JP2001309653A - Switching power supply device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a voltage level change of a secondary output after turning off a power supply. SOLUTION: A starting control circuit 2 which makes a starting circuit 1 inoperative when its power supply is turned off is provided. The starting control circuit 2 has a comparator 24, with which a comparison is made between a reference voltage which is set by a charging voltage of a smoothing capacitor C2 and a charging voltage which remains in an input smoothing capacitor C1. With the result of the comparison the starting circuit 1 is made inoperative by a transistor Tr2. With this control, after switching off of the power supply, a switching control circuit 10 does not repeat its starting operation and stopping operation by charging the smoothing capacitor C2 with the residual power remained in the input smoothing capacitor C1.

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 circuit provided as a power supply for various electronic devices, and is particularly suitable for stabilizing an output voltage when a power supply is turned off.

[0002]

2. Description of the Related Art It has been known that an electronic device such as a television receiver has a switching power supply circuit. In such a switching power supply circuit, for example, a flyback converter or a forward converter type switching converter is employed, and a switching control circuit for controlling a switching operation is provided on a primary side thereof. By the way, in the switching power supply circuit as described above, a start-up circuit for starting the operation of the switching control circuit when the power is turned on is required. The start-up circuit is usually configured using a switching element such as a transistor in order to shorten the start-up time and reduce the power loss.

FIG. 3 is a circuit diagram showing a configuration of a switching power supply circuit provided with a conventional starting circuit. In the power supply circuit shown in FIG. 3, when an AC input voltage VAC from a commercial AC power supply is supplied to a bridge rectifier circuit DBR via a switch S, a rectified output of the bridge rectifier circuit DBR is supplied to an input smoothing capacitor C1. It is flattened, and a DC input voltage VA is generated at both ends.

The positive side of the input smoothing capacitor C1 is
A converter is connected to the drain terminal of the switching element Q1 via the primary winding L1 of the converter transformer T, and the switching operation of the switching element Q1 obtains a primary current (switching current) intermittent in the primary winding L1. Have been.

[0005] The switching current flowing through the primary winding L1 of the converter transformer T induces an alternating voltage in the secondary winding L2 of the converter transformer T. The alternating voltage is converted into a rectifying diode D2 for output and a smoothing capacitor. The output is rectified and smoothed by C3 and output as a secondary DC output voltage (hereinafter simply referred to as "output voltage") VOUT.

On the primary side of the converter transformer T,
The auxiliary winding L3 is wound. Therefore, an alternating voltage is also excited in the auxiliary winding L3 by the switching current flowing through the primary winding L1, and this alternating voltage is applied to the rectifier diode D1.
Is supplied to the smoothing capacitor C2 and the power supply voltage supply terminal of the switching control circuit 10. The rectified voltage output from the rectifier diode D1 is set to have a higher voltage level than the Zener voltage of the Zener diode ZD1 described later.

The switching control circuit 10 is, for example, an IC
During operation, the drive terminal Drive
Output a switching pulse for performing switching control of the switching element Q1. Further, the output voltage VOUT is fed back to the switching control circuit 10, and the period of the drive pulse is varied according to the output voltage level so as to make the output voltage VOUT constant.

[0008] When the power is turned on, the starting circuit 1 enclosed by a broken line
Provided to activate the switching control circuit 10,
As described above, the transistor Tr1 is used to reduce the startup time and the power loss. In this case, the collector of the transistor Tr1 is connected to the positive electrode side of the input smoothing capacitor C1 via the limiting resistor R1, and a base resistor RB for supplying a base current is connected between the collector and the base. Further, a Zener diode ZD1 is connected between the base of the transistor Tr1 and the primary side ground as shown in the figure. Further, the emitter of the transistor Tr1 is connected to the positive electrode side of the smoothing capacitor C2 and the power supply terminal of the switching control circuit 10. At this time, the Zener voltage of the Zener diode ZD1 is set to a voltage higher than the operation start voltage of the switching control circuit 10, for example.

The operation of the above-described power supply circuit will be described below with reference to the timing chart shown in FIG. First, at the time of startup, the input smoothing capacitor C1 is charged from the AC input power supply via the bridge rectifier circuit DBR, and FIG.
At the time point t0 shown in the figure, the input voltage (DC input voltage) VA
Rises, the input voltage VA is applied to the collector of the transistor Tr1 via the limiting resistor R1. At the same time, the transistor Tr1 forming the starting circuit 1
A base current flows through the base through the base resistor RB. As a result, the transistor Tr1 is turned on, and the smoothing capacitor C2 is charged with a predetermined time constant via the resistor R1 and the transistor Tr1.

The charging voltage level of the smoothing capacitor C2 is equal to the operation start voltage Vcc1 of the switching control circuit 10.
At the time t1, when the terminal voltage Vcc of the power supply voltage supply terminal of the switching control circuit 10 shown in FIG. 4B reaches the operation start voltage Vcc1 of the switching control circuit 10, the switching control circuit 10 operates. State
As shown in FIG. 4D, a switching pulse for controlling the operation of the switching element Q1 is output from the drive terminal Drive to the gate of the switching element Q1.

Thus, the primary current flowing through the primary winding L1 of the converter transformer T is supplied to the switching control circuit 1
In response to the switching pulse from 0, the switching is controlled by the switching element Q1, and the power supply circuit starts operating as a switching converter.
An output voltage VOUT is output as shown in FIG.

When the power supply circuit starts operating as a switching converter, an alternating voltage is also excited in the auxiliary winding L3 of the converter transformer T, and the rectified voltage output from the rectifier diode D1 is changed by the alternating voltage to a smoothing capacitor C2. Supplied to At this time, the rectifier diode D1
The rectified voltage output from the Zener diode ZD1
After the operation as a switching converter is started, the starting voltage is supplied to the switching control circuit 10 from the auxiliary winding L3 of the converter transformer T. Along with this, the transistor Tr1 of the start-up circuit 1 is cut off, so that no power loss occurs in the start-up circuit 1 during a steady operation.

Accordingly, as shown in FIG. 4B, the terminal voltage Vcc of the switching control circuit 10 is changed after the operation of the switching control circuit 10 is started.
Although the voltage level temporarily drops due to the power consumption, the rectified voltage from the auxiliary winding L3 thereafter makes the terminal voltage Vcc stable.

On the other hand, for example, when the switch S is opened as a power-off operation at time t2 shown in FIG. 4, the power supply from the AC input power supply is cut off, and the voltage level of the input voltage VA shown in FIG. Will gradually decrease. When the level of the input voltage VA decreases,
Since the alternating voltage excited by the secondary winding L2 and the auxiliary winding L3 of the converter transformer T also decreases, the voltage of the capacitor C2 rapidly decreases, and the switching control circuit 10 shown in FIGS. Terminal voltage Vcc and secondary output voltage V
OUT voltage level drops. Then, at time t3 when the terminal voltage Vcc of the switching control circuit 10 decreases to the operation stop voltage Vcc2, the operation of the switching control circuit 10 stops, and the power supply circuit stops operating as a switching converter.

[0015]

By the way, when the above-mentioned power supply circuit stops operating as a switching converter, the current consumption of the power supply circuit becomes very small, so that the electric charge is still stored in the input smoothing capacitor C1. It has been in a state where it was done. That is, the voltage level of the input voltage VA is still maintained at a sufficiently high voltage level.

Therefore, for example, when the voltage level of the smoothing capacitor C2 becomes lower than the Zener voltage level of the Zener diode ZD1, the voltage remaining on the input smoothing capacitor C1 turns on the transistor Tr1 again. Thereby, the resistance R1, the transistor Tr1
, Charging of the smoothing capacitor C2 is performed, and the voltage level of the smoothing capacitor C2 rises. Then, for example, the terminal voltage V of the switching control circuit 10 shown in FIG.
At time t5 when cc reaches the operation start voltage Vcc1, the switching control circuit 10 is restarted. Therefore,
Also in this case, the drive terminal Dr of the switching control circuit 10
A switching pulse as shown in FIG. 4D is output again from ive, the power supply circuit starts operating as a switching converter bar, and the level of the secondary output voltage VOUT of the converter transformer T rises again. Will be.

In this case, the input smoothing capacitor C1
Is lower than that in the normal operation, the level of the alternating voltage excited by the secondary winding L2 is also low, and the voltage level of the output voltage VOUT is as shown in FIG.
As shown in (c), the voltage level is lower than the normal voltage level.

Similarly, the alternating voltage excited in the auxiliary winding L3 of the converter transformer T is also at a lower voltage level than in the normal operation, so that the terminal voltage Vcc of the switching control circuit 10 In this state, the voltage drops again as shown in FIG. 4B, and the operation of the switching control circuit 10 stops again at time t6 when the terminal voltage Vcc drops to the operation stop voltage Vcc2. Thereafter, during a period in which a certain amount of charge remains in the input smoothing capacitor C1, the starting circuit 1 may repeat the above-described operation a plurality of times.

For this reason, in the power supply circuit shown in FIG. 3, even after the power supply is turned off, the voltage level of the output voltage VOUT fluctuates, and the operation of the load circuit to which the output voltage VOUT is supplied becomes unstable. There was a disadvantage of becoming.

[0020]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and has an input voltage supply means for supplying a DC input voltage, a switching element, and a switching operation of the switching element. Switching converter means for transmitting a switching output obtained to the secondary side to the secondary side, output means for outputting an alternating voltage excited on the secondary side of the converter transformer as a secondary side DC output voltage, Drive control means for controlling the drive of the switching element in accordance with the voltage level of the side DC output voltage, and operation of supplying an operating voltage to the drive control means, charged by the alternating voltage excited on the primary side of the converter transformer When the power supply is started and the power supply is started, the charging operation of the operating voltage supply means is performed by the DC input voltage, and the drive control is performed. And a starting circuit for starting the stage.
When the power supply is turned off, a start control circuit for controlling the start circuit not to operate for at least a predetermined period is provided.

The starting control circuit includes: comparing means for comparing the detected voltage of the input voltage supply means with a predetermined reference voltage; and stopping means for stopping the operation of the starting circuit based on the comparison result of the comparing means. It was constituted by.

That is, according to the present invention, by providing a start-up control circuit for controlling the start-up circuit so that it does not operate when the power is turned off, the operation voltage supply means is lowered and the operation of the drive control means is stopped. In addition, it is possible to prevent the operation voltage supply unit from being charged by the voltage remaining in the input voltage supply unit.

[0023]

Embodiments of the present invention will be described below. FIG. 1 is a circuit diagram showing a configuration of a switching power supply circuit including a starting circuit according to an embodiment of the present invention. In the power supply circuit shown in FIG. 1, when an AC input voltage VAC from a commercial AC power supply is supplied to a bridge rectifier circuit DBR via a switch S, a rectified output of the bridge rectifier circuit DBR is supplied to an input smoothing capacitor C1. It is flattened, and a DC input voltage VA is generated at both ends.

The positive side of the input smoothing capacitor C1 is
A converter is connected to the drain terminal of the switching element Q1 via the primary winding L1 of the converter transformer T, and the switching operation of the switching element Q1 obtains a primary current (switching current) intermittent in the primary winding L1. Have been. In this embodiment, an FET (field effect transistor) is shown as the switching element Q1, but the switching element Q1 may be constituted by a high withstand voltage bipolar transistor or the like.

An alternating voltage is induced in the secondary winding L2 of the converter transformer T by the switching current flowing through the primary winding L1 of the converter transformer T, and the alternating voltage is output to the rectifier diode D2 for output and the smoothing capacitor C3. Rectified / smoothed and output as an output voltage VOUT.

On the primary side of the converter transformer T,
The auxiliary winding L3 is wound. Therefore, an alternating voltage is also excited in the auxiliary winding L3 by the switching current flowing through the primary winding L1, and this alternating voltage is applied to the rectifier diode D1.
Is supplied to the smoothing capacitor C2 and the power supply voltage supply terminal of the switching control circuit 10. In this case as well, the rectified voltage output from the rectifier diode D1 is set to be higher than the Zener voltage of the Zener diode ZD1 described later.

The switching control circuit 10 is, for example, an IC
And the like, and the switching element Q
It is assumed that the switching control of 1 is performed. For this reason,
During operation, a switching pulse is output from the drive terminal Drive to the gate of the switching element Q1.
The switching control circuit 10 has an output voltage VOUT
Is fed back, and the period or pulse width of the drive pulse is varied in accordance with the output voltage level so that the output voltage VOUT is made constant.

When the power is turned on, the starter circuit 1 enclosed by a broken line
Provided to activate the switching control circuit 10,
The transistor Tr1 is used to reduce the startup time and the power loss. In this case, the collector of the transistor Tr1 is connected to the positive side of the input smoothing capacitor C1 via the limiting resistor R1. As shown, a base resistor RB for supplying a base current is connected between the collector and the base of the transistor Tr1. In addition, the base has a Zener diode Z
D1 is connected as shown. Also, the transistor Tr1
Of the positive electrode of the smoothing capacitor C2,
The switching control circuit 10 is connected to a power supply voltage supply terminal. The Zener voltage of the Zener diode ZD1 is set to a voltage higher than, for example, the operation start voltage of the switching control circuit 10.

The power supply circuit of this embodiment is characterized in that a start control circuit 2 shown by a broken line is provided. The operating voltage of the comparator 24 provided in the activation control circuit 2 is supplied from a smoothing capacitor C2. A predetermined reference voltage is input to a positive-phase input terminal (+) of the comparator 24. Therefore, the comparator 2
The positive-phase input terminal 4 is connected to a connection point between the cathode of a Zener diode ZD2 whose anode is grounded to the primary ground and the other end of a resistor R4 whose one end is connected to a smoothing capacitor C2. The reference voltage input to the positive-phase input terminal of the comparator 24 is a detection voltage obtained by detection resistors R2 and R3 described later when the voltage level of the input voltage VA is set to a lower limit voltage value that guarantees the operation of the switching converter. A voltage value equal to or lower than the voltage is set as high as possible. Further, a voltage obtained by dividing the input voltage VA by the detection resistors R2 and R3 is input to a negative-phase input terminal (-) of the comparator 24 as a detection voltage.

The output of the comparator 24 is input to the base of the transistor Tr2. The collector of the transistor Tr2 is connected to the base of the transistor Tr1 provided in the starting circuit 1, and the emitter is grounded. Note that the start-up control circuit 2 and the start-up circuit 1 may be integrally formed by a one-chip IC. Further, it may be configured as a single chip with the switching control unit 10 configured by an IC.

Hereinafter, the operation of the power supply circuit according to the present embodiment will be described with reference to a timing chart shown in FIG. First, at the time of starting the power supply circuit of the present embodiment,
Since the voltage of the smoothing capacitor C2 is set to zero, the comparator 24 of the startup control circuit 2 is turned off. Therefore, the output of the comparator 24 is at a low level, and the transistor Tr2 is off.

Accordingly, as shown in FIG. 2A, when the input voltage (DC input voltage) VA rises at the time point t0, the operation of the starting circuit 1 becomes the same as the conventional power supply circuit. The voltage is applied to the collector of the transistor Tr1 via the limiting resistor R1 of the starting circuit 1, and a base current is input to the base thereof. As a result, the transistor Tr1 of the starting circuit 1 is turned on, and the smoothing capacitor C2 is charged through the transistor Tr1.

Normally, the rise of the input voltage VA shown in FIG. 2A is sufficiently faster than the charging of the smoothing capacitor C2 shown in FIG. 2B, so that the rise of the input voltage VA is higher than the reference voltage input to the positive-phase input terminal of the comparator 24. , The detection voltage input to the negative-phase input terminal is always higher. Therefore, even if the charging voltage level of the smoothing capacitor C2 rises and the comparator 24 enters the operating state, the output of the comparator 24 remains at the low level.

As described above, when the power supply circuit of the present embodiment is started up, the terminal voltage Vcc of the power supply voltage supply terminal of the switching control circuit 10 shown in FIG.
At the time t1 when the voltage reaches the operation start voltage Vcc1 of the switching control circuit 10, the switching control circuit 10 enters the operating state, and the switching pulse from the drive terminal Drive is output to the gate of the switching element Q1. As a result, the primary current flowing through the primary winding L1 of the converter transformer T is intermittently controlled by the switching element Q1 according to the switching pulse from the switching control circuit 10, and the power supply circuit starts operating as a switching converter. From the secondary side, as shown in FIG.
The output voltage VOUT is output.

After the operation of the switching converter is started, the operating voltage supply terminal of the switching control circuit 10 and the smoothing capacitor C2 are connected to the converter transformer T
A rectified voltage obtained by rectifying the alternating voltage excited by the auxiliary winding L3 by the rectifying diode D1 is supplied. As a result, during a steady operation, the transistor Tr1 is in a cutoff state, and no power loss occurs in the starting circuit 1.

Therefore, as shown in FIG. 2B, the terminal voltage Vcc of the switching control circuit 10 is changed after the operation of the switching control circuit 10 is started.
Although the voltage level temporarily drops due to the power consumption, the rectified voltage from the auxiliary winding L3 thereafter makes the terminal voltage Vcc stable.

Next, for example, when the switch S is opened as a power-off operation at time t2 shown in FIG. 2, the power supply from the AC input power supply is cut off, and the voltage of the input voltage VA shown in FIG. The level will decrease. When the detected voltage of the input voltage VA input to the negative-phase input terminal of the comparator 24 becomes equal to or lower than the reference voltage value input to the positive-phase input terminal, that is, equal to or lower than the comparator set level, the output of the comparator 24 becomes high level. As a result, the transistor Tr2 is turned on. Therefore, after the point in time t3 when the transistor Tr2 is turned on, the transistor Tr1 of the starting circuit 1 is not turned on at least until the transistor Tr2 is turned off.

Therefore, in the power supply circuit according to the present embodiment, even if the charging voltage level of the smoothing capacitor C2 falls below the Zener voltage of the Zener diode ZD1, the resistance R
1. The smoothing capacitor C2 is not charged via the transistor Tr1, and the operation of the switching control circuit 10 is stopped at time t3 when the charging voltage of the smoothing capacitor C2 becomes equal to or lower than the operation stop voltage Vcc2 of the switching control circuit 10. The power supply circuit shown in FIG. 1 stops operating as a switching converter.

In this embodiment, since the set level of the comparator 24 is set by the charging voltage of the capacitor C2, the transistor Tr1 when the power is turned off is turned off.
The off period is determined by the discharge relationship between the input smoothing capacitor C1 and the smoothing capacitor C2.
Therefore, for example, the discharging period of the smoothing capacitor C2 is long,
If the decrease in the charging voltage of the smoothing capacitor C2 is made slower than the decrease in the input voltage VA of the input smoothing capacitor C1, the output of the comparator 24 will maintain a high level after the power is turned off. Tr1
Never turn on. That is, since the starting circuit 1 does not restart and the level of the output voltage VOUT does not fluctuate after the power is turned off, the operation of the load circuit can be stabilized.

For example, when the discharge period of the input smoothing capacitor C1 is longer than the discharge period of the smoothing capacitor C2 and the decrease of the charging voltage of the smoothing capacitor C2 is faster than the decrease of the input voltage VA of the input smoothing capacitor C1, The output of the comparator 24 changes from high level to low level. Here, for example, at time t4 shown in FIG. 2, the output of the comparator 24 changes from high level to low level,
It is assumed that the transistor Tr1 of the starting circuit 1 is turned on. In this case, at this time point t4, if the charge remaining in the input smoothing capacitor C1 is small, the charging voltage (terminal voltage Vcc) of the capacitor C2 charged via the resistor R1 and the transistor Tr1 is, for example, as shown in FIG. Does not reach the drive start voltage Vcc1 of the switching control circuit 10, as shown by the broken line in FIG. Therefore, also in this case, the operation of the load circuit after the power is turned off becomes stable without restarting the switching control circuit 10 after the power is turned off.

On the other hand, the charge remaining on the input smoothing capacitor C1 at time t4 causes the charging voltage (terminal voltage Vcc) of the capacitor C2 to start operating at time t5, as shown by the solid line in FIG. 2C. When the voltage reaches the voltage Vcc1, the switching control circuit 10 starts operating again. At a time t6 when the terminal voltage Vcc decreases to the operation stop voltage Vcc2, the operation of the switching control circuit 10 is stopped again.

However, in the power supply circuit of this embodiment, the period (t3 to t3) from when the power supply is turned off to when the switching control circuit 10 is restarted in accordance with the operation of the comparator 24.
5) can be made longer than conventional power supply circuits.
Therefore, if the period from power-off to restarting of the switching control circuit 10 is at least longer than the period during which the voltage level of the output voltage VOUT output from the secondary side of the converter transformer T is sufficiently reduced, The switching control circuit 10 is restarted about once, and the voltage level of the output voltage VOUT can be prevented from rising to a level that affects the operation of the load circuit. That is, the same effect as when the starting circuit 1 is not restarted can be obtained. When actually configuring the power supply circuit, the capacitance of the smoothing capacitor C2 may be appropriately selected so that the voltage level of the output voltage VOUT does not increase to a level that affects the load circuit.

In this embodiment, the power supply circuit using the flyback method has been described as an example. However, the present invention is not limited to the flyback type power supply circuit. It is needless to say that the present invention can be applied to the power supply circuit of the above type and the resonance type power supply circuit.

The starter circuit 1 according to the present embodiment is configured such that the transistor T is controlled by the relationship between the Zener voltage of the Zener diode ZD1 and the rectified voltage output from the auxiliary winding L3 of the converter transformer T via the rectifier diode D1.
Although the power supply circuit in which the operation of the starting circuit 1 is controlled by cutting off the operation of r1 has been described as an example, the present invention is not limited to such a power supply circuit. For example, a separate comparator is provided in the start-up circuit, the voltage level of the power supply voltage supply terminal of the switching control circuit 10 is monitored, and when the power supply circuit is in a steady operation, the transistor T
The present invention can also be applied to a power supply circuit having a startup circuit that cuts off the operation of r1.

[0045]

As described above, the switching power supply circuit of the present invention is provided with the start-up control circuit for controlling the start-up circuit so as not to operate when the power is off, so that the charging voltage of the operating voltage supply means is reduced. After the operation of the drive control means is stopped, it is possible to prevent the operating voltage supply means from being charged by the residual voltage of the input voltage supply means via the starting circuit. In other words, after the power is turned off, the drive control means does not restart for at least the predetermined period.
It is possible to suppress a change in the secondary-side output voltage level that occurs when the drive control unit repeatedly starts and stops. Therefore, the operation of the load circuit connected to the secondary side does not become unstable due to the level fluctuation of the output voltage of the secondary side, and the operation of the electronic device equipped with the present invention after the power supply is turned off has good operation characteristics. Can be expected.

[Brief description of the drawings]

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

FIG. 2 is an operation timing chart of the power supply circuit shown in FIG.

FIG. 3 is a circuit diagram showing a configuration of a conventional power supply circuit.

4 is an operation timing chart of the power supply circuit shown in FIG.

[Explanation of symbols]

1 Startup circuit, 2 Startup control circuit, 10 Switching control circuit, 24 comparators, C1 C2 C3 smoothing capacitor, ZD1 ZD2 Zener diode, D
1 D2 Rectifier diode, DBR bridge rectifier circuit, L
1 Primary winding, L2 secondary winding, L3 auxiliary winding, Q1 switching element, R1 limiting resistor, R2R3 detecting resistor,
R4 resistance, RB base resistance, S switch, Tr1
Tr2 transistor, T converter transformer

Claims (2)

[Claims] 1. Switching converter means comprising: an input voltage supply means for supplying a DC input voltage; a switching element; and a converter transformer for transmitting a switching output obtained on a primary side by a switching operation of the switching element to a secondary side. Output means for outputting an alternating voltage excited on the secondary side of the converter transformer as a secondary-side DC output voltage; anddriving control of the switching element according to a voltage level of the secondary-side DC output voltage. Drive control means for performing, an operation voltage supply means charged by an alternating voltage excited on the primary side of the converter transformer, and supplying an operation voltage to the drive control means; A starting circuit that performs a charging operation of the operating voltage supply unit and starts the drive control unit; A switching power supply circuit, comprising: a start-up control circuit that controls the start-up circuit not to operate for at least a predetermined period when the power supply is off. 2. The starting control circuit according to claim 1, wherein said starting control circuit compares the detected voltage of said input voltage supplying means with a predetermined reference voltage, and stops the operation of said starting circuit based on a comparison result of said comparing means. The switching power supply circuit according to claim 1, further comprising: a stopping unit configured to stop the switching power supply.