JP2003092883A - Power supply unit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a power supply unit capable of reducing switching loss by lowering a switching cycle even with a small load. SOLUTION: This power supply unit comprises a switching circuit 22 which has a switching device Q1 provided at a primary side winding 21 of a transformer T and interrupts a DC voltage, a rectification device 30 which rectifies an AC voltage derived to a secondary side winding 31 of the transformer T by interrupting the DC voltage of the switching circuit 22, a smoothing capacitor 32 which smoothes rectification voltage outputted from the rectification device 30 and outputs the output voltage from an output terminal, a voltage detecting circuit 34 which detects the output voltage, and a photo coupler PH which transmits a detected signal of the voltage detecting circuit 34 to the switching circuit 22. In this power supply unit, the switching circuit 22 controls on/off of the switching device Q1 based on the detected signal from the photo coupler PH, and a voltage application circuit 46 is provided to reduce inverted voltage generated at a primary side coil 24 of the transformer T when the load of the output terminal 33 is small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power supply device for intermittently inputting a DC voltage to generate a DC voltage.

[0002]

2. Description of the Related Art Conventionally, a power supply device shown in FIG. 4 has been known. In FIG. 4, 1 is the primary winding 2 of the transformer H.
2 is a switching circuit having a switching element 3 for intermittently converting a DC voltage applied to the AC voltage into an AC voltage. The switching circuit 1 includes a switch element 4 for turning off the switching element 3, a capacitor 5 for making the switch element 4 conductive, a phototransistor PH1 of a photocoupler PH for charging the capacitor 5, and the like. Have

Reference numeral 6 is a first rectifying element for rectifying the AC voltage induced in the secondary winding 7 of the transformer H, and 8 is a smoothing capacitor for smoothing the rectified voltage of the first rectifying element 6. Reference numeral 9 is a detection circuit for detecting the output voltage of the output terminal P. The detection circuit 9 divides the output voltage of the output terminal P by a voltage dividing circuit 1
1 and a constant voltage IC circuit 12 and the like. Constant voltage IC
The voltage divided by the voltage dividing circuit 11 is input to the gate 12G of the circuit 12, and the constant voltage IC circuit 12 adjusts the output voltage of the output terminal P so that the voltage of the gate 12G becomes a predetermined voltage.

In this adjustment, for example, when the voltage of the gate 12G is equal to or higher than a predetermined value, the current flowing to the ground is increased to lower the output voltage to lower the voltage of the gate 12G to a predetermined value. When the value is less than the value, the output voltage is increased by decreasing the current flowing to the ground, and the voltage of the gate 12G is increased to a predetermined value.

A light emitting diode (transmission circuit) PH2 of the photocoupler PH emits light in accordance with a current (detection signal) flowing through the constant voltage IC circuit 12 and is transmitted to the switching circuit 1 on the primary side.

Further, in FIG. 4, 13 is a second rectifying element for rectifying the AC voltage induced in another secondary winding 14 of the transformer H, and 15 is a constant rectified voltage of the second rectifying element 13 for output. It is a constant voltage output circuit.

According to this power supply device, the switching element 3 is repeatedly turned on and off, whereby the rectified voltage is output from the first rectifier element 6 and the output voltage is output from the output terminal P. Now, when the output voltage of the output terminal P becomes higher, the current flowing through the constant voltage IC circuit 12 increases and the light emission amount of the light emitting diode PH2 of the photocoupler PH increases. Due to this increase in the amount of light emission, the current flowing through the phototransistor PH1 of the photocoupler PH increases, and the capacitor 5
Is charged in a short time, and the base voltage of the switch element 4 exceeds the threshold value in a short time. When the base voltage exceeds the threshold value, the switch element 4 is turned on and the switching element 3 is turned off.

On the contrary, when the output voltage of the output terminal P becomes low, the divided voltage of the voltage dividing circuit 11 becomes low, so that the current flowing through the constant voltage IC circuit 12 decreases and the photocoupler PH
The light emission amount of the light emitting diode PH2 is reduced. Due to this decrease in the amount of light emission, the phototransistor P of the photocoupler PH is
The current flowing through H1 decreases and the charging time of the capacitor 5 becomes longer, and the time until the base voltage of the switch element 4 exceeds the threshold value becomes longer. When the base voltage exceeds the threshold value, the switch element 4 is turned on and the switching element 3 is turned off.

That is, when the voltage of the output terminal P becomes higher, the ON time of the switching element 3 is shortened, and when the voltage of the output terminal P becomes lower, the ON time of the switching element 3 is lengthened to make the output terminal P longer. The voltage of P is made constant.

[0010]

However, in this power supply device, when the load connected to the output terminal P is a light load, the time during which the switching element 3 is on becomes short, so the switching element 3 is short. The switching frequency becomes higher. Therefore, there is a problem that switching loss increases.

The present invention has been made in view of the above problems, and an object thereof is to provide a power supply device capable of lowering the switching frequency and the switching loss even when the load is light. It is in.

[0012]

In order to achieve the above object, the invention of claim 1 has a switching device which is provided in a primary winding of a transformer to which a DC voltage is applied and which has a switching element for connecting and disconnecting the DC voltage. A circuit and a first rectifying element for rectifying an AC voltage induced in the secondary winding of the transformer by the intermittent DC voltage by the switching circuit,
A smoothing capacitor that smoothes the rectified voltage output from the first rectifying element and outputs the output voltage from the output terminal, a voltage detection circuit that detects the output voltage, and a detection signal of the voltage detection circuit that is switched on the primary side. A switching circuit for transmitting the voltage to the circuit, and the switching circuit includes a primary side auxiliary winding that generates an inverted voltage when the switching element is turned off, and a voltage generated in the primary side auxiliary winding of the switching element. A gate circuit which is applied to the gate to turn on the switching element; and an off circuit which turns off the switching element by lowering the gate voltage of the switching element based on a detection signal from the transmission circuit. A power supply device for setting the output voltage to a predetermined voltage by turning on / off a switching element by an off circuit, When the load connected to the terminals of the light load, characterized in that a voltage drop means to reduce the inverted voltage generated in the primary auxiliary winding.

According to a second aspect of the present invention, the voltage detection circuit comprises:
A constant voltage IC circuit that adjusts the output voltage so that the voltage input to the gate becomes a predetermined voltage; and a voltage dividing circuit that divides the output voltage and applies the voltage to the gate, wherein the voltage reduction unit includes: It has a voltage application circuit for applying a voltage higher than the predetermined voltage to the gate when the load is light.

According to a third aspect of the present invention, a second rectifying element is connected to one end of a secondary winding different from the secondary winding and rectifies an AC voltage induced in the secondary winding. And a constant voltage output circuit that outputs the rectified voltage output from the second rectifying element as a constant voltage, and the voltage applying circuit turns on / off by a remote signal from the outside, and the switch. And an input circuit for inputting the output voltage of the constant voltage output circuit to the gate only when the element is off.

According to a fourth aspect of the present invention, there is provided a switch element which is turned on / off by the remote signal to reduce the current flowing through the constant voltage IC circuit when the load is light.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of a power supply device according to the present invention will be described below with reference to the drawings.

[First Embodiment] In FIG. 1, 20 is 1
This is an electrolytic capacitor connected between the input terminals S1 and S2 on the next side, and a full-wave rectified voltage is applied to this electrolytic capacitor 20. Reference numeral 22 denotes a switching circuit, which includes a switching element Q1 connected to one terminal of the primary winding 21 of the transformer T and a control circuit 23 for controlling ON / OFF of the switching element Q1. Have

The control circuit 23 has a primary winding (primary auxiliary winding) 24 of the transformer T, a switch element 25, and a phototransistor PT of a photocoupler PH. The collector of the switch element 25 is connected to the base of the switching element Q1 and the emitter of the switch element 25 is grounded. A resistor R7 is connected between the collector and the emitter of the switch element 25, and a capacitor 26 is connected between the emitter and the base of the switch element 25. The collector of the switch element 25 is connected to the other terminal of the primary winding 21 of the transformer T via a resistor R6, and the base of the switch element 25 is connected to the emitter of the phototransistor PT.

The collector of the phototransistor PT has a transformer T via a diode D1 and a resistor R104.
Connected to one terminal of the primary winding 24 of
A capacitor C1 and a resistor R4 connected in series are connected between 04 and the gate of the switching element Q1.
The resistors R6, R4, R104 and the capacitor C1 constitute a gate circuit for turning on the switching element Q1. Further, the switch element 25, the capacitor 26, the phototransistor PT, and the like constitute an off circuit that turns off the switching element Q1.

Reference numeral 30 is a rectifying element connected to one terminal of the secondary winding 31 of the transformer T, reference numeral 32 is a smoothing capacitor for smoothing the rectified voltage output from the rectifying element 30,
The smoothing capacitor 32 is connected between the output terminal 33 and the ground. Output terminal 3 between output terminal 33 and ground
A voltage detection circuit 34 for detecting the output voltage of the No. 3 is connected.

The voltage detecting circuit 34 has a voltage dividing circuit 35 for dividing the output voltage of the output terminal 33, a constant voltage IC circuit 36 and the like. The voltage dividing circuit 35 has a resistor R connected in series.
107 and R108, and these resistors R107 and R1
The voltage divided by 08 is input to the gate 36G of the constant voltage IC circuit 36. The constant voltage IC circuit 36 adjusts the output voltage of the output terminal 33 by passing a current in the arrow direction so that the voltage of the gate 36G becomes a predetermined voltage. For example, when the voltage of the output terminal 33 becomes higher, the voltage of the gate 36G becomes higher than a predetermined value, and the constant voltage IC circuit 36 is controlled so that the gate voltage becomes a predetermined value.
Causes a large amount of current to flow in the direction of the arrow to lower the voltage of the output terminal 33. Conversely, when the voltage of the output terminal 33 becomes lower, the voltage of the gate 36G becomes lower than a predetermined value, and this gate voltage becomes a predetermined value. Thus, the current flowing through the constant voltage IC circuit 36 is reduced and the voltage of the output terminal 33 is increased.

That is, a current (detection signal) flows through the constant voltage IC circuit 36 in accordance with the output voltage of the output terminal 33 detected by the voltage detection circuit 34.

PD is a light emitting diode (transmission circuit) of the photocoupler PH, and this light emitting diode PD emits light according to the current flowing in the constant voltage IC circuit 36 and transmits it to the switching circuit 22 on the primary side.

A rectifying element (second rectifying element) 41 is connected to one terminal of the secondary winding 40 of the transformer T,
A smoothing capacitor 42 and a constant voltage output circuit 43 are connected to the output terminal of the rectifying element 41. The constant voltage output circuit 43 outputs the rectified voltage smoothed by the smoothing capacitor 42 to the output terminal 44 as a constant output voltage (5V).

A smoothing capacitor 45 and a voltage application circuit (voltage reduction means: switching frequency reduction means) 46 are connected between the output terminal of the constant voltage output circuit 43 and ground. The voltage application circuit (voltage reduction circuit) 46 is a transistor 47 that is turned on / off by a remote signal from the outside.
And a diode (input circuit) D3 whose cathode is connected to the gate 36G of the constant voltage IC circuit 36 and whose anode is connected to the collector of the transistor 47. The diode D3 inputs the output voltage of the constant voltage output circuit 42 to the gate 36G of the constant voltage IC circuit 36 when the transistor 47 is off.

The remote signal is an L level signal when the load is light, and an H level signal when the load is normal,
It is output from an external device (not shown).

Next, the operation of the power supply device of the above embodiment will be described with reference to the time chart shown in FIG.

The DC voltage applied to the primary winding 21 of the transformer T is interrupted by the switching element Q1, and this interruption induces an AC voltage in the secondary winding 31 of the transformer T. Then, a rectifying current is output from the rectifying element 30 as shown in FIG.
It is smoothed by 2 and is output from the output terminal 33 as a DC voltage of 12V (predetermined voltage).

When a normal load is connected to the output terminal 33, an H-level remote signal is externally input to the base of the transistor 47, and the transistor 47 becomes conductive. Due to the conduction of the transistor 47, the anode of the diode D3 becomes the ground potential, so that the voltage dividing circuit 35
The divided voltage (2.5V) is input to the gate 36G of the constant voltage IC circuit 36.

The constant voltage IC circuit 36 supplies a current corresponding to the output voltage of the output terminal 33 detected by the voltage detection circuit 34. This current causes the light emitting diode PD of the photocoupler PH to emit light, and a current flows through the phototransistor PT of the photocoupler PH. This current causes the capacitor 2
6 is charged, and the base voltage of the switch element 25 rises. When this base voltage exceeds the threshold value of the switch element 25, the switch element 25 is turned on and the switching element Q1 is turned off.

Now, for example, when the switching element Q1 is turned off at the time t1 shown in FIG. 2, the current flowing through the primary coil 21 of the transformer T sharply decreases, so that the voltage of the primary coil 24 of the transformer T increases. Vb is inverted, and thereafter, the voltage Vb rises at time t2. This primary coil 2
The base voltage VG of the switching element Q1 increases according to the voltage Vb of 4. When the base voltage VG exceeds the threshold value of the switching element Q1, the switching element Q1 is turned on (time point t3).

When the voltage Vb of the primary coil 24 is inverted, that is, when the switching element Q1 is turned off and the polarity of the transformer T is inverted, the rectifying element 30 outputs a rectified current.

By repeating these operations and the operation of the constant voltage IC circuit 36, a constant output voltage (12 V) is output from the output terminal 33.

Next, a case where a light load is connected to the output terminal 33 will be described.

When a light load is connected to the output terminal 33, an L level remote signal is input to the base voltage of the transistor 47 and the transistor 47 is turned off. Therefore, the output voltage (5 V) of the constant voltage output circuit 43 is equal to the resistance R1.
And to the gate 36G of the constant voltage IC circuit 36 via the diode D3. That is, the same state as when the output voltage of the output terminal 33 becomes high, the current flowing through the constant voltage IC circuit 36 increases, and as a result, the switching element Q1 turns off in a short time after turning on.

Now, for example, when the switching element Q1 is turned off at time t10 shown in FIG. 2, the voltage Vb of the primary coil 24 of the transformer T is reduced because the current flowing through the primary coil 21 of the transformer T is rapidly reduced. The voltage Vb is inverted, and thereafter the voltage Vb rises at time t11.

By the way, since the switching element Q1 is turned off in a short time after being turned on, the energy stored in the transformer T is small. Therefore, the inverted voltage Vb2
Is smaller than Vb1 and the inverted base voltage VG2 is also smaller than VG1. Then, as the voltage Vb rises, the base voltage VG also rises, but this base voltage VG rises and the maximum base voltage VGa is the switching element Q.
By presetting the number of turns of the primary coil 24, the resistance value of the resistor R4, and the capacitance of the capacitor C1 so that the threshold value of 1 is not exceeded, the switching element Q1 is not turned on and the base voltage VG is It vibrates.

On the other hand, since the capacitor C1 is charged through the resistor R6, the base voltage VG oscillates while rising as shown by the broken line in FIG. The rise of the broken line is determined by the time constants of the resistors R6 and R7 and the capacitor C1.

When the base voltage VG exceeds the threshold value of the switching element Q1, the switching element Q1 is turned on (time point t12).

As described above, the time (time point 10 to time point t12) from when the switching element Q1 is turned off to when it is turned on becomes long, and as a result, the output voltage of the output terminal 33 becomes 12 V or less. Further, since the time from when the switching element Q1 is turned off to when it is turned on becomes long, the switching frequency of the switching element Q1 is lowered and the switching loss can be reduced.

As described above, when the load is light, the inverted voltage Vb1 generated in the primary coil 24 of the transformer T is reduced (decreased) to reduce the switching frequency of the switching element Q1 to reduce the switching loss. It is a thing.

[Second Embodiment] FIG. 3 is a circuit diagram of a power supply device according to a second embodiment. In the second embodiment, the transistor Q4 is connected in parallel to the photodiode PD of the photocoupler PH, the base of the transistor Q4 is connected to the collector of the transistor 47 via the resistor R13, and the collector of the transistor Q4 is connected to the resistor R11. Is connected to the cathode of the photodiode PD, and the collector of the transistor 47 is connected to the resistor R1 via the diode D2.

According to the second embodiment, when the transistor 47 is on, that is, when a normal load is connected to the output terminal 33, the transistor Q4 is on and a current flows through the resistor R11. When the transistor 47 is off, that is, when a light load is connected to the output terminal 33, the transistor Q4 is off and the resistor R1
No current flows through 1. This ensures that a current flows through the constant voltage IC circuit 36 during a steady load, and that an unnecessary current of R11 does not flow during a light load to reduce loss.

[0044]

As described above, according to the present invention, it is possible to reduce the switching loss by reducing the switching frequency when the load is light.

[Brief description of drawings]

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

FIG. 2 is a time chart of the main part of FIG.

FIG. 3 is a circuit diagram showing a configuration of a second embodiment.

FIG. 4 is a circuit diagram showing a configuration of a conventional power supply device.

[Explanation of symbols]

21 Primary winding 22 Switching circuit 30 Rectifying element (first rectifying element) 31 Secondary winding 32 smoothing capacitor 33 output terminals 34 Voltage detection circuit 46 Voltage application circuit T transformer Q1 switching element PH photo coupler

Claims (4)

[Claims] 1. A switching circuit having a switching element provided on a primary winding of a transformer to which a DC voltage is applied and connecting and disconnecting the DC voltage, and a secondary side winding of the transformer due to the connection and disconnection of the DC voltage by the switching circuit. A first rectifying element that rectifies the AC voltage induced in the line, a smoothing capacitor that smoothes the rectified voltage output from the first rectifying element and outputs an output voltage from an output terminal, and a voltage that detects the output voltage. A detection circuit and a transmission circuit that transmits the detection signal of the voltage detection circuit to the switching circuit on the primary side are provided, and the switching circuit includes a primary side auxiliary winding that generates an inverted voltage when the switching element is turned off. A gate circuit for applying a voltage generated in the primary auxiliary winding to the gate of the switching element to turn on the switching element; An off circuit for turning off the switching element by lowering the gate voltage of the switching element based on a detection signal from the circuit, and turning the switching element on and off by the gate circuit and the off circuit causes the output voltage to reach a predetermined voltage. And a load connected to the output terminal is a light load.
A power supply device characterized by comprising voltage lowering means for reducing the inverted voltage generated in the secondary auxiliary winding. 2. The voltage detecting circuit, a constant voltage IC circuit for adjusting the output voltage so that the voltage input to the gate becomes a predetermined voltage, and a voltage dividing circuit for dividing the output voltage and applying it to the gate. 2. The power supply device according to claim 1, wherein the voltage reduction unit has a voltage application circuit that applies a voltage higher than the predetermined voltage to the gate when the load is light. 3. A second rectifying element, which is connected to one end of a secondary winding different from the secondary winding and rectifies an AC voltage induced in the secondary winding, and a second rectifying element. A constant voltage output circuit that outputs a rectified voltage output from the rectifying element as a constant voltage, and the voltage applying circuit is a switch element that is turned on / off by a remote signal from the outside, and this switch element is turned off. 3. The power supply device according to claim 2, further comprising an input circuit for inputting the output voltage of the constant voltage output circuit to the gate only when the voltage is present. 4. The power supply device according to claim 3, further comprising a switch element that is turned on / off by the remote signal to reduce a current flowing through the constant voltage IC circuit when the load is light.