JPH047668Y2 - - Google Patents

Description

[Detailed Description of the Invention] (a) Field of Industrial Application The present invention relates to a switching control type power supply circuit used as a power supply for television receivers and the like.

(b) Prior Art One type of switching control type power supply circuit is a blocking oscillation type, which has a configuration as shown in FIG. 2, for example. That is, in the power supply circuit shown in Fig. 2, the AC power input is input to the bridge rectifier circuit BD via the rush current limiting resistor _R1 , and the rectified output is smoothed by the capacitor _C1 . Unstable DC voltage is applied to the input winding N ₁ of converter transformer T, switching transistor
Applied to the series circuit of Q ₁ and current detection resistor R ₃ . The switching transistor _Q1 constitutes a blocking oscillation circuit with the feedback winding _N2 of the transistor T and the previous input winding _N1 , so that the feedback winding _N2 is connected to the capacitor _C5 and the diode _D5 . It is connected in series between the base and emitter of the switching transistor _Q1 and the current detection resistor _R3 via a parallel circuit and a resistor _R5 .

With respect to the DC voltage taken out by the diode D ₆ and the smoothing capacitor C ₆ from the detection winding N ₃ tightly coupled to the output winding N ₄ of the transformer T,
An error detection section consisting of a transistor Q ₃ , a Zener diode D ₇ , and resistors R ₇ to R ₁₂ is provided, and the output voltage obtained at point A is combined with each voltage described later to be applied to the base of the control transistor Q ₂ . The voltage is applied between the emitters. That is, the voltage at the point A is generated in the resistor _R3 by the voltage across the capacitor _C4 charged during the off period of the switching transistor _Q1 , and _the collector current Ii of the switching transistor _Q1 . and the voltage led to point B via resistor _R6 are combined and applied. In addition, the above capacitor C ₄
Charging is performed by a current Ic flowing from the d end of the feedback winding N ₂ through the illustrated path between the base and collector of the control transistor Q ₂ .

The voltage and current waveforms of each part of this power supply circuit are as shown in FIG. 3. In particular, switching transistor _Q1 is turned on and off in the steady state as follows. That is, now the switching transistor
During the on-period of Q ₁ , its collector current Ii increases over time, and thereby the voltage across resistor R ₃ also increases. Therefore, the above-mentioned combined voltage applied between the base and emitter of control transistor _Q2 during this on-period is in a negative state slightly lower than OV at the beginning of the on-period, assuming that line L is the reference potential. From this state, it changes in a positive direction. When the base-emitter rising voltage Vbe of the control transistor _Q2 is exceeded, the control transistor _Q2 is turned on.
Then, during the above ON period, the positive feedback current (base current) If from the feedback winding _N2 flows through the control transistor.
It will be bypassed by _Q2 , which will turn off the switching transistor _Q1 . Then, after a certain period of time has passed since the switching transistor _Q1 was turned off, it is turned on again by normal blocking oscillation operation.
From then on, the previous operations will be repeated.

(c) Problems that the invention aims to solve Now, in the power supply circuit shown in Figure 2, when the AC input voltage changes, the positive feedback current If supplied to the base of the switching transistor _Q1 also changes.
The following problems arise. That is, when the AC input voltage changes, the voltage applied to the input winding _N1 during the on period of the switching transistor _Q1 also changes, so the feedback of the on period determined by the turns ratio with the input winding _N1 changes. The voltage across winding N ₂ also changes.
When the voltage of this feedback winding changes, the impedance of the diode _D5 , resistor _R5 , etc. in the feedback current path is approximately constant, so the positive feedback current If flowing through this path will change as the voltage of the winding _N2 increases. will change in proportion to. In particular, when the input voltage is particularly high, the positive feedback current If increases significantly, and as a result, a large amount of excess charge accumulates at the base of _the switching transistor _Q1 . Time becomes longer. That is, in this case, the on/off duty ratio of the switching transistor _Q1 is not controlled accurately, and the diode is disconnected from the output winding _N4.
The DC output voltage taken by D ₃ and capacitor C ₇ will increase as the input voltage increases. For this reason, there was a problem in that the constant voltage control range for the AC input voltage could not be set widely.

Therefore, as one method to solve this problem, for example, in the power supply circuit of Japanese Patent Laid-Open No. 59-2570, the positive feedback current supplied from the feedback winding to the base of the switching transistor is transferred from the transistor, Zener diode, etc. The current is made constant by a circuit. However, in this method, the feedback current, that is, the base current of the switching transistor, flows directly through the constant current transistor, etc., so the power consumption in the transistor, etc. is large, and the number of parts increases, resulting in a considerable increase in cost. There were some drawbacks.

Therefore, the present invention is capable of setting a sufficiently wide constant voltage control range for input voltage in a blocking oscillation type switching control power supply circuit.
Moreover, it is an object of the present invention to realize this with a very simple configuration and without extra power consumption.

(d) Means for solving the problem In the present invention, means is provided for detecting a state in which the voltage generated in the feedback winding of the converter transformer exceeds a predetermined value, and the output voltage of this means is a DC output obtained from the transformer. The voltage is superimposed on the detected output voltage and applied to the control electrode of a control element for positive feedback current bypass connected between the base and emitter of the switching transistor.

(E) Effect According to the above configuration, when the input voltage increases and the voltage of the feedback winding increases, the operation of the control element is accelerated by the amount of the superimposed output voltage of the detection means, This eliminates the delay in turn-off timing of the switching transistor.

(F) Example Figure 1 shows an example of the present invention, and Figure 2 shows an example of the present invention.
Parts with the same symbols as in the diagram have the same configuration,
In addition, the following configurations have been added: First, the first point is the diode D ₅ in the positive feedback current path.
and between the connection midpoint C of the parallel circuit of the capacitor C ₅ and the resistor R ₅ and the base B of the control transistor Q ₂ ,
This is the point where the Zener diode _D9 , the diode _D10 , and the resistor _R13 are connected as shown. The second point is that a diode _D11 is connected in parallel to the capacitor _C4 between the base and emitter of the control transistor Q.
This is the point where they are connected as shown.

Now, in such an embodiment, the AC input voltage is relatively low, so the switching transistor
At low voltages on the feedback winding N ₂ during the on-period of Q ₁ , the Zener diode D ₉ and the diode D ₁₀
is turned off. Therefore, in this case, the above-described turn-off operation of the switching transistor _Q1 and the voltage at point A of the error detection section are changed in accordance with the fluctuation of the output voltage, thereby varying the timing of the turn-off operation. Operations such as constant voltage control are exactly the same as in the conventional example shown in FIG.

Here, the diode _D11 between the base and emitter of the control transistor _Q1 is used to fix the voltage across the capacitor _C4 , which is charged in the aforementioned current path, to about 0.7V. That is, without the diode D ₁₁ , the switching transistor Q ₁
When the voltage generated in the feedback winding N ₂ increases during the off-period of , the voltage of the capacitor C ₄ mentioned above also increases,
As a result, the base and emitter of the control transistor Q ₁ will be reverse biased in a more negative direction than necessary, degrading the constant voltage control sensitivity. To improve this, the diode D ₁₁ is provided. .

On the other hand, as the AC input voltage increases, the voltage during the ON period of the feedback winding _N2 increases, and the voltage at point A exceeds the sum of the Zener voltage of Zener diode _D9 and the rising voltage of diode _D10 . , becomes higher than the voltage at point B, both diodes D ₉ , D ₁₀
are both turned on. As a result, the voltage at point B becomes higher than when the diodes D ₉ and D ₁₀ and the resistor R ₁₃ are not provided. Therefore, even if the hall time of the control transistor Q ₁ becomes longer as described above due to the increase in the voltage during the ON period of the feedback winding N ₂ in this case, the values of the Zener diode D ₉ and the resistor R ₁₃ can be appropriately selected. If this is done, the turn-off timing of the transistor _Q1 can be advanced by that much.

Therefore, in this embodiment, the turn-off timing of the switching transistor _Q1 can be faithfully and variably controlled in accordance with the voltage change at the point A, thereby achieving accurate constant voltage control.

The reason why the diode D ₁₀ is provided in series with the Zener diode D ₉ is because if the diode D ₁₀ were not present, the charging of the capacitor C ₄ during the off-period of the switching transistor Q ₁ would be different from the path shown in FIG. 2. This is because the voltage at point _B changes due to _the current in this path, which adversely affects the constant voltage control operation.

In addition to this embodiment, the present invention can also be applied to the following blocking oscillation type power supply.
That is, it configures a control transistor to remain on (active) continuously during the on period of the switching transistor, and allows the control transistor to divide the amount of positive feedback current to the switching transistor in response to fluctuations in the DC output voltage. This is a power supply circuit that controls the turn-off timing of the switching transistor.

(g) Effects of the invention According to the power supply circuit of the invention, even if the positive feedback current to the switching transistor fluctuates due to changes in the input voltage, the turn-off timing of the switching transistor can be accurately controlled. The constant voltage control range for input voltage can be set sufficiently wide. Moreover, since the circuit section that performs such control is not configured to directly control the positive feedback current, power consumption in that circuit section does not become a problem.
Moreover, it can be realized at low cost with a very simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the power supply circuit of the present invention, FIG. 2 is a circuit diagram showing a conventional switching control type power supply circuit, and FIG. 3 is a voltage and current waveform diagram of each part thereof. T...Converter transformer, _N2 ...Feedback winding,
Q ₁ ... Switching transistor, Q ₂ ... Control transistor (control element), D ₉ ... Zener diode as detection means.

Claims (1)

[Claims for Utility Model Registration] In response to DC input, the input winding _N1 of the converter transformer T and the collector-emitter of the switching transistor _Q1 are connected in series, and the base-emitter of the switching transistor _Q1 is connected in series. A feedback winding _N2 of the converter transformer is connected to the feedback winding _N2 to form a blocking oscillation circuit, and a control element _Q2 is connected between the base and emitter to bypass the positive feedback current from the feedback winding N2, In the power supply circuit, the turn-off timing of the switching transistor Q ₁ is varied by controlling the control element Q ₂ according to the detected output voltage of the DC voltage obtained from the converter transformer T. One end C of the wire N ₂ and the control element Q ₂
A constant voltage diode _D9 and a current limiting resistor are connected between the control electrode of the switching transistor _Q1 and conductive when the positive feedback voltage generated in the feedback winding _N2 is equal to or higher than a predetermined value during the ON period of the switching transistor Q1. a series circuit including _R13 , connected between the other end d of the feedback winding _N2 and the control electrode, and connected to the switching transistor
A switching control type power supply circuit comprising: a capacitor _C4 that is charged during the off period of _Q1 ; and a diode _D11 connected in parallel to the _capacitor C4.