JP2000032749A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an RCC(Ringing Choke Converter) type switching power supply which automatically operates without requiring a signal from the apparatus, by changing the switching frequency during a light loading and a heavier loading. SOLUTION: The switching apparatus is provided with an oscillation frequency control circuit 5 including a second control element Tr2 for controlling the switching frequency of a switching element Q1 not to exceed the predetermined frequency, and an operation switching circuit for starting the operation of the oscillation frequency control circuit during the light load period, and for stopping the operation of the oscillation frequency control circuit 5 during the heavier load period. Operation of the oscillation frequency control circuit 5 is automatically switched without requiring a light load signal and a heavy load signal from the load.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an RCC (ringing choke converter) type switching power supply.

[0002]

2. Description of the Related Art An RCC type switching power supply is different from a separately-excited type switching power supply in that an expensive control element such as a power supply control IC is not used. However, the conventional switching power supply of the RCC system has a disadvantage that the oscillation frequency increases at light load and the efficiency deteriorates.

Therefore, an RCC type switching power supply as shown in FIG. 3 has been proposed. The circuit shown in FIG.
Primary winding Np1, secondary winding Ns, feedback winding Np2, Np
3, a switching element Q1 connected in series with the primary main winding Np1 to a primary DC power supply, a control circuit 7 connected to a control terminal of the switching element Q1 and a feedback winding, A switching frequency limiting circuit 8 connected to the circuit 7, the feedback windings Np2, Np3, and the phototransistor PT2; a voltage detecting circuit 9 for detecting an output voltage obtained by smoothing the output of the secondary output winding Ns; Dummy resistor R for stabilizing constant voltage control
With d, a normal RCC oscillation operation is performed.

Further, the following configuration prevents the efficiency from being reduced at light load. The plus of the secondary output terminal is connected to the collector of an NPN transistor Tr6 via a dummy resistor Rd, and the emitter of the transistor Tr6 is connected to the ground GND. Transistor Tr
The base of 6 is connected to a connection point of resistors R21 and R22 connected in series, one end of which is connected to ground GND.

On the other hand, a resistor R23 and a photodiode PD2 are connected between the plus side of the secondary output terminal and the ground GND. This resistor R23 and the photodiode P
One end of the resistor R22 is connected to the connection point of D2, and the collector of the transistor Tr7 is connected. The emitter of the transistor Tr7 is connected to the ground GND, and the base is connected to the connection point of the series-connected resistors R24 and R25, one end of which is connected to the ground GND. The other end of the resistor R25 is a signal terminal S.
The photodiode PD2 is photocoupled with the phototransistor PT2 connected to the switching frequency limiting circuit 8.

With such a configuration, the following operation is performed. When a standby signal is applied to the signal terminal S from the main body, the transistor Tr7 becomes conductive and the photodiode P
D2 is short-circuited, and the phototransistor PT2 is turned off. When the phototransistor PT2 is turned off, the switching frequency limiting circuit 8 performs an operation of limiting the switching frequency of the switching element Q1 via the control circuit 7, and keeps the switching frequency low.

[0007]

However, in the above-described conventional switching power supply, when the operation of the switching power supply is switched according to the load state of the main body, the main body transmits a signal indicating the load state. . Therefore, there is a problem that a special circuit, a signal line, and a signal terminal for outputting a signal indicating the load state to the main body side are required.

It is an object of the present invention to reduce the switching frequency of the switching element to improve the efficiency when the load is light such as when the main body is on standby, and to reduce the ripple noise of the output voltage by increasing the switching frequency when the load is heavy. Automatically without the need for a signal from the main unit
It is to provide a CC type switching power supply.

[0009]

In order to achieve the above object, the present invention provides a transformer having a primary main winding (Np1), a secondary output winding (Ns) and feedback windings (Np2, Np3). (T) and the primary main winding (Np
1) and the control terminal is connected to the feedback winding (Np
A switching element (Q1) connected to one end of 2);
A first control element (Tr1) connected to a control terminal of the switching element (Q1); and a first control element (T1).
r1) an output stabilization circuit (2) and an overcurrent protection circuit (3) connected between the control terminal of the feedback winding (Np2) and the output winding of the secondary output winding. In an RCC type switching power supply including the obtained secondary DC power supply and an output voltage detection circuit (4) connected to the secondary DC power supply, the switching element (Q
An oscillation frequency suppression circuit (5) having a second control element (Tr2) for controlling the switching frequency of 1) so as not to be higher than a predetermined frequency; and starting the operation of the oscillation frequency suppression circuit at light load. An operation switching circuit (6) for stopping the operation of the oscillation frequency suppression circuit (5) at the time of heavy load, so that the oscillation frequency suppression circuit does not need a light load signal and a heavy load signal from the load. The operation switching of (5) is automatically performed.

In the above configuration, when the load is light such as when the main body is in a standby state, the switching frequency of the switching element is reduced to improve the efficiency, and when the load is heavy, the switching frequency is increased to reduce the ripple noise of the output voltage. It is possible to provide an RCC switching power supply that automatically performs a signal indicating a load state from the main body without requiring the signal.

[0011]

(Embodiment 1) FIG. 1 is a circuit diagram of a switching power supply according to Embodiment 1 of the present invention. The same parts as those in FIG. 3 are denoted by the same reference numerals.

First, the oscillation frequency suppression circuit 5 will be described. The NPN-type control transistor Tr2 has an emitter connected to the ground GND and a collector connected from the cathode of the diode D5 to the gate of the switching element Q1 via the anode.

A Zener diode ZD2 and a resistor R are connected between the collector of the transistor Tr3 and the ground GND.
13 and a series circuit of a resistor R12 and a capacitor C6.
, And a parallel circuit with the resistor 14. This parallel circuit forms a time constant circuit. The connection point between the resistors R12 and R13 is connected to the base of the transistor Tr2. The base of the transistor Tr3 is connected from the cathode to the anode of the Zener diode ZD3 and the resistor R1.
5 and the collector of the transistor Tr4 through the emitter to the ground GND. A bias resistor 16 is connected between the emitter and the base of the transistor Tr3. Further, the transistor Tr
The emitter No. 3 is connected to the winding end of the feedback winding Np3 from the cathode to the anode of the diode D6.

A bias resistor R17 is connected between the base of the transistor Tr4 and the ground GND. Furthermore, the base of the transistor Tr4 is connected from the anode of the Zener diode ZD4 to the cathode of the diode D4 via the cathode and the resistor R18. The circuit of the transistors Tr3 and Tr4 forms a charging circuit for the bias capacitor C6 of the transistor Tr2.

Next, the circuit configuration of the operation switching circuit 6 for starting or stopping the operation of the oscillation frequency suppression circuit 5 will be described. The winding end of the feedback winding Np3 is connected to a diode D7.
Are connected to the ground GND via an anode to a cathode and a capacitor C7. Further, the diode D7
Is connected to ground GND via bias resistors R19 and R20. The base of the transistor Tr5 is connected to the connection point between the resistors 19 and R20. And the collector of the transistor Tr5 is
It is connected to the base of the transistor Tr2 of the oscillation frequency suppression circuit 5.

The circuit of this embodiment has the above-described configuration. Next, the operation of the circuit of this embodiment will be described. Primary main winding Np1, secondary output winding Ns and feedback winding Np
2. A normal RCC switching power supply circuit including a switching element Q1, a transistor Tr1, an output voltage detection circuit 4, and the like performs a normal RCC oscillation operation. In this normal RCC oscillation operation, when the input power (output power, load) is large, the switching frequency of the switching element Q1 is small, but when the input power (output power, load) is small, the oscillation frequency is large.

Next, the characteristics when the oscillation frequency suppression circuit 5 is added to the ordinary RCC circuit will be described.
Now, consider a case where the input power (output power, load) is reduced and the switching element Q1 is turned on. A positive voltage is generated at the winding start end of the feedback winding Np2. Due to this positive voltage, current flows through the diode D4, the resistor R18, the Zener diode D4, and the resistor R17, and the transistor Tr4 is turned on. However, the feedback winding N
Since the winding end of p3 is negative, no current flows through the transistors Tr3 and Tr4.

Next, it is assumed that the switching element Q1 is turned off by turning on the transistor Tr1 as described above. Then, the winding start end of the feedback winding Np2 becomes negative, a reverse bias is applied to the base of the transistor Tr4, and the transistor Tr4 turns off. However, there is a time delay before the transistor Tr4 is completely turned off, and the winding end of the feedback winding Np3 is positive.
Momentarily, the capacitor C6 is charged. This charging voltage turns on the transistor Tr2 and maintains the off state of the switching element Q1.

Maintaining the OFF state of the switching element Q1 extends the time until the next ON, which means that the oscillation cycle is lengthened and the oscillation frequency is lowered. That is, after the switching element Q1 is turned off by the transistor Tr1, the off time is extended by the transistor Tr2, and the oscillation frequency is suppressed low. When this oscillation frequency suppression circuit 5 operates,
As described above, the oscillation frequency is kept low, and the efficiency is improved when the input power is small, such as during standby, that is, when the load is light.

Next, a case where the operation of the oscillation frequency suppression circuit 5 is stopped will be described. Primary winding Np of transformer T
1 is not tightly coupled to the output winding Ns. That is, the primary winding Np1 has a slight leakage inductance.
At the moment when the switching element Q1 is turned off, a back electromotive force is generated in the leakage inductance, and the potential at the winding end of the primary winding Np1 rises. Therefore, the primary winding Np
The potential at the end of the winding of the feedback winding Np3 coupled to 1 also increases. The voltage rise value at that time is the switching element Q
It is proportional to the value of the current flowing through the primary winding Np1 at the moment when 1 is turned off. In other words, the feedback winding Np
The voltage at the winding end of No. 3 increases.

Under heavy load, the diode D of the operation switching circuit 6
The voltage of the rectifying and smoothing circuit constituted by the capacitor 7 and the capacitor C7, that is, the DC voltage of the cathode of the diode D7 with respect to the ground GND increases. Then, the resistor R19,
Due to the voltage division of R20, a bias is applied between the base and the emitter of the transistor Tr5, so that the transistor Tr5 is turned on and the transistor Tr2 is turned off. When the transistor Tr2 is turned off, the operation of keeping the oscillation frequency low cannot be performed, the operation of the oscillation frequency suppression circuit 5 stops, and the operation returns to the normal RCC operation.

The operation switching circuit 6 in the circuit of the present embodiment
The function of automatically stopping the operation of the oscillation frequency suppression circuit 5 at the time of heavy load, and shifting from a light load such as standby to a heavy load. Conversely, when shifting from a heavy load to a light load, the voltage at the winding end of the feedback winding Np3 decreases, the transistor Tr5 is turned off, and the transistor Tr2 can be turned on. Automatically switches to an operable state.

(Other Embodiments) Next, as another embodiment,
A modification of the collector connection of the transistor Tr5 of the operation switching circuit 6 will be described. FIG. 2 shows a transistor Tr5
Is connected to the collector of the transistor Tr3. When the load is heavy, the transistor Tr5 is turned on and the transistor T5 charges the bias capacitor C6.
The charging circuits of r3 and Tr4 are short-circuited, the transistor Tr2 of the oscillation frequency suppression circuit 5 is turned off, and the normal RC
C oscillation operation is performed. That is, the same effects as those of the first embodiment shown in FIG. 1 can be obtained by the configuration shown in FIG. 2 described here.

[0024]

As described above, the present invention reduces the switching frequency of the switching element to improve the efficiency during a light load such as when the main body is in standby, and increases the switching frequency during a heavy load to increase the output voltage. It is possible to provide an RCC type switching power supply that automatically reduces the ripple noise without requiring a signal from the main body.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram of an RCC switching power supply according to another embodiment of the present invention.

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

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 oscillation stabilization circuit 2 output stabilization circuit 3 overcurrent protection circuit 4 output voltage detection circuit 5 oscillation frequency suppression circuit 6 operation switching circuit 7 control circuit 8 switching frequency limiting circuit 9 output voltage detection path C1 to C7 capacitors D1 to D7 rectification Diode Np1 Primary winding of transformer Np2 Feedback winding of transformer Np3 Feedback winding of transformer Ns Secondary winding of transformer PD1, LPD2 Photodiode PT1, PT2 Phototransistor Q1 Switching elements R1 to R25 Resistance element S Signal terminal Sr Shunt regulator T transformer Tr1 to Tr7 Transistor ZD1 to ZD4 Zener diode

Claims (4)

[Claims] 1. A primary main winding (Np1) and a secondary output winding (Np1).
s) and a transformer (T) having feedback windings (Np2, Np3), and the primary main winding (Np1) connected to a primary DC power supply.
A switching element (Q1) having a control terminal connected to one end of the feedback winding (Np2) and a first control element (Tr1) connected to a control terminal of the switching element (Q1). , The first control element (Tr1)
Output stabilization circuit (2) and overcurrent protection circuit (3) connected between the control terminal of the first stage and the feedback winding (Np2).
An RCC type switching comprising: a secondary DC power supply obtained by rectifying and smoothing the output of the secondary output winding; and an output voltage detection circuit (4) connected to the secondary DC power supply. In the power supply, a second control is performed so that the primary winding (Np1) is not tightly coupled to the output winding (Ns) and the switching frequency of the switching element (Q1) is not higher than a predetermined frequency. Oscillation frequency suppression circuit (5) having a control element (Tr2) and operation switching for starting operation of the oscillation frequency suppression circuit under light load or stopping operation of the oscillation frequency suppression circuit (5) under heavy load A switching circuit for automatically switching the operation of the oscillation frequency suppression circuit without requiring a light load signal and a heavy load signal from a load. Ching power supply. 2. A primary main winding (Np1) and a secondary output winding (Np1).
s) and a transformer (T) having feedback windings (Np2, Np3); and a transformer connected to the primary main winding (Np1) so as to control on / off of a current flowing through the primary main winding (Np1) and oscillating. Switching element that turns on and off as part of the circuit (Q
1) and a load connected to a control terminal for controlling on / off of the switching element (Q1) and according to the amount of current flowing through the primary main winding (Np1) (that is, a load applied to the secondary output winding (Ns)). A second switching element (Tr2) that is part of an oscillation frequency suppression circuit (5) that delays the off period of the switching element (Q1) (depending on the size of the switching element (Q1));
An operation switching circuit (6) for switching the operation of the oscillation frequency suppression circuit (5) by turning on / off the second switching element (Tr2) of the oscillation frequency suppression circuit (5) according to the amount of current flowing through p1). A switching power supply, comprising: 3. The switching power supply according to claim 2, wherein the operation switching circuit (6) is a feedback winding (Np3) generated when a current flowing through the primary main winding (Np1) is switched.
And a third switching element (Tr5) that is turned on or remains off in accordance with the amount of voltage (i.e., in accordance with the amount of current flowing through the primary main winding (Np1)). A switching power supply characterized in that the operation of the oscillation frequency suppression circuit (5) is switched by turning on). 4. The switching power supply according to claim 3, wherein the voltage generated in the feedback winding (Np3) is due to a leakage inductance due to a coarse coupling between the primary main winding (Np1) and the secondary output winding (Ns). A switching power supply characterized in that: