JP3611246B2 - Switching power supply circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a switching power supply circuit.
[0002]
[Prior art]
FIG. 2 is a circuit diagram showing a conventional switching power supply circuit 200.
[0003]
In the conventional switching power supply circuit 200, a power supply voltage is supplied to the control circuit 4 from the first resistor R <b> 11 and the auxiliary power supply circuit 30.
[0004]
The resistor R <b> 11 is a starting resistor that supplies power to the control circuit 4 with a DC input voltage obtained via the primary side rectifier circuit 1 after the AC input power is turned on.
[0005]
The auxiliary power circuit 30 includes an auxiliary winding N3 of the transformer T, a diode D3, a capacitor C1, a resistor R2, a transistor Q3, and a Zener diode D4. After the FET 2 starts switching, power is not only supplied from the auxiliary power supply circuit 30 to the control circuit 4, but also supplied through the resistor R11.
[0006]
Next, the operation of the conventional example will be described.
[0007]
First, when an AC input voltage is input, the DC input voltage is input to the control circuit 4 as a power source via the primary side rectifier circuit 1. Initially, since FET2 is turned off and switching is not started, no voltage is generated in the auxiliary winding N3 of the transformer T. Therefore, when the AC input voltage is turned on, that is, when the DC input voltage is turned on, the resistor R11 supplies a starting current to the control circuit 4, thereby starting the control circuit 4.
[0008]
Then, after the control circuit 4 is activated, when the FET 2 starts switching and an alternating current starts to flow through the transformer T, an alternating voltage is generated in the auxiliary winding N3 of the transformer T, and is rectified by the diode D3, and the resistor A current begins to flow through R2, the transistor Q3 becomes conductive, and a voltage is generated at the output terminal of the auxiliary power circuit 30. In this case, the current flowing through the resistor R2 changes according to the level of the AC voltage generated in the auxiliary winding N3, that is, the voltage across the resistor R2 changes, so that the impedance of the transistor Q3 changes, and therefore the constant voltage. Will occur. As a result, power is supplied from the auxiliary power circuit 30 to the control circuit 4.
[0009]
[Problems to be solved by the invention]
In the conventional example, in the case of the control circuit 4 that requires power at the time of startup, it is necessary to sufficiently increase the current flowing through the resistor R11. Even during the steady operation in which the FET 2 starts switching and power is supplied from the auxiliary power supply circuit 30, current always flows through the resistor R 11, so that power loss always occurs in the resistor R 11. In addition, when the output current is a light load, sufficient power may not be obtained from the auxiliary winding N3. In this case, most of the power is supplied through the resistor R11, and thus the resistor R11. Therefore, there is a problem that a large resistor for high power is necessary, and there is a problem that the power supply is enlarged, and the switching power supply circuit 200 is increased in cost as the power supply is enlarged.
[0010]
In the present invention, a primary side rectifier circuit rectifies an AC power source, and a switching element controlled by a control circuit converts the rectified voltage into an AC voltage, and the converted AC power is converted into a secondary side rectifier circuit. In a switching power supply circuit for rectifying and outputting, switching can reduce power consumption of a resistor provided in a path of a DC voltage supplied to the control circuit, and thus prevent an increase in power supply size. The object is to provide a power supply circuit.
[0011]
[Means for Solving the Problems]
The present invention provides a main switching element that switches a DC input voltage, a rectifying / smoothing circuit that rectifies alternating current generated by switching the DC input voltage into a DC output voltage, and a time during which the main switching element is turned on and off. A control circuit that controls the DC output voltage to be constant by changing the first switching element, and when the DC input voltage is applied, the control circuit is powered via the first switching element. And a power supply circuit for on-off consisting of a constant voltage dropper type regulator for stopping power supply to the control circuit during a steady operation of switching of the main switching element, and a second switching element, It consists of a constant voltage dropper regulator that supplies power during the steady switching operation of the device. A voltage obtained by rectifying and smoothing the AC voltage generated in the auxiliary winding that outputs the AC voltage generated by switching the DC input voltage by the main switching element. If the voltage is lower, the on-time power supply circuit is turned on. If the voltage obtained by rectifying and smoothing the alternating voltage generated in the auxiliary winding is equal to or higher than a predetermined voltage, the second switching element is turned on. In this switching power supply circuit, power is supplied from the steady-state power supply circuit via two switching elements, and at the same time, the on-time power supply circuit is reliably turned off.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a circuit diagram showing a switching power supply circuit 100 according to an embodiment of the present invention.
[0013]
The switching power supply circuit 100 includes a primary side rectifier circuit 1, an FET 2, a transformer T, a secondary side rectifier circuit 3, a control circuit 4, an on-time power supply circuit 10, and a steady-state power supply circuit 20. Have
[0014]
The primary side rectifier circuit 1 is a circuit that rectifies an AC power source and generates a DC input voltage, and the FET 2 is a transistor that switches the DC input voltage.
[0015]
The secondary side rectifier circuit 3 is a rectifying / smoothing circuit that rectifies an alternating current generated by switching the DC input voltage by the FET 2 into a DC output voltage.
[0016]
The control circuit 4 is a control circuit that controls the DC output voltage to be constant by changing the time during which the FET 2 is turned on and off.
[0017]
The on-time power supply circuit 10 supplies power to the control circuit 4 when the DC input voltage is applied, and on the other hand, stops the power supply to the control circuit 4 during the steady operation of the FET 2 switching. It is. The on-time power supply circuit 10 includes a transistor Q1, a resistor R1, and a Zener diode D1.
[0018]
That is, the on-time power supply circuit 10 is a circuit that switches the path from the rectifying / smoothing circuit (primary rectifier circuit 1) to the control circuit 4 based on the DC voltage generated by the steady-state power supply circuit 20.
[0019]
The constant power supply circuit 20 rectifies and smoothes the AC voltage generated by the FET 2 switching the DC input voltage, sets the rectified and smoothed voltage to a predetermined constant voltage, and supplies power to the control circuit 4. This is a constant power supply circuit.
[0020]
The constant power supply circuit 20 includes an auxiliary winding N3 of the transformer T, a diode D3, a smoothing capacitor C1, a transistor Q2, a resistor R2, a Zener diode D1, and a diode D2.
[0021]
Next, the operation of the switching power supply circuit 100 will be described.
[0022]
First, when the switching power supply circuit 100 is started, the on-time power supply circuit 10 including the transistor Q1, the resistor R1, and the Zener diode D1 is controlled from the DC voltage obtained via the primary side rectifier circuit 1. Power is supplied to the circuit 4. That is, when the switching power supply circuit 100 is started up, a DC voltage obtained through the primary side rectifier circuit 1 is applied to the resistor R1 and the Zener diode D1, a current flows through the resistor R1, and a bias is applied to the transistor Q1. Is applied and the transistor Q1 is turned on. As a result, when the switching power supply circuit 100 is activated, power is supplied to the control circuit 4 via the power supply circuit 10 for on-loading.
[0023]
Thereafter, when the control circuit 4 operates and the switching of the FET 2 becomes a steady operation, the diode D3 rectifies the AC voltage generated in the auxiliary winding N3 of the transformer T, is smoothed by the capacitor C1, and generates a DC voltage. . When this DC voltage reaches a predetermined voltage, a current flows through the resistor R2, the transistor Q2 is turned on, the voltage at this time is applied to the control circuit 4, power is supplied to the control circuit 4, and the steady state starts. Is done.
[0024]
In this steady state, the output potential of the on-time power supply circuit 10 (the potential of the emitter of the transistor Q1) is higher than the potential on the cathode side of the Zener diode D1 (the potential of the base of the transistor Q1). Since it is lower by the amount, the base-emitter of the transistor Q1 is reverse-biased, and the transistor Q1 is turned off.
[0025]
Therefore, when the switching of the FET 2 is in a steady operation, the power supply to the control circuit 4 is performed only from the steady-state power supply circuit 20. Instead of the diode D2, a circuit in which a plurality of diodes are connected in series may be used, or a new Zener diode is used with a polarity opposite to that of the diode D2 in FIG. A difference may be provided between the output voltage of the power supply circuit 10 and the output voltage of the steady-state power supply circuit 20.
[0026]
Next, when the output current is light load or no load, the pulse width of the FET 2 is decreased or intermittent operation is performed, so that the voltage from the auxiliary winding N3 is decreased, and between the base and emitter of the transistor Q1. Becomes a forward bias, the transistor Q1 is turned on, and power is supplied to the control circuit 4 from the power supply circuit 10 for turning on.
[0027]
In this embodiment, the FET 2 is used as the semiconductor switching element, but a transistor, IGBT, or the like may be used instead.
[0028]
In the above embodiment, when the input power is turned on and when the output is lightly loaded or unloaded, power is supplied to the control circuit 4 via the power-on power circuit 10 and the switching of the FET 2 is in a steady operation. When power is supplied, only the steady-state power circuit 20 from the auxiliary winding N3 of the transformer T supplies power to the control circuit 4, so that the power consumption in the on-time power circuit 10 is small and the efficiency can be improved. In addition, since both of the power supply circuits 10 and 20 are constant voltage dropper type regulators, stable power can be supplied against fluctuations in input and output.
[0029]
【The invention's effect】
According to the present invention, the supply voltage of the power supply circuit supplied from the auxiliary winding is always set to a voltage slightly higher than the supply voltage of the start-up power supply circuit. Power consumption is low because it is supplied, and there is no adjustment factor in the switching power supply circuit. Even when the auxiliary winding voltage drops, the power supplied from the start-up power supply circuit is not delayed, so stable operation continues. The effect of doing.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a switching power supply circuit 100 according to an embodiment of the present invention.
FIG. 2 is a circuit diagram showing a conventional switching power supply circuit 200;
[Explanation of symbols]
1 ... Primary side rectifier circuit,
2 ... FET,
3 ... Secondary side rectifier circuit,
4 ... control circuit,
10 ... Power circuit for turning on,
20 ... Power circuit for steady state,
T ... Transformer,
N3: Auxiliary winding.

Claims (1)

A main switching element for switching a DC input voltage;
A rectifying and smoothing circuit for rectifying an alternating current generated by switching the direct current input voltage into a direct current output voltage;
A control circuit that controls the DC output voltage to be constant by changing a time during which the main switching element is turned on and off;
A first switching element that supplies power to the control circuit via the first switching element when the DC input voltage is turned on, and supplies the power to the control circuit during steady-state switching of the main switching element; A power circuit for power-on comprising a constant voltage dropper type regulator for stopping power supply;
A steady-state power supply circuit comprising a constant voltage dropper-type regulator that includes a second switching element and supplies power during the steady switching operation of the main switching element ;
And when the voltage obtained by rectifying and smoothing the AC voltage generated in the auxiliary winding that outputs the AC voltage generated by switching the DC input voltage by the main switching element is lower than a predetermined voltage, When the voltage obtained by rectifying and smoothing the alternating voltage generated in the auxiliary winding is equal to or higher than a predetermined voltage, the second switching element is turned on and the second switching element is turned on via the second switching element. A switching power supply circuit that supplies power from the steady-state power supply circuit and at the same time reliably turns off the on-time power supply circuit.

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