JP2011167012A - Circuit and method for prevention of inrush current - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inrush current preventing circuit and method, wherein the circuit is started if an input voltage when powered on reaches a detecting threshold, and the detecting threshold varies. <P>SOLUTION: The inrush current preventing circuit includes: a starting circuit 2; and a current limit circuit 3. The current limit circuit 3 prevents the generation of an inrush current when a DC power supply is turned on. The starting circuit 2 detects the input voltage, and starts the current limit circuit 3 when the input voltage reaches the detecting threshold. The starting circuit 2 includes a detecting threshold adjustor 21. The starting circuit 2 can adjust the detecting threshold by the detecting threshold adjustor 21. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an inrush current prevention circuit and an inrush current prevention method, and more particularly to an inrush current prevention circuit that starts by detecting an input voltage and an inrush current prevention method.

  An inrush current prevention circuit is used for power supply input of a device operating with a direct current power supply for the purpose of suppressing inrush current. As an example of such an inrush current prevention circuit, for example, an inrush current prevention circuit disclosed in Patent Document 1 can be cited.

  FIG. 4 is a circuit diagram showing an inrush current prevention circuit 400 which is an example of a normal inrush current prevention circuit. As shown in FIG. 4, the inrush current prevention circuit 400 is inserted between a DC input power supply 41, an input capacitor 50 and a load 51, and includes a resistor 42, a resistor 43, a capacitor 44, and a MOSFET (Metal Oxide Semiconductor Field). Effect Transistors) 45, a capacitor 46, a resistor 47, an output terminal 48, and an output terminal 49.

  The gate voltage of the gate of MOSFET (Metal Oxide Semiconductor Field Effect Transistors) 45 gradually increases due to the charging operation from the resistor 42 to the capacitor 44. When the gate voltage of the MOSFET 45 reaches a threshold value, the MOSFET 45 is turned on and current starts to flow, and the drain-source voltage starts to decrease. A decrease in the drain-source voltage is transmitted to the gate of the MOSFET 45 through the capacitor 46 and the resistor 47. As a result, the gate voltage of the MOSFET 45 is maintained at a substantially constant value. Therefore, the current of the MOSFET 45 is limited to a substantially constant value, and the charging voltage waveform of the input capacitor 50 is gradually charged with the ramp waveform.

  However, the inrush current prevention circuit 400 does not have the voltage detection function of the DC input power supply 41. For this reason, even when the DC input power supply 41 does not reach the allowable input voltage range of the load 51, the inrush current prevention circuit 400 starts charging the input capacitor 50. Therefore, when the load 51 is a DC-DC converter or the like, the load 51 may start operating in a state where the capacitor voltage does not reach the allowable input voltage range of the load 51. At this time, since the DC-DC converter of the load 51 does not reach the input allowable voltage range, there is a disadvantage that the DC-DC converter does not operate normally or fails to start.

  As a countermeasure against the drawback of the inrush current prevention circuit 400, there is a DC-DC converter disclosed in Patent Document 2. This DC-DC converter cuts off the output to the load by a cut-off circuit when the input voltage falls below the operating range voltage of the load. On the other hand, when the input voltage reaches the operating range voltage of the load, the output voltage is applied to the load.

JP 2005-45957 A Japanese Patent No. 2813735

  The value of the input voltage when the DC-DC converter described in Patent Document 2 applies the output voltage to the load (hereinafter referred to as a detection threshold) is determined by the breakdown voltage of the Zener diode or the like. However, the operating range voltage varies depending on the load to be combined. Therefore, in the DC-DC converter described in Patent Document 2, the elements must be appropriately replaced according to the load to be combined, and there is a drawback that it is difficult to apply widely.

  The present invention has been made in view of the above, and an object of the present invention is an inrush current prevention circuit and an inrush that are activated when an input voltage at power-on reaches a detection threshold and the detection threshold is variable. It is to provide a method for preventing current.

  An inrush current prevention circuit according to an aspect of the present invention includes a current limiting circuit that limits a current when a DC power supply is turned on, and a start-up circuit that starts the current limiting circuit when an input voltage reaches a detection threshold, The activation circuit includes at least a detection threshold adjustment unit that adjusts the detection threshold.

  An inrush current prevention method according to an aspect of the present invention includes a step of detecting an input voltage when a DC power source is turned on, and a current limit that limits a current when the DC power source is turned on when the input voltage reaches a detection threshold. And a step of activating the circuit, and controlling the activation of the current limiting circuit by adjusting the detection threshold.

  According to the present invention, it is possible to provide an inrush current prevention circuit and an inrush current prevention method which are activated when an input voltage at power-on reaches a detection threshold and the detection threshold is variable.

1 is a block diagram showing a configuration of an inrush current prevention circuit 100 according to a first embodiment. FIG. 1 is a circuit diagram showing a configuration of an inrush current prevention circuit 100 according to a first embodiment; FIG. 3 is an operation timing chart of the inrush current prevention circuit 100 according to the first embodiment. 3 is a circuit diagram of a normal inrush current prevention circuit 400. FIG.

Embodiment 1
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram illustrating a configuration of an inrush current prevention circuit 100 according to the first embodiment. As shown in FIG. 1, the inrush current prevention circuit 100 is inserted between the DC input power source 1, the input capacitor 6 and the load 7. The inrush current prevention circuit 100 is configured by a start circuit 2 and a current limiting circuit 3. The output terminal 4 of the inrush current prevention circuit 100 is connected to the input capacitor 6 and one end of the load 7. The output terminal 5 is connected to the input capacitor 6 and the other end of the load 7.

  The inrush current prevention circuit 100 prevents an inrush current from the DC input power source 1 when the power is turned on. The input voltage applied to the inrush current prevention circuit 100 when the power is turned on starts to increase immediately after the power is turned on, and reaches the output voltage of the DC input power supply 1 after a certain time.

  When the input voltage reaches the detection threshold, the starting circuit 2 makes the DC input power source 1 and the current limiting circuit 3 conductive. As a result, the current limiting circuit 3 is activated. The current limiting circuit 3 outputs an output voltage to the load 7 while preventing the occurrence of an inrush current. That is, the inrush current prevention circuit 100 as a whole is activated as the current limiting circuit 3 is activated.

  Here, by setting the detection threshold of the activation circuit 2 to the lower limit value of the operating range voltage of the load 7, the current limiting circuit 3 can be activated after the input voltage reaches the lower limit of the operating range voltage of the load 7. it can. In the inrush current prevention circuit 100, the detection threshold value can be changed by the detection threshold value adjustment unit 21 of the activation circuit 2. Therefore, the inrush current prevention circuit 100 can be applied to loads having different operating range voltages.

  Hereinafter, the configuration and operation of the inrush current prevention circuit 100 will be described in detail. FIG. 2 is a circuit diagram showing a configuration of the inrush current prevention circuit 100. As shown in FIG. 2, the startup circuit 2 includes a detection threshold adjustment unit 21, a resistor 22, and a switching circuit 23. One end of the resistor 22 is connected to the DC input power source 1 and the other end is connected to the switching circuit 23.

  The detection threshold adjustment unit 21 includes a variable resistor 24 and a resistor 25. The variable resistor 24 and the resistor 25 are connected in series to the DC input power source 1. The switching circuit 23 includes a transistor 26 and a Zener diode 27. A voltage between the variable resistor 24 and the resistor 25 is input to the base of the transistor 26. Transistor 26 is connected to resistor 22. The emitter of the transistor 26 is connected to one end of the Zener diode 27 and the current limiting circuit 3. The other end of the Zener diode 27 is connected to the negative terminal of the DC input power supply 1.

  The current limiting circuit 3 includes a resistor 31, a capacitor 32, a MOSFET 33, a capacitor 34, and a resistor 35. A resistor 31 and a capacitor 32 are connected between the emitter of the transistor 26 and the negative terminal of the DC input power supply 1. The gate of MOSFET 33 is connected to the emitter of transistor 26. A capacitor 34 and a resistor 35 are connected in series between the emitter of the transistor 26 and the drain of the MOSFET 33. The source of the MOSFET 33 is connected to the negative terminal of the DC input power supply 1. The drain of the MOSFET 33 is further connected to the output terminal 5.

  Next, the function of each element provided in the inrush current prevention circuit 100 will be described. The MOSFET 33 is controlled by the gate voltage Vg and suppresses the inrush current to the input capacitor 6. The resistors 22 and 31 generate the gate voltage Vg of the MOSFET 33. The capacitor 32 provides a time constant for the gate voltage Vg of the MOSFET 33. The capacitor 34 and the resistor 35 feed back the drain-source voltage Vds of the MOSFET 33 to the gate voltage Vg.

  Next, the operation of the inrush current prevention circuit 100 will be described. FIG. 3 is a time chart showing the operation of the inrush current prevention circuit 100. As shown in FIG. 3, when power is supplied to the inrush current prevention circuit 100, the input voltage Vin is applied to the inrush current prevention circuit 100. The input voltage Vin is divided by the variable resistor 24 and the resistor 25 of the detection threshold adjuster 21 to generate a control voltage Vc.

ここで、Ｒ２４は可変抵抗２４の抵抗値である。Ｒ２５は抵抗２５の抵抗値である。 In the inrush current prevention circuit 100, when the input voltage Vin reaches the detection threshold Vdet, the control voltage Vc input to the base of the transistor 26 is a voltage obtained by adding the Zener voltage Vz of the Zener diode 27 to the operation threshold voltage Vbe of the transistor 26. And the transistor 26 is turned on. Thus, the detection threshold Vdet is expressed by the following equation (1).

Here, R24 is the resistance value of the variable resistor 24. R25 is the resistance value of the resistor 25.

  That is, in the switching circuit 23 including the series connection of the transistor 26 and the Zener diode 27, when the control voltage Vc reaches the operation threshold voltage (Vbe + Vz) of the switching circuit, the transistor 26 is turned on. Further, as shown in Expression (1), the detection threshold value Vdet can be adjusted by changing the resistance value R24 of the variable resistor 24.

  Until the input voltage Vin reaches the detection threshold Vdet, the transistor 26 is OFF. Therefore, the gate voltage Vg is not applied to the gate of the MOSFET 33 (period T1 in FIG. 3).

  When the input voltage reaches the detection threshold Vdet, the transistor 26 is turned on, and the gate voltage Vg is applied to the gate of the MOSFET 33. The gate voltage Vg at this time has a ramp waveform due to the charging operation from the resistor 22 to the capacitor 32 (period T2 in FIG. 3).

  Next, when the gate voltage Vg of the MOSFET 33 reaches a threshold value, the MOSFET 33 is turned on and current starts to flow. At the same time, the drain-source voltage Vds of the MOSFET 33 starts to decrease. The fluctuation of the drain-source voltage Vds is transmitted to the gate of the MOSFET 33 via the capacitor 34 and the resistor 35. Therefore, the gate voltage Vg is maintained at a substantially constant value. As a result, the current flowing through the MOSFET 33 is limited to a substantially constant value. Therefore, the charging voltage Vq of the input capacitor 6 also has a ramp waveform (period T3 in FIG. 3).

  When the charging of the input capacitor 6 is completed, the gate voltage Vg of the MOSFET 33 starts to rise again. When the gate voltage Vg reaches the operating voltage value of the Zener diode 27, the gate voltage Vg is clamped to a constant value (period T4 in FIG. 3).

  As shown in Expression (1), the detection threshold Vdet is determined by the resistance values of the variable resistor 24 and the resistor 25 and the operating voltage of the Zener diode 27. Therefore, by setting the detection threshold Vdet in this configuration to be equal to or higher than the minimum value of the input voltage allowable range of the load 7, the inrush current prevention circuit 100 is started after the input voltage Vin reaches the operation guaranteed range of the load 7. Can do. Therefore, the load 7 can be normally started.

  Furthermore, in this configuration, the resistance value R24 of the variable resistor 24 is variable. Therefore, as shown in Expression (1), the detection threshold value Vdet can be appropriately adjusted by adjusting the resistance value R24. Thus, the inrush current prevention circuit 100 can be applied without replacing the resistor and the Zener diode each time according to the guaranteed operation range of the load. Therefore, according to this configuration, it is possible to provide an inrush current prevention circuit that can be applied to any load.

Other Embodiments The present invention is not limited to the above-described embodiments, and can be appropriately changed without departing from the spirit of the present invention. For example, the resistance value of the variable resistor 24 may be adjusted in advance, or may be controlled according to a control signal input from the outside. At this time, a control signal generated according to the operation compensation range of the load connected to the inrush current prevention circuit 100 can be used.

  In FIG. 2, the two resistors of the variable resistor 24 and the resistor 25 are connected in series. However, three or more resistors may be connected in series. Furthermore, if at least one of the resistors connected in series is a variable resistor, the detection threshold Vdet can be adjusted.

DESCRIPTION OF SYMBOLS 1 DC input power supply 2 Starting circuit 3 Current limit circuit 4, 5 Output terminal 6 Input capacitor 7 Load 21 Detection threshold value adjustment part 22, 25, 31, 35 Resistance 23 Switching circuit 24 Variable resistance 25 Resistance 26 Transistor 27 Zener diode 32, 34 Capacitor 33 MOSFET
41 DC input power supply 42, 43, 47 Resistor 44, 46 Capacitor 48, 49 Output terminal 50 Input capacitor 51 Load 100, 400 Inrush current prevention circuit Vg Gate voltage Vin Input voltage Vds Drain-source voltage

Claims (6)

A current limiting circuit that limits the current when the DC power is turned on;
A starting circuit for starting the current limiting circuit when an input voltage reaches a detection threshold;
The starting circuit is
At least a detection threshold adjustment unit for adjusting the detection threshold;
Inrush current prevention circuit. The detection threshold adjuster is
A plurality of resistors connected in series and generating a control voltage obtained by dividing the input voltage;
At least one of the plurality of resistors connected in series is a variable resistor;
The detection threshold changes when the control voltage changes according to the resistance value of the variable resistor.
The inrush current prevention circuit according to claim 1. The starting circuit is
A switching circuit for connecting the DC power supply and the current limiting circuit according to the control voltage;
The switching circuit is
When the control voltage reaches the operation threshold voltage of the switching circuit, the DC power supply and the current limiting circuit are connected,
The inrush current prevention circuit according to claim 2. The switching circuit is
A switching element controlled by the control voltage;
A Zener diode connected in series with the switching element,
The operation threshold voltage of the switching circuit is obtained by adding the Zener voltage of the Zener diode to the operation threshold voltage of the switching element,
The inrush current prevention circuit according to claim 3. A load is connected to the current limiting circuit,
The detection threshold is equal to a lower limit of a voltage range in which the load operates normally,
The inrush current prevention circuit according to any one of claims 1 to 4. Detecting the input voltage when the DC power is turned on;
When the input voltage reaches a detection threshold, starting a current limiting circuit that limits the current when the DC power is turned on, and at least comprising
Controlling the activation of the current limiting circuit by adjusting the detection threshold;
How to prevent inrush current.

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