JP2000116123A - Switching power unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power unit which can magnetically reset a transformer in a short time while the unit is maintained at a small size. SOLUTION: The switching power unit 1 is provided with a transformer 2 having at least first to third windings 2a-2c, first switch circuits 11 and 12 which switch the alternating current inputted to the first winding 2a, and a second switch circuit 21 which synchronously rectifies the voltage induced in the second winding 2b when the input alternating circuit is in a positive cycle. The power unit is also provided with a third switch circuit 23 which synchronously rectifies the voltage induced in the third winding 2c when the alternating current is in a negative cycle, a smoothing circuit 28 which smoothes the output voltages of the second and third switch circuits 21 and 23, and a switching control circuit 13. In addition, the power unit is also provided with current output inhibiting means 22 and 24 which inhibit the output of a current based on the exciting energy of the transformer 2 to the smoothing circuit 28 through the second or third winding 2b or 2c during a switching-off period, and a resetting means 18 which magnetically resets the transformer 2 during the switching- off period.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching power supply for generating a DC voltage by switching.
More specifically, the present invention relates to a switching power supply device suitable for directly generating a DC voltage from a commercial AC power supply by a so-called synchronous rectification method.

[0002]

2. Description of the Related Art As this kind of power supply device, Japanese Patent No. 274 is known.
A power supply circuit described in Japanese Patent No. 0495 is conventionally known. The power supply circuit 31 basically includes a transformer 2 having a primary winding 2a and secondary windings 2b and 2c, as shown in FIG.
On the side, switching FETs 11 and 12 and a switching control circuit 13 are provided. Further, an FET for synchronous rectification is provided on the secondary winding 2b, 2c side of the transformer 2.
21 and 23, respectively, and a diode 25 and a choke coil 26 are provided on the drain side of both FETs 21 and 23, respectively.
And a smoothing circuit including a capacitor 27. Further, between the output terminal of the output voltage VO and the input terminal e of the switching control circuit 13, a feedback control circuit for outputting to the switching control circuit 13 a control signal S4 for performing feedback control so that the output voltage VO becomes a constant voltage. 29 are provided.

In the power supply circuit 31, the switching control circuit 13 is configured to control one cycle T of the input AC VIN shown in FIG.
At this time, as shown in FIG. 2B, a drive signal S1 having a period T1 sufficiently shorter than the period T is supplied from the output terminal a to the FET.
Output to gates 11 and 12. Thereby, both FETs
The switching of the switches 11 and 12 is controlled to the ON state. During this switching ON period, the input AC VIN is applied to the primary winding 2a of the transformer 2.

In this case, during the period in which the input AC VIN is in the positive cycle, the secondary winding 2 is connected to the winding start terminal of the secondary winding 2b of the transformer 2 (the terminal indicated by the black dot in FIG. 4).
A positive voltage is induced in the winding end terminal of the terminal b. At this time, the switching control circuit 13 outputs a driving signal S2 synchronized with the driving signal S1 from the output terminal b as shown in FIG. Output to the gate of FET21. Therefore, as shown in FIG. 4, the current I1 based on the induced voltage of the secondary winding 2b is applied to the FET 21, the choke coil 26, the load (or the capacitor 27) not shown, and the winding end terminal of the secondary winding 2b. , The synchronous rectification and smoothing are performed, and the excitation energy is stored in the choke coil 26. Next, the switching control circuit 13 stops outputting the drive signal S1, and during this switching-off period, the freewheeling current I based on the excitation energy stored in the choke coil 26.
2 is the choke coil 26, load (or capacitor 2)
7) and a diode 25.

On the other hand, when the input AC VIN is in a negative cycle, a positive voltage is induced at the winding end terminal of the secondary winding 2c of the transformer 2 with respect to the winding start terminal of the secondary winding 2c. In this case, the switching control circuit 13
As shown in (d), a drive signal S3 synchronized with the drive signal S1 is output from the output terminal c to the FET 23. Accordingly, the current I3 based on the induced voltage of the secondary winding 2c is
As shown in the figure, the current flows through the FET 23, the choke coil 26, the load (or the capacitor 27), and the winding start side terminal of the secondary winding 2c, whereby synchronous rectification and smoothing are performed and the choke coil 26 Excitation energy is stored. Next, similarly to the positive cycle period, the switching control circuit 13 stops outputting the drive signal S1, and during this switching off period, the freewheeling current I2 based on the excitation energy accumulated in the choke coil 26 is supplied with the choke current. The current flows through a current path including the coil 26, the load (or the capacitor 27), and the diode 25. As described above, in the power supply circuit 31, the input AC VIN is directly switched by the FETs 11 and 12, and the induced voltages of the secondary windings 2b and 2c are synchronously rectified by the FETs 21 and 23, thereby providing a predetermined voltage. An output voltage VO is being generated.

[0006]

However, the conventional power supply circuit 31 has the following problems. That is, F
When a current flows through the primary winding 2a of the transformer 2 by switching of the ETs 11 and 12, the transformer 2 is excited according to a predetermined BH curve. Therefore, generally, in order to prevent the magnetic saturation of the transformer 2, the FE
During the switching-off period in which T11 and T12 are controlled to be in the off state, it is necessary to release the excitation energy from the transformer 2 to perform magnetic reset. However, during the switching-off period, since the FETs 11 and 12 are both controlled to be in the off state, the excitation energy of the transformer 2 is released through one of the secondary windings 2b and 2c. in this case,
For example, when the input AC VIN is in a positive cycle, on the secondary winding 2b side, since the FET 21 is controlled to be in the off state, the winding end side terminal of the secondary winding 2b, the diode 2
5. An outflow of a current (hereinafter, referred to as a “reset current”) based on the excitation energy through a current path including the FET 21 and the winding start side terminal is prevented.

On the other hand, generally, an FET has a built-in body diode capable of conducting a current from a source to a drain thereof. Therefore, during the switching off period in the positive cycle period of the input AC VIN, the reset current is supplied to the winding end side terminal of the secondary winding 2c and the FET 23.
Flows through a current path including the body diode, the choke coil 26, the load (or the capacitor 27), and the winding start side terminal of the secondary winding 2c. However, during this period,
Freewheeling current I2 is flowing through diode 25. For this reason, the voltage across the series circuit composed of the secondary winding 2c and the body diode of the FET 23 is equivalently equal to the forward voltage (about 0.5 to 1.5 V) of the diode 25. Is magnetically reset at a reset voltage close to 0V. In addition,
Also during the negative cycle period of the input AC VIN, the reset voltage when the reset current is released from the secondary winding 2b is:
For the same reason, the voltage is close to 0V.

In this case, the following equation must be satisfied in order for the transformer 2 to be magnetically reset within a predetermined reset time TR. In the same equation, B, A, N,
K and E are the magnetic flux density of the transformer 2 immediately before the input AC VIN flows during the switching-on period and the switching is controlled to be turned off, the cross-sectional area of the core of the transformer 2, the number of windings of the winding for releasing the reset current, respectively. Predetermined coefficient,
And the voltage across the winding which is the reset voltage when the excitation energy is released. B ・ A ・ NK ・ TR = TR ・ E ・ ・ ・ ・ ・ ・ Equation

In the above equation, the magnetic flux density B is uniquely determined by the switching condition, and the cross-sectional area A and the coefficient K are predetermined constant values. Also, the reset time T
R and the reset voltage E are in inverse proportion. Therefore, as described above, since the reset voltage E in the equation is substantially a voltage close to 0 V, when the number of turns N is a normal number of turns, the reset time TR becomes infinite.
In such a case, if the switching by the FETs 11 and 12 is continued, the transformer 2 is surely magnetically saturated. Therefore, in order to prevent magnetic saturation, the number of turns N is changed so that the reset time TR is shorter than the switching off period. However, in this case, the number of turns N becomes so unrealistic that the transformer 2 becomes extremely large. Therefore, the power supply circuit 3
The first problem is that, due to the increase in the size of the transformer 2, the essential advantage of the switching power supply that the size is reduced is lost.

The present invention has been made in view of such a problem, and has as its main object to provide a switching power supply device capable of magnetically resetting a transformer in a short time while keeping the size of the device small. .

[0011]

According to a first aspect of the present invention, there is provided a switching power supply device comprising: a first winding as a primary winding and at least a second winding and a third winding as a secondary winding. , A first switch circuit for switching an input AC input to the first winding, and a second switch for synchronously rectifying an induced voltage induced in the second winding during a positive cycle of the input AC. A third switch circuit for synchronously rectifying an induced voltage induced in the third winding during a negative cycle of the input AC; a smoothing circuit for smoothing output voltages of the second and third switch circuits; In a switching power supply device including a first switch circuit, a second switch circuit, and a switching control circuit for controlling a switching operation of the third switch circuit, a second switch circuit is provided. And a third switch circuit for preventing a current based on the exciting energy of the transformer from being output to the smoothing circuit through the second or third winding during a switching off period in which the switching control circuit controls both of them to be turned off. And a reset means for magnetically resetting the transformer during the switching off period.

According to a second aspect of the present invention, there is provided a switching power supply.
2. The switching power supply according to claim 1, wherein the second switch circuit is constituted by a first FET, the third switch circuit is constituted by a second FET, and the current output blocking means has a body diode whose direction is opposite to that of the body diode of the first FET. A third FET connected in series to the first FET so as to be oriented, and a second FET so that the body diode is oriented in a direction opposite to the direction of the body diode of the second FET.
And a fourth FET connected in series.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of a switching power supply according to the present invention will be described below with reference to the accompanying drawings. Note that the same components as those of the conventional power supply circuit 31 are denoted by the same reference numerals, and redundant description will be omitted.

As shown in FIG. 1, the power supply device 1 is a forward type AC / DC converter and includes a switching transformer 2. In addition, the power supply device 1 includes the FETs 11 and 12 and the switching control circuit 13 that constitute the first switch circuit according to the present invention on the primary winding 2a side of the transformer 2, and corresponds to reset means according to the present invention. 14, 15, capacitor 1
6 and a reset circuit 18 including a resistor 17.

The power supply device 1 has a body diode 22a built in the secondary winding 2b side of the transformer 2 and corresponds to a current output blocking means in the present invention.
T22 and the body diode 21a
And a F corresponding to the second switch circuit in the present invention.
The present invention is provided with an ET 21 and a FET 24 which is provided with a body diode 24a on the secondary winding 2c side of the transformer 2 and which corresponds to a current output blocking means in the present invention. And a FET 23 corresponding to the third switch circuit in the above. In this case, the drains of the FETs 21 and 23 are connected to each other, and the winding end side terminal of the secondary winding 2b and the winding start side terminal of the secondary winding 2c are connected to each other. Further, the power supply device 1 includes two FETs 2
There is provided a smoothing circuit 28 connected in parallel between the drains 1 and 23 and the winding end side terminal of the secondary winding 2 b, and this smoothing circuit 28 comprises a diode 25, a choke coil 26 and a capacitor 27.

Next, the overall operation of the power supply device 1 will be described. In the power supply device 1, the switching operation itself for generating the output voltage VO from the input AC VIN is performed in substantially the same manner as the conventional power supply circuit 31, so that the same symbols are used for the description of the operation and the like. A duplicate description will be omitted.

In this power supply device 1, the switching control circuit 13 outputs the drive signal S1 of the cycle T1 shown in FIG. 3B during the positive cycle period in one cycle T of the input AC VIN shown in FIG. FETs 11 and 12 from terminal a
Output to the gate. As a result, both FETs 11, 12
Is controlled to be turned on, and during this switching-on period, a current based on the input AC VIN flows through the primary winding 2 a of the transformer 2 and the series circuit of the FETs 12 and 11. At this time, the voltage of the center tap 2d of the primary winding 2a with respect to the terminal at the winding end side of the primary winding 2a is 1/2 of the voltage of the input AC VIN applied to the primary winding 2a. Therefore, the capacitor 16 is charged with the current flowing through the diode 14, so that the voltage across the capacitor 16 is maintained at approximately 1/2 of the voltage of the input AC VIN.

In this case, a positive voltage is induced at the winding start terminal of the secondary winding 2b of the transformer 2 with respect to the winding end terminal,
At this time, the switching control circuit 13 operates as shown in FIG.
The drive signal S2 shown in FIG.
Output to the gate. As a result, the current I1 based on the induced voltage of the secondary winding 2b, as shown in FIG.
2, 21, a choke coil 26, a load (or a capacitor 27) (not shown), and a current path composed of a terminal on the winding end side of the secondary winding 2b, whereby synchronous rectification and smoothing are performed and the choke coil 26 Excitation energy is stored. In the switching-on period, the transformer 2 is excited from an initial point PS in a non-excited state to a point P1 of a predetermined magnetic flux density, as shown by a BH curve in FIG. Next, the switching control circuit 13 stops outputting the drive signal S1, and during this switching-off period, the freewheeling current I2 based on the excitation energy stored in the choke coil 26 causes the choke coil 26, the load (or the capacitor 27). ) And a diode 25.

During the switching off period, the reset current is applied to the secondary winding 2b on the secondary winding 2b side.
b, a series circuit of a diode 25, FET21 and FET22 or a body diode 21a
And a current path composed of a series circuit of the body diode 22a and a winding start terminal of the secondary winding 2b. However, the FETs 21 and 22 are both controlled to be in the off state, and the body diode 21a and the body diode 22a are connected in series such that they are opposite to each other. As a result, the current path is cut off, so that the magnetic reset by the secondary winding 2b is prevented. Similarly, for the secondary winding 2c, the reset current is
End terminal of secondary winding 2c, FET 24 and FE
A series circuit of T23 or a series circuit of body diode 24a and body diode 23a, choke coil 2
6. Load (or capacitor 27) and secondary winding 2
An attempt is made to flow through the current path composed of the winding start side terminal of c.
However, both the FETs 23 and 24 are controlled to be in the off state, and the body diode 23a and the body diode 24a are connected in series such that they are opposite to each other. As a result, the current path is cut off, so that unlike the conventional power supply circuit 31, magnetic reset by the secondary winding 2c is also prevented.

On the other hand, in the primary winding 2a, the FETs 11, 1
2 are both in the off state, so that the FETs 11 and
The current path of the reset current via 12 is cut off.
Therefore, the reset current I4 is, as shown in FIG.
The current flows through the current path including the winding end terminal of the primary winding 2a, the diode 15, the capacitor 16 (or the resistor 17), and the center tap 2d of the primary winding 2a, whereby the magnetic reset of the transformer 2 is performed. At this time,
Since the capacitor 16 is charged by the current I4, the voltage between both ends of the capacitor 16 is gradually increased from approximately half the voltage of the input AC VIN. On the other hand, the reset voltage E between the winding end side terminal of the primary winding 2a and the center tap 2d is determined by the voltage across the capacitor 16 and the diode 1
5 is the sum of the forward voltage and the forward voltage. For this reason, unlike the conventional power supply circuit 31, the reset voltage E becomes a sufficiently high voltage. Therefore, according to the above equation, the reset time TR is a sufficiently short time, whereby the transformer 2 is magnetically reset from the point P1 in FIG. 2 to the state of the point P2 of the predetermined magnetic flux density within the switching off period. You. Thereafter, the transformer 2 outputs the drive signal S1
Point P in the same figure during the switching-on period
It is excited to the state of 3 and magnetically reset to the state of point P2 during the switching off period.

The voltage across the capacitor 16 is discharged through the resistor 17, charged by the input AC VIN through the diode 14 (or 15), and charged in the primary winding 2a.
Is maintained at a predetermined voltage value determined by charging with the reset current output from the controller. In this case, the capacitor 16
Of the resistor 17 (that is, the reset voltage E) and the reset time TR are determined by appropriately setting the resistance value of the resistor 17.
It is set to an arbitrary voltage value or time. In the positive cycle period and a negative cycle period described later, the feedback control circuit 29 receives the output voltage VO, detects an error voltage between the output voltage VO and a predetermined reference voltage, and responds to the error voltage. The error signal S4 is output to the input terminal e of the switching control circuit 13. As a result, the switching control circuit 13 transmits the PWM (Pulse-Wi
dth Modulation) signal as the driving signal S1
By outputting to the gates of the capacitors 1 and 12, the capacitor 2
7 is controlled to maintain the voltage at both ends at a predetermined voltage.

On the other hand, when the input AC VIN is in a negative cycle, a positive voltage is induced at the winding end terminal of the secondary winding 2c of the transformer 2 with respect to the winding start terminal of the secondary winding 2c. In this case, the switching control circuit 13
The drive signal S3 shown in FIG.
Output to 3, 24. For this reason, the current I3 based on the induced voltage of the secondary winding 2c, as shown in FIG.
4, 23, the choke coil 26, the load (or the capacitor 27), and the current path including the winding start side terminal of the secondary winding 2c, whereby synchronous rectification and smoothing are performed, and the excitation energy is supplied to the choke coil 26. Stored. In the switching-on period, the transformer 2 is excited from the point P2 at which the magnetic reset is performed in the positive cycle period to the point P4 having a predetermined magnetic flux density, as shown by the BH curve in FIG. Next, similarly to the positive cycle period, the switching control circuit 13 stops outputting the drive signal S1, and during this switching off period, the freewheeling current I2 based on the excitation energy accumulated in the choke coil 26 is supplied with the choke current. The current flows through a current path including the coil 26, the load (or the capacitor 27), and the diode 25.

During the switching off period, the reset current is applied to the secondary winding 2c on the secondary winding 2c side.
Attempts to flow through a current path consisting of a winding start terminal of c, a series circuit of the diode 25, FET23 and FET24 or a series circuit of the body diode 23a and the body diode 24a, and a winding end terminal of the secondary winding 2c. However, both the FETs 23 and 24 are controlled to be in the off state, and the body diode 23a and the body diode 24a are connected in series such that they are opposite to each other. As a result, the current path is cut off, so that the magnetic reset by the secondary winding 2c is prevented. Similarly, for the secondary winding 2b, the reset current is
Starting terminal of secondary winding 2b, FET22 and FET
21 or a series circuit of body diode 22a and body diode 21a, choke coil 2
6. Load (or capacitor 27) and secondary winding 2
An attempt is made to flow through the current path formed by the winding end side terminal b. However, the FETs 21 and 22 are both controlled to be in the off state, and the body diode 21a and the body diode 22a are connected in series such that they are opposite to each other. As a result, the current path is cut off, so that the magnetic reset by the secondary winding 2b is also prevented.

On the other hand, in the primary winding 2a, the current path through the FETs 11 and 12 is cut off in the same manner as in the positive cycle period, so that the reset current I5, as shown in FIG. 2a winding start terminal, diode 1
4. flow through a current path consisting of a capacitor 16 (or resistor 17) and a center tap 2d of the primary winding 2a;
Thereby, the magnetic reset of the transformer 2 is performed. In this case as well, as in the case of the positive cycle period, the reset voltage E becomes a sufficiently high voltage, so that the reset time TR becomes a sufficiently short time. Is magnetically reset from the point P4 to the state of the point P5 of a predetermined magnetic flux density. Thereafter, the transformer 2 is excited to the state at point P5 in the switching on period in synchronization with the drive signal S1 and the point P5 in the switching off period.
Magnetic reset to state 6.

As described above, according to the power supply device 1, during the switching off period, the FETs 21 to 24 output the reset current to the smoothing circuit 28 via the secondary winding 2b or the secondary winding 2c. , And a magnetic reset is performed by the reset circuit 18 via the primary winding 2a, and the reset voltage E at that time is prevented from lowering. Therefore, the magnetic reset can be performed by the high reset voltage E. Thereby, since it is not necessary to wind many windings 2a to 2c, the transformer 2 can be magnetically reset in a short time while keeping the size of the transformer 2 small.

It should be noted that the present invention is not limited to the above-described embodiment, and the configuration can be changed as appropriate. For example, in the power supply device 1 according to the embodiment of the present invention, the CRD
Although the reset circuit 18 of the type is used, the reset means in the present invention is not limited to this, and various reset circuits can be used. Further, the reset circuit 18 is not limited to the primary winding 2a, and may be disposed on the secondary winding 2b, the secondary winding 2c, or a separately provided reset winding. Furthermore, in the embodiment of the present invention, a power supply device of a type that outputs one type of output voltage VO has been described, but a plurality of output windings are formed in the transformer 2 and the secondary windings 2b, 2b in the power supply device 1 are formed. A plurality of rectifying / smoothing circuits on the 2c side may be provided corresponding to the respective output windings so that a plurality of output voltages different from each other can be output. In this case, any one of the plurality of windings may be used. Can be provided with a reset circuit. Also, the current output blocking means is not limited to the FET, but may be a transistor. Further, the FET 21 and the FET 22 may be integrally formed in one package.
23 and the FET 24 may be integrally formed in one package.

[0027]

As described above, according to the switching power supply device of the first aspect, during the switching off period of the second switch circuit and the third switch circuit, the current output blocking means sets the second winding of the reset current or the reset current. Since the output to the smoothing circuit via the third winding is blocked and the resetting means magnetically resets the transformer, the transformer can be magnetically reset in a short time, so that the number of turns of each winding of the transformer is increased. Therefore, it is possible to reduce the size of the transformer and, consequently, the size of the switching power supply device.

According to the switching power supply device of the second aspect, the second switch circuit, the third switch circuit, and the current output blocking means are each configured by an FET. Synchronous rectification can be performed.

[Brief description of the drawings]

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

FIG. 2 is a conceptual characteristic diagram of a BH curve for describing an operation of the power supply device 1 according to the embodiment of the present invention.

FIG. 3 is a diagram for explaining operations of the power supply device 1 according to the embodiment of the present invention and a conventional power supply circuit 31,
(A) is a voltage waveform diagram of the input AC VIN, (b) is a voltage waveform diagram of the drive signal S1, (c) is a voltage waveform diagram of the drive signal S2, and (d) is a voltage waveform diagram of the drive signal S3.

FIG. 4 is a circuit diagram of a conventional power supply circuit 31.

[Explanation of symbols]

 Reference Signs List 1 power supply device 2 transformer 2a primary winding 2b secondary winding 2c secondary winding 11 FET 12 FET 13 switching control circuit 14 diode 15 diode 16 capacitor 17 resistance 18 reset circuit 21 to 24 FET 21a to 24d body diode 28 smoothing circuit

Claims (2)

[Claims] 1. A transformer having a first winding as a primary winding and at least a second winding and a third winding as a secondary winding, and for switching an input AC input to the first winding. A first switch circuit, a second switch circuit for synchronously rectifying an induced voltage induced in the second winding during the positive cycle of the input AC, and a second switch circuit configured to induce an induced voltage in the third winding during the negative cycle of the input AC. A third switch circuit for synchronously rectifying an induced voltage to be generated, a smoothing circuit for smoothing output voltages of the second switch circuit and the third switch circuit, the first switch circuit, the second switch circuit, and the second switch circuit. A switching power supply device comprising: a switching control circuit that controls a switching operation of a three-switch circuit, wherein the second switch circuit and the third switch circuit A switching control circuit for preventing a current based on the excitation energy of the transformer from being output to the smoothing circuit through the second winding or the third winding during a switching off period in which both are controlled to be in an off state by the switching control circuit. A switching power supply device comprising: a current output blocking unit; and a reset unit for magnetically resetting the transformer during the switching off period. 2. The second switch circuit is configured by a first FET, the third switch circuit is configured by a second FET, and the current output blocking unit is configured such that a body diode has a direction opposite to a direction of a body diode of the first FET. The third F connected in series with the first FET so that
2. The switching power supply device according to claim 1, wherein the switching power supply device comprises an ET and a fourth FET connected in series to the second FET so that the body diode has a direction opposite to that of the body diode of the second FET. .