JP2988282B2 - Switching power supply - Google Patents

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 device having a switch for interrupting a DC voltage and an output transformer.

[0002]

2. Description of the Related Art FIG. 1 shows a conventional resonance type switching power supply for reducing switching loss. This switching power supply device includes an insulated gate field effect transistor (hereinafter, referred to as a first and a second switching element) connected between one end and the other end of a DC power supply 1 composed of a rectifying and smoothing circuit connected to an AC power supply. , A series circuit of Q1 and Q2, an output transformer T1, a resonance capacitor C1 connected in series to a primary winding N1 of the output transformer T1, a first and a second transistor Q1, First and second auxiliary capacitors Ca and Cb connected in parallel with Q2, and first and second transistors Q1 and Q2,
A control circuit 2 connected to the gate (control terminal) of Q2;
A rectifying / smoothing circuit including secondary windings N2a and N2b of the output transformer T1, diodes Da and Db connected to the secondary windings N2a and N2b, and a smoothing capacitor C0;
, An error amplifier 4, a reference voltage source 5, a light emitting diode 6, a phototransistor 7, a current detector 9, a current detection rectifier 9, a current detection resistor 10, and a starting resistor 1.
1, an auxiliary power supply capacitor 12, and a control power supply winding 1
3 and a control power supply rectifier 14. As is well known, the primary winding N1 of the transformer T1 has an exciting inductance and a leakage inductance and has secondary windings N2a and N2b.
Are electromagnetically coupled. Since the first and second transistors Q1 and Q2 have a structure in which the sources are connected to a substrate (bulk), the main switches include diodes D1 and D2 connected in anti-parallel to the main switch.

A series circuit of the primary winding N1 and the capacitor C1 is connected in parallel to a second transistor Q2.
The primary winding N1 and the secondary windings N2a and N2b of the transformer T1 are electromagnetically coupled via a core 15. Secondary winding N2a,
Both ends of N2b are connected to one end of a capacitor C0 via diodes Da and Db, and the center tap is connected to the capacitor C0.
Is connected to the other end. The output terminal of the smoothing capacitor C0 is connected to the load 3. One input terminal of the error amplifier 4 is connected to one end of the load 3, the other input terminal is connected to a reference voltage source 5, and a light emitting diode 6 is connected to this error output line. This error amplifier 4 outputs a voltage corresponding to the difference between the output detection voltage and the reference voltage.
The light emitting diode 6 emits light in response to the error output, and provides an optical signal to a phototransistor 7 optically coupled thereto.
The phototransistor 7 is connected between the feedback control terminal FB of the control circuit and the ground line.

The control circuit 2 has a control power supply terminal Vcc, a ground terminal GND, first and second output terminals OUT1, OUT2, and a current detection terminal IS, in addition to the feedback control terminal FB. The capacitor 12, which functions as a smoothing circuit and also functions as a control power supply, is connected to the main power supply 1 via a starting resistor 11 having a high resistance value and to a control power supply rectifier. Since the rectifying winding 13 for the control power supply is wound around the core 15 of the transformer T1 and is densely electromagnetically coupled to the secondary windings N2a and N2b, a voltage corresponding to the voltage of the secondary windings N2a and N2b is generated. . A rectifier 14 connected to the winding 13 rectifies the stabilized or constant voltage to charge the capacitor 12. A power supply terminal Vcc of the control circuit 2 is connected to one end of the capacitor 12, and a ground terminal GND.
Is connected to the other end of the capacitor 12, the voltage of the capacitor 12 becomes the power supply voltage of the control circuit 2. The first and second output terminals OUT1 and OUT2 of the control circuit 2 are connected to the gates of the first and second transistors Q1 and Q2. The current detection terminal IS is connected to one end of the current detection resistor 10. The current detector 8 is connected in series with the resonance capacitor C1, and here the resistor 1 is connected via the rectifier 9.
Since 0 is connected in parallel, a DC voltage corresponding to the current flowing through the capacitor C1 is obtained at the resistor 10. Note that a smoothing capacitor can be provided in the output stage of the rectifier 9 instead of the resistor 10 or together with the resistor 10.

A control circuit 2 for alternately turning on and off the first and second transistors Q1 and Q2 with a dead time is configured as shown in FIG. The VFO (variable frequency oscillator) 20 includes a resistor 21 and a capacitor 22 for forming a saw-tooth wave (triangular wave), and an oscillation control circuit 23 having an IC configuration. The oscillation control circuit 23 has terminals a, b, c, d, and e. A series circuit of a resistor 21 and a capacitor 22 is connected between the terminal a and the ground,
The terminal b is connected to the connection point between the resistor 21 and the capacitor 22. The oscillation control circuit 23 is a well-known circuit, and supplies a charging current supply circuit for the capacitor 22, a discharge control circuit for the capacitor 22, and a square wave pulse corresponding to a section having a positive slope of the voltage of the capacitor 22. A pulse forming circuit. The charging current supply circuit is connected to the terminals b, c, and d, controls the charging current in response to a voltage control signal of the terminal c connected to the feedback control terminal FB, and controls the terminal d when an overcurrent state occurs. Is formed so as to control the charging current in response to the current detection signal. The discharge control circuit includes a discharge switch connected between the terminal a and the ground, and connects the resistor 21 in parallel with the capacitor 22 when the capacitor 22 is charged to a predetermined voltage, and connects the capacitor 22 with a constant time constant. Discharge. The pulse forming circuit is provided between the terminal b and the terminal e, and generates a square wave pulse corresponding to the positive ramp voltage period of the triangular wave voltage shown in FIG. 3A of the capacitor 22 as shown in FIG. Occur. The period of the positive slope of the triangular wave shown in FIG. 3A and the on-period Ton of the square wave pulse of FIG. 3B vary depending on the output voltage or the load current. The off period Toff is kept at a fixed time width. Therefore, when the output voltage becomes higher than the predetermined value or when the load current becomes higher than the predetermined overcurrent level, FIG.
As shown by the dotted line, the repetition frequency of the triangular wave and the square wave pulse increases. The control of the charging current in the charging current supply circuit of the capacitor 22 is performed by the signal of the voltage detection terminal c at the time of non-overcurrent and by the signal of the current detection terminal d at the time of overcurrent. Such control is achieved by connecting terminals c and d to each other via respective diodes. In this case, the voltage at the terminal d during the overcurrent is set to be higher than the voltage at the terminal c. In order to detect an overcurrent, a current detection terminal IS is connected to one input terminal of a comparator 24.
The other input terminal of the comparator 4 is connected to a reference voltage source 25, and the output terminal of the comparator 24 is connected to a terminal d of the oscillation control circuit 23.

The output terminal e of the oscillation control circuit 23 is connected to the trigger input terminal T of the trigger type flip-flop 26 and the first and second AND gates 27 and 2 are connected.
8 is connected to one input terminal. The other input terminal of the first AND gate 27 is connected to the normal-phase output terminal of the flip-flop 26, and the other input terminal of the second AND gate 28 is connected to the negative-phase output terminal of the flip-flop 26. First and second AND gates 27 and 28
Are connected to first and second output terminals OUT1 and OUT2 via drive amplifiers 29 and 30. As a result, the first and second output terminals OUT1 and OUT2 are connected to FIG.
(C) A control voltage composed of a square wave pulse shown in (D) is obtained and supplied to the first and second transistors Q1 and Q2.

The control circuit 2 shown in FIG. 2 has an input drop detection circuit (UVLO) 31. The input drop detection circuit 31 is connected to the control power supply terminal Vcc and detects whether the control power supply voltage is equal to or lower than a predetermined value or higher than the predetermined value.

The control circuit 2 shown in FIG. 2 has a control power supply voltage (bias voltage) supply control circuit 32. The control power supply control circuit 32 includes a switch for selectively supplying the control power supply voltage and a predetermined voltage level (for example, 5
V), and to each circuit in the control circuit 2 in response to a signal indicating that the voltage of the control power supply terminal Vcc obtained from the input drop detection circuit 31 has become higher than a predetermined value. The power supply voltage or the bias voltage is supplied, and the supply of the control power supply voltage is interrupted in response to a signal indicating an overload.

In order to determine that the load 3 is overloaded based on a voltage detection signal, first and second comparators 33 and 34 are provided, and one input terminal of the first comparator 33 is provided with feedback control. The other input terminal is connected to the reference voltage source 35. When the load 3 in FIG. 1 is overloaded due to a short circuit or the like, the output voltage (load voltage) decreases, the light emission amount of the light emitting diode 6 decreases or becomes zero, and the phototransistor 7 is turned off or close to this, and the feedback is performed. When the voltage of the control terminal FB is equal to the reference voltage source 35
Higher than the voltage of As a result, the output of the first comparator 33 changes from a high level to a low level. A capacitor 36 is connected between one input terminal of the second comparator 34 and the ground, and a transistor 37 is connected in parallel with the capacitor 36.
The output terminal of the first comparator 33 is connected to the base of the first comparator 7. Since the first comparator 33 generates a high-level output in the non-overload (normal load) state, the transistor 37 is turned on, and the capacitor 36 is not charged and is at a low voltage. On the other hand, the first
Since the output of the comparator 33 becomes low level, the transistor 37 is turned off, and the voltage of the capacitor 36 becomes high. A current source 38 is provided between the control power supply terminal Vcc and the capacitor 36. The other input terminal of the second comparator 34 is connected to a reference voltage source 39. Therefore, when the voltage of the capacitor 36 becomes higher than the voltage of the reference voltage source 39 due to an overload, the output of the second comparator 34 changes from a low level to a high level, and the control power supply control circuit 32 is turned off. .

The primary winding N1 of the transformer T1 in FIG. 1 has a leakage inductance and an exciting inductance. The leakage inductance is equivalently connected in series with the primary winding N1, and the exciting inductance is equivalently connected in parallel with the primary winding N1. The first and second transistors Q1,
The ON control time Ton of Q2 is set to be shorter than a half wave of the resonance current waveform between the leakage inductance and the resonance capacitor C1.

FIG. 4 is a waveform diagram showing the state of each part in FIG. The first and second transistors Q1 and Q2 are shown in FIG.
On / off control is performed alternately by control signals shown in (A) and (B). Now, when transistor Q1 is on,
A drain current Id1 flows as shown at t1 to t3 in FIG. 4D by a circuit including the power supply 1, the first transistor Q1, the capacitor C1, and the primary winding N1 having inductance. The currents shown in FIGS. 4D and 4F do not include the currents of the diodes D1 and D2. The current in the section from t1 to t2 is obtained by biasing the current based on the series resonance of the leakage inductance and the capacitor C1 with the current component of the low frequency resonance based on the exciting inductance of the transformer T1. Drain current I flowing through transistor Q1
d1 has a waveform approximating a sine wave, enables zero current switching at turn-on, and reduces switching loss. The first transistor Q shown in the section from t3 to t4
During the on-to-off transition of 1 current flows through capacitor Ca and zero volt switching is achieved. That is, the current from t3 to t4 flows based on the resonance circuit including the DC power supply 1, the capacitor Ca, the capacitor C1, and the leakage inductance of the primary winding N1, and the resonance circuit including the capacitor Cb, the capacitor C1, and the leakage inductance. As a result, the voltage of the capacitor Ca, that is, the voltage between the drain and the source of the first transistor Q1 is reduced as shown in FIG.
As shown in (C), the voltage gradually increases in the interval from t3 to t4 to achieve zero volt switching. At time t5, even if the ON control signal is applied to the second transistor Q2, the current in the negative direction does not immediately flow through the primary winding N1. t4 to t5
In the section, the primary winding N1 is formed by the leakage inductance of the primary winding N1, the resonance capacitor C1 and the diode D2.
Current flows through 1. In the section from t5 to t7, the capacitor C
A resonance current shown in FIG. 4 (F) flows in a circuit including the primary winding N1 having a leakage inductance and the second transistor Q2. This results in a negative current in the primary winding N1. In the section between t7 and t8 in the negative half wave, the primary winding N1 having leakage inductance and the capacitor Cb
Of the resonance circuit of the capacitor C1 and the primary winding N1
, A capacitor Ca, a power supply 1, and a capacitor C1. As a result, the voltage of the capacitor Ca, that is, the voltage of the first transistor Q1, gradually decreases as shown in FIG. 4C due to the discharge of the capacitor Ca, and conversely, the voltage of the capacitor Cb, that is, the second transistor Q2.
Of FIG. 4 gradually increases as shown in FIG. t8
In the period from t9 to t9, a current flows through a circuit including the primary winding N1, the diode D1, the power supply 1, and the capacitor C1. FIG. 4 (G)
The current In1 of the primary winding N1 shown in FIG.
And the currents flowing through the diodes D1 and D2 and the currents flowing through the capacitors Ca and Cb and the currents flowing through the diodes D1 and D2. 1
If an AC current flows through the secondary winding N1 as shown in FIG. 4 (G), a current also flows through the secondary windings N2a and N2b accordingly,
The capacitor C0 is charged via the diodes Da and Db. During normal operation, the voltage of the capacitor C0 becomes substantially constant, and a stabilized voltage is supplied to the load 3.

In the device shown in FIG. 1, when the voltage of the load 3 becomes higher than a predetermined value, the output frequency of the VFO 20 shown in FIG. 2 becomes higher, and the first and second transistors Q1, Q2 are turned on.
Also has a higher on / off repetition frequency f. Conversely, when the voltage of the load 3 is lower than the predetermined value, the operation is reversed.

The voltage V of the primary winding N1 of the output transformer T1
The amplitude of n1 varies depending on the on / off frequency f of the first and second transistors Q1, Q2. FIG. 5 shows the relationship between the on / off frequency f, the power supplied to the secondary side of the transformer T1 by the resonance circuit of the inductance of the primary winding N1, and the capacitor C1. When the transistors Q1 and Q2 are turned on and off at a frequency higher than the inherent series resonance frequency f0 determined by the leakage inductance and the capacitor C1, the supply power P decreases. FIG. 6 is for explaining this. When the frequency f shown in the first half of FIG. 6 is low, the amplitude of the voltage Vn1 of the primary winding N1 is large. The amplitude of the voltage Vn1 decreases. As a result, voltage control and power control are achieved by controlling the on / off frequency f in the range of fa to fb.

The power supply device shown in FIG. 1 has an overcurrent preventing function and an overload preventing function in addition to a function of controlling the output voltage to be constant. When an overcurrent is detected based on the current detector 8, the on / off frequency of the first and second transistors Q1 and Q2 is increased in the same manner as when the load voltage rises, and the load 3
To reduce the amount of electric power supplied to the power supply.

When the load 3 enters an overload state such as a short circuit, the load voltage decreases, and the light emitting diode 6 enters a non-light emitting state or a low light emitting state close to this. As a result, the phototransistor 7 is turned off or in a state close thereto, a voltage higher than the voltage of the reference voltage source 35 is input to the first comparator 33 in FIG. 2, a low-level output is obtained, and the transistor 37 is turned off. The capacitor 36 is charged, the output of the second comparator 34 goes high, the switch of the control power supply control circuit 32 is turned off, and the drive amplifier 2
The supply of the control power supply voltage to each circuit such as 9 and 30 is stopped. As a result, the first and second transistors Q1,
The on / off control of Q2 is also interrupted and these are turned off. As a result, no voltage is obtained in the control power supply winding 13, and the voltage of the capacitor 12, that is, the control power supply terminal Vcc
Voltage also decreases. Since the capacitor 12 is connected to the DC power supply 1 through the starting resistor 11, the charging based on the control power supply winding 13 is gradually stopped and then gradually charged. Is detected, the control power supply control circuit 32 is turned on, a voltage is applied to each part of the control circuit 2, and the on / off operation of the first and second transistors Q1 and Q2 is started. If the overload state has not been eliminated, the control power supply control circuit 32 is turned off again, and the above operation is repeated.

[0016]

By the way, in the control circuit 2 shown in FIG. 2, the light incident on the phototransistor 7 is obtained at the time of starting, and the voltage of the capacitor 36 is referenced before the transistor 37 is turned on. The capacitor 36 is charged with a time constant by the constant current source 38 in order to prevent the second comparator 34 from erroneously determining an overload because the voltage becomes higher than the voltage of the voltage source 39. Therefore,
Even in a true overload state, the capacitor 36 is gradually charged by the constant current source 38. As a result, before the capacitor 36 is charged at the time of overload and the control power supply control circuit 32 is controlled to be turned off by the comparator 34, the voltage of the control power supply terminal Vcc decreases and the control power supply voltage cannot be turned off. May be. The control power supply winding 13
Is a secondary winding N2a to obtain a constant voltage.
It is tightly coupled to N2b. Therefore, when a short circuit or the like of the load 3 occurs, the voltage of the winding 13 decreases as well as the load voltage, and the voltage of the control power supply capacitor 12 may decrease rapidly. In the power supply device of FIG.
If the load 3 includes a large-capacity capacitor or has a strong soft-start function, a sufficient voltage is not obtained in the winding 13 at the time of start-up because the operation becomes the same as the overload state. It may not be able to start normally.

In order to solve the above-mentioned disadvantage, it is conceivable to configure a control power supply as shown in FIG. This FIG.
1 is the same as that of FIG. 1 except that a winding 40, a rectifier 41, a capacitor 42, a transistor 43, a Zener diode 44, and a resistor 45 are provided instead of the winding 13 and the rectifier 14 in FIG. Winding 40 is electromagnetically coupled tightly to primary winding N1, and loosely coupled to secondary windings N2a and N2b. A smoothing capacitor 42 is connected to the winding 40 via a bridge rectifier 41. This capacitor 42
Between the power supply terminal Vcc of the control circuit 2 and the transistor 4
3, a constant voltage control circuit 46 including a Zener diode 44 and a resistor 45 is connected. The transistor 43 of the constant voltage control circuit 46 is connected in series to the line, the Zener diode 44 is connected between the base of the transistor 43 and the ground line, and the resistor 45 is connected between the collector and the base of the transistor 43. Have been. In the circuit of FIG. 7, an input voltage, that is, a voltage substantially proportional to the voltage of the primary winding N1 is obtained in the control power supply winding 40. Therefore, even if the load 3 is overloaded due to a short circuit or the like, a voltage can be obtained at the winding 40. However, since the voltage of the winding 40 changes according to the change of the input voltage, it is necessary to provide the constant voltage control circuit 46, and the power loss here increases,
This causes a reduction in efficiency of the switching power supply device.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a switching power supply capable of preventing an abnormality in a control circuit and improving efficiency.

[0019]

In order to achieve the above object, the present invention provides a primary winding of a transformer connected between one end and the other end of a DC power supply via a switch; A switching power supply device comprising: a secondary winding; and a control circuit for turning on and off the switch so as to obtain a stabilized voltage at the secondary winding. A first control power supply winding densely electromagnetically coupled to a next winding; and a first control power supply connected between the first control power supply winding and a power supply terminal of the control circuit. Rectifier circuit, a second control power supply winding electromagnetically coupled more tightly to the primary winding than to the secondary winding, a second control power supply winding and the control circuit. A second control power supply rectifier circuit connected between the power supply terminal and the power supply terminal;
The second control power supply rectifier circuit is configured to supply power to the control circuit during a period in which a predetermined voltage cannot be supplied from the first control power supply rectifier circuit to the control circuit. The present invention relates to a switching power supply device characterized in that: It is preferable that the constant voltage circuit be provided at the output stage of the second control power supply rectifier circuit.

[0020]

According to the present invention, a constant voltage is obtained in the first control power supply winding during normal operation, and this voltage is supplied to the control circuit. Therefore, the power loss is reduced and the efficiency is increased as compared with the case where a constant voltage circuit is specially provided. Further, when a predetermined voltage cannot be obtained in the first control power supply winding tightly coupled to the secondary winding at the time of starting or due to a load short circuit or the like, a voltage of the second control power supply winding is used. The power supply voltage is supplied to the control circuit. Therefore, the control circuit can be operated even at the time of startup or at the time of load abnormality. When a constant voltage circuit is provided according to claim 2,
A constant voltage can be supplied to the control circuit even at the time of startup or overload. Although the constant voltage circuit causes power loss, the constant voltage circuit does not substantially function when the load is normal, so that the efficiency is hardly reduced.

[0021]

First Embodiment Next, a switching power supply according to an embodiment of the present invention will be described with reference to FIGS. However, in FIG. 8, substantially the same parts as those in FIGS. 1 and 7 are denoted by the same reference numerals, and description thereof is omitted. The switching power supply device of FIG. 8 includes the control power supply winding 40 of FIG.
1 except that a control power supply circuit is added. That is, the circuit of FIG. 8 includes a first control power supply winding 13, a first control power supply rectifier 14 as a first rectifier circuit, and a capacitor 12, and a second control power supply winding 40. And a second control power supply rectifier 41 as a second rectification / smoothing circuit, a smoothing capacitor 42, and a constant voltage circuit 46, all of which are connected to the control power supply terminal Vcc of the control circuit 2. In FIG. 8, the circuit between the first winding 13 and the control circuit 2 is the same as in FIG. 1, and the circuit between the second winding 40 and the control circuit 2 is the same as in FIG.

The first control power supply winding 13 is electromagnetically coupled to the secondary windings N2a and N2b more tightly than the primary winding N1 as in FIG. Is the secondary winding N2
a, N2b is more electromagnetically coupled to the primary winding N1 than to N2b. The number of turns of the windings 13 and 40 is set so that the voltage charged in the capacitor 12 by the first control power supply winding 13 is slightly higher than the output voltage of the constant voltage circuit 46. The specific configuration of the control circuit 2 is the same as that of FIG. 1 and is configured as shown in FIG.

FIG. 9 shows a part of the transformer T1 of FIG. 8 in principle. A bobbin 50 is arranged so as to surround the magnetic core 15. A primary winding N1 and a second control power supply winding 40 are arranged on one side of a partition wall 51 of the bobbin 50 so as to be tightly coupled to each other, and two secondary windings are provided on the other side of the partition wall 51. N2a, N2b and first control power supply winding 13
And are arranged so as to be tightly coupled to each other.

According to the circuit shown in FIG. 8, when the load 3 includes a large-capacity capacitor, the load 3 is overloaded at startup or when the load 3 is overloaded due to a short circuit or the like. In addition, a desired voltage cannot be obtained in the first control power supply winding 13. However, a desired voltage is obtained in the second control power supply winding 40 which is tightly coupled to the primary winding N1. As a result, the voltage of the capacitor 42 is
Becomes high, the transistor 43 enters a forward bias state, the constant voltage circuit 46 starts operating, and power is supplied to the control circuit 2 therefrom. When the load 3 becomes normal and a desired voltage is obtained in the first control power supply winding 13, the voltage of the capacitor 12 becomes higher than the voltage of the output of the constant voltage circuit 46, and the transistor 43 is in a reverse bias state. And the constant voltage circuit 46 becomes inactive. As a result, the power supply voltage of the control circuit 2 is changed to the first and second transistors Q1
, Q2 by the first control power supply winding 13 whose voltage is made constant.

As described above, since a substantially normal voltage is always supplied to the control circuit 2, the control circuit can always operate normally. In other words, the protection operation at the time of overload can be reliably achieved without substantially lowering the average efficiency.

[0026]

Second Embodiment Next, a switching regulator according to a second embodiment shown in FIG. 10 will be described. However, in FIG. 10, substantially the same parts as those in FIGS. 1, 7, and 8 are denoted by the same reference numerals, and description thereof will be omitted. The circuit of FIG. 10 is obtained by adding a capacitor C2 to the circuit of FIG. The capacitor C2 is connected in series with the capacitor C1, and this series circuit is connected between one end and the other end of the power supply 1.
The primary winding N1 includes first and second transistors Q1, Q2
And the first and second capacitors C1, C2
Is connected between the interconnection midpoints. Therefore, FIG.
The conversion circuit of 0 is the same as a well-known half bridge type.
Since the rest of the circuit in FIG. 10 is the same as in FIG. 8, the same operation and effect as in FIG. 8 can be obtained.

[0027]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) As shown in FIG. 11, an inductance Lr is connected in series with the primary winding N1, and this is used as an inductance for resonance with the capacitor C1 or this inductance Lr
And the leakage inductance of the primary winding N1 can be used as the resonance inductance with the capacitor C1. (2) An inductance can be connected in parallel to the primary winding N1 or a series circuit of the primary winding N1 and the inductance Lr, and this can be used similarly to the excitation inductance. (3) The transistors Q1 and Q2 can be bipolar transistors. Diodes D1, D2
Can be individual diodes. (4) Instead of providing the current detector 8, the voltage across the capacitor C1 is detected, whereby the load current can be detected. (5) Two different outputs can be obtained by changing the number of turns of the secondary windings N2a and N2b. Also, the output transformer T1
, One of the secondary windings N2a and N2b can be omitted. (6) A series circuit of the series resonance capacitor C1 and the primary winding N1 having leakage inductance can be connected in parallel to the first transistor Q1. That is, in FIG. 8, the power supply 1, the transistors Q1, Q2, the diode D1,
The polarity of D2 can be reversed. (7) The internal configuration of the control circuit 2 can be variously modified. (8) The present invention can be applied to conversion circuits other than the resonance type, such as half-bridge type, full-bridge type, forward type, and reverse type. (9) A diode for preventing backflow can be provided on the output line of the constant voltage circuit 46. (10) The constant voltage circuit 46 can be omitted. Even in this case, the power for the control power supply can be supplied from the winding 40 instead of the winding 13 at the time of startup or overload. During this period, the circuit does not become a constant voltage circuit, but there is almost no problem if it is short.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing a conventional switching power supply device.

FIG. 2 is a diagram illustrating a control circuit of FIG. 1;

FIG. 3 is a voltage waveform diagram showing states of points A to D of the control circuit of FIG. 2;

FIG. 4 is a waveform diagram showing a state of each unit in FIG. 1;

FIG. 5 is a diagram for explaining the relationship between the on / off frequencies of Q1 and Q2 in FIG. 1 and the transmission power to the secondary side.

FIG. 6 shows ON / OFF frequencies of Q1 and Q2 and primary winding N1.
FIG. 4 is a diagram showing a relationship with the voltage of FIG.

FIG. 7 is a circuit diagram showing another conventional switching power supply device.

FIG. 8 is a circuit diagram illustrating the switching power supply device according to the first embodiment.

FIG. 9 is a sectional view showing a part of the transformer in FIG. 8 in principle.

FIG. 10 is a circuit diagram illustrating a switching power supply device according to a second embodiment.

FIG. 11 is a diagram illustrating an LC series resonance circuit according to a modification.

[Explanation of symbols]

 T1 Output transformer C1 Resonant capacitor N1 Primary winding N2a, N2b Secondary winding N13, N40 First and second control power supply windings

Claims (2)

(57) [Claims] 1. A primary winding of a transformer connected between one end and the other end of a DC power supply via a switch, a secondary winding of the transformer, and stabilized by the secondary winding. A control circuit for turning on and off the switch so as to obtain a voltage, wherein the first power supply is electromagnetically coupled more tightly to the secondary winding than to the primary winding. A control power supply winding; a first control power supply rectifier circuit connected between the first control power supply winding and a power supply terminal of the control circuit; A second control power supply winding tightly electromagnetically coupled to the winding; and a second control power supply connected between the second control power supply winding and a power supply terminal of the control circuit. A rectifier circuit, and applies a predetermined voltage from the first control power supply rectifier circuit to the control circuit. Switching power supply apparatus characterized by being formed so as to supply power from the period can not be fed second control power rectifying circuit to the control circuit. 2. A power supply terminal connected between the second control power supply rectifier circuit and the power supply terminal of the control circuit and connected to the power supply terminal of the control circuit from the first control power supply rectifier circuit. A voltage regulating circuit formed so as to supply a voltage regulated to the power supply terminal only during a period in which a voltage cannot be supplied, and wherein the first control power supply rectifier circuit and the control circuit The switching power supply according to claim 1, wherein a constant voltage circuit is not provided between the switching power supply and the power supply terminal.