JPH0648907B2 - Self-excited switching regulator - Google Patents

Description

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

(b) Conventional Technology A self-excited switching regulator is generally used as a relatively small-capacity power supply circuit provided in an electronic device to supply power to the circuit.

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 and rectifies the commercial power supply AC. C1 is a smoothing capacitor for smoothing 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. A rectifying diode D1, a smoothing capacitor C2, and a dummy load R3 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 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 amplification factor of the base current times, the saturation state of Q cannot be maintained, the applied voltage of n1 decreases, and accordingly, the electromotive voltage of n3 decreases and Q sharply cuts off. To do. At that moment n
A reverse voltage is generated at 2, D1 conducts, the energy stored 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, this is induced in n3, and Q starts conducting 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. .
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 to prevent an unloaded state. However, in order to prevent the above intermittent oscillation,
The resistance value of the dummy load had to be set low to some extent, the amount of heat generated was large, and a relatively large resistor had to 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 The present invention relates to a primary winding of a transformer, a switching transistor for connecting and disconnecting a current flowing through the primary winding is connected in series, and an output of a feedback winding of the transformer is switched. A self-excited switching regulator connected to the base of the
Of the input power supply line is connected between the input power supply line and the ground of the input power supply, and a voltage divider circuit composed of second and third resistors is connected between the output of this voltage divider circuit and the emitter of the transistor. It is characterized in that a Zener diode is connected which is turned on when the absolute value reaches a constant value and causes a bias current to flow through the first resistor.

(e) Action In the switching regulator of the present invention, the voltage dividing circuit including the second and third resistors is connected between the input power source line and the ground of the input power source. The output potential of this voltage dividing circuit is the input voltage. Proportional to the power supply voltage. The first resistor is inserted between the emitter of the switching transistor and the ground of the input power supply, and the Zener diode connected between the output of the voltage dividing circuit and the emitter of the switching transistor has a constant absolute value of the input power supply voltage. When it reaches, the current is conducted and the bias current is applied to the first resistor. Therefore, when the absolute value of the input power supply voltage reaches a constant value and the Zener diode becomes conductive, a bias current is passed through the first resistor and the emitter potential of the switching transistor rises. Thus V _BE - by (emitter base voltage) decreases, the base current decreases, apparent h _fe
(Grounded emitter current amplification factor) decreases. Therefore, the energy stored in the transformer during one ON period of the transistor is reduced, and the excess storage energy in the transformer is not increased, and the balance between the storage and release of energy in the transformer is maintained. 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 releasing the stored energy of the transformer by the dummy load. Moreover, when the absolute value of the input power supply voltage is less than a certain value, the apparent h _{fe of the} switching transistor is
Does not decrease, the current capacity for the load is not limited.

(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 a diode bridge that rectifies the commercial power supply AC, and C1 is a smoothing capacitor that smoothes the rectified voltage. T is a transformer, and a switching transistor Q is provided on its primary winding n1.
Are connected in series. Further, the output of the feedback winding n3 passes through the current limiting resistor R2 and the switching transistor Q3.
Connected to the base of. Further, a starting resistor R1 is connected between the base of Q and the input power supply line Vcc. Input power line Vcc and input power ground (GND)
A second resistor R5 is interposed between the backflow prevention diode D2.
A voltage divider circuit including the third resistor R6 is connected. A first resistor R4 is inserted between the emitter of the switching transistor Q and the ground (GND) of the input power source, and a Zener diode ZD is connected between the emitter of Q and the output of the voltage dividing circuit. Secondary winding n2 of transformer T
A rectifying diode D1 and a smoothing capacitor C2 are connected to the.

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 h _fe times the base current tends to flow through the winding n1. An increase in the collector current causes an electromotive voltage in the winding n3,
A base current flows through Q through the current limiting resistor R2. After that, when the collector current increases and h _fe 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, the shortage of the base current is further promoted, and finally Q is cut off. 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, and the rectifying diode D1 starts conducting,
Supply power 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 is high, the resistors R5 and R6 form a resistance voltage dividing circuit, and therefore V
As cc rises, the cathode potential of the Zener diode ZD rises. Zener diode ZD anode-
When the voltage between the cathodes reaches the Zener voltage, it becomes conductive and the bias current Ia flows through the resistor R4. As a result, the emitter potential of Q rises and the base current Ib based on the electromotive voltage of the feedback winding n3 decreases. The decrease in the base current Ib causes a shortage of the base current within a short time after Q starts to conduct, and the peak current flowing through n1 decreases. In other words, the apparent h _{fe of the} transistor Q2
Is reduced, the energy stored in the transformer during the ON period is reduced. Therefore, the energy stored in the transformer is completely discharged from the secondary side during the off period of the transistor Q, 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 is low and the voltage applied to the Zener diode ZD is less than the Zener voltage, the bias current Ia does not flow. Therefore, the base current Ib due to the electromotive voltage 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 the apparent h _fe of the switching transistor does not decrease until the input power supply voltage reaches a certain value, the supply current capacity to the load does not decrease even when the input power supply voltage is low.

Since a transistor with a high h _fe can be used, the capacity of the 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, R6 ... Third resistance, ZD ... Zener diode.

Claims (1)

[Claims] 1. A self-excited switching system in which a switching transistor for connecting and disconnecting a current flowing through the primary winding is connected in series to the primary winding of the transformer, and the output of a feedback winding of the transformer is connected to the base of the switching transistor. In the regulator, a first voltage is provided between the emitter of the transistor and the ground of the input power source.
Of the input power supply line is connected between the input power supply line and the ground of the input power supply, and a voltage divider circuit composed of second and third resistors is connected between the output of this voltage divider circuit and the emitter of the transistor. A self-excited switching regulator characterized in that a Zener diode is connected which is turned on when an absolute value reaches a constant value and conducts a bias current to a first resistor.