JP2000324823A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve power conversion efficiency by compensating, for the amount of discharge to be discharged while a charging electric charge of inverse polarity that flows in when a main switching element is off, is discharged during an operation pause period for a capacitor that starts a control transistor that turns off the main switching element and making an overcurrent limit value nearly equal as in a heavy load when reducing a switching frequency in a light load in an RCC(ringing choke converter) type switching power supply. SOLUTION: A resistor for overcurrent protection is composed of divided resistors R51 and R52, and the connection point P51 is bypassed via ZD1 to TR51, thus reducing the charge current with positive polarity from control coil winding N12 when it is on at light load time, extending charge time to a prescribed voltage, making the overcurrent limitation value of a main switching element Q nearly equal to that at heavy load time, and fully reducing the switching frequency by a control circuit 59.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a so-called AC-D
The present invention relates to a switching power supply device suitably implemented as a C converter, a DC-DC converter, or the like.

[0002]

2. Description of the Related Art A small DC transformer is used for portable small electronic equipment and the like, which switches a DC current obtained by rectifying and smoothing a commercial AC or a DC current from a battery at a high frequency of, for example, about several hundred kHz. 2. Description of the Related Art A switching power supply device that converts a voltage to a desired voltage with high efficiency by a heater is widely used.

As a typical configuration of such a switching power supply device, a secondary output voltage is detected by a voltage detection circuit, and a control circuit controls a switching pulse width of a main switching element in accordance with a result of the detection. 2. Description of the Related Art A pulse width modulation (PWM) type switching power supply device that obtains a desired secondary output voltage is widely used.

Another typical configuration of a switching power supply device is to output the excitation energy stored in a transformer during an on period of a main switching element to a secondary circuit during an off period, and to terminate the output. A ringing choke converter (RCC) in which a ringing pulse generated later in a control winding of a transformer is fed back to a control terminal of the main switching element via a DC cut capacitor to turn on the main switching element again. Type switching power supply devices are also widely used.

In the switching power supply of the RCC system, as the load becomes heavier, the off-period and the on-period are automatically longer, ie, the switching frequency is reduced, and the secondary side output voltage is maintained at a predetermined constant voltage. So, P
A low-cost power supply does not require a complicated control circuit such as a WM type switching power supply device, and does not require a power supply circuit for operating the control circuit and generating a voltage that is a reference of a pulse width. Suitable for the device.

On the other hand, in the switching power supply, most of the loss is power consumption required for extracting the electric charge accumulated in the parasitic capacitance between the drain and source of the main switching element, iron loss of the transformer, and the like. In general, the higher the switching frequency, the higher the switching frequency. Therefore,
The switching power supply of the RCC system described above has a problem that the switching frequency increases as the load becomes lighter, so that the ratio of the loss to the converted power increases, and the power conversion efficiency decreases.

Therefore, as another prior art for solving such a problem, for example, Japanese Patent Application Laid-Open No. 9-47023 is disclosed.
And Japanese Utility Model Registration No. 3039391. In the prior art disclosed in Japanese Utility Model Registration No. 3039391, at a light load, a delay capacitor for dulling a ringing pulse is interposed in parallel with the control transistor.

Therefore, as described in the seventh to eighth lines of paragraph 0025 of the publication, the switching cycle can be extended only during the period in which ringing occurs, and the switching period at the time of light load is reduced. There is a problem that the switching frequency cannot be significantly reduced as compared with the switching frequency under heavy load.

FIG. 6 shows a simplified prior art switching power supply 1 disclosed in Japanese Patent Application Laid-Open No. 9-47023, in which the switching frequency under light load can be significantly reduced as compared with the switching frequency under heavy load. FIG. DC current obtained by rectifying commercial AC by a main power supply circuit (not shown) is supplied to input terminals p1, p
Input between the two. This DC current is supplied to the smoothing capacitor c
The main power supply voltage is output from the main power supply line 2 on the high level side and the main power supply line 3 on the low level side from the smoothing capacitor c11.

A transformer n is connected between the main power supply lines 2 and 3.
Is connected to a series circuit of the primary main winding n11 and the main switching element q. The main switching element q is
For example, it is realized by a bipolar transistor or a field effect transistor. In the example of FIG. 6, the field effect transistor is shown. A control circuit 9 is connected between the main power supply lines 2 and 3 via a starting resistor r3.

When the power is turned on, that is, when the power supply voltage is applied between the input terminals p1 and p2, the output voltage of the smoothing capacitor c11, that is, the main power supply voltage, increases, and the control voltage from the starting resistor r3 in the control circuit 9 is increased. When the divided voltage becomes equal to or higher than the threshold voltage of the main switching element q, for example, 3 V or more, the main switching element q is turned on, an upward voltage is applied to the primary main winding n11 in FIG. Is accumulated. As will be described later, when the main switching element q is turned off, a back electromotive force is generated in the primary main winding n11 in a downward direction due to the stored excitation energy, thereby causing the secondary main winding n21 to have an upward direction. Is induced.

The DC current induced in the secondary main winding n21 is supplied to a smoothing capacitor c13 via a diode d12.
, And after being smoothed by the smoothing capacitor c13, output terminals p3, p4
Is output to a load circuit (not shown). A voltage detection circuit 8 is interposed between the output power supply lines 6 and 7. The voltage detection circuit 8 includes a voltage dividing resistor and a photocoupler p.
c1 and the like, and the photocoupler p
The light emitting diode d13 of c1 is driven to turn on at a luminance corresponding to the output voltage, and the value of the output voltage is fed back to the primary side.

When the main switching element q is turned on, a voltage is induced in the control winding n12 in the same upward direction as that of the primary main winding n11. The induced current is a direct current cut capacitor c1, a bias resistor r2 and The signal is applied to the gate of the main switching element q via the control circuit 9, whereby the gate potential of the main switching element q is further raised, and the main switching element q is maintained in the on state.

The current induced in the control winding n12 when the main switching element q is turned on is controlled by the control circuit 9
The phototransistor tr1 of the photocoupler pc1
1 is supplied to one terminal of a capacitor c14, and the other terminal of the capacitor c14 is connected to the low-level main power supply line 3. Therefore, the capacitor c14 is charged with the positive polarity shown in FIG.
As the secondary output voltage increases, the charging current increases, and the voltage between the terminals of the capacitor c14 increases rapidly. The charge voltage of the capacitor c14 is supplied to the base of the control transistor tr12 interposed between the gate and the source of the main switching element q via the control circuit 9, and the charge voltage is equal to the threshold voltage of the control transistor tr12. For example, when the voltage becomes 0.6 V or more, the control transistor tr12 is turned on, whereby the gate potential of the main switching element q is rapidly reduced, and the main switching element q is driven off.

Therefore, the higher the secondary output voltage, that is, the lighter the load, the faster the output voltage of the capacitor c14 increases, and the faster the main switching element q is turned off.
Driven. The capacitor c14 is also supplied with a current induced in the control winding n12 via a resistor r12. The series circuit of the resistor r12 and the capacitor c14 is connected in parallel with the control winding n12 to form an overcurrent protection circuit. With this overcurrent protection circuit, even if the output voltage of the secondary-side smoothing capacitor c13 is low due to a short circuit between the output terminals p3 and p4, the on-period of the main switching element q is limited to a predetermined period. q is protected.

In the control winding n12, the numbers of turns of the control winding n12 and the secondary main winding n21 are designated by the same reference numerals, and if the secondary output voltage is vo, the main switching element q Is off, the downward direction in FIG.
When the voltage of (n12 / n21) vo is induced and the induced current flows through the resistor r12, the charge of the capacitor c14 is extracted, and the capacitor c1 is discharged.
4 is charged to a polarity opposite to that shown in FIG. 6, and a reset operation for the next on operation of the main switching element q is performed.

After the main switching element q is turned off, 1
When the output of the excitation energy stored in the next main winding n11 to the secondary side ends, ringing occurs mainly between the parasitic capacitance c15 of the control winding n12 and the control winding n12. The voltage (n12 / n21) is applied to the parasitic capacitance c15.
The electrostatic energy stored in vo is released, is converted into the excitation energy of the control winding n12 after 1/4 cycle of the oscillation, and then charges the parasitic capacitance c15 again.
An upward electromotive voltage of the voltage (n12 / n21) vo is generated in the control winding n12. The electromotive voltage, which is a ringing pulse, is set to be equal to or higher than the threshold voltage of the main switching element q, and the main switching element q is turned on again by the electromotive voltage. Thus, the main switching element q is automatically turned on / off continuously at the switching frequency corresponding to the load, and the desired secondary output voltage is output.

In the configuration of the ordinary switching power supply of the RCC type as described above, the switching power supply 1 has a switching frequency at a light load when the equipment on which the switching power supply 1 is mounted is in a standby state. The following configuration is provided in order to reduce the problem.
From the device side, a control signal is given to a control terminal p5. A series circuit of the light emitting diode d14 of the photocoupler pc2 and the resistor r13 is connected between the control terminal p5 and the output power line 7 on the low level side.
Therefore, when the control signal goes high during a heavy load in the non-standby state, the light emitting diode d14 is turned on, and the primary side is output to indicate that the light load is in the heavy load state.

On the other hand, on the primary side, the control circuit 9 is provided with a phototransistor tr13 of the photocoupler pc2. When the load is heavy, the phototransistor tr13 is turned on, and the oscillation frequency suppressing operation of the control circuit 9 is performed. Are stopped, the ringing pulse is applied to the main switching element q, and the normal RCC operation as described above is performed. On the other hand, at the time of the light load, the control terminal P
5 becomes low level, the light emitting diode d14 is turned off, and the phototransistor tr13 is turned off.
Then, the oscillation frequency suppressing operation is performed, the control transistor tr12 remains on, the ringing pulse is bypassed, and after a predetermined time, the control transistor tr12 is turned off, and the control transistor tr12 is turned off from the start resistor r3 in the control circuit 9. The main switching element q is turned on by the pressure value.

In this manner, the oscillation frequency at the time of light load is reduced, and the power consumption required for extracting the electric charge accumulated in the parasitic capacitance between the drain and the source of the main switching element q, the iron loss of the transformer n, etc. And the power conversion efficiency is improved.

In the configuration disclosed in JP-A-9-47023, the resistor r12 is composed of a series circuit of a resistor and a zener diode and a resistor provided in parallel with the series circuit. As the power supply voltage, that is, the output voltage of the smoothing capacitor c11 increases, the current flowing through the Zener diode increases, thereby compensating for the change in the output voltage. Therefore, in the present specification, for simplification of the description, the main power supply voltage is fixed, and such a configuration is replaced with the resistor r12.

[0022]

In the switching power supply device 1 configured as described above, during the ON period of the main switching element q, the charge of the capacitor c14 having the opposite polarity to that shown in FIG. 6 Determined by the time for charging to 0.6V with the polarity shown.

However, the charging time is a relatively long time from when the main switching element q is turned on to when the charge having the opposite polarity is extracted and the battery is charged to the positive polarity when the load is heavy.

On the other hand, at light load, after the control transistor tr12 bypassing the ringing pulse is turned off, the divided voltage in the control circuit 9 rises from the starting resistor r3, and the main switching element q is turned on again. An on-drive time is long, that is, an operation pause period is provided to reduce the oscillation frequency. During the operation pause period, the charge of the opposite polarity in the capacitor c14 is generated by the resistance r.
12 and the control winding n12, and the charging time is shorter than at the time of heavy load.

Therefore, when the load is light, the current limit value of the overcurrent protection circuit is smaller than when the load is heavy, so that the energy stored in the transformer n is small, and
There is a problem that the switching frequency cannot be sufficiently reduced in order to supply the necessary power to the secondary side.

In this regard, by using high current rating for each component and increasing the current limit values in the two operation modes of light load and heavy load as a whole, the switching frequency can be improved as intended initially. Although it is possible to lower the cost, it is not desirable in terms of cost.

An object of the present invention is to provide a switching power supply device capable of sufficiently lowering a switching frequency at light load with an economical configuration that does not use excessively high ratings for each component. is there.

[0028]

According to a first aspect of the present invention, there is provided a switching power supply which stores excitation energy in a transformer during an on-period of a main switching element, and a voltage induced in a control winding of the transformer. The first capacitor is charged by the current obtained by the constant resistance and the feedback current from the secondary side, and when the charged voltage reaches a predetermined voltage, the first control switching element drives the control terminal of the main switching element off. During the off period, the excitation energy stored in the transformer is output to an output circuit on the secondary side, and a ringing pulse generated in a control winding of the transformer after the output is completed is output to a second DC cut-off circuit. A ringing choke converter that feeds back to the control terminal of the main switching element via a capacitor and turns on the main switching element again In the switching power supply unit of the type, the constant resistance is formed by two divided resistors, and a switching frequency switching means for reducing the switching frequency of the main switching device than the heavy load when the load is light,
The overcurrent protection circuit, which is composed of a series circuit of the divided resistor and the first capacitor and is connected in parallel to the control winding, is provided with a first circuit which is caused by a decrease in the switching frequency at the time of light load. Charge compensating means for compensating the decrease in charge of the capacitor by reducing the charging current to the first capacitor by lowering the connection point between the divided resistors to the Zener voltage and reducing the charging current to the first capacitor. I do.

According to the above configuration, a normal RC with a heavy load is used.
At the time of C operation, the first capacitor is charged to the opposite polarity by the back electromotive force generated in the control winding when the main switching element is turned off, whereas at a light load, the switching frequency is reduced by the switching frequency switching means. During the operation suspension period from when the main switching element is once turned on / off to when it is turned on again, the first switching element is turned off.
Since the charge of the opposite polarity of the capacitor decreases, the charge corresponding to the decrease is compensated for by the charge compensating means.

In the compensation method, the constant resistance is composed of two divided resistors, and a charge compensating means composed of a zener diode and, for example, a transistor lowers a connection point between the divided resistors to a zener voltage. This is performed by reducing the positive charging current supplied from the control winding to the first capacitor.

Therefore, the first capacitor is charged with the positive voltage induced in the control winding by the turning on of the main switching element, and the predetermined voltage of the positive polarity capable of driving the first control switching element on. The time until the voltage becomes approximately equal to that under heavy load even under light load. The time is the ON time of the main switching element, and corresponds to a current flowing through the main switching element, such as a triangular waveform, which increases as the ON time of the main switching element increases. Therefore, the overcurrent limit value of the main switching element can be made substantially equal between a light load and a heavy load, and the performance of each component can be brought to near the rated value even at a light load. As a result, the switching frequency at light load can be sufficiently reduced without unnecessarily increasing the rated value of each component.

When the power is turned on, when the power is turned off, and when the input voltage is lowered, the voltage of the positive polarity induced in the control winding is low, so that the Zener diode does not turn on.
The bypass of the charging current to the first capacitor as described above does not occur. Thus, the time from when the power is turned on or when the input voltage drops until the first control switching element can be turned on does not become undesirably long. To prevent the transformer from saturating or generating noise due to the flow of current.

Further, in the switching power supply according to the second invention, the charge compensating means further includes a third capacitor in series with the Zener diode.

According to the above configuration, the zener diode can be turned on by the third capacitor even when the power is turned on or when the input voltage drops, and the saturation of the first capacitor is prevented while preventing the saturation of the transformer. The charging current can be reduced.

Still further, the switching power supply according to the third aspect of the present invention is characterized in that the excitation energy is stored in the transformer while the main switching element is on, and the constant resistance is reduced from the voltage induced in the control winding of the transformer. And the feedback current from the secondary side charges the first capacitor. When the charged voltage reaches a predetermined voltage, the first control switching element drives the control terminal of the main switching element off, and the first control switching element turns off the control terminal. During the period, the exciting energy stored in the transformer is output to the output circuit on the secondary side, and the ringing pulse generated in the control winding of the transformer after the output is completed is passed through the second capacitor for DC cutoff. A ringing choke converter that returns to the control terminal of the main switching element to turn on the main switching element again. A switching frequency switching means for lowering the switching frequency of the main switching element at a light load than at a heavy load, and a series circuit of the constant resistor and the first capacitor, which are parallel to the control winding. Charge compensating means for compensating for an overcurrent protection circuit configured to be connected to the first circuit and compensating for a decrease in the charge of the first capacitor caused by a decrease in the switching frequency at the time of the light load. A zener diode interposed between the resistor and the first capacitor and a short circuit between terminals of the zener diode.
An operation switching means that can be opened, and a charge compensating means for compensating for a decrease in the charge by reducing an induced voltage of the control winding by a zener voltage of the zener diode to reduce a charge current. And characterized in that:

According to the above configuration, similarly to the first invention, in compensating for the decrease in the charge of the opposite polarity of the first capacitor at the time of light load, the constant resistance constituting the overcurrent protection circuit. A zener diode is interposed between the zener diode and the first capacitor, and the operation switching means short-circuits the terminals of the zener diode under heavy load and supplies the positive induced voltage as it is. By opening the gap, the induced voltage of the positive polarity is reduced and supplied by the Zener voltage of the Zener diode.

Therefore, similarly to the first aspect, the time required for the first capacitor to be charged to the predetermined voltage by the positive voltage induced in the control winding by turning on the main switching element is reduced. The load can be made substantially equal to that under heavy load, and the overcurrent limit value of the main switching element can be made almost equal.

Further, the switching power supply according to the fourth aspect of the present invention stores the exciting energy in the transformer during the ON period of the main switching element, and uses a constant resistance from the voltage induced in the control winding of the transformer. The first capacitor is charged by the obtained current and the feedback current from the secondary side, and when the charged voltage reaches a predetermined voltage, the first control switching element turns off the control terminal of the main switching element.
f, and during the off period, the excitation energy stored in the transformer is output to the output circuit on the secondary side, and a ringing pulse generated in the control winding of the transformer after the output is completed is cut off for DC cut. In a ringing choke converter type switching power supply device in which the main switching element is fed back to the control terminal of the main switching element via a second capacitor and the main switching element is turned on again, the main switching element has a lighter load than a heavy load. A switching frequency switching means for lowering a switching frequency of a switching element; and a series circuit of the constant resistance and the first capacitor, wherein the overcurrent protection circuit is connected in parallel to the control winding. Compensating for a decrease in the charge of the first capacitor due to a decrease in the switching frequency at a light load. Load compensating means, a first diode interposed in series with the constant resistance, the first diode being in a forward direction with respect to a current flowing from the control winding during an ON period of the main switching element; and A second series circuit interposed in parallel with a first series circuit consisting of a first diode and having a second diode having a polarity opposite to that of the first diode, a first resistor, and a Zener diode; Operation switching means interposed in parallel with the zener diode and capable of short-circuiting or opening the terminals of the zener diode during the light load and the heavy load, respectively.
By switching the operation switching means so that the induced current of the opposite polarity at the time of off of the main switching element flowing through the first capacitor through the second series circuit becomes larger at the time of light load than at the time of heavy load. And charge compensating means for compensating for the decrease in the charge.

According to the above arrangement, the current flowing into the first capacitor during the ON period of the main switching element, that is, the current flowing in the first capacitor with a positive polarity is in the forward direction. As described above, the first diode is interposed, and therefore, the first diode performs an interrupting operation with respect to a reverse polarity current generated when the main switching element is turned off. Therefore, in parallel with the first series circuit including the first diode and the constant resistance, the second diode and the first diode having polarities opposite to those of the first diode are connected in parallel.
And a second series circuit comprising a Zener diode and an operation switching means interposed in parallel with the Zener diode. The operation switching means is cut off at the time of heavy load, and the inflow of the reverse polarity charging current into the first capacitor is suppressed. At the time of light load, the operation switching means is turned on to supply a large amount of the reverse polarity charging current.

Therefore, when the load is light, a large amount of charge having the opposite polarity is accumulated, and even if the charge having the opposite polarity is discharged due to the operation suspension period, the amount of the discharged charge is small. It is compensated in advance, and the overcurrent limit value of the main switching element can be made substantially equal to that at the time of heavy load.

Still further, in the switching power supply according to the fifth invention, the switching frequency switching means includes a second resistor interposed in series between the first control switching element and a second capacitor. A series circuit interposed between a connection point between the second resistor and the second capacitor and a control winding, the series circuit including a third resistor and a second control switching element. The control switching element is driven on at a light load, and during the on period of the main switching element at the light load, charge is accumulated in the second capacitor by a voltage induced in the control winding, and the ringing pulse When this occurs, a reverse bias is generated by the charge of the second capacitor to prevent the main switching element from being driven on.

According to the above configuration, the second control switching element is off at the time of heavy load, and the ringing pulse is mainly transmitted via the second capacitor and the second resistor without the influence of the series circuit. The control signal is supplied to the control terminal of the switching element, and the main switching element is turned on, so that the switching operation is continuously performed.

On the other hand, at the time of light load, the second control switching element of the series circuit is turned on,
In the second capacitor, more current induced in the control winding flows, and electric charge is accumulated. At this time, the potential of the control terminal of the main switching element can be maintained by the current flowing from the control winding via the second resistor. When the main switching element is turned off to release the excitation energy and a ringing pulse is generated, the ringing pulse is reverse-biased by the charging voltage of the second capacitor, and the control of the main switching element is performed via the second resistor. The on-driving of the main switching element by the ringing pulse is prevented.

Therefore, once the main switching element performs a switching operation under a light load, the next switching operation is performed in the same manner as when the power is turned on. That is, when the potential of the control terminal of the main switching element gradually rises due to the start-up voltage obtained by resistance division of the main power supply voltage and the threshold voltage at which the main switching element is turned on, the main switching element is turned on. Turn on.

As described above, when the load is light, the restart of the main switching element due to the ringing pulse as in the case of the heavy load is stopped, and the restart is performed gently in the same manner as when the power is turned on. , The switching frequency can be reduced. This suppresses a loss that increases in proportion to the switching frequency, such as power required for extracting electric charge stored in the floating capacitance between the drain and source of the main switching element, and achieves high power conversion efficiency even at light load. be able to.

Further, such a decrease in the switching frequency at the time of a light load can be realized by a simple configuration of the series circuit including the third resistor and the second control switching element and the second resistor. , And can be realized with a low-cost configuration.

[0047]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The first embodiment of the present invention will be described below with reference to FIG.

FIG. 1 shows an RC according to a first embodiment of the present invention.
It is a block diagram of the switching power supply device 51 of C system. The basic configuration of the switching power supply device 51 uses the switching power supply device of Japanese Patent Application Laid-Open No. 9-47023 shown in FIG.

A DC current obtained by rectifying a commercial AC by a main power supply circuit (not shown) is input between input terminals P1 and P2. This DC current is smoothed by the smoothing capacitor C11.
The main power supply voltage is output between the main power supply line 12 on the high level side and the main power supply line 13 on the low level side.

A series circuit of a primary main winding N11 of a transformer N and a main switching element Q is connected between the main power supply lines 12 and 13. The main switching element Q
Is realized by, for example, a bipolar transistor or a field effect transistor. In the example of FIG. 1, the field effect transistor is shown. The main power supply line 12,
The control circuit 59 is also connected between the control circuits 13 via a starting resistor R3.

When the power is turned on, that is, when the power supply voltage is applied between the input terminals P1 and P2, the output voltage of the smoothing capacitor C11, that is, the main power supply voltage rises, and the voltage from the starting resistor R3 to the control circuit 59 is increased. When the divided voltage becomes equal to or higher than the threshold voltage Vth of the main switching element Q, for example, 3 V, the main switching element Q is turned on and the primary main winding N1 is turned on.
1, an upward voltage in FIG. 1 is applied, and the excitation energy is accumulated. As will be described later, when the main switching element Q is turned off, a back electromotive force is generated in the primary main winding N11 in the downward direction due to the stored excitation energy, thereby causing the secondary main winding N21 to move in the upward direction. Is induced.

The DC current induced in the secondary main winding N21 is passed through a diode D12 to a smoothing capacitor C13.
, And after being smoothed by the smoothing capacitor C13, the output terminals P3,
The signal is output from P4 to a load circuit (not shown). A voltage detection circuit 18 is interposed between the output power supply lines 16 and 17. The voltage detection circuit 18 includes a voltage-dividing resistor, a photocoupler PC1, and the like. The light-emitting diode D13 of the photocoupler PC1 is driven to emit light at a luminance corresponding to the output voltage. 1
Feedback to the next side.

When the main switching element Q is turned on, a voltage is induced in the control winding N12 in the same upward direction as the primary main winding N11, and the induced voltage is equal to the DC cut capacitor C1, the bias resistor R2, and the like. The signal is applied to the gate of the main switching element Q via the control circuit 59, whereby the gate potential of the main switching element Q is further raised, and the main switching element Q is maintained in the on state.

The voltage induced in the control winding N12 when the main switching element Q is turned on is controlled by the control circuit 5
9, the phototransistor TR of the photocoupler PC1
The capacitor C14 is supplied to one terminal of the capacitor C14 through the other terminal 11, and the other terminal of the capacitor C14 is connected to the low-level main power supply line 13. Therefore,
The capacitor C14 is charged with the positive polarity shown in FIG.
As the secondary side output voltage increases, the charging current increases,
The voltage between the terminals of the capacitor C14 rises quickly. The charging voltage of the capacitor C14 is supplied to the base of the control transistor TR12 interposed between the gate and the source of the main switching element Q via the control circuit 59, and the charging voltage is equal to the threshold voltage of the control transistor TR12. For example, when the voltage becomes 0.6 V or more, the control transistor TR12 is turned on, whereby the gate potential of the main switching element Q is rapidly reduced, and the main switching element Q is driven off.

Therefore, the higher the secondary output voltage, that is, the lighter the load, the faster the charging voltage of the capacitor C14 increases, and the faster the main switching element Q is turned off.
Driven. The voltage induced in the control winding N12 is also applied to the capacitor C14 by two divided resistors R51 and R51.
52. These split resistors R5
1, a series circuit of R52 and the capacitor C14 is connected in parallel with the control winding N12 to form an overcurrent protection circuit. With this overcurrent protection circuit, the output terminals P3,
Even if the output voltage of the secondary-side smoothing capacitor C13 is low due to a short circuit between P4 and the like, the ON period of the main switching element Q is limited to a predetermined period, and the main switching element Q is protected.

In the control winding N12, the numbers of turns of the control winding N12 and the secondary main winding N21 are designated by the same reference numerals, and when the secondary output voltage is Vo, the main switching element Q Is off, the downward direction in FIG.
The voltage of (N12 / N21) Vo is induced, and the induced current flows through the dividing resistors R51 and R52, whereby the charge of the capacitor C14 is extracted, and the polarity of the capacitor C14 is opposite to that shown in FIG. And the reset operation for the next on operation of the main switching element Q is performed.

After the main switching element Q is turned off, 1
When the output of the excitation energy stored in the secondary main winding N11 to the secondary side ends, ringing occurs mainly between the parasitic capacitance C15 of the control winding N12 and the control winding N12, and the ringing occurs. The voltage (N12 / N21) is applied to the parasitic capacitance C15.
The accumulated electrostatic energy at Vo is released, converted into the excitation energy of the control winding N12 after 1/4 cycle of the oscillation, and then charged again to charge the parasitic capacitance C15.
An upward electromotive voltage of the voltage (N12 / N21) Vo is generated in the control winding N12. The electromotive voltage, which is a ringing pulse, is set to be equal to or higher than the threshold voltage Vth of the main switching element Q, and the main switching element Q is turned on again by the electromotive voltage. Thus, the main switching element Q is automatically turned on / off continuously at the switching frequency corresponding to the load, and a desired secondary-side output voltage is output.

The switching power supply 51 is added to the configuration of the ordinary RCC type switching power supply as described above.
The following configuration is provided to reduce the switching frequency when the device on which the switching power supply device 51 is mounted is in a standby state and at a light load. From the device side, a control signal is given to a control terminal P5. A series circuit of the light emitting diode D14 and the resistor R13 of the photocoupler PC2 is connected between the control terminal P5 and the low-level output power supply line 17. Therefore, when the control signal becomes high level under heavy load, the light emitting diode D14 is turned on, and the heavy load state is output to the primary side.

On the other hand, on the primary side, the control circuit 59 is provided with a phototransistor TR13 of the photocoupler PC2, and the connection point P51 of the division resistors R51 and R52 is bypassed to the low-level side main power supply line 13. A zener diode ZD1 and a control transistor TR51 are provided. At the time of the heavy load, the phototransistor TR13 is turned on, the oscillation frequency suppressing operation of the control circuit 59 is stopped, and the ringing pulse is given to the main switching element Q, so that the normal RCC operation as described above is performed. With the control transistor TR
When 51 is turned off, the capacitor C14 is charged to the positive polarity shown in FIG. 1 with a relatively large charging current.

On the other hand, at the time of the light load, the control signal to the control terminal P5 becomes low level, the light emitting diode D14 is turned off, and the phototransistor TR13 is turned off.
ff, the oscillation frequency suppressing operation is performed, the control transistor TR12 remains on, the ringing pulse is bypassed, and after a predetermined time, the control transistor TR1 is turned off.
2 is turned off, and the main switching element Q is turned on by the divided voltage in the control circuit 59 from the starting resistor R3.

At the time of this light load, the control transistor TR51 is also turned on to limit the potential of the connection point P51 of the divided resistors R51 and R52 to the Zener voltage Vzd1 of the Zener diode ZD1, so that the charging current to the capacitor C14 is reduced. Is shunted, and the capacitor C14 is charged to the positive polarity shown in FIG. 1 with a relatively small charging current.

Accordingly, as described above, the operation quiescent period occurs in the main switching element Q due to the bypass of the ringing pulse, and the discharge of the charge of the capacitor C14 proceeds, and it should remain in the capacitor C14 at the off timing. Even if the charge of the opposite polarity to that shown in FIG. 1 is reduced, the decrease in the charge is bypassed by shunting the positive charging current shown in FIG. 1 by the Zener diode ZD1 and the control transistor TR51 when on. It is compensated by suppressing the charging current.

Therefore, the main switching element Q is turned on.
The capacitor C14 is charged with the positive voltage induced in the control winding N12 by the control transistor TR12.
The time until the threshold voltage of the positive polarity, which enables the on-driving, becomes substantially equal to that at the time of heavy load even under light load. The time is the ON time of the main switching element Q, and the ON time of the main switching element Q, such as a triangular wave shape.
the main switching element Q that increases with the increase in n time
Corresponding to the current flowing through. Therefore, the overcurrent limit value of the main switching element can be made substantially equal between a light load and a heavy load.

In this manner, the power conversion efficiency is improved by reducing the power consumption required for extracting the electric charge accumulated in the parasitic capacitance between the drain and source of the main switching element Q and the core loss of the transformer N. In order to reduce the oscillation frequency at light load, the capacity of each component can be brought up to near the rated value even at light load, and it is not necessary to use parts with large rated values unnecessarily. It can be realized at low cost.

When the power is turned on, when the power is turned off, or when the input voltage is lowered, the voltage of the ringing pulse is low. Therefore, the Zener diode ZD1 is not turned on, and the bypass of the charging current to the capacitor C14 as described above is not performed. Does not occur. As a result, the time from when the power is turned on or when the input voltage drops until the control transistor TR12 can be turned on does not become undesirably long, and a large current flows through the primary main winding N11. This can prevent the transformer N from saturating and generating noise.

FIG. 2 shows a second embodiment of the present invention.
It is as follows if it explains based on.

FIG. 2 is an electric circuit diagram of a switching power supply 52 according to a second embodiment of the present invention. The basic configuration of the switching power supply device 52 is the one proposed by the present applicant in Japanese Patent Application No. 10-42654, and the configuration for suppressing the charging current to the capacitor C14 at light load is as follows. It is the same as the switching power supply device 51 described above, and the corresponding portions are denoted by the same reference numerals,
The description is omitted.

An activation circuit 14 including a capacitor C2, voltage dividing resistors R3 to R5, and a diode D4 is connected between the main power supply lines 12 and 13. When power is turned on, that is, when a power supply voltage is applied between the input terminals P1 and P2,
The output voltage of the smoothing capacitor C11, that is, the main power supply voltage increases, and the voltage dividing resistors R4, R
5 is equal to or higher than the threshold voltage Vth of the main switching element Q, the main switching element Q
n drives.

A snubber circuit 15 composed of a series circuit of a resistor R11 and a capacitor C12 is provided in parallel between the drain and the source of the main switching element Q. This snubber circuit 15 absorbs and removes vibration generated by leakage inductance between the primary main winding N11 and the other windings N12 and N21 when the main switching element Q is turned off.

When the main switching element Q is turned on, the current induced in the control winding N12 is supplied to the gate of the main switching element Q via the DC cut capacitor C1 and the bias resistor R2. The current induced in the control winding N12 when the main switching element Q is turned on is equal to the capacitor C1 and the bias resistance R2.
From the phototransistor T of the photocoupler PC1.
The signal is supplied to one terminal of the capacitor C14 via R11. The charging voltage of the capacitor C14 is given to the base of the control transistor TR12 interposed between the gate and the source of the main switching element Q.

A diode D11 for preventing backflow, a Zener diode ZD2, a resistor R1, a control transistor, and the like are provided between a connection point P6 between the capacitor C1 and the bias resistor R2 and the main power supply line 13 on the low level side. TR1
Are connected in series. A connection point P52 between the control transistor TR1 having the function of the control transistor TR51 and the resistor R1 is connected to the Zener diode ZD.
1 through a connection point P5 of the divided resistors R51 and R52.
1 connected.

The power supply voltage from the sub power supply circuit 19, which is a voltage corresponding to the secondary output current value as described later, is applied to the base of the control transistor TR1 via the resistor R14. A transistor TR15 is interposed between the base of the control transistor TR1 and the main power supply line 13 on the low level side.
The power supply voltage from the sub power supply circuit 19 is applied to the base of R15 via the resistor R17 and the zener diode ZD3.

That is, the base of the control transistor TR1 is connected to the phototransistor T of the photocoupler PC2.
Instead of R13, it is driven by the transistor TR15, the secondary side load becomes heavy, the power supply voltage from the sub power supply circuit 19 becomes high, and the Zener diode Z
When the voltage becomes equal to or higher than the zener voltage Vzd3 of D3, a current flows to the base of the transistor TR15,
5 turns on. Thereby, the control transistor TR1
Of the control transistor TR becomes low level.
1 is off, and the operation is performed in the heavy load operation mode.

On the other hand, when the charge on the secondary side becomes lighter and the charging voltage becomes lower than the zener voltage Vzd2, the base current of the transistor TR15 becomes zero and the transistor TR15 is turned off, thereby turning off the control transistor TR1. Is biased by the resistor R14, the control transistor TR1 is turned on, and the operation in the light load operation mode can be performed.

In this way, the load can be determined only on the primary side and the control transistor TR1 can be automatically controlled. Therefore, the operation mode for detecting the operation mode of the mounted device such as the control terminal P5 can be detected. It is not necessary to provide a special configuration, and cost can be reduced.

On the other hand, the sub-power supply circuit 19 includes a smoothing capacitor C16, two diodes D2 and D3, and a choke coil L. Diode D2
Extracts the induced current from one terminal of the control winding N12 while the main switching element Q is on, and charges the smoothing capacitor C16 via the choke coil L.
The flywheel diode D3 connects the connection point P10 between the choke coil L and the diode D2 to the control winding N1.
2 is connected to the other terminal. Therefore, when the main switching element Q is turned off and the polarity direction of the induced voltage of the control winding N12 is reversed, the diode D2 is turned off, and the exciting current in the choke coil L passes through the smoothing capacitor C16 via the flywheel diode D3. Charge. The inductance of the choke coil L is selected such that the exciting current becomes zero by the next turn-on of the main switching element under heavy load.

Thus, the smoothing capacitor C16
Is charged to a voltage corresponding to the secondary-side output current value as described above, and on / off driving of the control transistor TR1 becomes possible based on the output voltage of the smoothing capacitor C16.

In the switching power supply 52 constructed as described above, the transistor T
R15 is turned on, the control transistor TR1 is turned off, and the backflow prevention diode D11, the zener diode ZD2, the resistor R1, and the control transistor TR1 are turned off.
The normal RCC operation as described above is performed without being affected by the series circuit of the RCC.

On the other hand, at the time of the light load, the transistor TR15 is turned off, and the control transistor TR1 is turned off.
Is turned on, and the series circuit is connected between the connection point P6 and the main power supply line 13. The resistance value of the bias resistor R2 is selected to be, for example, 680Ω, while the resistance value of the resistor R1 is selected to be, for example, 150Ω. Therefore, when the main switching element Q is turned on, a large amount of current flows through the series circuit while the on state of the main switching element Q is maintained.
, The electric charge is accumulated with the control winding N12 side as +.

Therefore, even if the ringing pulse is generated at light load, the ringing pulse is reverse-biased by the voltage between the terminals of the capacitor C1 and applied to the main switching element Q. On startup is blocked. As a result, the switching power supply device 52 is substantially the same as the above-described conventional switching power supply device 1 except for the Zener diode ZD2 and the diode D11 for backflow prevention, which are not essential components for the reasons described later. By simply adding a simple configuration of the resistor R1 and the control transistor TR1, the switching frequency of, for example, about 80 kHz under heavy load increases to 400 to 500 kHz in the switching power supply 1 under light load. On the other hand, the power conversion efficiency can be reduced to about several kHz, and the power conversion efficiency under light load can be greatly increased. Further, in the switching power supply device 51 described above, the configuration in the control circuit 59 is relatively complicated. On the other hand, as described above, the switching power supply device 52 substantially includes the resistor R1 and the control transistor TR1. , And the control transistor TR1 has the function of the control transistor TR51, and is realized by an extremely simple configuration.

The power supply voltage from the sub power supply circuit 19 is also output to the connection point P7 of the voltage dividing resistors R3 and R4 of the starting circuit 14 via the diode D1 for preventing backflow. Correspondingly, the capacitor C2 is provided in the starting circuit 14.

Therefore, when the power supply is turned on in which the voltage between the terminals of the capacitor C2 is almost zero, the divided voltage of the main power supply voltage of, for example, several hundred volts by the voltage dividing resistors R3 to R5 is applied to the gate of the main switching element Q. Will be given.

On the other hand, when a predetermined time elapses after the power is turned on, the smoothing capacitor C16 is charged to a predetermined power supply voltage, for example, about 10 V, and the capacitor C2 is connected to the main power supply voltage and the output of the sub power supply circuit 19. The battery is charged to a voltage substantially corresponding to the difference from the voltage. Accordingly, as described above, even if the main switching element Q is not turned on by the ringing pulse due to the light load state and the voltage for the on-start is output from the starting circuit 14, the voltage dividing resistor It is possible to prevent the current from flowing from the main power supply to R3 to R5, and to drive the main switching element Q with the divided voltage of the output voltage of the sub power supply circuit 19 having a relatively low voltage. Thereby, the voltage dividing resistors R3 to R
5 can also be reduced, and the efficiency can be further improved.

The capacitor C2 and the voltage dividing resistor R
3 and the main power supply line 13 on the low level side.
Is provided in parallel with the voltage dividing resistors R3 to R5 and in a reverse bias direction. Therefore, when the main power supply voltage decreases, the smoothing capacitor C11-main power supply line 13-voltage dividing resistor R5
~ R3-Capacitor C2-Main power supply line 12-Smoothing capacitor C11-
Main power supply line 13-diode D4-capacitor C2-
A path from the main power supply line 12 to the smoothing capacitor C11 forms a discharge path for the capacitor C2. by this,
Even if the time from power-off to power-on is short, the capacitor C2 can be reliably discharged, the potential at the connection point P8 can be raised substantially equal to the main power supply voltage, and the main switching element Q can be reliably started. it can.

Here, the resistance values of the voltage dividing resistors R3 to R5 can be determined as follows. When the input voltage to the input terminals P1 and P2 is Vin, the output voltage of the smoothing capacitor C16, that is, the output voltage of the sub power supply circuit 19 is Vs, and the resistance values of the voltage dividing resistors R3 to R5 are indicated by the same reference numerals, respectively, At the start of operation Vin × [R5 / (R3 + R4 + R5)]> Vth (1) During steady operation under light load condition Vs × [R5 / (R4 + R5)]> Vth (2) The Zener diode ZD2 is light When the control transistor TR1 is turned on at the time of load, the resistance value of the voltage dividing resistor R5 in Equations (1) and (2) is equal to the value of the series circuit of the bias resistor R2 and the resistor R1 and the value of the parallel circuit of the voltage dividing resistor R5. This is a compensating Zener diode that is provided to prevent the above equations 1 and 2 from being satisfied. Therefore, the Zener voltage Vzd2 is selected to be equal to or higher than the threshold voltage Vth and equal to or lower than the induced voltage of the control winding N12 when the main switching element Q is turned on. However, due to design specifications,
If the above equations 1 and 2 can be satisfied even at a light load, the Zener diode ZD2 may be deleted, that is, the diode D11 and the resistor R1 may be short-circuited.

Further, a diode D1 for preventing backflow is provided.
1 is the off state of the main switching element Q at light load
During the period, a current flows to the connection point P6 through the path of the control transistor TR1, the resistor R1, and the zener diode ZD2, and is provided to prevent a negative bias to the gate of the main switching element Q from being released.

Therefore, for example, the supply of the base current to the control transistor TR1 is stopped during periods other than the ON period of the main switching element Q, and the operation of extracting the base current during the remaining OFF period is performed. If the current supply circuit can reliably prevent the current from flowing through the path during the off period, the diode D11 for preventing backflow can be omitted.

FIG. 3 shows a third embodiment of the present invention.
It is as follows if it explains based on.

FIG. 3 is a block diagram of a switching power supply 61 according to a third embodiment of the present invention. The switching power supply 61 is the same as the switching power supply 5 described above.
Similar to the first embodiment, corresponding parts are denoted by the same reference numerals, and description thereof will be omitted. This switching power supply 6
1, a resistor R equal to the combined resistance value of these divided resistors R51 and R52 is used instead of the divided resistors R51 and R52.
12 is used, and a zener diode ZD1 is interposed in series with the resistor R12. The control transistor TR51 is provided in parallel with the Zener diode ZD1.

Therefore, the transistor TR51 is turned on by the control circuit 59 at the time of heavy load, and the terminals of the Zener diode ZD1 are short-circuited. As a result, the induced current in the control winding N12 is reduced by a relatively small resistance value to the capacitor C14. On the other hand, when the load is light, it is turned off to open the terminals of the Zener diode ZD1, and the capacitor C14 has a relatively low voltage obtained by subtracting the Zener voltage Vzd1 from the induced voltage in the control winding N12. , Thus being charged with less current.

Also in this case, when the load is light, the capacitor C1 due to the operation suspension period of the main switching element Q is not used.
4, the overcurrent limit value of the main switching element Q can be made substantially equal between a light load state and a heavy load state by compensating for the discharge of the charge having the opposite polarity of 4.

It goes without saying that the Zener diode ZD1 and the control transistor TR51 and the resistor R12 may be interchanged. Also, when the light load power is turned on, the induced voltage in the control winding N12 is small, the Zener diode ZD1 does not turn on, and the capacitor C
Since the capacitor 14 is charged only with the secondary side feedback current via the photocoupler PC1, the capacitance of the capacitor C14 is selected so that the transformer N does not saturate in such a situation. Once the main switching element Q is turned on / off, the induced voltage of the control winding N12 becomes a predetermined value, and the Zener diode ZD1 turns on.

FIG. 4 shows a fourth embodiment of the present invention.
It is as follows if it explains based on.

FIG. 4 is a block diagram of a switching power supply 71 according to a fourth embodiment of the present invention. The switching power supply 71 is the same as the switching power supply 5 described above.
Similar parts to those in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted. This switching power supply 7
1, another capacitor C17 is provided in parallel with the capacitor C14, and the control transistor TR
51 and the Zener diode ZD1.

The control circuit 59 turns off the control transistor TR51 at the time of heavy load to make the capacity of only the capacitor C14 effective, and at the time of light load, the control transistor TR5
1 when the potential of the connection point P51 reaches the Zener voltage Vzd1, the capacitor C17 is charged,
The charging speed of the capacitor C14 can be reduced.
Also in this case, the overcurrent limit value of the main switching element Q can be made substantially equal between a light load and a heavy load.

In the switching power supply device 51, as described above, the zener diode ZD1 does not turn on when the power is turned on or the input voltage drops, and the charging current does not bypass the capacitor C14. If C17 is provided, the Zener diode ZD1 is set to o
The charging current of the capacitor C14 can be reduced while preventing the transformer N from being saturated.

FIG. 5 shows a fifth embodiment of the present invention.
It is as follows if it explains based on.

FIG. 5 is a block diagram of a switching power supply 81 according to a fifth embodiment of the present invention. The basic configuration of the switching power supply 81 is similar to the switching power supply 51 described above. However, the switching power supply 5
Each of 2, 61, and 71 has an overcurrent of the main switching element Q between a light load and a heavy load depending on the amount of charging of the capacitor C14 by the induced voltage of the control winding N12 when the main switching element Q is on. While the limit values are equalized, this switching power supply 81
The overcurrent limit value is equalized by adjusting the reverse polarity charging current to the capacitor C14 when the main switching element Q is turned off.

Therefore, a diode D21 is connected to the capacitor C14 in series with the control winding N12 so as to be in a forward direction with respect to the positive charging current when the main switching element Q is on. Is done. In parallel with the series circuit of the diode D21 and the resistor R12, a diode D22, a resistor R21 and the Zener diode ZD are connected.
1 is connected. Zener diode ZD1
Is provided in parallel with the control transistor TR51, and the diode D22 has a polarity opposite to that of the diode D21, and is in a forward direction with respect to a charging current having the opposite polarity when the main switching element Q is on.

However, the control transistor TR51 has an emitter connected to the control winding N12, that is, a diode D22.
And the collector is connected to the capacitor C14.
Connected to the side. Note that the resistor R21, the zener diode ZD1, and the control transistor TR51 may be interchanged.

Therefore, the main switching element Q is turned on.
At times, the induced voltage of the control winding N12 charges the capacitor C14 to a positive polarity through the diode D21 as usual. On the other hand, when the main switching element Q is off, the control transistor TR51 turns off when the load is light.
n, the induced voltage of the opposite polarity in the control winding N12 is reduced by the relatively small resistance value of the resistor R21.
In contrast, when the load is heavy, the control transistor TR51 is turned off, and the induced voltage of the opposite polarity is reduced by the zener voltage Vzd1 of the zener diode ZD1 to charge the capacitor C14 with the opposite polarity. .

Therefore, the capacitor C14 is charged with a larger amount of charge of the opposite polarity at the time of light load than at the time of heavy load, and the discharge amount due to the operation suspension period of the main switching element Q as described above is reduced. The increase can be compensated for in advance. Also in this case, the overcurrent limit value of the main switching element Q can be made substantially equal between a light load and a heavy load.

The switching power supplies 61, 71,
It goes without saying that the configuration of 81 can also be applied to the configuration of the switching power supply device 52.

[0104]

As described above, the switching power supply according to the first aspect of the present invention provides a current obtained by a constant resistance from the voltage induced in the control winding of the transformer by turning on the main switching element, and from the secondary side. Of the first by the feedback current of
When the charged voltage reaches a predetermined voltage, the control terminal of the main switching element is off-driven via the first control switching element, and the main switching element is turned on again by the ringing pulse generated thereby. In the switching power supply device of the ringing choke converter system described above, when the switching frequency switching means lowers the switching frequency of the main switching element at a light load than at a heavy load, the two divided resistors forming the constant resistance and A first capacitor is connected to an overcurrent protection circuit, which is composed of a series circuit of capacitors and connected in parallel to the control winding, by a back electromotive force generated in the control winding when the main switching element is turned off. Charge of the opposite polarity At the time of loading, the charge is reduced during the operation suspension period, and the charge is reduced by a charge compensating means composed of a zener diode and a transistor, for example. And compensates by reducing the positive charging current supplied from the control winding to the first capacitor.

Therefore, the first capacitor is charged with the positive voltage induced in the control winding by the turning on of the main switching element, and the first control switching element is turned off.
The time until the predetermined voltage of the positive polarity that can be driven n can be almost equal to that at the time of heavy load even under light load, and the overcurrent limit value of the main switching element is longer than that at the time of light load. The switching frequency at light load can be sufficiently reduced without making the rated values of the components substantially equal to those under load.

When the power is turned on, when the power is turned off, and when the input voltage is lowered, the voltage of the positive polarity induced in the control winding is low, so that the Zener diode does not turn on.
The bypass of the charging current to the first capacitor as described above does not occur. Thus, the time from when the power is turned on or when the input voltage drops until the first control switching element can be turned on does not become undesirably long. To prevent the transformer from saturating or generating noise due to the flow of current.

Further, the switching power supply according to the second aspect of the present invention includes the third capacitor in series with the Zener diode in the charge compensating means as described above.

Therefore, the zener diode can be turned on by the third capacitor even when the power is turned on or the input voltage drops, and the charging current of the first capacitor is reduced while preventing the transformer from being saturated. Can be done.

Further, as described above, the switching power supply according to the third invention compensates for the decrease in the charge of the first capacitor having the opposite polarity at the time of light load, similarly to the first invention. In doing so, a zener diode is interposed between the constant resistance and the first capacitor that constitute the overcurrent protection circuit, and under heavy load, the terminals of the zener diode are short-circuited and the positive induced voltage is supplied as it is. When the load is light, the terminals of the Zener diode are opened, and the induced voltage of the positive polarity is reduced and supplied by the Zener voltage of the Zener diode.

Therefore, similarly to the first invention, the time required for the first capacitor to be charged to the predetermined voltage by the positive voltage induced in the control winding by the turning on of the main switching element is defined as: Even under a light load, it can be made almost equal to that under a heavy load, and the overcurrent limit value of the main switching element can be made almost equal.

Further, as described above, the switching power supply according to the fourth aspect of the invention compensates for the decrease in the charge of the first capacitor having the opposite polarity at the time of light load, as in the first aspect of the invention. In doing so, a first series circuit comprising a first diode and a constant resistor is interposed so as to be in a forward direction with respect to a current for charging the first capacitor with a positive polarity,
With respect to the current having the opposite polarity, a second series circuit including a second diode, a first resistor, and a Zener diode is provided. When a heavy load is applied, the terminals of the Zener diode are cut off to supply a reverse current to the first capacitor. The inflow of the charging current of the polarity is suppressed, and at the time of a light load, the conduction is performed to supply a large amount of the charging current of the opposite polarity.

Therefore, when the load is light, a large amount of the charge of the opposite polarity is accumulated, and even if the charge of the opposite polarity is discharged due to the operation suspension period, the charge corresponding to the discharge is reduced. Is compensated in advance, and the overcurrent limit value of the main switching element can be made substantially equal to that at the time of heavy load.

As described above, the switching power supply according to the fifth aspect of the present invention supplies a ringing pulse to the control terminal of the main switching element via the second capacitor and the second resistor when the load is heavy. Perform normal switching operation to drive the switching element on,
At a light load, electric charge is accumulated in the second capacitor by a voltage induced in the control winding when the main switching element is turned on, the main switching element is turned off, and the emission of the excitation energy ends, and the ringing pulse is generated. Even if it occurs, the ringing pulse is reverse-biased by the charging voltage of the second capacitor, and o
Block n drive.

Therefore, when the load is light, the restart of the main switching element due to the ringing pulse as in the case of the heavy load is stopped, and once the main switching element performs the switching operation at the light load, the next switching operation is performed by the power supply. As in the case of closing, the switching is performed gently, and the switching frequency of the main switching element can be reduced.
As a result, it is possible to suppress a loss that increases in proportion to the switching frequency, such as power required for extracting the electric charge accumulated in the floating capacitance between the drain and the source, and to obtain high power conversion efficiency even at a light load.

Further, such a decrease in the switching frequency at the time of light load can be reduced by using a second resistor for increasing the impedance between the second capacitor and the main switching element for charging, It can be realized with a simple configuration with a series circuit including a third resistor connecting the connection point with the second resistor to the main power supply line and a second control switching element, and can be realized with a low-cost configuration. Can be.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating an electrical configuration of an RCC switching power supply device according to a first embodiment of the present invention.

FIG. 2 is an electric circuit diagram of a switching power supply according to a second embodiment of the present invention.

FIG. 3 is a block diagram showing an electrical configuration of a switching power supply according to a third embodiment of the present invention.

FIG. 4 is a block diagram showing an electrical configuration of a switching power supply according to a fourth embodiment of the present invention.

FIG. 5 is a block diagram showing an electrical configuration of a switching power supply according to a fifth embodiment of the present invention.

FIG. 6 is a simplified block diagram showing a typical conventional switching power supply of the RCC system.

[Explanation of symbols]

51, 52; 61, 71, 81 Switching power supply 12, 13 Main power supply line 14 Starter circuit 15 Snubber circuit 16, 17 Output power supply line 18 Voltage detection circuit 19 Sub-power supply circuit 59 Control circuit (switching frequency switching means) C1 capacitor ( C2, C12 Capacitor C11, C13, C16 Smoothing capacitor C14 Capacitor (first capacitor) C15 Parasitic capacitance C17 Capacitor (third capacitor) D1, D2, D4, D11, D12, D16 Diode D3 Flywheel Diode D13, D14 Light emitting diode D21 Diode (first diode) D22 Diode (second diode) L Choke coil N Transformer N11 Primary main winding N12 Control winding N21 Secondary main winding PC1, PC Photocoupler Q Main switching element R1 Resistance (third resistance) R2 Bias resistance (second resistance) R3, R4, R5 Voltage dividing resistance R11, R13, R14, R17 Resistance R12 Constant resistance R51, R52 Division resistance R21 Resistance ( TR1 Control transistor (second control switching element) TR11, TR13 Phototransistor TR12 Control transistor (first control switching element) TR15 Transistor TR51 Control transistor (charge compensation means, operation switching means) ZD1 Zener diode ( Charge compensation means) ZD2, ZD3 Zener diode

Claims (5)

[Claims] 1. A current obtained by a constant resistance from a voltage induced in a control winding of a transformer and a feedback current from a secondary side while accumulating excitation energy in a transformer during an ON period of a main switching element. When the charging voltage reaches a predetermined voltage, the first control switching element turns off the control terminal of the main switching element, and during the off period, the first control switching element is stored in the transformer. Outputting the excitation energy to the output circuit on the secondary side,
A ringing pulse generated in the control winding of the transformer after the output ends is fed back to the control terminal of the main switching element via a second DC cut capacitor, and the main switching element is turned on again. A switching power supply device of a choke converter type, wherein the constant resistance is formed by two divided resistors, a switching frequency switching means for lowering a switching frequency of the main switching element at a light load than at a heavy load; A charge of the first capacitor caused by a decrease in the switching frequency at the time of the light load is supplied to an overcurrent protection circuit which is configured by a series circuit of one capacitor and connected in parallel with the control winding. Is reduced to the Zener voltage at the connection point between the divided resistors, and the first capacitor Switching power supply apparatus characterized by comprising a charge compensation means for compensating by decreasing the charging current to the support. 2. The switching power supply device according to claim 1, wherein said charge compensating means further includes a third capacitor in series with the Zener diode. 3. A current obtained by a constant resistance from a voltage induced in a control winding of a transformer and a feedback current from a secondary side, while storing excitation energy in a transformer during an ON period of a main switching element. When the charging voltage reaches a predetermined voltage, the first control switching element turns off the control terminal of the main switching element, and during the off period, the first control switching element is stored in the transformer. Outputting the excitation energy to the output circuit on the secondary side,
A ringing pulse generated in the control winding of the transformer after the output ends is fed back to the control terminal of the main switching element via a second DC cut capacitor, and the main switching element is turned on again. A switching power supply unit of a choke converter type, comprising: a switching frequency switching means for lowering a switching frequency of the main switching element at a light load than at a heavy load; and a series circuit of the constant resistance and the first capacitor; Charge compensating means for compensating an overcurrent protection circuit connected in parallel to a line for a decrease in the charge of the first capacitor caused by a decrease in the switching frequency at the time of the light load. A Zener diode interposed between the constant resistance and the first capacitor; Operation switching means for short-circuiting / opening the terminals of the Zener diode, and reducing the charging current by lowering the induced voltage of the control winding by the Zener voltage of the Zener diode to reduce the charging current. And a charge compensating means for compensating for the component. 4. A current obtained by a constant resistance from a voltage induced in a control winding of the transformer, and a feedback current from a secondary side while accumulating excitation energy in the transformer while the main switching element is on. When the charging voltage reaches a predetermined voltage, the first control switching element turns off the control terminal of the main switching element, and during the off period, the first control switching element is stored in the transformer. Outputting the excitation energy to the output circuit on the secondary side,
A ringing pulse generated in the control winding of the transformer after the output ends is fed back to the control terminal of the main switching element via a second DC cut capacitor, and the main switching element is turned on again. A switching power supply unit of a choke converter type, comprising: a switching frequency switching means for lowering a switching frequency of the main switching element at a light load than at a heavy load; and a series circuit of the constant resistance and the first capacitor; Charge compensation means for compensating an overcurrent protection circuit configured in parallel with a line for a decrease in the charge of the first capacitor due to a decrease in the switching frequency at the time of the light load. The control is interposed in series with the constant resistance, and the control is performed during the ON period of the main switching element. A first diode that is forward with respect to the current flowing from the line, and a first series circuit that includes the constant resistance and the first diode, which is interposed in parallel with the first diode, and has a polarity opposite to that of the first diode. A second series circuit comprising a second diode, a first resistor, and a zener diode; a second series circuit interposed in parallel with the zener diode, for short-circuiting the terminals of the zener diode under the light load and the heavy load, respectively. Or an operation switching means that can be opened, wherein the induced current of the opposite polarity at the time of off of the main switching element flowing to the first capacitor through the second series circuit is compared with that at the time of light load at the time of heavy load. And a charge compensating means for compensating for the decrease in the charge by switching the operation switching means so as to increase the switching power supply. 5. The switching frequency switching means, comprising: a second resistor interposed in series between the first control switching element and a second capacitor; and a second resistor and a second capacitor. And a series circuit interposed between the control winding and a control winding, the series circuit including a third resistor and a second control switching element, wherein the second control switching element is turned on at a light load, During the ON period of the main switching element at light load, the voltage induced in the control winding causes the second
5. A charge is accumulated in said capacitor, and when said ringing pulse is generated, a reverse bias is generated by the charge of said second capacitor to prevent on-drive of said main switching element. The switching power supply device according to any one of the above.