JPH11206121A - Switching power unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To lower loss and lower noise, in a switching power unit 21 which is so arranged as to get a secondary current of a desired voltage, by switching a primary current of a transformer N. SOLUTION: At on state of a switching element Q1, the regenerated power of excitation energy accumulated in inductance components of a primary winding N1 is returned into the closed circuit by the primary winding N1, a switching element Q2, and a diode D1 in advance after being accumulated in a resonant capacitor C2 once by the off state of a switching element Q1, and at the time of the next switching element Q1, it is used for the removal of charge accumulated in the floating capacity, and the switching element is turned on softly. Moreover, by the excitation energy accumulated in a choke coil L1 at the time of on state of the switching element Q1, a smoothing capacitor C1 to serve as a power source is boosted and charged at the time of off state. Accordingly, the low power consumption and low noise can be materialized.

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 a portable small electronic device or the like, and 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. A typical prior art of such a switching power supply is disclosed in, for example, Japanese Patent Laid-Open No.
No. 40 is cited, and the main circuit diagram is shown in FIG. 10 and the main operation waveform is shown in FIG.

Referring to FIG. 10, a switching power supply 1 generally includes a power supply circuit 2 for supplying a direct current.
A switching circuit q1, q2, a transformer n, a smoothing capacitor c1, a resonance capacitor c2, a boosting capacitor c3, a secondary circuit including diodes d1, d2 and a smoothing capacitor c4, and a control circuit 3. This is a resonance type switching power supply device that is configured to generate a primary voltage higher than the output voltage of the power supply circuit 2 by resonance.

The power supply circuit 2 removes high-frequency noise components from a commercial power supply 4 by a filter 5, performs full-wave rectification by a diode bridge d, and outputs the resultant between power supply lines 6 and 7.

The output voltage of the power supply circuit 2 is input to a smoothing capacitor c 1 via a boosting reactor 8, smoothed, and output between power supply lines 9 and 7. A series circuit of switching elements q1 and q2 is connected between the power supply lines 9 and 7. Further, a series circuit of a primary winding n1 of a transformer n and a resonance capacitor c2 is connected to the switching element q2 in parallel. A capacitor c that forms a partial resonance circuit between source and drain of the switching elements q1 and q2 to reduce switching loss when the switching elements q1 and q2 are turned off.
5 and c6 are interposed.

The secondary winding n2 of the transformer n is provided with an intermediate tap n0. The intermediate tap n0 serves as a ground-level power supply line 10 for secondary output. Both terminals of the secondary winding n2 are connected to a secondary output high-level power supply line 11 via diodes d1 and d2, respectively. A smoothing capacitor c4 is interposed between the power supply lines 11 and 10. Therefore, on the secondary side of the transformer n, the induced AC current is subjected to full-wave rectification and smoothing, and is output between the power supply lines 11 and 10.

[0007] The voltage between the power supply lines 11 and 10 is taken into the control circuit 3, divided by the voltage dividing resistors r 1 and r 2 and inputted to the non-inverting input terminal of the comparator 12. A reference voltage from a reference voltage source 13 is applied to an inverting input terminal of the comparator 12, and the comparator 12 has a divided voltage of a voltage between the power supply lines 11 and 10 higher than the reference voltage. Indeed, a large output current is generated to drive the light emitting diode d3 of the photocoupler.

The phototransistor q of the photocoupler
Reference numeral 3 is connected to a high-level power supply via a resistor r3, and a potential at a connection point between the phototransistor q3 and the resistor r3 is input to a voltage controlled oscillator (VCO). The voltage controlled oscillator 14 oscillates at a frequency corresponding to the potential at the connection point, and outputs an oscillation output to the control signal generation circuit 15. The control signal generating circuit 15 alternately turns on / off the two switching elements q1 and q2 at the oscillation frequency of the voltage-controlled oscillator 14 and so as to have a slight dead time when on / off is switched.

In the switching power supply 1 constructed as described above, the source-drain voltage of the switching element q2 is vq2, the current is iq2, the current flowing through the switching element q1 is iq1, and the connection of the switching elements q1 and q2 is The current flowing from the point to the primary winding n1 or flowing out to the connection point is i2, the terminal voltage of the resonance capacitor c2 is vc2, the current flowing out of the power supply circuit 2 is i1, and the current flowing through the capacitor c3 is ic.
3, and the current flowing through the step-up reactor 8 is i3, which is shown in FIG.

Between times τ1 and τ4, the switching element q2 is turned off, the switching element q1 is turned on, and a closed circuit including the smoothing capacitor c1, the switching element q1, the primary winding n1, and the resonance capacitor c2. The current i2 flows due to the series resonance of. On the contrary,
During the on period of the switching element q2 from time τ5 to τ8, the current i2 flows through a series resonance circuit including a closed circuit including the resonance capacitor c2, the primary winding n1, and the switching element q2. The magnitude (maximum amplitude) of the resonance current i2 changes proportionally according to the magnitude of the load, whereby the output voltage can be adjusted to some extent in response to the load change.

On the other hand, the terminal voltage vc of the resonance capacitor c2
2 is an AC voltage having a cycle corresponding to the on / off operation of the switching elements q1 and q2 and having an amplitude that varies depending on the size of the load. When the voltage vc2 becomes lower than the output voltage of the power supply circuit 2, the power supply circuit 2, a current ic3 flows through a series circuit composed of the boost capacitor c3 and the resonance capacitor c2, and flows into the boost reactor 8 in a closed circuit of the boost capacitor c3, the boost reactor 8, the switching element q1, and the primary winding n1. When energy is accumulated and the switching element q1 turns off, the boost capacitor c
3, the step-up reactor 8, a smoothing capacitor c1,
In a closed circuit with the resonance capacitor c2, the smoothing capacitor c1
Is boosted and charged.

Therefore, the smoothing capacitor c1 can be charged to a voltage higher than the output voltage of the power supply circuit 2. As described above, the voltage v between terminals of the resonance capacitor c2
c2 changes depending on the magnitude of the load, and becomes lower at light load and no load. Therefore, the power supply capacitor c1
Of the switching elements q1 and q2 can be lowered.

With such a configuration, a power supply voltage higher than the output voltage of the power supply circuit 2 can be obtained, and an undesired increase in the power supply voltage from the smoothing capacitor c1 can be suppressed. The power factor can be improved by using the current i1 supplied from the circuit 2 as a waveform whose peak value changes according to the voltage amplitude.

[0014]

The switching power supply device 1 configured as described above has the following problems.

(I) The DC voltage of the secondary output is stabilized by changing the switching frequency of the switching elements q1 and q2. That is, the output voltage is controlled by lowering the switching frequency when the output load current is large, and increasing the switching frequency when the output load current is small, thereby causing the following problems. is there.

(A) The switching elements q1 and q2 are set to 0V
If they are not turned on, switching noise occurs. Thus, the switching element q
To turn on 1, q2 at 0V, drain-
It is necessary to extract the charge stored in the stray capacitance between the sources, and the energy required for this operation increases in proportion to the switching frequency. In FIG. 11, at time τ4
~ Τ5 and τ8 ~ τ9, the primary winding n1 of the transformer n
Such an operation is performed by the excitation energy stored therein. Therefore, it is necessary to store the extra energy in the primary winding n1 in an earlier process, and an iron loss corresponding to this is generated in the transformer n. Here, as described above, since the resonance amplitude becomes small at the time of a light load, the switching frequency needs to be higher than the resonance frequency, so that the iron loss increases and the power conversion efficiency decreases.

(B) When the switching frequency changes,
This is undesirable in terms of unnecessary radiation of the mounted equipment and malfunctions applied to the mounted equipment. For example, in a switching power supply device of PWM control, since the switching frequency is fixed, only the switching fundamental frequency and noise of its harmonic components are mainly generated. Unwanted radiation and malfunction are affected by these frequency components. You only have to take care of it. On the other hand, in the case of the above-described conventional technology, since the switching frequency changes continuously, it is necessary to design a device in consideration of the influence of noise on all frequencies.

(Ii) The output voltage of the smoothing capacitor c1 is 2
It increases with an increase in the secondary output voltage. For this reason, it is necessary to select an element having a high drain-source withstand voltage rating for the switching elements q1 and q2. In this regard, when a TFT (thin film transistor) is employed for the switching elements q1 and q2, as a general characteristic of the TFT, the higher the withstand voltage rating between the drain and the source, the higher the conduction resistance tends to be. Also, there is a problem that the power conversion efficiency is reduced.

An object of the present invention is to provide a switching power supply capable of improving power conversion efficiency and suppressing noise.

[0020]

In a switching power supply according to the present invention, a load state detecting means detects a secondary output voltage of a transformer, and a control circuit responds to the result of the detection by a control circuit. The switching of the primary current of the transformer to obtain a secondary current of a desired constant voltage, wherein the primary winding of the transformer is interposed between the DC power supply lines. A resonance circuit interposed between the primary winding and the main switching element in a series circuit including the primary winding and the main switching element; a diode provided in parallel with the resonance capacitor; A sub-switching element provided in parallel with a series circuit of the above, a smoothing capacitor for deriving an output voltage between the DC power supply lines, a resonance switch and a main switch. A choke coil for providing a direct current from a bridge rectifier circuit between the switching element and the switching element, wherein the control circuit alternately conducts the main switching element and the sub-switching element, and turns on the main switching element. With the sub-switching element of
By f, it is set within a predetermined period during which the exciting current flows through the parasitic diode of the main switching element.

According to the above configuration, when the main switching element is turned on, the primary winding of the transformer, the resonance capacitor, and the main switching element are connected in series between the power supply lines. A current is induced on the secondary winding side of the transformer, and the current is smoothed and output.
Excitation energy is stored in the inductance component of the primary winding. When the main switching element is turned off, the 1
A closed circuit is formed by the next winding, the resonance capacitor, and the sub-switching element. First, after the electric charge stored in the floating capacitance of the sub-switching element is extracted, the closed circuit is passed through the parasitic diode of the sub-switching element. Current flows through the sub-switching element,
The sub-switching element is soft-on. When the release of the excitation energy by the inductance component of the primary winding is completed, the voltage between the terminals of the resonance capacitor becomes maximum,
The resonance capacitor starts discharging, and with the discharging,
Since the resistance component in the closed circuit of the primary winding, the diode, and the sub-switching element is extremely small, the current flows back with little attenuation.

Therefore, the auxiliary switching element is turned off.
Then, the recirculated current flows in the closed circuit of the main switching element and the diode from the primary winding through the power supply line, and the electric charge accumulated in the stray capacitance of the main switching element is released. As a result, the current flows in the parasitic diode of the main switching element. The control circuit turns on the main switching element within a predetermined period during which the exciting current flows through the parasitic diode of the main switching element. Thereby, the main switching element can be soft-on at the next excitation timing.

Therefore, the two switching elements can be softly turned on, noise can be suppressed, and the regenerative power of the excitation energy stored in the inductance component of the primary winding when the main switching element is turned on can be reduced. When the main switching element is turned off, the current is once stored in the resonance capacitor, and then the resonance capacitor is discharged to allow the inside of the closed circuit using the sub-switching element to return, so that the stray capacitance when the next main switching element is turned on is reduced. Since it is used for extracting electric charges, power conversion efficiency can be improved.

When the main switching element is turned off, the parasitic diode of the sub-switching element is turned on, the excitation energy stored in the choke coil L1 is regenerated, and the smoothing capacitor is boosted and charged. Further, when the sub-switching element is turned off, the current flowing back in the closed circuit of the primary winding, the sub-switching element, and the diode turns on the parasitic diode of the main switching element, and , A diode, a primary winding, and a smoothing capacitor, so that the smoothing capacitor is also boosted and charged.

Furthermore, even if the load state changes, the length of the period in which the current is circulated only needs to be changed, so that the switching cycle can be kept constant, and even if the load becomes light, the iron loss can be reduced. Does not increase, it is possible to improve efficiency, and measures against unnecessary radiation and malfunction are also taken.
It can be realized relatively easily.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described.
The following is a description based on FIGS. 1 to 9.

FIG. 1 is a block diagram showing an electrical configuration of a switching power supply 21 according to an embodiment of the present invention. The switching power supply device 21 generally includes a power supply circuit 22, two switching elements Q1 and Q2, a transformer N, a smoothing capacitor C1, and a resonance capacitor C2.
, A diode D1, a choke coil L1, a secondary output circuit including diodes D2 and D3, a choke coil L2 and a smoothing capacitor C3, and a voltage detection circuit 2.
3, a feedback circuit including a photocoupler PC and a control circuit 24.

The power supply circuit 22 includes a commercial power supply 25, a filter 26, and a diode bridge D. After the high frequency noise component of the commercial AC from the commercial power supply 25 is removed by the filter 26, Bridge D
And is output between the output terminals 27 and 28.

On the other hand, on the primary side of the transformer N, a primary winding N1 of the transformer N and a resonance capacitor C are provided between power supply lines 29 and 30 from a smoothing capacitor C1 serving as a power supply.
2 and a series circuit of the switching element Q1 are connected. Further, a diode D1 is provided in parallel with the resonance capacitor C2. Furthermore, the power supply line 29, the resonance capacitor C2 and the switching element Q
1, a switching element Q2 is provided in parallel with a series circuit of the primary winding N1 and the resonance capacitor C2. The resonance capacitor C2
A connection point between the power supply circuit 22 and the switching element Q1 is connected to a high-level output terminal 27 of the power supply circuit 22 via a choke coil L1, and the power supply line 30 is connected to a ground-level output terminal of the power supply circuit 22. It is connected to terminal 28.

On the secondary side of the transformer N, the intermediate tap N0 of the secondary winding N2 is an output terminal on the low level side, and is connected to the output terminal 32 via the power supply line 31. Both ends of the secondary winding N2 are connected from the high-level power supply line 33 to the output terminal 34 via diodes D2 and D3 and a choke coil L2, respectively. A smoothing capacitor C3 is provided between the power supply lines 33 and 31.

Further, between the power supply lines 33 and 31,
A voltage detection circuit 23 is provided. The voltage detection circuit 23 drives the light emitting diode D4 of the photocoupler PC according to the output voltage Vo between the output terminals 34 and 32. The phototransistor Q3 of the photocoupler PC is connected to the control circuit 24. The control circuit 24 alternately drives the gates of the switching elements Q1 and Q2 with a larger duty as the output voltage Vo decreases. . The drive signal to the gates of the switching elements Q1 and Q2 has a dead time set when its level is switched so that the two switching elements Q1 and Q2 are not turned on at the same time. Thus, feedback control for maintaining the output voltage Vo constant regardless of the load state is realized by pulse width modulation.

The switching elements Q1 and Q2 are composed of, for example, MOSFETs, and have parasitic diodes D5 and D6 between the source and the drain, respectively. Further, the switching elements q1, q
A small-capacity capacitor as shown in FIG. 2 may be provided, and the voltage between the source and the drain may be gradually changed to reduce the power consumption.

FIG. 2 is a waveform diagram for explaining the operation of the switching power supply device 21 configured as described above, and FIGS. 3 to 8 are current path diagrams for explaining the operation. In FIG. 2, E _Q1G and E _Q2G represent the gate potentials of the switching elements Q 1 and Q 2, that is, the level of the drive signal from the control circuit 24. When the potentials E _Q1G and E _Q2G are at the high level, , The switching elements Q1 and Q2 are turned on. I _IN represents a current supplied from the power supply circuit 22 through the choke coil L1, and a direction flowing from the choke coil L1 to a connection point between the resonance capacitor C2 and the switching element Q1, as indicated by a broken line in FIG. Is the positive direction.
I _L represents the current flowing through the primary winding N1, as indicated by the solid line in FIG. 3, a direction flowing to the resonance capacitor C2 from the primary winding N1 and the positive direction. I _Q1 , I
_Q2 represents the current flowing through the switching elements Q1 and Q2, and the direction indicated by the solid line in FIG. 3 and the direction indicated by the solid line in FIG. E _Q1 , E _Q2
Are the drains of the switching elements Q1 and Q2, respectively.
Indicates the source-to-source voltage. E _C2 represents the voltage between the terminals of the resonance capacitor C2, and the direction charged from the + direction to the − direction shown in FIGS. 3 and 4 is defined as the positive direction. I _D1 and I _D2 represent secondary-side induced electromotive currents rectified by the diodes D2 and D3, and the direction in which the smoothing capacitor C3 is charged is the positive direction.

Between times t1 and t2, as shown in FIG. 3, the switching element Q1 is on and the output terminal 2
7 supplied via the choke coil L1 from the current I _IN
Is connected to the output terminal 28 via the switching element Q1.
Flows to As described later, the smoothing capacitor C1
In is the voltage E _C1 shown by the following formula is always charged by the voltage E _C1, current I _L, the smooth capacitor C1
It flows in a closed circuit of the primary winding N1, the resonance capacitor C2, the switching element Q1, and the smoothing capacitor C1. E _C1 = √2Ea + α (1) where Ea is the effective value of the commercial power supply 25 and α is
As a result of the current I _IN flowing between times t2 and ts described later,
This is the charging voltage that occurs.

The current induced in the secondary winding N2 becomes a charging current _ID1 that increases in an arc shape on the diode D2 side.
It is supplied from the choke coil L2 to the smoothing capacitor C3.

In this state, since the inductance of the primary winding N1 roughly resonates with the resonance capacitor C2, as shown in FIG. 2, the current I _L and the voltage E _C2 follow a sinusoidal curve. Increase. The current I _D1 also increases with similar to the curve on the current I _L.

On the other hand, the current I flowing through the choke coil L1
_IN increases linearly until the time t2, as shown in the following equation.

I _IN = (T _ON / L 1) × V _IN (2) where T _ON is a time starting from time t 6 described later, L 1 is an inductance of the choke coil L 1, and V _IN Is the output voltage of the power supply circuit 22 (output terminal 2
7, 28).

Further, a combined current represented by I _Q1 = I _L + I _IN (3) flows through the switching element Q1.

Between times t2 and t4, as shown in FIG. 4, first, at time t2, the switching element Q1 is turned off.
Then, the current _IQ1 flowing through the switching element Q1 becomes zero. However, the primary winding N1 the current I _L flows through the excitation energy stored in the current I _IN by the excitation energy stored in the choke coil L1
After the electric charge accumulated in the floating capacitance existing between the drain and the source of the switching element Q2 is extracted, the electric current flows through the diode D6 that is parasitic between the drain and the source of the switching element Q2.

Therefore, the positive current I generated by the current preserving action of the inductance component of the primary winding N1
_L flows in a closed circuit of the primary winding N1-resonant capacitor C2-switching element Q2-1-primary winding N1, and the excitation energy stored in the inductance component is regenerated to charge the resonance capacitor C2. You. Also, the excitation energy stored in the inductance component of the choke coil L1 is regenerated, and the current I _IN causes the smoothing capacitor C
Charged to 1.

From the time t2, the current I _IN flowing through the choke coil L1 decreases linearly as shown by the following equation, and becomes zero at the time ts.

[0043]

(Equation 1)

Here, T _6-2 is the time from time t6 to time t2 described later, E _C1 is the charging voltage of the smoothing capacitor C1, and T _OFF is the time starting from time t2. It is.

The time T from time t2 to time ts
_2-s can be represented by the following equation.

[0046]

(Equation 2)

During this period, the resonance capacitor C2 and the primary winding N1 are in a current resonance state, and the currents I _L , I _Q2 and the voltage E _C2 are equal to those in FIG.
As shown by, it changes along a sinusoidal curve. Further, the current I _L by the electromotive force direction is reversed induced in the secondary side, current I _D1 is next zero, the current I _D2 instead flows, changes in an arc shape.

At time t3, the control circuit 24 switches the switching element Q2 from off to on. At this time,
At the time t2, the currents I _L and I _IN have already flowed through the parasitic diode D6, and the drain-source voltage E _Q2 of the switching element _Q2 is 0V.
At time t3, the switching element Q2 can be turned on softly.

Between times t4 and t5, the primary winding N1
Is completed, the voltage _EC2 between the terminals of the resonance capacitor C2 becomes maximum, and as shown in FIG. 5, the switching element Q2 that has already been turned on causes the resonance capacitor C2-1 to have a primary winding. the closed circuit line N1- switching element Q2- resonance capacitor C2, the negative direction of the current I _L flows through the discharge of the resonant capacitor C2, the inductance component of the primary winding N1 again, and when on the switching element Q1 , The excitation energy in the opposite direction is stored. During this period, the resonance capacitor C2 and the primary winding N1 are in a current resonance state, and the current _IL and the voltage _EC2 change along a sinusoidal curve as shown in FIG. Accordingly, the current _ID2 also attenuates along the sinusoidal curve.

Between times t5 and t6, the diode D1 is replaced by the primary winding N1 and the switching element Q2 as shown in FIG.
And form a closed loop. Thus, since the conduction resistance of the closed loop such as by the resistance component of the on resistance and the primary winding N1 of the switching element Q2 and the diode D1 is small, so that a closed-loop negative direction of the current I _L continues to reflux .

[0051] During the time interval t6 to t8, as shown in Figure 7, first, the transistor Q2 is turned off at time t6, the current I _L, the primary winding N1- parasitic diode of the smoothing capacitor C1- transistor Q1 D5- The diode D1-1 flows through the closed circuit of the primary winding N1, and after extracting the electric charge accumulated in the floating capacitance of the transistor Q1,
Flows through the parasitic diode D5 of the switching element Q1,
The smoothing capacitor C1 is charged.

The smoothing capacitor C1 is, for example, 15
Is set to a relatively large capacity of about 00MyuF, the peak value of the current I _L contrast is 8A for example, about,
Even when the current I _IN and the current I _L are charged, the voltage between the terminals is kept constant without substantially changing. Thus the negative direction of the current I _L, as shown in Figure 2, decreases linearly toward the zero level. Thus, in a state where the voltage between the drain and the source of the switching element Q1 is 0 V, the control circuit 24 sets the gate of the switching element Q1 to the high level, and realizes the soft-on operation.

[0053] At this time, the path of the power supply circuit 22-a choke coil L1- diode D1-1 winding N1- smoothing capacitor C1- power circuit 22, start the current I _IN flows, I _L = I _Q1 + I _IN ... (6) As the supply current I _IN from the power supply circuit 22 increases, the current I _Q1 flowing through the closed loop decreases, and at time t8, I _L = I _IN .

At the time t6, the primary winding N
1, an electromotive voltage in the direction of charging the smoothing capacitor C1 is generated, and the current I _D1 also starts flowing on the secondary side.

[0055] During the time t8~t1, I _L <I _IN, and the addition by the switching element Q1 is on, as shown in Figure 8, current I _IN is, the resonant capacitor C
The current splits at the connection point between the switching element Q1 and the switching element Q1, and most of the current flows through the switching element Q1. When release of the excitation energy at time t1 in the primary winding N1 is completed, the negative direction of the current I _L becomes zero, then, as shown in FIG. 3, a smoothing capacitor C
Positive current I _L flows from 1, the charging of the resonance capacitor C2 is started.

In the switching power supply 21 configured as described above, the control circuit 24 detects the output voltage Vo between the output terminals 34 and 32 by the voltage detection circuit 23 as described above, and the output voltage Vo is When the load is a heavy load that is going to be lower than the rated voltage, the duty is increased, that is, the period between times t6 to t1 and times t1 to t5 is increased, whereby the currents I _D1 and I _D2 flowing through the secondary diodes D2 and D3 are reduced. Enlarge. When the load is light, the time between the times t6 to t1 and t1 to t5 is shortened. On the other hand, the switching period, that is, the period from time t1 to the next time t1 is constant, and the larger the duty, the shorter the period from time t5 to t6.
The time is set to be zero during the time t5 to t6.

Next, the power factor improving function will be described. First, the inductance of the choke coil L1 is determined when the secondary load current is at the rated maximum, that is, at the time t6 to t6.
The period between t1 and t1 to t5 is maximum, and the period from time t5 to t6
When the interval becomes zero, it is selected to satisfy the following equation.

T _2-s <T _2-6 (7) where T _2-s is a period from time t2 to time ts, and T _2-6 is a period from time t2 to time t6. is there.

The inductance of the choke coil L1, the period T2 _-s, and the output voltage E _C1 of the smoothing capacitor C1 have the relationship shown in Table 1.

[0060]

[Table 1]

On the other hand, the output voltage E _C1 is obtained by discharging the excitation energy accumulated in the choke coil L1 during the period T _6-2 from time t6 to time t2 within the period from time t2 to time ts. , The output voltage V of the power supply circuit 22
_IN is a voltage obtained by boosting, and as the potential difference between the voltage V _IN and the output voltage E _C1 increases, the choke coil L
There is a problem that the excitation energy to be stored in the coil 1 increases, the hysteresis loss of the choke coil L1 mainly increases, and the power conversion efficiency decreases. Further, when the output voltage E _C1 becomes high, it is necessary to use high withstand voltage elements for the switching elements Q1 and Q2.
In the FT, there is a problem that the conduction resistance increases, which also lowers the power conversion efficiency.

However, the absolute value | Iac | of the input current Iac injected from the commercial power supply 25 is represented by the following equation.

[0063]

(Equation 3)

Where To is the switching cycle, Ea is the output voltage of the commercial power supply 25, that is, the effective value of the input voltage of the switching power supply 21, and ω
Is the angular velocity of the commercial power supply 25.

Therefore, as the output voltage E _C1 of the power supply circuit 22 increases, the value of the second term in the parentheses in the above equation decreases, and the input current Iac can be approximated to a sine wave. That is, the relationship between the output voltage E _C1 , the power conversion efficiency of the switching power supply 21, and the degree of approximation of the waveform of the input current Iac from the commercial power supply 25 to the input voltage Ea is as shown in the following table.

[0066]

[Table 2]

In view of the above Table 2, the present inventor selects one example of the circuit constant of the switching power supply device 21 as follows, so that the switching power supply in which the countermeasures against the harmonics of the input current are taken is commonly called. As a power supply device called a one-converter, the highest power conversion efficiency of 82% was obtained, and the results shown in Table 3 and FIG. 9 were obtained.

Input voltage Ea from commercial power supply 25: 100
V (effective value) Commercial power frequency: 50 Hz Secondary output voltage Vo: 24 V Secondary output current: 6 A Inductance of choke coil L1: 33.5 μH Number of turns and inductance of primary winding N1: 11 turns, 41 μH secondary Number of turns of winding N2: 4 turns × 2 Inductance of choke coil L2: 7.7 μH Capacitance of resonance capacitor C2: 0.12 μF Switching cycle To: 10 μsec (100 kHz)

[0069]

[Table 3]

As is apparent from Table 3, the charging voltage E _C1 of the smoothing capacitor C ₁ is almost _independent of the load conditions, is low voltage, and has a substantially constant value.
1 and Q2, an element having a small conduction resistance and a low withstand voltage of about 300 V can be used.

As is apparent from FIG.
For the input voltage waveform from the commercial power supply 25 indicated by 1, the input current waveform is indicated by reference numeral α2, and the regulation value of class A defined by the standard (IEC1000) on the regulation of input current harmonics (IEC1000) In FIG. 9, the period of 13% or more exceeds the convex frame indicated by reference numeral α3).

As described above, o of switching element Q1
At time n, the regenerative power of the excitation energy stored in the inductance component of the primary winding N1 is temporarily stored in the resonance capacitor C2 by the off of the switching element Q1, and then the primary winding N1-switching element Q2- It is returned to the closed circuit by the diode D1 and used to remove the electric charge accumulated in the stray capacitance when the next switching element Q1 is turned on, is soft-on, and is connected to the choke coil L1 when the switching element Q1 is turned on. Since the stored excitation energy is used to boost and charge the smoothing capacitor C1 serving as a power supply at the time of off, low power consumption and low noise can be achieved.

The charging voltage E _{C1 of the} smoothing capacitor C1 is
Can be suppressed relatively low, the hysteresis loss of the choke coil L1 can be reduced, and an element having a small conduction resistance can be selected for the switching elements Q1 and Q2, thereby also reducing the loss. be able to. Furthermore, switching elements Q1,
When Q2 is on / off, the drain-source voltage E _Q1 ,
There is no crossing between E _Q2 and the drain current, which also prevents losses.

Furthermore, even if the load condition changes, the length between times t6 to t1 and t1 to t5 changes and the switching cycle is kept constant, so that the iron loss increases even when the load becomes light. Therefore, the efficiency can be improved, and measures against unnecessary radiation and malfunction can be realized relatively easily.

When the switching element Q1 is turned off, the inductance of the primary winding N1 of the transformer N and the resonance capacitor C2 resonate with each other, so that the primary winding N1
Ringing noise due to the leakage inductance between the coil and the secondary winding N2 can be prevented. Similarly,
Also when the switching element Q2 is turned off, the primary winding N
Since the ringing noise due to the leakage inductance between the primary winding N2 and the secondary winding N2 is smoothed by the smoothing capacitor C1, the occurrence of the ringing noise can be prevented.

[0076]

The switching power supply according to the present invention has the following features.
As described above, when the main switching element is turned on, the regenerative power of the excitation energy stored in the inductance component of the primary winding of the transformer is dissipated when the main switching element is turned off. It is returned and accumulated in the closed circuit with the switching element, and is used for extracting the electric charge accumulated in the floating capacitance of the main switching element when the next main switching element is turned on.

Therefore, the main switching element can be soft-on at the next excitation timing, noise can be suppressed, and power conversion efficiency can be increased.

When the main switching element is turned off, the parasitic diode of the sub-switching element is turned on, the excitation energy stored in the choke coil L1 is regenerated, and the smoothing capacitor is boosted and charged. Further, when the sub-switching element is turned off, the current flowing back in the closed circuit of the primary winding, the sub-switching element, and the diode turns on the parasitic diode of the main switching element, and , A diode, a primary winding, and a smoothing capacitor, so that the smoothing capacitor is also boosted and charged.

Further, even if the load condition changes, the length of the period in which the current is recirculated only has to be changed, and the switching cycle can be kept constant. Does not increase, it is possible to improve efficiency, and measures against unnecessary radiation and malfunction are also taken.
It can be realized relatively easily.

[Brief description of the drawings]

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

FIG. 2 is a waveform chart for explaining the operation of the switching power supply device shown in FIG.

FIG. 3 is a current path diagram for explaining an operation of the switching power supply device shown in FIG.

FIG. 4 is a current path diagram for explaining the operation of the switching power supply device shown in FIG.

FIG. 5 is a current path diagram for explaining the operation of the switching power supply device shown in FIG.

FIG. 6 is a current path diagram for explaining the operation of the switching power supply device shown in FIG.

7 is a current path diagram for explaining the operation of the switching power supply device shown in FIG.

FIG. 8 is a current path diagram for explaining the operation of the switching power supply device shown in FIG.

9 is a diagram showing input voltage / current waveforms when the switching power supply device shown in FIG. 1 is under heavy load.

FIG. 10 is a block diagram showing an electrical configuration of a typical conventional switching power supply device.

11 is a waveform chart for explaining the operation of the switching power supply device shown in FIG.

[Explanation of symbols]

Reference Signs List 21 switching power supply device 22 power supply circuit 23 voltage detection circuit 24 control circuit 25 commercial power supply 26 filter C1, C3 smoothing capacitor C2 resonance capacitor D diode bridge D1 to D3 diode D4 light emitting diode D5, D6 parasitic diode L1, L2 choke coil N transformer N1 Primary winding N2 Secondary winding PC Photocoupler Q1 Switching element (main switching element) Q2 Switching element (sub-switching element) Q3 Phototransistor

Claims (1)

[Claims] A load state detection means detects a secondary output voltage of a transformer, and a control circuit supplies a drive signal responsive to the detection result to a main switching element to switch a primary current of the transformer. A switching power supply device for obtaining a secondary current of a desired constant voltage, wherein a series circuit of the primary winding of the transformer and the main switching element, which is interposed between DC power supply lines, A resonance capacitor interposed between the secondary winding and the main switching element; a diode provided in parallel with the resonance capacitor; and a sub-switching element provided in parallel with a series circuit of the primary winding and the resonance capacitor. A smoothing capacitor for deriving an output voltage between the DC power supply lines, and a DC from a bridge rectifier circuit between the resonance capacitor and a main switching element. A choke coil that provides current flow, the control circuit alternately conducts the main switching element and the sub-switching element, and turns on the main switching element by turning off the sub-switching element. The switching power supply device is set within a predetermined period during which the exciting current flows through the parasitic diode.