JPH0777513B2 - Switching power supply start-up circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a switching power supply, and more particularly to the activation of a switching power supply with improved operation protection for an input voltage range of an input power supply and system reset operation of an output side load circuit. Regarding the circuit.

[Prior Art] A conventional switching power supply, as shown in FIG. 3, directly lowers an input voltage by a constant voltage element such as a Zener diode 4 and supplies it to a control circuit 13 to detect an internal voltage detection circuit of the control circuit 13. Was started in. However, in this method, the control circuit 13
Since all the current consumed by is supplied from the dividing resistor 2,
The power consumption of the dividing resistor 2 is large, and in order to stabilize the voltage of the filter capacitor 12 against the input voltage fluctuation, it is necessary to absorb the current change of the dividing resistor 2 by the Zener diode 4, so that the Zener diode 4 has a large power consumption. However, there were problems such as heat generation, efficiency reduction, and size increase of parts.

Therefore, in order to improve the power consumption of the example shown in FIG. 3, a system as shown in FIG. 4 in which the control circuit power supply is self-generated has been developed. In the example of FIG. 4, the voltage conversion transformer 15 is provided with a third winding 15a, and the rectifying diode 14 and the filter capacitor 12 form a power supply for the control circuit 13. By setting the voltage of the Zener diode 4 lower than the third winding voltage, the current amplification transistor 9 in the normal operation,
The power of the feeder resistor 8 can be reduced to zero, and the power can be significantly reduced. This circuit is commonly used (The control IC of the μPC1094 was developed for this circuit, The current before starting is suppressed to about 1mA, and the feeder resistance 8 and current amplification transistor 9 when power is applied
Has the effect of reducing the surge current. ).

As Although this system has the effect that the power consumption during normal operation is small, to make the power of the control circuit 13 on their own, the input voltage is instantaneously interrupted as between time t ₄ ~t ₅ in FIG. 5 Even in this case, the operation is continued until the voltage of the filter capacitor 12 becomes V _T2 (low voltage detection point of the control IC forming the control circuit 13) or less. The input voltage at this time is significantly lower than that during normal operation, and the output voltage also drops to some extent, but the DC / DC converter unit A also operates with the maximum pulse width.

Therefore, at this point, each element needs to have a sufficient margin in order to prevent the main switch 16 from being destroyed due to the saturation of the voltage conversion transformer 15. For this reason, the design of the voltage conversion transformer 15 becomes difficult and large. Further, when the input voltage is restored, the DC / DC converter section A rises again, so that there is a risk that the output side voltage is restored from a halfway voltage and the load side logic circuit malfunctions and system runaway occurs.

[Problems to be Solved] That is, the switching power supply described above has the drawback that the power consumption during normal operation is large in the example of FIG. 3 and large components are required, and the example of FIG. 4 requires a large transformer. There is a drawback in that the load side malfunctions when the input is interrupted and the system may run out of control.

The present invention has been made in consideration of the above problems,
An object of the present invention is to provide a start-up circuit for a switching power supply which aims to reduce power consumption, downsize a power exchange transformer, and prevent system malfunction at the time of momentary interruption by supplying a signal to a control circuit with a definite signal.

[Means for Solving the Problems] In order to achieve the above object, a switching power supply start-up circuit according to the present invention has a voltage control circuit, a main switch, a voltage exchange transformer, a secondary side rectifier circuit, and a smoothing filter. In a switching power supply circuit that forms a power supply for the control circuit by providing a third winding, a dividing resistor connected between the positive and negative electrodes of the input power supply and a base at the dividing point of this dividing resistance, A transistor having a collector connected to the positive electrode and an emitter connected to the voltage control circuit via a diode, a Zener diode having a cathode connected to the dividing point, and an on / off transistor connected to an on / off control terminal of the voltage control circuit. A base is connected to the anode of the Zener diode, and an emitter is connected to the negative electrode of the input power source. A turn-off transistor having a collector connected to the collector, a dropper power supply set to a voltage lower than the third winding voltage, and a reference voltage of the dropper power supply supplied from the dividing resistor, and the on-off transistor The control circuit is activated by the reference voltage determined by.

[Embodiment] Next, an embodiment of the present invention will be described with reference to the drawings.

It should be noted that, in the following, the same parts as those in the conventional art will be designated by the same reference numerals and overlapping description will be omitted.

FIG. 1 is a circuit diagram showing an embodiment of the present invention, 2 and 3 are input voltage dividing resistors, and B is a reference voltage detecting circuit.

The reference voltage detection circuit B forming the dropper power supply includes a bias resistor 5 connected to the anode side of the Zener diode 4, a bias resistor 7 connected in parallel with the dividing resistors 2 and 3, and a detection transistor 6 and an on / off transistor 11 connected directly. Consists of.

Now, when the input power supply 1 is turned on at time t ₁ in FIG. 2, the voltage of the Zener diode 4 and the filter capacitor 12 rises. When the voltage of the filter capacitor 12 reaches the low voltage detection point V _T2 of the IC forming the control circuit 13, the control circuit 13 becomes operable, but the output of the on / off transistor 11 is up to time t ₂ when the voltage of the Zener diode 4 is fixed. The operation has stopped due to. At time t ₂ , the start-up signal is sent from the on / off transistor 11 due to the confirmation of the voltage of the Zener diode 4, the control circuit 13 operates and the DC / DC converter unit A is started, and the voltage of the filter capacitor 12 becomes the voltage of the voltage conversion transformer 15. The voltage rises to the voltage determined by the third winding 15a (greater than the voltage of the Zener diode 4), and the feeder resistor 8 and the current amplification transistor 9 are cut off.

The input voltage when the on-off transistor 11 is activated is determined by the following formula.

However, in the above formula R ₂ R _3: the resistance value V ₄ of the resistor 2 and 3: zener voltage of the zener diode 4 V _BE6: an input-side voltage of the detection transistor 6.

The starting input voltage V _{T1 in} the above equation (1) is set to a voltage at which this circuit can control the output voltage.

Next, if a momentary interruption occurs at the input at time t ₄ , the input voltage V ₁
The output voltage does not show any abnormal fluctuation if the voltage exceeds the start input voltage V _T1 and recovers, but immediately below the start input voltage V ₁ , the on / off transistor 11 operates to stop the operation of the converter unit A. Therefore, the design input range of the DC / DC converter unit A can be limited to the starting input voltage V _T1 .

Now, the input voltage V _T1 has recovered and restarted at time t _{5
When T1} is exceeded, the converter section starts immediately and the output voltage rises again.

In this way, the output voltage V ₁ of this circuit does not generate an intermediate-level output voltage that causes the load-side logic circuit to malfunction because it stops and then rises. Since the system can be reset at the start-up, it is possible to prevent the system from running out of control, and to reduce power consumption during normal operation by cutting off the feeder resistor 8 and the current amplification transistor 9.

As described above, according to the present invention, the input voltage detection circuit and the reference voltage determination signal are added to the switching power supply that constitutes the power supply for the control circuit by providing the voltage conversion transformer with the third winding. By adopting a configuration that turns the control circuit on and off with a confirmation signal, the power consumption during normal operation is reduced, the design voltage range of the converter transformer is limited, the design is easy and compact, and the halfway situation of the output voltage is eliminated. This has the effect of preventing system malfunction.

In addition, the system can be reset by the rise of the output voltage due to the restart at the time of a momentary power failure, which has the effect of preventing system runaway.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, and FIG.
FIG. 3 is a circuit diagram showing an example of a conventional switching power supply circuit, FIG. 4 is a circuit diagram showing another example of a conventional switching power supply circuit, and FIG. 5 is a timing chart showing the operation of the example of FIG. 4. 1: Input power supply, 2, 3: Split resistance 4: Zener diode 5: Bias resistance, 6: Detection transistor 7: Bias resistance, 8: Feeder resistance 9: Current amplification transistor 10: Diode 11: On-off transistor 12: Filter capacitor 13 : Control circuit, 14: Rectifier diode 15: Voltage conversion transformer 16: Main switch, 17: Rectifier diode 18: Filter capacitor 19: Voltage detection circuit

Claims (1)

[Claims] 1. A switching device comprising a voltage control circuit, a main switch, a power exchange transformer, a secondary side rectification circuit and a smoothing filter, wherein a third winding is provided on the power exchange transformer to form a power supply for the control circuit. In the power supply circuit, a dividing resistor connected between the positive and negative electrodes of the input power source, a base is connected to the dividing point of this dividing resistor, a collector is connected to the positive electrode of the input power source, and a diode is connected to the voltage control circuit via a diode. A transistor having an emitter connected thereto, a Zener diode having a cathode connected to the division point, an on / off transistor connected to an on / off control terminal of the voltage control circuit, and a base connected to an anode of the Zener diode,
A dropper power supply having a detection transistor in which an emitter is connected to a negative electrode of the input power supply and a collector is connected to the on / off transistor, and the dropper power supply is set to a voltage lower than the third winding voltage; and a reference of the dropper power supply. A starting circuit for a switching power supply, wherein a voltage is supplied from the dividing resistor and the control circuit is started by the reference voltage determined by the on / off transistor.