JP3409287B2 - Self-excited switching power supply circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited switching power supply circuit, and more particularly to a flyback type self-excited switching power supply that outputs from a secondary output winding side when a current in a primary winding is stopped. Regarding the circuit.

[0002]

2. Description of the Related Art A switching power supply circuit is used as a stabilized power supply in a battery charger, an AC adapter or the like. The drive method (switching method) of the switching element can be roughly classified into a self-excited oscillation method and a separately excited oscillation method. The self-excited oscillation method is a method of converting the voltage appearing in the feedback winding of an inductance component such as a transformer As a positive feedback to the control terminal of the switching element, the oscillation operation is performed.

As such a conventional self-excited switching power supply circuit 100, a circuit as shown in FIG. 6 is known. That is, 1 is an unstable DC power supply whose voltage may fluctuate, and 1a is its high-voltage side terminal, 1b.
Is a low voltage side terminal. Further, 2a is a primary winding of the transformer 2, 2b is a feedback winding of the transformer 2, 2c is a secondary output winding of the transformer 2, and 3 is a field effect transistor for oscillation (hereinafter referred to as FET). ). Reference numeral 21 is a start-up resistor used to form a start-up circuit for applying a forward bias (in other words, a gate voltage equal to or higher than a threshold value or an ON voltage) to the gate of the FET 3 at the time of circuit start-up, and 6 is a gate Zener diode for preventing an excessive input to the gate 12, an electric resistance 23, a starting capacitor connected in series between the electric resistance feedback winding 2b and the gate of the FET 3, and a block 24 for preventing an excessive input to the gate. An electric resistance 5 is an oscillation control transistor element. Reference numeral 22 is a control resistor that constitutes an oscillation stabilizing circuit together with the control capacitor 11. Furthermore,
Reference numerals 4 and 13 shown on the output side are a rectifying diode and a smoothing capacitor, respectively, which form a rectifying and smoothing circuit, and rectify and smooth the output of the secondary output winding 2c to obtain a high voltage side output line 20a and a low voltage side. Output between the output lines 20b.

In the operation of this circuit, first, when a DC voltage is applied to the high voltage side terminal 1a and the low voltage side terminal 1b of the power source 1, the starting capacitor 12 is charged via the starting resistor 21 (in the figure). The bottom electrode is + and the top is-. Since the starting capacitor 12 to be charged is connected in series with the starting resistor 21, the terminal voltage of the starting capacitor 12 gradually increases based on the time constant of the RC circuit. At this time, since the Zener diode 6 is connected between the gate of the FET 3 and the low voltage side terminal 1b, a part of the current flowing through the starting resistor 21 also slightly flows through the Zener diode 6.

When the charging voltage of the starting capacitor 12 reaches the ON voltage, a forward bias voltage is applied to the gate of the FET 3 and the FET 3 is turned on (conducts).

When the FET 3 is turned on and a current starts to flow in the primary winding 2a connected in series, an induced electromotive force is generated in each winding of the transformer. At this time, the voltage as the drive signal generated in the feedback winding 2b charges the control capacitor 11 via the control resistor 22.

The induced voltage generated in the feedback winding 2b is superposed on the charging voltage of the starting capacitor 12 and
The gate voltage of T3 is maintained at a voltage higher than the threshold voltage (ON voltage). At this time, the Zener diode 6 blocks an excessive input to the gate.

When the control capacitor 11 is charged and the charged voltage reaches a predetermined bias voltage of the oscillation control transistor 5 or more, a base current flows through the oscillation control transistor 5 and the collector-emitter is conductive. And the electric current flows in the meantime. As a result, the gate of the FET 3 is substantially short-circuited with the low voltage side terminal 1b by the oscillation control transistor 5, and the FET 3 is turned off.

When the FET 3 is thus turned off and the current flowing through the transformer is substantially cut off, a so-called flyback voltage (induced back electromotive force) is generated in each winding. At this time, the flyback voltage generated in the secondary output winding 2c is rectified and smoothed by the smoothing rectification circuit formed by the rectifying diode 4 and the capacitor 13, and the output line 2
It is output as electric power supplied to the load connected between 0a and 20b.

On the other hand, the flyback voltage generated in the feedback winding 2b is applied to the secondary winding 2c by the load connected to the output side.
The flyback voltage generated in the feedback winding 2b charges the starting capacitor 12 (the lower electrode in the drawing is + and the upper electrode is −). At this time, the Zener diode 6 is
Reverse bias is applied to the gate of ET3, and low voltage terminal 1
It acts as a path for a charging current that charges the starting capacitor 12 from the b side.

When the discharge of the electrical energy accumulated in the secondary output winding 2c is terminated by the induced back electromotive force and the flyback voltage generated in the feedback winding 2b also drops,
The charging voltage held in the starting capacitor 12 until then is applied as a forward gate voltage to the gate of the FET 3, and the FET 3 is turned on. In this way, a series of oscillation operations are repeated.

In such a conventional self-excited switching power supply circuit 100, the voltage between the output lines 20a and 20b is monitored to change the oscillation cycle of the primary side, and the FET3.
The on-duty of the output lines 20a, 20b
A circuit is provided to stabilize the output voltage across.

That is, two voltage dividing resistors 30 and 31 are connected in series between the high voltage side output line 20a and the low voltage side output line 20b, and the intermediate tap 32 thereof is connected to the error amplifier 33.
By connecting to the inverting input terminal of, the divided voltage of the output voltage is input to the inverting input terminal. In addition, the error amplifier 33
A reference power supply 34 is connected between the non-inverting input terminal of the above and the low-voltage side output line 20b, and a reference voltage for comparison with the divided voltage of the output voltage is input to the non-inverting input terminal.

A photo coupler light emitting element 35 is connected to the output side of the error amplifier 33, and the photo coupler light emitting element 35 is connected.
Is connected to the high-voltage side output line 20a via the electric resistance 36, and is supplied with drive power. The resistor 37 and the capacitor 38 connected in series are an AC negative feedback element for stably operating the error amplifier 33.

On the other hand, one side (F
A photocoupler light receiving element 39 that is photocoupled with the photocoupler light emitting element 35 is connected between the gate side of ET3) and the base of the oscillation control transistor 5.

With these circuit configurations, for example, when the output voltage between the high voltage side output line 20a and the low voltage side output line 20b rises above the set value, the partial pressure input to the inverting input terminal of the error amplifier 33 also rises, The potential difference from the reference voltage is inverted and amplified, and the potential exceeds the light emission threshold of the photocoupler light emitting element 35.

As a result, the photocoupler light emitting element 35 emits light, and the photocoupler light receiving element 39 receives the light, so that one side of the starting resistor 21 and the base of the oscillation control transistor 5 are short-circuited. As a result, while the oscillation control transistor 5 is turned off, the control capacitor 11 charged through the control resistor 22 is connected to the starting resistor 2
The DC power supply 1 is also charged through the battery 1, and the charging speed is accelerated. When the charging speed is accelerated, the oscillation control transistor 5
Turns on quickly and the on-duty of the FET 3 decreases, so that the output voltage decreases to the set value.

Conversely, when the output voltage drops below the set value,
Since the photocoupler light emitting element 35 does not emit light, the one side of the starting resistor 21 and the base of the oscillation control transistor 5 are cut off, and the control capacitor 11 is charged only via the control resistor 22. Therefore, the charging time is delayed, the on-duty of the FET 3 increases, the output voltage rises to the set value, and the constant voltage control of the output voltage is performed through such a process.

[0019]

In this conventional self-excited switching power supply circuit 100, when the load is not connected to the output end or the load is in the standby state and only a slight load is applied, the self-excited switching power supply circuit 100 is self-excited. Since there is no flyback voltage enough to oscillate, it usually shifts to intermittent oscillation operation, but intermittent oscillation operation continues not only in this standby state but also with a certain load connected. there were.

In this intermittent oscillation operation, there is a problem that a ripple voltage of several hundred Hz is generated due to charging and discharging of the smoothing capacitor 13, and in order to eliminate the influence on the load, it works at a low frequency and is commensurate with the output power. It is necessary to provide an integrating circuit with a rated value, which is not a practical measure.

Therefore, in the conventional self-excited switching power supply circuit 100, the circuit constant of the internal element is adjusted to generate an internal load, and the self-excited oscillation is generated even in the standby state.
Since the continuous operation is performed, the on-duty increases and power is consumed, and power saving cannot be achieved.

The present invention has been made in view of the above problems, and in the standby state, intermittent oscillation operation is performed to save power consumption, and even when a low load is connected, An object of the present invention is to provide a self-excited switching power supply circuit in which ripple voltage of frequency is not generated.

[0023]

A self-excited switching power supply circuit according to the present invention comprises a transformer having a primary winding, a secondary output winding, and a feedback winding, and a DC power supply connected in series with the primary winding. Oscillation field-effect transistor, DC power supply connected in series with start-up resistor, oscillation control transistor whose series connection point is connected to the gate of oscillation field-effect transistor, and resistor connected in series to feedback winding And a capacitor, the oscillation stabilization circuit that connects the series connection point of the resistor and the capacitor to the base of the oscillation control transistor, the rectification smoothing circuit that rectifies and smoothes the output of the secondary output winding, and the rectification smoothing circuit. The output detection voltage, which represents the output voltage of the comparator circuit, is input from one input terminal of the comparison circuit and compared with the reference voltage input from the other input terminal of the comparison circuit to determine the output voltage of the rectification smoothing circuit. When the voltage is higher than a predetermined control voltage, the photocoupler light emitting element is turned on or off, and an output voltage detection circuit that outputs a judgment signal is photocoupled with the photocoupler light emitting element. It is equipped with a series connection point of the start-up resistor and the oscillation control transistor when inputting, and a photocoupler light receiving element that conducts between the base of the oscillation control transistor, and the on-duty of the oscillation field effect transistor is changed by the output voltage. In a self-excited switching power supply circuit that stabilizes the output voltage, the comparator circuit is connected to one input terminal of the comparison circuit via a switching element and delays the output detection voltage when the switching element becomes conductive. The output current flowing through the output line of the rectification smoothing circuit and the delay circuit input to one input terminal of Out, output current, and conducts when it is zero or very small value, the output current,
An output current detection circuit that controls the switching element is also attached so as to cut off when the value exceeds a very small value, and a load is not connected to the output of the rectification smoothing circuit or a low load is connected. In the standby state, the determination signal output from the photocoupler light emitting element is delayed to induce the self-excited oscillation operation by the oscillation field effect transistor to the intermittent oscillation operation.

In a standby state in which the load is not connected to the output of the rectifying / smoothing circuit or the load is not operating and is connected with a low load, zero or a small value is output to the output line of the rectifying / smoothing circuit. Only the current flows and the switching element becomes conductive, so that the delay circuit is connected to one input terminal of the comparison circuit, and the output detection voltage representing the output voltage is delayed and input to the comparison circuit.

Since the comparator circuit compares the output detection voltage input with a delay with the reference voltage, the output of the determination signal is output with a delay after the actual output voltage exceeds the control voltage,
Further, after the actual output voltage becomes equal to or lower than the control voltage, the output voltage is stopped with a delay, and as a result, in the self-excited switching power supply circuit, the actual output voltage fluctuates at a constant cycle above and below the control voltage,
Transition to intermittent oscillation operation.

In the intermittent oscillating operation, the time for which the oscillating field effect transistor is turned off (hereinafter referred to as "on time") is longer than the time for which the oscillating field effect transistor is turned on and a current is supplied from the DC power supply to the primary winding. Since the off time will be extremely long, the power consumption is reduced.

On the other hand, in an operating state in which a load is connected to the output of the rectifying / smoothing circuit, an output current of a minute value or more flows through the output line of the rectifying / smoothing circuit, and the switching element operates to cut off the delay circuit. Is disconnected from the input terminal of the comparison circuit.

Since the comparison circuit compares the output detection voltage representing the actual output voltage with the reference voltage, the determination signal is output when the actual output voltage becomes equal to or higher than the control voltage, and the output voltage becomes the control voltage. It acts only as a constant voltage control signal for If the circuit constants of internal elements are adjusted so that self-excited oscillation occurs, when a load that allows an output current to flow is connected and the standby state is exited, self-excited oscillation is performed and the ripple voltage at a low frequency Does not occur.

According to another aspect of the present invention, there is provided a self-excited switching power supply circuit, in which a transformer having a primary winding, a secondary output winding and a feedback winding, and a field effect transistor for oscillation connected to a DC power supply in series with the primary winding. And an oscillation control transistor connected in series with the starting resistor to the DC power supply, the series connection point of which is connected to the gate of the field effect transistor for oscillation, and a resistor and a capacitor connected in series to the feedback winding,
An oscillation stabilization circuit in which a series connection point of a resistor and a capacitor is connected to the base of an oscillation control transistor, a rectification smoothing circuit that rectifies and smoothes the output of the secondary output winding, and a high-voltage side output line of the rectification smoothing circuit. Output voltage detection circuit that compares the voltage-dividing resistor connected between the output line and the low-voltage side output line with the output detection voltage of the intermediate tap of the voltage-dividing resistor and the reference voltage, and makes the photocoupler light-emitting element emit light when the reference voltage is exceeded. And a photocoupler light-emitting element, and a photocoupler light-receiving element connected between the base of the oscillation control transistor and a series connection point of a start-up resistor and an oscillation control transistor. In the self-excited switching power supply circuit that changes the on-duty of the effect transistor and stabilizes the output voltage, the center tap of the voltage dividing resistor and the rectification smoothing circuit A delay capacitor connected via a switching element between the output lines on the pressure side, and the output current flowing through the high-voltage output line or the low-voltage output line of the rectifying and smoothing circuit is detected, and the output current is zero or a minute value. Between the high voltage side output line and the low voltage side output line, an output current detection circuit that controls the switching element is added so that it conducts when there is a certain value and shuts off when the output current increases beyond a small value. In the standby state where no load is connected or a low load is connected, the light emission timing of the photocoupler light emitting element is delayed to induce the self-excited oscillation operation by the oscillation field effect transistor to the intermittent oscillation operation. Characterize.

In the standby state, only zero or a minute output current flows through the high voltage side output line or the low voltage side output line,
By the conduction of the switching element, the delay capacitor is connected in parallel with the voltage dividing resistor between the intermediate tap and the low voltage side output line to form an integrating circuit. As a result, the output detection voltage of the intermediate tap is compared with the reference voltage later than the actual output detection voltage, and the light emission timing of the photocoupler light emitting element is delayed. As a result, the actual output voltage fluctuates above and below the control voltage in a constant cycle, and shifts to the intermittent oscillation operation. In the intermittent oscillation operation, the off time is extremely longer than the on time, so the power consumption is reduced.

On the other hand, in an operating state in which a load is connected between the high-voltage side output line and the low-voltage side output line, an output current of a minute value or more flows, and the switching element is cut off to disconnect the delay capacitor. .

Since the output voltage detection circuit compares the output detection voltage obtained by dividing the actual output voltage with the reference voltage, constant voltage control of the output voltage is performed. If the circuit constants of the internal elements are adjusted so that self-sustained pulsation occurs, in this operating state, self-sustained pulsation naturally occurs and a low-frequency ripple voltage does not occur.

According to another aspect of the self-excited switching power supply circuit of the present invention, a switching transistor and a control transistor, whose bases are connected to each other, are connected in parallel between the high voltage side output line and the low voltage side output line, and the collector of the switching transistor is connected to the switching transistor and the control transistor. A delay capacitor is connected, the switching transistor is used as a switching element, the collector and base of the control transistor are coupled, a current detection resistor is interposed in the low voltage side output line, and the emitter of the control transistor is connected to the rectifying and smoothing circuit side. , And an output current detection circuit by connecting the emitters of the switching transistors to the output side, respectively.

Since the collector and the base of the control transistor are connected to each other, no current flows between the base and the emitter, and the control transistor is kept at a constant potential difference.

In the standby state, almost no current flows through the low voltage side output line, so that a constant potential difference does not occur at both ends of the current detecting resistor, and the potential difference between the base and emitter of the switching transistor, which serves as a switching element, is the operating voltage. As described above, conduction is established between the collector and the emitter. As a result, the delay capacitor is connected in parallel with the voltage dividing resistor between the intermediate tap and the low voltage side output line.

In the operating state, a constant current flows through the low-voltage side output line, so the potential of the emitter of the switching transistor rises with respect to the potential of the emitter of the control transistor, and the potential difference from the base becomes less than the operating voltage. , The switching transistor is turned off. As a result, the delay capacitor is disconnected from the low voltage side output line.

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a diagram showing an outline of a main part of a self-excited switching power supply circuit 10 according to an embodiment of the present invention. The self-excited switching power supply circuit 10 according to the present embodiment has the same main circuit and circuit elements as those of the conventional self-excited switching power supply circuit 100 shown in FIG. , And the description thereof is omitted.

As shown in FIG. 1, the primary winding 2 of the transformer
a is connected in series with the oscillating field effect transistor (FET) 3 to the DC power source 1, and controls the current flowing through the primary winding 2a by the on / off operation of the FET 3.

Between the gate of the FET 3 and the high-voltage side terminal 1a of the DC power source 1, a starting resistor 21 for supplying a charging current at the time of starting the circuit and an electric resistor 2 for preventing an excessive input to the gate.
4 are connected in series.

The feedback winding 2b, the starting capacitor 12 and the electric resistance 23 are connected in series between the connection point of the starting resistance 21 and the electric resistance 24 and the low voltage side terminal 1b of the DC power supply 1. . Further, between the connection point and the low-voltage side terminal 1b, an oscillation control transistor 5 serving as a control switching element and a Zener diode 6 whose forward direction is the connection point direction from the low-voltage side terminal 1b are connected in parallel. It is connected.

In addition, the feedback winding 2b and the starting capacitor 1
A control resistor 2 is provided between the connection point between the two and the low-voltage side terminal 1b.
2 and the control capacitor 11 are connected in series, and the connection point is connected to the base of the oscillation control transistor 5 to form an oscillation stabilization circuit.

Further, between one side of the starting resistor 21 (gate side of the FET 3) and the base of the oscillation control transistor 5,
A photocoupler light-receiving element 39 that is photocoupled with the photocoupler light-emitting element 35 on the secondary side is connected, and when light from the photocoupler light-emitting element 35 is received, they are short-circuited.

A rectifying diode 4 in series with the secondary output winding 2c and a smoothing capacitor 13 in parallel with the secondary output winding 2c are connected to the secondary output winding 2c of the transformer. It comprises a digitalization circuit.

Voltage dividing resistors 30 and 31 are connected in series between the high voltage side output line 20a and the low voltage side output line 20b of the rectifying / smoothing circuit, and the intermediate tap 32 thereof is connected to the inverting input terminal of the error amplifier 33. The output detection voltage, which is the divided output voltage, is input to the inverting input terminal. Further, between the non-inverting input terminal of the error amplifier 33 and the low voltage side output line 20b,
The reference power supply 34 is connected, and the reference voltage for comparison with the output detection voltage is input to the non-inverting input terminal.

The output side of the error amplifier 33 is connected to the error amplifier 3
The photocoupler light emitting element 35 that blinks according to the output value of 3 is connected, and this photocoupler light emitting element 35 is photocoupled with the photocoupler light receiving element 39 on the primary side as described above.

Between the intermediate tap 32 and the non-inverting input terminal of the error amplifier 33, AC negative feedback elements 37 and 38 are connected between the output of the error amplifier 33 and one side of the delay capacitor 40. ing.

The other side of the delay capacitor 40 is connected to the collector of a switching transistor 41 which serves as a switching element. This collector is further connected to the high-voltage side output line 20a via the resistor 42, and the emitter is connected to the low-voltage side output line 20b, so that the switching transistor 41 is connected between the high-voltage side output line 20a and the low-voltage side output line 20b. Connected to.

On the rectifying / smoothing circuit side of the low-voltage side output line 20b connected to the emitter of the switching transistor 41,
The current detection resistor 43 is interposed, and the emitter of the control transistor 44 is connected to the low voltage side output line 20b of the current detection resistor 43 on the side of the rectifying and smoothing circuit.

The control transistor 44 also has a collector connected to the high voltage side output line 20a via a resistor 45, and is connected between the high voltage side output line 20a and the low voltage side output line 20b. The base of the control transistor 44 is coupled to the collector and is connected to the base of the switching transistor 41.

The self-excited switching power supply circuit 1 configured as described above is started via the starting resistor 21 when a DC voltage is first applied between the high voltage side terminal 1a and the low voltage side terminal 1b of the power supply 1 at the time of circuit startup. The charging capacitor 12 is charged (the lower electrode in the figure has a positive polarity and the upper electrode has a negative polarity). Since the starting capacitor 12 to be charged is connected in series with the starting resistor 21, the terminal voltage of the starting capacitor 12 gradually increases based on the time constant of the RC circuit. When the charging voltage of the starting capacitor 12 reaches the on-voltage, the forward bias voltage is applied to the gate of the FET3, and the FET3 is turned on (conducts).

Thus, the FET 3 is turned on and the primary winding 2
When a current starts to flow in a, an induced electromotive force is generated in each winding of the transformer. At this time, the induced voltage as the drive signal source generated in the feedback winding 2b charges the control capacitor 11 via the control resistor 22.

The induced voltage generated in the feedback winding 2b is superposed on the charging voltage of the starting capacitor 12 and
The gate voltage of T3 is maintained at a voltage higher than the threshold voltage (ON voltage). During this ON operation, an excessive input to the gate is blocked by the Zener diode 6.

When the charging voltage of the control capacitor 11 rises and the terminal voltage thereof exceeds a predetermined bias voltage of the oscillation control transistor 5, a base current flows in the oscillation control transistor 5 and the collector-emitter voltage is increased. It becomes conductive and current flows between them. Then, the gate of the FET 3 is substantially short-circuited by the oscillation control transistor 5 (here, the potential of the low voltage side terminal 1b, for example, 0 volt), and the FET 3 is turned off.

When the FET 3 is turned off and the current flowing through the primary winding 2a of the transformer is substantially cut off, a load of a predetermined size is connected between the high voltage side output line 20a and the low voltage side output line 20b. If so, a flyback voltage proportional to the load is generated in each winding of the transformer 2.

The flyback voltage generated in the secondary output winding 2c is rectified and smoothed by the rectifying and smoothing circuit composed of the diode 4 and the capacitor 13, and the high voltage side output line 20a.
Is output to the load connected between the low voltage side output line 20b and the low voltage side output line 20b.

On the other hand, the flyback voltage generated in the feedback winding 2b is proportional to the flyback voltage generated in the secondary output winding 2c by the load, and is controlled by the flyback voltage generated in the feedback winding 2b. Capacitor 11
Is discharged, and the starting capacitor 12 is charged (the lower electrode in the figure has a positive polarity and the upper electrode has a negative polarity).

At this time, the Zener diode 6 serves as a charging path for charging the starting capacitor 12 from the low voltage side terminal 1b side of the feedback winding 2b through the electric resistance 23, and reverse biases the gate of the FET3. It acts to keep the FET3 off.

When the discharge of the electrical energy accumulated in the secondary output winding 2c is terminated by the induced back electromotive force and the flyback voltage generated in the feedback winding 2b also drops,
The charging voltage held in the starting capacitor 12 until then is applied as a forward gate voltage to the gate of the FET 3 to turn on the FET 3. By repeating such a series of operations, the self-excited switching power supply circuit 10
Self-oscillates.

Here, in the operating state in which a predetermined load is connected between the high-voltage side output line 20a and the low-voltage side output line 20b, a current flows through the current detecting resistor 43 due to the load, and the emitter of the control transistor 44 flows in the emitter. On the other hand, the potential of the emitter of the switching transistor 41 rises. On the other hand, since the base voltage of the switching transistor 41 is equal to the base voltage of the control transistor 44 and is maintained at a constant potential with respect to the emitter of the control transistor 44, the switching transistor 41 does not reach the predetermined bias voltage and is in the off state. Has become. As a result, the delay capacitor 40 is separated from the input of the error amplifier 33, and the voltage division at the intermediate tap 32 is directly applied to the inverting input terminal of the error amplifier 33 as the output detection voltage representing the output voltage at that time. Is entered. That is, in the operating state where the load is connected, the conventional self-excited switching power supply circuit 1 shown in FIG.
The output detection voltage is used for constant voltage control of the output voltage.

For example, when the output voltage between the high-voltage side output line 20a and the low-voltage side output line 20b rises above a set value, the output detection voltage input to the inverting input terminal of the error amplifier 33 also rises, and the photocoupler light emitting element 35. When the photocoupler light receiving element 39 receives the light emission of, the one side of the starting resistor 21 and the base of the oscillation control transistor 5 are short-circuited.
As a result, the charging speed of the control capacitor 11 is accelerated while the oscillation control transistor 5 is turned off, the oscillation control transistor 5 is quickly turned on, and the output voltage drops.

On the contrary, when the output voltage drops below the set value,
Since the photocoupler light-emitting element 35 does not emit light, one side of the starting resistor 21 and the base of the control transistor 5 are cut off, and charging of the control capacitor 11 is delayed. As a result, the on-duty of the FET 3 increases and the output voltage rises to the set value.

In this way, the reference voltage to be compared with the output detection voltage can be set to a predetermined voltage, and the output voltage between the high voltage side output line 20a and the low voltage side output line 20b can be stabilized at the control voltage.

On the other hand, an external load is not connected between the high-voltage side output line 20a and the low-voltage side output line 20b, or even if it is connected, it is in a standby operation and only a small load is connected in a standby state. Then, since no current flows through the current detection resistor 43, the potentials of the emitters of the control transistor 44 and the switching transistor 41 become substantially equal. On the other hand, the base voltage of the switching transistor 41 is equal to the base voltage of the control transistor 44 kept at a constant potential with respect to the emitter of the control transistor 44, and the potential difference from the emitter of the switching transistor 41 is equal to or higher than the bias voltage. It is turned on and the collector and emitter are electrically connected. As a result, the delay capacitor 40 is connected in parallel with the voltage dividing resistor 31 between the low voltage side output line 20b and the intermediate tap 32,
An integrating circuit is formed. Therefore, the output detection voltage at the intermediate tap 32 lags behind the actual output voltage between the high-voltage side output line 20a and the low-voltage side output line 20b, and the error amplifier 33
Input to the inverting input terminal of.

The operation when the output detection voltage is input to the error amplifier 33 later than the actual output voltage will be described below.

First, when the circuit is started by inputting a DC power source, the output voltage between the actual high-voltage side output line 20a and the low-voltage side output line 20b (hereinafter simply referred to as output voltage) rises to the control voltage.

B. At this time, since the output voltage determined from the output detection voltage input with a delay has not reached the control voltage yet, the output voltage is output until the output detection voltage determines that the output voltage has reached the control voltage. The photocoupler light emitting element 35 stops emitting light in an attempt to increase the voltage.
As a result, the actual output voltage further rises above the control voltage. FIG. 2 shows the voltage waveform between the base and the emitter of the oscillation control transistor 5 at this time in units of a time width of 5 .mu.sec, and shows the oscillation portion of FIG. In the figure, the period during which the FET 3 rises to the right from the minimum value to the maximum value is the time when the FET 3 is on, and the control capacitor 11 is charged. As shown in this figure, the on-time gradually increases and the output voltage also increases.

C. Then, when it is determined that the output voltage has reached the control voltage by the output detection voltage that is input with a delay, the photocoupler light emitting element 35 emits light in order to reduce the output voltage, and When the capacitor 11 is charged rapidly, it turns on quickly, and FET3
ON time is shortened. That is, the oscillation control transistor 5 is quickly turned on, so that the F energy is compulsorily generated before the electrical energy for the next oscillation is generated.
ET3 turns off. As a result, self-sustained pulsation is stopped and the actual output voltage gradually decreases. During this time,
Even if the actual output voltage drops to the control voltage, the output detection voltage that is input with a delay does not determine that the output voltage is below the control voltage, so the actual output voltage exceeds the control voltage and continues to drop. .

D, the output detection voltage input with a delay also drops, and when it is determined that the output voltage is less than or equal to the control voltage, the photocoupler light emitting element is turned off in an attempt to increase the output voltage,
The operation of b is repeated.

By repeating steps c to d, the actual output voltage fluctuates at a constant cycle above and below the control voltage, resulting in intermittent oscillation operation. In the intermittent oscillation operation, the off time is extremely longer than the on time, so the power consumption is reduced. FIG. 3 shows the voltage waveform between the base and the emitter of the oscillation control transistor 5 during the intermittent oscillation operation in units of a time width of 2 msec, and it is shown that intermittent oscillation occurs at a constant cycle.

In the above-described series of operations according to the present embodiment, when the load connected between the high voltage side output line 20a and the low voltage side output line 20b is operating at a low voltage, the load is low. Is theoretically the feedback winding 2 as described above.
Since the flyback voltage of b is not sufficiently generated and does not contribute to the charging of the starting capacitor 12, the electric energy for the next oscillation is insufficient even when the delay capacitor 40 is disconnected and self-excited. Oscillation becomes easy to shift to intermittent oscillation. Therefore, in the present embodiment, in the state where the delay capacitor 40 is disconnected (that is, in the operating state where the low load is connected and the current flows through the output line),
The internal circuit constants are adjusted to generate a flyback voltage by the internal load, and the self-excited oscillation operation is continued even if the load is low.

FIG. 4 shows the voltage waveform between the base and the emitter of the oscillation control transistor 5 during the self-excited oscillation operation for 5 μsec.
In the above, the internal load is adjusted as described above, the delay capacitor 40 is intentionally disconnected,
The figure shows the state of self-excited oscillation even when an external load is not connected.

If a current due to a load flows through the high-voltage side output line 20a and the low-voltage side output line 20b as described above, self-excited oscillation occurs even with a slight load, so that a large ripple voltage does not appear in the output voltage due to intermittent oscillation. .

The switching element 4 detects the output current flowing through the high voltage side output line 20a or the low voltage side output line 20b.
The output current detection circuit that operates 1 is not limited to the combination of the current detection resistor 43 and the control transistor 44, and various other circuits can be adopted.

FIG. 5 shows a second embodiment using a current transformer as an output current detection circuit.
The same components as those of the self-excited switching power supply circuit 10 according to the embodiment are designated by the same reference numerals.

As shown in the figure, the primary winding 50a of the current transformer is interposed in the high voltage side output line 20a, and when an output current due to an external load flows in the high voltage side output line 20a, it is connected to the low voltage side output line 20b. An induced current flows through the next winding 50b. This induced current is generated between the output lines of the secondary winding 50b,
Smoothing capacitor 51 connected in parallel with the secondary winding 50b
And an electric resistance 52 and a diode 53 connected in series
Is smoothed and rectified by. When the smoothed and rectified induced current flows through the electric resistance 54 connected in series to the output side of the diode 53, the potential difference of the electric resistance 54 turns on the control transistor 55, lowers the base potential of the switching transistor 41, and turns off. Let as a result,
The delay capacitor 12 is insulated from the low voltage side output line 20b.

On the other hand, in the standby state where no output current flows through the high voltage side output line 20a, the current transformer is not excited, so that the base potential of the control transistor 55 becomes equal to that of the low voltage side output line 20b and the control transistor 55 is turned off. . As a result, the base of the switching transistor 41 is connected to the high-voltage side output line 20a via the resistors 56 and 57, and the potential difference between the base and the emitter connected to the low-voltage side output line 20b exceeds the bias voltage, and the switching transistor 41 becomes Turn on. As a result, the delay capacitor 12 includes the intermediate tap 32 and the low voltage side output line 20b.
Connected between. In this way, the switching transistor 41, which is a switching element, can be on / off controlled depending on the presence / absence of the output current.

In the above embodiment, the high voltage side output line 20.
The output voltage between the a and the low voltage side output line 20b is divided by the two voltage dividing resistors 30 and 31, and the division of the output voltage taken out from the intermediate tap 32 is used as the output detection voltage representing the output voltage. The output voltage between the high voltage side output line 20a and the low voltage side output line 20b may be directly used as the output detection voltage and compared with the reference voltage.

Further, in the above embodiment, the circuit constant is set so that the self-excited oscillation operation is performed even when the external load is not connected. However, there is a load such that at least a slight output current flows. As long as the self-excited oscillation operation is performed in the connected operation state, intermittent oscillation may be performed in the above-described state where the load is not connected from the outside.

[0079]

As described above, according to the present invention,
In the standby state where the external load is not connected or is in the standby state, the intermittent oscillation operation is performed to save power consumption,
On the other hand, in an operating state in which a low load is connected such that even a small amount of output current flows, it is possible to reliably shift to a self-excited oscillation operation in which a low-frequency ripple voltage does not occur.

In addition to this, in the invention of claim 2, a voltage dividing resistor provided for dividing the output voltage to obtain the output detection voltage representing the output voltage is used as a part of the delay circuit. Therefore, the delay circuit can be formed without increasing the number of extra circuit elements.

Also, the delay circuit can be formed with a simple structure in which the delay capacitor is connected in parallel to the voltage dividing resistor.

Further, according to the invention of claim 3, in addition to the invention of claim 2, an output current detection circuit can be provided with a simple structure in which a control transistor and a current resistance element are combined.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a self-excited switching power supply circuit 10 according to an embodiment of the present invention.

FIG. 2 is an enlarged waveform diagram showing a voltage waveform between a base and an emitter of an oscillation control transistor 5 during an intermittent oscillation operation.

FIG. 3 is a waveform diagram showing a voltage waveform between a base and an emitter of an oscillation control transistor 5 during an intermittent oscillation operation.

FIG. 4 is a waveform diagram showing a voltage waveform between a base and an emitter of an oscillation control transistor 5 during a self-excited oscillation operation in a state where a delay capacitor 40 is intentionally disconnected and an external load is not connected.

FIG. 5 is a circuit diagram showing a main part of a self-excited switching power supply circuit according to another embodiment.

FIG. 6 is a circuit diagram of a conventional self-excited switching power supply circuit 100.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 DC power supply 2 Transformer 2a Primary winding 2b Feedback winding 2c Secondary output winding 3 Oscillation field effect transistor 4 Rectifying diode (rectification smoothing circuit) 5 Oscillation control transistor 11 Control capacitor (oscillation stabilizing circuit) 13 Smoothing capacitor (rectification smoothing circuit) 20a High-voltage side output line 20b Low-voltage side output line 21 Starting resistor 22 Control resistor (oscillation stabilizing circuit) 30, 31 Voltage dividing resistor 32 Intermediate tap 33 Error amplifier (comparing circuit, output voltage Detection circuit) 35 Photocoupler light emitting element 39 Photocoupler light receiving element 40 Delay capacitor (delay circuit) 41 Switching transistor (switching element) 43 Current detection resistor (output current detection circuit) 44 Control transistor (output current detection circuit)

Claims (3)

(57) [Claims] 1. A primary winding (2a) and a secondary output winding (2)
c) and a transformer (2) having a feedback winding (2b), a DC power supply (1), an oscillating field effect transistor (3) connected in series with the primary winding (2a), and a DC power supply (1). ) Is connected in series with the starting resistor (21), and the series connection point is connected in series to the oscillation control transistor (5) connected to the gate of the oscillation field effect transistor (3) and the feedback winding (2b). An oscillation stabilization circuit that is composed of a resistor (22) and a capacitor (11) connected to each other, and a series connection point of the resistor (22) and the capacitor (11) is connected to the base of the oscillation control transistor (5), and a secondary output. A rectifying / smoothing circuit (4, 13) for rectifying and smoothing the output of the winding (2c), and an output detection voltage representing the output voltage of the rectifying / smoothing circuit (4, 13) are input to one input of a comparison circuit (33). Input from the terminal,
When it is determined that the output voltage of the rectifying / smoothing circuit (4, 13) is equal to or higher than a predetermined control voltage by comparing with the reference voltage input from the other input terminal of the comparison circuit (33), the photocoupler light emitting element ( 35) Light-emitting or extinguishing, and an output voltage detection circuit that outputs a judgment signal, and a photo-coupler with the photocoupler light-emitting element (35), when a judgment signal is input from the photocoupler light-emitting element (35), a starting resistor (21) and oscillation control transistor (5) connected in series, and oscillation control transistor (5)
Photocoupler light receiving element (39) for conducting between bases of
And a field effect transistor for oscillation (3) according to the output voltage.
In the self-excited switching power supply circuit that changes the on-duty of the output voltage to stabilize the output voltage, the comparison circuit (3
3) One input terminal of the comparison circuit (33) is connected to one input terminal of the comparison circuit (33) by delaying the output detection voltage when the switching element (41) is turned on. The output current flowing through the output lines of the input delay circuit (40) and the rectifying / smoothing circuit (4, 13) is detected, and when the output current is zero or a minute value, it conducts and the output current is a minute value. An output current detection circuit (43) for controlling the switching element (41) is further provided so as to be cut off when the voltage exceeds the limit, and a load is connected to the output of the rectification smoothing circuit (4, 13). No, or in a standby state in which a low load is connected, the determination signal output from the photocoupler light emitting element (35) is delayed to change the self-excited oscillation operation by the oscillation field effect transistor (3) to the intermittent oscillation operation. Induce Self-excited switching power supply circuit, characterized in that. 2. A primary winding (2a) and a secondary output winding (2)
c) and a transformer (2) having a feedback winding (2b), a DC power supply (1), an oscillating field effect transistor (3) connected in series with the primary winding (2a), and a DC power supply (1). ) Is connected in series with the starting resistor (21), and the series connection point is connected in series to the oscillation control transistor (5) connected to the gate of the oscillation field effect transistor (3) and the feedback winding (2b). An oscillation stabilization circuit that is composed of a resistor (22) and a capacitor (11) connected to each other, and a series connection point of the resistor (22) and the capacitor (11) is connected to the base of the oscillation control transistor (5), and a secondary output. A rectifying and smoothing circuit (4, 13) for rectifying and smoothing the output of the winding (2c), and a high voltage side output line (20a) of the rectifying and smoothing circuit (4, 13).
And a low voltage side output line (20b)
0, 31) and the output detection voltage of the intermediate tap (32) of the voltage dividing resistor (30, 31) are compared with the reference voltage, and the output voltage that causes the photocoupler light emitting element (35) to emit light when the reference voltage is exceeded. The detection circuit (33) and the photocoupler light emitting element (35) are photocoupled and connected between the series connection point of the starting resistor (21) and the oscillation control transistor (5) and the base of the oscillation control transistor (5). And a photocoupler light receiving element (39), and the field effect transistor (3) for oscillation is generated by the output voltage.
In the self-excited switching power supply circuit that changes the on-duty of the output voltage and stabilizes the output voltage, the intermediate tap (32) of the voltage dividing resistor (30, 31) and the low-voltage side output line of the rectifying and smoothing circuit (4, 13) ( During 20b),
A delay capacitor (40) connected via a switching element (41) and a high-voltage side output line (20a) of a rectifying and smoothing circuit (4, 13).
Alternatively, switching is performed so that the output current flowing through the low-voltage side output line (20b) is detected and conducted when the output current is zero or a minute value, and cut off when the output current increases beyond the minute value. An output current detection circuit (43, 44) for controlling the element (41) is additionally provided, and a load is not connected between the high voltage side output line (20a) and the low voltage side output line (20b), or a low load is applied. In a connected standby state, the light emission timing of the photocoupler light emitting element (35) is delayed so that the self-excited oscillation operation by the oscillation field effect transistor (3) is guided to the intermittent oscillation operation. Switching power supply circuit. 3. A switching transistor (41) and a control transistor (44), whose bases are connected to each other, are connected in parallel between the high voltage side output line (20a) and the low voltage side output line (20b), and the switching transistor (41) is connected. The collector of 41) is connected to the delay capacitor (40), the switching transistor (41) is used as a switching element, the collector and base of the control transistor (44) are coupled, and a current is detected in the low voltage side output line (20b). For the output current detection circuit (43, 44) by connecting the emitter of the control transistor (44) to the rectifying and smoothing circuit (4, 13) side and the emitter of the switching transistor (41) to the output side, respectively. 3. The self-excited switching power supply circuit according to claim 2, wherein