JPH10191629A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve efficiency at the time of a light load in a switching power supply which makes possible partial resonance operation by regeneration at the secondary side. SOLUTION: A regenerative switching element 6 connected in series to the secondary winding 4b of a transformer 4 keeps the gate connected to both a secondary side drive circuit 16 and an operation mode signal outputting circuit 17. For a rated load, the high level signal outputted by the operation mode signal outputting circuit 17 turns on the regenerative switching element 6 in accordance with the instruction of the secondary side drive circuit 16 before a main switching element 5 is switched on. Thereby, the transformer 4 is reverse-excited and a switching power supply 1 performs partial resonance operation by regeneration at the secondary side. When a load 3 is light, on the other hand, the operation mode signal outputting circuit 17 outputs a low level operation mode signal. As a result, because the 'on' state of the regenerative switching element 6 is restricted, a loss arising from a regenerative current is suppressed irrespective of the output of the secondary side drive circuit 16.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply for intermittently supplying input DC power to a load side, and more particularly to a switching power supply capable of performing partial resonance by secondary-side regeneration.

[0002]

2. Description of the Related Art In recent years, various electronic devices such as a personal computer, a video device and a small OA (Office Automation) device have been miniaturized. Switching power supplies are widely used because they are easy to achieve high efficiency.

As shown in FIG. 13, in a conventional switching power supply 101, an input side voltage source 102 is, for example,
DC input voltage V _i by rectifying and smoothing AC power
And one of the transformers 104 connected in series to each other.
It is applied to the secondary winding 104a and the main switching element 105. The main switching element 105 is turned on / off at a high frequency in accordance with an instruction from the primary side control circuit 108. After this high-frequency voltage is transmitted to the secondary winding 104b, the diode 106b and the smoothing capacitor 107
And the load 103 as a DC voltage V _O
Output to

The primary side control circuit 108 performs fixed frequency pulse width control (PWM) based on, for example, a divided voltage of the output voltage V _O , and turns on and off the conduction period of the main switching element 105. Adjust the period and percentage. As a result, the switching power supply 101 can stably supply a constant DC voltage V _O to the load 103. In the following, in distinction from a partial resonance circuit system based on secondary-side regeneration described below,
This method is called a PWM method.

By the way, in the switching power supply 101,
As the switching frequency of the main switching element 105 increases, the size of the transformer 104 and the smoothing capacitor 107 can be reduced. Therefore, in order to reduce the size of the switching power supply 101, further improvement of the switching frequency is required. However, a MOS (Metal Oxide Semiconduct) usually used as the main switching element 105 is used.
or) type field effect transistor (FET), the ideal FET 105a that actually operates as a switch
, A diode 105b and a parasitic capacitance 105c are parasitic. Therefore, the main switching element 105 is off, the parasitic capacitance 105c, the voltage of the sum of the input voltage V _i and the flyback voltage is applied, charges are accumulated. The flyback voltage is determined by the output voltage V _O and the transformer turns ratio of the transformer 104. When the main switching element 105 is turned on in this state, the charge accumulated in the parasitic capacitance 105c is short-circuited by the FET 105a. As a result, heat loss of the main switching element 105 and generation of noise are caused,
This may cause a rise in temperature and a decrease in efficiency. Since the loss at the time of switching increases in proportion to the switching frequency, the improvement of the switching frequency is limited, which is a factor that hinders the miniaturization of the switching power supply 101.

Therefore, conventionally, a method has been used in which a regenerative switching element 106 is provided on the secondary side of the transformer 104, and the main switching element 105 and the regenerative switching element 106 are alternately turned on. Thus, before the main switching element 105 is turned on, the charge stored in the parasitic capacitance 105c can be extracted by resonance.
As a result, even when the switching frequency is high, generation of heat loss and noise can be prevented. Hereinafter, this method is referred to as a partial resonance circuit method using secondary-side regeneration. In order to distinguish the FET 105a of the main switching element 105 from the FET 106a of the regenerative switching element 106, each of them is connected to the main FET 105a or the regenerative F
Called ET106a.

Here, the operation of the switching power supply 101 of the partial resonance circuit type by the secondary side regeneration will be briefly described. As shown in FIG. 14, the main switching element 105
Is on and the regenerative switching element 106 is off (t
In the period from a to tb), one of the transformers 104
Primary current I ₁ flowing through the next winding 104a is linearly increased by the input voltage V _i, storing excitation energy in the transformer 104.

At time tb, the main switching element 105 is turned off. Thereby, the main FET 105
drain of a - source voltage, i.e., the switching voltages V ₁ rises (time from tb to tc). As a result, conducting the secondary side of the diode 106b is energized energy of the transformer 104, (the period from tc to td) for emitted as secondary current I ₂ flowing to the output terminal OUT. Regenerative switching element 106 for a period of up to the excitation energy is released, i.e., the secondary current I ₂ is turned to the period until 0.

The discharge of the excitation energy of the transformer 104 is as follows.
It ends at time td. However, since the regenerative switching element 106 is on, the secondary winding 104b has
The output voltage V _O is forcibly applied. As a result, the secondary current I ₂ flows in the reverse direction, and reversely excites the transformer 104 (period from td to te).

When the regenerative switching element 106 is turned off at the time of te, resonance between the primary winding 104a and the parasitic capacitance 105c on the primary side has an amplitude determined by the flyback voltage and the reverse excitation current. Phenomenon occurs,
Reducing the switching voltage V _1. Moreover, (the period from tf to tg) when the switching voltages V ₁ becomes zero, the diode 105b conducts, regenerates inverse excitation energy to an input side voltage source 102. During this period, since the electric charge of the parasitic capacitance 105c is extracted by the resonance phenomenon, the source-drain voltage of the main FET 105a is 0V. Therefore, if the main switching element 105 is turned on during this period, heat loss and noise do not occur. At time tg, the regeneration of the reverse excitation current ends, and the excitation of the transformer 104 is started as at time ta.

The conventional switching power supply 101 comprises a main switching element 105 and a regenerative switching element 106.
At the desired timing, the primary side and the
The secondary side drive circuits 108 and 109 are realized by the circuit shown in FIG. That is, the voltage detection circuit 111 divides the output voltage V _O and outputs a voltage control signal. The voltage control signal is transmitted to a control signal transmission unit 1 such as a photocoupler.
12 electrically isolated by the control IC 113
To the feedback terminal FB.

The control IC 113 outputs a drive signal IC _OUT which is used as a reference when controlling ON / OFF of the switching elements 105 and 106. Drive signal IC _OUT in response to the feedback terminal voltage V _FB, which is pulse-width control in a fixed frequency, the lower the feedback terminal voltage V _FB, the high level period is set longer. Further, the delay circuit 114 delays the rise of the drive signal IC _OUT by a predetermined time and applies the same to the gate of the main FET 105a. On the other hand, the drive signal IC _OUT is transmitted to the secondary side drive circuit 116 via the drive signal transmission unit 115 including a photocoupler or the like. The secondary side drive circuit 116
The drive signal IC _OUT is inverted and the regenerative FET 106a
To the gate of The delay time of the delay circuit 114 is set in advance according to the time during which the reverse excitation energy is regenerated to the input (the time from td to te shown in FIG. 14). Thereby, both switching elements 105 and 10
6 can be turned on alternately at the timing shown in FIG. As a result, the main switching element 105 can perform zero-cross turn-on, and the switching power supply 101 can suppress generation of heat loss and noise even when the switching frequency is increased.

[0013]

However, in the switching power supply having the above configuration, the regenerative current increases at a light load, so that the efficiency at a light load is remarkably deteriorated as compared with the conventional PWM type switching power supply. There is a problem that.

Generally, for example, a personal computer or a small OA
In an electronic device such as a device or a video device, there is a large difference between power consumption during operation and power consumption during standby. Further, since these electronic devices store the previous state and various set values, the switching power supply must supply power even during standby. Therefore, when a switching power supply of a partial resonance circuit type using secondary-side regeneration is employed for these electronic devices, there is a possibility that the efficiency of the switching power supply may be degraded at a light load, such as during standby. In particular, among electronic devices, portable electronic devices such as notebook personal computers are driven by a battery, so that a decrease in efficiency directly leads to a reduction in the operating time of the electronic device. Therefore, when used in these applications, the reduction in efficiency at light load becomes a very serious problem.

In addition, when the switching power supply is housed in a portable electronic device or used in an AC adapter, the storage space for the switching power supply is extremely limited. In addition, if the efficiency is deteriorated and the switching power supply generates heat, the switching power supply itself and surrounding devices may be damaged. Therefore, it is necessary to provide a heat radiation member such as a heat radiation plate. As a result, the switching power supply becomes even larger.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a small and thin switching power supply that is highly efficient even at light load.

[0017]

According to a first aspect of the present invention, there is provided a switching power supply, comprising: a transformer having a load connected to a secondary side; and a power supply to a primary side of the transformer. And a regenerative switching element that passes a reverse excitation current to the secondary side of the transformer after the excitation energy accumulated by the power is released. It is characterized by taking.

That is, there is provided an operation mode switching means for limiting the conduction of the regenerative switching element during the light load mode in which the load is lighter than the predetermined level, as compared with the remaining rated load mode. .

In the above configuration, the operation mode switching means detects whether the load exceeds a predetermined level, for example, by detecting a load current. In the rated load mode in which the load exceeds a predetermined level, the operation mode switching means does not limit the conduction of the regenerative switching element. Therefore, after the excitation energy of the transformer is released, the regenerative switching element passes a reverse excitation current to the secondary side of the transformer from, for example, a smoothing capacitor provided on the load side to reversely excite the transformer. . In this state, when the regenerative switching element is cut off, the primary winding of the transformer resonates with the parasitic capacitance of the main switching element. As a result, the main switching element can be turned on, for example, when the current or the voltage is zero.

When the main switching element is zero-cross turn-on, no loss occurs at the time of turn-on. Therefore, compared with a switching power supply before the conventional regenerative switching element is provided, that is, a switching power supply of a PWM system, the loss at the time of switching and the like are reduced. Noise can be greatly reduced. As a result, the switching frequency of the main switching element can be improved without lowering the efficiency, and a small, thin, and highly efficient switching power supply can be realized.

On the other hand, in the case of the light load mode in which the load does not reach the predetermined level, the operation mode switching means includes, for example,
By applying a low-level operation mode signal to the regenerative switching element or cutting off the connection between the regenerative switching element and the drive circuit, conduction of the regenerative switching element is limited as compared with the rated mode. As a result, it is possible to limit the total of the reverse excitation current and suppress the loss caused by the reverse excitation current. As a result, the efficiency at the time of light load can be greatly improved as compared with the conventional switching power supply of the partial resonance circuit system using the secondary side regeneration. At light load, the conduction period of the main switching element is short, so that the loss at the time of switching is relatively low. Therefore, it is possible to realize a highly efficient switching power supply that is small and thin, even at a light load.

If the operation mode switching means limits the conduction of the regenerative switching element at light load, the operation mode switching means is caused by the reverse excitation current as compared with the conventional switching power supply of the partial resonance circuit type using the secondary regeneration. Since the loss can be reduced, a sufficient effect can be obtained in realizing a highly efficient switching power supply even under a light load.

On the other hand, a switching power supply according to a second aspect of the present invention is the switching power supply according to the first aspect of the present invention.
The operation mode switching means is characterized in that the regenerative switching element is cut off at least during a period after the excitation energy of the transformer is released.

In this configuration, in the light load mode, the regenerative switching element is cut off at least during a period after the excitation energy is released. Therefore, since no reverse excitation current flows, the efficiency at light load can be further improved as compared with the switching power supply according to the first aspect of the present invention.

According to a third aspect of the present invention, there is provided a switching power supply according to the first or second aspect of the invention.
The operation mode switching means includes a determination unit that determines whether a load current supplied to the load exceeds a predetermined value.

In this configuration, the determining unit determines that the load is light if the load current does not exceed a predetermined value. The determination unit can be easily realized by, for example, a comparator that compares a voltage between both ends of a resistor connected in series with a load and a predetermined reference voltage. Therefore, the operation mode switching means can accurately determine whether the load is a light load and limit the conduction of the regenerative switching element.

On the other hand, a switching power supply according to a fourth aspect of the present invention is the switching power supply according to the first or second aspect of the invention.
The operation mode switching means includes a determination unit that determines whether a peak value of the secondary current of the transformer exceeds a predetermined value.

In this configuration, the determining section determines whether or not the peak value of the secondary current of the transformer exceeds a predetermined value. The determination unit can be easily realized by, for example, a comparator that compares a voltage between both ends of a resistor provided in a flow path of the secondary current and a predetermined reference voltage.
Thus, the operation mode switching means can accurately determine whether the load is light, and restrict the conduction of the regenerative switching element, with a simple configuration.

According to a fifth aspect of the present invention, there is provided a switching power supply according to the first or second aspect of the invention.
The operation mode switching means includes a determination unit that determines whether a time until the excitation energy of the transformer is released exceeds a predetermined value.

In this configuration, the determination section determines whether or not a period until the excitation energy of the transformer is released exceeds a predetermined value. As the load is smaller, the ratio between the conduction period and the interruption period of the main switching element is adjusted so that less excitation energy is stored in the transformer.
As a result, the period required for releasing the excitation energy is shortened. Therefore, the determination unit can accurately determine whether the load is smaller than the predetermined level based on whether the period exceeds a predetermined value. Whether or not the excitation energy of the transformer has been released can be identified by the direction of the current flowing through the secondary winding of the transformer. Whether or not the predetermined time has elapsed can be relatively easily determined by, for example, an RC circuit having a predetermined time constant. As a result, the operation mode switching means can accurately determine whether the load is a light load and limit the conduction of the regenerative switching element while having a simple configuration.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an embodiment of the present invention.
This will be described below with reference to FIG.
That is, the switching power supply according to the present embodiment is, for example, a personal computer such as a notebook personal computer or a small OA (Offi
ce Automation) When a small, thin, and highly efficient power supply is required, such as when embedded in various portable electronic devices such as equipment or video equipment, or when used as an AC power supply, It is a power source that is preferably used.

As shown in FIG. 1, the switching power supply 1 includes a transformer 4 for electrically insulating an input side voltage source 2 and a load 3 from each other. The input side voltage source 2 provided on the primary side of the transformer 4 is, for example, a DC power source obtained by rectifying and smoothing an AC power source or a battery itself. Input voltage V
_i can be applied. The main switching element 5 is connected in series to the primary winding 4a. The main switching element 5 is realized by a field effect transistor (FET), and includes an ideal FET 5a and an FET 5a.
Is represented by a diode 5b and a parasitic capacitance 5c that are parasitic between the source and the drain of the semiconductor device.

On the other hand, on the secondary side of the transformer 4, one end of the secondary winding 4b is connected to the output terminal OUT, and the other end is connected to the ground terminal GND via the regenerative switching element 6 having the FET 6a. It is connected. Also, both terminals O
A smoothing capacitor 7 such as an electrolytic capacitor is provided between UT and GND. Further, both terminals OUT.
GND is connected to the load 3 of the switching power supply 1.

In addition, the switching power supply 1 is provided with a control unit 8 for controlling the conduction / cutoff of the switching elements 5 and 6. Specifically, the control unit 8 includes, for example, a voltage detection circuit 11 that generates a voltage control signal by dividing the output voltage V _O and a photocoupler, for example. A control signal transmitting unit 12 for transmitting the voltage control signal to the primary side, and a pulse width control (PWM) is performed based on the voltage control signal received from the control signal transmitting unit 12 to control both switching elements 5 and 6.
And a control IC 13 for generating a drive signal IC _{OUT serving} as a reference when driving. Also, drive signal IC
The rise of the _OUT by a predetermined delay time td1 is delayed, (referred to as main FET in the following) FET provided in the main switching element 5 and the delay circuit 14 to be applied to the gate of 5a, 2-order the drive signal IC _OUT A driving signal transmitting unit 15 for transmitting the driving signal IC _OUT to a secondary side driving circuit 16 for inverting the received driving signal IC _OUT and driving an FET (hereinafter, referred to as a regenerative FET) 6 a provided in the regenerative switching element 6. Is provided. The drive signal transmission section 15 is also formed of a photocoupler or the like, and electrically insulates the primary side and the secondary side.

Furthermore, the switching power supply 1 according to this embodiment, based on the load 3, such as by outputting an operation mode signal V _M, the operation mode signal output to limit the conduction of the regenerative switching element 6 in the light load A circuit (operation mode switching means) 17 is provided. When the operation mode signal output circuit 17 does not determine that the load 3 is a light load (hereinafter, referred to as a rated load mode), the switching power supply 1
Performs the partial resonance operation by the secondary-side regeneration as in the related art. On the other hand, when it is determined that the load is light (hereinafter, referred to as a light load mode), the operation mode signal output circuit 17 reduces the gate voltage V _GS2 of the regenerative FET 6a or regenerates the secondary drive circuit 16 with the regenerative FET 6a. The conduction between the regenerative switching element 6 and the FET 6a is limited as compared with the rated load mode. Thereby, the reverse excitation current is cut off, and the switching power supply 1 is switched to an operation in which the transformer 4 is not reversely excited. Hereinafter, the operation in the light load mode will be referred to as a PWM operation in order to distinguish it from the partial resonance operation due to the secondary side regeneration.

The operation mode signal output circuit 17 does not limit the conduction of the regenerative switching element 6 in the rated load mode, and the regenerative switching element 6
6. Therefore, before describing the configuration of the operation mode signal output circuit 17 and the operation of the switching power supply 1 in the light load mode, the configuration of the other parts and the operation in the rated load mode will be described.

That is, for example, as shown in FIG. 2, the voltage detection circuit 11 includes an output terminal OUT and a ground terminal GN.
D and a series resistor 11a and 11b, and a shunt regulator 11c having a reference terminal R connected to a connection point between the resistors 11a and 11b.
The cathode terminal K of the shunt regulator 11c is
The control signal transmission unit 12 is connected to the output terminal OUT via the light emitting diode unit 12b of the photocoupler 12a and the resistor 11d. Note that the anode terminal A of the shunt regulator 11c is grounded.

[0038] For example, such as by variations in the input voltage V _i and the load 3, the output voltage V _O increases, the potential of the reference terminal R of the shunt regulator 11c increases, the impedance of the shunt regulator 11c is lowered. Therefore, the current flowing through the light emitting diode unit 12b increases, and the light amount of the light emitting diode unit 12b increases. As a result, the impedance of the phototransistor section 12c of the photocoupler 12a decreases, and the control I
The voltage V _FB of the feedback terminal FB of C13 is reduced. When the output voltage V _O decreases, the impedance of the shunt regulator 11c increases, and the control IC
13, the feedback terminal voltage V _FB is increased.

On the other hand, the feedback terminal FB of the control IC 13 is grounded via the phototransistor 12c of the photocoupler 12a. The control IC 13
Is an oscillator 13a for generating a triangular wave V _OSC having a predetermined frequency and level, and a feedback terminal voltage V _FB
13b for comparing the output of the comparator 13b with the triangular wave V _OSC, and npn transistors 13c and pn for power-amplifying the output of the comparator 13b by a push-pull operation.
and a p-type transistor 13d. As shown in FIG. 3, the period T and the level of the triangular wave V _OSC are constant, and the comparator 13b outputs the feedback terminal voltage V _{OSC.
While FB} is smaller than the triangular wave V _OSC , the transistor 13
A high-level drive signal IC _OUT is output via c · 13d. As a result, the control IC 13 can adjust the period t _on when the drive signal IC _OUT is at a high level according to the feedback terminal voltage V _FB .

The delay circuit 14 is composed of, for example, a diode and a resistor provided in parallel between the input and output (both are not shown). Drive signal IC _OUT
Rises, a current flows through the gate of the main FET 5a via the resistor. Therefore, the gate voltage V _GS1 of the main FET 5a gradually increases due to the time constant of the gate-source capacitance of the main FET 5a and the resistance, and when the threshold voltage is reached, the main FET 5a conducts. As a result, the main switching element 5 is connected to the drive signal IC
_From the rise of _OUT , conduction occurs with a delay of a predetermined delay time td1 determined by the time constant. On the other hand, when the drive signal IC _OUT falls, the diode of the delay circuit 14 conducts, and the capacitance between the gate and source of the main FET 5a is rapidly pulled out. As a result, the main switching element 5
It is cut off simultaneously with the fall of the drive signal IC _OUT . This allows the delay circuit 14 to delay the rising of the drive signal IC _OUT by the predetermined time td1 and transmit the delayed signal to the main switching element 5.

On the other hand, the secondary-side drive circuit 16 is, for example, an inverter circuit. After receiving the drive signal IC _OUT via the drive signal transmission unit 15 in an electrically insulated manner, the secondary drive circuit 16 outputs the drive signal IC _OUT . It can be inverted and applied to the gate of the regenerative FET 6a.

Next, the operation of the switching power supply 1 in the rated load mode will be described. As described later, in the rated load mode, the regenerative switching element 6 is not affected by the operation mode signal output circuit 17, and
Conduction / interruption is performed only in accordance with an instruction from the next drive circuit 16.

As shown in FIG. 4, the delay time td1 has elapsed from the time t1 when the drive signal IC _OUT has risen, and at the time t2, the gate voltage V of the main FET 5a has been reached.
_GS1 becomes high level, and the main FET 5a conducts.
In this state, since the drive signal IC _OUT is at a high level,
The secondary side drive circuit 16 applies a low-level gate voltage V _GS2 to the gate of the regenerative FET 6a,
Is shut off. Thus, the primary current I _D1 of the primary winding 4a of the transformer 4, the input voltage V _i of the input-side voltage source 2 is applied, increases linearly, storing excitation energy to the transformer 4 (t2 To t3).

When the time t _on elapses from the time t1, the control IC 13 lowers the drive signal IC _OUT (time t3). At this point, the diode of the delay circuit 14 becomes conductive, and immediately reduces the gate voltage V _GS1 of the main FET 5a. As a result, the main FET 5a is cut off simultaneously with the fall of the drive signal IC _OUT , and the transformer 4
Of the primary winding 4a, the drain of the main FETs 5a - source voltage, i.e., rises switching voltage V _1. When the drive signal IC _OUT becomes low level, 2
The secondary side drive circuit 16 makes the regenerative FET 6a conductive. As a result, the excitation energy stored in the transformer 4 becomes 2
It is discharged to the output terminal OUT as the secondary current I _D2 (t
3 to t4).

At time t4, the emission of the excitation energy ends, and the secondary current I _D2 becomes zero. However, in this state, since the regenerative FET 6a is conducting, the output voltage V _O is forcibly applied to the secondary winding 4b.
Therefore, the secondary current I _D2 flows in the reverse direction and reversely excites the transformer 4 (period from t4 to t5).

Further, when the switching cycle T elapses from the time point t1, the control IC 13 outputs the drive signal I
C _OUT is restarted (at time t5). This allows
The gate voltage V _GS2 of the regenerative FET 6a decreases, and the regenerative switching element 6 is cut off. Meanwhile, at this point,
The gate voltage V _GS1 of the main FET 5a is kept at a low level by the delay circuit 14, and the main switching element 5
Is shut off. In this state, during the period from the time t4 to the time t5, the reverse excitation energy stored in the transformer 4 becomes the primary current I _{D1 and the} primary winding 4
From a, an attempt is made to regenerate to the + side of the input side voltage source 2.
As a result, the primary winding 4a and the parasitic capacitance 5c of the main FET 5a resonate at an amplitude determined by the flyback voltage and the reverse excitation current, and the charge stored in the parasitic capacitance 5c is extracted (from t5 to t6). Period until).

After the charge is extracted, the primary current I _{D1 as} a regenerative current is regenerated to the input side voltage source 2 via the diode 5b parasitic on the main FET 5a (from t6 to t).
Up to 8). During this period, the primary-side current _ID1 flows to the input-side voltage source 2, so that no charge is accumulated in the parasitic capacitance 5c. Therefore, the drain-source voltage V _DS1 of the main FET 5a is kept at approximately 0V.

When the delay time td1 has elapsed from the time point t5, the gate voltage V _GS1 of the main FET 5a becomes high level, and the main switching element 5 conducts (time point t7). The delay time td1 of the delay circuit 14 is set so as to fall within the time from t6 to t8. As a result, the main switching element 5 can perform zero-cross turn-on, and can prevent heat loss and noise at the time of turn-on.

When the main switching element 5 is turned on, the primary current I _D1 linearly increases, and after the regeneration of the reverse excitation current is completed, the excitation of the transformer 4 is started again. After this,
The switching power supply 1 repeats the operation after t2. Thus, the power supplied from the input side voltage source 2 is intermittently turned on / off by the main switching element 5 and transmitted to the secondary side of the transformer 4. On the secondary side of the transformer 4, the intermittent power is smoothed by the smoothing capacitor 7. As a result, the switching power supply 1 can apply the DC voltage V _O to the load 3 via the output terminal OUT.

Further, the voltage detection circuit 11 connected to the output terminal OUT generates a voltage control signal based on the output voltage V _O, and outputs the voltage control signal via the control signal transmission unit 12 to the feedback terminal FB of the control IC 13. Is applied. Thus, the feedback terminal voltage V _FB increases as the output voltage V _O increases. Further, as shown in FIG. 3, the control IC 13 drives the high-level drive signal IC during a period in which the feedback terminal voltage V _FB is smaller than the triangular wave V _OSC.
Output _OUT . Since the level and frequency of the triangular wave V _OSC are constant, the higher the feedback terminal voltage V _{FB, the shorter} the period t _on during which the drive signal IC _OUT is at a high level. As a result, the ON period of the main switching device 5 is adjusted according to the output voltage V _O, the switching power supply 1, regardless of variations in the input voltage V _i and the load 3, an output voltage V _O of a given value it can.

In the rated load mode, before the main switching element 5 is turned on, the regenerative switching element 6 is turned on to reversely excite the transformer 4. Thereby, the main switching element 5 can perform zero-cross turn-on. As a result, compared with the case where the regenerative switching element 6 is not provided, that is, as compared with the case of the conventional PWM type switching power supply, the generation of heat loss and noise during switching can be greatly reduced.

On the other hand, if the operation mode signal output circuit 17 determines that the load 3 is light, the conduction of the regenerative switching element 6 is limited. As a result, the switching power supply 1 performs the PWM operation as in the case where the regenerative switching element 6 is not provided. In this state, the reverse excitation current is reduced due to the conduction limitation of the regenerative switching element 6. Therefore, the loss due to the reverse excitation current can be prevented, and the efficiency at the time of light load can be kept substantially the same as the conventional PWM type switching power supply. As a result, the conventional 2
Efficiency at light load can be greatly improved as compared with a switching power supply of a partial resonance circuit system using secondary regeneration.

The operation mode signal output circuit 17 is connected to the load 3
Various methods can be used to detect the state of the main FET 5a and to limit the conduction of the main FET 5a. Below,
Each configuration example of the operation mode signal output circuit 17 and the operation of the switching power supply 1 in the light load mode will be described in detail.

For example, the operation mode signal output circuit 17
As a first configuration example, there is a circuit for detecting a load current. Specifically, as shown in FIG. 5, the operation mode signal output circuit 17a is provided on the load 3 side with respect to the smoothing capacitor 7 shown in FIG. 1, and the output terminal OUT and the ground terminal GND.
And resistors 21 and 22 connected in series with each other. A connection point between the resistor 21 on the output terminal OUT side and the resistor 22 on the ground terminal GND side is connected to an inverting input terminal of a comparator (determination unit) 23. On the other hand, a resistor 24 is provided between the smoothing capacitor 7 and the resistor 22, and a non-inverting input terminal of the comparator 23 is connected to a connection point between the resistors 22 and 24. Resistance values of the resistors _{21 · 22 · 24 R 21 ·} R 22 ·
R ₂₄ is the load current I _LIM1 when switching to the light load mode.
It is set according to.

In the above configuration, while the load current I _O flows to the load 3, the non-inverting input terminal of the comparator 23
A voltage of I _{OR 24} is applied. On the other hand, the inverting input terminal has a divided voltage V _O · R ₂₂ / (R ₂₁
+ R ₂₂ ). Therefore, when the voltage V _O · R ₂₂ / (R ₂₁ + R ₂₂ ) is higher than the voltage I _O · R ₂₄ , that is, when the load current I _O is I _LIM1 = R ₂₂ · V _O /
If (R ₂₁ + R ₂₂ ) · R ₂₄ ｝ or less, comparator 2
3 outputs an operation mode signal V _M at the low level indicating the light-load mode. Thereby, the regenerative switching element 6 shown in FIG.
Always shut off.

On the other hand, when the load current I _O exceeds I _LIM1 ,
Operation mode signal output circuit 17a outputs an operation mode signal V _M of the high level indicating the rated load mode. As a result, the regenerative switching element 6 is connected to the secondary drive circuit 1
Conduction / interruption is controlled by the six outputs.

For example, when the load current I _O exceeds I _LIM1 as in the period up to t11 shown in FIG. 6, the operation mode signal output circuit 17a outputs the high-level operation mode signal V indicating the rated load mode. _M is output. In this state, the switching power supply 1 is operating in the rated load mode, and the main switching elements 5 and the regenerative switching elements 6 are alternately conductive as in FIG. As a result, the main switching element 5 can perform zero-cross turn-on, and switching loss and switching noise at the time of turn-on are suppressed low. Further, since the load is heavier than in the light load mode, the control IC 13 sets the pulse width t _on of the drive signal IC _OUT longer.
Therefore, the period during which the regenerative switching element 6 is conducting is relatively short, and the reverse excitation current during the conducting period does not have a very large value. As a result, heat generation due to the reverse excitation current is suppressed.

Meanwhile, as the subsequent t11 period, the load current I _O is below the predetermined level, the operation mode signal output circuit 17a outputs an operation mode signal V _M at the low level indicating the light load mode . In this state, the regenerative FE
The gate voltage V _{GS2 of} T6a is always kept at the low level regardless of the output voltage of the secondary side drive circuit 16. As a result, while the main switching element 5 is cut off, no reverse excitation current is generated, and the switching power supply 1 performs the PWM operation.

In this state, the load is lighter than in the rated load mode.
_OUT of the pulse width t _on, is set shorter. Therefore, assuming that the regenerative switching element 6 conducts during the period in which the main switching element 5 is shut off, as in the rated load mode, a large amount of reverse exciting current flows,
The amount of heat generated by the switching power supply 1 tends to increase.

However, the switching power supply 1 according to the present embodiment always shuts off the regenerative switching element 6 in the light load mode, so that no heat is generated by the reverse excitation current. As a result, the amount of heat generated by the switching power supply 1 under a light load can be significantly reduced as compared with the related art.
In the light load mode, since the load 3 is light, the load current _IO is small. Therefore, the loss and noise due to the switching of the main switching element 5 are kept at extremely low values.

The second operation mode signal output circuit 17
As a configuration example, a secondary current flowing through the secondary winding 4b,
That is, the drain current I _D2 of the regenerative switching element 6
A circuit that detects the state of the load 3 based on the peak value of the load 3 can be used.

Specifically, as shown in FIG. 7, the operation mode signal output circuit 17b is connected to the smoothing capacitor 7 shown in FIG.
Connected to the transformer 4 and between the output terminal OUT and the ground terminal GND.
32. Also, the resistor 31 on the output terminal OUT side
And the connection point between the ground terminal GND and the resistor 32 are connected to an inverting input terminal of a comparator (determination unit) 33. On the other hand, a resistor 34 is provided between the regenerative switching element 6 shown in FIG. 1 and the resistor 32, and a non-inverting input terminal of the comparator 33 is connected to a connection point between the resistors 32 and 34. I have. The comparator 33 has a hysteresis and can keep the output at a high level from when the output goes to a high level until the non-inverting input terminal voltage becomes 0V. Further, the output of the comparator 33 is applied to the gate of the regenerative switching element 6 via the delay circuit 35. The delay circuit 35
Is composed of, for example, a diode and a resistor connected in parallel, similarly to the delay circuit 14. The diode limits the current flowing from the comparator 33 to the gate of the regenerative FET 6a. Thus, the delay circuit 35 can delay the point in time when the output signal becomes low level for a predetermined time td2 after the output of the comparator 33 becomes low level. Note that each resistor 31
The resistance values R ₃₁ , R _32, and R ₃₄ of ₃₂ and ₃₄ are set according to the peak value I _LIM2 of the drain current when switching to the light load mode. Further, similarly to the delay circuit 14, the resistance value of the delay circuit 35 is set according to the delay time td2 of the delay circuit 35.

In the above configuration, as shown in FIG. 8, when the gate voltage V _GS1 of the main FET 5a goes low,
The main switching element 5 is cut off, and the current _ID2 flows through the secondary winding 4b of the transformer 4. As a result, the voltage I _D2 · R ₃₄ is input to the non-inverting input terminal of the comparator 33, and the divided voltage V _OR · ₃₂ / (R ₃₁ + R ₃₂ ) by the resistors 31 and 32 is input to the inverting input terminal. Applied.

The voltage V _O · R ₃₂ / voltage is higher than the voltage I _D2 · R _34.
When (R ₃₁ + R ₃₂ ) is larger, that is, when the peak value of the drain current I _D2 is equal to the predetermined threshold I _LIM2 = R ₃₂ · V
_{When O} / {(R ₃₁ + R ₃₂ ) · R ₃₄ } or less, the comparator 33 derives a low-level output V _CMP , and when the threshold value I _LIM2 is exceeded, derives a high-level output V _CMP .

For example, when the drain current I _D2 exceeds the current I _LIM2 , as at time t21, the comparator 33 determines _whether the drain current I _D2 becomes 0V.
The output V _CMP is maintained at a high level (from t21 to t22).
Period until). Therefore, the operation mode signal output circuit 1
7b during this period, and outputs an operation mode signal V _M of the high level indicating the rated load mode. Further, the delay circuit 35 keeps the output at a high level from the time when the drain current _ID2 becomes 0 V until a predetermined time td2 elapses (a period from t22 to t23). As a result, during the period from t21 to t23, conduction / interruption of the regenerative switching element 6 is controlled in accordance with the drive signal IC _OUT . It should be noted that the delay time td2 of the delay circuit 35 is the same as that in the case where the peak value of the drain current I _D2 is the lowest in the light load mode, that is, even when the threshold value I _LIM2 is used. until such time as the IC _OUT becomes high level, it is set to be able to maintain the operation mode signal V _M to the high level.

[0066] Beyond the point of the t23, the operation mode signal V _M becomes a low level. However, when the load 3 is heavy, the peak value of the drain current _ID2 becomes equal to the threshold value _ILIM2.
Since exceeds, the operation mode signal output circuit 17b is the main switching element 5 then at the time it was interrupted, and outputs an operation mode signal V _M of the high level again (time point t24).

Therefore, when the load 3 is heavier than the predetermined level, the switching power supply 1 can perform the partial resonance operation by the secondary-side regeneration as in the case of the operation mode signal output circuit 17a shown in FIG.

On the other hand, even during the peak time as in the period after t25, the drain current I _D2 is equal to the threshold value I _{D2.
When LIM2} is not _exceeded , the comparator 33 always derives a low-level output. As a result, the operation mode signal output circuit 17b outputs the regenerative F signal regardless of the drive signal IC _OUT.
The gate voltage V _{GS2 of the} ET 6a is always kept at a low level. Therefore, the regenerative switching element 6 is always shut off, and the switching power supply 1 can switch to the PWM operation as in the case of the operation mode signal output circuit 17a.

As still another configuration example, the period required to release the energy stored in the transformer 4 to the output is as follows:
There is a configuration in which the operation mode signal output circuit 17 detects. In general, the smaller the load 3, the smaller the main switching element 5
, The peak value of the secondary-side drain current _ID2 decreases. Therefore, the smaller the load 3, the shorter the time required to output the energy stored in the transformer 4. As a result, the predetermined period td
The operation mode signal output circuit 17 can determine whether or not the load 3 is lighter than a predetermined level depending on whether or not the secondary side drain current I _D2 becomes 0 within three. Note that the duty ratio is a ratio of the ON period t _on of the main switching element 5 to the switching cycle T.

Specifically, as shown in FIG. 9, the operation mode signal output circuit 17c is connected to the resistors 31, 32,.
It has the same resistors 41, 42, 44 as 34. The connection point between the resistor 41 and the resistor 42 is connected to the non-inverting input terminal of the comparator 43.
Is connected to the inverting input terminal of the comparator 43. The resistance value R ₄₁ · R of the resistors 41 and 42
_{The reference numeral 42} is set so that the divided voltage R ₄₂ · V _O / (R ₄₁ + R ₄₂ ) is as close as possible to 0V.

Further, the output of the comparator 43 is connected to a reset terminal R of a set-reset flip-flop (hereinafter, abbreviated as RS-FF) 45 corresponding to the determination section described in the claims. . Also, R
The timer circuit 46 is also connected to the reset terminal R of the S-FF 45. The timer circuit 46 as a trigger a low-level drive signal transfer unit 15 shown in FIG. 1, a predetermined period td3, the output signal V _TIM high level can be applied to the reset terminal R. Further, an output signal of the secondary side drive circuit 16 is applied to a set terminal S of the RS-FF 45.

In the above configuration, when the control IC 13 changes the drive signal IC _OUT to a low level at time t31 shown in FIG. 10, the main switching element 5 is cut off.
As a result, the exciting energy stored in the transformer 4 while the main switching element 5 is conducting is the current flowing through the secondary winding 4b of the transformer 4, that is, the regenerative FET 6
It is emitted as the drain current I _D2 of a.

Therefore, the voltage I _D2 · R ₄₄ is applied to the inverting input terminal of the comparator 43, and the divided voltage R ₄₂ · V _O / (R
₄₁ + R ₄₂ ) is applied. As described above, the resistance value R _41
R42 is set such that the divided voltage becomes substantially zero. As a result, the comparator 43, while the drain current I _D2 of the regenerative FET6a is negative, (the period from t33 to t34) which outputs a signal V _CMP of high level. on the other hand,
The timer circuit 46 performs a period from when the drive signal IC _OUT changes to a low level to when a predetermined time td3 elapses.
Output a high-level output signal V _TIM (from t31 to t
Up to 32).

In the rated load mode, the duty of the drive signal IC _OUT is set to be large. Therefore, at least one of the output signal V _TIM of the timer circuit 46 and the output signal V _{CMP of the} comparator 43 is at a low level. As a result, the reset terminal R of the RS-FF 45 is always kept at low level,
The FF 45 applies the same signal as the output of the secondary drive circuit 16 to the gate of the regenerative FET 6a.

On the other hand, if the secondary-side drain current I _D2 becomes 0 V between the time when the load 3 is lightened and the main switching element 5 is cut off and the above-mentioned predetermined time td3 elapses, at time t35 As at the time point, the comparator 43 derives the high-level output signal V _CMP even though the output signal V _TIM of the timer circuit 46 is at the high level.
Accordingly, a high-level signal is input to the reset terminal R of the RS-FF 45. As a result, RS-FF4
5 keeps the gate voltage V _GS2 of the regenerative FET 6a at a low level until a high level is input to the set terminal S, that is, until the main switching element 5 is cut off again. As a result, the regenerative switching element 6 is cut off from when the drain current _ID2 becomes 0 V until the main switching element 5 is cut off. Therefore, while the load 3 is lower than the predetermined level, the reverse excitation current in the light load mode can be reduced as in the case of the operation mode signal output circuit 17a shown in FIG.

The fourth configuration example of the operation mode signal output circuit 17 is a circuit for determining the weight of the load 3 based on the magnitude of the reverse exciting current. Specifically, as shown in FIG.
The operation mode signal output circuit 17d includes a resistor 31 shown in FIG.
・ It has the same resistors 51, 52 and 54 as 32 and 34. The connection point between the resistors 51 and 52 is connected to the non-inverting input terminal of the comparator 53, and the connection point between the resistors 54 and 52 is connected to the inverting input terminal of the comparator 53. If the threshold value I _LIM3 of the reverse excitation current when discriminating between the light load mode and the rated load mode is too small, the main FET 5 in the rated load mode may be used.
The source-drain voltage V _DS1 of a cannot reach 0. On the other hand, if it is too large, useless regenerative current flows, so that efficiency cannot be improved.

[0077] the reverse excitation current I _2P required for turning at zero voltage, as shown in the following equation (1), I _2P = ^{_{[(V i 2 -n 2 · V}} O 2) · C / L ^1/2 · n (1) In the above equation (1), n is the primary / secondary turns ratio of the transformer 4, L is the primary inductance of the transformer 4, and C is the parasitic capacitance 5c of the main switching element 5, respectively. ing.

As an example of each of the above numerical values, n = 5, V _O =
Assuming that 18 V, C = 300 pF, and L = 170 μH, the input side voltage source 2 rectifies and smoothes the AC voltage of 100 V AC to generate V _i , that is, V _i = 140 V
In the case of (DC), I _2P = 0.71A. Similarly, when the input-side voltage source 2 to produce a V _i from the AC voltage of AC230V, i.e., if the V _i = 320 V (DC),
I _2P = 2.04A. Therefore, the threshold I
_LIM3, like become 2A order of 0.2 A, is set by adjusting the resistance value R ₅₁ · R ₅₂ · R ₅₄ for each resistance.

In this configuration, as shown in FIG.
When the gate voltage V _{GS1 of the} ET 5a becomes low level, the main switching element 5 is cut off, and the current _ID2 flows through the secondary winding 4b of the transformer 4. Thus, the inverting input terminal of the comparator 53, the voltage I _D2 · R ₅₄ are inputted. On the other hand, a divided voltage V _O · R ₅₂ / (R ₅₁ + R ₅₂ ) by the resistors 51 and 52 is applied to the non-inverting input terminal.

When the non-inverted input terminal voltage V _O · R ₅₂ / (R ₅₁ + R ₅₂ ) is higher than the inverted input terminal voltage I _D2 · R ₅₄ , that is, the drain current I _D2 which becomes the reverse excitation current
Is a predetermined threshold value I _LIM3 = R ₅₂ · V _O / ｛(R ₅₁ +
R ₅₂ ) · R ₅₄ }, the comparator 53
The low-level output V _CMP is derived, and the high-level output V _CMP is derived while the output V _CMP is less than the threshold value I _LIM3 .

When the load 3 is a rated load (period before t41), the off period of the main switching element 5 is set relatively short. Therefore, the peak value of the reverse excitation current does not reach threshold value _ILIM3 . In this state, the comparator 53 always outputs an operation mode signal V _M at the high level, the regeneration switching element 6 is repeated connection / disconnection in accordance with an instruction of the secondary drive circuit 16. As a result,
The switching power supply 1 has an operation mode signal output circuit 17a.
As in the case of the above, a partial resonance operation is performed by the secondary-side regeneration.

On the other hand, when the load 3 is a light load (period after t41), the off-period of the main switching element 5 is set relatively long, so that the peak value of the reverse excitation current is _reduced to the threshold value I _{LI M3} . (At time t42). As a result, the comparator 53 derives the low-level output V _CMP , so that the gate voltage V _GS2 of the regenerative FET 6a drops to the low level regardless of the instruction of the secondary side drive circuit 16, and the regenerative switching element 6 Cut off. Therefore, the reverse excitation energy stored in the transformer 4 is equal to t6 shown in FIG.
As in the period from to t8, the voltage is regenerated to the input side voltage source 2. Furthermore, after the drive signal IC _OUT rises,
When the delay time td1 of the delay circuit 14 has elapsed, the main switching element 5 conducts, and the excitation of the transformer 4 starts (at time t43).

In the above configuration, when the load 3 is a light load, the reverse excitation current is suppressed to a predetermined value I _LIM3 or less, and is greatly reduced as compared with the conventional switching power supply indicated by a broken line in the figure. You. Thus, the loss due to the regenerative current can be limited to a predetermined value or less, and the efficiency at light load can be improved as in the case of the operation mode signal output circuits 17a to 17c. However, in the case of the operation mode signal output circuit 17d according to the present embodiment, even in the light load mode, the regenerative current flows until the threshold value _ILIM3 is reached. Therefore, the efficiency of the switching power supply 1 at light load is slightly lower than that of the operation mode signal output circuits 17a to 17c.

As described above, the switching power supply 1 according to the present embodiment is a switching power supply 1 capable of performing a partial resonance operation by secondary-side regeneration, and has a main power supply for intermittently supplying power to the primary side of the transformer 4. It includes a switching element 5 and a regenerative switching element 6 for passing a reverse excitation current to the secondary side of the transformer 4 after the excitation energy of the transformer 4 is released. Furthermore, in the switching power supply 1 according to the present embodiment, during the light load mode in which the load 3 is lighter than the predetermined level, the conduction of the regenerative switching element 6 is smaller than in the remaining rated load mode. An operation mode signal output circuit 17 for limiting is provided.

In the above configuration, in the rated load mode, the operation mode signal output circuit 17
Does not limit the conduction of Therefore, after the excitation energy of the transformer 4 is released, the regenerative switching element 6
For example, a reverse excitation current is passed from the smoothing capacitor 7 to the secondary side of the transformer 4 to reversely excite the transformer 4. In this state, when the regenerative switching element 6 is cut off, the primary winding 4a of the transformer 4 resonates with the parasitic capacitance 5c of the main switching element 5. As a result,
For example, the main switching element 5 can be turned on when the current or the voltage is zero. When the main switching element 5 performs zero-cross turn-on, no loss occurs at the time of turn-on, so that loss and noise at the time of switching can be significantly reduced as compared with the conventional PWM type switching power supply 1. As a result, the switching frequency of the main switching element 5 can be improved without reducing the efficiency,
A thin and highly efficient switching power supply 1 can be realized.

[0086] On the other hand, if the light load mode, the operation mode signal output circuit 17, for example, to apply an operation mode signal V _M of the low level to a regenerative switching element 6, the regenerative switching element 6 and the secondary-side driving circuit 16 To limit the conduction of the regenerative switching element 6 compared to the rated mode. As a result, it is possible to limit the total of the reverse excitation current and suppress the loss caused by the reverse excitation current. As a result, the efficiency at the time of light load can be greatly improved as compared with the conventional switching power supply of the partial resonance circuit system using the secondary side regeneration. At a light load, the conduction period of the main switching element 5 is short, so that the switching loss is
It is kept relatively low. Therefore, it is possible to realize the switching power supply 1 that is small and thin and highly efficient even under light load.

When the operation mode signal output circuit 17 limits the conduction of the regenerative switching element 6 at a light load, the reverse excitation current can be reduced as compared with the conventional switching power supply of the secondary resonance type partial resonance circuit. Therefore, the effect of realizing the high-efficiency switching power supply 1 even at a light load can be sufficiently obtained.

However, as shown in the operation mode signal output circuits 17a to 17c shown in FIGS. 5 to 10, the operation mode signal output circuit 17 operates at least during the period after the excitation energy of the transformer 4 is released. It is desirable to shut off the switching element 6. This makes it possible to completely cut off the reverse excitation current at the time of a light load, so that, for example, as compared with a case where a slight reverse excitation current flows even at a light load as in the operation mode signal output circuit 17d shown in FIG. The efficiency under load can be further improved.

Various methods are conceivable as a method of detecting the state of the load 3 by the operation mode signal output circuit 17. For example, as in the operation mode signal output circuit 17a shown in FIG. 5, a configuration including a comparator 23 that determines whether the load current I _O supplied to the load 3 exceeds a predetermined value, or FIG. as in the operation mode signal output circuit 17b shown, the peak value of the secondary current I _D2 is a predetermined threshold value I
A configuration including a comparator 33 for determining whether or not _LIM2 has been exceeded, or a period from the time when the transformer 4 emits the excitation energy until the end of the emission, as in the operation mode signal output circuit 17c shown in FIG. RS-F which detects the state of the load 3 according to whether or not exceeds a predetermined value.
There is a configuration including F45. In either configuration, the operation mode signal output circuit 17 can accurately determine whether the load 3 is a light load and limit the conduction of the regenerative switching element 6 while having a simple configuration.
In addition, even if the configuration is other than the above, the same effect as that of the present embodiment can be obtained as long as the conduction of the regenerative switching element 6 can be limited at light load.

In this embodiment, the control IC 13
However, based on the voltage control signal obtained by dividing the output voltage V _O , the frequency is fixed, the pulse width of the drive signal IC _OUT is controlled, and the ratio between the conduction period and the cutoff period of the main switching element 5 is adjusted. But it is not limited to this.
For example, the ratio may be adjusted based on the load current I _O , or may be adjusted by controlling the frequency with a fixed pulse width. Further, in the present embodiment,
When the source-drain voltage V _{DS1 of the} main FET 5a is 0, the main switching element 5 is turned on.
It may be turned on when the drain current I _D1 is 0.
Regardless of the control method of the main switching element 5, if the operation mode signal output circuit 17 can limit the conduction of the regenerative switching element 6 in the light load mode and suppress the reverse excitation current, the same effect as in the present embodiment can be obtained. .

Further, in the present embodiment, an electronic device, an AC adapter, or the like has been described as an example of application of the switching power supply 1. However, the present invention is not limited to this, and any device requiring a DC voltage can be widely applied. However, the switching power supply 1 according to the present embodiment is particularly suitable for the above-described applications because it is small and thin, but has high efficiency even under light load.

[0092]

As described above, the switching power supply according to the first aspect of the present invention regenerates during the light load mode in which the load is lighter than the predetermined level, compared with the remaining rated load mode. This is a configuration including operation mode switching means for restricting conduction of the switching element.

In the above configuration, the operation mode switching means limits the conduction of the regenerative switching element during the light load mode as compared with the remaining period. As a result, the loss caused by the reverse excitation current can be suppressed. Therefore, there is an effect that a high-efficiency switching power supply can be realized even under a light load while being small and thin.

As described above, in the switching power supply according to the second aspect of the present invention, in the configuration of the first aspect, the operation mode switching means includes at least a period after the excitation energy of the transformer is released. , The regenerative switching element is cut off.

In this configuration, in the light load mode, the regenerative switching element is cut off at least during a period after the excitation energy is released, and the reverse excitation current can be completely prevented. Therefore, there is an effect that the efficiency at light load can be further improved as compared with the switching power supply according to the first aspect of the present invention.

As described above, in the switching power supply according to the third aspect of the present invention, in the configuration of the first or second aspect, the operation mode switching means includes a load current supplied to the load exceeding a predetermined value. This is a configuration that includes a determination unit that determines whether or not the operation is performed.

Therefore, the operation mode switching means can limit the conduction of the regenerative switching element by accurately determining whether or not the load is a light load while having a simple configuration. As a result, there is an effect that a highly efficient switching power supply can be realized with a simple configuration and even under a light load.

As described above, in the switching power supply according to the fourth aspect of the present invention, in the configuration of the first or second aspect, the operation mode switching means is arranged so that the peak value of the secondary current of the transformer is a predetermined value. Is provided with a determination unit that determines whether or not the value exceeds the value.

Therefore, the operation mode switching means can accurately determine whether the load is a light load and limit the conduction of the regenerative switching element, while having a simple configuration. As a result, there is an effect that a highly efficient switching power supply can be realized with a simple configuration and even under a light load.

According to a fifth aspect of the present invention, there is provided the switching power supply according to the first or second aspect of the present invention, wherein the operation mode switching means sets a time until the excitation energy of the transformer is released. Has a determination unit for determining whether or not the value exceeds a predetermined value.

Therefore, the operation mode switching means can limit the conduction of the regenerative switching element by accurately determining whether or not the load is light, while having a simple configuration. As a result, there is an effect that a highly efficient switching power supply can be realized with a simple configuration and even under a light load.

[Brief description of the drawings]

FIG. 1, showing an embodiment of the present invention, is a circuit diagram illustrating a main part of a switching power supply.

FIG. 2 is a configuration diagram showing a main part of a control IC in the switching power supply.

FIG. 3 is a waveform chart for explaining the operation of the control IC.

FIG. 4 is a waveform chart showing the operation of each unit in the switching power supply.

FIG. 5 is a circuit diagram showing a main part of an operation mode signal output circuit in the switching power supply.

FIG. 6 is a waveform diagram showing an operation of each unit in the switching power supply including the operation mode signal output circuit.

FIG. 7 shows another embodiment and is a circuit diagram showing a main part of the operation mode signal output circuit.

FIG. 8 is a waveform chart showing the operation of each unit in the switching power supply including the operation mode signal output circuit.

FIG. 9 is a circuit diagram showing still another embodiment and showing a main part of the operation mode signal output circuit.

FIG. 10 is a waveform chart showing the operation of each unit in the switching power supply including the operation mode signal output circuit.

FIG. 11 is a circuit diagram showing still another embodiment and showing a main part of the operation mode signal output circuit.

FIG. 12 is a waveform diagram showing an operation of each unit in the switching power supply including the operation mode signal output circuit.

FIG. 13 shows a conventional example, and is a configuration diagram showing a partial resonance type switching power supply by secondary-side regeneration.

FIG. 14 is a waveform chart showing the operation of each unit in the switching power supply.

FIG. 15 is a circuit diagram showing the switching power supply in more detail.

[Explanation of symbols]

Reference Signs List 1 switching power supply 4 transformer 5 main switching element 6 regenerative switching element 17 operation mode signal output circuit (operation mode switching means) 23 comparator (judgment unit) 33 comparator (judgment unit) 45 set-reset flip-flop (judgment unit)

Claims (5)

[Claims] 1. A transformer having a load connected to a secondary side, a main switching element for intermittently supplying electric power to the primary side of the transformer, and a power supply comprising: a power supply; In a switching power supply having a regenerative switching element for passing a reverse excitation current to the secondary side of a transformer, a load is lighter than a predetermined level in a light load mode, compared with a period of a remaining rated load mode. A switching power supply comprising an operation mode switching means for restricting conduction of the regenerative switching element. 2. The switching power supply according to claim 1, wherein said operation mode switching means cuts off a regenerative switching element at least during a period after the excitation energy of said transformer is released. 3. The switching device according to claim 1, wherein said operation mode switching means includes a determination unit for determining whether a load current supplied to the load exceeds a predetermined value. Power supply. 4. The apparatus according to claim 1, wherein said operation mode switching means includes a determination section for determining whether or not a peak value of a secondary current of said transformer exceeds a predetermined value. 2. The switching power supply according to 2. 5. The method according to claim 1, wherein the operation mode switching means includes a determination unit for determining whether a time until the excitation energy of the transformer is released exceeds a predetermined value. Item 3. The switching power supply according to Item 1 or 2.