JPS5943908B2 - switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching power supply using a flyback converter or a forward converter.

Generally, the typical operation of a flyback converter is to release the magnetic energy accumulated in the transformer during the ON period of the switching circuit to the load side during the OFF period, and the switching frequency of the switching circuit and the output power are inversely proportional to each other. be.

It can be seen that it is possible to make the flyback converter constant voltage by utilizing this property and controlling the operating frequency to vary in inverse proportion to the output power as the load fluctuates. Conversely, if constant voltage control is performed, the operating frequency changes in inverse proportion to the load current as the load changes. However, such characteristics of the flyback converter cause disadvantages such as a decrease in efficiency and an increase in the size of the transformer when load fluctuations are large. On the other hand, in a forward converter, output control is performed by changing the on period or off period of the switching circuit, that is, by changing the duty ratio, and even if only the operating frequency of the switching circuit is changed, the output can be controlled. It generally has the property of not being able to be controlled.

However, in the case of a self-oscillating oscillation circuit, it is relatively easy to change the oscillation frequency, but it is difficult to change the duty ratio, which poses a problem in simplifying the circuit configuration. Furthermore, in flyback converters and forward converters, the voltage supplied to them is limited by the withstand voltage of the switching transistor in the switching circuit, but with the current transistor characteristics, it is not possible to directly rectify the 200V commercial power supply voltage in Europe etc. Supplying the same to the switching circuit resulted in insufficient withstand voltage, which caused a practical problem.

In view of the above points, the present invention has a configuration in which two switching circuits are connected in series with respect to the supply voltage, thereby enabling stable operation at a high supply voltage, and output control using a simple circuit configuration. Regarding the switching power supply that has been made executable.

Embodiments of the switching power supply according to the present invention will be described below with reference to the drawings.

FIG. 1 shows a first embodiment of the present invention, which is a self-oscillating flyback converter.

In this figure, the oscillation transformer T, has primary windings L1, 2
It has a secondary winding L2 and two feedback windings Lf, Lf2. Blocking oscillator 1 as a switching circuit
are a transistor Q, a feedback winding Lf connected to its base side, a primary winding L1 connected to its emitter side, resistors R1 to R3, a capacitor C1 and a diode D,
Similarly, the blocking oscillator 2 includes a transistor Q2 and a feedback winding Lf connected to the base side of the transistor Q2.
2, the primary winding L connected to the collector side, and the resistor R4
to R6, a capacitor C2, and a diode D2. A positive voltage + is supplied to the collector of the transistor Q, and a negative voltage - is supplied to the emitter of the transistor Q2, so that the blocking oscillators 1 and 2 are connected in series with the supplied DC voltage. Furthermore, the emitter of transistor Q is clamped to negative voltage by diode D3, and the collector of transistor Q2 is clamped to negative voltage by diode D3.
4, it is clamped to positive voltage +. In addition, the transistor Q
1 accumulation time (storage time) of transistor Q2
If necessary, the capacitance of capacitor C is set to be larger than the capacitance of capacitor C2 in order to make the capacitor longer than C2.
On the other hand, a diode D5 is connected to the secondary winding L2 of the transformer T.
and a rectifying and smoothing circuit of capacitor C3 are connected, and the DC output voltage E.

is supplied between output terminals A and B. In this case, the DC output voltage E. A flyback voltage proportional to appears in the feedback winding Lf2, but this flyback voltage is rectified by a control circuit 3 consisting of a diode D6, a capacitor C4, and a constant voltage diode ZDl, and becomes a DC negative voltage F.
c is applied to the base of the transistor Q2 via a constant voltage diode ZDl. In the above configuration, each locking oscillator 1, 2 is connected to the primary winding L of the transformer T,
A return winding Lf commonly used and coupled to the primary winding L1
Since the transistors Q, .
The turn-on timing of Q2 is substantially the same. Now, when transistors Ql and Q2 are both turned on by the DC bias of resistors R2 and R5, current flows through the path of transistor Q1, primary winding L, and transistor Q, and this causes each feedback winding Lf, Lf2 An induced voltage is generated in the direction that further forward biases transistors Q, , Q2, and transistor Q
,, Q2 are in a saturated state, a DC voltage is applied to the primary winding L, and the transformer T1 is excited. transistor Q
,, Capacitors Cl and C2 are inserted between the base of Q2 and the feedback windings Lfl and Lf2, respectively, and the transistor Q2 is negatively biased by the constant voltage diode ZD. Therefore, after a certain period of time, the transistor Q2 turns off from the on state earlier than transistor Q.

On the other hand, the base of transistor Q1 changes from forward bias to reverse bias due to turn-off of transistor Q2, but transistor Q1 still remains on due to carrier storage in the base region. In such a state, the induced current in the transformer T1 flows through the diode D4 and the transistor Q1.
, circulates along the path of the primary winding L1, and both ends of the primary winding L are almost in a short-circuited state. Therefore, the base of the transistor Q1 is not reverse biased to a large extent, so that the storage time becomes long, and an effective length of the rest period that is not too short can be obtained. In this way, transistor Ql
, Q2's carrier storage and drive waveforms due to the difference in their turn-off times. During the rest period, the primary winding L1 is essentially in a short-circuit state, so during this period, the transformer T1 is The stored magnetic energy is conserved. Now, after the predetermined rest period ends, the transistor Q1 is also turned off, and the primary winding L1 becomes substantially open.

At this time, the emitter voltage of the transistor Q1 is pulled to the negative side by the induced voltage of the primary winding L, but the voltage does not become more negative than - due to the diode D3. The collector of transistor Q2 likewise does not rise above a positive voltage + due to diode D4. In this way, on the primary side of the transformer T, when both transistors Ql and Q2 are on, the primary winding L1 is excited, when the second transistor Q is on, and when the transistor Q is off, the primary winding L is short-circuited. The three operations of turning off the three transistors Ql and Q2 and opening the primary winding L are repeated in sequence.

As a result, the voltage across the primary winding L becomes as shown in FIG. The presence of D4 makes it 2V-. By switching the primary side of the oscillation transformer T as described above, the voltage induced in the secondary winding L2 is rectified and smoothed by the diode D5 and capacitor C3, resulting in a DC output voltage E.
. The output voltage is supplied to the load via output terminals A and B. In this case, the DC output voltage E. Since a flyback voltage proportional to is generated in the feedback winding Lf2, this is rectified to obtain the DC output voltage E. DC negative voltage proportional to E. c is obtained by the control circuit 3, and the transistor Q2 is obtained via the constant voltage diode ZDl.
controls the base of In other words, as the load becomes lighter, the DC output voltage E. As becomes larger, 1Ec1 becomes weaker, and as a result, the base of the transistor Q2 is biased more negatively through the constant voltage diode ZDl.
Therefore, the on period of transistor Q2 becomes shorter and the oscillation frequency becomes higher. At this time, the above-mentioned rest period exists after the on-period, and this rest period is constant regardless of the oscillation frequency, so the voltage waveform applied to the primary winding L, as shown in FIG. 2B, The duty ratio decreases as the oscillation frequency increases. As a result, the DC output voltage E. decreases due to the synergistic effect of increasing the oscillation frequency and decreasing the duty ratio, and stabilization control is executed. According to the first embodiment, the blocking oscillator 1
, 2 are connected in series, and clamping diodes D3 and D4 are provided, so that transistors Q, , Q2 with low breakdown voltage can be used.

Therefore, it is possible to directly rectify the 200 series commercial power supply, which has been difficult in the past, and use it as a supplied DC voltage. Also,
Since a pause time is provided to short-circuit the primary winding T1 of the transformer T1 by utilizing the difference in turn-off timing of the transistors Ql and Q2, it is possible to change the oscillation frequency and the duty ratio. Therefore, the range of variation in oscillation frequency can be made smaller than in conventional flyback converters. Therefore, it is possible to reduce the size and weight of the transformer, improve its efficiency, and stabilize its operation under no load. FIG. 3 shows a second embodiment of the present invention, which is a self-oscillating forward converter.

In this figure, a blocking oscillator 1A includes a transistor Q and an oscillation transformer T1 connected to the base side of the transistor Q.
The feedback winding Lf, and the primary winding L connected to the emitter side
1, resistors R, , R2, R7, capacitor C, and diode D6. This diode D, strongly excites the base of the transistor Q1 during forward bias,
It is turned off during reverse bias to prevent strong reverse bias from being applied and to maintain the base at a high impedance, thereby lengthening the storage time of transistor Q. The connections between the locking oscillator 2 and the clamping diodes D3 and D4 are the same as in the first embodiment. On the other hand, the secondary winding L2 of the oscillating lance T has a polarity opposite to that shown in FIG.
A rectifying and smoothing circuit consisting of is connected, and a DC output voltage E.

is supplied between output terminals A and B. This DC output voltage E. is added to the error amplifying circuit 5, which is composed of the transistor Q, the constant voltage diode ZD, the light emitting element DFl of the photocoupler 4, the variable resistor VR, and the resistors R8 to R, and the setting specified by the constant voltage diode ZD2. compared to voltage. In addition, the base circuit of the transistor Q2 includes a transistor Q4 and a photo coupler 4.
A control circuit 6 consisting of a light receiving element QFl diode DlOl capacitor C6 and resistors Rl2 and Rl3 is provided, and this control circuit 6 is coupled to an error amplification circuit 5 via a photocoupler 4. In the above configuration, switching on the primary side of the oscillation transformer T1 is performed in the same manner as in the first embodiment described above, and the voltage induced in the secondary winding L2 is transferred to the diodes D8, D, the capacitor C5, and the choke coil Lc. The DC output voltage is rectified and smoothed by E.

The output voltage is supplied to the load via output terminals A and B. In this case, the DC output voltage E. When becomes larger than a predetermined set value, transistor Q3 becomes conductive and light emitting element DF emits light,
As a result, the light receiving element QF has a low resistance. Therefore, the transistor Q4 becomes conductive, and a negative voltage rectified and smoothed by the diodes D, O and the capacitor C6 is applied to the base of the transistor Q2 via the transistor Q4.
This increases the oscillation frequency of the locking oscillator 2. In this case as well, the rest period from when transistor Q2 is turned off to when transistor Q is turned off is almost constant regardless of the oscillation frequency, so the duty ratio of the voltage waveform applied to primary winding L1 is It decreases as the frequency increases. Therefore, the DC output voltage E. will decrease, and stabilization control will be executed. In the second embodiment as well, the blocking oscillators 1A and 2 are connected in series with respect to the supplied DC voltage, and the clamping diodes D3 and D4 are provided, so transistors with low breakdown voltage are used as the transistors Ql and Q2. It is possible.

Furthermore, since a rest period is provided in which the primary winding T1 of the transformer T1 is short-circuited, it is possible to change the duty ratio by changing the oscillation frequency. Therefore, there is an advantage that stabilization control of the forward converter can be executed with a simple circuit configuration. Note that it is clear that the configurations of the error amplification circuit and the control circuit can be changed as appropriate.

As described above, according to the switching power supply of the present invention, 2
The configuration in which the switching circuits are connected in series with respect to the supply voltage enables stable operation at high supply voltages, and output control can be performed with a simple circuit configuration.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a first embodiment of a switching power supply according to the present invention, FIG. 2 is a waveform diagram for explaining its operation, and FIG. 3 is a circuit diagram showing a second embodiment. 1, 1A, 2... Blocking oscillator, 3, 6...
Control circuit, 4... Photo coupler, 5... Error amplifier circuit, C1 to C6... Capacitors, D1 to D,.

Claims (1)

[Claims] 1. An oscillation transformer T_1 having a primary winding, a secondary winding, a first feedback winding, and a second feedback winding, and the collector of the first transistor Q_1 connected to one end of a power supply. the first feedback winding is connected between the base and emitter, and the emitter of the first self-excited oscillator is connected in series with one end of the primary winding, and the emitter of the second transistor Q_2 is supplied. The second feedback winding is connected between the base and the emitter, and the other end of the primary winding is connected in series to the collector to connect the first self-excited type between the supply power sources. a second self-excited oscillator connected in series with the oscillator; and a diode forming a closed circuit together with the primary winding and the first or second transistor, wherein both the first and second transistors are turned on. a second state where one of the first or second transistors is on and the other is off, and a third state where both the first and second transistors are off. is repeated, and in the second state, the primary winding is equivalently brought into a short-circuited state by a series circuit of the on-state of the first or second transistor and the diode. Switching power supply. 2. The switching power supply according to claim 1, wherein the first and second self-excited oscillators have different base constants so that turn-off timings of the first and second transistors are different. 3. Claim 1, wherein a feedback control signal corresponding to the output on the secondary winding side is applied to the base of the transistor that is turned off in the second state among the first or second transistors. Switching power supply listed.