JPH0732598B2 - Switching power supply - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply device that controls the on-time of a switching transistor so that the output voltage becomes constant even if the input voltage or load changes.

[Conventional technology]

A conventional device of this type is shown in FIG. 2, in which 1 is a DC power supply and S ₁ is a power switch. Transformer in series with power switch S ₁
The primary winding T _{1 of} 11 and the collector-emitter path of the switching transistor 5 are connected, and the emitter of the switching transistor 5 is connected to the current detection resistor 6. An output circuit composed of a rectifying and smoothing circuit having a rectifying diode 12 and a smoothing capacitor 13 is connected to the secondary winding T ₂ of the transformer 11, and a load 14 is connected to the output terminals TA and TB of this output circuit. To be done. The auxiliary winding T ₃ of the transformer 11 is a control circuit for controlling on / off of the switching transistor 5.
The output of the control circuit 10 is connected to the base of the switching transistor 5. The control circuit 10 is connected so that the output voltage of the rectifying / smoothing circuit including the diode 12 and the capacitor 13 is fed back. Trance
The primary winding T _{1 of} 11 has a diode 4, a resistor 2, and a capacitor 3
A series circuit with a snubber circuit consisting of is connected in parallel. The base of the bypass transistor 8 is connected to the connection point between the emitter of the switching transistor 5 and the current detection resistor 6 via the resistor 7, and the collector of the bypass transistor 8 is connected to the base of the switching transistor 5. . The resistor 9 connected between the connection point between the power switch S ₁ and the primary winding 11 of the transformer 11 and the base of the switching transistor 5 is a starting resistor.

When the power switch S ₁ is turned on in FIG. 2, first, the base current flows through the switching resistor 5 through the starting resistor 9, and the collector current of the switching transistor 5 begins to flow. Primary winding T ₁ and auxiliary winding T _{3 of} transformer 11
Is wound in the polarity shown in the figure, a voltage is generated in the primary winding T ₁ and a voltage is also generated in the auxiliary winding T ₃ . Auxiliary winding T ₃
The voltage generated at is supplied to the control circuit 10 and fed back to the base current of the switching transistor 5 via the control circuit 10. As a result, the switching transistor 5 is turned on by the positive feedback action that the base current of the switching transistor 5 increases and the collector current increases, and the collector current linearly increases due to the inductance of the transformer 11. At this time, the secondary winding of the transformer 11
A voltage is also induced in T ₂ , but no current flows because the diode 12 is in the opposite rectification direction. Since the collector current becomes constant after the switching transistor 5 is turned on, the voltage induced in the auxiliary winding T ₃ is reduced, the base current is reduced, and the switching transistor 5 is rapidly turned off. When the switching transistor 5 turns off, the transformer
The energy stored in 11 is discharged to the secondary side and a current flows through the diode 12 to charge the capacitor 13.
After the switching transistor 5 is turned off, the base current is supplied again through the starting resistor 9 and the above-described operation is repeated, and the constant voltage Vo is output from the secondary side output terminals TA and TB of the transformer 11. The output voltage Vo on the secondary side is fed back to the control circuit 10, and when the output voltage Vo decreases, the ON time of the switching transistor 5 is lengthened, and when the output voltage Vo increases, the ON time of the switching transistor 5 is shortened. To do.

When the switching transistor 5 is turned off, a counter electromotive force is generated in the primary winding T ₁ of the transformer 11, but this energy is stored in the capacitor 3 through the diode 4, and the energy stored in the capacitor 3 is stored in the resistor 2 by the resistor 2. It is supposed to consume. Also, it is prevented that a current that exceeds the safe operation area flows through the switching transistor 5 to damage the switching transistor 5 and an excessive current flows through the primary winding T ₁ of the transformer 11 to saturate the transformer 11. Therefore, the current flowing through the switching transistor 5 is monitored by the current limiting resistor 6. When an excessive current flows through the switching transistor 5 and the voltage drop of the current detection resistor 6 increases, a base current flows through the resistor 7 into the transistor 8 to turn on the transistor 8. When the transistor 8 is turned on, the base current of the switching transistor 5 is bypassed by the transistor 8, so that the switching transistor 5 is turned off.

[Problems to be Solved by the Invention]

By the way, when the output of the secondary side of the transformer 11 is not yet activated, a transient that causes the output of the secondary side to suddenly rise, such as when the power switch S ₁ is turned on or when the output is short-circuited due to a load failure. In the state, the control circuit 10 transfers excessive energy to the transformer 11 so as to increase the output voltage.
To enter. The energy of this transformer 11 returns to the collector of the switching transistor 5 as a back electromotive force. That is, the energy from the transformer 11 is stored in the capacitor 3 of the snubber circuit, but the amount of the energy stored in the capacitor 3 becomes an amount more than that consumed by the resistor 2 during the off period of the switching transistor 5 and the capacitor 3 It will remain 3. As a result, the terminal voltage of the capacitor 3 rises every time the switching transistor 5 is repeatedly turned on and off, and eventually exceeds the rated value of the collector voltage of the switching transistor 5, which has the drawback of damaging the switching transistor.

It is possible to connect a Zener diode to the collector of the switching transistor in order to suppress the overvoltage on the collector of the switching transistor, but in this case, when the switching transistor 5 turns on the charge of the parallel equivalent capacitance of the Zener diode. However, there is a drawback that the loss increases due to the discharge.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a switching power supply device that eliminates the above-mentioned drawbacks of the conventional device and prevents the switching transistor from being damaged due to the collector voltage of the switching transistor rising above a rated value.

[Means for Solving the Problems]

According to the present invention, a switching transistor is connected in series to a primary winding of a transformer, an output circuit including a rectifying / smoothing circuit is connected to a secondary winding of the transformer, and the primary winding of the transformer is connected in parallel. Is connected to a diode and a snubber circuit including a parallel connection circuit of a resistor and a capacitor connected in series to the diode, and a collector of a bypass transistor driven by a current detection resistor connected to the emitter of the switching transistor is connected to In a switching power supply device connected to the connection point between the base of the switching transistor and the output of a control circuit that controls on / off of the switching transistor, between the connection point of the diode and the snubber circuit and the base of the bypass transistor. The overvoltage charged in the capacitor of the snubber circuit. It is accomplished by connecting a surge suppression circuit with over voltage suppression element.

[Action]

The power supply voltage Vin and capacitor stationary time snubber circuit output voltage Vo is stable while being charged voltage Vco, high voltage Vc ₁ than the steady state in the capacitor of the snubber circuit at the time of power-on and outputs a short circuit Is charged with. Set the operating voltage Vs of the overvoltage suppression element of the surge suppression circuit to (Vin + Vco) <
By selecting Vs ≦ (Vin + Vc ₁ ), when (Vin + Vc ₁ ) exceeds Vs, the overvoltage suppressing element becomes conductive and the base potential of the bypass transistor becomes high. As a result, the bypass transistor is turned on with a small voltage drop due to the current detection resistor, in other words, the current limiter level is lowered, which shortens the on time of the switching transistor and prevents excessive energy from being injected into the transformer. To prevent and suppress the voltage applied to the collector of the switching transistor.

〔Example〕

Next, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows an embodiment of the present invention, which is shown in FIG.
The same parts as those shown in the drawings are designated by the same reference numerals and the description thereof will be omitted.

1 is different from FIG. 2 in that a surge suppression circuit 15 is connected between a connection point of a diode 4 and a snubber circuit composed of a resistor 2 and a capacitor 3 and a base of a bypass transistor 8. . The surge control circuit 15 is composed of a series circuit of a Zener diode 15a and a resistor 15b.When the Zener diode 15a is a Zener voltage V _ZD and the voltage drop due to the resistor 15b is V _R , the operating voltage V _S of the surge suppressor 15 is V _S =
It becomes V _ZD + V _R. The operating voltage V _S of the surge suppression circuit 15 is selected to be larger than (Vin + Vco), where Vco is the voltage charged in the capacitor 3 of the snubber circuit during steady state when the power supply voltage Vin and the output voltage V _O are stable. .

Now, when the power switch S ₁ is turned on, the output voltage V _O has not risen yet. Therefore, the control circuit 10 lengthens the ON time of the switching transistor 5 so that the output voltage V _O rises quickly to a predetermined voltage, and as a result, excessive energy is input to the transformer 11. At this time, transformer 11
Although the energy consumed on the secondary side of the switch is small, the input energy is large, so excess energy becomes counter electromotive force at the moment when the switching transistor 5 is turned off and bounces back to the collector of the switching transistor 5. Come on. This energy is charged in the capacitor 3 through the diode 4 and consumed by the resistor 2.
Since more energy is stored in the capacitor 3 than is consumed by the resistor 2 when the power is turned on, the terminal voltage of the capacitor 3 gradually increases as the switching transistor 5 is repeatedly turned on and off. Then, when the terminal voltage of the capacitor 3 rises to Vc ₁ which exceeds the steady state voltage Vco and the sum of the power supply voltage Vin and the terminal voltage Vc ₁ of the capacitor 3 exceeds the Zener voltage V _ZD of the Zener diode 15a, the Zener diode 15a becomes conductive, that is, the current I _S flows through the resistor 15b and the base-emitter path of the transistor 8 and the Zener diode 15a becomes conductive. As a result, the voltage of the cathode of the diode 4 is suppressed to V _ZD + V _R + V _BE , and the voltage applied to the cathode of the switching transistor 5 is also suppressed to the rated value or less. Where V _R is resistor 15b
If the resistance value of R is R _S , then V _R = I _S · R _S , and V _BE is the base-emitter voltage of the transistor 8.

After the Zener diode 15a of the surge suppression circuit 15 becomes conductive, the control circuit 10 then supplies the base current to the switching transistor 5 and the collector current starts to flow. At this time, since the base potential of the transistor 8 is increased by the surge control circuit 15, the transistor 8 is turned on by a collector current smaller than that in the steady state. When the transistor 8 is turned on, the base current of the switching transistor 5 is bypassed and the switching transistor 5 is turned off. Therefore, the on time of the switching transistor 5 is shortened and excessive energy is not input to the transformer 11. In this way, the voltage applied to the collector of the switching transistor 5 is suppressed below the rated value of the collector voltage, and the collector current value of the switching transistor 5 is also suppressed within the rated value, so that it operates in the safe operation area. Further, since the transformer 11 does not flow an excessive current, the saturation phenomenon does not occur.

When the output is short-circuited or the load fluctuates, the output voltage changes abruptly, which causes excessive energy to be input to the transformer 11 as when the power is turned on. Thus, the switching transistor 5 is protected.

In addition, although the example using the Zener diode as the overvoltage suppressing element has been described in the embodiment, this may be a surge absorbing element such as a surge absorber, the switching element may be a MOSFET, and a flyback type switching power supply device. However, the same effect can be obtained with the forward method.

〔The invention's effect〕

As described above, according to the present invention, the connection point between the snubber circuit that absorbs the counter electromotive force generated in the primary winding of the transformer and the diode, and the base of the bypass transistor that bypasses the base current of the switching transistor. By connecting the surge suppression circuit between the and, even when there is excessive energy input to the transformer, the voltage applied to the collector of the switching transistor is suppressed below the rated voltage to prevent damage to the switching transistor. can do. This has the effect that the capacitance of the switching transistor can be reduced and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a switching power supply device showing an embodiment of the present invention, and FIG. 2 is a circuit diagram of a switching power supply device showing a conventional example. 2: Resistor, 3,13: Capacitor, 4,12: Diode, 5: Switching transistor, 6: Current detection resistor, 8: Bypass transistor, 10: Control circuit, 11: Transformer, 15: Surge suppression circuit, 15a: Zener diode.

Claims (1)

[Claims] 1. A switching transistor is connected in series to a primary winding of a transformer, an output circuit consisting of a rectifying and smoothing circuit is connected to a secondary winding of the transformer, and a diode and a diode are connected in parallel to the primary winding of the transformer. A snubber circuit consisting of a parallel connection circuit of a resistor and a capacitor connected in series to this diode is connected, and the collector of the bypass transistor driven by the current detection resistor connected to the emitter of the switching transistor is turned on. -In a switching power supply device connected to a connection point between a control circuit for controlling off and a base of a switching transistor, between the connection point of the diode and the snubber circuit and the base of the bypass transistor, the snubber circuit Overvoltage suppressing element that conducts due to overvoltage charged in a capacitor Switching power supply apparatus characterized by connecting the surge suppression circuit having.