JP3171068B2 - Switching power supply - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a switching power supply of a ringing choke converter (RCC) type.

[0002]

2. Description of the Related Art FIG. 3 shows a conventional switching power supply circuit based on the RCC system. In the figure, T is a transformer,
It has a primary main winding NI, a secondary output winding N2, and a feedback winding N3 with a center tap. The rectifier diode D1 and the smoothing capacitor C1 constitute a primary DC power supply S1. The negative (-) terminal of the primary DC power supply S1 and the center tap of the feedback winding N3 are connected to form a common ground GND.
It has become. The positive (+) terminal of the primary DC power supply S1 is connected to the ground GND from the winding start end to the winding end of the primary main winding N1 and from the collector to the emitter of the switching transistor Tr1. The base of the switching transistor Tr1 is connected to the winding start end of the feedback winding N3 via a circuit element such as a diode. Also, the plus (+) terminal of the primary DC power supply S1 is
It is connected to the ground GND via the starting resistor R1 and the base of the switching transistor Tr1, and from the collector to the emitter of the control transistor Tr2. The winding end of the feedback winding N3 is connected to ground via a diode D3, a resistor R4, a base of a phototransistor PT and a control transistor Tr2, and a parallel circuit of a resistor R3 and a capacitor C3. This series of circuits constitutes a charging time constant circuit of the capacitor C3.

On the other hand, on the secondary output winding N2 side of the transformer T, a secondary DC power supply S2 is formed by rectification and smoothing by a diode D2 and a capacitor C2, and an output voltage V _{O is obtained.}
To the load. An output voltage detection circuit k including an operational amplifier OP, a photodiode PD, and the like is connected to the secondary DC power supply S2. This photodiode PD is photocoupled with the phototransistor PT.

The general circuit configuration of a conventional switching power supply is as described above. Next, the operation will be described.

The output voltage detection circuit k on the secondary output winding N2 side
Always detects a change in the output voltage V _O , and when a difference from a predetermined voltage occurs, the operational amplifier OP amplifies the difference to change the amount of current flowing through the photodiode PD. The amount of current of the phototransistor PT photocoupled with the PD is adjusted, the charge time constant of the capacitor C3 is changed, the control transistor Tr2 is turned on / off, and the duty of the switching transistor Tr1 is changed to output. The voltage V _O is maintained at a predetermined voltage.

For example, when the output voltage V _O rises, the current flowing through the photodiode PD increases, and the current flowing through the phototransistor PT also increases, so that the capacitor C3 is charged earlier and the control transistor Tr2 is turned on earlier. Therefore, the switching transistor Tr
1 turns off earlier, the on-period becomes shorter, and the output voltage V _O is controlled in a decreasing direction. On the contrary, the output voltage V
_{When O} decreases, the ON period of the switching transistor Tr1 becomes longer, and the output voltage V _O is controlled to increase. In any case, the output voltage V _O is controlled to be a predetermined voltage.

[0007]

However, in the conventional switching power supply of the RCC system, although the constant voltage control is performed by the feedback control, when the input voltage becomes high and the input power (load) becomes small, the switching transistor becomes low. The switching frequency of Tr1 is increased, and therefore, the duty cycle of switching transistor Tr1 is shortened, and the ON period is shortened. As a result, capacitor C3 cannot be instantaneously charged to the threshold voltage, and control transistor Tr2 is turned on. Therefore, there is a problem that the switching transistor Tr1 cannot be turned off, and the output voltage V _O increases more and more.

Therefore, the present invention provides a charging time constant circuit, in which a bypass capacitor is connected in parallel to the collector resistance of a phototransistor to rapidly charge the time constant capacitor and quickly bring the control element to a threshold voltage. Let me reach
An object of the present invention is to provide a switching power supply capable of turning on and off a switching element instantaneously.

[0009]

In order to achieve the above object, the present invention provides a primary main winding (N1) and a secondary output winding (N1).
2) Transformer (T) having feedback winding (N3)
A switching element (Q) connected in series with the primary main winding (N1) to a primary DC power supply (S1), and a control terminal connected to one end of the feedback winding (N3); ) Connected to the control terminal (TR)
1) an output stabilizing circuit (2) connected between a control terminal of the control element (TR1) and the feedback winding (N3).
And the overcurrent protection circuit (3), and the control element (TR
A time constant circuit (1) connected to the control terminal of (1), a secondary DC power supply (S2) obtained by rectifying and smoothing the output of the secondary output winding (N2), and the secondary DC power supply (S2), an output voltage detection circuit (5) having a photodiode. The output stabilization circuit (2) includes a diode (D3) and a resistor (R4). ) And a parallel circuit of a capacitor (C4) and a series circuit of a phototransistor (PT).

According to the present invention, since the bypass capacitor (C4) is connected in parallel to the collector resistance (R4) of the phototransistor (PT) of the output stabilizing circuit (2) functioning as a charging time constant circuit, the input voltage can be reduced. Even if the switching frequency increases due to an increase or a decrease in input power (light load), a charging current for charging the time constant capacitor (C3) flows instantaneously via the bypass capacitor (C4), and the control element (TR1) ) Is quickly turned on and the switching element (Q) is rapidly turned off, so that the output voltage V _O can be maintained at a constant voltage following the increased switching frequency.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIG. In the figure, S1 is a primary DC power supply, which is obtained by rectifying a commercial frequency power supply voltage by a rectifier diode D1 and smoothing it by a smoothing capacitor C1. T is a transformer having a primary main winding N1, a secondary output winding N2, and a feedback winding N3. The negative (-) terminal of the primary DC power supply S1 and the winding end of the feedback winding N3 are connected to form a common ground GND.

The plus (+) terminal of the primary DC power supply S1 is connected to the ground G through the winding start and end of the primary main winding N1 and the drain to source of the switching element Q via the source.
Connected to ND. The control terminal of the switching element Q is connected to the winding start end of the feedback winding N3 via a circuit element such as a diode.

A series circuit of a starting resistor R1 and a bias resistor R2 is connected between the plus (+) terminal of the primary DC power supply S1 and the ground GND, and these connection points are connected to the control terminal of the switching element Q. In addition to being connected, it is connected to the collector of the control transistor TR1 constituting the control element. The control transistor TR1
Are connected to the ground GND. A time constant circuit 1 including a resistor R3 and a time constant capacitor C3 is connected between the emitter of the control transistor TR1 and the ground GND.

An output stabilizing circuit (charging time constant circuit) 2 is connected between the base of the control transistor TR1 and the beginning of the feedback winding N3. The output stabilizing circuit 2 includes a series circuit of a diode D3, a parallel circuit of a collector resistor R4 and a bypass capacitor C4, and a phototransistor PT. An overcurrent protection circuit 3 including a diode and a resistor is connected between the cathode of the diode D3 and the base of the control transistor TR1. Further, an oscillation stabilizing circuit 4 composed of a diode and a capacitor is connected between the winding start end of the feedback winding N3 and the ground GND.

On the other hand, on the secondary output winding N2 side of the transformer T, the back electromotive force of the primary main winding N1 is applied to a diode D2,
The secondary DC power supply S2 is obtained by rectification and smoothing by the capacitor C2, and supplies the output voltage V _O to the load. An output voltage detection circuit 5 is connected between a plus (+) terminal and a minus (-) terminal of the secondary DC power supply S2. The output voltage detection circuit 5 includes a resistor R5, a photodiode PD
And a parallel circuit of a series circuit of the shunt regulator SR and a series circuit of the divided resistors R6 and R7. The connection point between the division resistors R6 and R7 is connected to the reference terminal of the shunt regulator SR.

The circuit of this embodiment has the above configuration, and the operation will be described next. When the primary DC power supply S1 is connected to the commercial frequency power supply, the switching element Q first conducts. Then, a positive voltage is induced in the primary main winding N1. Due to the induced voltage of the primary main winding N1, a voltage is also induced in the secondary output winding N2, but the current is blocked by the reverse polarity of the rectifier diode D2 and no current flows on the secondary output winding N2 side. On the other hand, a current flows through the output stabilization circuit 2, the overcurrent protection circuit 3, and the time constant circuit 1 of the feedback winding N3, and charges the time constant capacitor C3. The transistor TR1 is turned on to turn off the switching element Q. When the switching element Q is turned off, a back electromotive force is generated in the primary main winding N1, and a voltage is induced in the secondary output winding N2 by the back electromotive force, and the induced voltage is rectified and smoothed. The next DC power supply S2 is obtained and supplies the output voltage V _O to the load.

On the other hand, with the discharge of the charging voltage of the time constant capacitor C3 and the disappearance of the back electromotive force of the primary main winding N1, the switching element Q is turned on again by the kick voltage generated in the feedback winding N3 to perform the switching operation. Will continue.

The input power is P ₁ , the primary main winding N
Assuming that the inductance value of ₁ is L ₁ , the on-duty of the switching element Q is D, and the input voltage is V ₁ , the switching frequency f is as follows.

F = D ² V ₁ / 2L ₁ P ₁ According to this equation, when the input voltage V ₁ increases and the input power P ₁ decreases and the load becomes light, the switching frequency f increases. I can understand. As described above, when the switching frequency f is increased, the charging period of the time constant capacitor C3 is shortened. Therefore, it is necessary to supply a sufficient charging current instantaneously.

In the circuit of this embodiment, since the bypass capacitor C4 is connected in parallel with the collector resistor R4 of the output stabilizing circuit (charging time constant circuit) 2, when the switching frequency f increases, the bypass capacitor C4 The impedance Z (1 / 2πfC) becomes small, and the charging current does not flow mainly by bypassing the bypass capacitor C4. The phototransistor PT is turned on instantaneously, and at the same time, the time constant capacitor C3 is rapidly lowered to the threshold value. By charging, the control transistor TR1 can be turned on and the switching element Q can be turned off. Figure 3 shows a phototransistor passing current I _PT characteristics for the input power P _1, the passing current I _PT characteristics of a conventional example of a prior broken line is not connected a bypass capacitor C4, the solid line after connecting the bypass capacitor C4 present 9 shows a passing current _IPT characteristic of the embodiment. In this embodiment, the phototransistor passing current I _PT is larger than that in the conventional example, and it can be predicted that the charging current of the time constant capacitor C3 is correspondingly increased and rapid charging is performed.

[0021] Thus, by connecting the bypass capacitor C4, even when high switching frequency f is the input power P ₁ become to a light load decreases, the time since the constant capacitor C3 can rapidly charging, switching The controllability of the element Q can be maintained.

[0022]

According to the present invention, since the bypass capacitor (C4) is connected in parallel to the collector resistance (R4) of the photodiode (PT) of the output stabilization circuit (2) functioning as a charging time constant circuit, Even if the switching frequency increases due to a rise in voltage or a reduction in input power (light load), the charging current flowing through the bypass capacitor (C4) rapidly charges the time constant capacitor (C3) mainly to the threshold value. Since the control element (TR1) is quickly turned on and the switching element (Q) is quickly turned off, the output voltage V _O can be kept at a constant voltage following the increased switching frequency.

[Brief description of the drawings]

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

FIG. 2 is a diagram showing characteristics of a phototransistor passing current with respect to input power of the switching power supply of this embodiment and a conventional switching power supply.

FIG. 3 is a circuit diagram of a conventional switching power supply.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Time constant circuit 2 Output stabilization circuit 3 Overcurrent protection circuit 4 Oscillation stabilization circuit 5 Output voltage detection circuit S1 Primary DC power supply S2 Secondary DC power supply Q Switching element TR1 Control transistor C3 Time constant capacitor PD Photodiode PT Phototransistor SR Shunt regulator R1 Starting resistance

Claims (1)

(57) [Claims] 1. A primary main winding (N1) and a secondary output winding (N1).
2) Transformer (T) having feedback winding (N3)
A switching element (Q) connected in series with the primary main winding (N1) to a primary DC power supply (S1), and a control terminal connected to one end of the feedback winding (N3); ) Connected to the control terminal (TR)
1) an output stabilizing circuit (2) connected between a control terminal of the control element (TR1) and the feedback winding (N3).
And the overcurrent protection circuit (3), and the control element (TR
A time constant circuit (1) connected to the control terminal of (1), a secondary DC power supply (S2) obtained by rectifying and smoothing the output of the secondary output winding (N2), and the secondary DC power supply An output voltage detection circuit (5) connected to (S2) and having a photodiode, wherein the output stabilization circuit (2) includes a diode (D3);
A parallel circuit of a resistor (R4) and a capacitor (C4);
A switching power supply comprising a series circuit with a phototransistor (PT).