Switching power supply

Abstract: A switching power supply has one or more transformers (1,2) of which the primary and secondary windings are respectively isolated from each other. Switching elements (3,6) provide on-off control of DC voltage applied to the primary winding of the or each transformer (1,2). A current detection circuit (15) detects voltage drop caused by the resistance of the secondary winding (1B,2B) of the or each transformer (1,2), the current detection circuit (15) outputting current detection signals proportional to the secondary load current of the or each transformer.

Description: The present invention relates to a Switching Power Supply equipped with current detection circuits both for protection against excess current and current sharing function in paralell operation. In general, a conventional Switching Power Supply for stabilizing D.C.output voltage by switching elements thereof is coupled with excess current protective circuitry for suppressing the secondary excess current of a transformer in an overload condition. On the other hand, in the event of parallel operation of multiple Switching Power Supplies, it is desirable, for reliability's point of view, that a current sharing function may be integrated therewith in order to equalize load current from each Power Supply. For the aforementioned excess current protective circuitry and current sharing function, a current detection circuit is indispensable to monitor the secondary output load current of a transformer, which, for example, is disclosed in Japanese Patent Application Laid-open No.61-231875. Said Japanese Patent Application discloses such circuit arrangements that between the primary or secondary lines of a transformer is interposed a current detector such as a resistor or a current transformer, and the current flowing therethrough is supplied to a current detection circuit to output current detection signals in proportion to load current thereof. However, the above conventional art has a few problems described below. First, in the event of using a resistor as a current detector, power consumption by said resistor itself causes a large loss of said power supply itself. Secondly, specifically in the case of a power supply with a large output current, the current flowing through the resistor also increases, thus such resistor must have a larger permissible power capacity, which prevents the saving of space of the power supply. On the other hand, in the event of using a current transformer as a current detector, said loss of power supply becomes less than in using a resistor. However, power supply for super power needs a large transformer as well as said resistor, which prevents the space saving and low-cost manufacturing of the power supply. JP-A-03-17855 discloses use of an auxiliary winding magnetically coupled with a main winding for detecting the current flowing in the main winding. WO-A-85/04059 discloses a switching power supply which uses an integrating circuit including a resistor for detecting secondary current. Accordingly, it is the main object of the present invention to provide a switching power supply which can realize the saving of space even for a larger output current. It is another object of the present invention to provide a switching power supply which can realize low-cost manufacturing even for a larger output current. According to a first aspect of the present invention, there is provided a switching power supply comprising: two identical transformers of which the primary and secondary windings are each connected in series; corresponding diodes each of which is coupled with a respective one terminal of the secondary windings respectively; one or more switching elements for on-off control of D.C. input voltage applied to said primary winding; said transformers inducing, in use, an output voltage in the secondary winding in response to the D.C. input voltage applied to the primary winding, said secondary winding having a winding resistance; and a detection circuit for sensing current flow in said secondary winding without the need of separate current sensing elements; wherein, in association with the switching operation of said switching elements, one diode coupled with the secondary winding of said one transformer is turned on, while the other diode coupled with the secondary winding of said other transformer is turned off, in use whereby the electric current is supplied from the secondary winding across the diode to a load, thereby generating a voltage drop caused by said winding resistance in the secondary winding, and; said detection circuit is non-magnetically coupled with the respective connection points between said secondary windings and said diodes, thereby detecting only the voltage drop caused by the winding resistance of the secondary winding of one transformer through the detection of a difference in output voltage generated between both the terminals of said secondary winding. Other objects, features and advantages of the invention will be apparent to those skilled in the art from the following description of the preferred embodiments of the invention, wherein reference is made to the accompanying drawings, of which: FIG. 1 is a circuit diagram showing the first embodiment of a switching power supply of the invention; FIG. 2 is a waveform illustrating the primary and secondary voltage of a transformer of the first embodiment of the invention; and FIG. 3 is a circuit diagram showing a second embodiment of a switching power supply of the invention. Hereinafter is explained a first embodiment of the invention with reference to FIGS. 1 and 2. In FIGS.1 and 2 showing a first embodiment of the invention, reference numerals 1 and 2 designate transformers, of which the primary and secondary sides are isolated, wherein the primary windings 1A and 2A are connected in series, while the secondary windings 1B and 2B are connected in series respectively. Said primary windings 1A and 2A are connected in series to D.C.power supply 4 via a switching element or MOS FET 3 in order to apply D.C.input voltage V-IN thereto. Further, series circuit of a blocking capacitor 5 and switching element or MOS FET 6 is connected in parallel with said series circuit of said primary windings 1A,2A. Between each drain and source of said FETS 3,6 are provided capacitors 7,8 having discrete inherent capacitances. Although not shown in FIGS, each FET is integrated with body diodes therein, and driving signals from pulse width control circuit 9 are alternately supplied to discrete gates of FETS3,6, so that output voltage V-OUT can be held constant by controlling pulse width of said driving signals. On the other hand, the voltage V1,V2 induced in the secondary windings 1B,2B of transformers 1,2 are rectified by rectifier circuit of center-tap type fabricated by connecting diodes 10,11 to said secondary windings 1B,2B respectively. After that, said voltages V1,V2 are smoothed by smoothing capacitor 12 to be supplied to load 13 as D.C.output voltage V-OUT. Incidentally,reference numeral 14 designates voltage detection circuit connected to opposite terminals of said smoothing capacitor 12, which supplies detected voltage to pulse width control circuit 9, corresponding to variation of said D.C.output voltage V-OUT. Reference numeral 15 designates current detection circuit for protection against excess current provided on the secondary sides of transformers 1,2. Said current detection circuit 15 detects voltage drop in the secondary windings 1B,2B of transformers 1,2 in order to output current detection signals in proportion to the secondary load current iL of transformers 1,2. In other words, to the connnection points of the secondary winding 1B with diode 10 and the secondary winding 2B with diode 11 are connected respectively one of terminals of resistors 16,17, while the other terminals thereof are connected to each other. Further, between said other terminal of resistors 16,17 and the connection point of the secondary windings 1B and 2B is interposed capacitor 18 to form filter circuit comprising resistors 16,17 and capacitor 18. Reference numeral 19 designates conventional amplifier circuit comprising resistors 20,21 and operational amplifier 22, which amplifies output signals from said filter circuit at predetermined amplification factor defined by said resistors 20,21 to output the same as current detection signals. Said current detection signals from current detection circuit 15 are supplied to negative input terminal of the operational amplifier 23, while predetermined reference voltage is applied to the positive input terminal thereof, thus higher electric potential of current detection signals compared to said reference voltage permits the output terminal of said operational amplifier 23 to supply High-level control signals to pulse width control circuit 9. And said control signals force pulse width control circuit 9 to narrow the pulse width of the driving signals to each FET 3,6in order to limit load current iL. Hereinafter is described the action of the structure thus far described. First, FET 3 is on-off controlled by means of driving signals from pulse width control circuit 9 on the primary sides of transformers 1,2, whereby D.C input voltage V-IN is intermittently applied to the primary windings 1A,2A of transformers 1,2. On the other hand, FET 6 is turned-on/off alternately with respect to FET 3, while each FET 3,6 is so controlled that there exists certain dead band in changing over from switching-on to switching-off of each FET 3,6. At this time, driving signals from pulse width control circuit 9 are supplied in order that capacitor 7 may be discharged by accumulated energy in the primary windings 1A,2A of transformers 1,2 prior to turning-on of FET 3, while capacitor 8 may be discharged by accumulated energy in the primary windings 1A,2A of transformers 1,2 prior to turning-on of FET 6, whereby switching loss can be minimum to attain so called "Zero-Voltage Switching". With the above serial action of FETs 3,6, the voltage induced in the secondary windings 1B,2B is rectified and smoothed by diodes 10,11 and smoothing capacitor 12 to supply predetermined D.C output voltage V-OUT to load 13. And pulse width control circuit 9 controls the pulse width of driving signals supplied to each FET 3,6 with reference to detected voltage from voltage detection circuit so that said D.C.output voltage V-OUT will be held constant. On the other hand, at the both terminals of the secondary windings 1B,2B of transformers 1,2 symmetrically emerge output voltage V1,V2 having such waveforms as shown in FIG.2 respectively. At this time, electrical nature of transformers 1,2 shows that products of output voltage V1 and time, output voltage V2 and time in turning-on of FET 3 are respectively equal to those in turning-off of FET 3. However, winding resistance in each secondary windings 1B,2B causes voltage drop VA when electric current flows through said secondary windings 1B,2B. That is to say, if the output voltages V1,V2 are negative, current supply to load 13 is interrupted by diodes 10,11, thus electric current flows through one of said secondary windings 2B, therefore, no voltage drop caused by the resistance therein occurs. On the other hand, if the output voltage V1,V2 are positive, current supply to load 13 is permitted by diodes 10,11, thus electric current flows through said one of said secondary windings 2B, whereby a voltage drop will occur in proportion to load current iL at both terminals of the secondary windings 1B, 2B. In order to detect said voltage drop VA caused in each secondary winding 1B,2B, the current detection circuit 15 eliminates such signals included within a switching frequency region from each output voltage V1,V2 by means of said filter circuit comprising resistors 16,17 and capacitor 18, whereby only the D.C.signal corresponding to said voltage drop VA is supplied to amplifier circuit 19. Thereafter, said amplifier circuit 19 amplifies said D.C.signals from said filter circuit in order to obtain current detection signals in proportion to load current iL. In accordance with the structure of the present embodiment, said current detection signals can be utilized for protection against excess current. That is, if load current iL exceeds predetermined value because of, for example, short-circuiting of load 13, electric potential of said current detection signals is also to become high in proportion to said load current iL. Therefore, High-Level control signals from said operational amplifier 23 will be output to said pulse width control circuit 9, which immediately narrows the pulse width of driving signals to FET 3 in order to limit the supply of load current iL. As hereinbefore described, the present embodiment employs the winding resistance in the secondary windings 1B,2B of transformers 1,2 as current detector, so that load current iL can be monitored by means of the voltage drop caused in said secondary windings 1B,2B. Accordingly, conventional resistors or transformers for detection of electric current are no longer necessary, an as a result, parts can be saved. Especially, the present embodiment enables large-power supply to easily realize the saving of space and low-cost manufacturing thereof. Further, said current detection circuit 15 detects the current in the secondary windings 1B,2B of transformers 1,2, of which the detection is approximately equal to direct detection of load current iL. Therefore, constant current/voltage dropping characteristic can be easily obtained in utilizing said current detection circuit 15 for protection against excess current. In Fig. 3 showing a second embodiment of the invention, current sharing function is applied to said switching power supply. Again, the same portions as those of the foregoing embodiment are designated as common reference numerals, and their repeated detail description will be omitted. The current detection signals from said current detection circuit 15 is divided by resistors 41,42 to be supplied to negative input terminal of the operational amplifier 43. On the other hand, to the positive input terminal of said operational amplifier 43 is connected current sharing terminal PC and is supplied said current detection signals divided by resistors 44,45. Further, between the negative terminal and output terminal of said operational amplifier 43 is interposed resistor 46, while said output terminal of operational amplifier is connected via diode 47 to the positive terminal of operational amplifier 48, to which is supplied reference voltage. Additionally, to the negative input terminal of said operational amplifier 48 are supplied detection signals from voltage detection circuit 14, while the output terminal thereof is connected to pulse width control circuit 9. Since each Power Supply device allows the current detection circuit 15 integrated therewith to output current detection signals in proportion to load current iL thereof, the electric potential at said current sharing terminal PC varies depending upon the variation of said load current iL. Therefore, in the case of single operation, such Power Supply having smaller load current iL has corresponding smaller electric potential at its current sharing terminal PC as compared with that of such Power Supply having larger load current iL, On the other hand, in the case of parallel operation, as each current sharing terminal of each Power Supply device is connected to one another, the potential at current sharing terminal PC of such Power Supply with smaller load current iL inevitably rises, whereas the electric potential at current sharing terminal PC of such Power Supply with larger load current iL inevitably falls. Accordingly, each Power Supply having smaller load current iL permits the electric potential at the positive input terminal of operational amplifier 43 to rise, thus the output terminal of said operational amplifier 43 becomes High so as to raise the potential at the positive input terminal provided on the reference voltage side of operational amplifier 48. On the other hand, each Power Supply having larger load current iL allows the potential at the positive input terminal of operational amplifier 43 to inevitably fall, thus the output terminal of said each operational amplifier 43 becomes Low-Level, nevertheless, current flow from the reference voltage into operational amplifier 43 is interrupted by diode 47 so that the electric potential of reference voltage is held constant at the positive input terminal of said operational amplifier 48. Consequently, each Power Supply having smaller load current iL permits the reference potential of operational amplifier 48 to rise, whereby pulse width control circuit 9 can control the driving signals to each FET 3,6 in order to permit more load current iL to flow thereinto in accordance with the output signals from said operational amplifier 48. Incidentally, the present invention should not be limited to the aforementioned embodiments thus far described, but can be modified within the scope of the claims. For example, besides Zero-Voltage-Swithichg or Forward Switching Power Supply, the present invention may be applied to Push and Pull or Full Bridge Switching Power Supply. Further, current detection signals from current detection circuit may be utilized for such functions other than for said protection aganist excess current or current sharing.

Claims: A switching power supply comprising: two identical transformers (1.2) of which the primary (1A,2A) and secondary (1B,2B) windings are each connected in series; corresponding diodes (10,11) each of which is coupled with a respective one terminal of the secondary windings (1B,2B) respectively; one or more switching elements (3,6) for on-off control of D.C. input voltage applied to said primary winding (1A,2A); said transformers (1,2) inducing, in use, an output voltage in the secondary winding (1B,2B) in response to the D.C. input voltage applied to the primary winding (1A,2A), said secondary winding (1B,2B) having a winding resistance; and a detection circuit (15) for sensing current flow in said secondary winding (1B,2B) without the need of separate current sensing elements; wherein, in association with the switching operation of said switching elements (3,6), one diode (10) coupled with the secondary winding (1B) of said one transformer (1) is turned on, while the other diode (11) coupled with the secondary winding (2B) of said other transformer (2) is turned off, in use whereby the electric current is supplied from the secondary winding (1B) across the diode (10) to a load (13), thereby generating a voltage drop (VA) caused by said winding resistance in the secondary winding (1B), and; said detection circuit (15) is non-magnetically coupled with the respective connection points between said secondary windings (1B,2B) and said diodes (10,11), thereby detecting only the voltage drop (VA) caused by the winding resistance of the secondary winding (1B) of one transformer (1) through the detection of a difference in output voltage generated between both the terminals of said secondary winding (1B,2B). A switching power supply according to claim 1, further comprising a pulse width control circuit (9) for supplying driving signals to said one or more switching elements to hold the D.C. output voltage constant, said pulse width control circuit forcedly narrowing the pulse widths of the driving signals in order to limit the supply of load current when the output current detection signals reach at least a predetermined level. A switching power supply according to claim 1, further comprising: a current sharing terminal (PC) connected to a transmitting line for carrying the output detection signals from the current detection circuit (15); and, a pulse width control circuit (9) for supplying the driving signals to said one or more switching elements (3,6) to hold the D.C. output voltage constant and controlling pulse widths of the driving signals supplied to the switching elements (3,6) so that as an electric potential at the current sharing terminal increases more load current will be allowed to flow.

Inventors: MIMURA HIROSHI NEMIC-LAMBDA K, MIMURA, HIROSHI, MIMURA, HIROSHI, NEMIC-LAMBDA K.K.
Applicants: NEMIC LAMBDA KK, NEMIC-LAMBDA KABUSHIKI KAISHA
Application year: 1993
Main classification: H02M3
IPC codes: H02M3