JP2000236662A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance efficiency while reducing the size by driving a switching element at a high switching frequency, when the ripple voltage is higher than a threshold and driving it with a low switching frequency, thereby when the voltage is lower switching the switching frequency automatically depending on the load conditions. SOLUTION: Ripple voltage of a primary smoothing capacitor C1 is low, when the load is light, and is increased when the load is heavy. A signal of H level is delivered from an output terminal c, when the ripple voltage detected at the detection terminals a, b of a ripple voltage detection circuit 5 is lower otherwise a signal of L level is delivered. Upon receiving the signal of H level, a switching frequency switching circuit 2 operates a PWM control circuit 1 at a low switching frequency and operates it at a high switching frequency upon receiving the signal of L level. Since the switching frequency can be switched automatically depending on the load conditions, the switching power supply can be reduced in size while enhancing the efficiency.

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 FIG. 4 shows a conventional general switching power supply. In FIG. 4, a switching power supply includes a commercial AC voltage input terminal V _IN and a rectifying bridge diode D1.
A primary smoothing capacitor C1, a transformer T1 having a primary winding N1 and a secondary winding N2, and a primary smoothing capacitor C1.
A switching element Q1 connected in parallel to both ends of a switching element Q1 via a primary winding N1 of a transformer T1;
PWM control circuit 1 connected to the control terminal of
Switching frequency switching circuit 2 connected to control circuit 1
And a load signal terminal S for inputting a load signal indicating a load state of the main unit (load) to the switching frequency switching circuit 2.
A rectifier diode D2 having an anode connected to one end of the secondary winding N2, a secondary smoothing capacitor C2 connected between the cathode of the rectifier diode D2 and the other end of the secondary winding N2, A positive output voltage terminal (+) and a negative output voltage terminal (-) connected between both terminals of the smoothing capacitor C2, respectively, and a voltage detection circuit 3 connected between the output voltage terminals (+) and (-). And an optical coupling element 4 connected between the voltage detection circuit 3 and the PWM control circuit 1.

Here, in order to improve the efficiency of the switching power supply, it is necessary to lower the switching frequency of the switching element Q1 to reduce the switching loss of the switching element Q1. However, if the switching frequency is set to be low even when the main unit is in a heavy load state, there is a problem that the switching power supply becomes large.

In order to solve this problem, in the above conventional example, the switching frequency switching circuit 2 receives a load signal indicating a load state from a load signal terminal S, and changes the switching frequency of the PWM control circuit 1 to a low switching for light load. The frequency is switched to a high switching frequency for heavy loads.

With the above configuration, the switching element Q1 is driven at a high switching frequency under a heavy load to reduce the size of the switching power supply, and conversely, the switching power supply is low under a light load such as a standby mode of the main unit. By driving the switching element Q1 at the switching frequency, it is possible to reduce switching loss and improve efficiency.

[0006]

However, in the configuration of the conventional switching power supply described above, a special signal generating circuit, a signal terminal and a signal line for outputting a load signal indicating a load state from the main unit are required. There was a problem that.

Therefore, an object of the present invention is to reduce the size of the switching power supply by increasing the switching frequency when the load is light, such as in the standby mode of the main unit, without requiring a signal from the main unit and automatically. And to provide a switching power supply.

[0008]

To achieve the above object, a switching power supply according to the present invention comprises a commercial AC voltage input terminal, a rectifying bridge diode connected to the commercial AC voltage input terminal, a primary winding. , A transformer having a secondary winding, a switching element connected from the positive electrode of the rectifying bridge diode to the positive electrode via the primary winding of the transformer, and a control terminal of the switching element. A PWM control circuit for adjusting the on-duty of the switching element; a primary smoothing element having a negative electrode connected to a connection point between a negative electrode of the switching element and a negative electrode of the rectifying bridge diode, and a positive electrode connected to a positive electrode of the rectifying bridge diode; A capacitor, a rectifier diode connected to an anode at one end of the secondary winding, a cathode of the rectifier diode, A secondary smoothing capacitor connected between the other end of the next winding, positive and negative output voltage terminals respectively connected to both terminals of the secondary smoothing capacitor, and a secondary smoothing capacitor connected between the output voltage terminals; A voltage detection circuit, an optical coupling element connected between the voltage detection circuit and the PWM control circuit, and a switching frequency of the PWM control circuit based on a load signal indicating a load state. A frequency switching circuit for switching between a frequency and a high switching frequency for heavy load, and a voltage switching circuit connected to both terminals of the primary smoothing capacitor, detecting a ripple voltage of the primary smoothing capacitor, and detecting a load state according to the magnitude of the ripple voltage. And a ripple voltage detecting circuit for transmitting a load signal indicating the following to the switching frequency switching circuit.

[0009]

The ripple voltage appearing on the primary smoothing capacitor increases in proportion to the load of the main unit. In the above configuration,
A ripple voltage is detected by a ripple voltage detection circuit, and if the ripple voltage is larger than a threshold value, a higher switching frequency,
If the ripple voltage is smaller than the threshold value, a signal is transmitted to the switching frequency switching circuit so as to switch to a lower switching frequency, and the switching element is driven through the PWM control circuit, thereby automatically changing the switching frequency according to the load state. It is possible to switch.

[0010]

FIG. 1 is a drawing which best illustrates the features of the present invention. Hereinafter, description will be made with reference to FIG. The same parts as those in FIG. 4 are denoted by the same reference numerals. Only the differences from the conventional example will be described.

The switching power supply circuit shown in FIG. 1 is different from the general switching power supply circuit shown in FIG. 4 in that a ripple voltage detection circuit is connected to both ends of a primary smoothing capacitor C1 through detection terminals a and b except for a load signal terminal S. 5 are provided. The output terminal c of the ripple voltage detection circuit 5 is connected to the switching frequency switching circuit 2.

The operation of the circuit shown in FIG. 1 will be described. The ripple voltage appearing on the primary smoothing capacitor C1 increases in proportion to the load of the main unit. That is, when the load is light,
The ripple voltage of the primary smoothing capacitor C1 is small, and conversely, when the load is heavy, the ripple voltage of the primary smoothing capacitor C1 increases. When the ripple voltage detected at the detection terminals a and b of the ripple voltage detection circuit 5 is smaller than the threshold value, an H-level signal is output from the output terminal c. On the other hand, when the ripple voltage is larger than the threshold value , And outputs an L-level signal from the output terminal c.

The switching frequency switching circuit 2 operates the PWM control circuit 1 at a low switching frequency when receiving an H level signal, and operates at a high switching frequency when receiving an L level signal. As described above, the switching frequency is automatically switched according to the load state.

FIG. 2 shows the ripple voltage detecting circuit 5 of the circuit shown in FIG.
FIG. In FIG. 2, the ripple voltage detection circuit 5 includes a capacitor C3 connected in series between the detection terminals a and b, resistors R1 and R2 and a resistor R3 connected in series between the detection terminals a and b, a cathode, and an anode. , A diode D3 connected in the order of anode and cathode from the connection point of the resistors R1 and R2, a capacitor C4 connected between the cathode of the diode D3 and the detection terminal b, and a base. Is connected to the cathode of the diode D3, the collector is connected to the cathode of the constant voltage diode ZD1, and the emitter is the detection terminal b.
And an NPN transistor Tr1 connected to the NPN transistor Tr1.

Here, for the sake of explanation, a connection point between the resistors R1 and R2 is referred to as d and a base terminal e of the transistor Tr1. FIG. 3 shows the operation waveform. In FIG. 3, (1)
Is between a and b, (2) is between d and b, (3) is between e and b,
(4) shows the voltage waveform between c and b, respectively. Also,
Indicates a waveform under light load, and indicates a waveform under heavy load.

The operation will be described below with reference to the circuit diagram of FIG. 2 and the operation waveform diagram of FIG. As shown in FIG. 3A, both ends of the primary smoothing capacitor C1 are superimposed on a DC voltage,
A small ripple voltage appears at a light load, and a small ripple voltage appears at a heavy load. FIG. 3B shows a voltage waveform obtained by extracting only the ripple voltage with a high-pass filter including the capacitor C3 and the resistors R1 and R2. Further, the diode D
3. A waveform obtained by rectifying and smoothing this voltage by the capacitor C4 is shown in FIG. Since the voltage between eb in FIG. 3 (3) is smaller than the base-emitter ON voltage Vbe (on) of the transistor Tr1 at light load, the transistor Tr
Reference numeral 1 denotes an off state, and the voltage between c and b becomes the Zener voltage Vzd1 of the constant voltage diode ZD1. At this time, the voltage of the output terminal c is at the H level (FIG. 3 (4)).

On the other hand, at the time of heavy load, the voltage between eb reaches the base-emitter ON voltage of the transistor Tr1 in FIG. 3 (3), the transistor Tr1 is turned on, and both ends of the constant voltage diode ZD1 are connected between the base and emitter of the transistor Tr1. Lower to the saturation voltage Vce (sat). The voltage of the output terminal c at this time is at the L level (FIG. 3 (4))
). In this way, the ripple voltage detection circuit 5 outputs an H level signal from the output terminal c under a light load and an L level signal under a heavy load.

As described above, receiving this output signal,
The switching frequency switching circuit 2 and the PWM control circuit 1 drive the switching element Q1 at a low switching frequency at a light load, and at a high switching frequency at a heavy load.

[0019]

As described above, according to the present invention,
For efficiency and miniaturization, lower the switching frequency under light load conditions and increase the switching frequency under heavy load conditions.Special signal generation circuits, signal terminals, signal lines, etc. It is possible to provide a switching power supply that can be automatically performed simply by adding a simple circuit without providing the switching power supply.

[Brief description of the drawings]

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

FIG. 2 is a diagram illustrating in detail a ripple voltage detection circuit of the switching power supply according to the embodiment of the present invention.

FIG. 3 is a diagram showing operation waveforms of the switching power supply according to the embodiment of the present invention.

FIG. 4 is a diagram showing a switching power supply according to a conventional example of the present invention.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 PWM control circuit 2 switching frequency switching circuit 3 voltage detection circuit 4 optical coupling element 5 ripple voltage detection circuit V _IN commercial AC voltage (+) Positive output voltage terminal (−) Negative output voltage terminal T1 Transformer N1 Primary of transformer T1 Winding N2 Secondary winding of transformer T1 Q1 Switching element D1 Rectifying bridge diode D2 Rectifying diode D3 Diode ZD1 Constant voltage diode C1 Primary smoothing capacitor C2 Secondary smoothing capacitor C3, C4 Capacitor R1, R2, R3 Resistance Tr1 NPN transistor a , B A ripple voltage detection terminal of the ripple voltage detection circuit 5 c A signal output terminal of the ripple voltage detection circuit 5 S Load signal terminal

Claims (2)

[Claims] 1. A commercial AC voltage input terminal, a rectifying bridge diode connected to the commercial AC voltage input terminal, a transformer having a primary winding and a secondary winding, and a positive electrode of the rectifying bridge diode A switching element connected to the positive electrode through the primary winding, a PWM control circuit connected to a control terminal of the switching element, and adjusting an on-duty of the switching element according to a load state; and a negative electrode of the switching element. A primary smoothing capacitor having a negative electrode connected to a connection point of the rectifier bridge diode with the negative electrode and a positive electrode connected to the positive electrode of the rectifier bridge diode; a rectifier diode connected to an anode at one end of the secondary winding; A secondary smoothing capacitor connected between the cathode of the rectifier diode and the other end of the secondary winding; Positive and negative output voltage terminals respectively connected to both terminals of the capacitor, a voltage detection circuit connected between the output voltage terminals, and an optical coupling connected between the voltage detection circuit and the PWM control circuit A switching power supply having a low switching frequency for a light load and a high switching frequency switching circuit for a heavy load based on a load signal indicating a load state, both ends of the primary smoothing capacitor. A ripple voltage detection circuit that detects a ripple voltage of the primary smoothing capacitor and transmits a load signal indicating a load state to the switching frequency switching circuit according to the magnitude of the ripple voltage. Switching power supply. 2. A converter for converting a commercial AC voltage into a DC, a PWM controller for PWM-controlling the DC converted by the converter, and a control result by the PWM controller to a secondary load. A transformer, a switching frequency switching circuit for switching the switching frequency of the PWM control means, and detecting the magnitude of the DC ripple voltage converted by the conversion means, generating a signal for switching the switching frequency from the detection result, the switching frequency A switching power supply, comprising: a ripple voltage detection circuit that transmits a signal generated to a switching circuit.