JPH10337017A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a switching power supply in which power loss is reduced at the time of steady operation through a starting circuit. SOLUTION: An AC power supply 1 is turned on to supply a starting voltage from a starting circuit 28 (40) to an auxiliary power supply 16 which actuates a PWM control circuit 11 to turn a switching element 5 connected in series with the primary winding of a transformer on/off and apply a pulse voltage to the primary winding 6 of the transformer 15. A load 41 is driven with an AC voltage induced in the secondary winding of the transformer 15 and the auxiliary power supply 16 is operated steadily with a voltage induced in the tertiary winding. Upon elapsing a predetermined time after steady operation of the auxiliary power supply 16 is detected or the AC power supply 1 is turned on, starting voltage supply from the starting circuit 28 (40) to the auxiliary power supply 16 is interrupted.

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.

[0002]

2. Description of the Related Art An auxiliary power supply circuit is connected to a tertiary winding of a transformer, a control circuit is operated by an output voltage from the auxiliary power supply, and a switching element connected to the primary winding of the transformer is turned on / off by the control circuit. A switching power supply for supplying power to a load connected to a secondary winding of a transformer is disclosed in, for example, Japanese Patent Application Laid-Open No. Hei 8-2666053. FIG. 3 shows an example of such a conventional switching power supply device.

In FIG. 3, an AC power supply 1 is connected to a primary-side rectifying / smoothing circuit 17 composed of a bridge diode 2 and an electrolytic capacitor 3. The output side of the primary side rectifying and smoothing circuit 17 is connected to the primary winding 6 of the transformer 15 via the switching element 5. Transformer 15-2
The secondary winding 8 is connected to a load 41 via a secondary-side rectifying / smoothing circuit 18 including a diode 9 and an electrolytic capacitor 10.

A tertiary winding 7 of a transformer 15 has an auxiliary power supply circuit 16 composed of a diode 13 and an electrolytic capacitor 12.
And a PWM (pulse width modulation) control circuit 11 for controlling on / off of the switching element 5 is connected to the auxiliary power supply circuit 16. A starting resistor 4 is connected between the output terminals of the auxiliary power supply circuit 16 and the bridge diode 2. The feedback circuit 14 is a circuit for comparing the output voltage Vout with a reference voltage and feeding back an error signal to the PWM control circuit 11 in order to supply a stable voltage to the load side.

In the configuration shown in FIG. 3, when the AC power supply 1 is turned on and an AC voltage is input, the AC voltage is rectified by the primary side rectifying and smoothing circuit 17 to become a DC voltage. It is supplied to a series circuit.
Further, the DC voltage is simultaneously applied to the auxiliary power supply circuit 16 via the starting resistor 4, and the electrolytic capacitor 12 is charged with the starting current from the starting resistor 4.

As a result, the potential of the electrolytic capacitor 12 gradually increases, and when the voltage reaches the operation start voltage of the PWM control circuit 11, the PWM control circuit 11 starts operating. PWM
When the control circuit 11 starts operating, the switching element 5 performs an on / off operation, outputs an AC voltage to the secondary winding 8 and the tertiary winding 7 of the transformer 15, and outputs a secondary rectification smoothing circuit 18.
And supplies a voltage to the auxiliary power supply circuit 16 to maintain the PWM control circuit 11 in an operating state.

[0007]

In the above-described conventional switching power supply device, even if the auxiliary power supply circuit enters a steady operation state in which power is supplied from the tertiary winding of the transformer,
Since a voltage corresponding to the difference between the DC output voltage of the primary rectifying and smoothing circuit and the DC output voltage of the auxiliary power supply circuit is applied between both terminals of the starting resistor, there is a problem that power loss occurs in the starting resistor. Was.

[0008]

According to a first aspect of the present invention, there is provided a switching power supply device, wherein a rectified output of an AC power supply is connected to a primary winding of a transformer via a switching element. When the auxiliary power supply circuit is provided in the tertiary winding and the AC power supply is turned on in the auxiliary power supply circuit,
A starting circuit for supplying a starting voltage to the auxiliary power circuit is provided, and a control circuit that operates at an output of the auxiliary power circuit turns on and off the switching element to supply power from a secondary winding of the transformer to a load. In the power supply device, voltage detecting means for detecting an output of a tertiary winding of the transformer, and starting voltage supply cutoff means for cutting off supply of a starting voltage from the starting circuit to the auxiliary power supply circuit by an output of the voltage detecting means. Are provided.

Therefore, when the AC power supply is turned on, a start-up voltage is supplied to the auxiliary power supply circuit via the start-up circuit, and the PWM is supplied to the auxiliary power supply circuit.
The control circuit is operated to drive the switching element on and off. As a result, a pulse voltage is supplied to the primary winding of the transformer, an output voltage is generated in the secondary and tertiary windings to drive the load, and a voltage is also supplied to the auxiliary power supply circuit to control the control circuit. Set to a steady operating state. In this state, the output of the tertiary winding of the transformer is detected by the voltage detection means, and the detection output of the voltage detection means activates the start-up voltage supply cut-off means, thereby reducing the start-up voltage from the start-up circuit to the auxiliary power supply circuit. The supply is cut off so that no power is lost in the starting circuit.

According to a second aspect of the present invention, a rectified output of an AC power supply is connected to a primary winding of a transistor via a switching element, and an auxiliary power supply circuit is provided in a tertiary winding of the transformer. A starting circuit that supplies a starting voltage when the AC power supply is turned on is provided in the circuit, and a control circuit that operates at the output of the auxiliary power supply circuit turns on and off the switching element to load the secondary winding of the transformer. A switching power supply device for supplying power to the AC power supply, characterized in that a timer means is provided for interrupting the supply of the starting voltage from the starting circuit to the auxiliary power supply circuit after a lapse of a predetermined time from the turning on of the AC power supply.

When the AC power supply is turned on, a start-up voltage is supplied to the auxiliary power supply circuit via a start-up circuit, and the PWM control circuit is operated to drive the switching element on and off. As a result, a pulse voltage is supplied to the primary winding of the transformer, an output voltage is generated in the secondary and tertiary windings to drive the load, and power is also supplied to the auxiliary power supply circuit, thereby controlling the control circuit. Set to a steady operating state.

On the other hand, when a predetermined time has elapsed since the AC power supply was turned on, the timer means operates, and the supply of the start-up voltage from the start-up circuit to the auxiliary power supply circuit is cut off. In this state, since the voltage is supplied from the tertiary winding of the transformer to the auxiliary power supply circuit, the control circuit maintains a steady operation state even when the starting voltage is cut off. As described above, since the starting voltage is cut off after a lapse of a certain period of time after the AC power supply is turned on, it is possible to prevent power loss in the starting circuit in the steady operation state.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION

(Embodiment 1) FIG. 1 is a circuit diagram of Embodiment 1 of the present invention, in which parts corresponding to those of the conventional example shown in FIG.

FIG. 1 differs from the conventional example shown in FIG. This starting circuit 28
Is provided between a series circuit of a transistor 20, a diode 22, and a resistor 23 provided between one output terminal of the primary-side rectifying / smoothing circuit 17 and the output terminal of the auxiliary power supply circuit 16, and between the base and collector of the transistor 20. A resistor 19;
A constant voltage diode 21 provided between the base and the other output terminal of the primary side rectifying / smoothing circuit 17, a transistor 24 provided in parallel with the constant voltage diode 21, a base of the transistor 24 and an auxiliary power supply circuit 16; A series circuit of a resistor 25 and a diode 27 provided between input terminals of
It comprises a capacitor 26 provided between the connection point of the resistor 25 and the diode 27 and the emitter of the transistor 24.

When the AC power supply 1 is turned on in the starting circuit 28, the DC terminal rectified by the primary side rectifying / smoothing circuit 17 turns the base terminal of the transistor 20 high, turning on the transistor 20. As a result, a DC voltage is applied to the auxiliary power supply circuit 16 via the diode 22 and the starting resistor 23, and the electrolytic capacitor 1 of the auxiliary power supply circuit 16
2 and the PWM control circuit 11 starts operating. P
When the WM control circuit 11 starts operating, the switching element 5 is turned on and off, a pulse voltage is applied to the primary winding 6 of the transformer 15, an AC voltage is induced in the secondary winding 8, and secondary rectification is performed. The load 41 is driven by the DC voltage rectified by the smoothing circuit 18.

A voltage is also induced in the tertiary winding 7 of the transformer 15, and the induced voltage causes the auxiliary power supply circuit 1 to operate.
When the power supply 6 starts, the positive bias rectified and smoothed by the diode 27 and the capacitor 26 is applied to the base terminal of the transistor 24, and the transistor 24 is turned on. As a result, the base terminal of the transistor 20 becomes low, and the transistor 20 is turned off.
The power supply via is stopped. Thereafter, a current flows through the resistor 19, and the value of the resistor 19 is equal to the value h of the value of the resistor 23.
_fe (constant of transistor 20) times, the loss is 1
/ H _fe , and the power loss by the starting circuit 28 can be reduced.

(Embodiment 2) FIG. 2 shows Embodiment 2 of the present invention.
The same reference numerals are given to portions corresponding to the conventional example shown in FIG. 3, and description thereof will be omitted. 2 differs from the conventional example in the configuration of the starting circuit 40.

The starting circuit 40 is a primary-side rectifying / smoothing circuit 1
7, a transistor 32, a diode 32 and a resistor 3 provided between one output terminal of the
3, a resistor 29 provided between the base and collector of the transistor 30, a constant voltage diode 31 provided between the base and the other output terminal of the primary rectifying and smoothing circuit 17, and a constant voltage A transistor 34 provided in parallel with the diode 31, a series circuit of resistors 35, 36 and 38 provided between both output terminals of the primary side rectifying and smoothing circuit 17, a diode 39 connected in parallel with the resistor 38,
It is composed of a capacitor 37 connected in parallel with a series circuit of the resistors 35 and 36, and a connection point of the resistors 35 and 36 is connected to the base of the transistor 34.

The operation of the power supply by the start circuit 40 is basically the same as that of the start circuit 28 shown in the first embodiment, and the difference is in the method of stopping the power supply. In the starting circuit 40, when the AC power supply 1 is turned on, the DC voltage rectified by the primary side rectifying and smoothing circuit 17 causes
The base terminal of the transistor 30 becomes high, and the transistor 30 is turned on.

As a result, the diode 32 and the starting resistor 33
A DC voltage is applied to the auxiliary power supply circuit 16 via the
The control circuit 11 starts operating. When the PWM control circuit starts operating, the switching element 5 is turned on and off, a pulse voltage is applied to the primary winding 6 of the transformer 15, and an AC voltage is applied to the secondary winding 8 and the tertiary winding 7,
The load 4 is controlled by the DC voltage rectified by the secondary side rectifying and smoothing circuit.
1 is driven, and a voltage is supplied to the auxiliary power supply circuit 16 to keep the PWM control circuit in the operating state.

On the other hand, when the AC power supply 1 is turned on, the DC voltage rectified by the primary side rectifying and smoothing circuit 17 causes
The capacitor 37 starts charging with the time constant of the resistor 38 and the capacitor 37. The terminal voltage of the capacitor 37 is the resistance 3
5. The voltage is divided by the resistor 36 and applied to the base terminal of the transistor 34.

Therefore, the voltage at the base terminal of the transistor 34 rises with a time constant, and when the voltage reaches V _BE , the transistor 34 turns on, the base potential of the transistor 30 goes low to turn off the transistor 30, and the starting resistor From 33, the supply of power to the auxiliary power supply circuit 16 is stopped. The resistor 38 and the capacitor 3 are connected so that the transistor 34 is turned on after the auxiliary power supply circuit 16 starts supplying power.
It is necessary to set a time constant of 7.

After power is supplied to the auxiliary power supply circuit 16 by the tertiary winding 7 of the transformer 15,
The starting circuit 40 stops its operation and does not return unless the AC power is turned on again. Therefore, when the output voltage decreases during overload, the supply voltage of the auxiliary power supply circuit 16 also decreases, and the PWM
Since the operation of the control circuit 11 can be stopped, the circuit can be protected by an overload.

[0024]

The switching power supply of the present invention supplies a starting voltage to the auxiliary power supply circuit by the starting circuit at the time of starting.
When the auxiliary power supply circuit is started, this is detected and the supply of the start-up voltage by the start-up circuit is cut off, so that a loss of electric power by the start-up circuit after the steady operation state is attained can be eliminated. In addition, since the supply of the starting voltage by the starting circuit is interrupted after a lapse of a predetermined time from the start, it is possible to reduce the power loss due to the starting circuit after the steady operating state. Therefore, it is particularly effective as a switching power supply device for wide input.

[Brief description of the drawings]

FIG. 1 is a circuit diagram according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram according to a second embodiment of the present invention.

FIG. 3 is a circuit diagram of a conventional example.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 AC power supply 5 Switching element 6 Primary winding 7 Tertiary winding 8 Secondary winding 11 PWM control circuit 12 Electrolytic capacitor 14 Feedback circuit 15 Transformer 16 Auxiliary power supply circuit 17 Primary rectification smoothing circuit 19,23,25, 29, 33, 35, 36, 38
Resistance 20, 24, 30, 34 Transistor 21, 31 Constant voltage diode 26, 37 Capacitor

Claims (2)

[Claims] A rectified output of an AC power supply is connected to a primary winding of a transformer via a switching element, an auxiliary power supply circuit is provided in a tertiary winding of the transformer, and the AC power supply is connected to the auxiliary power supply circuit. When the power supply is turned on, a starting circuit for supplying a starting voltage to the auxiliary power supply circuit is provided, and a control circuit operated by an output of the auxiliary power supply circuit turns on and off the switching element to supply power from a secondary winding of the transformer to a load. And a voltage detecting means for detecting an output of a tertiary winding of the transformer, and starting to cut off supply of a starting voltage from the starting circuit to the auxiliary power supply circuit by an output of the voltage detecting means. A switching power supply device comprising a voltage supply cutoff means. 2. A rectified output of an AC power supply is connected to a primary winding of a transistor via a switching element, an auxiliary power supply circuit is provided in a tertiary winding of the transformer, and the AC power supply is connected to the auxiliary power supply circuit. A switching power supply for providing a start-up circuit for supplying a start-up voltage when turned on, and a control circuit operating at an output of the auxiliary power supply circuit for turning on and off the switching element and supplying power to a load from a secondary winding of the transformer; A switching power supply device, further comprising: timer means for interrupting supply of a start-up voltage to an auxiliary power supply circuit from the start-up circuit a predetermined time after the AC power supply is turned on.