JPH0546789B2 - - 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 the blocking oscillation type disclosed in Japanese Patent Application Laid-Open No. 145775/1982. We previously proposed a power supply circuit. That is, in the power supply circuit of 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 to obtain an unstable output. DC voltage of input winding N of converter transformer T
1. Applied to the series circuit of the switching transistor Q1 and the current detection resistor R3. The switching transistor Q1 constitutes a blocking oscillation circuit with the feedback winding N2 of the transformer T and the previous input winding N1, and therefore the feedback winding (N
2) is connected in series between the base and emitter of the switching transistor Q1 and the current detection resistor R3 via a capacitor C5 and a resistor R5.

Further, for the DC voltage taken out by the diode D6 and the smoothing capacitor C6 from the detection winding N3 tightly coupled to the output winding N4 of the transformer T, the transistor Q3, the Zener diode D7, and the resistors R7 to R12 The output voltage obtained at point A is combined with each voltage described later and applied between the base and emitter of the control transistor Q2. 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 is generated through the capacitor C3 and the resistor R6. It is combined with the voltage led to point B and applied. In addition,
The charging of the capacitor C4 is performed by d of the feedback winding N2.
This is done by a current Ic flowing from the end side through the illustrated path between the base and collector of the control transistor Q2.

Note that the diode D11 between the base and emitter of the control transistor Q1 is for fixing the voltage across the capacitor C4, which is charged as described above, to about 0.7V.

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 Q
During the ON period of 1, the collector current Ii increases with time, and the voltage across the resistor R3 also increases accordingly. 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 on-period, the positive feedback current (base current) If from the feedback winding N2 is bypassed by the control transistor Q2, which turns off the switching transistor Q1. After that, switching transistor Q1
After a certain period of time has elapsed since the turn-off, it is turned on again by normal blocking oscillation operation, and the previous operation is repeated thereafter. and,
In such an operation, the timing of the above-mentioned turn-off operation is changed by changing the voltage at point A in response to fluctuations in the DC output voltage taken out from the output winding N4 by diode D8 and capacitor C7. This means that the output voltage is stabilized.

(c) Problems to be solved by the invention In the power supply circuit shown in Fig. 2, when the power supply is turned on and off using a small, low-rated switch element with a small current capacity, the switch element It is conceivable to connect a switch between points C and B, and when the power is turned off, close the switch element to forcibly turn on the control transistor Q2 and stop the operation of the switching transistor Q1.

However, the voltage at point C as seen from the reference line L is positive centered around the DC=0 level in Figure 3 f.
Since the voltage is a rectangular wave voltage with negative polarity, even if the switch element is directly connected between points C and B, it cannot operate properly. This is because even if the switch element is closed during a period in which the rectangular wave voltage is negative, that is, a period when the switching transistor Q1 is off, no base current can be supplied to the base of the control transistor Q2. Therefore, in this case, after the switch element is closed and the switching transistor Q1 is once turned on by the blocking oscillation operation, the base current is supplied to the control transistor Q2 through the switch element, and at that point This is because the control transistor Q2 turns on for the first time and the switching transistor Q1 stops, so the operation during this period becomes unstable.

Therefore, it is an object of the present invention to provide a switching control type power supply circuit that can reliably stop the switching transistor and thereby accurately turn off the power supply even if a small and low-rated switching element is used. purpose.

(d) Means for Solving Problems In the present invention, in the above-mentioned blocking oscillation type switching control power supply circuit, a rectifying and smoothing circuit is installed at the connection point between the base of the switching transistor and the feedback winding side of the converter transformer. Connect a diode and a capacitor for power supply, and connect a switch element for turning the power on and off between the connection midpoint of the capacitor and diode and the base of the control transistor connected between the base and emitter of the switching transistor. did.

(E) Effect According to the above configuration, a positive DC voltage obtained by rectifying and smoothing the rectangular wave voltage of the feedback winding is obtained at the midpoint of the connection between the diode and the capacitor, and when the switch element conducts, the voltage increases. Since the base current is supplied to the control transistor using the power supply and the control transistor is completely turned on, the switching transistor is reliably kept in the off state.

(F) Embodiment Figure 1 shows an embodiment of the switching control type power supply circuit of the present invention, in which the parts with the same symbols as in Figure 2 have the same configuration, but the following configuration is added. has been done. That is, the first point is between the base of the switching transistor Q1 and the feedback winding N.
resistor R5 and capacitor C3 connected between
A rectifier diode D9 and a smoothing capacitor C8 are connected between the connection midpoint C and the reference line L,
A photocoupler is connected between the connection midpoint D between the capacitor and the diode and the base of the control transistor Q2 as a switch element for turning the power on and off.
This is the point where the collector-emitter of the light receiving transistor Q4 in the PC is connected to the current limiting resistor R13. The second point is that the starting resistor is divided into two resistors R4' and R4'', and the midpoint E of the connection is connected to point D.

Here, the light emitting diode D10 in the photocoupler PC is turned on and emits light when a control signal for turning off the power is applied from a remote control signal processing circuit (not shown).

Therefore, when the control signal is not applied and the light emitting diode D10 is off, the light receiving transistor Q4 is also off, so in this case the power supply circuit is in the on (operating) state, as explained in FIG. works. At this time, a positive DC voltage is obtained at point D by rectifying and smoothing the rectangular wave voltage at point C (see FIG. 3 f) using diode D9 and capacitor C8. Therefore, when the light receiving transistor Q4 is turned on in response to the control signal, a base current flows to the control transistor Q2 via this transistor Q4. At this time, since this current flows directly from the charge stored in the capacitor (C8), that is, from the low impedance DC power source, even when the AC input voltage is low and therefore the DC voltage is relatively low,
It is large enough to turn on control transistor Q2. Therefore, the control transistor Q2 is reliably turned on by the conduction of the light-receiving transistor Q4, which short-circuits the base and emitter of the switching transistor Q1, so that the switching transistor Q1 is turned off.

When the switching transistor Q1 is turned off, the potential at point C becomes negative, but the current from the capacitor C9 flows through the light receiving transistor Q4 immediately after that. During the off-period of the switching transistor Q1, the current Is flowing from the input side to the base side of the switching transistor Q1 via the starting resistor R4' flows through the light receiving transistor Q4 to the base of the control transistor Q2. , this control transistor continues to be on, thus ensuring that the switching transistor Q1 remains off thereafter.

Here, the connection middle point E of starting resistors R4' and R4''
The reason why is connected to point D is that after the switching transistor Q1 is once turned on by the conduction of the control transistor Q2, the potential is lowered to the point C and the switching transistor Q1 is connected to the point after that. This is to ensure the off state. That is, in the state after the switching transistor Q1 is turned off, the potential Ed at point D is
The voltage between the base and emitter of the control transistor Q2 is V _BE , the voltage between both ends of the resistor R13 is V _R 13, and the voltage between the collector and emitter of the light receiving transistor Q4 is
If Vc _E 1, then Ed = V _BE + V _R 13 + Vc _E 1. On the other hand, the potential Ef at point F is Ef = Vc _{E 2, assuming the collector-emitter voltage of control transistor Q2 is Vc E 2} . The potential Ec at the point is Ec=V _BE +V _R 13+Vc _E 1-Vc _E 2/R4″+R5・R5 ≒V _BE +V _R 13/R4″+R5・R5, and is determined by the partial voltage of resistors R4″ and R5. However, even if the connection between points D and E is cut, the values of the starting resistors R4' and R4'' (in this case, one resistor is sufficient) should be appropriately selected, and By eliminating resistor R13, switching transistor Q1 can remain off after being turned off. However, in this case, the potential Ec at point C is Ec=V _BE +Vc _E 1+V _D (V _D is the rising voltage of D9)
Therefore, the cost will be higher than in the above case.

Note that during the period when the switching transistor Q1 is held in the off state, the photo coupler PC
Needless to say, the aforementioned control signal continues to be applied to the light emitting diode D10 inside.

(G) Effects of the Invention As described above, according to the switching control type power supply circuit of the present invention, the operation of the switching transistor is completely stopped by the power on/off switch element, and the power is turned off reliably. can be maintained in the same state. Moreover, since a small and low-rated switch element can be used as the switch element, it is suitable for remote control of turning on and off the power at a low cost.

[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, Q1...Switching transistor, Q2...
...Control transistor, D9... Rectifier diode, C8... Smoothing capacitor, PC... Photo coupler as a switch element.

Claims (1)

[Claims] 1. The input winding of a converter transformer and the collector-emitter of a switching transistor are connected in series with respect to a DC input, and the feedback winding of the transformer is connected between the base-emitter of this transistor. In addition to constructing a blocking oscillation circuit,
By connecting a control transistor between the base and emitter to bypass the positive feedback current from the feedback winding, and controlling this control transistor according to fluctuations in the DC voltage obtained from the transformer,
In the power supply circuit in which the turn-off timing of the switching transistor is variable,
A rectifying and smoothing diode and a capacitor are connected to the connection point between the base of the switching transistor and the feedback winding side, and the power is turned on and off between the connection point between the capacitor and the diode and the base of the control transistor. A switching control type power supply circuit that connects switching elements for 2. Claim 1, wherein the connection point between the base of the switching transistor and the feedback winding side is connected to the other end of a starting resistor whose one end is connected to the DC input side. Switching control type power supply circuit described in . 3. Claim 2, characterized in that the starting resistor is composed of a plurality of resistors constituting a voltage dividing circuit, and the midpoint of one of the resistors is connected to the midpoint of the connection between the diode and the capacitor. Switching control type power supply circuit described in .