JPH03222671A - Switching power supply - Google Patents

Abstract

PURPOSE:To suppress surge voltage and to reduce loss by performing ON/OFF control of a first switching element and further performing ON/OFF control of a second switching element within the OFF interval of the first switching element. CONSTITUTION:A first capacitor 18 is connected in parallel with a reverse parallel circuit of a first switching element 7 and a first diode 17, while furthermore a second capacitor 20 having larger capacitance than the first capacitor 18 is connected through a parallel circuit of a second diode 21 and a second switching element 22 in parallel with the reverse parallel circuit. A control circuit 8 performs ON/OFF control of the first switching element 7 and further performs ON/OFF control of the second switching element 22 within OFF interval of the first switching element 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a voltage resonant switching power supply device. [Prior Art] A switching regulator connects a transformer and a switching element in series, generates alternating current on the secondary side of the transformer by controlling on/off of the switching element, and obtains a direct current output by rectifying the alternating current. Widely used. FIG. 14 shows an example of a conventional voltage resonance type switching regulator. A DC power supply 1 of this switching regulator consists of a full-wave rectifier 3 and a smoothing capacitor 4 connected to an AC power supply terminal 2. A transformer 5 is connected between one end and the other end (ground terminal) of this DC power supply 1.
A series circuit of a primary winding (main winding 1t) 6 and a switching element 7 made of a field effect transistor is connected. A control terminal (gate electrode) of the switching element 7 is connected to a control circuit 8. The secondary winding 9 of the transformer 5 is
diode 10.1], reactor 12, and capacitor 1
It is connected to DC output terminals 15 and 16 via an output rectifying and smoothing circuit 14 consisting of 3 and 3. A diode 17 is connected in antiparallel to the switching element 7 in order to cause a current to flow in the opposite direction to the switching element 7. In this example, since the switching element 7 is an N-channel insulated gate field effect transistor in which the substrate is connected to the source,
It has a built-in diode. Therefore, it is not necessary to connect diode 17 externally, but it is shown separately for ease of understanding. The capacitor 18 connected in parallel to the switching element 7 has the effect of slowing down the voltage rise of the switching element 7. The on-time of the switching element 7 is controlled based on detection by a voltage detection circuit 19 connected to the output terminal 15. [Problem to be Solved by the Invention 1] By the way, a surge voltage is generated in the primary winding 6 when the switching regulator is turned off, but this is absorbed by the capacitor 18. Further, at this time, the rise of the voltage of the switching element 7 becomes gradual as shown in FIG. 15, resulting in the effect of reducing switching loss. However, when the voltage of the capacitor 18 finally reaches the power supply voltage Vin, the energy of the capacitor 18 is released at turn-on, resulting in a loss. SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a switching power supply device that can easily achieve both surge absorption and loss reduction. [Means for Solving the Problems] The present invention for achieving the above object comprises: a DC power supply; a transformer connected between one end and the other end of the DC power supply;
first and second main terminals are connected to a control terminal, the first main terminal is connected to one end of the DC power supply via the transformer, and the second main terminal is connected to the other end of the DC power supply. a first switching element connected to the transformer; a first capacitor comprising a separate capacitor or stray capacitance associated with the transformer in resonance with the inductance of the transformer; and an output circuit coupled to the transformer. and,
a second capacitor or a voltage source capable of applying a voltage to the first capacitor to prevent or slow a change in the voltage of the first capacitor during an off period of the first switching element; a second capacitor or voltage source connected in series with the second capacitor or voltage source for selectively relating the first capacitor or voltage source to the first capacitor during off-periods of the first switching element; a switching element, and a control circuit that controls on/off the first switching element and controls on/off the second switching element within the off period of the first switching element. This relates to switching power supplies. Note that, as shown in claim 2, constant voltage control can be performed by changing the on-time width of the first switching element. Moreover, as shown in claim 3, constant voltage control can be performed by changing the on-time width of the second switching element. Moreover, as shown in claim 4, constant voltage control can be performed by changing the voltage of the second capacitor or voltage source. [Function] According to the invention of each claim, it becomes possible to absorb surge voltage by the function of the first and second capacitors,
One first switching element can be turned off and turned on at zero voltage. [First Embodiment] Next, a voltage resonant switching regulator according to a first embodiment of the present invention will be described with reference to FIGS. 1 to 3. However, the parts in FIG. 1 that are common to FIG. 14 are given the same reference numerals and their explanations will be omitted. The switching regulator shown in FIG. A second capacitor 20 having a large capacity is also connected in parallel through a parallel circuit of a second diode 2 and a second switching element 22. That is, the upper end of the second capacitor 20 is connected to the lower end of the primary winding 6, and the lower end of the second capacitor 20 is connected to the power supply through a parallel circuit of the second diode 2 and the second switching element 22. It is connected to the bottom end of 1. Note that a small-capacity capacitor 23 for noise removal is connected in parallel to the diode 2. In this embodiment, the second switching element 22 is a P-channel insulated gate field effect transistor with a built-in diode by connecting the substrate to the source, so the diode 2 can be omitted. Diode 2] is shown separately for ease of understanding. FIG. 2 shows the control circuit 8 of FIG. 1 in detail,
a first control signal forming circuit 3 which forms a control signal for the second switching element 22 based on voltage detection of the second capacitor 20; and a first control signal of the first control signal forming circuit 3. and a second control signal forming circuit 32 that forms a second control signal having a certain time relationship with the first switching element 7 and supplies it to the first switching element 7. The first control signal forming circuit 3] includes a voltage dividing circuit consisting of resistors 33a and 34b connected to both ends of the second capacitor 20, and one input terminal connected to the voltage dividing point of this voltage dividing circuit, It consists of a voltage comparator 35 whose other input terminal is connected to a reference voltage source 34 , and a drive circuit 36 connected between the output terminal of this comparator 35 and the second switching element 22 . Note that the reference voltage source 34 consists of a voltage dividing circuit including a phototransistor 38 and a resistor 39 connected between a power supply terminal 37 and the ground, and an inverting input terminal of a comparator 35 is connected to this voltage dividing point. There is. The phototransistors 3 and 8 are optically coupled to a light emitting element (not shown) that emits light in response to the output voltage detected by the voltage detection circuit 19 shown in FIG. The second control signal forming circuit 32 includes a trailing edge detection circuit 40 connected to the comparator 35 and a delay circuit 41 connected to the trailing edge detection circuit 40.
, a clock pulse generator 42 , a flip-flop 43 that is set by the output of the delay circuit 41 and reset by the clock pulse, and a drive connected between the output terminal of the flip-flop 43 and the first switching element 7 Mouth R
It consists of 44. [Work 1 Before the time tl in Figure 3, the first switching element 7 is on, so the voltage between both terminals (train and source) is as shown in Figure 3 (D). When the clock pulse shown in FIG. 3A is generated from the clock pulse generator 42 of FIG. 2 at a very low time t1, the flip-flop 43 is reset and the first switching element 7 is turned off. When the first switching element 7 is turned off, a flyback voltage is generated in the primary winding 6 of the transformer 5, but the inductance of the primary winding 6 and the
The current ■1 shown in FIG. 3 (F) according to the first resonant frequency of the LC resonant circuit with the capacitor 18 of
The energy of the primary winding 6 is transferred to the first capacitor 18 by the resonance action flowing in the closed circuit consisting of the capacitor 18 and the DC power supply 1, so that the first capacitor 18 is charged, and this charging voltage Vc gradually increases. It becomes expensive. Since the voltage of the first capacitor 18 is the same as the voltage between both terminals of the first switching element 7, the voltage of the first switching element-7 also gradually increases. This makes it possible to turn off the first switching element 7 at substantially zero volts. During the off period, a charging current also flows through the second capacitor 20 connected in parallel to the first capacitor 18, and the second capacitor 20 is charged to a certain value. This second capacitor 20
The charge is only discharged through the second switching element 22, and this discharge period is part of the off period. Therefore, during the ON period of the first switching element 7, the second
The capacitor 20 does not discharge and maintains a relatively high charging voltage as shown in FIG. 3(H). By the way, during a period in which the voltage Vcl of the first capacitor 18 is lower than the voltage VC2 of the second capacitor 20, the second diode 2]- is in a reverse bias state, and the primary winding 6
Even if a flyback voltage occurs in the second capacitor 20
The voltage V of the first capacitor 18 at which the current I2 does not flow through
When C1 becomes somewhat higher than the voltage Vc2 of the second capacitor 20, the second diode 2 becomes forward biased and turns on at time t2. As a result, the first and second capacitors 18.20 are substantially connected in parallel, a resonant circuit consisting of the primary winding 6 and the first and second capacitors 18.20 is formed, and a second resonant circuit is formed. A current flows in the primary winding 6 according to the frequency. Since the capacitance of the second capacitor 20 is sufficiently larger than the capacitance of the first capacitor 18, t2~
The change in the voltage of the second capacitor 20 during the t5 period becomes extremely small. When a charging current flows through the second capacitor 20, this voltage Vc2 gradually increases and crosses the reference voltage Vr at time t3, and the output of the comparator 35 in FIG. 2 becomes as shown in FIG. 3(B). This is reversed, and the second switching element 22 is turned on. Even if the second switching element 22 is turned on at time t3, if the resonant voltage of the second capacitor 20 has not reached its peak, the current 2 continues to flow through the second diode 2. When the voltages of the first and second capacitors 18.20 reach their peak at time t4, the voltage of FIG. 3 (F
) (G) current 1] and I2 begin to flow in the opposite direction. At this time, since the second switching element 22 is on, the capacitor 2
A discharge current of 0 flows. The discharge current of the first capacitor 18 flows in a closed circuit consisting of the first capacitor 18, the primary winding 6, and the power supply 1. Voltage V of second capacitor 20
As shown in FIG. 3(H), c2 becomes the reference voltage V at time t5.
3, the output of the comparator 35 is inverted as shown in FIG. As a result, after time t5, the operation shifts to the first resonant frequency based on the resonant circuit of the primary winding 6 and the first capacitor 18, and the voltage Vc of the first capacitor 18 increases.
gradually decreases and reaches zero at time t6 in FIG. In addition,
During the first half period during this period t5 to t6, when the voltage of the first capacitor 18 is higher than the voltage of the power supply 1, the energy of the first capacitor 18 is released to the power supply 1, and the voltage of the first capacitor 18 increases. During the latter half of the period when the voltage is lower than the voltage of the power supply 1, the energy of the first capacitor 18 is released to both the power supply and the load. Thereafter, the energy stored in the primary winding 1]6 is released in the circuit consisting of the primary winding 6 and the first diode 17 during the period t-6 to t7, and then the positive energy is transferred through the first switching element 7. Current flows in the direction.
The trailing edge detection circuit 40 in FIG. 2 detects the trailing edge time t5 of the comparison output pulse in FIG. . At time t6 when the second switching element 7 is turned on, the voltage across the first switching element 7 and the voltage of the first capacitor 18 are zero, so switching loss at turn-on is reduced and the capacitor 18 energy losses are also reduced. When the clock pulse shown in FIG. 3A occurs again at time ta, the same operation as in the period t1 to t8 occurs repeatedly. For example, when the output voltage becomes lower than a predetermined value, the output voltage of the voltage detection circuit 19 increases, the light output of the light emitting element built therein increases, and the resistance value of the phototransistor 38 in FIG. 2 decreases. , the reference voltage V" becomes higher. As a result, the period during which the voltage VC2 of the second capacitor 20 is the reference voltage in FIG. 3(H) becomes shorter,
Eventually, the off period of the first switching element 7 becomes shorter and the output voltage returns to its original level. Since the clock pulses in FIG. 3A are generated at a constant cycle, if the off-period of the first switching element 7 changes, the on-period also inevitably changes. This makes it possible to keep the frequency constant and change the duty. Note that if the frequency is constant, noise countermeasures will be easier. [Second Example] Next, a switching regulator of a second example shown in FIG. 4 will be described. However, in FIG. 4 and FIGS. 5 to 12, which will be explained later, the same parts as those in FIG. 1 and FIG. 14 are designated by the same reference numerals, and the explanation thereof will be omitted. In this embodiment, a tertiary winding 5 is electromagnetically coupled to a primary winding 6.
0 is provided, and a second capacitor 20 is connected in parallel to this. Note that a parallel circuit including a second diode 2 and a second switching element 22 is connected between the tertiary winding 50 and the second capacitor 20. The second capacitor 20 is connected to the tertiary winding 5 as shown in FIG.
Even if it is connected through 0, it is equivalently connected in parallel to the primary winding [6]. First and second switching elements 7.2
The on/off control of No. 2 is performed in the same manner as in the first embodiment. The second diode 2 is turned on when a voltage higher than the charging voltage of the second capacitor 20 is generated in the tertiary winding 50 during the off period. This second embodiment also provides the same effects as the first embodiment. [Third Example] In the switching regulator of the third example shown in FIG. 5, a second capacitor 20 is connected in parallel to the primary winding 6. The parallel circuit of the second switching element 22, the diode 2], and the capacitor 23 is connected in series to the second capacitor 20, as in FIG. In this circuit as well, the first switching element 7 and the second switching element 22 are driven on and off in the same manner as in FIG. When the first switching element 7 is turned from on to off, a current flows due to the resonance between the primary winding 6 and the first capacitor 18, and the second diode 2 and the second switching element 22 are turned on. By this, the second capacitor 20 is connected in parallel with the primary winding [6].
and two capacitors 18,? Current flows due to resonance due to zero. When the second switching element 22 is turned off, current flows again through the resonant circuit of the primary winding 6 and the first capacitor 18. In the circuit shown in FIG. 5, a reactor L1 is connected in series with the primary winding 6 to assist resonance. This kind of glue. Tors can also be inserted in other embodiments. The circuit of FIG. 5 is substantially equivalent to the circuits of FIGS. 1 and 4, and therefore has the same effect. [Fourth Embodiment] In the switching regulator of the fourth embodiment shown in FIG.
l! has been done. A parallel circuit of the second diode 2], the second switching element 22, and the capacitor 23 is connected to the second capacitor 20 in the same manner as in FIG. In this embodiment, the secondary winding 9 has the function of coupling the second capacitor 20 to the primary winding 6 in addition to the function of the output winding. In the circuit shown in FIG. 6, the second capacitor 2q is equivalently connected in parallel to the primary winding 6 during the off period, and the same effect as in the circuits shown in FIGS. 1, 4, and 5 is obtained. can get. [Fifth Embodiment] The switching regulator of the fifth embodiment shown in FIG.
It has a parallel circuit with 7a. Therefore, 1/2 of the voltage during the off period is shared between the two first switching elements 7.7a. Since the two first switching elements 7.7a are driven on and off at the same time, they operate substantially the same as the embodiment of FIG. 5. Note that the capacitor 18a has the same function as the capacitor 18, and the diode 17a has the same function as the diode 17. The reactor 51 connected in series to the secondary winding 9 is for easily generating resonance. [Sixth Example 1] In the switching regulator of the sixth example shown in FIG. 8, the lower end of the tertiary winding 1] 50 is connected to the upper end of the primary winding [6], and the upper end of the tertiary winding 50 A second capacitor 20 is connected between the power supply 1 and the lower end thereof. Note that a parallel circuit including a second diode 2], a second switching element 22, and a capacitor 23 is connected in series to the second capacitor 20. Even if the second capacitor 20 is related to the primary winding 6 in this manner, it is equivalent to the circuits shown in FIGS. 1 and 4 to 7, and similar effects can be obtained. [Seventh Embodiment f14] The configuration of the main circuit of the switching regulator according to the seventh embodiment of the present invention shown in FIG. 9 is the same as that in FIG. 4. However, the first and second switching elements 7. of FIG.
The control circuit 22 is different from that in FIG. In order to detect the current of the first switching element 7, a resistor R1 is connected in series with the first switching element 7, and this resistor R1
In order to detect the voltage across the first switching element 7, voltage dividing resistors R2 and R3 are connected between the upper end of the first switching element 7 and the ground. Further, in order to apply a control signal to the gate of the first switching element 7, a resistor R4 is connected between the gate and the ground, and the gate is connected to the power supply terminal 64 via a resistor R41.
It is connected to the. A transistor 62 is connected in parallel to the resistor R4, and its base is connected to the voltage dividing point of the resistors R2 and R3. A resistor R5 is connected in series with this to detect the current of the second switching element 22, and a voltage dividing resistor R6 is connected between both terminals of the resistor R5 and the second switching element 22 to detect the voltage across the terminal. , R7 are connected. Further, a resistor R8 is connected between the gate of the second switching element 22 and the ground, a transistor 63 is connected in parallel to this resistor R8, and the second switching element 22
The gate of is connected to the power supply terminal 64 via the resistor R9-RIG. Further, in order to control the output voltage, a phototransistor 65 is connected in parallel to the resistor RIG, and is optically coupled to a light emitting element (not shown) of the voltage detection circuit 19. 1 work] When the first switching element 7 is turned on and the current flowing through it gradually increases and the detection voltage based on the resistor R1 exceeds a certain value, the transistor 62 is turned on and the gate is closed. The first switching element 7 is turned off because it is connected to ground. Resonance between the primary winding 6 and the first capacitor 18 during turn-off occurs in the same manner as in the first embodiment, and the voltage of the first capacitor 18 gradually increases due to the resonance. During the off period of the first switching element 7, a downward voltage is first generated in the tertiary winding 50,
A closed circuit consisting of the secondary winding 50, the second capacitor 20, and the second diode 2 is formed. This allows the first
Each capacitor 18.20 is charged by resonance based on the second capacitor 18.20 and the transformer 5, and then discharged. The second switching element 22 has a second
The ON control signal is applied almost in synchronization with the conduction of diode 2], but both f! second diode 2 at pressure
The second switching element 22 is not turned on during the period in which the second switching element 22 is turned on.
The discharge current of the capacitor 20 flows through a closed circuit consisting of the secondary winding 50, the second switching element 22, and the resistor R5. This current increases with time, and the divided voltage output by resistors R6 and R7 also gradually increases. As a result, transistor 6
3 becomes smaller, and the second switching element 22
The second switching element 22 turns off at the point when it is no longer possible to maintain the on state. Since the phototransistor 65 is optically coupled to a light emitting element (not shown) of the voltage detection circuit 19, if the output voltage becomes too high, for example, the resistance value of the phototransistor 65 increases and the gate voltage decreases. Thereby, control of the turn-off point of the second switching element 22 is achieved, and it becomes possible to control the turn-off of the first switching element 7 similarly to the first embodiment. The circuit of FIG. 9 is substantially the same as the first embodiment except for the direction in which the control signal is formed, and can obtain the same effects. [Eighth Embodiment] FIG. 10 shows a control circuit of a switching regulator according to an eighth embodiment. This main circuit is the same as that in FIG. 1, and only the control circuit 8 in FIG. 1 is the same as in FIG. It has been modified as shown. The control circuit 8a in FIG. 10 is the control circuit 8 in FIG.
10, a current detection resistor 20 is connected in series with a capacitor 20, and this resistor 70 is connected in series with a current detection resistor 20. A current direction detection circuit 71 is connected. This current direction detection circuit 71 is a circuit that detects the point in time when the direction of the current flowing through the resistor 70 via the capacitor 20 is reversed, and detects the point in time t4 in the first figure, which corresponds to the point in time t4 in FIG. 1] Generates the output shown in Figure (C). The first timer 72 connected to the current direction detection circuit 71 is
In response to the leading edge of the pulse in Fig. 1] Fig. (C), Fig. 1] Fig. (D
), a pulse W7aT1 is generated at a first fixed time. On the other hand, the comparator circuit 35 generates an output pulse as shown in FIG. 1 (A) as in the case of FIG. This pulse is sent to the flip-flop 7 via a trigger circuit 73.
4 set input, and the flip flop 74 is the first]
The level becomes high as shown in Figure (B). The output of the current direction detection circuit 71 is connected to the reset terminal of the flip-flop 174 via a trigger circuit 75. Therefore, the flip-flop 74 connects the first and second capacitors 7.2
It is reset at time t4 when 0 becomes a discharge state, and the pulse shown in FIG. 1 (E) is generated. Since the output of the flip-flop 74 and the output of the first timer 72 are input to the OR gate 76, the output shown in FIG. is supplied to the switching element 22 as an on control signal. OR gate 7
The output of 6 is a trailing edge (falling edge) detection circuit 40 and a delay circuit 41.
is applied to the second timer 77 via. The second timer 77 is triggered at time t6, which is delayed by Td from time t5 in the first figure, and generates a pulse for a second fixed time T2 as shown in FIG. This pulse is supplied to the first switching element 7 via the drive circuit 44. According to this embodiment, the time when the second switching element 22 is turned off can be stably determined based on the output of the timer 72. In addition, since the on-time width of the first switching element 7 is set to a constant time T2 by the second timer 77, L
It becomes easier to satisfy the C resonance condition. In addition, the first
As a modification of Figure 0, the trigger is at time t3 in Figure 1, and the first]
As shown by the chain line in Figure (D>), the period t3 to t5 may be set to T1. In this case, the flip-flop 74 can be omitted.The period t3 to t4 is almost determined by the circuit constants, so after the time t4 [Ninth Embodiment] In the switching regulator of the ninth embodiment shown in FIG.
0a is connected. This voltage source 20a functions similarly to the second capacitor 20, and has the effect of clamping this voltage as shown in FIG. 13(D) when the first capacitor 18 is charged to a certain value. When the voltage of the first capacitor 18 is clamped, the voltage of the first capacitor 1
Interruption of the resonant operation between the winding 8 and the primary winding 6 occurs, making it possible to control the time width of the winding 7. The on/off control of the second switching element 22 is performed based on the detection of the current I2 shown in FIG. 13(G) flowing through the voltage source 20a. Since it functions in the same way as the second capacitor 20, which has a larger capacity than the voltage source 20a4, the ninth embodiment has the same effects as the other embodiments. [Modifications] The present invention is not limited to the above-described embodiments, and the following modifications are possible, for example. (1) The first switching element 7.7a and the second switching element 22 can be replaced with bipolar transistors or the like. (2) The diode 1] can be removed from the flow smoothing circuit 14. (3) The output rectifying and smoothing circuit 14 may be omitted to provide a DC-AC converter (inverter). (4) The output can also be taken out by using the transformer 5 as a single-turn transformer. (5) A reactor may be connected in series to the second capacitor 20 to facilitate resonance. (6) It is possible to connect a diode in series with the first switching element 7. (7) The first capacitor 18 can be connected in parallel to the primary winding +1]6. In addition, the first capacitor 18
may be a stray capacitance of the primary winding 6 or the like. (8) The second capacitor 20 in FIGS. 4, 5, 6, 7, and 8 can be replaced by the voltage source 20a as in FIG. 12. (9) It is possible to perform constant voltage control by controlling the voltage value of the voltage source 20a in FIG. 12. (10) Constant voltage control may be performed by controlling both the on-time width of the first switching element 7 and the on-time width of the second switching element 22. (1)) Constant voltage control can be performed by keeping the on time width of the second switching element 22 constant and changing the on time width of the first switching element 7. [Effect of the Invention 1] As is clear from the above, according to the invention of each claim, suppression of surge voltage and reduction of loss can be easily achieved.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a switching regulator according to the first embodiment of the present invention, Fig. 2 is a block diagram showing details of the control circuit shown in Fig. 1, and Fig. 3 is a waveform showing the state of each part in Fig. 1. Figures 4, 5, 6, 7, 8, 9 and 10 are 2nd, 3rd, 4th, 5th, 6th, 7th and 8th Figure 1 is a waveform diagram showing the state of each part in Figure 10, Figure 12 is a circuit diagram showing the switching regulator of the embodiment of
The figure is a circuit diagram showing a switching regulator according to the ninth embodiment of the present invention, FIG. 13 is a waveform diagram showing the states of each part in FIG. 12, and FIG.
15 is a circuit diagram showing a conventional switching regulator, and FIG. 15 is a waveform diagram showing the voltage of the switching element in FIG. 14. 1... Power supply, 5...-Transformer, 6... Primary winding, 7
... first switching element, 8 ... control circuit, 9
...Secondary winding, 17...-first diode, 18.
...first capacitor, 20...second capacitor,
2]...Second diode, 22...Second switching element.

Claims (1)

[Scope of Claims] [1] A direct current power source, a transformer connected between one end and the other end of the direct current power source, and first and second main terminals and a control terminal, the first a first switching element whose main terminal is connected to one end of the DC power supply via the transformer, and whose second main terminal is connected to the other end of the DC power supply; and a first switching element that resonates with the inductance of the transformer. a first capacitor comprising an independent capacitor or stray capacitance associated with said transformer, and an output circuit coupled to said transformer; a second capacitor or voltage source capable of applying a voltage to the first capacitor for blocking or slowing voltage changes; a second switching element connected in series with the second capacitor or voltage source for selectively relating a voltage source to the first capacitor; and controlling the first switching element on and off; and a control circuit for controlling on/off of the second switching element within the off period of the first switching element. [2] The control circuit according to claim 1, wherein the control circuit is configured to change the on-time width of the first switching element so as to control the output voltage of the output circuit to be constant. Switching power supply. [3] The control circuit according to claim 1, wherein the control circuit is configured to change the on-time width of the second switching element so as to control the output voltage of the output circuit to be constant. Switching power supply. [4] The switching power supply device according to claim 1, further comprising means for controlling the voltage of the second capacitor or the voltage source in order to control the output voltage of the output circuit to be constant.