RINGING CONVERTER

Abstract: A ringing converter of the type having an output voltage stabilizing circuit (ZD1, C2) connected on the primary side of a transformer (T1). 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 (T1). 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: RINGING CONVERTER This invention relates to a DC-to-DC converter, particularly of the type referred to as a ringing converter. A ringing converter is possessed of 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 an apparatus of the type in which the circuitry internally of the apparatus is to be DC-separated from the power supply side. A ringing converter of such type may comprise a blocking oscillator consisting of 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 applied to the blocking oscillator is converted into a prescribed AC voltage thereby, the 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 have been unfit for application to power supplies connected to electronic circuits that demand relatively stable power supply voltages, According to the present invention there is provided a ringing converter comprising a circuit for stabilizing the output voltage thereof connected to the primary side of a transformer of a blocking oscillator of the ringing converter, a circuit for detecting primary current which is proportional to load current on the secondary side of the transformer, thereby to sense change in output voltage caused by fluctuation in said load current on the secondary side as detected by said circuit, and means to feed back a signal indicative of the sensed change to the blocking oscillator to stabilize the output voltage. In a development the secondary winding of the transformer has the same number of turns as a primary winding which applies an input signal to a switching transistor of the blocking oscillator. A preferred embodiment of the present invention provides a ringing converter having a stable output voltage. The output voltage will exhibit little fluctuation even when the voltage on the input side fluctuates, and even when there is a change in the load current on the output side. A preferred embodiment mazes esit possible to compensate for a voltage drop caused by the resistance of the secondary winding. These and other features and advantages of a ringing converter according to 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 throughout the figures thereof. Figs. 1 and 2 are circuit diagrams each showing a conventional ringing converter; Fig. 3 is a waveform diagram showing the base potential of a transistor for producing oscillation, Fig. 4 is a circuit diagram showing an embodiment by way of example of a ringing converter according to the present invention; and Fig. 5 is a waveform diagram showing the conduction time of a transistor Q1 included in the circuit of Fig. 4, as well as the shape of a voltage V4. 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 a conventional ringing converter, that the present invention seeks to overcome or alleviate at least to some extent. A ringing converter of the type to which the present invention is applied in the present example has the general circuit configuration shown in Fig. 1. Here a blocking oscillator is constructed by a transformer and a transistor Q1 When the switch SW is closed, a minute current flows into the base of transistor Q1 thro ugh a resistor R1, rendering the transistor Q1 slightly conductive so that a minute voltage is impressed upon the primary winding P1 of transrorn.ur T1. As a result, a voltage is induced in the second primary winding P2 of the transformer, causing an increase in the base current of the transistor Q1 via the resistor R2, so that the transistor conducts to a greater degree. Since the arrangement is for positive feedback, transistor Q1 attains the fully conductive state instantaneously. The current flowing in the primary winding P1, namely the collector current of the transistor Q1' increases with time from the moment of conduction, but an upper limit is imposed upon the collector current owing to a limitation upon the current amplification factor of transistor Q1. This 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, owing to the effects of positive feedback, the transistor Q1 is rendered completely non-conductive instantaneously. When transistor Q1 is cut off in this manner, a voltage is developed owing to the magnetic energy which has been stored in the primary winding P1 up to the moment of cut-off. This voltage charges a capacitor C1 through a secondary winding S and a diode D1, giving rise to an output voltage Vout. At the same time, the base of transistor Q1 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 P2 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 D1, the current returning to zero abruptly upon lapse of the recovery time. When this occurs a vcry small kick voltage is generated by the primary winding P2, so that a current flows into the base of transistor through resistor R2, along with a current from resistor R1, thereby again rendering transistor Q1 conductive. The result is oscillation sustained in the manner described. With the conventional arrangement of this type, any fluctuation in the load current 1out or in the input voltage Vin will give rise to a large fluctuation in the output voltage Vout. The arrangement of Fig. 2 represents an improvement over the ringing converter arrangement of Fig. 1. The conducting action of transistor Q1 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 ZD1 and a voltage developed by a capacitor C when the 2 transistor Q1 is rendered non-conductive. Specifically, as far as the relationship between the Zener diode ZD1, voltage V2 and the operation of transistor Q1 is concerned, it is obvious in terms of direct currents that transistor Q1 operates in such a-manner that the voltage V2 does not become larger than the Zener voltage VZD Transistor Q1 operates in the same manner even in one period of oscillation. When transistor Q1 conducts, the collector current increases with time but, owing to the emitter resistance, a voltage ascribable to the collector current appears and is applied to the base potential. This voltage VBE, as shown in Fig. 3, is limited by the Zener voltage VZD1 and voltage V2, so that the transistor Q1 tends to be shifted toward the cut-off state owing to the insufficient base current. Thus, the voltage VBE decides the transistor conduction time. In accordance with circuit operation, voltage V2 grows in size as the conduction time increases. Since the circuit operates with negative feedback, however, the conduction time decreases as the voltage V2 increases. Accordingly, although voltage V2 fluctuates by the minute change a VBE in the base-emitter voltage VBE, stabilization is achieved in theory. Furthermore, if it is assumed that primary winding P2 and secondary winding S have the same number of turns, their respective outputs V2, Vout will also be the same if the diodes Dl, D2 drop equivalent voltages. Thus, if V2 is stabilized, the same will be true for Vout Nevertheless, the current flowing in primary winding P2 is very small, with the current flowing in secondary winding S being larger in magnitude and fluctuating constantly owing to the fluctuation in load. This means that the voltage drop due to the resistance of thesecon H ,mTinding is not of a negligible magnitude, the result being an error in the output voltage. A ringing converter which is in accordance with the present invention, and which is constructed to eliminate or reduce 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 composed of transistors Q2, Q3 and of a resistor R7 is added to the ringing converter having the output voltage stablizing 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 P1 is approximately proportional to the current IoUt flowing through the secondary winding S. Specifically, the arrangement is such that the change in the output voltage Vout attributed to the change in load, as well as the winding resistance of the secondary winding S, are compensated for on the primary side, thereby to suppress fluctuation of the output voltage Vout To be more specific, the mean value of the emitter current of transistor Q1 can be detected as the voltage V3 by means of the resistor R3 and capacitor C3 which are connected to the emitter, but this value expresses also the mean value of the current of primary winding P1 when the base current is neglected. In the abovementioned differential amplifier composed of transistors Q2, Q3 and of resistor R7, transistor Q3 conducts when its base potential exceeds that of transistor Q2, thereby drawing a base current from transistor Q1 which is therefore cut off. Resistor R4, corresponding to the emitter resistance of transistor Q1 in the circuit of Fig. 2, detects the emitter current (which is approximately equivalent to the current flowing through primary winding P1) which increases with time once transistor Q1 starts conducting, the voltage V4 developed by resistor R4 being applied to the input terminal (the base of transistor Q3) of the differential amplifier. If the bases of transistors Q2, Q3 are at the same potential when transistor Q1 makes the transistion from the conductive to the nonconductive state,the voltage V2 can be expressed as follows: 2 VZD1 + V6+ V5 - V3 V V4=VZD1 + V6 VV4 . (#) so that V5 V3 Ignoring the base current of transistor Q2 gives: EMI10.1 Substituting for the above in Eq. (1) gives: EMI10.2 The relationship between the voltage V2 and the transistor conduction time is as shown in Fig. 5. Since the voltage V4 can be set to a negligibly small value, and since the voltage V3 is proportional to the output current Iout, V2 can be written as follows: EMI10.3 where k is the proportion factor. Letting primary winding P2 and secondary winding S have the same number of turns, and letting diodes D2, D1 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: Vout = V2 + VD2 - r*Iout - VD1 = V2 - r*Iout .. (5) Substituting for Eq. 4 in Eq. 5 gives: EMI10.4 By suitably selecting the various constants so as to establish the relation R6 k = r, the relation Vout = VZD1 can be 5 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 embodiment of the 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 theembodin#nt is therefore so constructed as to detect, on 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 deposite fluctuations in load and in 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.

Claims: Claims: 1. A ringing converter comprising a circuit for stabilizing the output voltage thereof connected to the primary side of a transformer of a blocking oscillator of the ringing converter, a circuit for detecting primary current which is proportional to load current on the secondary side of the transformer, thereby to sense change in output voltage caused by fluctuation in said load current on the secondary side as detected by said circuit, and means to feed back a signal indicative of the sensed change to the blocking oscillator to stabilize the output voltage. 2. A ringing converter according to claim 1, in which the secondary winding of the transformer has the same number of turns as a primary winding which applies an input signal to a switching transistor of the blocking oscillator.

Inventors: HATTORI, MASAYUKI, NAKAMURA, SHIGEO
Applicants: FANUC LTD
Application year: 1981
Main classification: H02M3
IPC codes: H02M3