fluctuating of DC current, is needed. When the power is supplied to the switching regulator from an inverter main circuit, thus property is particularly necessary. As is well known, the inverter main circuit is designed to obtain DC power from AC power through a rectifier or a power rectifier. When the AC current is terminated, regenerated energy is recirculated to the DC power circuit to suppress voltage drop during deceleration of a power regenerative induction motor which is connected to the inverter as a load. In this case, the inverter receives the DC control power from the DC power circuit of the inverter. During motor deceleration, the DC voltage level in the DC power circuit of the inverter main circuit fluctuates significantly. This makes it difficult for the switching regulator to performance consistantly. SUMMARY OF THE INVENTION Therefore, it is an object of the invention to provide a switching regulator with simplified circuit structure for convenience of development and designing, which can make designing of the circuit easier with simplified circuit structure. Another object of the invention is to provide a switching regulator which operates stably even under substantial fluctuation of input voltage. In order to accomplish the above-mentioned and other objects, a switching regulator, according to the present invention, comprises a switching section which is switched ON and OFF at a given frequency corresponding to the duty cycle for driving a transformer, and a voltage control section for regulating DC current of the switching section or DC voltage at the output of the transformer. The switching section serves as transformer for stepping up or down the DC input voltage. On the other hand, the voltage control section controls current flowing through the switchign section for stabilizing the DC input voltage at the switching section. In the preferred construction, the switching section and the voltage control section are connected in series. According to one aspect of the invention, a switching regulator comprises a transformer having a primary side and a secondary side, first means for switching ON and OFF at a given frequency and a given duty cycle for driving the transformer, and second means, independent of the first means, for regulating one of the DC voltages in the primary side and secondary side. In the preferred circuit layout, the first and second means are arranged in series. The second means may include means for setting a reference voltage. The second means may further include comparator means comparing the DC voltage and the reference voltage to produce HIGH level comparator signal when the DC voltage is higher than the reference voltage, and a first switching element switched between ON and OFF, the first switching element being responsive to the HIGH level comparator signal to be turned OFF. The second means may also comprise a DC reactor and a second switching element which has variable ON/OFF ratio depending upon the level of current flowing in the reactor. The second means may further comprise a shunt resistor connected to the reactor in series, and may detect the current level in the reactor by detecting voltage level at the shunt resistor for turning OFF. The second means may further comprise a photo coupler associated with the shunt resistor to turn ON in response to a voltage higher than a predetermined value being aaplied to the shunt resistor for turning OFF of the second switching element in response to turning ON of the photo coupler. Preferably, the aforementioned first and second switching elements comprise a single common switching element. The single common switching element may comprise a field effect transistor. The first means may comprise a switching transistor connected to a pulse generator means for producing the given frequency and the given duty cycle of pulse train. In preferred embodiment, the pulse generating means generates a given constant frequency and a pulse train having a given constant duty cycle. The pulse generating means may generate a rectangular pulse train having a high frequency for applying high frequency power to the primary side of the transformer. In practice, the transformer includes the primary winding in the primary side and a plurality of secondary windings in the secondary side, each of the secondary windings being associated with means for converting high frequency induced current into DC current having voltage proportional to the peak level of the high frequnecy power applied to the primary winding. BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be understood more fully from the detailed description given herebelow and from the accompanying drawings of the preferred embodiment of the invention, which, however, should not be taken to limit the invention to the specific embodiment but are for explanation and understanding only. In the drawing: The sole figure is a circuit diagram of the preferred embodiment of a switching regulator according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, the preferred embodiment of a switching transistor of the present invention, has input terminals P and N connected to a DC power source (not shown) , such as a control power source of an inverter circuit, to be supplied DC power. The input terminal P is connected to the primary winding 22a of a transformer 22 at a center tap via a fuse 21. The ends of the primary winding 22a are connected to FETs 23 and 24 in series. Sources of the FETs 23 and 24 are connected to a DC reactor 25. A smoothing capacitor 26 is connected between the center tap of the primary winding 22a and the reactor 25. The other end of the reactor 25 is connected to the drain of FET 28 via a shunt resister 27. The source of FET 28 is connected to the input terminal N. In addition, between the junction between the reactor 25 and the shunt resister 27 and the center tap of the primary winding 22a, a flywheel diode 29 is disposed. Gates of the FETs 23 and 24 are connected to an oscillator 32 which generates a pulse train having a constant frequecy and a duty cycle of 50%. Therefore, the FETs are driven ON and OFF for applying high frequency current to the primary winding 22a of the transformer 22. High frequency current in the primary winding 22a induces current in secondary windings 22A, 22B and 22C. Rectifiers 33A, 33B and 33C and smoothing capacitors 34A, 34B and 34C which are respectively connected to the secondary windings 22A, 22B and 22C for outputting DC current having levels proportional to the primary voltage applied to the primary winding 22a. In this way, the output voltage of the secondary winding is controlled by the secondary voltage of the secondary windings 22A, 22B and 22C, which is, in turn, controlled by the primary current in the primary winding 22a. ON/OFF ratio of the FET 28 is controlled by a voltage control circuit 35. The voltage control circuit 35 includes a voltage setting switch 36 for setting a desired voltage. The voltage setting switch 36 is connected to a comparator 37 which provides a desired degree of a histerisis. The comparator 37 compares the primary voltage to the set voltage of the voltage setting switch 36. The comparator 37 is associated with a photo coupler 38 which is designed to extract the HIGH level comparator output. The photo coupler 38 comprises a photodiode 38a connected to the comparator 37 so as to emit light while the comparator output is held HIGH level, and a phototransistor 38b responsive to the light from the photodiode 38a to turn ON. The phototransistor 38b is connected to a buffer amplifier 39 which has histerisis characteristics. The voltage control circuit 35 also includes a photo coupler 40 which is associated with the shunt resistor 27. The photo coupler 40 functions for detecting excessive voltage levels or excessive current levels. The output of the photo coupler 40 is connected to the buffer amplifier 39 in parallel relationship with the photo coupler 38. Similarly to the photo coupler 38, the photo coupler 40 comprises a photodiode 40a connected to the shunt resistor 27 so as to emit light when the voltage level or current level in the shunt resistor is in excess of a given value, and a phototransistor 40b responsive to the light from the photodiode 40a to turn ON. The buffer amplifier 39 is connected to the gate of the FET 28 to turn OFF the FET 28 in response to HIGH level output of the photo coupler 38. In the preferred embodiment of the switching regulator constructed as set forth above, the transformer 22, the FETs 23 and 24 and the oscillator 32 form a switching section which does not have a voltage control function. On the other hand, the DC reactor 25, the FET 28 and the voltage control circuit 35 forms a voltage control circuit which functions for voltage control and protection against excessive current. The switching section turns the FETs 23 and 24 ON and OFF at a constant frequency by the pulse train signal of the oscillator 32 at a constant frequency. By switching FETs 23 and 24 at a high and constant frequency, rectangular waveform output can be obtained at the output of the primary winding 22a of the transformer 22. The rectangular wave output from the primary winding has constant frequency which corresponds to the frequency of the pulse train and having a signal level corresponds to the level of the DC primary voltage applied to the primary winding. The voltage control section controls the ON/OFF ratio of the FET 28 so that the DC primary voltage is adjusted to the set voltage of the voltage setting switch 36. The effect of the voltage control section set forth above, is that the DC primary voltage to be applied to the primary winding 22a can be held substantially constant irrespective of fluctuation of DC input voltage. Therefore, the switching operation performed by the switching section and the voltage regulating operation performed by the voltage control circuit can be made into mutually independent operations. Therefore, in designing the switching regulator circuit, the switching section and the voltage control section can be designed separately from each other. Thus, when designing the switching section, only capacity, saturation and turn ratio of the transformer need to be considered for obtaining the desired performance. Similarly, in designing the voltage control section, fluctuation range of the DC input voltage and specification of the reactor 25, such as capacity and so forth, are the only factors which need to be considered in order to stably control the DC primary voltage to be applied to the primary winding of the transformer. Therefore, by knowing necessary data about circuit components, such as DC input voltage fluctuation range and so forth, designing of the circuit can be made by relative simple calculation and the requirement for extensive error basis testing is eliminated. This is advantagous since only adjustment of the performance of the voltage control section is necessary in view of the fluctuation of the DC input voltage, which makes it easier to design the circuit of the switching regulator to be used as the control power source for the inverter. This being because, the fluctuation of the DC input voltage does not affect theto transform step up performance in the switching section. In practice, satisfactory regulation of the DC primary voltage can be obtained by setting the range of variation of the ON/OFF ratio of FET 28 to cover the maximum fluctuation range of the DC input voltage. In addition, since voltage control is in practice performed by controlling current flowing through the reactor 25, protection of the circuit against excessive current can be performed simply by adjusting the ON/OFF ratio of the FET 28. For example, excessive current may be generated upon ON-set of power supply, at which an inrush current is generated, or upon shorting at the output side of the transformer. Against such excessive current, satisfactory circuit protection can be obtained by detecting the current level at the reactor 25. Furthermore, in the shown embodiment, since the DC primary voltage applied to the primary winding 22a can be satisfactorily regulated, transformer can be driven at a constant and fixed frequency and duty cycle. This makes designing of the transformer and oscillator simpler. This further provide flexibility in selecting operating parameters, such as voltage, current, power, operating frequency and so forth of the FETs 23 and 24 and other circuit components making circuit design easier. While the present invention has been disclosed in terms of the preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention set out in the appended claims. For example, though the shown embodiment is directed to a switching regulator having a transformer with a center tap which requires two switching transistors, the invention is also applicable in switching regulator having a single switching transistor. Furthermore, the shown embodiment can be modified to detect the voltage for voltage control of one of the DC outputs of the output side DC circuit of the transformer.

Claims: WHAT IS CLAIMED IS: 1. A switching regulator comprising: a transformer having a primary side and a secondary side; first means for switching ON and OFF at a given frequeucy and a given duty cycle for driving said transformer; and second means, independent of said first means, for regulating one of the DC voltages in said primary side and secondary side. 2. A switching regulator as set forth in claim 1, wherein said first and second means are arranged in series. 3. A switching regulator as set forth in claim 1, wherein said second means includes means for setting a reference voltage so as to regulate said DC voltage at said reference voltage. 4. A switching regulator as set forth in claim 3, wherein said second means further includes comparator means comparing said DC voltage and said reference voltage to produce a HIGH level comparator signal when said DC voltage is higher than said reference voltage, and a first switching element switched between ON and OFF, said switching element being responsive to said HIGH level comparator signal to be turned OFF. 5. A switching regulator as set forth in claim 1, wherein said second means comprises a DC reactor and a second switching element which has variable ON/OFF ratio depending upon level of current flowing said reactor. 6. A switching regulator as set forth in claim 5, wherein said second means further comprises a shunt resistor connected to said reactor in series, and detects said current level in said reactor by detσ- voltage level at said shunt resistor for turning C_._. 7. A switching regulator as set forth in claim 6, wherein said second means further comprises a photo coupler associated with said shunt resistor to turn ON in response to the voltage aaplied to on said shunt resistor exceeding a predetermined value and means for -turning OFF of said switching element in response to turning ON of said photo coupler. 8. A switching regulator as set forth in claim 7, wherein said first and second switching elements comprise a single common switching element. 9. A switching regulator as set forth in claim 8, wherein said single common switching element comprise a field effect transistor. 10. A switching regulator as set forth in claim I, wherein said second means comprises a DC reactor and a switching element which has variable ON/OFF ratio depending upon the level of current flowing in said reactor. 11. A switching regulator as set forth in claim 10, wherein said second means further comprises a shunt resistor connected to said reactor in series, and detects said current level in said reactor by detecting voltage level at said shunt resistor for turning OFF. 12. A switching regulator as set forth in claim II, wherein said second means further comprises a photo coupler associated with said shunt resistor to turn ON in response to the voltage applied to said shunt resistor exceeding a predetermined value and means for turning OFF of said switching element in response to turning ON of said photo coupler. 13. A switching regulator as set forth in claim 1, wherein said first means comprises a switching transistor connected to a pulse generator means for producing said given frequency and said given duty cycle of pulse train. 14. A switching regulator as set forth in claim 13, wherein said pulse generating means generates a given constant frequency and a given constant duty cycle of pulse train. 15. A switching regulator as set forth in claim 14, wherein said pulse generating means generates a rectangular pulse train having a high frequency for applying high frequency power to said primary side of said transformer. 16. A switching regulator as set forth in claim 15, wherein said transformer includes said primary winding in said primary side and a plurality of secondary winding in said secondary side, each of said secondary winding being associated with means for converting high frequency induced current into DC current having voltage proportional to the peak level of said high frequency power applied to said primary winding.

Inventors: IWASA MASAO, KITO YASUTAMI, IWASA, MASAO, KITO, YASUTAMI
Applicants: OTIS ELEVATOR CO, OTIS ELEVATOR COMPANY
Application year: 1988
Main classification: H02M3
IPC codes: H02M3