JP3465491B2 - Switching power supply - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a self-excited flyback converter type switching power supply device, and more particularly to improvement of overcurrent protection means thereof.

[0002]

2. Description of the Related Art A general flyback converter type switching power supply device has an overcurrent protection function because the maximum collector current of its switching transistor depends on the amplification factor h _FE , but it is not enough. There is a problem that the start point of overcurrent protection shifts when the input voltage fluctuates (for example, when the input voltage increases, the start point of overcurrent protection shifts to the larger side).

Therefore, many switching power supply devices have already been proposed which can solve such a problem with a simple circuit. FIG. 4 shows a first conventional example. In FIG. 4, 2 is a transformer, 2a is a primary winding, and 2b.
Is a secondary winding, 2c is a bias winding, 4 is a diode, 6
Is a MOSFET as a switching element, 8 is a resistor,
10 is a time constant circuit, 12 is a resistor, 14 is a capacitor, 1
6 is a capacitor, 18 is a resistor, and 20 is a transistor as a control element.

A MOSFET 6 as a switching element is connected in series to the primary winding 2a of the transformer 2, and the output of the bias winding 2c of the transformer 2 is fed back to the gate of the MOSFET 6 via a capacitor 16 and a resistor 18. There is.

Further, a time constant circuit 10 consisting of a resistor 12 and a capacitor 14 connected in series with each other is connected to both ends of a bias winding 2c of the transformer 2 and MOSFE.
A transistor 20 as a control element is connected in parallel between the gate and the source of T6 so that the voltage of the capacitor 14 is applied to the base of the transistor 20.

The operation will be described below. When the input voltage Vi is applied, the gate voltage is applied to the MOSFET 6 through the starting resistor 8 and the MOSFET 6 becomes conductive. As a result, a voltage is applied to the primary winding 2a of the transformer 2, and at the same time, a voltage Vb is generated in the bias winding 2c. This is the MOSFET through the capacitor 16 and the resistor 18.
Applied to the gate of T6, MOSFET 6 turns on rapidly.

At this time, the voltage of the secondary winding 2b of the transformer 2 is applied in the opposite direction to the diode 4, so that no current flows in the secondary winding 2b and energy is accumulated in the transformer 2. At the same time, a charging current flows through the capacitor 12 that constitutes the time constant circuit 10 through the resistor 12, and the base potential of the transistor 20 gradually rises.

When the potential of the capacitor 14 reaches a predetermined value and the transistor 20 starts to conduct, the gate voltage of the MOSFET 6 decreases and the MOSFET 6 cannot be maintained in the ON state, so that the voltage of the primary winding 2a decreases and the bias voltage increases. Winding 2
The voltage Vb of c also decreases. Since this is positive feedback, M
OSFET 6 turns off rapidly. When the MOSFET 6 is turned off, the energy stored in the transformer 2 is supplied from the secondary winding 2b to the output side through the diode 4.

After that, the voltage Vb of the bias winding 2c becomes zero.
Then, the voltage from the starting resistor 8 again causes the MOSFET
6 is turned on, and the above operation is repeated.

In such a flyback converter system, the output on the secondary side is MO, as is well known.
There is a basic relationship of being proportional to the ON period of the SFET 6.

In this switching power supply device, overcurrent protection is performed by utilizing this in reverse. That is, the ON period of the MOSFET 6 (this is also the time until the transistor 20 in the OFF state is turned ON)
Is defined by the speed at which the voltage of the capacitor 14 rises, that is, the time constant T of the time constant circuit 10 (= R1 · C1, where R1 is the resistance value of the resistor 12 and C1 is the capacitance of the capacitor 14). If the upper limit of the ON period of the MOSFET 6 is determined by the time constant T, a current above the set value is not output to the secondary side (that is, overcurrent protection). For example, if the time constant T is reduced, the transistor 20 turns on quickly and the MOSFET 6 turns off quickly. Therefore, the protection mode is entered with a smaller load current.

In the switching power supply device as described above, since the overcurrent protection starting point is basically determined by the time constant T of the time constant circuit 10, the variation of the overcurrent protection starting point due to the variation of the input voltage Vi is suppressed. be able to.

However, more strictly speaking, since it is a self-excited type, when the input voltage Vi fluctuates, for example, rises, the oscillation frequency rises, and a large amount of output is exerted. Therefore, from this point of view, overcurrent protection still remains. Since the starting point will change, it can be said that there is still room for improvement in such points.

Further, as an example in which the above-mentioned improvement is actually made, there is a switching power supply device described in JP-A-63-87171.

The second conventional example will be described with reference to FIG. FIG. 5A is a circuit diagram of the switching power supply device described in the publication, and FIGS. 5B to 5E are configuration examples of impedances used in the switching power supply device.

In FIG. 5A, the same parts as those in FIG. 4 are designated by the same reference numerals, and the description thereof will be omitted. 16a is a starting diode, 28 is an output voltage control photocoupler,
Reference numeral 11 denotes an impedance circuit, and the impedance circuit 11 discloses configuration examples as shown in FIGS. 17 is a diode, 19 is a diode,
Reference numeral 22 is a resistor, 23a is a resistor, 23b is a resistor, 23c is a resistor, and 24 is a Zener diode.

The starting diode 16a is replaced with the capacitor 16
Since the operation is the same as that of FIG. 4 except that it is connected in parallel with, the description thereof will be omitted. The voltage induced in the bias winding 2c when the switching element 6 is on charges the capacitor 14 via the resistor 12. Further, while the switching element 6 is off, the capacitor 14 is discharged through the impedance circuit 11 contrary to the above operation.
In this publication, the reason why the starting point of the overcurrent protection fluctuates is not analyzed in detail, and it is presumed that it may be because the capacitor 14 is not sufficiently discharged. It is described that the impedance of charging and discharging is changed to correct the variation of the starting point of overcurrent protection.

However, in the configuration example of each impedance circuit shown in FIGS. 5B to 5E, the charging loop is the resistor 1
Since some pass through 2 and some pass through the impedance circuit 11, if one constant (for example, the resistance 12) is set again and then another constant (for example, the impedance circuit 11) is reset, the total current value changes and switching This is not preferable because it interferes with the overcurrent protection operation of the power supply device and the means for setting the starting point of overcurrent protection becomes complicated.

Further, significant improvement effect for the following reasons for the input voltage fluctuation of the overcurrent protection point in the configuration example of FIG. 5 (B) ~ (D) was not observed. Capacitor 1
The discharge of No. 4 is performed by the voltage induced in the bias winding 2c during the off period. This voltage is a flyback voltage in the flyback converter system and is proportional to the output voltage but is proportional to the input voltage. Absent. Therefore, if the output voltage is constant, the residual charge of the capacitor 14 at the end of the off period is almost constant regardless of the input voltage.

Next, the state changes during the ON period and the capacitor 1
The charging of No. 4 starts, but at this time, each charging loop is only a resistor, which is no different from the conventional example of FIG. However, although there is a tendency that the charging time is shortened due to the increase of the oscillation frequency due to the increase of the input voltage, that is, the shortening of the cycle, this phenomenon is equivalent to the first conventional example of FIG.
Furthermore, since charging / discharging is performed via linear impedance, there is no difference from the operation of the conventional example,
Therefore, with respect to the input voltage fluctuation at the overcurrent protection point, almost the same result as that of the conventional example of FIG. 4 was obtained. The effect of FIG. 5E will be described later.

Further, there is a switching power supply device described in Japanese Patent Publication No. 7-114545 as another improvement example of the conventional example of FIG. The third conventional example will be described with reference to FIG. FIG. 6 is a circuit diagram of the switching power supply device described in the above-mentioned publication. In FIG. 6, the same parts as those in FIG. 4 are given the same numbers and their explanations are omitted. 10a
Is a time constant circuit. In the conventional example, in order to correct the input voltage fluctuation at the overcurrent protection point, the time constant circuit is configured by connecting the resistor 12 in parallel to the series circuit of the resistor 22 and the Zener diode 24. Therefore, the nonlinear impedance Z represented by the following equation (1) is obtained,
It is disclosed that the input voltage fluctuation at the overcurrent protection point can be corrected.

Z = Vr / {(Vr-Vz) / R2 + (Vr / R1)} (1) where Vr is a voltage applied to the nonlinear impedance Z, and Vr
z is the Zener voltage of the Zener diode 24, R1 is the resistance value of the resistor 12, and R2 is the resistance value of the resistor 22.

It is possible to correct the input voltage fluctuation at the overcurrent protection point by using such a non-linear impedance with respect to the voltage. From the equation (1), at least with respect to charging of the capacitor 14, a loop passing through R1 (resistor 12) and R2 are connected. It turns out that there is a loop through (resistor 22),
If one constant (for example, the resistor 12) is set and then another constant (for example, the series circuit of the resistor 22 and the Zener diode 24) is reset, the total current value changes, which interferes with the overcurrent protection operation of the switching power supply device. However, it is not preferable like the second conventional example of FIG. 5 because the means for setting the starting point of overcurrent protection becomes complicated.

Further, FIG. 6 is changed to FIGS. 5 (A), (B) to (E).
Comparing with the above, it can be seen that the configuration is equivalent to the case where the component (E) of FIG. 5 is used. Therefore, the effect of FIG.
The effect is similar to that of FIG.

[0025]

As described above, in each conventional example, the overcurrent protection point changes due to the fluctuation of the input voltage, and the charging / discharging loop passes through the resistor 12 and another loop. However, since it affects the action of charging and discharging the capacitor 14, it interferes with the overcurrent protection device of the switching power supply device and the means for setting the starting point of the overcurrent protection becomes complicated, which is not preferable.

An object of the present invention is to eliminate the above-mentioned conventional drawbacks and provide a switching power supply device with stable overcurrent protection.

[0027]

In order to solve the above-mentioned problems, a switching power supply device according to the present invention has a switching element connected in series to a primary winding of a transformer, and an output of a bias winding of the transformer is connected to the switching element. A time constant circuit having a resistor and a capacitor connected in series and defining the maximum value of the ON period of the switching element in the control circuit of the switching element is provided at both ends of the bias winding. A control element for forcibly turning off the switching element according to the output of the circuit is provided, and the resistor forming the time constant circuit is formed of a plurality of resistors connected in series, and one resistor among the resistors is connected in parallel with the resistor. Overcurrent protection by connecting a Zener diode with a polarity that blocks the voltage generated in the resistor during the ON period of the switching element
Is configured to do .

With the above-mentioned structure, the input voltage fluctuation at the overcurrent protection point can be corrected and the charge / discharge loop of the capacitor can be unified with a simple circuit structure.
The charge / discharge characteristics of the capacitor can be set independently of each other, and the starting point of overcurrent protection can be easily set. Furthermore, when the input voltage of the switching power supply unit drops , the voltage induced in the bias winding also decreases, but even if the Zener diode stops conducting, there is a charging / discharging loop due to the resistance, so the control element The operation can be secured, and the overcurrent protection operation of the switching power supply does not become abnormal.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention according to claim 1 of the present invention is such that a switching element is connected in series to a primary winding of a transformer, and an output of a bias winding of the transformer is fed back to a control electrode of the switching element. A time constant circuit having a resistor and a capacitor that define the maximum ON period of the switching element in the switching element control circuit and are connected in series is provided at both ends of the bias winding, and switching is performed by the output of this time constant circuit. A control element for forcibly turning off the element is provided, and a resistor forming the time constant circuit is configured by a plurality of resistors connected in series with each other, and an ON period of the switching element is parallel to one resistor among them. the resistance by connecting a polarity of the zener diode to inhibit a voltage generated in a configuration for performing the overcurrent protection, a simple configuration with overcurrent protection port in Correcting the input voltage variation of the cement, it is possible to unify the charging and discharging loop of the capacitor can be set independently of each other charge-discharge characteristics of the capacitor, in which the starting point of the overcurrent protection can be easily set. In addition, the voltage induced in the bias winding decreases as the input voltage of the switching power supply decreases, but even if the Zener diode stops conducting, there is a charge / discharge loop due to the resistance, so the operation of the control element is ensured. Therefore, the overcurrent protection operation of the switching power supply device does not become abnormal.

According to the invention described in claim 2, a semiconductor element whose conduction is controlled by an input voltage is connected in parallel with one of a plurality of resistors forming the time constant circuit according to claim 1, It is possible to precisely correct the fluctuation of the starting point of overcurrent protection with respect to the input voltage, and since a non-linear element such as a Zener diode is not used,
The change in the starting point of the overcurrent protection with respect to the input voltage can be corrected more smoothly.

An embodiment of the switching power supply device of the present invention will be described below with reference to FIGS. 1, 2 and 3.

(Embodiment 1) Embodiment 1 of the present invention will be described with reference to the circuit diagram of FIG. 1 in FIG.
0b is a time constant circuit, 25 is a Zener diode, 26 is a resistor, and 27 is a resistor. Others are the same as those of the conventional example shown in FIG. To do.

Next, the operation will be described. In FIG. 1, the impedance R that controls the charging of the capacitor 14 is composed of a Zener diode 25, a resistor 26, and a resistor 27, and its value R is expressed by the following equation (2).

R = R27 × Vr / (Vr−Vz25) (2) where R27 is the resistance value of the resistor 27 and Vz25 is the zener voltage of the zener diode 25.

Examining equation (2) and FIG. 1, it can be seen that the loop for charging and discharging the capacitor 14 is only through the resistor 27, and the resistor 26 can be set independently. Also,
In the equation (2), the impedance R is the voltage V as in the equation (1).
It can be seen that the impedance becomes a nonlinear impedance that changes with r (the impedance R decreases in value as Vr increases). Therefore, the switching element 6 is quickly turned off when the input voltage Vi rises, and the input voltage fluctuation at the overcurrent protection point can be corrected.

When the input voltage Vi drops and the voltage Vr drops below the Zener voltage Vz25, the Zener diode 25 becomes non-conductive, but the Zener diode 25
Since the resistor 26 is connected in parallel with, the charging loop for the capacitor 14 at this time passes through the series circuit of the resistor 26 and the resistor 27.

Therefore, since the charging / discharging loop for the capacitor 14 always exists regardless of the change of the input voltage Vi, the operation of the control element 20 is always ensured, and the overcurrent protection operation of the switching power supply device does not become abnormal. . As can be seen from the above description, the resistor 26 is the Zener diode 2
5 exerts its effect only when the Zener diode 25 is in the non-conducting state, so that when the Zener diode 25 is in the conducting state, it has no influence on the overcurrent protection operation, and the constant is determined independently of the resistor 27 and the Zener diode 25. This makes it possible to simplify the means for setting the overcurrent protection point.

Although the overcurrent protection operation has been described above, it goes without saying that a photocoupler 28 may be added so as to control the charging current of the capacitor 14 for normal stabilizing operation.

(Embodiment 2) Embodiment 2 of the present invention will be described with reference to the circuit diagram of FIG. 2A and the impedance configuration examples of FIGS. 2B and 2C. Further, FIG. 3 is a circuit diagram of another embodiment.

Next, the configuration and the operation thereof will be described. Reference numeral 10c is a time constant circuit, 29 is a voltage detection circuit Q for detecting the input voltage Vi, and is configured as follows. 3 is a diode, 30 is a resistor, 31 is a resistor, 32a is a photo diode, 32b is a phototransistor, and 32a and 32b.
A photocoupler is constituted by, and 34 is a transistor.

When the voltage detection circuit Q is configured as shown in FIG. 2B, when a current flows from the input voltage Vi to the resistor 30 and the photodiode 32a is turned on, the phototransistor 32b has (c) to (c) depending on its luminous intensity. The resistance between (d) decreases. Therefore, when the input voltage Vi rises, the resistance between (c) to (d) also decreases together with the increase of the voltage Vr, so that the capacitor 14 is charged in a shorter time and the ON period of the switching element 6 becomes shorter. . Further, since the input voltage Vi is directly detected, it is possible to more accurately correct the variation in the starting point of the overcurrent protection with respect to the input voltage.

Furthermore, since a highly nonlinear element such as a Zener diode is not used, it is possible to smoothly correct fluctuations in the starting point of overcurrent protection with respect to the input voltage. Even when the voltage detection circuit Q is composed of (C), the resistance between (c) and (d) of the transistor 34 decreases in accordance with the increase of the input voltage Vi, so that the ON period of the switching element 6 can be controlled. The diode 3 may be inserted on the emitter side of the transistor 34, or may be omitted if the breakdown voltage of the transistor 34 is not hindered.

Although the input voltage Vi is directly detected in FIG. 2, the voltage Vb generated in the bias winding 2c during the ON period of the switching element 6 is similar to the input voltage Vi. Even if it is configured as shown in FIG. 3, its operation does not change.

In the embodiment of FIG. 3, the input voltage Vi is
Since it is not directly detected, the precision is inferior to that of the embodiment of FIG. 2, but the same precision as that of FIG. 1 can be secured, and since the input voltage Vi is not used, the operation is simpler and safer. Moreover, since a highly nonlinear element such as a Zener diode is not used, the fluctuation correction of the starting point of the overcurrent protection with respect to the input voltage can be smoothly performed.

[0045]

As described above , according to the present invention, it is possible to correct the input voltage fluctuation at the overcurrent protection point and to unify the charging / discharging loop of the capacitor with a simple circuit configuration. The charge / discharge characteristics can be set independently of each other, and the starting point of overcurrent protection can be easily set. In addition, the voltage induced in the bias winding decreases as the input voltage of the switching power supply decreases, but even if the Zener diode stops conducting, there is a charge / discharge loop due to the resistance, so the operation of the control element is ensured. Therefore, the overcurrent protection operation of the switching power supply device does not become abnormal. Alternatively, since the semiconductor element whose conduction is controlled by the input voltage of the switching power supply device is connected in parallel with one of the resistors forming the time constant circuit, the starting point of the overcurrent protection against the input voltage can be more accurately set. It is possible to correct the fluctuations of the input current, and since a non-linear element such as a Zener diode is not used, the fluctuation of the starting point of the overcurrent protection with respect to the input voltage can be corrected more smoothly.

[Brief description of drawings]

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

FIG. 2A is a circuit diagram of another embodiment of the switching power supply device of the present invention, and FIGS. 2B and 2C are circuit diagrams showing an impedance configuration example used in the circuit.

FIG. 3 is a circuit diagram according to another embodiment.

FIG. 4 is a circuit diagram of a conventional switching power supply device.

FIG. 5A is a circuit diagram of another conventional switching power supply device, and FIGS. 5B to 5E are circuit diagrams of impedance configuration examples used in the circuit.

FIG. 6 is a circuit diagram of another conventional switching power supply device.

[Explanation of symbols]

2 transformers 2a Primary winding 2b Secondary winding 2c bias winding 4 diode 6 switching elements 8 resistance 10b time constant circuit 10c time constant circuit 14 Capacitor 16 capacitors 18 Resistance 20 Control element 25 Zener diode 26 Resistance 27 Resistance 29 Voltage detection circuit 30 resistance 31 resistance 32a photo diode 32b phototransistor 34 transistor

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Claims (2)

(57) [Claims] 1. A switching element is connected in series to a primary winding of a transformer, an output of a bias winding of the transformer is fed back to a control electrode of the switching element, and a switching element control circuit has an ON period of the switching element. A time constant circuit having a resistor and a capacitor that define the maximum value of is connected in series to each other is provided at both ends of the bias winding, and a control element that forcibly turns off the switching element by the output of the time constant circuit is provided. A polarity that configures the resistor forming the time constant circuit with a plurality of resistors connected in series with each other, and inhibits the voltage generated in the resistor in parallel with one resistor in the ON period of the switching element. Switching power supply device that connects the Zener diode of the above to provide overcurrent protection . 2. One of a plurality of resistors forming a time constant circuit
The switching power supply device according to claim 1, wherein a semiconductor element whose conduction is controlled by an input voltage is connected in parallel with the number.