JPH07241073A - Switching power supply - Google Patents

Abstract

PURPOSE:To enhance the circuit efficiency by precharging a capacitor with a voltage induced in an auxiliary winding of a transformer and operating a switching control circuit with an output voltage from the capacitor when the output voltage from the auxiliary winding drops thereby reducing loss of a starting resistor. CONSTITUTION:When a voltage detector 301 detects output voltage drop of an auxiliary rectifying circuit 120 due to decrease of the duty ratio of ON/OFF control signal for a switching element 14 delivered from a PWM control section 20, a switch control circuit 302 opens a switch K2 and closes a switch K3. Consequently, the output voltage from a charging capacitor C1 is fed through the switch K3 to the power supply terminal Vcc at the control section 20. Since the control section 20 can sustain the operation with an operating voltage fed from the charging capacitor C1 even if the output voltage from the auxiliary rectifying circuit 120 drops somewhat, significant loss is not generated in the starting circuit 8.

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, and more particularly to a switching power supply device capable of reducing loss. In particular, it is useful for a DC-DC converter.

[0002]

2. Description of the Related Art FIG. 8 is a circuit diagram showing an example of a conventional forward type switching power supply device. In this switching power supply device 501, when the power switch S is closed and the DC power supply 1 is turned on, the starting circuit 8 operates. That is, since a current flows through the Zener diode 4 through the resistor 3, the Zener voltage is applied to the base of the transistor 6. Then, since the transistor 6 is turned on, a current flows through the starting resistor 5 and the diode 7, and the PW
The operating voltage is supplied to the power supply terminal Vcc of the M control unit 20. As a result, the PWM control unit 20 starts its operation and sends the on / off control signal Vb to the switching element 14,
The switching element 14 is turned on / off. When the switching element 14 is turned on / off, the primary winding 10 of the transformer 9
A DC current flows intermittently in the secondary winding 11 and a voltage is generated in the secondary winding 11 and the auxiliary winding 12.

The voltage appearing in the secondary winding 11 is rectified and smoothed through a rectifying diode 21, a freewheeling diode 22, a choke coil 23 and a smoothing capacitor 24, and an output voltage Vo is loaded (not shown). Is output to.

On the other hand, the voltage induced in the auxiliary winding 12 is rectified and smoothed by the auxiliary rectifier circuit 120. That is, the rectifying diode 16, the free wheeling diode 17,
It is rectified by the choke coil 18 and the smoothing capacitor 19 and supplied to the power supply terminal Vcc of the PWM control unit 20.
This voltage is more stable than the voltage supplied from the starting circuit 8 and causes the PWM control unit 20 to operate stably.

The output voltage Vo is fed back to the PWM controller 20 via the output error detector 25 and the photocoupler 26, and the PWM controller 20 turns on / off the signal V.
The duty ratio of b is controlled to stabilize the output voltage Vo.

Since the voltage output from the auxiliary rectifier circuit 120 is higher than the voltage output from the starting circuit 8, the diode 7 is reverse biased and the starting circuit 8 stops its operation. At this time, the power consumption in the starting circuit 8 becomes the minimum, and only the loss of the resistor 3 and the Zener diode 4 is required.
When a sufficient voltage is not generated in the auxiliary winding 12 due to a decrease in the duty ratio of the on / off signal Vb, the voltage output from the auxiliary rectifier circuit 120 decreases. Then, the diode 7 is in the forward bias state, the starting circuit 8 operates again, and the voltage is supplied from the starting circuit 8 to the PWM control unit 20.

[0007]

However, the DC power supply 1
Since the voltage of 1 is significantly higher than the operating voltage of the PWM control unit 20, when the starting circuit 8 operates, the voltage shared by the starting resistor 5 becomes large and the loss in the starting resistor 5 becomes large. For example, as the start-up circuit 8, 158 of "Transistor Technology SPECIAL No. 28 CQ Publisher" is used.
When the starting circuit shown on the page is used, the loss in the starting resistor 5 is 5.1 W at the rated input. Therefore, an object of the present invention is to provide a switching power supply device capable of reducing the loss due to the starting resistance.

[0008]

In the switching power supply device of the present invention, a primary winding of a transformer and a switching element are connected in series to a direct current power supply, and a starting circuit is connected in parallel with the direct current power supply. An operating voltage is supplied from the starting circuit to the switching control circuit, and in a steady state, the operating voltage is supplied from the auxiliary winding of the transformer to the switching control circuit via an auxiliary rectifier circuit, and the switching control circuit operates the switching element. In a switching power supply device which is turned on / off to intermittently supply a direct current to a primary winding of a transformer and takes out an output from a secondary winding of the transformer,
A capacitor charged by the voltage induced in the auxiliary winding of the transformer is provided, and when the output voltage from the auxiliary rectifier circuit drops, an operating voltage is supplied from the capacitor to the switching control circuit, Is characterized in that an operating voltage is supplied from the starting circuit to the switching control circuit when the output voltage of the device is reduced.

[0009]

In the switching power supply device of the present invention, the capacitor is charged by the voltage appearing in the auxiliary winding of the transformer, and when the output voltage from the auxiliary rectifying circuit on the auxiliary winding side drops, the capacitor changes to the switching control circuit. Supply operating voltage. Therefore, the switching control circuit can continue to operate with the operating voltage supplied from the capacitor even if the output voltage from the auxiliary rectifier circuit drops a little, and there is no need to re-start the starting circuit. Avoids large losses in the circuit. When the output voltage from the capacitor decreases, the starting circuit is connected again, and the operating voltage is supplied from the starting circuit to the switching control circuit.

[0010]

EXAMPLES The present invention will be described in more detail with reference to the examples shown in the drawings. The present invention is not limited to this.

First Embodiment FIG. 1 shows a switching power supply device 1 according to a first embodiment of the present invention.
It is a block diagram showing 01. In this switching power supply device 101, a DC power supply 1, a power switch S,
The primary winding of the transformer 9 and the switching element 14 are connected in series.

Further, the power switch S and the transformer 9
The connection point of the primary winding is connected to the power supply terminal Vcc of the PWM control unit 20 via the starting circuit 8 and the switch K1. A rectifying / smoothing circuit H1 is connected to the secondary winding of the transformer 9, and a voltage Vo is applied from the rectifying / smoothing circuit H1.
Is output to the load (not shown).

The output voltage Vo is output to the output error detector 25.
And the PWM control unit 2 via the photo coupler 26.
It is fed back to 0.

The auxiliary winding of the transformer 9 is connected to the PWM control unit 2 via an auxiliary rectifier circuit 120 and a switch K2.
0 power supply terminal Vcc. The auxiliary winding of the transformer 9 is connected to the charging capacitor C1 via the charging diode D1. Further, the connection point between the charging diode D1 and the charging capacitor C1 is connected to the power supply terminal Vcc of the PWM control unit 20 via the switch K3.

The output voltage from the auxiliary rectifying circuit 120 and the output voltage from the charging capacitor C1 are input to the voltage detector 301. The output of the voltage detector 301 is connected to a switch control circuit 302 that controls the opening and closing of the switches K1 to K3.

Next, the operation of the switching power supply device 101 will be described. In the initial state, the switch K1 is closed and the switches K2 and K3 are open. FIG. 2 shows a substantial circuit configuration when the power switch S is closed. At this time, the start-up circuit 8 operates and the PWM control unit 2
The operating voltage is supplied to the power supply terminal Vcc of 0. As a result, the PWM control unit 20 starts its operation, sends the on / off control signal Vb to the switching element 14, and turns on / off the switching element 14. When the switching element 14 is turned on / off, a direct current intermittently flows in the primary winding of the transformer 9, and a voltage is generated in the auxiliary winding and the secondary winding.
The voltage generated in the secondary winding is rectified and smoothed by the rectification / smoothing circuit H1, and the output voltage Vo is output to the load.

When the PWM control section 20 is activated, the switch control circuit 302 opens the switch K1 and closes the switch K2. The substantial circuit configuration at this time is shown in FIG. The voltage induced in the auxiliary winding is rectified and smoothed by the auxiliary rectifier circuit 120, and the PWM control unit 2 is switched through the switch K2.
0 to the power supply terminal Vcc. This voltage is more stable than the voltage supplied from the starting circuit 8 and causes the PWM control unit 20 to operate stably. Further, the voltage induced in the auxiliary winding is given to the charging capacitor C1 through the charging diode D1 to charge the charging capacitor C1.

When the voltage detector 301 detects that the output voltage from the auxiliary rectifier circuit 120 has dropped due to a decrease in the duty ratio of the on / off control signal Vb, the switch control circuit 302 causes the switch control circuit 302 to switch. Open the device K2 and close the switch K3. The substantial circuit configuration at this time is shown in FIG.
Shown in. At this time, the output voltage from the charging capacitor C1 is supplied to the power supply terminal Vcc of the PWM control unit 20 via the switch K3.

Further, when the voltage detector 301 detects that the output voltage from the charging capacitor C1 has dropped below a predetermined value, the switch control circuit 302 causes the switch K to switch.
3 is opened and the switch K1 is closed. The substantial circuit configuration at this time is shown in FIG. At this time, the DC power source 1 causes the starting circuit 8 to operate again, and the operating voltage is supplied from the starting circuit 8 to the PWM control unit 20.

In the above switching power supply device 101,
Even if the output voltage from the auxiliary rectifier circuit 120 is slightly lowered, the PWM control unit 20 can continue to operate with the operating voltage supplied from the charging capacitor C1, and it is necessary to restart the starting circuit 8. Therefore, the starting circuit 8 does not have to generate a large loss.

-Second Embodiment- FIG. 6 is a circuit diagram of a switching power supply device according to a second embodiment of the present invention. This switching power supply device 201 has the following three points as the conventional switching power supply device 5 shown in FIG.
Different from 01. A switch 34 is provided between the power switch S and the starting circuit 8. An anode of the charging diode 27 is connected to one end of the auxiliary winding 12, a charging capacitor 28 is connected between the cathode of the charging diode 27 and the ground, and a backflow prevention diode is connected to one end of the charging capacitor 28. 29
Is connected to the anode and the cathode of the backflow prevention diode 29 is connected to one end of the starting resistor 5. A voltage detector 32 is connected to the charging capacitor 28, a switch control circuit 35 is connected to the voltage detector 32, and the switch control circuit 35 controls the opening / closing of the switch K1.

Next, the operation of this switching power supply device 201 will be described. In the initial state, the switch 34 is closed. When the power switch S is closed, the starting circuit 8 operates. That is, a current flows through the Zener diode 4 through the resistor 3 and the Zener voltage Vz is applied to the base of the transistor 6. Then, since the transistor 6 is turned on, a current flows through the starting resistor 5 and the diode 7, and the operating voltage is supplied to the power supply terminal Vcc of the PWM control unit 20.

Anode voltage VSUB1 of diode 7
Is the base-emitter voltage of the transistor 6 which is VBE.
Then, VSUB1 = Vz−VBE. If numerical examples are shown, Vz = 12V, VBE =
When 0.7V and VF = 0.6V, VSUB1 = 10.
It is 7V.

As a result, the PWM control section 20 starts its operation and outputs the ON / OFF control signal Vb to the switching element 14.
To turn on / off the switching element 14. When the switching element 14 is turned on / off, a current intermittently flows in the primary winding 10 of the transformer 9, and a voltage is generated in the secondary winding 11 and the auxiliary winding 12.

The voltage generated in the secondary winding 11 is rectified and smoothed through the rectifying diode 21, the free wheeling diode 22, the choke coil 23, and the smoothing capacitor 24, and the output voltage Vo is output to the load.

On the other hand, the voltage generated in the auxiliary winding 12 is rectified by the auxiliary rectifying circuit 120 and supplied to the power supply terminal Vcc of the PWM control section 20. This voltage VSUB
2 is more stable than the voltage supplied by the starting circuit,
The PWM control unit 20 is operated stably. Voltage VSUB2
Is the cycle in which the switching element 14 is turned on and off, T,
If the ON period is TON, VSUB2≈ (TON / T) × (n3 / n1) × Vin. If numerical examples are shown, n1 = 200, n3 = 6
0, Vin = 100, T = 25μs, TON = 12.5μ
When s, VSUB2≈15V. VSUB2 is
It is set higher than the VSUB1. Therefore, the diode 7 is turned off by the reverse bias, and the transistor 6 is also turned off.

The voltage generated in the auxiliary winding 12 is supplied to the charging capacitor 2 through the charging diode 27.
8 to charge the charging capacitor 28. The voltage V28 across the charging capacitor 28 is the same as that of the transistor 6
Is off and no current flows from the charging capacitor 28, so if the voltage of the DC power supply 1 is Vin, the number of windings of the primary winding 10 is n1, and the number of windings of the auxiliary winding 12 is n3, then V28≈ (N3 / n1) × Vin. To show a numerical example, n1 = 200 turns, n3 =
When 60 turns and Vin = 100V, V28≈30V
Is.

When the voltage V28 across the charging capacitor 28 reaches a predetermined voltage, the switch control circuit 35 causes the switch 3 to operate.
Open 4

The PWM control section 20 includes an output error detection section 25.
The duty ratio of the on / off signal Vb is controlled according to the error signal fed back from the photo coupler 26 to stabilize the output voltage Vo.

Now, when the load current Io decreases, TON
Becomes smaller. Then, the output voltage from the auxiliary rectifier circuit 120 decreases. Then, when VSUB2 becomes lower than VSUB1, the diode 7 is forward biased and the starting circuit 8 operates. Then, via the transistor 6 and the diode 7, the voltage V28 across the charging capacitor 28 is applied.
Is supplied to the power supply terminal Vcc of the PWM control unit 20.

Voltage V28 across charging capacitor 28
(30V in the above numerical example) is the voltage Vi of the DC power supply 1.
Since it is smaller than n (100 V in the above-mentioned numerical example), the voltage shared by the starting resistor 5 is also small, and the loss in the starting resistor 5 is smaller than before.

When the output voltage from the auxiliary rectifier circuit 120 further decreases and the voltage V28 across the charging capacitor 28 drops below a predetermined value, the voltage drop is detected by the voltage detector 32.
Is detected by the switch control circuit 302,
Close. Then, the starting circuit 8 is restarted and the DC power supply 1
The operating voltage is supplied to the PWM control unit 20 via the starting circuit 8.

FIG. 7 shows a graph of the power consumption of the switching power supply 201 of the second embodiment and the power consumption of the conventional example. The horizontal axis represents the load current Io extracted from the secondary winding 11. The vertical axis represents the sum of the power consumption of the PWM control unit 20 and the power consumption of the auxiliary rectifier circuit 120. The solid line L1 shows the characteristic according to the second embodiment. The broken line La shows the characteristic of the conventional example. In the switching power supply device 201 of the second embodiment, the load current Io is 0.00
In the range of 3A to 0.17A, the charging capacitor 2
Since power is supplied from 8 to the PWM control unit 20, the sum of power consumption is 1.4 W, which is smaller than 10.6 W of the conventional example. The voltage Vin of the DC power supply 1 is 1
35V, the number of turns n1 of the primary winding 10 of the transformer 9 is 14 turns, the number of turns n2 of the secondary winding 11 is 7 turns, the number of turns n3 of the auxiliary winding 12 is 4 turns, and a choke coil 18 is 13 μH, and the starting resistance is 5 = 1.5 KΩ. The power consumption of the PWM control unit 20 is 1 W when the operating voltage of 15 V is supplied.

[0034]

According to the switching power supply device of the present invention, the capacitor is charged by the voltage induced in the auxiliary winding of the transformer, and when the output voltage from the auxiliary winding drops, the output from the capacitor is output. Since the switching control circuit is operated by using the voltage, the loss in the starting resistance can be reduced and the circuit efficiency can be improved as compared with the conventional case.

[Brief description of drawings]

FIG. 1 is a block diagram showing a switching power supply device according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing an operation of the switching power supply device of FIG.

FIG. 3 is an explanatory diagram showing an operation of the switching power supply device of FIG.

FIG. 4 is an explanatory diagram showing an operation of the switching power supply device of FIG.

5 is an explanatory diagram showing an operation of the switching power supply device of FIG. 1. FIG.

FIG. 6 is a circuit diagram of a switching power supply device according to a second embodiment of the present invention.

FIG. 7 is a characteristic diagram of the switching power supply device of FIG. 6 and a conventional example.

FIG. 8 is a circuit diagram showing a conventional switching power supply device.

[Explanation of symbols]

 101, 201 Switching power supply device 1 DC power supply 3 Resistance 4 Zener diode 5 Starting resistance 6 Transistor 7 Diode 8 Starting circuit 9 Transformer 10 Primary winding 11 Secondary winding 12 Auxiliary winding 14 Switching element 16, 21 Rectifying diode 17 , 22, freewheeling diode 18, 23 choke coil 19, 24 smoothing capacitor 25 output error detection unit 26 photocoupler 27, D1 charging diode 28, C1 charging capacitor 29 backflow prevention diode 32, 301 voltage detector 34, K1 to K3 Switch 35,302 Switch control circuit 120 Auxiliary rectifier circuit S Power switch

Claims (1)

[Claims] 1. A primary winding of a transformer and a switching element are connected in series to a DC power source, and a starting circuit is connected in parallel with the DC power source, and an operating voltage is supplied from the starting circuit to a switching control circuit at the time of starting. However, in a steady state, an operating voltage is supplied from the auxiliary winding of the transformer to the switching control circuit via an auxiliary rectifier circuit, and the switching element is turned on / off by the switching control circuit to turn on / off the primary winding of the transformer. DC is intermittently supplied to the
In a switching power supply device for extracting an output from a secondary winding, a capacitor charged by a voltage induced in an auxiliary winding of the transformer is provided, and when the output voltage from the auxiliary rectifier circuit decreases, the switching from the capacitor is performed. A switching power supply device, which supplies an operating voltage to a control circuit, and supplies the operating voltage from the starting circuit to the switching control circuit when the output voltage from the capacitor decreases.