JPH0828967B2 - Switching regulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a switching regulator used in a constant voltage power supply circuit or the like.

(B) Conventional Technology In general, a switching regulator is composed of a transformer, a switching element for connecting and disconnecting a current flowing through a primary winding of the transformer, a switching control circuit for controlling the switching element, and the like. In addition, the power supply of the switching control circuit can be roughly classified into a type that supplies from the primary side and a type that supplies from the secondary side.

Generally, a dedicated IC including a pulse generation circuit and a pulse width control circuit is used as the switching control circuit, and the power supply voltage for the switching control circuit is lower than the input power supply voltage of the switching regulator. Therefore, when power is supplied to the switching control circuit from the primary side, an auxiliary power supply circuit for generating a power supply voltage for the switching control circuit is provided. In addition, when the power of the switching control circuit is supplied from the secondary side, the power supply voltage is not supplied to the switching control circuit unless the switching regulator is started. A start-up circuit that provides a start-up voltage is provided.

The general structure of a switching regulator of the type that supplies power to the switching control circuit from the secondary side
Shown in the figure.

In FIG. 5, 3 is a transformer, and its primary winding
A switching element Q is connected in series with N1. The starting circuit 1 supplies a power supply voltage for the switching control circuit 2 to the switching control circuit 2 via a diode when the input power supply voltage reaches a predetermined value. As a result, the switching control circuit 2 controls the switching element Q to be turned on / off. Q
Turns on / off the secondary winding of transformer 3.
An electromotive voltage is generated in N2. At the same time, the switching control circuit power supply circuit 4 rectifies and smoothes the electromotive voltage of the other winding N3 on the secondary side and makes it a constant voltage, and supplies power to the switching control circuit 2 via the diode. In the steady state, the output voltage of the switching control circuit power supply circuit 4 is the start circuit 1
Since it is set higher than the output voltage of, the starting circuit 1 operates only at the time of starting.

In the switching regulator of the type in which the power of the switching control circuit 2 is supplied from the secondary side as described above, the starting circuit 1 is provided, and the power is supplied from the primary side to the switching control circuit 2 only at the time of starting, and after the starting of the switching regulator. Is configured so that the switching control circuit power supply circuit 4 always supplies power to the switching control circuit 2.

(C) Problems to be Solved by the Invention However, the conventional switching regulator having such a starting circuit has the following problems.

FIG. 6 shows the voltage change of each part with respect to the change of the input power supply voltage Vin shown in FIG. Here, V2 is the output voltage of the switching control circuit power supply circuit 4, and V3 is the starting circuit 1.
Is the output voltage of. Further, Von indicates the minimum operable voltage of the switching control circuit 2. The starting circuit 1 is basically a constant voltage circuit, and outputs a constant voltage when the input power supply voltage Vin is a predetermined value or more as shown in FIG. When the output voltage V3 of the starting circuit 1 exceeds Von (more precisely, when it exceeds the value obtained by adding the voltage drop of the diode), the switching control circuit 2 starts operating. Input power supply voltage Vi
If n rises, the output voltage V2 of the switching control circuit power supply circuit 4 gradually rises and is made constant with a predetermined value. Under the condition of V2> V3, the switching control circuit 2 is driven by the output voltage of the switching control circuit power supply circuit 4, and power is not supplied from the starting circuit 1.

However, if Vin drops here, Vin1 to V in Fig. 6
If the range of in2 is maintained, V2 <V3 is satisfied, so that power is continuously supplied from the starting circuit 1 to the switching control circuit 2. The start-up circuit 1 has the ability to supply power only for a short time, and if such a state continues, there is a risk that circuit components (such as resistors) of the start-up circuit 1 will be burned out. Further, in such a state, the specified output voltage cannot be obtained, so that the load circuit may operate abnormally.

An object of the present invention is to provide a switching regulator in which a switching control circuit is driven immediately by a power supply circuit for a switching control circuit after being started, and a driving time of the switching control circuit by the starting circuit is stopped to a minimum time. To provide.

(D) Means for Solving the Problems The present invention is directed to a switching control by rectifying and smoothing an electromotive voltage of an output winding of a transformer and a startup circuit that supplies a startup voltage to a switching control circuit by making an input power supply voltage a constant voltage. In a switching regulator equipped with a power supply circuit for switching control circuit that supplies power to the circuit, the output voltage of the power supply circuit for switching control circuit becomes higher than the output voltage of the starter circuit when the switching control circuit operates. A feature is that a step-down circuit is provided at the input part of the starting circuit so that the output voltage of the starting circuit exceeds the operable bottom voltage of the switching control circuit.

(E) Function FIG. 1 shows the configuration of the switching regulator of the present invention. Further, FIG. 2 shows a change in voltage of each part with a change in the input power supply voltage Vin.

The difference from the configuration of the conventional switching regulator shown in FIG. 5 is that resistors R1 and R2 are added to the input section of the starting circuit 1.
The step-down circuit consisting of

Since the starting circuit 1 supplies the divided control outputs of the resistors R1 and R2 with a constant voltage to supply power to the switching control circuit 2, the output voltage V3 of the starting circuit 1 gradually rises as the input power supply voltage Vin rises. As shown in FIG. 2, the switching control circuit 2 operates when V3 exceeds Von (minimum voltage for starting operation of the switching control circuit 2) (more precisely, exceeds Von plus the voltage drop of the diode). To start. At this time, since the input power supply voltage Vin has already reached near the rated voltage, the output voltage V2 of the switching control circuit power supply circuit 4 reaches a predetermined constant voltage immediately after the switching control circuit 2 starts operating, and V2> V3 Then, the starting circuit 1 stops operating. After that, if the output voltage V2 of the switching control circuit power supply circuit does not fall below Von even if the input power supply voltage Vin of the switching regulator drops, the switching control circuit operates by V2. Vi
If n further decreases and the output voltage V2 of the switching control circuit power supply circuit 4 falls below Von, the output voltage V3 of the starter circuit at that time falls below Von. And V3 do not work and the switching regulator stops.
Therefore, the switching control circuit operates only by the output voltage V3 of the starting circuit 1 because V3 rises and V3 rises to Vo.
Only when it reaches n, the driving time of the switching control circuit by the starting circuit stops at the minimum time. As described above, the start-up circuit 1 operates only at the start-up, and although the switching control circuit 4 operates, an abnormal state in which current is supplied from the start-up circuit 1 to the switching control circuit 2 does not occur. Therefore, the above-mentioned inconvenience does not occur even when the input power supply voltage drops.

(F) Embodiment FIG. 3 is a circuit diagram of a switching regulator which is an embodiment of the present invention, and FIG. 4 shows a voltage change of each part according to a change of the input power supply voltage Vin.

In FIG. 3, resistors R1 and R2 are step-down circuits, and ZD is a zener diode that forms a constant voltage circuit together with R1 and R2. A resistor R3, a transistor Q2, a diode D1 and a capacitor are connected between the input power supply line and the power supply terminal of the switching control circuit 2, and the base of the transistor Q2 and the cathode of the zener diode ZD are connected to form a starting circuit. There is.

A switching element Q1 is connected to the primary winding N1 of the transformer 3, and the switching control circuit 2 controls on / off of the switching element Q1 to interrupt the current flowing through N1. One winding N2 on the secondary side of the transistor 3
Rectifier diode D3, flywheel diode D4,
The choke coil L and the smoothing capacitor C1 are connected. In addition, the rectifier diode is installed on the other winding N3 on the secondary side.
D2, a smoothing capacitor C2, and a series regulator 5 for stabilizing the rectified and smoothed output are connected. This D
2, C2 and 5 form a power supply circuit for the switching control circuit.

The operation of the switching regulator described above is as follows.

First, consider the case where the input power supply voltage Vin gradually rises. When Vin gradually rises, the cathode voltage V3 of the Zener diode ZD gradually rises as shown in FIG. 4, and along with this, the power supply voltage V1 of the switching control circuit 2 also rises. When the input power supply voltage Vin reaches Va and V1 reaches the operation start minimum voltage Von of the switching control circuit 2, the fourth
As shown in the figure, the switching control circuit 2 starts to operate for the first time. As a result, the capacitor voltage V2 'of the switching control circuit power supply circuit rises. On the other hand, the cathode voltage V3 of ZD never exceeds the Zener voltage Vz. As a result, V1> V3 and the switching control circuit 2 becomes V2 ′.
Power is supplied from the side and the transistor Q2 is turned off.

After that, if the input power supply voltage Vin decreases, it decreases as shown in FIG. 4, and the input power supply voltage Vin reaches Vb and V1
When <Von, the operation of the switching control circuit stops and V2 ′ also becomes 0. At this time, V3> V1 again, but since the switching control circuit 2 has already stopped at this time, a large current does not flow through the transistor Q2 and the resistor R3 (10 mA or less), and there is no risk of damage to the resistor R3.

Thus, even if the input power supply voltage Vin is lower than the rated voltage, the power supply from the starting circuit to the switching control circuit 2 is not maintained.

(G) Effects of the Invention As described above, according to the present invention, the switching control circuit is activated by the start-up circuit, and once the switching regulator starts to be activated, the switching control circuit power supply circuit immediately supplies power to the switching control circuit. As a result, the starting circuit operates only for a short time at the time of starting. For this reason, even if the input power supply voltage drops after startup, burning of the components of the startup circuit is prevented, and a highly safe switching regulator is configured.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing the configuration of the present invention, and FIG. 2 is a diagram showing the relationship of the voltage change of each part with respect to the fluctuation of the input power supply voltage. FIG. 3 is a circuit diagram of a switching regulator which is an embodiment of the present invention, and FIG. 4 is a diagram showing a voltage change of each part with respect to a fluctuation of the input power supply voltage. FIG. 5 is a diagram showing a configuration of a conventional switching regulator, and FIG.
The figure is a diagram showing changes in voltage of each part with respect to changes in the input power supply voltage. R1, R2 ... Step-down circuit.

Claims (1)

[Claims] 1. A starter circuit for converting an input power supply voltage to a constant voltage to supply a starter voltage to a switching control circuit, and a switching control circuit for rectifying and smoothing an electromotive voltage of an output winding of a transformer and supplying power to the switching control circuit. In a switching regulator equipped with a power supply circuit, when the switching control circuit is operating, the output voltage of the starter circuit is switching-controlled only when the input power supply voltage is such that the output voltage of the power supply circuit for switching control circuit becomes higher than the output voltage of the starter circuit. A switching regulator in which a step-down circuit is provided in an input portion of a starter circuit so as to exceed a minimum operable voltage of the circuit.