JPH0993916A - Switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a switching power supply which reduces the range of fluctuation of the protecting operation starting current against the change of input voltage by connecting an over-current compensating circuit formed of a diode and a resistor between a current control terminal and a feedback coil. SOLUTION: Since an over-current compensating circuit 7 consisting of a series circuit of a diode Do, a resistor Ro and a zener diode Zo is connected between the winding end portion b of a feedback coil N3 and a current control terminal CLM, a current flows toward the feedback coil N3 via the over-current compensating circuit from the current limit terminal CLM and when an input voltage Vin becomes large, potential of the point b becomes further negative, allowing the current flowing to the point b from the current limit terminal CLM to increase. Thereby, the time required to reach the threshold value for stoppage of oscillation is shortened, realizing quick rejection of over-current and initiation of the protecting operation. Therefore, the width of fluctuation of the protecting operation starting current can be reduced.

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 in which fluctuations in load current due to fluctuations in input voltage are suppressed.

[0002]

2. Description of the Related Art Regarding a conventional switching power supply circuit,
This will be described with reference to FIG. In the figure, reference numeral 1 is a primary DC power supply, which is an AC power supply Ac rectified and smoothed by a rectifying diode D1 and a smoothing capacitor C1. T is a transformer, which comprises a primary winding N1, a secondary winding N2, and a feedback winding N3. A primary winding N1, a switching element Q composed of a field effect transistor, and a source resistance R1 are connected in series to both ends of the primary DC power supply 1. This source resistance R
A series circuit of dividing resistors R2 and R3 is connected to both ends of 1. A capacitor C2 is connected in parallel with both ends of the dividing resistor R2. The source resistor R1, the dividing resistors R2 and R3, and the capacitor C2 constitute the overcurrent detection circuit 2.

A series circuit of a rectifying diode D2 and a smoothing capacitor C3 is connected to both ends of the feedback winding N3, which constitute a tertiary DC power supply 3 and supply a voltage to the switching power supply control integrated circuit Ic. There is. This integrated circuit Ic has an oscillation circuit which internally oscillates at several hundred kHz,
It has an oscillation control circuit, an overcurrent protection operation circuit, and the like, and has external terminals such as an oscillation control terminal Vout, a power supply voltage terminal Vcc, a ground terminal GND, a current limiting terminal CLM, and a feedback terminal Fb. Pt is a phototransistor forming a photocoupler, and is a feedback terminal Fb of the integrated circuit Ic and a winding start end of the feedback winding N3 (ground G).
ND). The integrated circuit Ic, the phototransistor Pt, and the switching element Q form the switching circuit 4.

Next, the primary winding N1 side and the feedback winding N
The circuit connection on the 3 side will be described. The oscillation control terminal Vout of the integrated circuit 1c is connected to the gate of the switching element Q. The power supply voltage terminal Vcc is connected to the positive terminal of the primary DC power supply 1 and the positive terminal of the tertiary DC power supply 3 via the current limiting resistor R4. The ground terminal GND is connected to the source of the switching element Q and the negative terminal of the tertiary DC power supply 3, respectively. The current limiting terminal CLM is connected to the connection point of the dividing resistors R2 and R3. Next, the circuit configuration on the secondary winding N2 side will be described. A series circuit of a rectifying diode D3 and a smoothing capacitor C4 is connected to both ends of the secondary winding N2 to form a secondary DC power supply 5. A series circuit of a photodiode Pd and a shunt regulator Sh forming a photocoupler is connected to both ends of the smoothing capacitor C4 so that the photodiode Pd is in the forward direction and the shunt regulator Sh is in the reverse direction. The photodiode Pd constitutes a photocoupler with the phototransistor Pt. In addition, the smoothing capacitor C4
A series circuit of resistors R5 and R6 is connected to both ends of. The reference voltage terminal of the shunt regulator Sh is connected to the connection point of the resistors R5 and R6. These photodiode Pd, shunt regulator Sh,
The resistors R5 and R6 form a constant voltage control circuit 6.

Next, the operation of the above-mentioned conventional switching power supply circuit will be described.

When the variable AC power supply Ac is connected and the input voltage Vin of the primary DC power supply 1 is applied, the primary winding N1,
Switching element Q and source resistance R1 (and resistance R
2, current flows through R3). This source resistance R1
The voltage across is determined by the current flowing through it. The potential at the connection point a between the resistor R2 and the resistor R3 is determined by the charging / discharging time constants of the resistor R2 and the capacitor C2.

On the other hand, when the output voltage on the side of the secondary winding N2 increases, the current is shunted through the series circuit of the photodiode Pd and the shunt regulator Sh so that the connection point between the resistors R5 and R6 has a constant potential. . This shunt current is
A current is passed through the phototransistor Pt by the photocoupler to bring the potential of the feedback terminal Fb of the integrated circuit Ic close to the ground potential, and the oscillation control terminal Vo of the integrated circuit Ic.
The duty of the oscillating frequency of the switching element Q is controlled through ut to operate to lower the voltage. And
The current flowing through the source resistance R1 increases in the negative direction, and the voltage of the source resistance R1 increases in the negative direction,
The potential of the current control terminal CLM connected to the connection point a rises to the minus side and the threshold value (-of the integrated circuit Ic
When it exceeds 0.2 V, the overcurrent protection is activated.

The load circuit on the secondary winding N2 side constitutes a constant voltage / constant power control circuit. When the input voltage Vin fluctuates, the current flowing through the primary winding N1 side is inversely proportional to the input voltage Vin, The current flowing through the resistor R1 also varies in inverse proportion.

On the other hand, since the threshold reaching level is detected by the voltage drop of the current flowing into the current control terminal CLM, the input voltage Vin is the overcurrent which starts the protection operation by this threshold. (Protection operation current) increases linearly as shown by the solid line in FIG. In the figure, when the input voltage Vin is changed from 90V to 300V, the overcurrent (protection operation current) that enters the protection operation is 2
Increased from A to 5A, the fluctuation current width is 3A.
The protection operation current in FIG. 4 means the load current on the secondary winding N2 side at the oscillation stop point of the integrated circuit Ic.

[0010]

However, the conventional switching power supply circuit has a drawback that the fluctuation range of the overcurrent protection operation is large with respect to the fluctuation of the input voltage.

Therefore, an object of the present invention is to provide a switching power supply in which the fluctuation width of the protection operation start current is reduced with respect to the fluctuation of the input voltage.

[0012]

In order to achieve the above object, the present invention provides at least a primary DC power supply (1), a transformer (T) having a primary winding, a secondary winding and a feedback winding, and at least A switching power supply control integrated circuit (Ic) having a power supply voltage terminal, a ground terminal, an oscillation control terminal, a current limiting terminal (CLM), and a feedback terminal, and the transformer (T) connected to the primary DC power supply (1). The output of the primary winding, the switching element (Q) connected to the oscillation control terminal and the overcurrent detection circuit (2), and the secondary winding of the transformer (T) is rectified and smoothed to supply a DC voltage to the load. A secondary direct current power supply (5), a constant voltage control circuit (6) connected to the secondary direct current power supply (5), and an output of the feedback winding to rectify and smooth the direct current to the integrated circuit (Ic). Three supplying voltage A phototransistor (which is photocoupled to the DC power supply (3) and the photodiode of the constant voltage control circuit (6) and is connected between the tertiary DC power supply (3) and the feedback terminal of the integrated circuit (Ic). Pt), an overcurrent correction circuit (7) connected between the feedback winding and the connection point of the current limiting terminal (CLM) and the overcurrent detection circuit (2), and a switching power supply. .

Further, the present invention is the switching power supply according to claim 1, wherein the overcurrent correction circuit (7) comprises a series circuit of a diode and a resistor.

Further, the present invention is the switching power supply according to claim 1, wherein the overcurrent correction circuit (7) comprises a series circuit of a diode, a resistor and a Zener diode.

According to the present invention, in the above-mentioned configuration, by connecting an overcurrent correction circuit including a diode and a resistor between the current control terminal of the integrated circuit for controlling the switching power supply and the feedback winding, switching is performed. The potential of the current control terminal of the integrated circuit for power supply control is quickly pulled to the threshold value to start the protection operation, and the protection operation current due to the fluctuation of the input voltage, that is, the fluctuation width of the load current is reduced.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment circuit of a switching power supply of the present invention will be described below with reference to FIG. Since the circuit of this embodiment uses all the circuits of the conventional example shown in FIG. 6 and an improved circuit is added thereto, the circuit description and the circuit symbols and numbers shown in FIG. 6 are used as they are.

In FIG. 1, a series circuit composed of a diode Do, a resistor Ro and a zener diode Zo is connected between the winding end b of the feedback winding N3 and the current control terminal CLM (connection point a). In this case, the diode Do
Has its cathode, and the Zener diode Zo has its anode facing the winding end end b of the feedback winding N3. The diode Do, the resistor Ro, and the zener diode Zo form the overcurrent correction circuit 7. In some cases, Zener diode Zo
May be excluded to form an overcurrent correction circuit including a series circuit of a diode Do and a resistor Ro.

The voltage / current waveform of the circuit of this embodiment shown in FIG. 1 is shown in FIG. A in the figure is a voltage waveform between the drain and source of the switching element Q, B in the figure is a drain current waveform in the switching element Q, C is a voltage waveform at point a, and D in the figure is b.
The voltage waveform of a point is shown.

In the circuit of this embodiment, a diode Do, a resistor Ro and a zener diode Zo are provided between the winding end b of the feedback winding N3 and the current control terminal CLM (connection point a).
Is connected to the overcurrent correction circuit 7, the current flows from the current limiting terminal CLM through the overcurrent correction circuit 7 toward the feedback winding N3, and when the input voltage Vin becomes large, The potential becomes further negative, and the amount of current flowing from the current limiting terminal CLM (point a) to point b also increases. This means that the amount by which the potential of the current limiting terminal CLM (point a) is pulled in negatively increases, the threshold value for oscillation stop is reached sooner, the overcurrent is blocked, and the protection operation starts earlier. means. FIG. 3 illustrates this fact. That is, the resistance R1 due to the input voltage Vin
The change characteristics of the terminal voltage V _CLM of the current limiting terminal CLM (point a) with respect to the current I _R1 passing through are shown in the solid line in the case of the conventional example, and the broken line x in the case of the present embodiment. At I _R1 , the threshold value of −0.2 V is reached, the overcurrent protection operation is started, and the oscillation is stopped. The broken line x is bent in the middle because of the Zener current of the Zener diode Zo. The broken line Y represents an overcurrent correction circuit including a series circuit of a diode Do and a resistor Ro excluding the Zener diode Zo. Again,
As compared with the conventional example (solid line), the threshold value of -0.2 V is reached earlier. FIG. 4 shows the above operation as a protection operation current characteristic with respect to the input voltage Vin. In the conventional circuit (solid line), the protection operation current monotonically increases as the input voltage Vin increases, but in the broken line x of the present embodiment, the Zener effect causes the protection operation current to change from an increase to a decrease. Thus, the fluctuation width of the protection operation current is smaller than that of the conventional example. That is, while the conventional example is 3A, the present example is 1A. A broken line Y indicates a diode Do and a resistor Ro excluding the Zener diode Zo.
This is the case of the overcurrent correction circuit including the series circuit of. Also in this case, the fluctuation width of the protection operation current is narrower than that of the conventional example.

FIG. 5 shows the above operation by changing the voltage across the resistor R1. The resistor R1 is turned on during the ON period.
It is understood that the negative potential of the voltage between both ends is deeper in the present embodiment (broken line) than in the conventional example (solid line). This is because the current control terminal CLM (connection point a) becomes more negative due to the overcurrent correction circuit 7, and the time to reach the threshold value is shortened.

[0021]

According to the present invention, since the overcurrent correction circuit including a diode and a resistor is connected between the current control terminal and the feedback winding of the switching power supply control integrated circuit, the switching power supply control integrated circuit is integrated. The potential of the current control terminal of the circuit can be quickly pulled to the threshold value to start the protection operation, and the protection operation current due to the fluctuation of the input voltage, that is, the fluctuation width of the load current can be reduced.

[Brief description of drawings]

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

2 shows voltage / current waveforms of the embodiment circuit shown in FIG. 1, where A is a drain-source voltage waveform diagram of the switching element Q, B is a drain current waveform diagram of the switching element Q, and C is an overcurrent. Voltage waveform diagram of the connection point a of the dividing resistor R2 and the dividing resistor R3 of the current detection circuit, D is a voltage waveform diagram of the winding end end a of the feedback winding

FIG. 3 is a characteristic diagram of a current limiting terminal voltage with respect to a current passing through a source resistance in the embodiment circuit shown in FIG.

[Fig. 4] Protection operating current characteristic diagram against input voltage

FIG. 5: Characteristic diagram of voltage across source resistance

FIG. 6 is a conventional switching power supply circuit diagram.

[Explanation of symbols]

1 primary DC power supply 2 overcurrent detection circuit 3 tertiary DC power supply 4 switching circuit 5 secondary DC power supply 6 constant voltage control circuit 7 overcurrent correction circuit AC AC power supply D1, D2, D3 diode Q switching element Sh shunt regulator C1, C2, C3, C4 Capacitor T Transformer N1 Primary winding N2 Secondary winding N3 Feedback winding Ic Switching power supply control integrated circuit Vout Oscillation control terminal Vcc Power supply voltage terminal GND Ground CLM Current limiting terminal Fb Footback terminal Pt Phototransistor Pd Photodiode

Claims (3)

[Claims] 1. A primary DC power supply (1), a transformer (T) having a primary winding, a secondary winding and a feedback winding, and at least a power supply voltage terminal, a ground terminal, an oscillation control terminal, a current limiting terminal (CLM). ), A switching power supply control integrated circuit (Ic) having a feedback terminal, a primary winding of the transformer (T) connected to the primary DC power supply (1), and a switching element (Q) connected to the oscillation control terminal. ) And an overcurrent detection circuit (2), a secondary DC power supply (5) for rectifying and smoothing the output of the secondary winding of the transformer (T) and supplying a DC voltage to a load, and the secondary DC power supply ( 5) a constant voltage control circuit (6), a tertiary DC power supply (3) for rectifying and smoothing the output of the feedback winding to supply a DC voltage to the integrated circuit (Ic), and the constant voltage control Photo of circuit (6) A phototransistor (Pt) photocoupled to the diode and connected between the tertiary DC power supply (3) and the feedback terminal of the integrated circuit (Ic), the current limiting terminal (CLM) and the overcurrent detection circuit. A switching power supply comprising an overcurrent correction circuit (7) connected between the connection point (2) and the feedback winding. 2. The switching power supply according to claim 1, wherein the overcurrent correction circuit (7) comprises a series circuit of a diode and a resistor. 3. The switching power supply according to claim 1, wherein the overcurrent correction circuit (7) comprises a series circuit of a diode, a resistor and a Zener diode.