JPH0444509B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching control type power supply circuit, particularly to a turn-off timing control type power supply circuit.

Blocking oscillation type power supplies have been realized as switching control type power supplies because they do not require an external drive circuit or a drive transformer. This blocking oscillation type is further classified into various types depending on the control method.
There is a circuit as shown in Fig. 1, which was proposed in the publication (see Japanese Patent Publication No. 1).

First, the power supply circuit shown in FIG. 1 will be briefly explained, and the problems to be solved by the present invention will be presented.

The power supply circuit in Figure 1 can be roughly divided into input rectifier section 1
It is composed of a blocking oscillation section 2, a converter transformer 3, an error detection section 4, a control circuit section 5, and an output rectification section 6, and basically performs the following operations. That is, when the power switch SW is turned on, the starting power Is is supplied from the input rectifier 1 to the base of the switching transistor TR4 to start the blocking oscillator 2, and in the steady state after starting, the control circuit 5 performs the above-mentioned operation. The turn-off timing of the switching transistor TR4 is controlled according to the output of the error detection section 4. Among them, the operation particularly when turning off the switching transistor TR4 is as follows.

That is, when the switching transistor TR4 is turned on, the current Ii flowing between its collector and emitter generates a negative voltage L1, which increases over time, at point A on the upper end side of the current detection resistor R11 as a reference line. This voltage is led to point B via resistor R16 and capacitor C7, and on the other hand, C
The error detection transistor TR1 detects the voltage generated at the point.
The voltage obtained by comparing and amplifying the voltage of the Zener diode D5 is led to point B, and by combining both the positive and negative voltages, a positive voltage that decreases over time is obtained at point B. When this voltage becomes lower than the positive voltage obtained at point D by the turn-off capacitor C5, which has been charged in advance to the polarity shown in the figure, the control transistors TR2 and TR3 are sequentially turned on, thereby causing the above-mentioned A reverse bias current Id flows from the capacitor C5 between the base and emitter of the switching transistor TR4, turning off the transistor TR4. At this time, since the voltage at point C changes depending on the input power supply voltage and the load condition of the output rectifier 6, the positive bias level at point B changes, which causes the switching transistor to change. The turn-off timing of TR4 changes to perform constant voltage control.

In addition, in the above operation, the voltage at point C is the voltage at the detection winding when the switching transistor TR4 is turned off.
It is obtained by rectifying and smoothing the rectangular wave voltage generated at N _C by diode D6 and capacitor C3. Further, the capacitor C5 is charged by the current Ir flowing from the feedback winding N _B through the diode D7 when the switching transistor TR4 is off.

Although the operation of the circuit shown in FIG. 1 at turn-off is generally as described above, this circuit has the following drawbacks.

First, during the off period of the switching transistor TR4, the base and emitter of the transistor are reverse biased by the voltage generated in the feedback winding N _B , and at this time, the turn-off capacitor C5 is biased to a level smaller than that voltage. Low value (polarity shown)
Therefore, point D, that is, the control transistor
The emitter of TR2 is at almost the same potential as the reference line L1. On the other hand, in this state, since no negative voltage is led from point A to point B, point B has a positive potential that is sufficiently higher than the reference line L1.
For this reason, the base-emitter of the control transistor TR2 is heavily reverse biased, and therefore, a transistor with a high base-emitter reverse withstand voltage must be used as the transistor TR2.

Next, a feedback winding N _B and a detection winding N _C are provided separately, and the charging path of the turn-off capacitor C5 (see Ir in the figure) is electrically floating from the reference line L1 mentioned above. Therefore, both the above windings N _B ,
There must be sufficient insulation between N _{and C} , therefore,
The converter transformer 3 becomes large and its manufacturing process becomes complicated.

Furthermore, a current limiting resistor R16 and a DC blocking capacitor C7 are essential for guiding the voltage at point A to point B in order to generate a voltage varying as described above at point B.

Therefore, the present invention proposes a switching control type power supply circuit that eliminates all of these drawbacks, and the details thereof will be explained below.

FIG. 2 shows an embodiment of the power supply circuit of the present invention, and parts corresponding to those in FIG. 1 are given the same figure numbers. In this embodiment, the input rectifier 1 and the output rectifier 6 are the same as those shown in FIG. The structure is as follows.

First, the first feature is that a feedback winding N _B and a detection winding N _C are successively provided in the converter transformer 3 so as to have the polarities shown in the figure, and one end 1 of the feedback winding N _B is connected to a switch via a resistor R14. Transistor TR4
The middle point j of the two windings N _B and N _C is connected to the base of the switching transistor TR4 via a positive feedback current limiting circuit SK.

Next, the second feature is that a turn-off capacitor C5 is connected between the connection midpoint j and the point E on the upper end side of the current detection resistor R11 via a diode D7, and during the connection between the capacitor and the diode. Point F
is connected to one end M side of a pair of control transistors TR2 and TR3, which are configured to have a conductivity type opposite to that of the one in FIG.
The other end G side of this transistor pair is directly connected to the base of the switching transistor TR4.

Furthermore, the third feature is that a diode D6 and a capacitor C3 for taking out the detected voltage are connected between both ends K and i of the windings N _B and N _C , and the DC voltage obtained at this capacitor is connected to the resistors R3, VR, and R4. The first partial pressure is
The point shown in the figure is that it is applied to the base of the error detection transistor TR1 of the opposite conductivity type.

Note that the capacitor C6 and resistor R15 connected to the collector of the switching transistor TR4
is for shaping the waveform of the voltage generated in the input winding N1 when TR4 is off. Furthermore, the resistor R14 inserted on the emitter side of the transistor TR4 is for current feedback to speed up the turn-off of this transistor TR4.

Now, in this embodiment, the operation of the switching transistor TR4 from activation to turn-on is the same as in the case of FIG. 1, so the explanation will be omitted. The explanation will focus on the operation.

First, in a steady state, when the switching transistor TR4 is turned on (the principle of operation will be explained later), a current Ii flows through this transistor.
(FIG. 6D) flows, and this current Ii generates a negative voltage at point E that increases with time. Here, during the off period before the switching transistor TR4, the turn-off capacitor C5 is charged to the polarity shown in the figure by the current Ir flowing from the feedback winding N _B. and the control transistor
The emitter of TR2, that is, point M, has a negative potential (V _M in FIG. 6) corresponding to the sum of the potential at point E and the voltage across the capacitor C5 with respect to line L2, so this potential V _M switching transistor
During the ON period of TR4, it decreases (the negative value increases) as time passes.

On the other hand, the voltage division midpoint N between the resistors R7 and R8 connected between the collector of the error detection transistor TR1 and the line L2 is a negative potential (the sixth V _N ) in Figure 2).

Therefore, when the potential at point M further decreases by the voltage V _BE between the base and emitter of control transistor TR ₂ compared to the potential at point N, TR ₂ turns on and TR3 also turns on. Therefore, turn-off capacitor C5 → detection resistor R11 → resistor R
14 → Between emitter and base of switching transistor TR4 → Control transistor TR3 → Resistor R
10→A reverse bias current flows through the path of the capacitor C5, and the switching transistor TR4 is turned off. After that, this transistor TR4
is held off by the reverse voltage on the feedback winding N _B (FIG. 6E) until it is then turned on again.

Here, during the off period of the switching transistor TR4, both the control transistors TR2 and TR3 are off, so the above-mentioned reverse voltage of the feedback winding N _B is not applied to the point M. The point has a potential lower than the line L2 by the voltage across the capacitor C5 (see FIG. 6). Therefore, in this state, the control transistor TR2
The reverse bias voltage applied between the base and emitter of is V _MN in FIG. 6, which is much lower than that in FIG. 1.

Note that turning on the switching transistor TR4 in a steady state is achieved by reversing the resonant current due to the inductance and distributed capacitance of the input winding N1 in the direction of the current Ii during the off period. Further, when the switching transistor TR4 is turned on, the positive feedback current If flows through the illustrated path. All of these are the same as in the case of FIG.
Further, the voltage and current waveforms at each part are as shown in FIG.

FIG. 3 shows another embodiment of the invention. In the figure, the same symbols are attached to the parts corresponding to those in Figure 2, but one end of the positive feedback current limiting circuit SK is connected to the k end of the winding N _c . The difference from the case shown in FIG. 2 is that one end of the detection voltage extraction capacitor C3 is connected in series with the turn-off capacitor C5. That is, in this embodiment, the windings N _B ,
By using both N and _C for feedback and detection, it is possible to supply sufficient positive feedback current to the switching transistor TR4, and the capacitor C3 for taking out the detection voltage can be of small capacity. However, the operation is basically the same as in the case of FIG.

In the embodiment shown in FIG. 3, during the ON period of the switching transistor TR4, the reference line L is
The potential difference between the connection point between 2 and the diode D6 of the capacitor C3 changes, and the potential at the N point changes accordingly, but at this time, the error detection transistor
TR1 becomes more conductive and its collector current increases.

Therefore, the potential at the N point moves in a direction that quickly turns on the control transistor TR2. However, on average, the output voltage will remain approximately constant.

FIG. 4 also shows another embodiment of the invention. In this embodiment, a single winding NBC is used for feedback and detection, the diode corresponding to D7 in Figure 3 is deleted, and the charging voltage of the turn-off capacitor C5 is divided by the capacitance with the smoothing capacitor C3. The other parts are the same as the circuit shown in FIG.

FIG. 5 shows yet another embodiment, which differs from the circuit shown in FIG. 4 in the following points. That is, the charge of the turn-off capacitor C5 is connected from the k end of the winding NBC for feedback and detection to the resistor R17 and the diode D.
This is done by means of a current flowing through 7. At this time, the resistor R17 adjusts the magnitude of the charging current to the capacitor C5 so that the voltage across the capacitor C5 is lower than the voltage across the capacitor C3.

In the circuits shown in Figures 1 to 5, in order to supply a sufficient reverse bias current between the base and emitter of switching transistor TR4 when it is turned off, a capacitor C5 with a considerably large capacity must be used. Must be. However, in the embodiment shown in FIG. 4, the capacitance is divided by capacitors C3 and C5, so in that case, the capacitance of C3 must also be increased. However, in this case, even though the capacitors C3 and C5 are connected in series, the combined capacitance of both ends up becoming large, which deteriorates the responsiveness of the error detection unit 4 to fluctuations in the detected voltage. . Therefore, the above-mentioned embodiment shown in FIG. 5 solves this drawback.

Note that in the above embodiment, two control transistors are used.
Although Tr ₂ and Tr ₃ are provided, if one of them, Tr _2, has a large current rating, the other Tr ₃ , which acts as a current amplification device, can be omitted.

The switching control type power supply circuit of the present invention as described above has the following features compared to the conventional circuit shown in FIG. Since this can be made as small as possible, an inexpensive transistor with a relatively low reverse breakdown voltage can be used as the control transistor.

(b) Use the same winding for both feedback and detection, or
Alternatively, since the feedback winding and the detection winding are provided continuously, the converter transformer can be made smaller.
Its production becomes easier.

(c) There is no need to separately provide a resistor or capacitor for guiding the voltage generated in the current detection resistor of the switching transistor to the control transistor, and the number of parts can therefore be reduced.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a conventional switching control type power supply circuit, and FIGS. 2, 3, 4, and 5 are circuit diagrams showing different embodiments of this type of power supply circuit according to the present invention. FIG. 6 is a waveform diagram showing voltage and current waveforms at each part. DESCRIPTION OF SYMBOLS 1...Input rectification section, 2...Blocking oscillation section, 3...Converter transformer, 4...Error detection section, 5...Control circuit section, 6...Output rectification section, TR
2, TR3...Control transistor, TR4...Switching transistor, R11...Resistor for current detection, C5...Capacitor for turn-off.

Claims (1)

[Claims] 1. The input winding of the converter transformer and the collector of the switching transistor are connected to the input rectifier.
a blocking oscillator section including a current detection resistor connected in series between the emitter and the current detection resistor in this order, and a feedback winding of the converter transformer connected between the base and emitter of the switching transistor; a turn-off capacitor, which is charged by the voltage generated in the feedback winding during the period, and whose one end is connected to the input rectifier side end of the current detection resistor; and the other turn-off capacitors. The collector and emitter of the switching transistor are connected between the end side and the base of the switching transistor, and a variable DC voltage obtained by comparing and amplifying the DC voltage that changes according to the output voltage with a constant reference voltage is connected to the base of the switching transistor. and a control circuit section having a control transistor to which the voltage is applied, and when the voltage generated at the other end of the turn-off capacitor reaches a predetermined value determined by the variable DC voltage, the control transistor becomes conductive. A switching control type constant voltage power supply circuit, wherein a charging voltage of the turn-off capacitor is applied between the base and emitter of the switching transistor, thereby cutting off the switching transistor.