JP2000232778A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the output characteristics of a flyback-system switching power supply. SOLUTION: For a control circuit 40 of a switching power supply, a diode 41, a resistor 42, a capacitor 43, a resistor 44, a transistor 45, a transistor 46, a resistor 47, and a resistor 48 are added other than a conventional control circuit 10 and a capacitor 10a. When the switching power supply results in a further excessive load current, as compared with a drooping characteristic region, a cut-off current characteristic is provided, thus preventing intermittent operation and damage in the power supply that have occurred conventionally.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flyback type switching power supply.

[0002]

2. Description of the Related Art FIG. 3 is a circuit diagram showing a conventional example of this type of flyback type switching power supply. FIG.
Wherein 1 is a rectifier for rectifying alternating current such as commercial power,
2, a capacitor for smoothing the rectified voltage of the rectifier 1; 3, a transformer comprising a primary winding 3a, a secondary winding 3b, and a tertiary winding 3c; 4, a polarity of the primary winding 3a of the transformer 3 as shown; A power MOSFET having a drain terminal connected to its side, 5 is a diode for rectifying a voltage of the polarity shown in the secondary winding 3b of the transformer 3, 6 is a capacitor for smoothing the rectified voltage via the diode 5, and 7 is A feedback circuit comprising a shunt regulator 8 and a diode side 9a of a photocoupler 9 according to a well-known technique, 10 is a current mode control circuit (hereinafter simply referred to as a control circuit 10), and 10a is connected to a terminal CS of the control circuit 10 as shown in the figure. A capacitor that is connected and constitutes a part of the control circuit 10.

The feedback circuit 7 outputs an operation amount necessary for adjusting the voltage between both ends of the capacitor 6 to V _O based on a set value of the shunt regulator 8 as a feedback signal. Is transmitted to the terminal FB of the control circuit 10 through the transistor side 9b. Here, capacitor 1
Reference numeral 1 denotes a noise filter for the feedback signal.

The terminal RS of the control circuit 10 has a power M
Connected to the source terminal of OSFET4, power MOSF
A current detection resistor 12 is connected between the source terminal of ET4 and the illustrated ground potential.

Further, the voltage of the illustrated polarity of the tertiary winding 3 c of the transformer 3 is rectified by the diode 13, and the rectified value is smoothed by the resistor 14 and the capacitor 15 to obtain a value obtained by the control circuit 1.
0 power is applied between the terminal VCC of the control circuit 10 and the terminal GND of the control circuit 10 connected to the illustrated ground potential. Note that the resistor 16 from the positive electrode side of the rectifier 1 is a resistor required at the time of starting described later.

FIG. 4 is a detailed circuit configuration diagram of the control circuit 10 shown in FIG. In this control circuit 10, the power M
A trigger signal from the oscillator 21 for setting a cycle obtained by adding the on-period and the off-period of the OSFET 4 is sent to the input terminal S of the flip-flop element 23, and a trigger signal from the comparator 22 described later is transmitted to the flip-flop element 23.
To the input terminal R. The output from the output terminal Q of the flip-flop element 23 turns on / off the power MOSFET 4 via the gate drive element 31.

Further, the feedback signal is input to the comparator 22 via the transistor side 9b of the photocoupler 9 and the resistor 25. Here, the other end of the resistor 25 is connected to a voltage stabilized by the VCC voltage via the regulator circuit 24. The output voltage of the regulator circuit 24 is supplied to the comparator 22 by the resistor 2.
6 and the resistor 27, the voltage across the current detection resistor 12 through a filter comprising a resistor 28 and a capacitor 29, and a capacitor 10a connected to an output of a soft start circuit 30 described later. Value is input.

The operation of the flyback switching power supply shown in FIG. 3 will be described below with reference to the detailed circuit diagram of the control circuit 10 shown in FIG. A tertiary winding 3c is provided in the transformer 3 as a control power supply for the control circuit 10, and a voltage generated in the tertiary winding 3c is applied to a diode 13, a resistor 14,
The rectified value is supplied via the capacitor 15 and the rectified value is supplied.

However, since the rectified value is not generated unless the switching power supply starts operating, power is supplied from the positive electrode of the rectifier 1 to the control / protection circuit 10 via the resistor 16 at the time of startup. When the switching power supply is started, power is supplied from the tertiary winding 3c to the control circuit 10, and since the control circuit 10 has a standby function, the resistance is set so that the standby current immediately before the start is minimized. Sixteen resistance values are selected.

Further, immediately before the switching power supply is started, the voltage across the capacitor 6 is zero. Therefore, the feedback signal also has a maximum value in the direction of increasing the voltage across the capacitor 6. When started in this state, the switching operation starts with the ON period of the power MOSFET 4 being maximum, and as a result, the power MOSFET 4
There is a possibility that an excessive peak current will flow in No. 4 to cause damage.

Therefore, in the soft start circuit 30 formed by a comparator, a current source, and the like, the switching power supply is started and the VCC rises to some extent (for example, until the VCC rises to 50% to 70% of the normal value). The voltage across the capacitor 10a input to the comparator 22 is adjusted so that the on-period of the power MOSFET 4 is gradually expanded (soft-started) from the minimum value (≒ 0). The voltage across the capacitor 10a becomes zero when the soft start is completed.

In a normal operating state of the switching power supply, a load (not shown) having a resistance value R
_In order to set the voltage supplied to _L ) to a desired value V _O , the control circuit 10 controls the feedback signal from the feedback circuit 7 shown in FIG. 3, the voltage divided by the resistors 26 and 27, and the current detection. The comparator 22 compares the voltage between both ends of the resistor 12 with a value passed through a filter including a resistor 28 and a capacitor 29, and turns on the power MOSFET 4 via the flip-flop 23 and the gate drive element 31 based on the result of the calculation. The period and the off period are controlled.

[0013]

FIG. 5 is an explanatory diagram showing the output characteristics of the conventional switching power supply shown in FIG. 3, and shows a constant voltage region in which the load current ranges from zero to the rated current _IO (see FIG. 5). ), The load voltage becomes the voltage V _O across the capacitor 6. Further, in the drooping characteristic region where the load current exceeds the rated current I _O , as shown in FIG. 5, the voltage across the current detection resistor 12 in the control circuit 10 is reduced so that the load current decreases as the load current increases. The on-period and the off-period of the power MOSFET 4 are controlled based on a value passed through a filter including a resistor 28 and a capacitor 29.

However, when the load current further increases, the load voltage also tends to decrease. At this time, the tertiary winding 3c of the transformer 3 connects the diode 13 and the resistor 1 to each other.
4, the rectified value (= VCC) via the capacitor 15 is also reduced, and as a result, the power MOSFET is controlled by the soft start circuit 30 of the control circuit 10 and the capacitor 10a.
The operation of gradually increasing the ON period of T4 from the minimum value (≒ 0) is performed. For example, when the load is short-circuited (load resistance value R _L ≒ 0) due to some factor, the soft start circuit 30 is activated. There is a problem that the switching power supply shown in FIG. An object of the present invention is to provide a switching power supply that solves the above-mentioned problems.

[0015]

According to a first aspect of the present invention, there is provided a transformer comprising a primary winding, a secondary winding and a tertiary winding which are flyback-coupled to the primary winding; Power MOSFET with drain terminal connected to one end of wire
A current detection resistor having one end connected to a source terminal of the power MOSFET; a primary DC power supply connected between the other end of the primary winding and the other end of the current detection resistor; A feedback circuit for transmitting a rectified value of the voltage of the line as a feedback signal; a voltage of the tertiary winding as a power source; and setting the rectified value of the secondary winding to a predetermined value ( _VO ) based on the feedback signal. While controlling
A function of transmitting a gate signal having a drooping characteristic to a load supplied from the secondary winding to the power MOSFET when a voltage across the detection resistor exceeds a predetermined value; A control circuit having a function of soft-starting the operation at the start of the operation, the control circuit,
A function of monitoring a voltage of the tertiary winding and limiting a current of the load when the voltage drops below a predetermined value is added.

According to a second aspect of the present invention, in the first aspect, the monitoring of the voltage of the tertiary winding is stopped when the voltage of the primary DC power supply is lower than a predetermined value. According to the present invention, by adding a function of monitoring the voltage of the tertiary winding after the switching power supply is completely started and limiting the current of the load when the voltage drops below a predetermined value. In addition, the above-described intermittent operation when the load of the switching power supply is overloaded (for example, when the load is short-circuited) can be prevented.

[0017]

FIG. 1 is a circuit diagram of a flyback type switching power supply according to an embodiment of the present invention, in which components having the same functions as those of the conventional circuit shown in FIG. It is attached.

That is, the switching power supply shown in FIG. 1 includes a rectifier 1, a capacitor 2, a transformer 3, and a power MOSF.
ET 4, diode 5, capacitor 6, feedback circuit 7, shunt regulator 8, photocoupler 9, capacitor 11, current detection resistor 12, diode 13, resistor 14, capacitor 15, resistor 16, and control circuit 40
A control circuit 10, a capacitor 10a, a diode 41, a resistor 42, a capacitor 43, a resistor 44, a transistor 45, a transistor 46, a resistor 47, and a resistor 48.

The operation of the flyback type switching power supply shown in FIG. 1 will be described with reference to the explanatory diagram of the output characteristics of the switching power supply shown in FIG. 2 and the detailed circuit configuration diagram of the control circuit 10 shown in FIG. This will be described below.

First, an AC voltage is applied to the illustrated input,
When the switching power supply starts to start, the voltage between both ends of the capacitor 2 becomes a predetermined value (for example, 80% of the normal value).
Until the voltage rises to about), the NPN transistor in which the voltage obtained by dividing the voltage between both ends of the capacitor 2 by the resistors 47 and 48 is applied to the base terminal is in a non-conductive state. Then, the soft start circuit 30 of the control circuit 10 and the capacitor 10a perform an operation of gradually increasing the on-period of the power MOSFET 4 from the minimum value (≒ 0), thereby completing the startup.

In the normal operation state of this switching power supply, the voltage between both ends of the capacitor 10a is zero as described above.
An NPN transistor in which a voltage obtained by dividing the voltage between both ends of the capacitor 2 by a resistor 47 and a resistor 48 is applied to a base terminal is in a conductive state, but the voltage generated by the tertiary winding 3c is reduced by a diode 41, a resistor 42, By applying the rectified voltage via the capacitor 43 to the base terminal of the PNP transistor 45 via the resistor 44, the transistor 45 is in a reverse bias state and is non-conductive.
Therefore, like the switching power supply of the conventional circuit, FIG.
As shown in the figure, the operation is performed in the constant voltage region or the drooping characteristic region.

Next, while the switching power supply is operating in the drooping characteristic region, if the load current further increases, the load voltage also tends to decrease. At this time, the tertiary winding 3c of the transformer 3 switches the diode 13, the resistor 14 , The rectified value (= VCC) through the capacitor 15 is also reduced.
The operation of gradually increasing the on-period of the power MOSFET 4 from the minimum value (≒ 0) by the soft start circuit 30 and the capacitor 10a of the control circuit 10 described above is attempted. When the relationship between the voltage at both ends and the rectified voltage of the voltage generated by the tertiary winding 3c via the diode 41, the resistor 42, and the capacitor 43 is smaller than the rectified voltage <the voltage at both ends of the capacitor 10a, the transistor 45 is turned on. As a result, the transistor 45 and the transistor 46 form a discharge circuit of the voltage across the capacitor 10a, and the rectified voltage ≒ the voltage across the capacitor 10a.

Therefore, in this state, the switching power supply shown in FIG. 1 operates in the current limiting region as shown in FIG. 2, the load current is limited to a substantially constant value, and the load voltage is The voltage corresponds to the resistance value _{RL of the} load.

In the circuit configuration shown in FIG.
The OSFET 4 and the current detection resistor 12 for detecting the current of the power MOSFET 4 are connected to the source terminal. A power MOSFET with a current sense terminal is connected between the current sense terminal and the source terminal of the power MOSFET. It may be replaced with a current detection resistor.

[0025]

According to the present invention, the function of monitoring the voltage of the tertiary winding after the switching power supply is completely started and limiting the current of the load when the voltage drops below a predetermined value is provided. With this addition, the switching power supply has current-limiting characteristics, and is prevented from being damaged even when the load is overloaded (for example, when the load is short-circuited).

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of a flyback type switching power supply showing an embodiment of the present invention.

FIG. 2 is a characteristic diagram for explaining the operation of FIG. 1;

FIG. 3 is a circuit configuration diagram of a flyback type switching power supply showing a conventional example.

FIG. 4 is a partial detailed circuit configuration diagram of FIG. 3;

FIG. 5 is a characteristic diagram illustrating the operation of FIG. 3;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Rectifier, 2 ... Capacitor, 3 ... Transformer, 4 ... Power MOSFET, 5 ... Diode, 6 ... Capacitor, 7 ...
Feedback circuit, 8 ... Shunt regulator, 9 ...
Photocoupler, 10 control circuit, 10a, 11 capacitor, 12 current detection resistor, 13 diode, 14 resistor 14, 15 capacitor, 16 resistor, 21 oscillator, 22 comparator, 23 flip-flop element, 24: regulator circuit, 25 to 28: resistor, 29
... capacitor, 30 soft start circuit, 31 gate drive element, 40 control circuit, 41 diode, 42
... Resistance, 43 ... Condenser, 44 ... Resistance, 45, 46 ...
Transistors, 47, 48 ... resistors.

Claims (2)

[Claims] 1. A transformer comprising a primary winding, a secondary winding and a tertiary winding flyback-coupled to the primary winding, and a power having a drain terminal connected to one end of the primary winding. A MOSFET; a current detection resistor having one end connected to a source terminal of the power MOSFET; a primary DC power supply connected between the other end of the primary winding and the other end of the current detection resistor; A feedback circuit that sends a rectified value of the voltage of the winding as a feedback signal; a voltage of the tertiary winding as a power source; and a rectified value of the secondary winding based on the feedback signal and a predetermined value ( _VO ).
When the voltage across the detection resistor exceeds a predetermined value, a gate signal having a drooping characteristic to a load supplied from the secondary winding is supplied to the power MO.
In a switching power supply including a control circuit having a function of sending the signal to an SFET and a function of soft-starting when the power MOSFET starts a switching operation, the control circuit monitors a voltage of the tertiary winding and controls the voltage. A switching power supply characterized by adding a function of limiting the current of the load when the voltage drops below a predetermined value. 2. The switching power supply according to claim 1, wherein when the voltage of the primary side DC power supply is lower than a predetermined value,
A switching power supply, wherein monitoring of the voltage of the tertiary winding is stopped.