JPH05115172A - Switching power supply startup circuit - Google Patents

Abstract

(57) [Summary] [Purpose] To reduce the required booster chopper capacity. [Structure] After the input voltage is detected by the input voltage detection circuit 4, the step-up chopper 1 is started, and at the same time, the timer 5 is started. Before the timer setting time elapses, the output of the step-up chopper 1 completely rises and is set. When the time elapses, the timer 5 activates the DC / DC converter 2. As a result, the boost chopper 1 requires a small capacity.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a step-up chopper and a DC / DC converter.
The present invention relates to a starting circuit for a switching power supply that is composed of a DC converter.

[0002]

2. Description of the Related Art Generally, it is not necessary to combine a step-up chopper and a DC / DC converter as a switching power supply, but when the fluctuation of the input power supply is large, or when the input power supply is turned on or the input voltage greatly drops. For example, a boost chopper is added to the input side of the switching power supply to stabilize the input voltage of the DC / DC converter.

The operation of a switching power supply in which a boost chopper is attached to the input side of a conventional DC / DC converter will be described.

In such a conventional switching power supply,
When the input power is turned on, the boost chopper starts. Figure 4
As shown in the output current-output voltage characteristics of the step-up chopper, the output current flows from the input power supply to the output as it is when the output voltage is higher than the input power supply voltage. The output current Ia is limited by the current protection circuit. A current excluding the current flowing from the Ia to the load of the step-up chopper (in this case, the DC / DC converter) flows into a capacitor provided between the step-up chopper and the DC / DC converter, and the capacitor is charged to generate DC / DC. The converter output rises. But I
When the load current of the step-up chopper is larger than a, a current [load current-Ia] flows from the capacitor and discharges this capacitor, so that the output voltage of the DC / DC converter decreases and the output voltage of the step-up chopper cannot be controlled. .. If the load of the step-up chopper is a resistive load, the load current becomes smaller when the output voltage of the step-up chopper is low, so that the load current <Ia is always satisfied, and there is no problem. However, when the load of the step-up chopper is a DC / DC converter, the input power of the DC / DC converter is constant power, and therefore, as shown in the graph of the input voltage-input current characteristics of the DC / DC converter in FIG.
When the input voltage is low, a large input current flows. DC /
When the input current of the DC converter, that is, the load current of the step-up chopper is maximum, the current that flows at the starting voltage of the DC / DC converter is Ib. Therefore, in the conventional switching power supply, the output capacitance of the step-up chopper needs to have a large output capacitance such as Ib instead of the current In ₁ required in the steady state.

[0005]

In the conventional technique as described above, the output capacitance of the step-up chopper must be increased to Ib with respect to the output capacitance In ₁ required in the steady state in FIG. Therefore, the choke coil that constitutes the boost chopper,
Diodes and trunk transistors with large current capacity must be used. Therefore, the cost becomes high,
There was a problem that the outer shape became large.

Therefore, an object of the present invention is to provide a starting circuit for a switching power supply which solves the above problems.

[0007]

The present invention is based on the fact that, in order to solve the above problems, the DC / DC converter should be started when the output of the boost chopper is equal to or higher than Va in FIG. And a DC / DC for inputting the output of the boost chopper and the boost chopper for boosting the input voltage
A switching power supply configured by a converter includes means for activating the DC / DC converter in a state in which a predetermined time has elapsed after the boost chopper starts to be activated and the output of the boost chopper has completely risen. Further, in a switching power supply composed of a boost chopper for boosting an input voltage and a DC / DC converter for inputting the output of the boost chopper, the output of the boost chopper is DC after the start of the boost chopper.
/ DC converter is provided with a means for activating the DC / DC converter when the voltage exceeds the maximum input power of the DC / DC converter.

[0008]

According to the present invention, the activation of the DC / DC converter is delayed for a predetermined time after the activation of the boost chopper, and the DC / DC converter is activated with the output of the boost chopper completely rising. The output of the boost chopper is DC /
The DC / DC converter is activated when the voltage exceeds the maximum input power of the DC converter.

[0009]

Embodiments of the present invention will now be described in detail with reference to the drawings.

(Embodiment 1) FIG. 1 is a circuit block diagram of Embodiment 1 of the present invention, in which reference numeral 1 is a step-up chopper, which uses an input power source as an input, a choke coil 13 and a diode 1.
4 and a transistor 15. Reference numeral 2 is a DC / DC converter, and 3 is a capacitor provided between the boost chopper 1 and the DC / DC converter 2. Reference numeral 4 denotes an input voltage detection circuit provided on the input side of the boost chopper 1, and when the input power is turned on, the input voltage detection circuit 4 detects the input voltage. When this circuit 4 detects the input voltage,
A signal is input to the on / off input terminal of the boost chopper 1 to start the boost chopper 1, and at the same time, a signal is input to the timer 5 to start the operation of the timer 5. As shown in FIG. 3, the output of the boost chopper 1 completely rises before the set time of the timer 5 is reached, and DC / D
The converter 2 can be started by inputting a signal to the on / off input terminal of the C converter.

(Second Embodiment) FIG. 2 is a circuit block diagram of a second embodiment of the present invention, in which a voltage detection circuit 12 receives an output voltage of the step-up chopper 1 and outputs D by an output signal.
The activation of the C / DC converter 2 is controlled. Reference numerals 8 and 9 are resistors connected in series, and both ends thereof are connected in parallel to the output end of the boost chopper 1. Reference numerals 10 and 7 are resistors and a Zener diode which are also connected in series, and both ends thereof are connected in parallel to the output terminal of the boost chopper 1.
6 is a comparator, 11 is a resistor provided between the output terminal and the non-inverting input terminal of the comparator 6, the comparator 6 inputs the voltage of the Zener diode 7 to the inverting input terminal, and the resistor 8 to the non-inverting input terminal. Input the voltage between 9 and the value of these resistors 8, 9 and 10 and Zener diode 7
When the output voltage of the boost chopper 1 rises to an arbitrary voltage Von between Va and Vn ₁ in FIG. 4 after the input power supply is turned on by appropriately selecting the voltage of Can be output to start the DC / DC converter 2.

As described above, 1) In the first embodiment, since the DC / DC converter 2 is activated after the output of the boost chopper 1 has completely risen, it is not necessary to increase the capacity of the boost chopper 1.

2) In the second embodiment, the DC / DC converter 2 is started at a voltage higher than Va in FIG.
It is not necessary to increase the capacity of.

3) In either of the embodiments, the required capacity of the boost chopper 1 can be made smaller than in the conventional case, so that the cost and size of the switching power supply can be reduced.

[0015]

As described above, according to the present invention, the DC / DC converter can be started even with a booster chopper having a smaller capacity than the conventional one, and the switching power supply can be made low cost and downsized.

[Brief description of drawings]

FIG. 1 is a circuit block diagram of a first embodiment of the present invention.

FIG. 2 is a circuit block diagram of a second embodiment of the present invention.

FIG. 3 is a diagram showing a time chart of the operation in the present invention.

FIG. 4 is a diagram showing output characteristics of a boost chopper and input characteristics of a DC / DC converter.

[Explanation of symbols]

 1 Step-up chopper 2 DC / DC converter 4 Input voltage detection circuit 5 Timer

Claims (2)

[Claims] 1. A switching power supply comprising a boost chopper for boosting an input voltage and a DC / DC converter for inputting the output of the boost chopper, wherein the boost chopper has a predetermined time elapsed after the start of activation of the boost chopper. A starting circuit for a switching power supply, comprising means for activating the DC / DC converter in a state where the output of the above has completely risen. 2. A switching power supply comprising a boost chopper for boosting an input voltage and a DC / DC converter for inputting the output of the boost chopper, wherein the output of the boost chopper is DC after the start of activation of the boost chopper.
A starting circuit for a switching power supply, comprising means for starting the DC / DC converter when the voltage exceeds the maximum input power of the / DC converter.