JP2002095249A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce power loss in a switching power supply and supply power to a switching control circuit with stability all the time regardless of the level of output voltage. SOLUTION: The control power circuit (12) of the switching power supply is provided with an accumulating circuit (22) that is charged at startup with a current flowing from a direct-current power supply (1) to primary winding (2a) and a main switching element (3), a comparing means (23) that produces an output signal when the charging voltage (VC1) of the accumulating circuit (22) reaches a starting reference voltage level (V1) and stops the output signal when the voltage (VC1) of the accumulating circuit (22) drops to a stopping reference voltage level (V2) lower than the starting reference voltage level (V1), and an auxiliary switching element (24) that is connected in series with the main switching element (3) and is brought into on-state on an output signal from the comparing means (23). Even if the output voltage fluctuates, the accumulating circuit (22) supplies the control circuit (10) with power for driving having a certain charging voltage (VC2) after the auxiliary switching element (24) is switched to on-state.

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, and more particularly to a switching power supply capable of supplying stable power to a switching control circuit with low loss and regardless of output voltage.

[0002]

2. Description of the Related Art FIGS. 4 and 5 show a flyback type switching power supply which has been widely used in the past.
Shown in The switching power supply shown in FIG. 4 includes a DC power supply (1) composed of a rectifier circuit or a battery (storage battery) connected to an AC power supply, a primary winding (2a) and a secondary winding (2b).
And a transformer (2) having a tertiary winding (2c), a MOS-FET (MOS field effect transistor) (3) as a main switching element, an input capacitor (4), a rectifying diode (6), and a smoothing element. Rectifying smoothing circuit with capacitor (7)
(5), drive capacitor (8), rectifier diode (9)
And a control circuit (1) for controlling ON / OFF of the MOS-FET (3).
0) and a starting resistor (11). Tertiary winding (2c) of transformer (2), drive capacitor (8), rectifier diode
(9) and the starting resistor (11) constitute a control power supply circuit (12) for supplying drive power to the control circuit (10). The primary winding (2a) of the transformer (2) and the MOS-FET (3) are connected in series to the DC power supply (1). Input capacitor (4) is DC power supply
It is connected between the output terminals of (1). The rectifying / smoothing circuit (5) is connected to the secondary winding (2b) of the transformer (2). The drive capacitor (8) is connected to the power input terminal (+ V _CC ) of the control circuit (10) and the DC power supply.
It is connected between the cathode terminal of (1). Rectifier diode (9)
Is connected in series with the tertiary winding (2c) of the transformer (2) between the cathode terminal of the DC power supply (1) and the power supply input terminal (+ V _CC ) of the control circuit (10). The starting resistor (11) is connected between the anode terminal of the DC power supply (1) and the power supply input terminal (+ V _CC ) of the control circuit (10). A power switch (13) is provided between the DC power supply (1) and the input capacitor (4).

The operation of the switching power supply shown in FIG. 4 is outlined as follows. When the power switch (13) is turned on, a current flows from the DC power supply (1) to the driving capacitor (8) via the starting resistor (11), and the driving capacitor (8) is charged to the power supply voltage V _CC. . The power supply voltage V _CC is supplied to the power supply input terminal (+ V _CC ) of the control circuit (10) to drive the control circuit (10), and the control circuit (10) performs gate control on the gate terminal of the MOS-FET (3). signal V _G is given. With this, MOS-
The FET (3) starts the on / off operation, and the voltage E of the DC power supply (1) is intermittently applied to the primary winding (2a) of the transformer (2) and the voltage is applied to the secondary winding (2b). Is induced. Transformer (2) 2
The voltage induced in the secondary winding (2b) is rectified and smoothed by the rectifying diode (6) and the smoothing capacitor (7) of the rectifying / smoothing circuit (5), and a DC output having a value different from the voltage E of the DC power supply (1). The voltage V _O is applied to the load (14). At the same time, the transformer
A voltage is also induced in the tertiary winding (2c) of (2), and this voltage is supplied to the power input terminal of the control circuit (10) via the rectifier diode (9).
(+ V _CC ). Therefore, after the start-up, the control circuit is controlled by the DC power supplied from the tertiary winding (2c) of the transformer (2) to the power input terminal (+ V _CC ) via the rectifier diode (9).
The operation of (10) is continued. The control circuit (10) controls the MOS-FET (3) according to the DC output voltage V _O supplied to the load (14).
By pulse width modulation of the gate control signal V _G to be applied to the gate terminal of the (PWM), which controls the on-off period of the MOS-FET (3).

The switching power supply shown in FIG. 5 has the same main circuit configuration as the switching power supply shown in FIG. 4, but a tertiary winding (2c) for power supply of a control circuit (10).
Also, the rectifier diode (9) is omitted, and a constant current circuit (15) is connected instead of the starting resistor (11). In the switching power supply device shown in FIG. 5, when the power switch (13) is turned on, DC power is supplied from the DC power supply (1) to the constant current circuit (15), and the constant current circuit (15) is driven. At this time, a constant current flows from the constant current circuit (15) to the driving capacitor (8), and the driving capacitor (8) is charged to the power supply voltage V _CC . Power supply voltage V _CC is the power supply input terminal of the control circuit (10) (+ V _CC )
And the control circuit (10) is driven, and the control circuit (10) supplies a gate control signal V _G to the gate terminal of the MOS-FET (3).
Is given. This turns on the MOS-FET (3).
The off operation starts, and after that, a constant current is supplied to the driving capacitor (8) by the constant current circuit (15).
A constant power supply voltage V _CC is always supplied to the power supply input terminal (+ V _CC ) of the control circuit (10).

[0005]

By the way, in the conventional switching power supply device shown in FIG. 4, power supply to the control circuit (10) after starting is generated in the tertiary winding (2c) of the transformer (2). This is performed by a voltage V ₃ proportional to the DC output voltage V _O. Accordingly, for example, when the DC output voltage V _O is varied by changing the ON duty of the MOS-FET (3), the value of the power supply voltage V _CC supplied to the control circuit (10) is also proportional to the DC output voltage V _O. Therefore, stable power cannot always be supplied to the control circuit (10). In particular, when the DC output voltage V _O is low, the power supply voltage V _CC of the control circuit (10) becomes less driving voltage, since the control circuit (10) may stop, substantially DC output voltage V _O It could not be changed.
In addition, when the main switching element constituting the switching power supply is an element having large power consumption such as a bipolar transistor, a driving capacitor having a large capacitance (8)
Is required, and the power consumption of the starting resistor (11) is increased in order to charge the driving capacitor (8) in a short time. FIG.
In the conventional switching power supply shown in (1), the constant current circuit (1
5) and for supplying drive power of the control circuit (10) by the driving capacitor (8), the driving power of a constant voltage value V _CC to a control circuit (10) regardless of the value of the DC output voltage V _O Although there is an advantage that the current can be supplied, most of the current flowing through the constant current circuit (15) is consumed by the constant current circuit (15), so that there is a problem that power consumption is large.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a switching power supply device which can supply stable power to a switching control circuit with low loss and regardless of output voltage.

[0007]

In the switching power supply according to the present invention, a DC power supply (1) is connected to a transformer (2).
The next winding (2a) and the main switching element (3) are connected in series, the control circuit (10) is driven by the power supplied from the control power supply circuit (12), and the main circuit is output by the control circuit (10). By controlling ON / OFF of the switching element (3), a DC output is extracted from the secondary winding (2b) of the transformer (2) via the rectifying / smoothing circuit (5). The control power supply circuit (12) includes a power storage circuit (22) charged by a current flowing from the DC power supply (1) through the primary winding (2a) and the main switching element (3), and a charging voltage of the power storage circuit (22). (V _C1) is started reference voltage level (V ₁₎ to generate an output signal upon reaching and charging voltage (V _C1) is started reference voltage level (V ₁₎ lower than stopping reference voltage of the power storage circuit (22) Level (V
Comparison means (23) for stopping the output signal when falling to ₂ )
And an auxiliary switching element (24) connected in series with the main switching element (3) and turned on by an output signal of the comparing means (23). The storage circuit (22), after the auxiliary switching element (24) is turned on,
The driving power having the charging voltage (V _C2 ) is supplied to the control circuit (10).

When the main switching element (3) is turned on when the power is turned on, a current flowing from the DC power supply (1) through the primary winding (2a) of the transformer (2) and the main switching element (3) is applied. The power storage circuit (22) is charged. When the charging voltage (V _C1 ) of the power storage circuit (22) reaches the activation reference voltage level (V ₁ ), an output signal is generated from the comparing means (23), and the auxiliary switching element (24) is turned on. When the auxiliary switching element (24) is turned on, driving power having a charging voltage (V _C2 ) is supplied from the power storage circuit (22) to the control circuit (10) through the auxiliary switching element (24), and the control circuit (10) is driven.
Thereafter, when the charging voltage (V _C1 ) of the storage circuit (22) decreases to a stop reference voltage level (V ₂ ) lower than the start reference voltage level (V ₁ ), the output signal from the comparison means (23) stops. The auxiliary switching element (24) is turned off from the on state, and the electric storage circuit (22) is charged again from the DC power supply (1) via the primary winding (2a) of the transformer (2) and the main switching element (3). Is done.
By repeating the above operation, the power storage circuit (22) is constantly charged to a substantially constant power supply voltage, and the control circuit (10) is continuously driven. Thus, the primary winding (2a) of the transformer (2)
Since the current supplied to the control circuit (10) is divided from the current flowing to the control circuit (10), the power loss is small. Also, even if the output voltage fluctuates, the charging voltage of the power storage circuit (22) is maintained at a substantially constant value, so that stable power is always supplied to the control circuit (10) without being affected by the fluctuation of the output voltage level. can do.

In one embodiment of the present invention, a control power supply circuit
(12) is provided with starting means (21) for giving an ON signal from the DC power supply (1) to the control terminal of the main switching element (3) via the primary winding (2a) at the time of starting. The storage circuit (22)
The charge of the storage capacitor (25) charged by the current flowing through the main switching element (3) and the charge of the storage capacitor (25) when the auxiliary switching element (24) is in the ON state pass through the auxiliary switching element (24). And a power supply capacitor (26) that is charged. The comparing means (23) generates an output signal when the charging voltage (V _C1 ) of the storage capacitor (25) reaches the startup reference voltage level (V ₁ ), and outputs the output signal to the storage capacitor (2
The output signal is stopped when the charging voltage (V _C1 ) of 5) falls to the stop reference voltage level (V ₂ ) lower than the start reference voltage level (V ₁ ). The power supply capacitor (26) supplies driving power having a charging voltage (V _C2 ) to the control circuit (10).

When the power is turned on, a current flows from the DC power supply (1) to the starting means (21) via the primary winding (2a) of the transformer (2), and the control terminal of the main switching element (3) is turned on. A signal is given to turn on. At the same time, the DC power supply
From (1), the storage capacitor (25) is charged via the primary winding (2a) of the transformer (2) and the main switching element (3).
When the charging voltage (V _C1 ) of the storage capacitor (25) reaches the starting reference voltage level (V ₁ ), an output signal is generated from the comparing means (23), and the auxiliary switching element (24) is turned on. When the auxiliary switching element (24) is turned on, the charging voltage (V _C1 ) of the storage capacitor (25) is changed to the auxiliary switching element.
The voltage is applied to the power supply capacitor (26) via (24), and the power supply capacitor (26) is charged. The charging voltage (V _C2 ) of the power supply capacitor (26) is applied to the control circuit (10) as driving power, and the control circuit (10) is driven. After that, the charging voltage (V _C1 ) of the storage capacitor (25) changes to the starting reference voltage level (V ₁ ).
When the voltage drops to a lower stop reference voltage level (V ₂ ), the output signal from the comparing means (23) stops, the auxiliary switching element (24) changes from the on state to the off state, and the DC power supply (1) changes the transformer (2). ) Through the primary winding (2a), the starting means (21) and the main switching element (3).
Is charged again. By repeating the above operation,
The storage capacitor (25) and the power supply capacitor (26) are constantly charged to a substantially constant power supply voltage, and the control circuit (10) is continuously driven.

The comparing means (23) is a hysteresis comparator having a hysteresis characteristic. When the output signal of the comparing means (23) stops and the main switching element (3) changes from the OFF state to the ON state, the auxiliary switching element (23). 24) changes from the on state to the off state. Since the auxiliary switching element (24) is turned off when the current flowing through the main switching element (3) is the minimum value, the switching loss of the auxiliary switching element (24) can be reduced.

The positive terminal of the storage capacitor (25) and the negative terminal of the power supply capacitor (26) are connected via a first backflow prevention diode (27), and the positive terminal of the storage capacitor (25) is Since the terminal and the positive terminal of the power supply capacitor (26) are connected via the second backflow prevention diode (28), the circuit operation can be performed reliably.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a switching power supply according to the present invention will be described below with reference to FIGS. However, in FIG. 1, the same portions as those in FIG. 4 are denoted by the same reference numerals, and the description thereof will be omitted. As shown in FIG. 1, the switching power supply according to the present embodiment has a starting resistor (21) as starting means, a hysteresis comparator (23) as comparing means, and a shunt M as an auxiliary switching element.
OS-FET (24), storage capacitor (25), power supply capacitor (26), and first backflow prevention diode (27)
A second backflow prevention diode (28) and a reference power supply (29)
And a control power supply circuit (12) having the following. The storage capacitor (25), the power supply capacitor (26), and the first and second backflow prevention diodes (27, 28) constitute a power storage circuit (22). The starting resistor (21) is connected to the drain of the MOS-FET (3).
Connected between gate terminals. Hysteresis comparator
In (23), the non-inverting input terminal (+) is connected to one end of the storage capacitor (25) and the inverting input terminal (-) is connected to the reference power supply (2).
9), generates a high voltage (H) level ON signal when the charging voltage V _{C1 of the} storage capacitor (25) reaches the startup reference voltage level V ₁ , and charges the storage capacitor (25). When the voltage V _C1 falls to the stop reference voltage level V ₂ lower than the start reference voltage level V ₁ , it has a history characteristic of generating an off signal of a low voltage (L) level. Shunt MOS-
The FET (24) has a drain terminal connected to the source terminal of the MOS-FET (3) and a source terminal connected to the storage capacitor (22).
And the gate terminal is connected to the output terminal of the hysteresis comparator (23). Storage capacitors (2
5) has one end connected to the source terminal of the MOS-FET (3) via the first backflow prevention diode (27) and the other end connected to the cathode terminal of the DC power supply (1). Power supply capacitor (2
6) has one end connected to one end of the storage capacitor (25) via the second backflow prevention diode (28) and to the power input terminal (+ V _CC ) of the control circuit (10), The other end is connected to the drain terminal of the shunt MOS-FET (24) and to the GND terminal (GND) of the control circuit (10). Other configurations are substantially the same as those of the conventional switching power supply device shown in FIG.

[0014] In the above configuration, the DC power supply on the power switch (13) at time t ₀ as shown in FIG. 2 (A)
When the voltage E of (1) is applied to the input capacitor (4), a current flows from the DC power supply (1) to the starting resistor (21) through the primary winding (2a) of the transformer (2), and the MOS- A voltage signal higher than the threshold is applied to the gate terminal of the FET (3), and the FET (3) is turned on. At this time, as shown in FIG. 2B, a current I _D1 having a constant value flows through the MOS-FET (3). At the same time, as shown in FIG. 3A, a DC power supply (1), a power switch (13), a primary winding (2a) of a transformer (2), a starting resistor (21), and a MOS-FET (3 ), 1st
A current flows through the path of the backflow prevention diode (27) and the storage capacitor (25), and the storage capacitor (25) is charged. As a result, as shown in FIG. 2C, the charging voltage V _C1 of the storage capacitor (25) increases linearly, and the voltage V D between the drain and source terminals of the shunt MOS-FET (24) increases.
_DS2 rises linearly as shown in FIG.

As shown in FIG. 2 (C), when the charging voltage V _{C1 of the} storage capacitor (25) reaches the starting reference voltage level V ₁ at time t ₁ , a high voltage (H) is output from the hysteresis comparator (23). ) A level ON signal is generated and the shunt MOS-
The FET (24) is turned on, and the voltage V _DS2 between the drain and source terminals of the shunting MOS-FET (24) becomes substantially 0 V as shown in FIG. At this time, as shown in FIG. 3B, a DC power supply (1), a power switch (13), a primary winding (2a) of a transformer (2), a starting resistor (21), and a MOS-FET (3) Current also flows in the path of the shunt MOS-FET (24),
Since energy is stored in (2), the current I _D1 flowing through the MOS-FET (3) linearly increases as shown in FIG. At the same time, as shown in FIG. 3C, the current flows through the path of the storage capacitor (25), the second backflow prevention diode (28), the power supply capacitor (26), and the shunting MOS-FET (24). Flows, and as shown in FIGS. 2C and 2E, the storage capacitor 25 and the power supply capacitor 26 are charged to a substantially equal voltage. The charging voltage V _C2 of the power supply capacitor (26) shown in FIG. 2 (E) is supplied to the power input terminal (+ V _CC ) of the control circuit (10) as driving power as shown in FIG. 3 (D). , Control circuit (1
0) is driven. Thus, the time t ₁ after the operation of the switching power supply of flyback type in the steady state is performed. That is, as shown in FIG. 2B, the MOS-FET (3)
Is in the ON state, the current I _D1 increases linearly, energy is stored in the transformer (2) and the DC output voltage V _O Is supplied, and when the MOS-FET (3) is off, the current I _D1
Becomes substantially zero, and the DC output voltage V _O is supplied from the secondary winding (2b) of the transformer (2) to the load (14) via the rectifying and smoothing circuit (5). During this operation, the power of the power storage capacitor (25) and the power supply capacitor (26) is consumed by the control circuit (10), and as shown in FIGS. 2 (C) and (E), The voltages V _C1 and V _{C2 of the} capacitor (25) and the power supply capacitor (26) decrease at substantially the same gradient.

As shown in FIG. 2B, at time t _{2 the} MO
The S-FET (3) changes from the off state to the on state, the current I _D1 increases linearly from 0, and the charging voltage V _C1 of the storage capacitor (25) becomes the stop reference voltage as shown in FIG. When the voltage drops to the level V ₂ (V ₁ <V ₂ ), a low voltage (L) level off signal is generated from the hysteresis comparator (23) and the shunting MOS-FET (24) is turned off from the on state. , As shown in FIG.
The voltage V _DS2 between the drain and source terminals of (24) is a DC power supply
The voltage rises linearly to the voltage difference between the voltage E of (1) and the voltage of the primary winding (2a) of the transformer (2). At this time, as shown in FIG. 3A, a DC power supply (1), a power switch (13), a primary winding (2a) of a transformer (2), a starting resistor (21), and a MOS-FET (3) ,
The first backflow prevention diode (27) and the storage capacitor (2
A current flows in the path of 5), and the storage capacitor (25) is charged again. This causes the charging voltage V _C1 of the storage capacitor (25) to rise linearly as shown in FIG. 2 (C).

As shown in FIG. 2C, when the charging voltage V _{C1 of the} storage capacitor (25) reaches the starting reference voltage level V ₁ again at time t ₃ , the high voltage (H) is output from the hysteresis comparator (23). H) A level ON signal is generated and the diverting MO
The S-FET (24) is turned on again from the off state, and as shown in FIG. 2D, the voltage V _DS2 between the drain and source terminals of the shunting MOS-FET (24) is the voltage of the DC power supply (1). Approximately 0 V from the voltage difference between E and the voltage of the primary winding (2a) of the transformer (2).
Descend to At this time, as shown in FIG. 3B, a DC power supply (1), a power switch (13), and a primary winding (2) of a transformer (2).
a), starting resistor (21) and MOS-FET (3), shunt MO
Since a current also flows through the path of the S-FET (24), FIG.
As shown in (1), the current I _D1 flowing through the MOS-FET (3) continuously increases linearly. At the same time, as shown in FIG. 3C, a storage capacitor (25), a second backflow prevention diode (28), a power supply capacitor (26), a shunt MOS-FE
A current flows in the path of T (24), and as shown in FIGS. 2 (C) and 2 (E), the storage capacitor (25) and the power supply capacitor (26) are again charged to a substantially uniform voltage. The charging voltage V _C2 of the power supply capacitor (26) shown in FIG. 2 (E) is, as shown in FIG. 3 (D), a power supply input terminal (+ V _CC ) of the control circuit (10) as driving power.
Supplied to As a result, the control circuit (10) is continuously driven, and as shown in FIGS. 2C and 2E, the charging voltages V _C1 and V _C2 of the storage capacitor (25) and the power supply capacitor (26).
Decrease at substantially the same slope. After this, the operation at time t ₂ and t ₃ are repeated alternately, continuously since the power storage capacitor (25) and the power supply capacitor (26) is always charged at a substantially constant voltage, the control circuit (10) Driven.
Note that the operation of the control circuit (10) in the steady state is the same as that of the conventional switching power supply device shown in FIG.

In this embodiment, the primary winding of the transformer (2) is
Charges the storage capacitor (25) with the current flowing through the wire (2a)
And the charging voltage V_C1The hysteresis comparator (23)
And the starting reference voltage level by the shunt MOS-FET (24).
Le V₁And stop reference voltage level V _TwoHold between and feed
To charge the capacitor (26) to a substantially constant power supply voltage,
The control circuit (10) is continuously driven. Thus, the tiger
Control circuit (10) from the current flowing through the primary winding (2a) of the
Since the current supplied to the power supply is divided, the power loss is small. Ma
Also, by changing the on-duty of the MOS-FET (3)
DC output voltage V _OOf the power supply capacitor (26)
Since the charging voltage is always kept at a substantially constant level,
Output voltage V_OIndependent of the control circuit (10)
Can supply power. Therefore, the DC output voltage V_OVariable
It is possible to Furthermore, the MOS-FET (3) is turned off.
MOS-FET (24)
Changes from the on state to the off state, the MOS-FET (3)
Current I flowing through_D1Is 0, the shunt MOS-FET (2
4) is turned off and the switch of the shunt MOS-FET (24) is
There is an advantage that the switching loss can be reduced.

The embodiments of the present invention are not limited to the above embodiments, and various modifications are possible. For example, in the above-described embodiment, the transformer (2) having two windings (2a, 2b) is used, but an auto-transformer (single winding) that takes out output from a part or extension of the primary winding (2a) A multi-output transformer having a plurality of output windings may be used. In each of the above embodiments, a mode in which a MOS-FET is used as a main switching element has been described. However, a bipolar transistor, an IGBT (insulated gate bipolar transistor), a J-FET (junction field effect transistor), a thyristor, or the like is used. Other switching elements may be used. In each of the above embodiments, the power storage circuit (22) is configured by the power storage capacitor (25), the power supply capacitor (26), and the first and second backflow prevention diodes (27, 28). A choke coil for energy storage may be connected instead of the capacitor (25). Further, in each of the above embodiments, the embodiment in which the present invention is applied to the flyback type switching power supply device is shown, but the present invention is also applicable to a forward type switching power supply device.

[0020]

According to the present invention, the current supplied to the switching control circuit is diverted from the current flowing through the primary winding of the transformer, so that a switching power supply with low power loss and low loss can be obtained. Further, since there is no need to provide a winding for supplying control power to the transformer, the winding structure of the transformer can be simplified, and stable power can be constantly supplied to the switching control circuit even when the output voltage is varied. Therefore, it is possible to realize a switching power supply device in which the variable range of the output voltage is wide and the operation is stable over the entire range.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram showing an embodiment of a switching power supply device according to the present invention.

FIG. 2 is a timing chart of the voltage and current of each unit shown in FIG.

FIG. 3 is an energization path diagram of FIG.

FIG. 4 is an electric circuit diagram showing an example of a conventional switching power supply device.

FIG. 5 is an electric circuit diagram showing another example of the conventional switching power supply device.

[Explanation of symbols]

(1) DC power supply, (2) transformer, (2a) primary winding, (2b) secondary winding, (2c) tertiary winding,
(3) ·· MOS-FET (main switching element), (4) ·
・ Input capacitor, (5) ・ ・ Rectifying smoothing circuit, (6) ・ ・
Rectifier diode, (7) ・ ・ Smoothing capacitor, (8) ・ ・ Drive capacitor, (9) ・ ・ Rectifier diode, (10) ・
・ Control circuit, (11) ・ ・ Start-up resistor, (12) ・ ・ Control power circuit, (13) ・ ・ Power switch, (14) ・ ・ Load, (1
5) ··· Constant current circuit, (21) · · · Starting resistor (starting means), (22) · · · Storage circuit, (23) · · · Hysteresis comparator (comparing means), (24) · · MOS shunt F
ET (auxiliary switching element), (25) ··· capacitor for storage, (26) · · · capacitor for power supply, (27) · · · 1st
(28) ..Second backflow prevention diode, (29) .. Reference power supply

Claims (5)

[Claims] 1. A primary winding of a transformer and a main switching element are connected in series to a DC power supply, a control circuit is driven by power supplied from a control power supply circuit, and the main circuit is driven by an output of the control circuit. In a switching power supply device for taking out a DC output from a secondary winding of the transformer via a rectifying / smoothing circuit by controlling on / off of a switching element, the control power supply circuit comprises: A power storage circuit charged by a current flowing through the main switching element, an output signal is generated when a charging voltage of the power storage circuit reaches a startup reference voltage level, and the charging voltage of the power storage circuit is higher than the startup reference voltage level. A comparing means for stopping the output signal when the output signal drops to a low stop reference voltage level; An auxiliary switching element that is turned on by the output signal of the comparing means, wherein the power storage circuit supplies driving power having a charging voltage to the control circuit after the auxiliary switching element is turned on. A switching power supply device characterized by the above-mentioned. 2. The control power supply circuit further includes a start-up means for applying an ON signal from the DC power supply to the control terminal of the main switching element via the primary winding at the time of start-up. A power storage capacitor charged by a current flowing through the switching element; and a power supply capacitor charged with the charge of the power storage capacitor via the auxiliary switching element when the auxiliary switching element is on. The means generates an output signal when the charging voltage of the storage capacitor reaches a startup reference voltage level, and when the charging voltage of the storage capacitor drops to a stop reference voltage level lower than the startup reference voltage level. Stopping the output signal, wherein the power supply capacitor supplies driving power having a charging voltage to the control circuit. Item 2. The switching power supply device according to Item 1. 3. The switching power supply according to claim 1, wherein the comparing unit is a hysteresis comparator having a hysteresis characteristic. 4. The auxiliary switching element changes from an on state to an off state when the output signal of the comparing means stops and the main switching element changes from an off state to an on state. A switching power supply device according to the item. 5. A positive terminal of the power storage capacitor and a negative terminal of the power supply capacitor are connected via a first backflow preventing diode, and a positive terminal of the power storage capacitor and a power supply capacitor of the power supply capacitor are connected to each other. 3. The switching power supply device according to claim 2, wherein the positive terminal is connected via a second reverse current prevention diode.