JPS5819925A - Power supply device - Google Patents

Abstract

PURPOSE:To protect an output transistor TR from overcurrent, by detecting the overcurrent of the output TR and lowering the voltage between both terminals of a capacitor, which protects the output TR from overcurrent for making of a power supply switch, to limit the collector current of the output TR. CONSTITUTION:When the current of a TR10 exceeds a certain value, a TR40 becomes conductive to lower the charging voltage of a capacitor 20. When this voltage between both terminals is lowered, the same operation as power supply is performed, and the conduction time of the TR10 is shorter independently of the variance of the output voltage, and the output voltage of a transformer 12 is lowered. Even if the TR40 is made conductive momently by noise, the output voltage is lowered only momently, and the output voltage is restored to the original state automatically when noise disappears. Consequently, it is unnecessary to increase the capacity of a capacitor 41, and the TR10 is protected from overcurrent surely.

Description

[Detailed Description of the Invention] The present invention relates to a switching power supply device, and an object of the present invention is to provide a power supply device that does not malfunction due to noise and whose transistors are not destroyed due to overcurrent. .

FIG. 1 shows an example of a conventional switching power supply circuit used in a television receiver. This power supply circuit is an insulated DC-DC converter using a pulse width control method, in which a voltage obtained by rectifying an AC input power source is switched in and out by an output transistor 10, and a stabilized voltage is transferred from the secondary side of a transformer 12 to the primary side. It is to be taken out by insulating and separating it from the side.

In the figure, 1 is an AC input power supply, 2, 3, and 4.5 are diodes forming a rectifier circuit, and 6 is a smoothing capacitor. Further, 7 is an integrated circuit element constituting a control circuit, which includes an error amplification circuit 71 composed of a differential amplification circuit, a pulse width control circuit 72, a support circuit 73, and a protection circuit 74 when the power is turned on. , the pulse width is controlled according to fluctuations in the output voltage and applied as an output to the base of the drive transistor 80.

The transistor 8 performs a switching operation using the drive transformer 9 as a load, and drives the output transistor 10 via the drive transformer 9. A rectifier diode 11 rectifies and smoothes the voltage generated in the third winding, which is proportional to the secondary voltage of the transformer 12, together with a smoothing capacitor 13, supplies it as a power supply voltage to the control circuit 7, and outputs it. The voltage is divided by resistors 16, 17, and 18 and applied to terminal (2) of the control circuit 7 as a detection voltage. Here, the terminal (2) of the control circuit 7 is a reference voltage terminal of the gate difference amplifier circuit 71, and a reference constant voltage is applied by the constant voltage diode 19. A differential amplifier circuit constituting the error amplifier circuit 71 compares the two voltages and controls the pulse width of the pulse output from the terminal (2).

Further, a resistor 14 and a capacitor 46 inserted between the terminals ■ and ■ of the control circuit 7 are elements that determine the oscillation frequency of the oscillation circuit 73.

Also, a capacitor 2o connected to the terminal of the control circuit 7
This is to start the conduction period of the transistor 10 from a narrow state when the power switch is turned on, and to prevent the transistor 1o from being destroyed by an overcurrent. As a result, when the power switch is turned on, the ON period of the transistor 10 is gradually extended by the time constant for charging the capacitor 20.

In addition, 21 is a transformer which insulates the horizontal retrace noise generated in the flyback transformer (FBT) of the horizontal deflection circuit, takes it out from the secondary side, applies it to the terminal ■ of the control circuit 7, and supplies it to the oscillation circuit 73. The oscillation frequency is made to match the horizontal scanning frequency. As a result, it is possible to prevent any interference from occurring in the signal of the television receiver. These are a diode and a capacitor that rectify and smooth the voltage and supply it to the drive transistor 8 as a power supply voltage.

A transistor 24 is temporarily turned on when the power switch is turned on and supplies the control circuit 7 and the drive transistor 8 with the power supply voltage formed by the diodes 2 to 6 and the capacitor 6, and 26 is constant after the power switch is turned on. This is a transistor that becomes conductive after a period of time and makes the transistor 24 non-conductive.

Now, when you turn on the power switch, transistor 2 instantly turns on.
4 is conductive, and as a result, 100 is connected across the capacitor 23.
A voltage of approximately V is charged and becomes the power supply voltage of the transistor 8. This voltage is also applied to the capacitor 13 via the resistor 26.
In addition, a voltage of about 7V is generated across the capacitor 13, which becomes the power supply voltage of the control circuit 7. Said capacitor 2
By appropriately selecting the values of the resistors 26, 27, and 28 and the constant voltage diode 29, the voltage of 3 can be kept constant regardless of voltage fluctuations of the AC input power source 1. In addition,
At this time, the collector voltage of the transistor 26 is 70~ao
It is about V.

When the transistor 24 becomes conductive in this way and the power supply voltage is supplied to the control circuit 7 and the transistors 8 and 1o, a pulse voltage is generated in the transformer 12, and the secondary side diode 30.
A DC voltage of about 16 volts can be obtained by the capacitor 31, and a DC voltage of about 27 V can be obtained by the diode 32 and capacitor 33, and a voltage of about 12 V rectified by the diode 11 is generated across the capacitor 13. Here, since the constant voltage value of the constant voltage diode 29 is set to a voltage of 6 to 7V lower than 12V, a large base current that saturates the base of the transistor 260 is supplied to the base of the transistor 260, and the transistor 26 becomes completely conductive. The base of the transistor 24 is grounded through the resistor 28, the diode 34 becomes conductive, and the transistor 24 becomes non-conductive and stops operating. After this, the circuit of FIG.
It operates using a DC voltage obtained from a diode 22 and a capacitor 23.

In this way, the reason why the power supply voltage is first extracted from the transistor 24 and the transistor 24 is made non-conductive after the power supply circuit operates normally is that the voltage is rectified from the diode 11 to the error amplifier circuit 71 and the transistor 24 is This is to prevent the voltage stabilization operation from becoming unstable when both voltages from Σ are applied. Furthermore, if the transistor 24 is constantly operated, there is a risk that the transistor 24 will be destroyed due to constant heat loss of about 4W.
To prevent this, a heat sink is required, which increases costs. However, if conduction is made only immediately after the power switch is turned on as described above, the heat sink becomes unnecessary.

The basic configuration and basic operation of the universal switching power supply circuit have been explained above.

By the way, in FIG. 1, 36 is a thyristor for protecting the transistor 1o from overcurrent, and the transistor 8
The resistor 36 is inserted between the base and ground of the transistor 10, and its gate is connected to one end of a resistor 36 connected to the emitter of the transistor 10. Transistor 10 due to overload of the load circuit, etc.
When an overcurrent flows through the resistor 36, a voltage proportional to the overcurrent is generated in the resistor 36, and when this voltage exceeds a certain value, the thyristor 3
6 conducts and grounds the base of transistor 89. This causes the transistors to become non-conductive and transistor 1.
0 also becomes non-conductive. As a result, the transistor 10 can be protected from overcurrent. Here, 37 and 38 are bias resistors, and 39 is a capacitor for preventing the thyristor 36 from malfunctioning due to noise.

In this way, in the circuit of FIG. 1, in order to reliably prevent the thyristor 36 from being erroneously turned on due to noise, the capacitance of the capacitor 39 is increased, and the transistor 1
Even if an overcurrent flows in the evening 1o, there will be a time delay before the thyristor 36 becomes conductive, and there is a risk that the transistor 10 will be destroyed. In this case, the thyristor 36 may malfunction because priority is given to preventing destruction of the transistor 10.

If the thyristor 36 malfunctions, it will not return to normal unless the power switch is turned off, which is inconvenient for the user.

The present invention eliminates the above-mentioned conventional drawbacks and eliminates noise.
It is an object of the present invention to provide a power supply device that prevents a main power supply path from erroneously stopping its operation and that can reliably protect transistors from overcurrent. , .

An example of this will be described below with reference to FIGS. 2 to 4.

In FIG. 2, elements having the same functions as those in FIG. 1 are designated by the same numbers. In FIG. 2, the thyristor 31 is deactivated and a transistor 40 is provided in its place. The collector of this transistor 4o is connected to the capacitor 20 and the terminal of the control circuit 7.
The emitters are connected to the ground, and the base is connected to the connection point between the emitter of the transistor 1o and the resistor 36. Here, 41 is the first
Similar to the capacitor 39 in the figure, the capacitor 1.42.4s for removing noise is a bias resistor.

In FIG. 2, the other configurations are the same as in FIG. 1,
The basic operation is also exactly the same.

Now, when an overcurrent flows through the transistor 1o and exceeds a certain value, the transistor 40 becomes conductive and reduces the charging voltage of the capacitor 20. When the voltage across the capacitor 20 decreases, the same operation as when the power is turned on occurs, and as the conduction period of the transistor 10 becomes narrower, the output voltage of the transformer 12 decreases regardless of fluctuations in the output voltage.

0 FIG. 3 shows the relationship between the output current of the load circuit and the output voltage, and FIG. 4 shows the relationship between the output current and the collector current of the transistor 10 in FIG. 2. In FIG. 3, the solid line A is the characteristic shown in FIG. 2, and the broken line B is the characteristic shown in FIG. 1 in the conventional example.

In the circuit shown in Figure 1, the output voltage becomes zero when the output current exceeds the steady-state current by a certain value, but in the circuit shown in Figure 2, the output voltage gradually decreases until the load circuit is completely closed. Even if a short circuit occurs, the load current is limited to a nearly constant value, and the fourth
As shown in the figure, the collector current of the transistor 1o is also limited to a constant value. In Figure 2, even if the transistor 40 becomes conductive momentarily due to noise, the output voltage only momentarily drops; when the noise disappears, it automatically returns to the original output voltage, and the output voltage is fixed at zero. It will not be done. Therefore, there is no need to increase the capacitance of the capacitor 41, and the transistor 10 can be reliably protected from overcurrent.

As is clear from the above embodiments, according to the present invention, when an overcurrent flows through the output transistor 10, this is detected, and the voltage across the capacitor is applied to protect the transistor 1G from the overcurrent when the power switch is turned on. , thereby reducing the output voltage of the transistor 1.
By limiting the collector current to zero, the transistor 710 can be protected from overcurrent. and,
According to the present invention, even if the transistor 4o becomes conductive momentarily due to noise, the output voltage only momentarily drops, and automatically returns to the original voltage when the noise disappears. Therefore, the capacitance is used as a noise countermeasure capacitor. There is no need to make it big.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of a power supply device in a conventional example, FIG. 2 is a circuit diagram of a power supply device in an embodiment of the present invention, and FIGS. 3 and 4 are characteristic diagrams for explaining the operation of the device of the present invention. be. 1... AC power supply, 2, 3, 4, 5...
Diode, 7... Control circuit, 71...
・Error amplification circuit, 72... Pulse width control circuit, 73... Lu... Oscillation circuit, 8, 10... Misery...-
Switching transistor, 9, 12...transformer, 11, 22...e diode, 13, 23
... Capacitor, 2o... Capacitor, 36... Resistor for overcurrent detection, 4o...
・Transistor.

Claims (1)

[Claims] A switching transistor, a means for converting and extracting the switching output of the switching transistor, further rectifying and smoothing it and outputting it as a DC voltage, and an error amplifier circuit that detects a voltage proportional to the DC output voltage and compares it with a reference voltage. , means for controlling the switching transistor using the output voltage of the error amplifier circuit to stabilize the DC output voltage; a capacitor for starting the conduction period of the switching transistor from a narrow state when a power switch is turned on; The device is characterized by comprising means for detecting a flowing current, and means for lowering the DC output voltage by lowering the voltage across the capacitor using the output of the detecting means, thereby limiting the collector current of the switching transistor. power supply.