JPH0229826Y2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to a starting circuit for a switching regulator, and particularly to a starting circuit for a switching regulator with low power loss.

(b) Prior art As shown in Fig. 1, a switching regulator converts commercial AC power into direct current using a direct rectifying and smoothing circuit 1, and converts the commercial AC power into direct current using a high frequency, for example 20KHz~, from a control circuit 2.
The switching transistor 3 is switched with a 50KHz pulse, and a voltage stabilized by the ferrite transformer 4, which is the collector load of the switching transistor 3, is obtained from the secondary side.
Such a switching regulator can replace the power transformer with a small and lightweight ferrite transformer 4, contributing to miniaturization of the power supply.

The control circuit 2 of the switching regulator described above is composed of a pulse oscillation circuit, a pulse width modulation circuit, and a drive circuit, and is often used at a low voltage of about 5 to 10V. Therefore, as shown in Figure 1,
The DC voltage of the rectifying and smoothing circuit 1 is connected in series with a resistor 5 and a Zener diode 6.
The stabilized voltage is applied to the control circuit 2 for operation. However, as the power supply capacity increases, the current consumption of the control circuit 2 increases, and the heat generation of the Zener diode 6 and voltage drop resistor 5 increases.
This had the drawback of deteriorating the overall efficiency of the switching regulator.

In order to eliminate this drawback, power for the control circuit 2 is often supplied from a commercial AC power supply using another auxiliary transformer 7 as shown in FIG. However, in this method, since the commercial AC power source has a low frequency of 50 or 60 Hz, the shape and weight of the auxiliary transformer 7 are relatively large, which goes against the purpose of reducing the size and weight of the switching regulator.

In order to solve this problem, it is conceivable to supply power to the control circuit 2 from a ferrite transformer 4 as shown in FIG. In FIG. 3, 1 is a rectifying and smoothing circuit, 2 is a control circuit, 3 is a switching transistor, 4 is a ferrite transformer, 8 is an auxiliary winding of the ferrite transformer 4, 9 is a rectifying and smoothing circuit of the auxiliary winding 8, 10 is a relay, 11 is a timer, 12
is resistance. In such a circuit, when the power is turned on, a current is supplied to the control circuit 2 through the resistor 12 to operate the control circuit 2 and start switching the switching transistor 3. When switching begins, a voltage is also generated in the auxiliary winding 8 of the ferrite transformer 4, which is converted into direct current by the rectifier and smoothing circuit 9. When the time for this direct current to rise sufficiently has passed, a timer 11 is activated and a relay 10 automatically switches the power source of the control circuit 2 to the auxiliary winding 8, thereby eliminating unnecessary power loss. However, this method requires a timer 11 and a relay 10,
The aim of reducing size and weight is still not sufficient.

The starting circuit of the switching regulator, which has been improved in consideration of this point, is composed of a starting transistor 13, as shown in FIG. It is divided by a Zener diode 15 and fixed biased at a Zener voltage _VZ . Therefore, when the power is turned on, the starting transistor 13 is forward biased and becomes conductive, and current is supplied to the control circuit 2 through the starting transistor 13. Subsequently, a voltage is generated in the auxiliary winding 8 provided in the ferrite transformer 4 at the same time as the switching transistor 3 starts operating, and a rectified voltage of the auxiliary power source is obtained in the rectifying and smoothing circuit 9 composed of a diode and a capacitor.
When this rectified voltage gradually rises and a voltage exceeding V _Z -V _BE is applied to the emitter of the starting transistor 13, the starting transistor 13 is reverse biased and shut off, and the control circuit 2 is operated with this rectified voltage.

The above starting circuit is disclosed in Japanese Utility Model Publication No. 56-27544.

However, in such a starting circuit, if the output voltage of the rectifying and smoothing circuit 1 does not rise to a level sufficient to operate the switching regulator and is left at an intermediate potential, the starting resistor 16 connected to the collector of the starting transistor 13 Since current continues to flow through the starting resistor 16, a high-power resistor must be used.

(c) Purpose of the invention The present invention has been made in view of the above-mentioned drawbacks and provides a switching regulator starting circuit which completely eliminates the conventional drawbacks.

(d) Structure of the invention The starting circuit of the switching regulator according to the invention includes a rectifying and smoothing circuit connected to an AC power supply, a transformer, a switching transistor, and a control circuit, and a high-frequency control output from the control circuit. A voltage dividing resistor that divides the output voltage of the rectifying and smoothing circuit by applying a pulse to the switching transistor to cause the switching transistor to miter and obtain a stabilized voltage on the secondary side of the transformer; at least two Zener diodes connected in series between a voltage dividing point and ground, and a starting transistor having a base connected to the connection point of the Zener diodes, and the starting transistor is made conductive when power is turned on to control the control. A current is supplied to the circuit, and when a rectified voltage obtained from an auxiliary winding of the transformer rises, the starting transistor is cut off, the starting transistor is cut off by the rectified voltage, and the control circuit is operated by the rectified voltage. has been done.

(E) Embodiment As shown in FIG. 5, the starting circuit of the switching regulator according to the present invention is composed of a rectifying and smoothing circuit 21 connected to an AC power supply, a transformer 24, a switching transistor 23, and a control circuit 22. Voltage dividing resistor 2 in switching regulator
5 and 26, at least two Zener diodes 27 and 28 connected in series, a starting transistor 29, and a starting resistor 30.

The voltage dividing resistors 25 and 26 divide the output voltage of the rectifying and smoothing circuit 21, and the starting transistor 2
It has the function of making 9 conductive.

Zener diodes 27 and 28 are voltage dividing resistors 2
The upper Zener diode 27 is connected in series between the voltage dividing points 5 and 26 and the ground, and the upper Zener diode 27 is connected to the voltage dividing resistors 25 and 2.
When the voltage divided by 6 exceeds the Zener voltage, it becomes conductive and makes the starting transistor 29 conductive, and the lower Zener diode 28 functions to fix the base potential of the next stage starting transistor 29 to a predetermined Zener voltage. have

The starting transistor 29 is a Zener diode 2
The base is connected to the connection points 7 and 28, the starting resistor 30 is connected between the collector and the power supply, and the emitter is connected to the control circuit 22. When the starting transistor 29 becomes conductive, it supplies the output voltage of the rectifying and smoothing circuit 21 to the control circuit 22 via the starting resistor 30.

The emitter of the starting transistor 29 is supplied with a rectified voltage obtained from the auxiliary winding 31 of the transformer 24 through a rectifying and smoothing circuit 32. This rectified voltage serves as a driving power source for the control circuit 22 during normal operation.

Next, the operation of the starting circuit according to the present invention will be explained.

When the power is turned on, the output voltage of the rectifying and smoothing circuit 21 increases, and the voltage dividing point also increases in proportion to the voltage dividing ratio of the voltage dividing resistors 25 and 26. The voltage developed across the voltage dividing resistor 26 exceeds the Zener voltage of the Zener diode 27, and the forward voltage of the starting transistor 29 also exceeds the Zener voltage of the Zener diode 27.
When V _BE is exceeded, the start-up transistor 29 becomes conductive. Then, current is supplied to the control circuit 22 via the starting transistor 29 and the starting resistor 30, and the control circuit 22 starts up and the switching transistor 2
The switching operation No. 3 is started. Subsequently, when the switching regulator starts operating, a rectified voltage is obtained from the auxiliary winding 31 of the transformer 24 via the rectifying and smoothing circuit 21. This rectified voltage is supplied to the control circuit 22 and is set approximately 2V higher than the Zener voltage of the Zener diode 28, so the base-emitter of the starting transistor 29 is reverse biased and cut off, and the control circuit 22 operates only with the rectified voltage. Continue.

(f) Effects of the invention According to the invention, a starting transistor 29, voltage dividing resistors 25, 26, Zener diodes 27, 28,
A starting circuit with fewer components can be realized by using only the starting resistor 30, and the purpose of reducing the size and weight of the switching regulator can be achieved.

Furthermore, even if the output voltage of the rectifying and smoothing circuit 21 does not rise sufficiently during startup and is left at an intermediate potential, unless the voltage at the voltage dividing point exceeds the Zener voltage of the Zener diode 27, the startup transistor 29 will not conduct and the startup resistor 30 will not conduct. Since no current flows subsequently, the starting resistor 30 can be realized with a small resistor, and wasteful power consumption can be prevented.

[Brief explanation of the drawing]

1 to 4 are circuit diagrams illustrating a conventional switching regulator starting circuit, and FIG. 5 is a circuit diagram illustrating a switching regulator starting circuit according to the present invention. Explanation of main drawing numbers: 21 is a rectifier and smoothing circuit, 22 is a control circuit, 23 is a switching transistor, 2
4 is a transformer, 25 and 26 are voltage dividing resistors, 27 and 2
8 is a Zener diode, and 29 is a starting transistor.

Claims (1)

[Scope of utility model registration request] It comprises a rectifying and smoothing circuit connected to an AC power source, a transformer, a switching transistor, and a control circuit, and a high frequency control pulse outputted from the control circuit is applied to the switching transistor to switch it and stabilize the secondary side of the transformer. The switching regulator that obtains the output voltage of the rectifying and smoothing circuit includes a voltage dividing resistor that divides the output voltage of the rectifying and smoothing circuit, and at least two Zener diodes connected in series between the voltage dividing point of the voltage dividing resistor and ground. and a starting transistor whose base is connected to the connection point of the Zener diode, and when the power is turned on, the starting transistor is made conductive to supply current to the control circuit, and the rectified voltage obtained from the auxiliary winding of the transformer is A starting circuit for a switching regulator, characterized in that when the starting transistor is turned on, the starting transistor is cut off, the starting transistor is cut off by the rectified voltage, and the control circuit is operated by the rectified voltage.