JPH0732601B2 - Self-excited switching regulator - Google Patents

Description

TECHNICAL FIELD The present invention relates to a self-excited switching regulator used as a power supply circuit.

(B) Conventional Technology As a power supply circuit of a relatively small capacity provided in an electronic device to supply power to the circuit, a self-excited switching regulator is generally used.

FIG. 2 is a circuit diagram of a conventional self-excited switching regulator which constitutes a so-called ringing choke converter. In the figure, DB is a diode bridge that rectifies the commercial power supply AC. C1 is a smoothing capacitor that smoothes this. T is a transformer, and a switching transistor Q is connected in series to its primary winding n1. Further, the output of the feedback winding n3 of the transformer is connected to the base of the switching transistor Q via the resistor R2. A resistor R1 is connected between the base of the switching transistor Q and the input power supply line Vcc. Secondary winding of transformer T
A rectifying diode D1, a smoothing capacitor C2 and a dummy load R3 are connected to n2.

The self-excited switching regulator configured as described above operates as follows. First, when the commercial power supply AC is input and the input power supply line Vcc rises to a predetermined voltage level, a base current flows through Q through the starting resistor R1 and Q becomes conductive. As a result, the collector current flowing in n1 increases, and an electromotive voltage is generated in n3 in the direction of increasing the base current of Q. At this time, energy is stored in the transformer T. After that, when the collector current reaches the multiplication factor of the base current, the saturation state of Q cannot be maintained, the applied voltage of n1 decreases, and the electromotive voltage of n3 decreases accordingly.
Cuts off sharply. At the moment n2, a reverse voltage is generated, D1 conducts, the energy accumulated in the transformer T is released, and power is supplied to the load. After that, when the current of D1 becomes 0, a surge voltage is generated in n2, which is n3.
Is induced and Q starts normal again.

Self-excited oscillation is performed by repeating the above.

(C) Problem to be Solved by the Invention By the way, such a conventional self-excited switching regulator is designed so that the input AC power supply voltage is not constant and can be used within a certain voltage range. ing.
Generally, the higher the input power supply voltage is, the more energy is stored in the transformer during one ON period of the switching transistor, and the smaller the load supply current is, the less energy is released from the transformer during the OFF period of the switching transistor. . Therefore, when the input power supply voltage is high and there is no load, energy remains in the transformer,
Self-excited oscillation becomes unstable and intermittent oscillation occurs.
If the oscillation operation becomes unstable in this way, there is a problem that the output voltage fluctuates and ripples occur. Therefore, conventionally, as shown in FIG. 2, a dummy load R3 is connected to the secondary side of the transformer so as not to be in an unloaded state. However, in order to prevent the above-mentioned intermittent oscillation, the resistance value of the dummy load must be set low to some extent, the amount of heat generated is large, and a relatively large resistor must be used. Therefore, the efficiency of the power supply circuit is low and the power supply circuit is large.

An object of the present invention is to provide a self-excited switching regulator that performs stable self-excited oscillation operation against changes in the input power supply voltage without using a special dummy load.

(D) Means for Solving the Problem In the self-excited switching regulator of the present invention, a switching transistor for connecting and disconnecting a current flowing through the primary winding of the transformer is connected in series, and a feedback winding of the transformer is provided. In a self-excited switching regulator in which a line output is connected to the base of the switching transistor, and a starting resistor is connected between the input power line and the base of the switching transistor, a first excavator is connected between the emitter of the switching transistor and the ground of the input power source. One resistor is inserted, and a series circuit of a second resistor and a backflow prevention diode, which is energized in a normally used input voltage range, is connected between the input power supply line and the emitter of the switching transistor.

(E) Action In the switching regulator of the present invention, the first resistor is inserted between the emitter of the switching transistor and the ground of the input power source, and the second resistor is connected between the input power source line and the emitter of the switching transistor. . Therefore, the emitter potential of the transistor has a value obtained by dividing the input power supply voltage by the first and second resistors. For this reason, for example, when the input power supply voltage rises, the emitter potential of the switching transistor also rises, so that V _BE (emitter-base voltage) decreases and the base current decreases, resulting in an apparent hfe (grounded emitter current). Amplification rate)
Is reduced. Therefore, the energy stored in the truss during one ON period of the transistor is reduced, and the balance between the storage and release of energy to the transformer is maintained without increasing the excessive stored energy in the transformer. In this way, the energy stored in the transformer during one ON period of the transistor is reduced, so that intermittent oscillation is suppressed without forcibly discharging the stored energy of the transformer by the dummy load.

Further, since the backflow prevention diode is connected in series to the second resistor, when the switching transistor is turned off, the electromotive voltage induced in the feedback winding of the transformer causes the feedback winding, the first resistance, and the The current does not flow in the current path including the resistance of No. 2 and the starting resistor, and the reverse bias between the base and the emitter of the switching transistor is surely performed, so that the switching operation is not unstable,
In addition, wasteful power consumption is also suppressed.

(F) Embodiment FIG. 1 is a circuit diagram of a self-excited switching regulator which is an embodiment of the present invention. In the figure, DB is commercial power AC
C1 is a smoothing capacitor that smoothes the rectified voltage. T is a transformer, and a switching transistor Q is connected in series to its primary winding n1. The output of the feedback winding n3 is the current limiting resistor.
It is connected to the base of the switching transistor Q via R2. In addition, the base of Q and the input power line Vcc
A starting resistor R1 is connected between them. The first resistor R4 is inserted between the emitter of the switching transistor Q and the ground (GND) of the input power supply, and the second resistor R5 and the backflow prevention diode D2 are connected between the emitter of Q and the input power supply line Vcc. Has been done. A rectifying diode D1 and a smoothing capacitor C2 are connected to the secondary winding n2 of the transformer T.

The self-excited switching regulator configured as described above operates as follows. First, when the commercial power supply AC is turned on and the voltage of the input power supply line Vcc reaches a predetermined value,
A base current starts to flow in the transistor Q through the starting resistor R1. As a result, a collector current of hfe times the base current tends to flow in the winding n1. An increase in the collector current causes an electromotive voltage in the winding n3, and a base current flows in Q through the current limiting resistor R2. After that, when the collector current increases and hfe decreases and the base current becomes insufficient, the transistor Q cannot maintain the saturated state and the collector current starts to decrease. As a result, the electromotive voltage of n3 is lowered, and the shortage of the base current is further promoted to finally bring Q into the cutoff state. Energy P which is stored in the transformer during the ON period of the transistor Q becomes _{P = 1 / 2L1 · I P1} 2.

Here, L1 is the self-inductance of the primary winding n1 of the transformer, and I _P1 is the peak current value flowing in n1.

Since the transistor Q is turned off, a counter electromotive voltage is generated in the secondary winding n2, the rectifying diode D1 starts conducting, and power is supplied to the load. After that, when the current of n2 becomes 0, a reverse current flows during the reverse recovery time of the diode D1, and a surge voltage is generated in n2, which is induced in n3 and Q starts to conduct again. The self-oscillation operation is performed by repeating the above.

Now, considering the case where the voltage of the input power supply Vcc rises, the resistors R4 and R5 form a resistor voltage divider circuit, so Vcc
As Q rises, the emitter potential of Q rises. As a result, the base current Ib based on the electromotive voltage of the feedback winding n3 decreases, so that the base current becomes insufficient within a short time after Q starts to conduct, and the peak current flowing through n1 decreases. In other words, the apparent hfe of the transistor Q decreases, so that the energy stored in the transformer during the ON period decreases. Therefore, during the off period of the transistor Q, the energy stored in the transformer is completely discharged from the secondary side, the transformer does not reach a saturation state, and normal oscillation operation is maintained.

On the contrary, when the voltage of the input power supply Vcc drops, the emitter potential of Q drops. Therefore, the base current Ib due to the supervoltage of n3 increases, and the peak current flowing through n1 increases due to the effect opposite to the above. Therefore, sufficient energy is stored in the transformer during one ON period of Q.

As described above, intermittent oscillation is suppressed when the input power supply voltage is high with no load or light load, and the transformer can be sufficiently excited when the input power supply voltage is low, limiting the current capacity for the load. There is no.

(G) Effects of the Invention As described above, according to the present invention, the following effects are achieved.

Even if the input power supply voltage is high with no load or light load,
Since intermittent oscillation can be suppressed without using a dummy load, loss due to the dummy load and increase in size of the device can be avoided.

Since a transistor with a high hfe can be used, the capacity of current supplied to the load can be maintained even when the input power supply voltage is low or when the temperature is low.

Since the intermittent oscillation is suppressed even if the input power supply voltage is high,
It can support a wide range of input power supply voltages.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a self-excited switching regulator which is an embodiment of the present invention, and FIG. 2 is a circuit diagram of a conventional self-excited switching regulator. T ... transformer, n1 ... primary winding, n2 ... secondary winding, n3 ... feedback winding, Q ... switching transistor, R4 ... first resistance, R5 ... second resistance.

Claims (1)

[Claims] 1. A primary winding of a transformer is connected in series with a switching transistor for connecting and disconnecting a current flowing through the primary winding, and an output of a feedback winding of the transformer is connected to a base of the switching transistor, and an input power supply is provided. In a self-excited switching regulator in which a starting resistor is connected between a line and the base of the switching transistor, a first resistor is inserted between the emitter of the switching transistor and the ground of the input power source to connect the input power line and the switching transistor. A self-excited switching regulator characterized in that a series circuit of a backflow prevention diode and a second resistor that is energized in a normally used input voltage range is connected between the emitters.