JP2002272097A - Switching power supply unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power supply unit that can prevent generation of a short-circuiting current in a state where a power voltage is unstable like when a power supply is turned on. SOLUTION: The switching power supply unit has a main switch 2 for converting a DC voltage to a high frequency voltage, a rectifier circuit including at least a switch 4 for synchronous rectification for synchronizing the high frequency voltage with the main switch 2 for rectifying, a filtering circuit for filtering the output of the rectifier circuit, and a control means for controlling the main switch 2 and the switch 4 for synchronous rectification. By the control means, the switch 4 for synchronous rectification is fixed to OFF state for a fixed period after a power supply Vcc to the control means is turned on, thus preventing generation of the short-circuiting current since the switch 4 for synchronous rectification is fixed to the OFF state for a fixed period after power is turned on, allowing an output voltage outputted from the filter circuit to smoothly rise when the power supply is turned on, and at the same time preventing the breakdown of the switch 4 for synchronous rectification.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a switching power supply, and more particularly, to a synchronous rectification type switching power supply.

[0002]

2. Description of the Related Art Heretofore, a so-called DC / DC converter has been known as a switching power supply device. In a typical DC / DC converter, a DC input is once converted into an AC using a switching circuit, then transformed (step-up or step-down) using a transformer, and further converted into a DC using an output circuit. This makes it possible to obtain a DC output having a voltage different from the input voltage.

Here, a switching element such as a transistor is used in an output rectifier used in a DC / DC converter, and is sometimes controlled in synchronization with a switching circuit on the input side. A DC / DC converter having such an output rectifier is generally called a synchronous rectification type switching power supply.

FIG. 5 is a circuit diagram showing a conventional synchronous rectification type switching power supply.

As shown in FIG. 5, a conventional synchronous rectification type switching power supply comprises a main switch 2 provided on a primary side of a transformer 1 and a synchronous rectification switch provided on a secondary side of the transformer 2. The main switch 2 has a duty controlled by the primary control circuit 5 so that the output voltage Vo has a predetermined value, and the synchronous rectification switches 3 and 4 have a secondary control. The main switch 2 is driven by the circuit 6 in the same phase and in the opposite phase. The drive circuits 7, 8 and the transformer 9 insulate the primary control circuit 5 and the secondary control circuit 6 from each other.
The primary control circuit 5 and the load are insulated by an insulation circuit 10.

[0006] The primary side control circuit 5 monitors the output voltage Vo via the insulating circuit 10 and, based on this, controls the control signal C1.
Generate The control signal C <b> 1 is supplied to the gate electrode of the main switch 2 by the drive circuit 12 after being given a predetermined delay by the delay circuit 11. Further, the control signal C1 is also transmitted to the secondary side via the drive circuits 7, 8 and the transformer 9, and becomes a control signal C2. The control signal C2 is supplied to the secondary side control circuit 6.

[0007] The secondary side control circuit 6 receives a control signal C2 and drives the synchronous rectification switch 3; a drive circuit 13 for inverting the control signal C2;
And a drive circuit 16 that receives the output of the inverting circuit and drives the synchronous rectification switch 4.
As a result, the synchronous rectification switch 3 becomes the main switch 2
And the synchronous rectification switch 4 is driven in the opposite phase to the main switch 2.

Thus, the high-frequency voltage generated on the secondary side of the transformer 1 is rectified by the synchronous rectification switches 3 and 4, further smoothed by a smoothing circuit comprising a choke coil 17 and a capacitor 18, and
Supplied to

[0009]

However, in the conventional switching power supply circuit shown in FIG. 5, the power supply voltage Vcc supplied to the secondary-side control circuit 6 is the same as when power is turned on.
Is unstable, the outputs of the drive circuits 13 and 16 are uncertain, and the synchronous rectification switches 3 and 4 are not properly driven.

For example, although the synchronous rectification switch 4 needs to be driven in the opposite phase with respect to the main switch 2 as described above, the output of the drive circuit 16 is uncertain, so that the main switch 2 May be turned on at the same time during the period when is turned on. In this case,
A short-circuit current flows through the secondary winding of the transformer 1. When such a short-circuit current occurs, not only does the output voltage Vo not rise smoothly when the power is turned on, but also the switches 3 and 4 for synchronous rectification may be destroyed.

Accordingly, an object of the present invention is to provide a switching power supply device capable of preventing a short-circuit current from occurring when the power supply voltage is unstable such as when power is turned on.

[0012]

SUMMARY OF THE INVENTION The object of the present invention is as follows.
A main switch for converting a DC voltage to a high-frequency voltage, a rectifier circuit including at least a synchronous rectification switch for rectifying the high-frequency voltage in synchronization with the main switch, a smoothing circuit for smoothing an output of the rectifier circuit, A switching power supply device comprising: a switch and control means for controlling the operation of the synchronous rectification switch, wherein the control means, for a certain period of time after turning on power to the control means,
The switching power supply device is characterized in that the synchronous rectification switch is fixed in an off state.

According to the present invention, since the synchronous rectification switch is fixed to the off state for a certain period after the power is turned on, it is possible to prevent the occurrence of a short-circuit current. Thus, when the power is turned on, the output voltage output from the smoothing circuit can be smoothly increased, and the switch for synchronous rectification can be prevented from being destroyed.

In a preferred embodiment of the present invention, the certain period is set substantially equal to a period from when the power is turned on to when the voltage of the power is stabilized.

In a further preferred aspect of the present invention, the power supply is generated based on an output voltage of the smoothing circuit.

In a further preferred aspect of the present invention, the control means drives the synchronous rectification switch in a phase opposite to that of the main switch after the predetermined period has elapsed.

In a further preferred aspect of the present invention, the switching power supply further includes a transformer, wherein the main switch is connected to a primary side of the transformer, and the synchronous rectifying switch is connected to the secondary side of the transformer.
Connected to the next side.

In a further preferred aspect of the present invention, the control means has a drive circuit for driving the synchronous rectification switch, and the input terminal of the drive circuit for the certain period after the power is turned on. Is clamped to one logic level, thereby fixing the synchronous rectification switch to the off state.

In a further preferred aspect of the present invention, the control means includes means for generating a control signal for controlling the main switch, a resistor connected in series between both ends of the power supply, and the control signal. And a series circuit including a diode and a capacitor connected in parallel to the transistor, and the input terminal of the drive circuit is connected to the node.

[0020]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
A preferred embodiment of the present invention will be described in detail.

FIG. 1 is a circuit diagram of a switching power supply device 20 according to a preferred embodiment of the present invention.

As shown in FIG. 1, the switching power supply 20 according to the present embodiment is a so-called forward converter, like the conventional switching power supply.
The difference is that the secondary side control circuit 6 is replaced with a secondary side control circuit 21 as compared with the conventional switching power supply device. The other components are the same as those of the conventional switching power supply device. Therefore, the same components as those of the conventional switching power supply device are denoted by the same reference numerals as in FIG. 5 and detailed description thereof is omitted.

The secondary side control circuit 21 receives a control signal C2 and drives the synchronous rectification switch 3;
Resistor 14 for inverting control signal C2 and transistor 1
5, a driving circuit 16 for receiving the output of the inverting circuit and driving the synchronous rectification switch 4, and a driving circuit 1
6 and a clamp circuit 22 connected to the input terminal of the control circuit 6. The clamp circuit 22 includes a diode 23 having an anode connected to the input terminal of the drive circuit 16, a diode 24 having an anode connected to the cathode of the diode 23 and a cathode connected to the power supply terminal, and a connection point between the diodes 23 and 24. And a capacitor 25 provided between the ground terminal. Here, the power supplied to the power terminal of the secondary side control circuit 21 is generated based on the output voltage Vo. Therefore, the voltage of the power supplied to the power terminal of the secondary side control circuit 21 is substantially proportional to the output voltage Vo.

The clamp circuit 22 serves to clamp the input terminal of the drive circuit 16 to a low level for a certain period based on a time constant determined by the resistor 14 and the capacitor 25 when the power supply voltage Vcc is applied. Here, it is preferable that the certain period substantially coincides with a period from when the power supply voltage Vcc is applied to when it is stabilized. Therefore, the constants of the resistor 14 and the capacitor 25 may be set so that they match.

Next, the operation of the switching power supply 20 according to this embodiment will be described.

First, the operation when the power supply voltage Vcc is stable will be described.

When the power supply voltage Vcc is stable, the primary side control circuit 5 monitors the output voltage Vo via the insulating circuit 10, and generates a control signal C1 having an appropriate duty based on the output voltage Vo. I do. Such a control signal C1
Is given a predetermined delay by the delay circuit 11,
The power is supplied to the gate electrode of the main switch 2 by the drive circuit 12. Further, the control signal C1 is also transmitted to the secondary side via the drive circuits 7, 8 and the transformer 9 to control the control signal C1.
The control signal C2 is supplied to the secondary side control circuit 21.

The control signal C2 supplied to the secondary side control circuit 21 is applied to the input terminal of the drive circuit 13 and the gate electrode of the transistor 15, whereby the synchronous rectification switch 3 is driven in the same phase as the main switch 2. The switch 4 for synchronous rectification is driven in the opposite phase to the main switch 2. Here, the timing shift between the control signal C1 and the control signal C2 caused by the interposition of the drive circuits 7, 8 and the transformer 9 is adjusted by the delay circuit 11.

Thus, the high frequency voltage generated on the secondary side of the transformer 1 by the switching of the main switch 2 is rectified by the synchronous rectification switches 3 and 4 and further smoothed by a smoothing circuit comprising a choke coil 17 and a capacitor 18. Is done. More specifically, while the main switch 2 is on, the transformer 1
Current flows through a loop composed of the choke coil 17, the load 19, the switch 3 for synchronous rectification, and the secondary winding of the transformer 1, and the output current Io is given to the load 19. With choke coil 17
Energy is stored in On the other hand, during a period in which the main switch 2 is off, a current flows through a loop composed of the choke coil 17, the load 19, and the synchronous rectification switch 4 due to energy stored in the choke coil 17, and the output current flows through the load 19. When Io is given, the energy stored in the choke coil 17 is released.

Next, the operation when the power supply voltage Vcc is applied will be described.

When the power supply voltage Vcc is turned on, as described above, the input terminal of the drive circuit 16 is clamped at a low level for a certain period based on the time constant determined by the resistor 14 and the capacitor 25. For this reason, during the period, the synchronous rectification switch 4 is maintained in the off state, and therefore, a short-circuit current flows when the synchronous rectification switch 4 is simultaneously turned on while the main switch 2 is on. Is gone. The rectification operation during this time is as follows.
This is performed by the body diode of the switch 4 for synchronous rectification.

Then, when the clamp is released after the above-mentioned fixed period has elapsed since the supply of the power supply voltage Vcc, the above-described operation when the power supply voltage Vcc is stable is performed.

After the clamp is released, the diode 2
3, the charged state of the capacitor 22 is maintained, so that the clamp circuit 22 does not affect the operation of the secondary side control circuit 21. Then, when the power supply voltage Vcc is cut off, the capacitor 22 is rapidly discharged via the diode 24, so that the clamp operation can be performed the next time the power is turned on.

As described above, according to the present embodiment, the synchronous rectification switch 4 is forcibly turned off for a certain period after the supply of the power supply voltage Vcc.
Does not cause a short-circuit current to flow through the secondary winding of the transformer 1 due to instability. Therefore, when the power is turned on, the output voltage Vo rises smoothly, and the short-circuit current does not damage the synchronous rectification switches 3 and 4.

Further, in this embodiment, the time constant circuit for determining the period to be clamped uses the resistor 14 which constitutes a part of the inversion circuit for inverting the control signal C2. Growth is minimized.

FIG. 2A shows the driving circuit 13 of the conventional switching power supply device when the power supply voltage Vcc is applied.
FIG. 2B is a graph illustrating an output waveform of the drive power supply 16 and a waveform of the output voltage Vo. FIG. 5 is a graph showing a waveform of an output voltage Vo.

As shown in FIG. 2A, it can be seen that the rising waveform of the output voltage Vo is largely disturbed in the conventional switching power supply. This is because the output waveform of the drive circuit 16 is disturbed because the power supply voltage Vcc is unstable. On the other hand, as shown in FIG. 2B, in the switching power supply device 20 according to the present embodiment, it can be confirmed that the output voltage Vo increases smoothly.

Next, another preferred embodiment of the present invention will be described.

FIG. 3 is a circuit diagram of a switching power supply 30 according to another preferred embodiment of the present invention.

As shown in FIG. 3, the switching power supply 30 according to the present embodiment is a so-called flyback converter, and a switch 31 for synchronous rectification is provided on the secondary side of the transformer 1. The operation of the synchronous rectification switch 31 is controlled by the secondary side control circuit 32. The secondary-side control circuit 32 has a configuration in which the drive circuit 13 is deleted from the secondary-side control circuit 21 used in the switching power supply device 20 according to the above embodiment, and the other configuration is the same as the secondary-side control. This is the same as the circuit 21.

In the switching power supply 30 according to the present embodiment, the switch 31 for synchronous rectification is driven by the secondary side control circuit 32 in a phase opposite to that of the main switch 2. As a result, the capacitor 18 is charged by the voltage generated on the secondary side of the transformer 1 while the main switch 2 is on, and is discharged when the main switch 2 is off.

Also in the switching power supply device 30 according to the present embodiment, the synchronous rectification switch 31 is forcibly fixed to off by the clamp circuit 22 for a certain period after the supply of the power supply voltage Vcc. As a result, a short-circuit current does not flow through the secondary winding of the transformer 1 due to the instability of the power supply voltage Vcc. When the power is turned on, the output voltage Vo rises smoothly and is synchronized by the short-circuit current. The rectifying switch 31 is not destroyed.

Next, still another preferred embodiment of the present invention will be described.

FIG. 4 is a circuit diagram of a switching power supply 40 according to still another preferred embodiment of the present invention.

As shown in FIG. 4, the switching power supply device 40 according to the present embodiment is a flyback converter that does not use a transformer, unlike the switching power supply device 30 according to the above embodiment. Specifically, the switching power supply device 40 according to the present embodiment includes:
A main switch 41 and a synchronous rectification switch 42 connected in series between the input power sources; a choke coil 17 connected between a node of the main switch 41 and the synchronous rectification switch 42 and one end of the load 19; , A control circuit 43 that monitors the output voltage Vo and adjusts the duty of the control signal C so that the output voltage Vo becomes a constant value, and a drive that receives the control signal C and drives the main switch 41. A circuit 44, an inverting circuit 45 including a resistor 14 and a transistor 15 for inverting the control signal C, a driving circuit 46 for receiving the output of the inverting circuit 45 and driving the synchronous rectification switch 42, and an input terminal of the driving circuit 46 And a clamp circuit 22 connected thereto.

As in the above embodiments, the clamp circuit 22 includes a diode 23 having an anode connected to the input terminal of the drive circuit 46 and a diode 2 having an anode connected to the cathode of the diode 23 and a cathode connected to the power supply terminal.
4 and a capacitor 25 provided between the connection point of the diodes 23 and 24 and the ground terminal.

Also in the switching power supply 40 according to the present embodiment, the switch 42 for synchronous rectification is driven in a phase opposite to that of the main switch 41. As a result, energy is accumulated in the choke coil 17 during the period when the main switch 41 is on, and is discharged during the period when the main switch 41 is off.

Also in the switching power supply device 40 according to the present embodiment, the switch 42 for synchronous rectification is forcibly fixed to off by the clamp circuit 22 for a certain period after the supply of the power supply voltage Vcc. As a result, the main switch 4
No short-circuit current flows through the switch 1 and the synchronous rectification switch 42, and when the power is turned on, the output voltage Vo rises smoothly and the short-circuit current destroys the main switch 41 and the synchronous rectification switch 42. Disappears.

The present invention is not limited to the above embodiments, and various changes can be made within the scope of the invention described in the claims, and these are also included in the scope of the present invention. It goes without saying that it is a thing.

For example, in each of the above embodiments, the period in which the clamp circuit 22 clamps the input terminals of the drive circuits 16 and 46 to a low level and the period from when the power supply voltage Vcc is applied to when it is stabilized are substantially set. However, it is not essential that they coincide with each other, and the clamp period of the clamp circuit 22 may be set shorter than the period from when the power supply voltage Vcc is applied to when it is stabilized, Conversely, it may be set longer. However, if the clamp period by the clamp circuit 22 is too short, the effect of the present invention may not be sufficiently obtained. Conversely, if the clamp period by the clamp circuit 22 is too long, the body diode of the switch for synchronous rectification may be damaged. There is a possibility that the loss due to the current flowing increases. Therefore, as described above, the clamp circuit 22
Is most preferably set so as to substantially coincide with the period from when the power supply voltage Vcc is applied to when it is stabilized.

The specific circuit configuration of the clamp circuit 22 shown in each of the above embodiments is the same as that of the drive circuits 16 and 4.
6 is an example of a circuit configuration for clamping the input terminal 6 to a low level for a certain period, and may have a different circuit configuration as long as it performs the same function.

Further, the application of the present invention is not limited to the switching power supply shown in each of the above embodiments.
The present invention is also applicable to a synchronous rectification type switching power supply having another circuit configuration.

Further, in the switching power supply device 20 according to the above embodiment, the synchronous rectification switch 4 is fixed in the OFF state for a certain period.
In addition, the synchronous rectification switch 3 may be fixed to the off state during the period.

[0054]

As described above, according to the present invention, the synchronous rectification switch is forcibly turned off only for a predetermined period when the power supply voltage is unstable, such as when the power is turned on. Can be prevented from occurring. As a result, the output voltage Vo can be smoothly increased when the power is turned on, and the synchronous rectification switch can be prevented from being destroyed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a switching power supply device 20 according to a preferred embodiment of the present invention.

FIG. 2A is a graph showing output waveforms of drive circuits 13 and 16 and output voltage Vo when a power supply voltage Vcc is applied in a conventional switching power supply device;
(B) is a graph showing the output waveforms of the drive circuits 13 and 16 and the output voltage Vo when the power supply voltage Vcc is turned on in the switching power supply device 20 according to the preferred embodiment of the present invention.

FIG. 3 is a circuit diagram of a switching power supply device 30 according to another preferred embodiment of the present invention.

FIG. 4 is a circuit diagram of a switching power supply device 40 according to still another preferred embodiment of the present invention.

FIG. 5 is a circuit diagram showing a conventional synchronous rectification type switching power supply device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Transformer 2 Main switch 3, 4 Synchronous rectification switch 5 Primary side control circuit 6 Secondary side control circuit 7, 8 Drive circuit 9 Transformer 10 Insulation circuit 11 Delay circuit 12, 13, 16 Drive circuit 14 Resistance 15 Transistor 17 Choke Coil 18 Capacitor 19 Load 20 Switching power supply 21 Secondary control circuit 22 Clamp circuit 23, 24 Diode 25 Capacitor 30 Switching power supply 31 Synchronous rectification switch 32 Secondary control circuit 40 Switching power supply 41 Main switch 42 Synchronous rectification Switch 43 Control circuit 44, 46 Drive circuit 45 Inverting circuit C1, C2, C Control signal

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Katsuhiko Shimizu 1-13-1, Nihonbashi, Chuo-ku, Tokyo TDC Corporation F-term (reference) 5H006 AA03 AA05 BB00 CA02 CB07 DC05 GA01 5H730 AA20 AS01 BB13 BB23 BB43 DD04 DD26 EE19 FD01 FG01 XC05 XC09 XC12 XX05 XX15

Claims (7)

[Claims] 1. A main switch for converting a DC voltage to a high-frequency voltage, a rectifier circuit including at least a synchronous rectification switch for rectifying the high-frequency voltage in synchronization with the main switch, and a smoother for smoothing an output of the rectifier circuit. Circuit and
A switching power supply device comprising: a main switch and control means for controlling an operation of the synchronous rectification switch, wherein the control means controls the synchronous rectification switch for a certain period of time after power is supplied to the control means. A switching power supply device fixed in an off state. 2. The switching power supply device according to claim 1, wherein the predetermined period is set substantially equal to a period from when the power is turned on to when the voltage of the power is stabilized. 3. The power supply according to claim 1, wherein the power supply is generated based on an output voltage of the smoothing circuit.
A switching power supply device according to claim 1. 4. The apparatus according to claim 1, wherein the control unit drives the synchronous rectification switch in a phase opposite to that of the main switch after the predetermined period has elapsed.
The switching power supply device according to any one of the above. 5. The apparatus according to claim 1, further comprising a transformer, wherein the main switch is connected to a primary side of the transformer, and the synchronous rectification switch is connected to a secondary side of the transformer. 5. The switching power supply device according to any one of 4. 6. The control means has a drive circuit for driving the synchronous rectification switch, and clamps an input terminal of the drive circuit to one logic level for the certain period after the power is turned on. The switching power supply according to any one of claims 1 to 5, wherein the switch for synchronous rectification is fixed to an off state. 7. The control means for generating a control signal for controlling the main switch, a resistor connected in series between both ends of the power supply and a transistor receiving a gate of the control signal, the transistor comprising: The switching power supply according to claim 6, further comprising a series circuit including a diode and a capacitor connected in parallel to the transistor, wherein the input terminal of the drive circuit is connected to the node. .