Abstract:
A ringing converter of the type having an output voltage stabilizing circuit connected on the primary side of a transformer. The converter is further provided with a circuit for detecting the primary current, which is proportional to the load current on the secondary side of the transformer. This enables detection of a change in the output voltage due to fluctuation in the load current on the secondary side, as detected by the detecting circuit. A signal indicative of the detected change is fed back to an oscillator circuit in order to stabilize the output voltage.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a DC-to-DC converter, particularly, to the type referred to as a ringing converter. 
     A ringing converter has a simple circuit construction and has its input and output sides DC-separated by means of a transformer. For this reason a ringing converter is often used as a low-power DC-to-DC converter in apparatus of the type in which the internal circuitry of the apparatus is to be DC-separated from the power supply. A ringing converter of such type may comprise a blocking oscillator including a single transistor and a transformer having primary, secondary and positive feedback windings, and a rectifier circuit for rectifying the AC voltage developed by the secondary winding of the transformer. In operation, a direct current which applied to the blocking oscillator is converted into a prescribed AC voltage thereby, the prescribed AC voltage then being rectified by the rectifier circuit in order to obtain a DC voltage of a prescribed voltage value. A disadvantage encountered in the conventional ringing converters is that the DC voltage output fluctuates widely when there is a change in the load current on the output side, or when there is a fluctuation in the DC voltage input. The conventional ringing converters therefore are not suitable for application to power supplies connected to electronic circuits that demand relatively stable power supply voltages. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a ringing converter having a stable output voltage. 
     Another object of the present invention is to provide a ringing converter whose output voltage exhibits little fluctuation even when the input voltage fluctuates. 
     Still another object of the present invention is to provide a ringing converter whose output voltage exhibits little fluctuation even when there is a change in the load current. 
     A further object of the present invention is to provide a novel ringing converter having an output stabilizing circuit, in which it is possible to compensate for a voltage drop caused by the resistance of the secondary winding. 
     According to the present invention, the foregoing and other objects are attained by providing a ringing converter of the type having a circuit for stabilizing the converter output voltage connected to the primary side of the transformer constituting the blocking oscillator circuit, and having a circuit for detecting the primary current, which is proportional to the load current on the secondary side of the transformer, thus enabling detection of a change in the output voltage caused by fluctuation of the load current on the secondary side as detected by the detecting circuit. A signal indicative of the detected change in the output voltage is fed back to the blocking oscillator circuit, whereby the DC voltage output of the ringing converter is stabilized. 
     These and other features and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawings, in which like reference characters designate the same or similar parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 are circuit diagrams of a conventional ringing converter; 
     FIG. 3 is a graph of the base potential of a transistor for producing oscillation; 
     FIG. 4 is a circuit diagram of an embodiment of a ringing converter according to the present invention; and 
     FIG. 5 is a graph of the conduction time of the transistor Q 1  included in the circuit of FIG. 4, as well as the shape of the voltage V 4  of the circuit of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will now be described, referring first to FIGS. 1 through 3 for a discussion of the short-comings encountered in the conventional ringing converter that the present invention seeks to overcome. 
     A ringing converter of the type to which the present invention is drawn has the general circuit configuration shown in FIG. 1. Here a blocking oscillator is constructed by a transformer T 1  and a transistor Q 1 . When the switch SW is closed, a minute current flows into the base of transistor Q 1  through a resistor R 1 , rendering the transistor Q 1  slightly conductive so that a minute voltage is impressed upon the primary winding P 1  of transformer T 1 . As a result, a voltage is induced in the second primary winding P 2  of the transformer, causing an increase in the base current of the transistor Q 1  via the resistor R 2 , so that the transistor conducts to a greater degree. Since the arrangement is for positive feedback, transistor Q 1  attains the fully conductive state instantaneously. The current flowing in the primary winding P 1 , namely the collector current of the transistor Q 1 , increases with time from the moment of conduction, but an upper limit is imposed upon the collector current due to the current amplification factor of transistor Q 1 . This upper limit gives rise to an increase in the collector potential with time, which in turn diminishes the base current, thereby further elevating the collector potential. Again, due to the effects of positive feedback, the transistor Q 1  is rendered completely non-conductive instantaneously. When transistor Q 1  is cut off in this manner, a voltage is developed due to the magnetic energy which has been stored in the primary winding P 1  up to the moment of cut-off. This voltage charges a capacitor C 1  through a secondary winding S and a diode D 1 , giving rise to an output voltage V out . At the same time, the base of transistor Q 1  is negatively biased so that the transistor is held in the non-conductive state. When the magnetic energy drops off to zero, the voltages developed by the primary winding P 2  and secondary winding S attempt to fall off to a value of zero. At this time, however, a current flows in the direction of the broken arrow for a period equivalent to the recovery time of the diode D 1 , the current returning to zero abruptly upon lapse of the recovery time. When this occurs a very small kick voltage is generated by the primary winding P 2 , so that a current flows into the base of transistor Q 1  through resistor R 2 , along with a current from resistor R 1 , thereby again rendering transistor Q 1  conductive. The result is sustained oscillation in the manner described. With the conventional arrangement of this type, any fluctuation in the load current I out  or in the input voltage V in  gives rise to a large fluctuation in the output voltage V out . 
     The arrangement of FIG. 2 represents an improvement over the ringing converter arrangement of FIG. 1. The conducting action of transistor Q 1  is the same as in the circuit of FIG. 1 as far as conduction start-up and the continuation of oscillation are concerned. However, a difference is observed in connection with a Zener diode ZD 1  and a voltage developed by a capacitor C 2  when the transistor Q 1  is rendered non-conductive. Specifically, as far as the relationship between the Zener diode ZD 1 , voltage V 2  and the operation of transistor Q 1  is concerned, it is obvious, in terms of direct currents, that transistor Q 1  operates in such a manner that the voltage V 2  does not become larger than the Zener voltage V ZD1 . Transistor Q 1  operates in the same manner even in one period of oscillation. When transistor Q 1  conducts, the collector current increases with time but, due to the emitter resistance, a voltage ascribable to the collector current appears and is applied to the base potential. This voltage V BE , as shown in FIG. 3, is limited by the Zener voltage V ZD1  and voltage V 2 , so that the transistor Q 1  tends to be shifted toward the cut-off state due to the insufficient base current. Thus, the voltage V BE  determines the transistor conduction time. In accordance with circuit operation the voltage V 2  grows in size as the conduction time increases. Since the circuit operates with negative feedback, however, the conduction time decreases as the voltage V 2  increases. Accordingly, although voltage V 2  fluctuates due to the minute voltage change ΔV BE  in the base-emitter voltage V BE , stabilization is achieved. Furthermore, if it is assumed that primary winding P 2  and secondary winding S have the same number of turns, their respective outputs V 2 , V out  will also be the same if the diodes D 1 , D 2  drop equivalent voltages. Thus, if V 2  is stabilized, the same will be true for V out . Nevertheless, the current flowing in primary winding P 2  is very small, and the current flowing in secondary winding S is larger and fluctuates constantly due to fluctuation in load. This means that the voltage drop due to the resistance of the secondary winding is not of a negligible magnitude, the result being an error in the output voltage. 
     The ringing converter of the present invention, constructed to eliminate the foregoing shortcomings, will now be described with reference to FIGS. 4 and 5. 
     It will be appreciated from FIG. 4 that a differential amplifier comprising transistors Q 2 , Q 3  and resistor R 7  is added to the ringing converter having the output voltage stabilizing circuit of FIG. 2. Though the oscillation and stabilization principles are the same as described in connection with FIG. 2, the present arrangement is constructed based on the fact that the current flowing in the primary winding P 1  is approximately proportional to the current I out  flowing through the secondary winding S. Specifically, the arrangement is such that the change in the output voltage V out  attributed to the change in load and the winding resistance of the secondary winding S, are compensated for on the primary side, thereby suppressing fluctuation of the output voltage V out . To be more specific, the mean value of the emitter current of transistor Q 1  can be detected as the voltage V 3  by means of the resistor R 3  and capacitor C 3  which are connected to the emitter, but this value expresses also the mean value of the current of primary winding P 1  when the base current is neglected. In the abovementioned differential amplifier comprising transistors Q 2 , Q 3  and resistor R 7 , transistor Q 3  conducts when its base potential exceeds that of transistor Q 2 , thereby drawing a base current from transistor Q 1  which is therefore cut off. Resistor R 4 , corresponding to the emitter resistance of transistor Q 1   in the circuit of FIG. 2, detects the emitter current (which is approximately equivalent to the current flowing through primary winding P 1 ) which increases with time once transistor Q 1  starts conducting. The voltage V 4  developed by resistor R 4  is applied to the input terminal (the base of transistor Q 3 ) of the differential amplifier. The bases of transistors Q 2 , Q 3  are at the same potential when transistor Q 1  makes the transistion from the conductive to the nonconductive state, the voltage V 2  can be expressed as follows: 
     
         V.sub.2 =V.sub.ZD.sbsb.1 +V.sub.6 +V.sub.5- V.sub.3- V.sub.4 =V.sub.ZD.sbsb.1 +V.sub.6- V.sub.4                        (1) 
    
     so that 
     
         V.sub.5 =V.sub.3 
    
     Ignoring the base current of transistor Q 2  gives: ##EQU1## Substituting the above in Eq. (1) gives: ##EQU2## The relationship between the voltage V 2  and the transistor conduction time is as shown in FIG. 5. 
     Since the voltage V 4  can be set to a negligibly small value, and since the voltage V 3  is proportional to the output current I out , V 2  can be written as follows: ##EQU3## where k is the proportion factor. 
     Letting primary winding P 2  and secondary winding S have the same number of turns, and letting diodes D 2 , D 1  develop equivalent voltage drops, and further, letting r represent the internal resistance of the secondary winding S, the output voltage V out  may be expressed as follows: 
     
         V.sub.out =V.sub.2 +V.sub.D.sbsb.2 -r·I.sub.out -V.sub.D.sbsb.1 =V.sub.2 -r·I.sub.out                            (5) 
    
     Substituting Eq. 4 in Eq. 5 gives: ##EQU4## By suitably selecting the various constants so as to establish the relation ##EQU5## the relation V out  =V ZD .sbsb.1 can be established. This shows that the output voltage will be constant regardless of any fluctuation in the load current. 
     It will be evident from the foregoing detailed description that the present invention is based on the fact that the load current of the secondary side of the transformer is proportional to the current on the primary side of the transformer in a ringing converter having an output voltage stabilizing circuit, and that the invention is therefore so constructed as to detect, in the primary side, the load current which prevails on the secondary side to thereby compensate for a variation in the output voltage caused by a fluctuation in the load current, said compensation being effected also on the primary side. Thus it is possible to obtain a stabilized, constant output voltage even with fluctuations in load and supply voltage, the results being superior to those seen in the prior art. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiment thereof except as defined in the appended claims.