JP2003224977A - Switching power supply - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a switching power supply apparatus which can supply stable operation voltage with low power loss to a control circuit which controls a switching device. <P>SOLUTION: This switching power supply apparatus has a control circuit 4, which controls ONs/OFFs of a switching transistor Q1 to obtain a prescribed value of a DC output voltage supplied to a load 7, a starting power supply circuit 8, by which a DC input voltage is stepped down and supplied to the control circuit 4 as an operation voltage at a start time, a rectifying/smoothing circuit 9 by which a voltage, generated in an auxiliary winding 3c of a switching transformer 3 in a steady operation state, is converted into a DC voltage and supplied to the control circuit 4 as an operation voltage, and a limiting means (a diode D3 and a Zener diode ZD1), which limits the output voltage of the rectifying/smoothing circuit 9 to a prescribed value. <P>COPYRIGHT: (C)2003,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 that outputs a DC stable voltage.

[0002]

2. Description of the Related Art Various circuits have been proposed as circuits for supplying an operating voltage to a control circuit for controlling on / off of a switching element of a switching power supply (hereinafter referred to as an operating voltage supply circuit). Therefore, a switching power supply device including the general operating voltage supply circuit shown in FIG. 2 will be described below as a configuration example of a conventional switching device.

An AC power supply 1 such as a commercial power supply is connected to the input side of a rectifier circuit 2 which is formed by connecting four diodes in a bridge. Further, the output side of the rectifier circuit 2 is a smoothing capacitor C.
Connected to 1. With such a configuration, the AC voltage output from the AC power supply 1 is rectified by the rectifier circuit 2,
It is smoothed by the smoothing capacitor C1 and becomes a DC input voltage.

Then, the primary winding 3a of the switching transformer 30 and an n-type channel FET (Field Effect Transis) are used.
A series circuit with the switching transistor Q1 which is tor) is connected to both ends of the smoothing capacitor C1. One end of the primary winding 3a of the switching transformer 30 is connected to the positive polarity side of the smoothing capacitor C1, and the source of the switching transistor Q1 is connected to the negative polarity side of the smoothing capacitor C1.

A resistor R6 is provided at both ends of the smoothing capacitor C1.
Also connected is an operating voltage supply circuit including a serial connection body of the zener diode ZD2 and the zener diode ZD2. The cathode of the Zener diode ZD2 is connected to the positive side of the smoothing capacitor C1 via the resistor R6, and the anode of the Zener diode ZD2 is connected to the negative side of the smoothing capacitor C1.

The secondary winding 3b of the switching transformer 30 is connected to a rectifying / smoothing circuit 50 composed of a rectifying diode D1 and a smoothing capacitor C2. That is, one end of the secondary winding 3b of the switching transformer 30 is connected to the positive polarity side of the smoothing capacitor C2 via the rectifying diode D1, and the other end of the secondary winding 3b of the switching transformer 30 is negative polarity of the smoothing capacitor C2. Connected to the side. Then, a resistor R1 and a resistor R2 are provided on both ends of the smoothing capacitor C2.
DC output voltage detection circuit 6 and load 7 consisting of a series connection of
And are connected in parallel.

Further, the control circuit 4 includes a zener diode ZD2 having both ends, a connection node between the resistors R1 and R2,
And the gate of the switching transistor Q1.

The thus configured switching power supply device of FIG. 2 operates as follows. When the switching transistor Q1 is in the ON state, the DC input voltage output from the smoothing capacitor C1 is supplied to the primary winding 3a of the switching transformer 30, and the excitation energy is stored in the primary winding 3a of the switching transformer 30. On the other hand, when the switching transistor Q1 is off, the excitation energy stored in the primary winding 3a of the switching transformer 30 is extracted from the secondary winding 3b of the switching transformer 30. Therefore, the switching transformer 30
The voltage output from the secondary winding 3b is a rectangular wave AC voltage. This rectangular wave AC voltage is applied to the rectifying and smoothing circuit 50.
It is rectified and smoothed by and becomes a DC output voltage. Then, this DC output voltage is supplied to the load 7. The DC output voltage detection circuit 6 detects the DC output voltage and outputs the divided voltage of the DC output voltage to the control circuit 4.

The control circuit 4 operates by using the Zener voltage output from the operating voltage supply circuit composed of the resistor R6 and the Zener diode ZD2 as the operating voltage to control the switching transistor Q1 by PWM [Pulse Width Modulation]. That is, the control circuit 4 generates a pulse signal (hereinafter referred to as a PWM control signal) according to the output signal of the DC output voltage detection circuit 6 and supplies the PWM control signal to the gate of the switching transistor Q1. Since the switching transistor Q1 is turned on / off according to the PWM control signal, the DC output voltage is feedback-controlled to a predetermined value.

The above-mentioned switching power supply device of FIG. 2 has the advantage that the operating voltage supply circuit has a simple structure and can be manufactured at a low cost. On the other hand, it is compared with the voltage across the smoothing capacitor C1, that is, the DC input voltage. Then, since the Zener voltage of the Zener diode ZD2 is small, there is a drawback that the power consumption in the resistor R6 becomes large.

FIG. 3 shows a switching power supply device having an operating voltage supply circuit that solves this drawback. Figure 3 now
Switching power supply devices are becoming the mainstream. In FIG. 3, the same parts as those of the switching power supply device of FIG. 2 are designated by the same reference numerals, and the description thereof will be omitted.

The switching power supply device of FIG. 3 is different from the switching power supply device of FIG. 2 in that the switching transformer 3 is provided instead of the switching transformer 30.
This is that the auxiliary winding 3c of the switching transformer 3, the starting power supply circuit 80, and the rectifying / smoothing circuit 90 are provided as operating voltage supply circuits instead of the series connection body of the resistor R6 and the Zener diode ZD2.

The startup power supply circuit 80 comprises a resistor R3, a resistor R4, a resistor R6, a resistor R7, an NPN transistor Q2, and a Zener diode ZD1. The cathode of the Zener diode ZD1 is connected to the positive side of the smoothing capacitor C1 via the resistor R4, and the anode of the Zener diode ZD1 is connected to the negative side of the smoothing capacitor C1. The base of the transistor Q2 is connected to the resistor R4 and the Zener diode ZD1 via the resistor R6.
The collector of the transistor Q2 is connected to the positive polarity side of the smoothing capacitor C1 via the resistor R3, and the emitter of the transistor Q2 is connected to the operating voltage positive input side of the control circuit 4 via the resistor R7. To be done. Further, the anode of the Zener diode ZD1 is connected to the operating voltage negative input side of the control circuit 4.

The rectifying / smoothing circuit 90 includes a diode D2,
And a smoothing capacitor C3. One end of the auxiliary winding 3c of the switching transformer 3 is connected to the positive side of the smoothing capacitor C3 via the rectifying diode D2, and the other end of the auxiliary winding 3c of the switching transistor 3 is connected to the negative side of the smoothing capacitor C3. It The positive side of the smoothing capacitor C3 is connected to the operating voltage positive input side of the control circuit 4, and the negative side of the smoothing capacitor C3 is connected to the operating voltage negative input side of the control circuit 4.

Next, the operation of the switching power supply device shown in FIG. 3 will be described. The operation of the parts other than the operating voltage supply circuit is the same as that of the switching power supply device of FIG. 2, so the description thereof will be omitted and only the operating voltage supply circuit will be described.

A reverse bias corresponding to the voltage across the smoothing capacitor C1 is applied to the Zener diode ZD1. The voltage across the smoothing capacitor C1 gradually increases at startup, and when the voltage across the smoothing capacitor C1 exceeds a predetermined value, the voltage across the Zener diode ZD1 becomes the Zener voltage.

Further, the cathode voltage of the Zener diode ZD1 is stepped down by the resistor R6 and becomes the base voltage of the transistor Q2, and the voltage across the Zener diode ZD1 becomes the Zener voltage, so that the transistor Q2 is turned on. Then, when the transistor Q2 is turned on, the voltage of the smoothing capacitor C1 is dropped by the resistor R3, the internal impedance between the collector and the emitter of the transistor Q2, and the resistor R7, and is supplied to the control circuit 4 as an operating voltage.

When the operating voltage is supplied from the power source circuit 80 for activation as described above, the control circuit 4 starts its operation to perform PWM.
A control signal is generated and the PWM control signal is sent to the gate of the switching transistor Q1 to start the on / off operation of the switching transistor Q1.

When the switching transistor Q1 starts the on / off operation, a voltage is induced in the auxiliary winding 3c of the switching transformer 3, the voltage is rectified by the diode D2 and smoothed by the smoothing capacitor C3, and the control circuit 4 produces an operating voltage. Is supplied to. When the voltage across the smoothing capacitor C3 rises and reaches a predetermined value, a transition is made from startup to steady operation.

When the operating voltage is supplied from the rectifying / smoothing circuit 90 to the control circuit 4 in this manner, the voltage at the connection node between the smoothing capacitor C3 and the resistor R7 becomes large, and the resistor R7 is added.
The potential difference between the two ends becomes smaller. Therefore, during the steady operation, the startup power supply circuit 80 supplies almost no power to the control circuit 4. That is, during steady operation, most of the power supply to the control circuit 4 is performed by the rectifying / smoothing circuit 90.

Since the switching power supply device of FIG. 3 has a configuration using a flyback converter, if the DC output voltage supplied to the load 7 is controlled to be constant, the voltage across the smoothing capacitor C3 will also be increased. It becomes constant and a stable operating voltage is supplied to the control circuit 4.

[0022]

However, the switching power supply device using the flyback converter is
Since it is not efficient when the load capacity is large, a switching power supply device having a large load capacity is generally configured to use a forward converter or a push-pull converter. However, when a switching power supply device using a forward converter or a push-pull converter is provided with an operating voltage supply circuit similar to that of the switching power supply device of FIG. 3, smoothing is performed even if the DC output voltage is controlled to be constant. The induced voltage of the auxiliary winding 3c of the switching transformer 3 fluctuates due to the voltage change of the capacitor C1 and the fluctuation of the load 7, and the voltage of the smoothing capacitor C3 also fluctuates. For this reason, in a switching power supply device using a forward converter or a push-pull converter, even if a DC output voltage is controlled to be constant when an operating voltage supply circuit similar to that of the switching power supply device of FIG. 3 is provided. The stable operating voltage could not be supplied to the control circuit 4.

In view of the above problems, it is an object of the present invention to provide a switching power supply device capable of supplying a stable operating voltage to a control circuit for controlling a switching element with low power loss.

[0024]

In order to achieve the above object, in the switching power supply device according to the present invention,
A switching transformer having a secondary winding, a secondary winding, and an auxiliary winding; a switching element that converts a DC input voltage into an AC voltage by an ON / OFF operation and supplies the AC voltage to a primary winding of the switching transformer; A first conversion circuit that converts the AC voltage induced in the secondary winding of the switching transformer into a DC voltage and outputs the DC voltage, and turns on / off the switching element so that the output voltage of the first conversion circuit has a predetermined value.
A control circuit for off control, a power supply circuit for starting which supplies the control circuit with an operating voltage by stepping down the DC input voltage at startup, and a voltage generated in the auxiliary winding of the switching transformer during steady operation into a DC voltage. A second conversion circuit that converts the voltage and supplies it as an operating voltage to the control circuit; and a limiting unit that limits the output voltage of the second conversion circuit to a predetermined value.
Is provided.

The second conversion circuit rectifies a voltage generated in the auxiliary winding of the switching transformer, a capacitor for smoothing the input voltage and outputting the smoothed voltage to the control circuit, the diode and the capacitor. And means for suppressing the peak value of the charging current of the capacitor, which is provided between and.

Further, the startup power supply circuit includes a transistor that is turned on when the DC input voltage is equal to or higher than a predetermined value, and a means that turns the transistor off in a steady operation regardless of the value of the DC input voltage. And a circuit that supplies an operating voltage to the control circuit when the transistor is turned on.

[0027]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of the present invention will be described below with reference to the drawings. As a switching power supply device according to the present invention, a switching power supply device using a forward converter will be described as an example here.
A switching power supply device using a forward converter according to the present invention is shown in FIG. In addition, in FIG.
The same parts as those of the switching power supply device are designated by the same reference numerals and the description thereof will be omitted.

The switching power supply device of FIG. 1 differs from the switching power supply device of FIG. 3 in that a forward converter is used instead of the flyback converter.
The point is that the starting power supply circuit 8 and the rectifying / smoothing circuit 9 are used instead of the starting power supply circuit 80 and the rectifying / smoothing circuit 90, respectively.

Secondary winding 3b of switching transformer 3
Is connected to a rectifying / smoothing circuit 5 including a rectifying diode D1, a commutation diode D4, and a smoothing capacitor C2.
That is, one end of the secondary winding 3b of the switching transformer 30 is connected to the commutation diode D4 via the rectifying diode D1.
Of the coil L1 and one end of the coil L1
The other end of 1 is connected to the positive polarity side of the smoothing capacitor C2. On the other hand, the other end of the secondary winding 3b of the switching transformer 30 is connected to the anode of the commutation diode D4 and the negative polarity side of the smoothing capacitor C2.

The starting power supply circuit 8 includes a diode D3,
It is composed of a resistor R3, a resistor R4, a transistor Q2, and a zener diode ZD1. Zener diode Z
The cathode of D1 is the smoothing capacitor C1 via the resistor R4.
Of the zener diode ZD1 is connected to the positive side of the smoothing capacitor C1.
And the base of the transistor Q2 and the diode D3
Of the transistor Q2 is connected to the connection node between the resistor R4 and the Zener diode ZD1, the collector of the transistor Q2 is connected to the positive side of the smoothing capacitor C1 via the resistor R3, and the emitter of the transistor Q2 and the diode D3 are connected.
Is connected to the input side of the operating voltage positive electrode of the control circuit 4. Further, the anode of the Zener diode ZD1 is connected to the operating voltage negative input side of the control circuit 4.

The rectifying / smoothing circuit 9 includes a rectifying diode D2.
And a smoothing capacitor C3 and a resistor R5. One end of the auxiliary winding 3c of the switching transformer 3 is connected to the positive side of the smoothing capacitor C3 via the rectifying diode D2 and the resistor R5. On the other hand, the switching transistor 3
The other end of the auxiliary winding 3c is connected to the negative side of the smoothing capacitor C3. The positive side of the smoothing capacitor C3 is connected to the operating voltage positive input side of the control circuit 4, and the negative side of the smoothing capacitor C3 is connected to the operating voltage negative input side of the control circuit 4.

Next, the operation of the switching power supply device shown in FIG. 3 will be described. Here, only the operation of the operating voltage supply circuit, which is a characteristic part of the present invention, will be described, and the description of the operation of parts other than the operating voltage supply circuit will be omitted.

A reverse bias corresponding to the voltage across the smoothing capacitor C1 is applied to the Zener diode ZD1. The voltage across the smoothing capacitor C1 gradually increases at startup, and when the voltage across the smoothing capacitor C1 exceeds a predetermined value, the voltage across the Zener diode ZD1 becomes the Zener voltage.

Further, since the cathode voltage of the Zener diode ZD1 becomes the base voltage of the transistor Q2 and the voltage is not induced in the auxiliary winding 3c of the switching transformer 3 at the time of starting, the voltage across the Zener diode ZD1 becomes the Zener voltage. As a result, the transistor Q2 is turned on. When the transistor Q2 is turned on, the voltage of the smoothing capacitor C1 is dropped by the internal impedance between the resistor R3 and the collector-emitter of the transistor Q2, and is supplied to the control circuit 4 as an operating voltage.

When the operating voltage is supplied from the startup power supply circuit 7 as described above, the control circuit 4 starts its operation to generate the PWM control signal, and sends the PWM control signal to the gate of the switching transistor Q1. , Start the on / off operation of the switching transistor Q1.

When the switching transistor Q1 starts the on / off operation, a voltage is induced in the auxiliary winding 3c of the switching transformer 3, this voltage is rectified by the diode D2, stepped down by the resistor R5 and smoothed by the smoothing capacitor C3 to operate. The voltage is supplied to the control circuit 4. When the voltage across the smoothing capacitor C3 rises and reaches a predetermined value, a transition is made from startup to steady operation.

Then, the circuit constant is set so that the diode D3 is forward-biased during the steady operation. As a result, during steady operation, the emitter voltage of the transistor Q2 becomes higher than the base voltage, and the transistor Q2 is turned off, so that power is supplied to the control circuit 4 by the rectifying / smoothing circuit 9.
Made only by. Further, by setting the circuit constants as described above, the voltage across the smoothing capacitor C3 becomes equal to the forward voltage of the diode D3 and the Zener diode ZD1.
The smoothing capacitor C1 is limited to the sum of the Zener voltage of
The stable operating voltage can be supplied to the control circuit 4 even when the induced voltage of the auxiliary winding 3c of the switching transformer 3 fluctuates in accordance with the voltage change of V.sub.1 or the change of the load 6.

Further, in the switching power supply device of FIG. 3, since the transistor Q2 is in the ON state during the normal operation, power (about 0.2 W) is consumed, but in the switching power supply device of FIG. 1, as described above. Since the transistor Q2 is in the off state during the steady operation, it does not consume power. Thereby, energy saving can be achieved.

Moreover, since the resistor R5 suppresses the peak value of the charging current of the smoothing capacitor C3, the smoothing capacitor C
The voltage across 3 is stabilized. This further stabilizes the operating voltage of the control circuit 4.

[0040]

According to the present invention, the output voltage of the second conversion circuit, which converts the voltage generated in the auxiliary winding of the switching transformer during steady operation into a DC voltage and supplies it as an operating voltage to the control circuit for controlling the switching element. Since the switching power supply device is provided with the limiting means for limiting the value to a predetermined value, the second conversion is performed even when the induced voltage in the auxiliary winding of the switching transformer changes due to a change in the DC input voltage or a load change. The output voltage of the circuit becomes a predetermined value. As a result, it is possible to supply a stable operating voltage to the control circuit during steady operation with no power loss.

According to the invention, the second conversion circuit includes a diode for rectifying a voltage generated in the auxiliary winding of the switching transformer, a capacitor for smoothing the input voltage and outputting the smoothed voltage to the control circuit, Means for suppressing the peak value of the charging current of the capacitor provided between the diode and the capacitor, so that the voltage across the capacitor is stabilized. As a result, the operating voltage of the control circuit is further stabilized.

Further, according to the present invention, the starting power supply circuit for stepping down the DC input voltage at the time of starting and supplying it as the operating voltage to the control circuit includes a transistor which is turned on when the DC input voltage becomes a predetermined value or more, and a steady state. Means for turning off the transistor regardless of the value of the DC input voltage during operation, and a circuit for supplying the operating voltage to the control circuit when the transistor is turned on. The transistor does not consume power. Thereby, energy saving can be achieved.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a configuration example of a conventional switching power supply device.

FIG. 3 is a diagram showing another configuration example of a conventional switching power supply device.

[Explanation of symbols]

1 AC power supply 2 rectifier circuit 3 switching transformer 4 control circuit 5, 9 Rectifying and smoothing circuit 6 DC output voltage detection circuit 7 load 8 Power supply circuit for startup C1 to C3 capacitors D1-D4 diode L1 coil Q1 switching transistor Q2 transistor R1 to R5 resistance ZD1 Zener diode

Claims (3)

[Claims] 1. A switching transformer having a primary winding, a secondary winding, and an auxiliary winding, and a DC input voltage is converted into an AC voltage by an ON / OFF operation and supplied to the primary winding of the switching transformer. Switching element, a first conversion circuit that converts the AC voltage induced in the secondary winding of the switching transformer into a DC voltage and outputs the DC voltage, and the output voltage of the first conversion circuit has a predetermined value. A control circuit for controlling ON / OFF of a switching element, a power supply circuit for starting which reduces the DC input voltage to supply it to the control circuit as an operating voltage at the time of starting, and an auxiliary winding of the switching transformer during steady operation. A switching power supply device comprising: a second conversion circuit that converts a voltage into a DC voltage and supplies the DC voltage as an operating voltage to the control circuit. A switching power supply device comprising a limiting means for limiting the input voltage to a predetermined value. 2. The second conversion circuit includes a diode for rectifying a voltage generated in the auxiliary winding of the switching transformer, a capacitor for smoothing the input voltage and outputting the smoothed voltage to the control circuit, the diode and the capacitor. The means for suppressing the peak value of the charging current of the capacitor provided between the switching power supply device and the switching power supply device according to claim 1. 3. A transistor in which the power supply circuit for starting is turned on when the DC input voltage exceeds a predetermined value,
Means for turning off the transistor regardless of the value of the DC input voltage during steady operation, and supplying the operating voltage to the control circuit when the transistor is turned on. The switching power supply device according to claim 2.