JPS6110473Y2 - - Google Patents

Description

[Detailed description of the invention] The invention is equipped with a DC voltage input terminal to which two types of DC input voltages of different levels are supplied, and a substantially constant output voltage can be obtained no matter which DC input voltage is supplied. The present invention relates to a power supply circuit that prevents this from occurring.

2. Description of the Related Art Conventionally, a switching regulator circuit shown in FIG. 1 has been known as a power supply circuit for obtaining a constant DC output voltage from two types of DC input voltages that are significantly different from each other, such as 12V and 24V. In this Figure 1, the DC input voltage is at terminal 1.
is supplied to terminal 3 through switching circuit 2.
is derived as the output voltage. This output voltage is compared with a reference voltage in the detection circuit 4, and a signal having a pulse width corresponding to the comparison error is supplied from the PWM circuit 5 to the switching circuit 2. The switching circuit 2 is switched on and off by the signal supplied from the PWM circuit 5. By controlling the length of the on/off time using this signal, the output voltage is maintained at a predetermined value.

In a circuit like the one shown in Fig. 1, if we try to obtain a constant output voltage even when the input voltage becomes a lower voltage, the on period in the switching circuit 2 becomes longer and a large current flows through the switching element. flows. For this reason, a special switching element is required. Furthermore, the circuit configuration becomes complicated because it includes the detection circuit 4 and the PWM circuit 5.

The present invention was devised in view of the above-mentioned problems, and it is possible to provide the power supply circuit as mentioned at the beginning with a very simple circuit configuration without using any special parts. This is how it was done.

The present invention will be explained below with reference to examples and FIGS. 2 and 3.

In FIG. 2, terminal 1 is supplied with a DC input voltage of 12V or 24V. This input voltage is supplied to the drive circuit 8 and the control circuit 11, respectively. The drive circuit 8 drives an inverter circuit 9, which will be described later, and includes an oscillator 13, a transistor 21,
23, resistors 17, 18, 19, a capacitor 14, and a transformer 26. Note that when the circuit shown in FIG. 2 is used as a power supply circuit for a television receiver, the oscillator 13 may be constructed by using a horizontal oscillation circuit or the like provided in the television receiver without providing an independent oscillator. You can do it.

The output of the oscillator 13 is connected to a capacitor 14 and a resistor 1.
The signal is supplied to the transistor 23 via a buffer amplifier composed of the transistors 7, 18, 19 and the transistor 21. And this transistor 23
After being amplified by , the signal is supplied to the base of a transistor 24 constituting the inverter circuit 9 via a transformer 26 .

A DC input voltage supplied to the control circuit 11 is supplied to the base of a transistor 22 via a resistor 20 and a Zener diode 29. The emitter of the transistor 22 is grounded, and the collector is connected to the base of the transistor 23 of the drive circuit 8 described above. This control circuit 11 has an input voltage of 12V
The drive circuit 8 is controlled by detecting whether the voltage is 24V or 24V. Therefore, the Zener diode 29 has a Zener voltage V _Z of 12V<V _Z <
24V, for example V _Z =18V. Therefore, when the input voltage is 12V, the transistor 22 is off, and when the input voltage is 24V, the transistor 22 is on. Thereby, the driving state of the driving circuit 8 is automatically controlled according to the input voltage.

The inverter circuit 9 is a so-called separately excited type inverter circuit driven by the above-mentioned drive circuit 8, and is composed of a transistor 24, a capacitor 15, and a transformer 27. transformer 27
is a single-turn step-up transformer having an intermediate terminal M, to which a DC input voltage is supplied. One end B of the transformer 27 is connected to the collector of the transistor 24, and the other end C is connected to the anode of a diode 28 constituting the rectifier circuit 10.

The rectifier circuit 10 includes a diode 28 and a capacitor 1
It consists of 6. The diode 28 is for rectification, and the capacitor 26 is for smoothing. The cathode of the diode 28 is connected to the output terminal 3,
The output of the power supply circuit shown in FIG. 2 is derived from the output terminal 3. Note that the output may be derived after being stabilized as necessary.

Next, the operation of the above configuration will be explained.

First, when the input voltage supplied to the input terminal 1 is 12V, the transistor 22 is in the off state as described above, so the point A shown in FIG. It changes like this.
The transistor 23 is turned on and off depending on the level of the signal a, and the output of the transistor 23 is supplied to the transistor 2 through the transistor 26.
A current i ₁ shown in FIG. 3 flows between the collector emitter of No. 4. At this time, the voltages at terminals B and C of the transformer 27 change as shown in signal b and signal c shown in FIG.
A current i ₂ flows through 8.

Note that the pulse width T of signal b and signal c (Fig. 3)
_Z is determined by the inductance of the transformer 27 and the capacitance of the capacitor 15. This pulse width T ₂ is T ₁ with respect to the pulse width T ₁ of the output signal a of the oscillator 13.
> T ₂ . The winding ratio between the terminals BM and MC of the transformer 27 is selected so that the average value of the signal c is 12V and the flat part is about 24V.

When the input voltage supplied to the input terminal 1 is 24V, the transistor 22 is turned on as described above. As a result, the transistor 23 is grounded through the collector-emitter of the transistor 22, so that the transistor 23 is always turned off regardless of the output of the oscillator 13. transistor 23
Since transistor 24 is always in an off state,
is always in the off state. At this time, the input voltage supplied to the terminal M of the transformer 27 is directly supplied to the anode of the diode 28 via the terminals MC.

As mentioned above, when the input voltage is 12V, the inverter circuit is activated to boost the voltage from 12V.
Since 24V is obtained, and when the input voltage is 24V, the inverter circuit is deactivated and 24V is sent out as is, a constant (one type) output voltage can be obtained from two types of voltage.

Since the present invention is as described above, it is a special power supply circuit that can obtain a nearly constant output voltage no matter which of two types of DC input voltages of different levels are supplied to the DC voltage input terminal. It can be provided with a very simple circuit configuration without using special parts. Further, according to the power supply circuit according to the present invention, the output voltage can be kept almost constant, so if a conventionally known feedback type stabilization circuit is connected to the output terminal, the allowable range of the input value of the feedback type stabilization circuit can be adjusted. There is no need to increase the voltage, and therefore a stable voltage can be obtained using a feedback type stabilizing circuit with a simple circuit configuration.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing a switching regulator as a conventional example, Fig. 2 is a circuit diagram showing an embodiment of the present invention, and Fig. 3 is a waveform diagram explaining the operation of the circuit shown in Fig. 2. be. In addition, in the symbols used in the drawings, 9 is an inverter circuit, 11 is a control circuit, 27 is a transformer,
M is an intermediate terminal.

Claims (1)

[Claims for Utility Model Registration] A step-up transformer having an intermediate terminal, and a DC voltage input connected to the intermediate terminal of the step-up transformer, which is configured to be supplied with two types of DC input voltages of different levels. an inverter output switching element connected to one end of the step-up transformer, a rectifier circuit connected to the other end of the step-up transformer, and a drive for supplying switching pulses for driving the inverter to the output switching element. a circuit that is connected between the DC voltage input terminal and the drive circuit and determines the level of the DC input voltage supplied to the DC voltage input terminal, and when the DC input voltage is lower than a predetermined value, the drive circuit a control circuit that supplies switching pulses from the output switching element to the output switching element and prevents the switching pulse from being supplied from the drive circuit to the output switching element when the DC input voltage is higher than the predetermined value; The circuit includes a Zener diode whose cathode side is connected to the DC voltage input terminal,
It is composed of a transistor connected between the anode side of the Zener diode and the drive circuit, and when the DC input voltage is lower than the predetermined value, the transistor turns off, thereby switching the output from the drive circuit. A switching pulse is supplied to the element, and the DC input voltage is stepped up by the step-up transformer, and when the DC input voltage is higher than the predetermined value, the transistor is turned on by the Zener voltage of the Zener diode. As a result, the switching pulse is not supplied from the drive circuit to the switching element, and therefore the DC input voltage is applied to the output terminal as it is without being boosted. A power supply circuit configured to obtain a nearly constant output voltage regardless of the input voltage level.