JP2003333845A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply having high power supply efficiency in which a size can be reduced by a simple arrangement. <P>SOLUTION: A DC power supply 45 and a main switching element 44 are connected to the primary winding 41 side of a main transformer 40, and a rectification element 52 and a commutation element 54 are connected to the secondary winding side 42. The main transformer 40 is provided with a tertiary winding 43, and a circuit 65 for rectifying/smoothing a voltage induced in the tertiary winding 43 when the main switching element 44 is turned on. A driving means, i.e., a transistor 62, detects a voltage drop of the tertiary winding 43 for the potential of a capacitor 66 when the main switching element 44 is turned off and turns the commutation element 54 on. A discharging switch element or a transistor 68 turned on by a control signal synchronized with the on/off signal of the main switching element 44 is connected with the gate of the commutation element 54. <P>COPYRIGHT: (C)2004,JPO

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 which converts a DC voltage into a desired voltage and supplies it to an electronic device, which is a synchronous rectification type switching power supply device.

[0002]

2. Description of the Related Art Conventionally, in a switching power supply device,
A diode has been used as a rectifying element for converting the electric power converted into AC power by the main switch element into DC power again. In recent years, there has been a demand for lower output voltage and higher efficiency of a switching power supply device, and instead of a diode having a large forward voltage drop as a rectifying element, synchronous rectification using a MOS-FET having a small conduction resistance and a small loss is used. Type switching power supply devices have come into use.

A conventional synchronous rectification type switching power supply device includes a main transformer 10 having a primary winding 11 and a secondary winding 12 as disclosed in, for example, FIG. 5 and Japanese Patent Application Laid-Open No. 7-308064. A DC power supply 15, a main switch element 14 and a control circuit 16 which is a drive circuit are connected to the primary winding 11 side, and a rectifying element 2 is connected to the secondary winding 12 side.
2. A commutation element 24 is provided, and a choke coil 26 is further provided.
And the output capacitor 28 is connected. A MOS-FET is used for the rectifying element 22 and the commutation element 24, and the transformer 1 is turned on / off by turning on / off the main switching element 14.
The voltage generated at 0 is used to turn on / off the MOS-FETs of the rectifying element 22 and the commutation element 24. Further, the main transformer 10 is provided with a tertiary winding 13, and a rectifying / smoothing circuit 32 for rectifying and smoothing the voltage of the tertiary winding 13 is provided.
And a voltage detection circuit 34 for detecting the voltage of the tertiary winding 13.
And a drive circuit 36 that is on / off controlled by the output of the voltage detection circuit 34. In the switching power supply device, the drive circuit 36 generates a drive signal which is turned on / off complementarily with the output of the control circuit 16 to turn on / off the commutation element 24.

Further, FIG. 6 and Japanese Patent Laid-Open No. 2000-26405.
A circuit configuration of a switching power supply device as disclosed in Japanese Patent Publication No. 2 is also proposed. Here, the same members as those in FIG. 5 are designated by the same reference numerals and the description thereof will be omitted. In this device, the excitation energy stored in the main transformer 10 while the main switching element 14 is on is changed to the main switching element 1
The voltage generated when 4 is discharged during the off period (hereinafter referred to as a reset voltage) is input to the gate of the FET of the commutation element 24, the commutation element 24 is turned on, and the control device 16 causes the main switch to switch. The transistor of the discharge circuit 38 is turned on by the signal having the same phase as the control signal for turning on the element 14, and the charge of the gate capacitance of the commutation element 24 is discharged.

[0005]

In the former case of the above-mentioned conventional technique, the commutation element 24 is driven through the drive circuit 36 by using the voltage generated in the tertiary winding 13 when the main switching element 14 is turned on. I'm turning it off. Therefore, there is a delay time from when the voltage is generated in the tertiary winding to when the commutation element 24 is turned off. During the delay time until the commutation element 24 is turned off, a through current flowing through the commutation element 24 and the rectification element 22 is generated, which causes a reduction in the efficiency of the switching power supply.

In the latter case of the above-mentioned conventional technique, the commutation element 24 is turned on by the reset voltage generated in the secondary winding 12 when the main switching element 14 is turned off.
In this case, a certain time is required for the voltage generated in the secondary winding 12 to rise to the threshold voltage of the FET of the commutation element 24 after the main switching element 14 is turned off. Therefore, the commutation element 24 cannot be turned on during that time, and a current flows through the body diode of the FET of the commutation element 24 until the commutation element 24 is turned on, which lowers the power supply efficiency.

Further, in the former case of the above-mentioned conventional technique, since the reset voltage is stored in the capacitor of the rectifying / smoothing circuit 32, the capacitor is connected in parallel with the inductance of the main transformer 10 while the main switch element 14 is off. It will be in the state of being. In the latter case of the above-mentioned conventional technique, the reset voltage is also used to set the MOS- of the commutation element 24.
While the input capacitance (Ciss) of the FET is being charged and the main switch element 14 is off, the input capacitance (Ci) of the commutation element 24 is parallel to the inductance of the main transformer 10.
ss) is connected.

Therefore, in both the former case and the latter case of the conventional technique, a large capacitor is connected in parallel with the inductance of the main transformer 10 while the main switch element 14 is off.

Here, the excitation energy stored in the main transformer 10 while the main switch element 14 is on is
The time required to discharge the main switch element 14 during the off period is determined by the resonance period of the inductance component of the main transformer 10 and the capacitor component connected to it.

Therefore, when a large capacitor is connected in parallel with the inductance of the main transformer 10, the resonance cycle becomes longer and the time required to release the excitation energy stored in the main transformer 10 becomes longer. If the main switch element 14 is turned on before the release of the excitation energy is completed, the main transformer 10 is demagnetized and the main transformer 10 is saturated. Therefore, in each of the above-mentioned conventional techniques, the switching frequency cannot be increased. Therefore, large parts are used as parts such as the main transformer, the output choke coil, the output capacitor, and the input filter, which causes a problem that the power supply device becomes large and the cost increases.

The present invention has been made in view of the above conventional technique, and an object of the present invention is to provide a switching power supply device which can be downsized with a simple structure and has high power supply efficiency.

[0012]

According to the present invention, a DC power source and a main switching element such as an FET are connected to the primary winding side of a main transformer, and a rectifying element such as an FET and a commutation element are connected to the secondary winding side. A forward converter of a synchronous rectification system in which elements are connected and a main switching element is turned on and off by a control circuit. A tertiary winding is provided in a main transformer, and this tertiary winding is generated when the main switching element is turned on. A rectifying / smoothing circuit for rectifying and smoothing the voltage is provided, voltage detecting means for detecting a voltage drop of the tertiary winding with respect to the potential of the rectifying / smoothing circuit when the main switch element is turned off is provided, and an output of the voltage detecting means is provided. It is a switching power supply device provided with a drive means for turning on a commutation element by using it.

Further, a discharging switch element such as a transistor is provided between the gate and source of the FET, which is a commutation element, and the discharging switch is synchronized with the ON signal for the main switching element of the control circuit. By turning on the element, the commutation element is turned off. An insulating circuit such as a transformer or a photocoupler may be provided in a path for transmitting a signal for turning off the commutation element from the control circuit.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a switching power supply device according to an embodiment of the present invention. This switching power supply device has a DC coil on a primary winding side of a main transformer 40 having a primary winding 41 and a secondary winding 42. Power supply 4
5, the primary winding 41 and the main switching element 44 of the MOS-FET are connected in series. The positive side terminal of the DC power supply 45 is connected to the dot side terminal of the primary winding 41, and the non-dot side terminal is the main switch element 44.
Connected to the drain of. The source of the main switch element 44 is connected to the negative terminal of the DC power supply 45, and the gate of the main switch element 44 outputs a drive signal of a control circuit 46 that performs PWM (Pulse Width Modulation) control as a drive circuit. Are connected.

On the secondary winding side of the main transformer 40, M
A rectifying element 52 composed of an OS-FET and a MOS
A commutation element 54 composed of -FET is provided in series. In the secondary winding 42, the terminal on the dot side is connected to the drain of the commutation element 54, and the drain on the rectification element 52 is connected to the terminal on the side without the dot.
The source of No. 4 and the source of the rectifying element 52 are connected. A choke coil 56 and an output capacitor 58 are connected in series between the drain and source of the commutation element 54, and both ends of the output capacitor 58 are connected to the output terminal.

Further, the main transformer 40 is provided with a tertiary winding 43, and the terminal on the dot side of the tertiary winding 43 is connected to the gate of the rectifying element 52 via the resistor R1. , Connected to the anode of the diode 60. The cathode of the diode 60 is connected to the emitter of the pnp transistor 62 and also to the base of the transistor 62 via the resistor R2. To the base of the transistor 62, the dot-attached terminal of the tertiary winding 43 is connected via a capacitor 64. Furthermore, the cathode of the diode 60 is connected to the dotless side terminal of the tertiary winding 43 via the capacitor 66. Diode 60 and capacitor 66
Thus, the rectifying / smoothing circuit 65 is configured.

The collector of the transistor 62 is connected to the gate of the commutation element 54 and also to the collector of the npn transistor 68 which is a discharge switching element. The emitter of the transistor 68 is connected to the non-dotted terminal of the tertiary winding 43 and the source of the commutation element 54. At the base of the transistor 68, a control signal having the same phase as the drive signal of the main switch element 44 by the control circuit 46, a drive transformer 70 as an insulating circuit, and a resistor R3.
Be entered via.

Next, the operation of the switching power supply device of this embodiment will be described with reference to FIGS. First, when the drive signal of the main switching element 44 is output from the control circuit 46 and the main switching element 44 is turned on, as shown in the period T1 of FIG. 2, the dots are placed on the secondary winding 42 and the tertiary winding 43. , A voltage obtained by multiplying the input voltage by the transformer turns ratio is generated. Then, the rectifying element 52 is turned on,
A current flows through the choke coil 56 and the output capacitor 58. At the same time, while the voltage of the tertiary winding 43 is higher than the voltage of the capacitor 66, the capacitor 6 is connected via the diode 60.
Charge 6 After the capacitor 66 is charged, the period T
A constant voltage is maintained at 1.

When the control signal of the control circuit 46 turns off the main switch element 44, the voltage of the tertiary winding 43 starts to decrease as shown in the period T2 of FIG. It is transmitted to the base of the transistor 62, which is the driving means of the commutation element 54, via the.
When the voltage of the tertiary winding 43 decreases from the voltage of the capacitor 66 by the threshold value of the transistor 62, the transistor 62 turns on and charges the input capacitance Ciss of the gate of the commutation element 54. While the voltage of the tertiary winding 43 is decreasing, the transistor 62 is on. At this time, the transistor 6
The base current of 2 charges the capacitor 64.

When the reset voltage of the tertiary winding 43 reaches its peak, the base current of the transistor 62 stops flowing and the transistor 62 turns off. After that, as shown in a period T3 of FIG. 2, the voltage of the tertiary winding 43 increases toward zero. At this time, the voltage of the capacitor 64 is
As the voltage of the tertiary winding 43 rises, it flows to the capacitor 66 through the discharging resistor R2 and is discharged.
Since 4 is relatively small compared to the capacitor 66, the voltage of the capacitor 66 hardly changes.

Even if the voltage of the tertiary winding 43 becomes zero, the gate voltage of the commutation element 54 is maintained because the gate voltage of the commutation element 54 does not have the discharge path of the input capacitance Ciss, as shown in period T4 of FIG. The diversion element 54 maintains the ON state.

Then, the control circuit 46 outputs a control signal for turning off the commutation element 54, which is a signal in phase with the signal for turning on the main switching element 44. This control signal is
It is input to the base of the transistor 68 via the drive transformer 70, and the transistor 68 is turned on. As a result, the input capacitance Ciss of the commutation element 54 is discharged.

According to the switching power supply device of this embodiment, even when the reset voltage generated in the main transformer 40 rises slowly, the transistor 62 is driven by detecting the voltage drop of the tertiary winding 43 at that time, and the capacitor 62 is driven. 66
Since the input capacitance of the commutation element 54 is charged with the electric charge stored in the commutation element 54, high-speed operation of the commutation element 54 is enabled, and it is possible to prevent current from flowing in the body diode of the FET that is the commutation element 54. The loss of the device can be reduced. In addition, the discharge of the input capacitance of the commutation element 54
Since the transistor 68, which is a discharging switch element, is used for the control signal having the same phase as the drive signal of the main switch 44, the commutation element 54 cannot be turned off before the rectification element 52 is turned on, and a through current flows. There is no chance that it will end.
Furthermore, a large capacity capacitor 66 is used, or a commutation element 54 has a large input capacity MOS-F.
Even when ET is used, while the main switch element 44 is off, these capacitive components have the main transformer 40.
Since it is not connected in parallel with the inductance of
There is no influence on the resonance cycle when the main transformer 40 releases the excitation energy. Therefore, the switching frequency can be increased.

The switching power supply device of the present invention is not limited to the above embodiment, and the rectifying / smoothing circuit 65 includes an npn transistor 72, a Zener diode 74, and a resistor 76 instead of the diode 60 as shown in FIG. It is also possible to combine the assembled rectifier circuit with the capacitor 66. Further, as shown in FIG. 4, in addition to the diode 60, a rectifier circuit including a diode 78 and a choke coil 80 may be combined with a capacitor 66. As a result, a more efficient rectifier circuit can be obtained.

The synchronous rectifying element is an n-channel M
Other than the OS-FET, a p-channel MOS-FET may be used, and an element having a similar function such as an IGBT may be used. Further, as the voltage detecting means, the drive circuit for turning on the commutation element, and the discharge switching element, elements having the same functions as those in the above-described embodiment can be appropriately selected. The control signal of the discharging switch element includes a control signal having the same phase as the drive signal of the main switching element and a signal having an appropriate phase shift, and the same switching may be performed by a signal having an opposite phase. The drive transformer as an insulation circuit may also be replaced with a photocoupler,
Any signal may be used as long as it can transmit signals while achieving insulation. Further, the insulating circuit is not limited to the path for transmitting the signal for turning off the commutation element, but may be provided for the signal path for turning on / off the main switch element.

[0026]

According to the switching power supply device of the present invention, the ON operation of the commutation element can be speeded up, and the loss of the power supply device can be suppressed. Further, by providing the discharge switching element controlled by the control circuit of the main switching element, it is possible to appropriately turn off the commutation element,
It is possible to prevent a through current with the rectifying element and to improve the efficiency of the power supply device, which contributes to higher frequency and smaller size of the power supply device.

[Brief description of drawings]

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

FIG. 2 is a timing chart showing the operation of the switching power supply device of this embodiment.

FIG. 3 is a circuit diagram showing another example of a rectifying / smoothing circuit used in the switching power supply device of the present invention.

FIG. 4 is a circuit diagram showing still another example of the rectifying / smoothing circuit used in the switching power supply device of the present invention.

FIG. 5 is a schematic circuit diagram of a conventional switching power supply device.

FIG. 6 is a schematic circuit diagram of another conventional switching power supply device.

[Explanation of symbols]

40 main transformer 41 Primary winding 42 Secondary winding 43 Third winding 44 Main switch element 45 DC power supply 46 Control circuit 52 Rectifying element 54 Commutation element 56 choke coil 65 Rectification smoothing circuit

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Claims (3)

[Claims] 1. A DC power supply and a main switching element are connected to a primary winding side of a main transformer, a rectifying element and a commutation element are connected to a secondary winding side, and a main switching element is turned on by a control circuit. In a switching power supply device of a synchronous rectification type that is turned off, a tertiary winding is provided in the main transformer, and a rectifying / smoothing circuit that rectifies and smoothes a voltage generated in the tertiary winding when the main switching element is turned on is provided. A switching power supply device comprising a driving means for detecting a voltage drop of the tertiary winding with respect to a potential of the rectifying / smoothing circuit when the element is turned off and turning on the commutation element. 2. A discharge switch element is connected to a gate of the commutation element, and the discharge switch element is turned on by using a signal synchronized with an ON signal for the main switch element of the control circuit. The switching power supply device according to claim 1, wherein the commutation element is turned off. 3. The switching power supply device according to claim 2, wherein an insulating circuit is provided in a path for transmitting a signal for turning off the commutation element from the control circuit.