JPH0549259A - Switching power supply - Google Patents

Abstract

PURPOSE:To realize a switching power supply in which stabilized operation is realized even when DC input voltage drops. CONSTITUTION:In a switching power supply in which DC input voltage to the primary winding of a transformer 4 is turned ON/OFF through a switching element 6 and the voltage induced in the secondary winding of the transformer 4 is rectified/smoothed through a rectifying/smoothing circuit, the switching power supply comprises a control circuit 8 for turning the switching element 6 ON/OFF and a step-up type DC-DC converter 17 which steps up DC input voltage to the primary winding of the transformer 4 and feeds thus stepped up voltage, as power supply, to the control circuit 8.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply, and more particularly to relaxation of DC input voltage range conditions.

[0002]

2. Description of the Related Art As a power source that can be easily obtained from an automobile, there is a DC 12V output of a battery for starting an engine. Therefore, in-vehicle electronic devices often incorporate a switching power supply, for example, as shown in FIG.

In FIG. 4, 1 is an input filter,
A diode 2 for protecting the reverse connection of the input voltage is connected in parallel to the input terminal, an input voltage of DC 12V is applied to the input terminal, and a capacitor 3 is connected to the output terminal. The positive terminal of the capacitor 3 is added to one end of the primary winding 5 of the transformer 4, and the negative terminal is connected to the common potential point. The other end of the primary winding 5 of the transformer 4 is an FET
It is connected to the common potential point via 6. A series regulator 7 is connected to the positive terminal of the capacitor 3. This series regulator 7 is FET6
The electric power is supplied to the control circuit 8 for switching and driving. One end of the secondary winding 9 of the transformer 4 is connected to the anode of the diode 10, and the other end is connected to the anode of the diode 11 and one end of the capacitor 13.
The cathode of the diode 10 is connected to one end of the choke coil 12 and the cathode of the diode 11. The other end of the choke coil 12 is connected to the other end of the capacitor 13. These diodes 10, 11, choke coil 12 and capacitor 13 form a rectifying / smoothing circuit. In FIG. 4, the output voltage of the rectifying / smoothing circuit is fed back to the control circuit 8 so that the output voltage becomes a desired value.
The feedback loop for controlling the ON / OFF cycle of T6 is omitted.

FIG. 5 is a diagram showing a specific example of the series regulator 7 used in FIG. 4, including a transistor 14 and a Zener.
It is composed of a diode 15, a resistor 16 and the like. The collector of the transistor 14 is connected to the power supply line, the emitter is connected to the power supply target, and the base is connected to the cathode of the Zener diode 15. The anode of Zener diode 15 is connected to the common potential point,
The gate is connected to the power supply line via the resistor 16.

By the way, the input voltage range of the switching power supply incorporated in such an on-vehicle electronic device is often designed so that the minimum voltage is about 10V with respect to the rated voltage of 12V. The reasons are: (1) If the voltage is set to 10 V or less, the current capacity becomes large, and the size of the component becomes large. (2) The operation guarantee voltage of the control circuit 8 is about 10 V.
When the voltage becomes 0 V or less, the reliability of the operation of the control circuit 8 becomes unreliable.

[0006]

However, when the vehicle-mounted electronic device using the switching power supply shown in FIG. 4 is actually driven by the engine starting battery of the automobile, the output voltage of the battery is started when the engine is started. It may drop to about 6V and may be reset depending on the configuration of the electronic device. The present invention solves such conventional problems, and an object thereof is to realize a switching power supply that can obtain stable operation even when the DC input voltage decreases.

[0007]

SUMMARY OF THE INVENTION The present invention which solves the above-mentioned problems is achieved by switching on and off a DC input voltage input to a primary winding of a transformer with a switching element.
In a switching power supply for rectifying and smoothing a voltage induced in a secondary winding by a rectifying and smoothing circuit, a control circuit for driving the switching element on and off and a DC input voltage input to a primary winding of the transformer are stepped up. And a step-up type DC-which supplies power to the control circuit.
It is characterized by being composed of a DC converter and.

[0008]

The power supply for the switching element control circuit is supplied through the DC-DC converter that steps up the DC input voltage input to the primary winding of the transformer to a predetermined voltage. As a result, even if the DC input voltage input to the primary winding of the transformer drops, the control circuit of the switching element operates stably, and a stable DC voltage is output from the rectifying / smoothing circuit connected to the secondary winding of the transformer. Is output.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of the present invention.
The same parts as those in FIG. In FIG.
A step-up type DC-DC converter 17 steps up the output voltage of the series regulator 7 to a predetermined voltage and supplies it to the control circuit 8.

FIG. 2 is a circuit diagram showing a specific example of FIG.
In FIG. 2, the input filter 1 includes input terminals T1 and T2.
The capacitor C1 connected between the output terminals T3 and T4
Capacitors C2 and C, which are connected in series between and have their midpoints grounded
Capacitor C between the series circuit of 3 and output terminals T3 and T4.
A capacitor C4 connected in parallel with the series circuit of C2 and C3.
And a coil L1 connected between the input terminal T1 and the output terminal T3, and a coil L2 connected between the input terminal T2 and the output terminal T4. Series Regulator 7
As the Zener diode that composes, a Zener voltage of 6V is used, and it stabilizes at + 5V even if the DC input voltage input to the primary winding 5 of the transformer 4 changes between + 12V and + 6V. The output voltage can be output. D
As the C-DC converter 17, a series regulator
A well-known one configured to step up the + 5V output voltage of the power supply 7 to + 12V and supply it to the control circuit 8 is used. As such a DC-DC converter, a commercially available one that is integrated as a power supply unit may be used, or may be composed of individual parts. As the control circuit 8,
For example, μPC494 configured as an integrated circuit capable of operating at + 12V is used.

FIG. 3 is a peripheral circuit diagram for driving the FET 6 of FIG. The control signal output terminal of the control circuit 8 is connected to the bases of the transistors Q1 and Q2, and
It is connected to a common potential point via a resistor R1. The collector of the transistor Q1 is connected to the + 12V power supply line, and the collector of the transistor Q2 is connected to the common potential point. The emitters of the transistors Q1 and Q2 are commonly connected, and the connection point is connected to the gate of the FET 6 via the resistor R2. The connection point between the resistor R2 and the gate of the FET6 is connected to the common potential point via the resistor R3.

The operation of FIG. 2 will be described. When the DC voltage input from the engine starting battery to the input filter 1 rises to around + 5V, the series regulator 7 supplies the DC-DC converter 17 with a voltage stabilized at + 5V. Start supply. Then, the DC-DC converter 17 starts supplying + 12V power to the control circuit 8. When the + 12V power is supplied, oscillation starts inside the control circuit 8 and the FET 6 is driven on and off. A voltage is induced in the secondary winding 9 of the transformer 4 by driving the FET 6 on and off, and a predetermined DC voltage is output from the rectifying and smoothing circuit. Here, since + 12V power is supplied to the control circuit 8 while the regulated voltage of + 5V is being output from the series regulator 7,
For example, the switching operation of the FET 6 does not stop even if the output voltage of the battery drops to about 6V due to the start of the engine, and a predetermined DC voltage is continuously output from the rectifying / smoothing circuit.

Although the forward type has been described in the above embodiment, the technical idea of the present invention is to apply it to various switching power supplies such as flyback type, push-pull type, half bridge type and full bridge type. It is applicable.

Further, the above embodiment is a primary control system in which the switching operation is controlled by the primary side of the transformer, but the input side and the output side of the DC-DC converter are electrically insulated from each other and the withstand voltage is increased. Is sufficient, it is also effective for the secondary control method in which the switching operation is controlled by the secondary side of the transformer.

Further, although the FET is used as the switching element in the above-mentioned embodiment, for example, an ordinary transistor may be used.

Further, in the above embodiment, the series regulator and the DC-DC converter are separately constructed, but they may be integrated.

[0017]

As described above, according to the present invention,
Since the step-up type DC-DC converter is provided in the system for supplying power to the control circuit for controlling the on / off operation of the switching element such as FET, the range of stable DC input voltage can be expanded. Therefore, by incorporating the switching power supply of the present invention, for example, as a power supply circuit of an on-vehicle electronic device that uses an engine starting battery of an automobile as a driving power supply, even if the battery output voltage is lowered at the time of engine starting, the effect thereof is reduced. It is possible to operate stably without receiving.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific example of FIG.

FIG. 3 is a peripheral circuit diagram for driving the FET 6 of FIG.

FIG. 4 is a circuit diagram showing a conventional example.

5 is a diagram showing a specific example of a series regulator used in FIG.

[Explanation of symbols]

 1 Input Filter 4 Transformer 6 Switching Element (FET) 8 Control Circuit 17 DC-DC Converter (Step-up Type)

Claims (1)

[Claims] 1. A switching power supply in which a DC input voltage input to a primary winding of a transformer is turned on and off by a switching element and a voltage induced in a secondary winding of the transformer is rectified and smoothed by a rectifying and smoothing circuit, A control circuit for driving the switching element on and off, and a step-up type DC-DC converter for stepping up a DC input voltage input to the primary winding of the transformer and supplying it as a power source to the control circuit. A switching power supply characterized by comprising: