JP2501364Y2 - Step-up switching regulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Field of Industrial Application The present invention relates to a step-up switching regulator that boosts an input voltage and supplies a power supply voltage to an electronic device.

(B) Conventional Technology Conventionally, a so-called chopper type switching regulator has been used as a switching regulator that does not require insulation between an input and an output. In general,
The chopper type switching regulator is composed of a switching transistor that controls the amount of electric power passing through, a diode that rectifies and smoothes the switched DC voltage, a choke coil, and a capacitor. And
When a voltage higher than the input voltage is required, a boost switching regulator is used.

FIG. 2 shows a circuit of a conventional step-up switching regulator. In FIG. 2, IN is an input terminal and OUT is an output terminal. Choke coil between this input terminal and output terminal
A series circuit of L1 and diode D1 is provided, and a switching transistor Q1 is provided between the connection point of choke coil L1 and diode D1 and ground. In the figure, reference numeral 1 denotes a switching control circuit that controls on / off of the switching transistor Q1 and operates using a voltage (Vcc) supplied from an input terminal as a power source. A capacitor C1 that smoothes the ripple component of the input voltage of the input terminal IN is connected between the input terminal and the ground, and a capacitor C2 that smoothes the output voltage is provided on the output side of the diode D1. A variable resistor VR1 for output voltage feedback is further provided between the output side of the diode D1 and the ground. Further, an output control transistor Q2 is provided at the final stage of the output terminal. In the figure, 2 is a comparator, which is a resistor R6 provided between the input terminal and the ground, the output voltage of the constant voltage generating circuit including the Zener diode ZD1, and a resistor R4 provided between the output side of the diode D1 and the ground. , And compare with the divided voltage of R5. This comparator 2
Transistor between the output of Q2 and the output control transistor Q2
Q4 and resistor R3 are connected.

The operation of the circuit shown in FIG. 2 is as follows. First, when the rated voltage is supplied to the input terminal IN, the switching control circuit 1 turns on / off the switching transistor Q1. Energy is stored in the choke coil L1 during the on period of Q1, and the energy of the choke coil L1 is released through the diode D1 during the off period of Q1. Capacitor at this time
C2 smoothes the output voltage. The switching control circuit 1 PWM-controls the switching transistor Q1 according to the voltage of the output (REF) of the variable resistor VR1. That is, the input voltage of the REF terminal is compared with the internal reference voltage, and based on the result, the terminal S
The output duty is stabilized by setting the on-duty ratio of the output rectangular wave. In such a normal state, the output voltage of the voltage divider circuit consisting of resistors R4 and R5 is R6 and Zener diode ZD1.
It is set higher than the reference voltage generated more, the output of the comparator 2 becomes a high potential (power supply voltage), the transistor Q4 is turned on, and the transistor Q2 is turned on.

When the switching control circuit 1 fails for some reason, the switching transistor Q1 is destroyed, etc. and the switching operation is stopped, the input voltage is continuously supplied to the emitter of the output control transistor Q2 through the choke coil L1 and the diode D1. It will be. In this state, the output voltage of the voltage divider circuit consisting of resistors R4 and R5 becomes lower than the reference voltage of Zener diode ZD1, and comparator 2
Becomes a low potential (ground potential) and the transistor Q4 is turned off. As a result, the output control transistor Q2 is also turned off and the output of the DC voltage is cut off. In this way, an uncertain voltage lower than the rated voltage is not supplied to the circuit of the load device.

(C) Problems to be Solved by the Invention As described above, in the step-up switching regulator provided with the protection circuit so that the input voltage is not directly output when the switching operation is stopped, it is uncertain in the electronic device as the load. Since the power supply voltage is not supplied, there is an effect that the failure or malfunction can be prevented.

However, the conventional protection circuit considers the decrease of the output voltage as the stop of the switching operation and shuts off the output when the output voltage falls below a certain voltage. was there. That is,
The Zener voltage value of the Zener diode ZD1 varies, and the Zener current of the Zener diode ZD1 changes due to the variation of the resistance value of the resistor R6, which also changes the Zener voltage value and the variation of the resistance values of the resistors R4 and R5. As a result, the voltage division ratio of the resistors varies and the detected voltage of the comparator 2 also varies to some extent. Therefore, when the difference between the input voltage and the output voltage is small due to the specifications of the switching regulator, and the variation width of the detected voltage by the comparator 2 becomes larger than the difference between the input voltage and the output voltage, the reliable protection does not work and is sufficient. There was a problem that reliability could not be secured.

An object of the present invention is to provide a step-up switching regulator capable of reliably interrupting the voltage output when the switching operation is stopped even when the difference between the input voltage and the output voltage is small.

(D) Means for Solving the Problems A step-up switching regulator of the present invention is a series circuit of a choke coil and a diode provided between an input terminal and an output terminal, and a connection point between the choke coil and the diode and ground. In a step-up switching regulator consisting of a switching transistor provided and a switching control circuit that controls the switching transistor to turn on and off by using an input voltage as a power supply, a CR time constant circuit in which a capacitor is connected to the ground side and a resistor is connected to the input terminal side. And a diode for charging and discharging the charge of the capacitor between the collector or drain of the switching transistor and the capacitor, including an output control transistor connected between the diode and the output terminal, and the charging voltage of the capacitor. Above when falling Characterized by providing an output control circuit for turning on the power control transistor.

(E) Function In the step-up switching regulator of this invention, CR
The capacitor of the time constant circuit is charged with the input voltage from the input terminal with a constant time constant. However, since a diode for charging and discharging the capacitor is provided between the collector or drain of the switching transistor and the capacitor, the charge of the capacitor is discharged through the diode and the switching transistor when the switch transistor is turned on. . During normal switching operation, the charge charged in the capacitor is quickly discharged during the ON period of the switching transistor, so that the output voltage of the capacitor becomes almost equal to the ground potential. The output control circuit turns on the output control transistor when the charging voltage of the capacitor is approximately the ground potential. If the switching operation is stopped while the switching transistor is off for some reason, the capacitor rises with a constant time constant and exceeds a constant voltage. As a result, the output control circuit turns off the output control transistor.

As described above, the voltage of the capacitor changes with the voltage width of the ground potential from the input voltage depending on the presence / absence of the switching operation, so that the capacitor can be reliably operated even when the difference between the input voltage and the output voltage is small.

(F) Embodiment FIG. 1 shows a circuit of a step-up switching regulator according to an embodiment of the present invention. In FIG. 1, IN is an input terminal and OUT is an output terminal. This input terminal IN and output terminal O
A series circuit of choke coil L1, diode D1 and output control transistor Q2 is connected between UTs. A switching transistor Q1 is provided between the connection point of the choke coil L1 and diode D1 and ground, a capacitor C1 is provided between the input terminal and ground to smooth the ripple component of the input voltage, and a smooth output is provided between the output of diode D1 and ground. The capacitor C2 is provided. A variable resistor VR1 for output voltage feedback is further provided between the output of the diode D1 and the ground. A CR time constant circuit consisting of a resistor R1 and a capacitor C3 is provided between the input terminal and ground, and the capacitor C3 and switching transistor Q1
A diode D2 for charging and discharging the capacitor C3 is connected between the collectors of the. Further, a resistor R2 and a transistor Q are provided between the capacitor C3 and the base of the output control transistor Q2.
A transistor circuit composed of 3 and a resistor R3 is provided. This transistor circuit and output control transistor Q2
Corresponds to the output control circuit according to the present invention. In the figure, 1 is a switching control circuit that operates using the voltage at the input terminal as the power supply (Vcc), and is a variable resistor that detects the output voltage.
Depending on the output voltage of VR1, the switching transistor Q1 is set to P
WM control.

The operation of the circuit shown in FIG. 1 is as follows. First,
When the rated voltage is applied to the input terminal IN, the switching control circuit 1 turns on / off the switching transistor Q1. During the ON period of Q1, the input current flows through the choke coil L1 and energy is stored in the choke coil L1. During the off period of Q1, the energy of choke coil L1 is released through diode D1. At that time, the capacitor C2 smoothes the output voltage. On the other hand, since the input voltage is also supplied to the CR time constant circuits R1 and C3, the capacitor C3 starts charging with a constant time constant, but during the ON period of the switching transistor Q1, the capacitor C3 is connected via Q1 and the diode D2. The charge of is discharged. Therefore, the operation in which the charging of the capacitor C3 is started after Q1 is switched from on to off and the capacitor C3 is discharged again by turning on Q1 is repeated. CR time constant circuit Since the discharge time constant of the circuit of C3, D2, Q1 is extremely small compared to the charge time constant of R1, C3, the capacitor C3
The charging voltage of will be kept almost at ground potential. Therefore, the base potential of the transistor Q3 becomes almost the ground potential and becomes conductive, and the base current flows through the output control transistor Q2 through the resistor R3 and the transistor Q3. As a result, Q2 becomes conductive and the output voltage is supplied from the output terminal OUT.

If the switching operation is stopped while Q1 remains off due to a failure of the switching control circuit 1 or destruction of the switching transistor Q1 for some reason, the charge of the capacitor C3 will not be discharged and its voltage will have a constant time constant. Rises at. When the voltage of the capacitor C3 exceeds the input voltage minus the forward drop voltage of the diode D1, the emitter-base voltage of the transistor Q2, and the emitter-base voltage of the transistor Q3.
The base current of Q3 becomes 0 and turns off, and the output control transistor Q2 turns off accordingly. Therefore, the voltage output is cut off. When the switching transistor Q1 is destroyed and the collector-emitter is short-circuited, the cathode potential of the diode D1 becomes the ground potential, and the electric charge of the capacitor C2 is quickly discharged to the output side. Also indeterminate voltage is not delivered to the load.

Although a bipolar transistor is used as the switching transistor Q1 in the embodiment, a MOS-FET can be used as the switching transistor if the output part of the switching control circuit 1 is of the voltage output type. At that time, the diode D2 may be connected to the drain as a source ground.

(G) Effect of the Invention According to this invention, the circuit configuration is not such that the output is cut off when the output voltage falls below a certain value, but according to the capacitor charging voltage that greatly changes due to the continuation / stop of the switching operation of the switching transistor. Since the output control transistor is controlled, even if the difference between the input voltage and the output voltage is small, the abnormality of the switching operation can be detected promptly and the output can be cut off, so that the reliability of the power supply circuit is improved. Further, since the comparator and the reference voltage generation circuit for giving the reference voltage are not required,
The circuit configuration is simplified and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a step-up switching regulator according to the present invention, and FIG. 2 is a circuit diagram of a conventional step-up switching regulator. 1 ... Switching control circuit, 2 ... Comparator, R
1, C3 …… CR time constant circuit, D2 …… Discharge diode, Q2…
... Output control transistor.

Claims (1)

(57) [Scope of utility model registration request] 1. A series circuit of a choke coil and a diode provided between an input terminal and an output terminal, a switching transistor provided between a connection point of the choke coil and the diode and ground, and the switching using an input voltage as a power source. A step-up switching regulator consisting of a switching control circuit that controls on / off of a transistor, with a capacitor connected to the ground side and a resistor connected to the input terminal side.
A CR time constant circuit, a diode for charging and discharging the capacitor is provided between the collector or drain of the switching transistor and the capacitor, and includes an output control transistor connected between the diode and the output terminal. A step-up switching regulator provided with an output control circuit for turning on the output control transistor when the charging voltage of the capacitor drops.