JPH0370467A - Switching regulator - Google Patents

Abstract

PURPOSE:To lower the cost easily by providing a constant voltage circuit, which becomes ON when the charged voltage of a capacitor exceeds a certain value, and a transistor which becomes ON by the currents flowing in this constant voltage circuit, between the output of a CR circuit and the power input of a switching control circuit. CONSTITUTION:When the charged voltage of a capacitor C1 exceeds a Zener voltage, Zener currents flow to the Zener diode ZD, and cathode voltage falls. Hereby, the base current flows to a transistor(TR)Q1, and TRQ1 is turned on. When TRQ2 becomes on, energy is accumulated in a transistor 3. At this time, the generated voltage by an auxiliary winding N3 is rectified and smoothed, and power is supplied to a control IC 7 and a totem pole circuit. Moreover, if TRQ2 becomes on, the energy supplied from secondary winding N2 is smoothed and supplied to load.

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application This invention relates to a switching regulator used in power supply circuits and the like.

(1)) Prior Art In general, a switching regulator is composed of a transformer, a switching transistor that switches on and off the primary winding current of the transformer, a switching control circuit that controls the switching transistor, and the like.

The configuration of the main parts of a conventional switching regulator is shown in the third section.
As shown in the figure. In FIG. 3, Q2 is a switching transistor that cuts off the current flowing through the primary winding N1 of the transformer 3. Switching control circuit 2 controls on/off of switching transistor Q2. A resistor R2 and a transistor Q3 are connected between the input power line and the power terminal of the switching control circuit 2, and a capacitor C2 is connected between the power terminal of the switching control circuit 2 and ground. A resistor R3 is connected between the collector and base of the transistor Q3, and a Zener diode ZD is connected between the base of Q3 and the ground, thereby forming a starting circuit 1'. When the input power is turned on, starting power is supplied to the switching control circuit 2 by this starting circuit 1'. When the switching control circuit 2 is activated and the switching transistor Q2 is turned on and off, an electromotive voltage is generated in the secondary winding N2 of the transformer 3, and along with this, an electromotive voltage is generated in the secondary winding N2 of the transformer 3.
An electromotive voltage is generated at the switching control circuit 3, and the rectifying and smoothing circuit 4 rectifies and smoothes this to supply power to the switching control circuit 2. The output voltage of this rectifying and smoothing circuit 4 is set somewhat higher than the output voltage of the starting circuit 1'. Therefore, after starting, the switching control circuit 2 operates mainly by power supply from the auxiliary winding N3 and the rectifying and smoothing circuit 4.

(C1 Problem to be solved by the invention) However, in a switching regulator having a conventional starting circuit as shown in FIG. 3, a high-voltage transistor is required as transistor Q3 provided in the starting circuit, and an expensive There was a problem in that the overall cost of the device was high.In other words, switching regulators are normally supplied with a power supply voltage obtained by rectifying the commercial power supply using a diode bridging device, etc.
In the case of an AC 200V system, a high voltage transistor with a collector-emitter maximum rated voltage V CEO of approximately 400V must be used.

If we were to construct a startup circuit without using transistor Q3, we would need capacitor C2 to sufficiently charge between the gate and source of switching transistor Q2.
Another problem arises in that a large-capacity device must be used as a storage device, which increases the size of the entire device.

SUMMARY OF THE INVENTION An object of the present invention is to provide a switching regulator in which a starting circuit is configured using transistors having a relatively low breakdown voltage, and the cost can be easily reduced.

(Means for Solving Problem d1) This invention includes a switching control circuit that controls on/off a switching transistor connected to a primary winding of a transformer, and a switching control circuit that supplies power from an input power source to start the switching control circuit. In a switching regulator equipped with a starting circuit, a CR circuit consisting of a series circuit of a resistor and a capacitor is connected to the human power source, and a constant voltage circuit that becomes conductive when the charging voltage of the capacitor exceeds a certain value, and a constant voltage circuit connected to this constant voltage circuit. The present invention is characterized in that a transistor which is made conductive by a flowing current is provided between the output of the CR circuit and the power input of the switching control circuit to form a startup circuit.

fe) Function An example of the configuration of the switching regulator of the present invention is shown in FIG. In Fig. 1, 1 is a starting circuit, in which a CR circuit 5 consisting of a series circuit of a resistor R1 and a capacitor C1 is connected between the input power source and the ground, and a constant voltage that becomes conductive when the charging voltage of the capacitor C1 exceeds a certain value. A circuit 6 and a transistor Q1 made conductive by the current flowing through the constant voltage circuit 6 are provided between the output of the CR circuit and the power source of the switching control circuit 2.

Now, considering the case where the input power is turned on, a charging current flows to 01 through R1, and the charging voltage of C1 increases.

When the charging voltage of C1 exceeds a certain value, the constant voltage circuit 6 becomes conductive, and at the same time, the transistor Q1 also becomes conductive. Therefore, the starting voltage is supplied to the switching control circuit 2, and the switching control circuit 2 starts operating and turns on/off the switching transistor Q2. As a result, the voltage generated from the rectifying and smoothing circuit 4 is supplied to the switching control circuit 2, and thereafter, the switching control circuit 2 is operated by the output voltage of the rectifying and smoothing circuit 4.

By configuring the circuit as described above, the voltage applied to the transistor Q1 in the startup circuit does not exceed the charging voltage of the capacitor C1, and a transistor with low breakdown voltage can be used.

(f) Embodiment FIG. 2 shows a circuit diagram of a switching regulator which is an embodiment of the present invention. In Figure 2, Q2 is transformer 3
This is a switching transistor that cuts off the current flowing through the primary winding N1. Transistors Q4 and Q5 constitute a so-called totem pole circuit, and control IC7
is a switch that controls switching transistor Q2 on/off via this totem pole circuit and resistor R5.
This is a control IC for a notching regulator. Also, D.B.
is a diode prefuge which rectifies the AC input and supplies input power to the primary t'vAN1 of the transformer and the starting circuit l.

The starting circuit l consists of a resistor R1 and a capacitor C1.
It consists of an R circuit 5, a constant voltage circuit 6 consisting of a resistor R4 and a Zener diode ZD, and a transistor Q1.

A rectifying and smoothing circuit consisting of a diode D2 and a capacitor C4 is connected to the secondary winding N2 of the transformer 3 to supply DC voltage to the load. A resistor R6, a rectifier diode D1, and a smoothing capacitor C3 are connected to the auxiliary winding N3 of the transformer, and the output thereof is connected to the power supply terminal of a totem pole circuit consisting of a control IC 7 and transistors Q4 and Q5.

The operation of the switching regulator configured as above is as follows. First, when the AC input is turned on, the rectified voltage is applied to the primary winding N1 of the transformer and the starting circuit l.
is supplied to When the charging voltage of the capacitor C1 exceeds the Zener voltage of the Zener diode ZD, a Zener current flows through ZD and its cathode potential decreases. This causes a base current to flow through the transistor Q1, turning Ql on. Therefore, transistor Q4. A power supply voltage approximately equal to the Zener voltage of ZD is supplied to the totem pole circuit consisting of Q5 and the control IC 7, and they start operating. This controls on/off of the switching transistor Q2.

When Q2 is turned on, current flows through the primary winding N1, and energy is stored in the transformer 3 during this period. Further, at this time, the electromotive voltage of the auxiliary winding N3 is rectified and smoothed by the diode D1 and the capacitor C3, and power is supplied to the control IC 7 and the totem pole circuit. Also, Q2
When N2 turns off, the energy released from the secondary winding N2 of the transformer during the off period is smoothed by N2 and supplied to the load.

By configuring the circuit as described above, the voltage applied to the transistor Q1 in the starting circuit 1 does not exceed the charging voltage of the capacitor C1, and the charging voltage of C1 is determined by the Zener voltage of the Zener diode ZD.
The maximum applied voltage to l is approximately equal to the Zener voltage of ZD. Since the Zener voltage of this Zener diode ZD is the power supply voltage required when starting the control IC 7, it is a relatively low voltage. Therefore, the transistor Q1 can be of a low breakdown voltage (for example, the maximum rating vCEo is 50 V), and can be constructed at low cost.

In the embodiment shown in FIG. 2, Ql becomes conductive only after the charging voltage of capacitor C1 reaches a predetermined value.
When Ql is conductive, the charging voltage of C1 changes to control IC7.
When the transistor Q4 is turned on, the gate and source of the switching transistor Q2 can be sufficiently charged through Q4. Therefore, a capacitor with relatively small capacitance can be used as the capacitor C1.

(g1 Effects of the invention According to this invention, low voltage transistors can be used as the transistors used in the startup circuit, so the startup circuit can be configured using inexpensive transistors, reducing the cost of the entire circuit. In addition, since the transistor in the startup circuit becomes conductive after the capacitor charging voltage of the CR circuit reaches a predetermined value, sufficient drive current can be supplied from the capacitor to the switching control circuit and the switching transistor. Therefore, the electrostatic capacitance of the capacitor in the starting circuit can be relatively low, and the overall size can be reduced.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an example of the configuration of the present invention. FIG. 2 is a circuit diagram of a switching regulator according to an embodiment of the present invention. FIG. 3 is a circuit diagram of the main parts of a conventional switching regulator. 1, l'-starting circuit, 3-transformer, 5-CR circuit, 6 constant voltage circuit.

Claims (1)

[Claims] (1) In a switching regulator that is equipped with a switching control circuit that controls on/off the switching transistor connected to the primary winding of the transformer, and a startup circuit that supplies power from the input power source and starts the switching control circuit, the input A CR circuit consisting of a series circuit of a resistor and a capacitor is connected to the power supply, and a constant voltage circuit that becomes conductive when the charging voltage of the capacitor exceeds a certain value, and a transistor that becomes conductive due to the current flowing in this constant voltage circuit are connected to the CR circuit. A switching regulator comprising a starting circuit provided between an output of the circuit and a power input of the switching control circuit.