JP2000188862A - Input circuit for dc-dc converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the scale and cost of an input circuit for DC-DC converter by adding various functions to the input circuit for preventing the input of a rush current to an input capacitor, protecting DC input power supply when the power is reversely connected, preventing malfunctions when the input voltage drops, holding an output voltage when the input is instantaneously interrupted, and so on, to the input circuit. SOLUTION: The gate of a MOSFET 15 which also functions to protect input power supply 11, when the power supply 11 is reversely connected by inverting its positive and negative sides and when the input voltage becomes lower than a specific value, is connected to the gate of another MOSFET 17. The MOSFET 17 is an element which functions to suppress rush currents and protect the power supply 11 when the input voltage becomes lower than the specific value, and a Zener diode 22 which protects the gate of the MOSFET 17 from overvoltages and a capacitor 23 for soft starting, are connected between the gate and source of the MOSFET 17. In addition, a transistor 24 controls the equivalent resistances between the drains and sources of the MOSFETs 15 and 17 and prevents the input of rush currents by means of a resistor 16 for detecting input current provided between its base and emitter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an input circuit of a DC-DC converter, and more particularly, to a DC-DC converter having various functions such as input reverse connection protection, input voltage drop protection, output voltage holding, and rush current prevention. The input circuit.

[0002]

2. Description of the Related Art Conventionally, in a DC-DC converter, a large-capacity capacitor used for smoothing is provided in an input circuit. Therefore, when the power is turned on, a large charging current flows as an inrush current to the capacitor.

As a method for preventing the inrush current, for example, an inrush current prevention circuit disclosed in Japanese Patent Application Laid-Open No. 5-146143 has been proposed. FIG. 6 is a circuit diagram of a conventional inrush current prevention circuit.

As shown in FIG. 6, a rush current prevention circuit includes a DC input power supply E, a power supply smoothing capacitor C, and a load Z.
o, and a current detection resistor R connected in series between a DC input power supply E and a smoothing capacitor C.
s and the current limiting element Q1.

In this inrush current prevention circuit, when the switch S1 is turned on, the smoothing capacitor C and the load Zo
The input voltage is applied to the current limiting element Q1 through the switch. When the switch S1 is turned on, no current flows through the current detection resistor Rs because the current limiting element Q1 is off.

Therefore, the reference voltage generated at the resistor R1 is applied to the input of the error amplifier IC1 with the same polarity as the input of the error amplifier IC1. As a result, the output of the error amplifier IC1 is output as the increased voltage, and the voltage is applied to the gate of the current limiting element Q1. As a result, the current limiting element Q1 starts flowing the current I for charging the smoothing capacitor C.

[0007] The current I also flows through the current detection resistor Rs to cause a voltage drop. Until the voltage reaches the reference voltage, the current limiting element Q1 operates to keep the current I flowing. When the voltage due to the current detection resistor Rs becomes larger than the reference voltage due to the resistor R1, the voltage applied to the gate of the current limiting element Q1 decreases because the polarity of the voltage applied to the input of the error amplifier IC1 is inverted. Work in the direction you want to.

That is, since the reference voltage and the voltage drop by the current detection resistor Rs are controlled to be the same, as a result, the current for charging the smoothing capacitor C becomes a constant value, and the peak value of the current can be suppressed. it can.

[0009] In a DC-DC converter, an electric circuit as a load may be broken by connecting a DC input voltage source in reverse. As a device for protecting from the reverse connection, for example, a reverse connection protection circuit for a DC power supply disclosed in Japanese Patent Application Laid-Open No. 7-46756 has been proposed. FIG. 7 is a circuit diagram of a conventional reverse connection protection circuit of a DC power supply.

As shown in FIG. 7, the reverse connection protection circuit of the DC power supply forms a reverse connection protection circuit by combining a diode D1 and a fuse F. In this reverse connection protection circuit, the supply current of the DC power supply reversely connected to the positive and negative voltage supply terminals 1 and 2 flows through the diode D1 having a low load impedance, and the DC current reversely connected to the electric circuit 4 is It is not supplied and reverse connection protection of the electric circuit 4 is performed.

When the diode D1 is destroyed, the supplied direct current of the reverse connection flows through the diode D2 and the fuse F, and the fuse F is instantaneously blown and cut off.
The supply of the direct current of the reverse connection to the power supply is cut off, and the electric circuit 4 is protected.

[0012]

However, in the rush current prevention circuit, when the DC power supply is reversely connected, the reverse connection DC current flows through the parasitic diode of the MOSFET which is the current limiting element Q1, so that the DC power supply Reverse connection protection cannot be performed. In the DC power supply reverse connection protection circuit, the diode D1 and the fuse F protect the DC power supply by supplying a reverse connection current thereto.
The diode D1 was destroyed or the blown fuse F had to be replaced.

When the input voltage drops, DC-DC
The converter may malfunction, and if there is a momentary interruption of input, etc., there is no function to hold the input voltage, so it was necessary to attach another circuit to hold the output voltage of the DC-DC converter. .

That is, in the case of the conventional input circuit, respective functions are provided to prevent rush current, protect the reverse connection of the DC power supply, prevent malfunction when the input voltage drops, and hold the output voltage in the event of an instantaneous interruption of input. The circuit I had was attached individually.
As a result, the circuit scale is increased and the cost is increased.

SUMMARY OF THE INVENTION An object of the present invention is to reduce the size of the circuit and reduce the cost, prevent inrush current, protect the reverse connection of the DC power supply, prevent malfunction when the input voltage drops, and maintain the output voltage in the event of a momentary loss of input. An object of the present invention is to provide an input circuit of a DC-DC converter having the function of (1).

[0016]

In order to achieve the above-mentioned object, an input circuit of a DC-DC converter according to the present invention comprises an input circuit connected between a DC input power supply and an input capacitor connected between inputs of the DC-DC converter. DC-D connected to
The input circuit of the C converter has the functions of preventing inrush power input to the input capacitor, protecting against reverse connection of the DC input power supply, preventing malfunction when input voltage drops, and maintaining output voltage in case of momentary input loss. It is characterized by having a circuit configuration.

[0017] With the above configuration, the input circuit of the DC-DC converter connected between the DC input power supply and the input capacitor connected between the inputs of the DC-DC converter can supply the rush power to the input capacitor. It also has functions of input prevention, protection against reverse connection of DC input power, prevention of malfunction when input voltage drops, and holding of output voltage in case of momentary input loss.

As a result, not only the circuit size is reduced and the cost is reduced, but also the circuit size is not increased and the cost is not increased. It is possible to provide functions such as prevention of output and holding of output voltage in case of instantaneous interruption of input.

[0019]

Embodiments of the present invention will be described below with reference to the drawings.

(First Embodiment) FIG. 1 is a circuit diagram of an input circuit of a DC-DC converter according to a first embodiment of the present invention. As shown in FIG. 1, an input circuit 10 of the DC-DC converter is connected between an input power supply 11 and an input capacitor 12. Input capacitor 12
Is connected between the inputs of the DC-DC converter 13, a switch 14 and an N-channel MOSFET are connected between the negative terminal of the input power supply 11 and the negative terminal of the input capacitor 12.
(Metal-oxide-semiconductor
r fielddef transistor)
15, input current detection resistor 16, and N-channel MOS
FET 17 is connected in series.

Source of MOSFET 15 and detection resistor 1
A shunt regulator 18 and a resistor 19 are connected in series between the connection point 6 and the positive terminal of the input power supply 11.
A MOSFET is provided between a connection point between the source of the MOSFET 17 and the detection resistor 16 and a positive electrode of the input power supply 11.
Resistors 20 and 21 for applying a bias to the resistors 15 and 17 are connected in series.
5, 17 gates are connected.

The MOSFET 15 is an element having both a function of protecting the input power supply 11 when the polarity is reversed and a function of protecting the input voltage when the input voltage becomes lower than a specified value. , MOSFE
It is connected to the gate of T17.

The MOSFET 17 is a device having both a function of suppressing an inrush current and a function of protecting the input voltage when the input voltage becomes lower than a specified value. Is connected to the capacitor 23 for soft start.

The NPN transistor 24 has a collector connected to the gates of the MOSFETs 15 and 17 through a current limiting resistor 25, and a resistor 35 connected in series to a base. Transistor 24 includes MOSFETs 15 and 1
7, the equivalent resistance between the drain and source is controlled, and an input current detection resistor 16 is connected between the base and the emitter to prevent the input of an inrush current.

On the other hand, the gates of the MOSFETs 15 and 17
Between the sources, the collector of the NPN transistor 26 is connected to the gate through a resistor 27, the emitter is connected to the source, and the base of the NPN transistor 26 is connected to the positive electrode of the input power supply 11 through a Zener diode 28 and a resistor 29. .

The connection point between the Zener diode 28 and the resistor 29 is connected to the emitter of a PNP transistor 30. The collector of the PNP transistor 30 is MOSF
The ET 17 is connected to the connection point of the current detection resistor 16, and the base is connected to the connection point of the shunt regulator 18 and the resistor 19. This shunt regulator 18 operates by the detection voltage of the input voltage detection resistors 31 and 32.

Both transistors 30 and 26 are MOSFE
The equivalent resistance between the drain and source of T15 and T17 is controlled to protect the input voltage when the input voltage becomes lower than a specified value and to maintain the output voltage when the input voltage is momentarily interrupted.
This is performed for the DC converter 13.

Next, the operation of the input circuit 10 of the DC-DC converter will be described.

First, the operation of input reverse connection protection will be described. As shown in FIG. 1, when the positive and negative electrodes of the input power supply 11 are normally connected, the switch 14 is turned on (ON).
Then, through the resistors 20 and 21, the input current detection resistor 16, and the parasitic diode 15 p of the MOSFET 15,
A bias voltage is applied to the gate of the MOSFET 15,
The conduction between the drain and the source of the MOSFET 15 is conducted, and the input voltage is supplied to the circuits thereafter.

When the positive and negative electrodes of the input power supply 11 are connected in reverse, no current flows due to the parasitic diode 15p of the MOSFET 15, so that the gate of the MOSFET 15
No bias voltage is applied between the sources, and MOSFET1
5 is in an off state (OFF). Therefore, no voltage is applied to the circuits thereafter, and the circuit is protected against reverse connection of the input power supply 11.

Next, a description will be given of the operation of protection when the input voltage becomes lower than the specified value and the operation of holding the output voltage when the input voltage is momentarily interrupted.

FIG. 2 is a graph showing the relationship between the input voltage of the DC-DC converter and the voltage of the input power supply. As shown in FIG. 2, the input voltage Vin of the input power supply 11
Is applied, the NPN transistor 26 operates and M
The gate and source of the OSFETs 15 and 17 are short-circuited, and the drain and source of the MOSFETs 15 and 17 are turned off.

Therefore, the DC-
No voltage is applied to the DC converter 13 and the DC-DC
Converter 13 does not start. When the input voltage Vin is detected by the input voltage detection resistors 31 and 32, and the divided voltage becomes equal to or higher than the reference voltage Vref of the shunt regulator 18, the shunt regulator 18 conducts. As a result, conduction between the collector and the emitter of the PNP transistor 30 is conducted, and the NPN transistor 26 is turned off.

Here, the reference voltage Vref is Vref = Vref =
{Resistance 32 / (Resistance 31 + Resistance 32)} × VT (VT:
This is a specified value, that is, the voltage across the input voltage detecting resistors 31 and 32 at which the shunt regulator 18 is turned on.

When the transistor 26 is turned off, the MOS
The gates of the FETs 15 and 17 are biased by the resistors 20 and 21, the conduction between the drains and the sources of the MOSFETs 15 and 17 is conducted, and the voltage Vc across the capacitor 12 which is the input voltage of the DC-DC converter 13 rises (FIG. 2).
reference).

As described above, when the input voltage Vin becomes the prescribed value VT
When the voltage becomes lower than that, the DC-DC converter 13 is protected by turning off the drain-source of the MOSFET 17. Further, even when the input voltage Vin is momentarily interrupted in the short-circuit state and the input voltage Vin drops, the drain-source of the MOSFET 17 is turned off by the same operation, so that the input voltage of the DC-DC converter 13 Of the DC-D
Since the output voltage of the C converter 13 does not immediately decrease,
The output voltage is held for a certain time.

Next, the inrush current preventing operation will be described. FIG. 3 is an explanatory diagram showing the relationship between the inrush current and time in a graph. As shown in FIG. 3, when the switch 14 is turned on and the input voltage becomes the specified value VT, the NPN transistor 26 is turned on.
Is turned off, a voltage obtained by dividing the voltage Vin of the input power supply 11 by the resistors 20 and 21 is applied to the gates of the MOSFETs 15 and 17, the drain-source of the MOSFETs 15 and 17 conduct, and the input capacitor 12 is charged. The current Ir flows.

At this time, the capacitor 23 is
T15 and 17 are prevented from rising suddenly. Current Ir
Flows beyond the set value, the voltage drop is increased by the input current detection resistor 16 and the voltage between the base and the emitter of the NPN transistor 24 is increased.
The gate-source potentials of the gates 15 and 17 are reduced.

As a result, the equivalent resistance of MOSFETs 15 and 17 increases, and the value of current Ir is limited.

When the input capacitor 12 is further charged,
The voltage drop due to the input current detection resistor 16 decreases, and NP
Since the base-emitter voltage of the N-transistor 24 is reduced, the equivalent resistance between the collector and the emitter of the transistor 24 is also increased, and the gate-source potential of the MOSFETs 15 and 17 is raised.

As a result, the equivalent resistance of the MOSFETs 15 and 17 decreases, and the limitation on the current Ir is released.

As a result of such a restriction, the input power supply 11
Is limited to a constant value (see FIG. 3), and the voltage Vc across the input capacitor 12 rises linearly (see FIG. 2).

As described above, the input circuit 10 of the DC-DC converter includes two N-channel MOSFETs 15,
17 and the current detection resistor 16 and the three transistors, the MOSFE can be used even when the input power supply is reversely connected.
T is turned off, and protection is performed without applying a reverse voltage to the input of the DC-DC converter.

Further, compared to the conventional inrush current prevention circuit system in which a reverse connection current of a DC power supply is protected by a diode and a fuse, the diode is not destroyed, so that the safety is high and the blown fuse is blown. You will also be free from the troublesome task of replacing.

Further, since the input voltage is held when the input voltage is momentarily interrupted, there is no need to provide a separate circuit for holding the output voltage of the DC-DC converter, and the circuit scale is not increased and the cost is increased. No.

Furthermore, the rush current flowing through the input capacitor when the input power is turned on is limited by the MOSFET,
Since the current flowing through the OSFET is detected by the control transistor using the current detection resistor and the equivalent resistance of the MOSFET is controlled, the inrush current at power-on can be made constant.

(Second Embodiment) FIG. 4 is a circuit diagram of an input circuit of a DC-DC converter according to a second embodiment of the present invention. As shown in FIG. 4, the input circuit 40 of the DC-DC converter has the same basic configuration as the input circuit 10 of the DC-DC converter, but the input voltage becomes lower than a specified value. Protection when the input voltage is momentarily interrupted and the output voltage holding operation when the input voltage is momentarily interrupted.
We are even more creative.

In the input circuit 40 of the DC-DC converter, the connection point between the input voltage detecting resistors 31 and 32 is connected to the reference of the shunt regulator 41, and the cathode of the shunt regulator 41 is connected to the photo diode Is connected. The resistor 44 has a role of flowing an idling current to the shunt regulator 41. The phototransistor of the photocoupler 43 is connected between the base and the emitter of the NPN transistor 26.

When the input voltage of the input power supply 11 is applied,
The NPN transistor 26 operates to operate the MOSFET 15,
Short-circuit between gate and source of MOSFET17, MOSFET1
The area between the drain and the source is turned off. Therefore, no voltage is applied to the DC-DC converter 13 connected behind the DC-DC converter 13, and the DC-DC converter 13 does not start.

When the input voltage is divided by the resistors 31 and 32 and the divided voltage becomes equal to or higher than the reference voltage Vref of the shunt regulator 41, conduction between the cathode and the anode of the shunt regulator 41 occurs, and
A current flows through the photodiode 3. As a result, the phototransistor of the photocoupler 43 turns on and the NPN transistor 26 turns off.

When the NPN transistor 26 is turned off,
The gates of the MOSFETs 15 and 17 are biased by the resistors 20 and 21, and the drains of the MOSFETs 15 and 17 are
The connection between the sources is conducted, and the voltage across the capacitor 12, which is the input voltage of the DC-DC converter 13, rises (see FIG. 2).

As described above, when the input voltage becomes lower than the specified value, the DC-DC converter 13 is protected by turning off the drain and source of the MOSFET 17. Also, when the input voltage drops due to momentary interruption in the short-circuit state of the input voltage, the MOSFET 1
7 is turned off, and the input voltage of the DC-DC converter 13 is held at the voltage across the input capacitor 12, so that the output voltage of the DC-DC converter 13 does not immediately decrease and the output voltage is constant. Hold for time.

Therefore, in this embodiment, PN
By using the photocoupler 43 instead of the P transistor 30, the input voltage detection unit and the control units of the MOSFETs 15 and 17 can be insulated.

(Third Embodiment) FIG. 5 is a circuit diagram of an input circuit of a DC-DC converter according to a third embodiment of the present invention. As shown in FIG. 5, the input circuit 50 of the DC-DC converter further protects the input voltage when the input voltage becomes lower than a specified value and holds the output voltage when the input voltage is momentarily interrupted. , And a structure for providing hysteresis. Other configurations are DC-D
This is the same as the input circuit 10 of the C converter.

In the input circuit 50 of the DC-DC converter, the connection point between the input voltage detection resistors 51 and 52 is connected to the inverting input of the operational amplifier 53, and the connection point between the Zener diode 54 and the resistor 55 that generate the reference voltage is It is connected to the non-inverting input of the operational amplifier 53. The resistor 56 is connected between the output and the non-inverting input for the hysteresis operation of the operational amplifier 53. The output of the operational amplifier 53 is a resistor 5
7 is connected to the base of the NPN transistor 26.

When the voltage obtained by dividing the input voltage of the input power supply 11 by the input voltage detecting resistors 51 and 52 is smaller than the Zener voltage Vth of the Zener diode 54 that generates the reference voltage, the output of the operational amplifier 53 is High. And the NPN transistor 26 operates and the MOSFET 1
Short between the gate and source of 5, 17 and MOSFE
The connection between the drain and source of T15 and T17 is turned off.

Therefore, the DC-
No voltage is applied to the DC converter 13 and the DC-DC
Converter 13 does not start.

On the other hand, the input voltage is changed to the input voltage detecting resistor 5.
When the voltage divided by 1 and 52 becomes higher than the Zener voltage Vth of the Zener diode 54 that generates the reference voltage, the output of the operational amplifier 53 becomes low (Low), and the NPN transistor 26 is turned off. Subsequent M
The operation of the OSFETs 15 and 17 is similar to the above operation.

As described above, the input circuits 10, 40 and 50 of the DC-DC converter according to the present invention
And the input capacitor 12 connected between the inputs of the DC-DC converter 13, and between the negative terminal of the input power supply 11 and the negative terminal of the input capacitor 12
It has a basic configuration in which a switch 14, an N-channel MOSFET 15, an input current detection resistor 16, and an N-channel MOSFET 17 are connected in series.

The MOSFET 15 is an element having both a function of protecting the input power supply 11 when the polarity of the input power supply 11 is reversed and a function of protecting the input power supply 11 when the input voltage becomes lower than a specified value. The element has both a function of suppressing an inrush current and a function of protecting the input voltage when the input voltage becomes lower than a specified value.

That is, the input circuit of the conventional DC-DC converter is designed to prevent inrush current, protect the reverse connection of the DC power supply, hold the output voltage in case of momentary interruption of input, and protect the input low voltage.
Although the circuits having the respective functions are individually provided, the input circuits 10 and 4 of the DC-DC converter according to the present invention are provided.
0, 50 can have these functions together with the simple circuit configuration described above.

Thus, the size of the DC-DC converter can be reduced, and the cost can be reduced.

[0063]

As described above, according to the present invention, the DC-D connected between the DC input power supply and the input capacitor connected between the inputs of the DC-DC converter.
The input circuit of the C converter has the functions of preventing the input of inrush power to the input capacitor, protecting against reverse connection of the DC input power supply, preventing malfunction when the input voltage drops, and holding the output voltage in the event of a momentary input loss. In addition to miniaturizing the circuit size and keeping the cost low, it is possible to provide functions such as prevention of inrush current, protection of reverse connection of the DC power supply, prevention of malfunction when the input voltage drops, and holding of the output voltage in the event of a momentary loss of input.

[Brief description of the drawings]

FIG. 1 is a DC-DC according to a first embodiment of the present invention.
It is a circuit diagram of an input circuit of a converter.

FIG. 2 is an explanatory diagram showing a graph of a relationship between an input voltage of a DC-DC converter and a voltage of an input power supply.

FIG. 3 is an explanatory diagram showing a relationship between an inrush current and time in a graph.

FIG. 4 is a DC-DC according to a second embodiment of the present invention;
It is a circuit diagram of an input circuit of a converter.

FIG. 5 is a DC-DC according to a third embodiment of the present invention;
It is a circuit diagram of an input circuit of a converter.

FIG. 6 is a circuit diagram of a conventional inrush current prevention circuit.

FIG. 7 is a circuit diagram of a conventional DC power supply reverse connection protection circuit.

[Explanation of symbols]

10, 40, 50 DC-DC converter input circuit 11 Input power supply 12 Input capacitor 13 DC-DC converter 14 Switch 15, 17 N-channel MOSFET 16 Input current detection resistor 18, 41 Shunt regulator 19, 20, 21, 27, 29, 35, 42, 44, 5
5, 56, 57 Resistance 22, 28, 54 Zener diode 23 Capacitor 24, 26 NPN transistor 25 Current limiting resistor 30 PNP transistor 31, 32, 51, 52 Input voltage detection resistor 43 Photocoupler 53 Operational amplifier

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[Procedure amendment]

[Submission date] October 22, 1999 (1999.10.
22)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claims

[Correction method] Change

[Correction contents]

[Claims]

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5H410 BB02 CC02 DD02 EA11 EB32 EB37 FF03 FF05 FF22 LL04 LL07 LL18 5H730 AA15 AA17 AA20 FD11 FD41 FF19 XC09 XX02 XX11 XX13 XX22 XX33 XX42 5H740 A02

Claims (6)

[Claims] An input circuit of a DC-DC converter connected between a DC input power supply and an input capacitor connected between inputs of a DC-DC converter, wherein an input of inrush power to the input capacitor is prevented. D having a circuit configuration having functions of protection against reverse connection of a DC input power supply, prevention of malfunction when input voltage drops, and holding of output voltage in case of momentary input loss.
Input circuit of C-DC converter. 2. A first MOSFET for protecting against reverse connection of the DC input power supply between a negative terminal of the DC input power supply and a negative terminal of the input capacitor, and a first MOSFET for detecting an input current. 2. The input circuit of a DC-DC converter according to claim 1, wherein an input current detection resistor and a second MOSFET for preventing input of inrush power to the input capacitor are connected in series. 3. The first MOSFET and the second MO.
Between the SFETs, a shunt regulator and the first
3. The input circuit of a DC-DC converter according to claim 2, wherein a PNP transistor for controlling an equivalent resistance between a drain and a source of the second MOSFET is connected. 4. The input circuit of a DC-DC converter according to claim 3, wherein a photocoupler is connected instead of said PNP transistor. 5. The input circuit according to claim 3, wherein a Zener diode for generating a reference voltage and an operational amplifier are connected instead of the shunt regulator and the PNP transistor. 6. The first MOSFET is connected to a negative terminal of the DC input power supply via a switch, and an NPN transistor for controlling an equivalent resistance between a drain and a source of the first and second MOSFETs is provided. The D according to any one of claims 2 to 5, further connected.
Input circuit of C-DC converter.