JPH1056732A - Rush current limiting circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the efficiency of a rush current limiting circuit for switching regulator. SOLUTION: When a switch 23 is closed, a capacitor 26 is charged from a rectifier 24 through a resistor 25. When the capacitor 26 is charged, NMOSs 28 and 35 are maintained in turned-off states. Thin a drive circuit 29 starts the switching of the NMOS 28 and, when the NMOS 28 is maintained in a turned-on state, exciting energy is stored in a transformer 27. When the NMOS 28 is turned off, the transformer 27 discharges the exciting energy and a flyback voltage is generated in a winding 27a. Therefore, the voltage S28 at the drain D of the NMOS 28 becomes the sum of the voltage V26 of the capacitor 26 and the flyback voltage VF. A voltage having the equal value to that of the flyback voltage VF is stored in a capacitor 31. The flyback voltage VF stored in the capacitor 31 is supplied to the gate G of the NMOS 35 after the voltage is damped by means of a resistor 33 and a Zener diode 34. When the voltage VF is supplied to the gate G, the NMOS 35 is turned on and the resistor 25 is short-circuited, resulting in the operation of a switching regulator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switching regulator having a smoothing capacitor, which limits the current flowing into the smoothing capacitor by a resistor when the power is turned on, and short-circuits the resistor in a steady state. It is related to the circuit.

[0002]

2. Description of the Related Art Conventionally, techniques in such a field include:
For example, there is one described in the following literature. Literature; Haruo Togawa, Practical Power Circuit Design Handbook, (1
992), CQ Publishing Company, p. 196 In a general switching regulator, a commercial power supply (for example, AC 100 V) is rectified and smoothed directly or after stepping down by a transformer to generate a DC voltage, and this DC voltage is converted into a power transistor or the like. High-speed switching is performed by the power FET, and further rectification and smoothing are performed to obtain a desired DC voltage. In particular, when the commercial power supply voltage is directly rectified and smoothed, the current flowing into the smoothing capacitor is limited by charging the smoothing capacitor via the limiting resistor when the power is turned on. After the charging of the smoothing capacitor is completed, the thyristor connected in parallel with the limiting resistor is turned on to short-circuit the limiting resistor, thereby entering a steady operation state.

FIG. 2 is a schematic circuit diagram showing an example of a switching regulator having a conventional inrush current limiting circuit described in the above-mentioned document. This switching regulator is a power plug 1 that is inserted into a commercial power outlet.
have. One terminal of the power plug 1 is connected to one terminal of the switch 3 via the fuse 2.
The other terminal of the switch 3 is connected to one input terminal of the diode bridge 4. The other terminal of the power plug 1 is connected to the other input terminal of the diode bridge 4. The negative output terminal of the diode bridge 4 is
Connected to ground. + Of diode bridge 4
The side output terminal is connected to the + side of the electric field capacitor 6 via the resistor 5. The negative side of the electrolytic capacitor 6 is connected to the ground. Further, the positive side of the electric field capacitor 6 is the hot side of the winding 7a of the transformer 7 (indicated by “•”).
It is connected to the. The cold side of the winding 7 a is connected to the drain D of an N-channel MOSFET (hereinafter referred to as NMOS) 8. The source S of the NMOS 8 is connected to the ground. The gate G of the NMOS 8 is connected to the drive circuit 9. The winding 7b of the transformer 7 is connected to a rectifying / smoothing circuit (not shown), and an output voltage is output from the rectifying / smoothing circuit.

The hot side of the winding 7a of the transformer 7 is
It is connected to the cold side of the winding 7c of the transformer 7. The hot side of the winding 7c is the anode A of the diode 10.
And the cathode K of the diode 10 is connected to the resistor 11
To the gate G of the thyristor 12.
Further, the cathode K of the thyristor 12 is connected to the positive side of the electric field capacitor 6. The gate G of the thyristor 12 is connected to the cathode K of the thyristor 12 via the capacitor 13. A resistor 14 is connected in parallel with the capacitor 13. The anode A of the thyristor 12 is connected to the + output terminal of the diode bridge 4. The rush current limiting circuit includes the resistor 5, the winding 7c of the transformer 7, the diode 10, the resistor 11, the thyristor 12, the capacitor 13, and the resistor 14.

Next, the operation of FIG. 2 will be described. When the power plug 1 is inserted into the outlet of the commercial power supply and the switch 3 is turned on, the commercial power supply is rectified by the diode bridge 4 and a full-wave rectified waveform is applied to the anode A of the thyristor 12. At this time, since the drive circuit 9 is designed so that the operation is stopped, the NMOS 8 does not perform the switching action. Therefore, since no voltage is generated in the winding 7c of the transformer 7, there is no voltage for firing the thyristor 12, and the thyristor 12 is in an off state. As a result, the inrush current of the electric field capacitor 6 is limited by the resistor 5, and the electric field capacitor 6 is charged with a time constant determined by the resistance value of the resistor 5 and the capacitance of the electric field capacitor 6. Next, when the drive circuit 9 starts operating when the electric field capacitor 6 is almost completely charged, the NMOS 8 starts the switching operation and the winding 7c
Voltage is generated. This voltage is rectified by the diode 10, attenuated to about 1 V by the resistors 11 and 14, supplied to the gate G of the thyristor 12, and the thyristor 12 is turned on. Then, the resistor 5 is short-circuited, and the switching regulator enters a steady operation state. The noise generated when the power is turned on or the like is generated by the capacitor 13.
The second gate G is prevented from entering the gate G.

[0006]

However, the inrush current limiting circuit of the switching regulator of FIG. 2 has the following problems (1) to (3). (1) When the resistance between the anode A and the cathode K when the thyristor 12 is in the ON state (hereinafter referred to as ON resistance) is 1Ω.
Since the power is high before and after, the power loss in the thyristor 12 is relatively large. (2) Since it is necessary to provide the winding 7c on the transformer 7 in order to generate a voltage for firing the thyristor 12, the configuration of the transformer 7 becomes complicated, resulting in an increase in the cost and efficiency of the transformer 7. (3) Gate G after turning on thyristor 12
In order to prevent the gate current from flowing, the circuit for driving the gate G becomes complicated, which is difficult to realize. Therefore, the inrush current limiting circuit of the switching regulator has a circuit configuration in which a gate current always flows through the gate G of the thyristor 12, so that useless power is consumed.

[0007]

According to the present invention, in order to solve the above-mentioned problems, a DC voltage stored in a smoothing capacitor is repeatedly switched and supplied to a primary winding of a transformer, and a secondary winding of the transformer is provided. A switching regulator that transfers power to the line, a resistor that limits a charging current that flows into the smoothing capacitor from the DC power supply unit when the power is turned on, and a resistor that is generated after the voltage of the smoothing capacitor reaches a predetermined voltage. And a short-circuit means for short-circuiting the resistor based on a control signal to be performed, wherein the short-circuit means is configured as follows. That is, the short-circuit means includes a capacitor for storing a flyback voltage generated in a primary winding of the transformer, an attenuating means for attenuating the flyback voltage stored in the capacitor to generate the control signal, A MOSFET having an enhancement type characteristic that is turned on based on a signal to short-circuit the resistor. According to the present invention, since the inrush current limiting circuit is configured as described above, the charging current flows into the smoothing capacitor via the resistor from the DC power supply unit when the power is turned on, and the smoothing capacitor is charged. When the voltage of the smoothing capacitor reaches a predetermined voltage, the DC voltage stored in the smoothing capacitor is repeatedly switched and supplied to the primary winding of the transformer.
At this time, a flyback voltage is generated in the primary winding of the transformer. This flyback voltage is stored in a capacitor. The flyback voltage stored in this capacitor is
The voltage is attenuated by the attenuating means to a voltage for turning on the MOSFET and supplied to the MOSFET as a control signal. Then MO
The SFET turns on and shorts the resistor. Therefore,
The above problem can be solved.

[0008]

FIG. 1 is a schematic circuit diagram of a switching regulator having an inrush current limiting circuit according to an embodiment of the present invention. This switching regulator has a power plug 21 inserted into an outlet of a commercial power supply. One terminal of the power plug 21 is connected to one terminal of the switch 23 via the fuse 22. The other terminal of the switch 23 is connected to one input terminal of the diode bridge 24. Power plug 2
The other terminal of 1 is connected to the other input terminal of the diode bridge 24. -Of the diode bridge 24
The side output terminal is connected to the ground. Note that a portion from the commercial power supply to the diode bridge 24 is a DC power supply section. Further, this switching regulator includes a resistor 25, a smoothing capacitor 26, a transformer 27, an NMOS 2
8, a drive circuit 29, a diode 30, and short-circuit means. The short-circuit means includes a capacitor 31, a resistor 32, a resistor 33, a Zener diode 34, and an NMOS 3
5 is comprised. The resistor 33 of this short-circuit means
And the Zener diode 34 constitute an attenuation means. Also, the short circuit means and the resistor 25 constitute an inrush current limiting circuit.

The + output terminal of the diode bridge 24 is connected via a resistor 25 to the + side of a smoothing capacitor 26. The negative side of the smoothing capacitor 26 is connected to the ground. The resistor 25 has a function of limiting the charging current flowing from the diode bridge 24 to the smoothing capacitor 26 when the power is turned on. Further, the positive side of the smoothing capacitor 26 is connected to the hot side of the primary winding 27a of the transformer 27. The cold side of the primary winding 27a
It is connected to the drain D of the NMOS. NMOS
The source S of 28 is connected to the ground. NMO
The ground G in S28 is connected to the drive circuit 29. The secondary winding 27b of the transformer 27 is connected to a rectification / smoothing circuit (not shown), and an output voltage is output from the rectification / smoothing circuit. The cold side of the primary winding 27a of the transformer 27 is connected to the anode A of the diode 30.
And the cathode K of the diode 30 is connected to the hot side of the primary winding 27a via the capacitor 31. The capacitor 31 has a function of storing a flyback voltage generated in the primary winding 27a. A resistor 32 is connected in parallel with the capacitor 31 to discharge the capacitor 31. The resistance of the resistor 32 is set to a value at which the capacitor 31 discharges in a few seconds after the switch 23 is turned off.

Further, the cathode K of the diode 30 is connected via a resistor 33 to the cathode K of a zener diode 34. Anode A of Zener diode 34
Is connected to the + side of the smoothing capacitor 26. Then, the voltage of the cathode K of the Zener diode 34 becomes the control signal cs. Cathode K of Zener diode 34
Is connected to the gate G of the NMOS 35. NMO
The source S of S35 is connected to the + side of the smoothing capacitor 26. The drain D of the NMOS 35 is connected to the + output terminal of the diode bridge 24. This NMO
S35 has an enhancement type characteristic, and the control signal cs
And has a function of short-circuiting the resistor 25 by being turned on based on. FIG. 3 is a time chart for explaining the operation of FIG. 1, in which voltage is plotted on the vertical axis and time is plotted on the horizontal axis. The operation of FIG. 1 will be described with reference to FIG. First, when the plug 21 is inserted into the outlet of the commercial power supply and the power is turned on, that is, the switch 23 is turned on, a charging current flows from the + output terminal of the diode bridge 24 to the smoothing capacitor 26 via the resistor 25, 26 is charged. At this time, the smoothing capacitor 26
Is set to a sequence in which the operation of the drive circuit 29 is stopped until the voltage on the + side of the power supply reaches a certain value (usually 90 to 100% of the peak value of the input AC voltage) determined by the input AC voltage of the commercial power supply. , NMOS 28 are off. The NMOS 35 is also in the off state because the gate G and the source S are at the same potential. Therefore, the inrush current flowing into the smoothing capacitor 26 when the switch 23 is turned on can be sufficiently reduced by selecting the value of the resistor 25. Therefore, the resistance 25
Fuse 12 due to inrush current generated when no fuse is provided
Of the switch 13, burning of the contact of the switch 13, and deterioration of the diode bridge 24, the smoothing capacitor 26, and the like.

Next, when the voltage on the positive side of the smoothing capacitor 26 reaches the predetermined value or more, the drive circuit 29 operates,
When the drive signal S29 is supplied to the gate G of the NMOS 28 and the switching operation of the NMOS 28 starts,
At time t1, the NMOS 28 is turned on, and the excitation energy is stored in the transformer 27. At time t2, the NMOS 28 is turned off and the transformer 27
Emits excitation energy, so that a flyback voltage is generated from the hot side to the cold side of the primary winding 27a.
Therefore, as shown in FIG. 3, the voltage S28 at the drain D of the NMOS 28 is several tens of volts to several hundred volts,
And the flyback voltage VF is added. Therefore, a DC voltage equal to the value of the flyback voltage VF is stored in the capacitor 31 which forms a snubber circuit together with the diode 30 and the resistor 32. Its polarity is positive on the side of the diode 30 connected to the cathode K.
The flyback voltage VF stored in the capacitor 31 is increased by 10 by the resistor 33 and the Zener diode 34.
It is attenuated to a value of about V and applied to the gate G of the NMOS 35. Then, the NMOS 35 is turned on, the resistor 25 is short-circuited, and the switching regulator enters a steady operation state.

As described above, in the present embodiment, the resistance 33 is calculated based on the flyback voltage VF stored in the capacitor 31 of the snubber circuit connected to the primary winding 27a of the transformer 27.
And a constant voltage control signal c by the zener diode 34
Since the control signal cs is generated and the control signal cs is applied to the gate G of the NMOS 35 connected in parallel with the resistor 25 to keep the NMOS 35 in the ON state, the following advantages (1) to (3) are obtained. There is. (1) Since the on-resistance of the NMOS 35 can be easily obtained at about 0.2 Ω (however, the withstand voltage between the drain and the source is about 500 V), the power loss is 1/1 of that of the conventional thyristor 12 in FIG. It is reduced to about 5. (2) Unlike the conventional method using a thyristor, an auxiliary winding (that is, the winding 7c in FIG. 2) is not required for the transformer, so that the space for other windings of the transformer is relatively increased. And the efficiency is improved. (3) Since the voltage of the NMOS 35 is controlled, the gate current does not always flow, unlike the thyristor 12 in FIG. 2, and almost no drive power is required. Since there are the advantages (1) to (3) as described above, it is expected that the efficiency of the inrush current limiting circuit and the overall efficiency of the switching regulator are improved.

The present invention is not limited to the above embodiment,
Various modifications are possible. For example, there are the following modifications. (A) Zener diode 34 in FIG. 1 may be a resistor. However, the resistance value needs to be set so as to be within the range of the standard value between the gate G and the source S of the NMOS 35. (B) In the embodiment, a circuit for supplying power from the + output terminal of the diode bridge 24 has been described as an example. However, a circuit for connecting the + output terminal to the ground and supplying power from the − output terminal May be configured. However, the polarity of the connection, such as the smoothing capacitor 26, whose polarity is distinguished is all opposite to that in the case of FIG. MOSFET that short-circuits the resistor that limits the charging current flowing into the smoothing capacitor
Is a P-channel type MOSF with enhancement type characteristics
ET will be used.

[0014]

As described above in detail, according to the present invention, the short-circuit means for short-circuiting the resistor for limiting the charging current flowing into the smoothing capacitor stores the flyback voltage generated in the primary winding of the transformer. A capacitor and an attenuating means for attenuating the flyback voltage to generate a control signal;
Since there is provided a MOSFET which is turned on based on this control signal and short-circuits the resistor, the following effects (1) to (3) are obtained. (1) Since the on-resistance of the MOSFET can be easily obtained at about 0.2Ω (however, the withstand voltage between the drain and the source is about 500 V), the power loss is reduced by 1 / compared to the case of using a conventional thyristor. It can be reduced to about 5. (2) Unlike the case where a thyristor is used, it is not necessary to provide a winding for generating a current to be supplied to the gate of the thyristor in the transformer. Can be improved. (3) Since the voltage of the MOSFET is controlled, the gate current does not always flow unlike the conventional thyristor, and the drive power can be reduced. The effects (1) to (3) as described above can improve the efficiency of the inrush current limiting circuit and the overall efficiency of the switching regulator.

[Brief description of the drawings]

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

FIG. 2 is a circuit diagram of a conventional switching regulator.

FIG. 3 is a time chart of FIG. 1;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 21 Power plug (DC power supply part) 22 Fuse (DC power supply part) 23 Switch (DC power supply part) 24 Diode bridge (DC power supply part) 25 Resistance 26 Smoothing capacitor 27 Transformer 31 Capacitor 33 Resistance (attenuation means) 34 Zener diode (attenuation means) 35 NMOS (MOSFET)

Claims (1)

[Claims] 1. A switching regulator for repeatedly switching a DC voltage stored in a smoothing capacitor, supplying the DC voltage to a primary winding of a transformer, and transferring power to a secondary winding of the transformer. A resistor for limiting a charging current that rushes into the smoothing capacitor from a power supply unit, and short-circuit means for short-circuiting the resistor based on a control signal generated after the voltage of the smoothing capacitor reaches a predetermined voltage. In the inrush current limiting circuit, the short-circuit means includes a capacitor for storing a flyback voltage generated in the primary winding of the transformer, and an attenuation for attenuating the flyback voltage stored in the capacitor to generate the control signal. Means, a MOSFET having an enhancement type characteristic that is turned on based on the control signal and short-circuits the resistor; An inrush current limiting circuit, comprising: