JPH07213049A - Switching power unit - Google Patents

Abstract

PURPOSE:To provide a step-up switching power unit with high efficiency by reducing the loss generated in a rectifier element without complicating the circuit constitution. CONSTITUTION:An n-channel MOS FET is used for a switching transistor Q1, and a p-channel MOS FET is used for a transistor Q2 as a rectifier element. A current path is formed by connecting a resistor R3 between the source and gate of the transistor Q2, and further the gate of the transistor Q2 is connected to an output terminal, where the gate of the switching transistor Q1 of a control circuit 3 is connected, through a capacitor C3. Hereby, since MOS FET is used for the rectifier element, the loss occurring in the rectifier element lessens. The transistor as a rectifier element can be driven with the same control signal as a switching transistor, having only the resistor element and the capacitive element added. Therefore, a switching power unit simple in circuit constitution and high in efficiency can be gotten.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a step-up type switching power supply device which realizes high efficiency by reducing loss generated in a rectifying element.

[0002]

2. Description of the Related Art There are various circuit systems for a step-up type switching power supply device that obtains an output voltage higher than an input voltage. However, in the case of a relatively small power supply device, a circuit configuration such as that shown in FIG. An insulation type switching power supply device is often used. The circuit configuration of the conventional switching power supply device shown in FIG. 4 is as follows. In FIG. 4, 1
Is the input terminal on the high potential side of the switching power supply device, 2 is the output terminal on the high potential side, and the input and output terminals on the low potential side of the switching power supply device are shown in common with the ground. The input terminal 1 is connected to the ground via the capacitor C1, and the input terminal 1 is further connected to the drain of the switching transistor Q1 of the N-channel MOS FET via the choke coil L1. The source of the switching transistor Q1 is connected to the ground, and the gate is connected to the output terminal of the control circuit 3.

The connection point between the choke coil L1 and the drain of the switching transistor Q1 is connected to the anode of the diode D1 and the cathode of the diode D1 is connected to the output terminal 2. A smoothing capacitor C2 is connected between the output terminal 2 and ground, and a series circuit of resistors R1 and R2 is connected. The connection point of the resistors R1 and R2 is connected to the voltage detection terminal of the control circuit 3. In the circuit having the above configuration, when the switching transistor Q1 is turned off, the flyback voltage is generated in the choke coil L1 and the input voltage V
A high voltage in which the flyback voltage is superimposed on _IN charges the smoothing capacitor C2 via the diode D1. Then, the voltage across the smoothing capacitor C2 is supplied from the output terminal 2 to the outside as the output voltage V _O.

[0004]

The conventional switching power supply device shown in FIG. 4 uses a diode D1 as a rectifying element. However, the diode element has its PN
Due to the structure of the junction, there is a forward drop voltage V _{F in the} forward direction in which the current flows. Therefore, a loss corresponding to the product of the forward drop voltage V _F and the forward current I _F of the diode D1 occurs in the diode D1. In a switching power supply device using a diode element as a rectifying element as shown in FIG. 4, particularly when a large output current flows, the loss generated in this diode element accounts for a large proportion of the loss of the entire switching power supply device. However, it has been an obstacle in improving the efficiency of the switching power supply. Therefore, the present invention is
An object of the present invention is to provide a step-up type switching power supply device that reduces loss generated in a rectifying element without complicating the circuit configuration and realizes high efficiency.

[0005]

According to the present invention, a choke coil and a switching element formed by a field effect transistor are connected in series, and energy generated by a flyback voltage generated in the choke coil when the switching element is turned off is rectified and smoothed. Thus, in the step-up switching power supply device that obtains an output voltage higher than the input voltage, a field effect transistor having a polarity opposite to that of the switching element is used as a rectifying element, and the gate and source of the transistor rectifying element are A switching power supply device characterized in that a current path is formed by a passive element other than a capacitive element, and a control signal applied to the gate of the switching element via the capacitive element is applied to the gate of the transistor rectifying element. is there.

[0006]

EXAMPLE The voltage generated in the main current path in the ON state of the transistor element operating in the saturation region is much smaller than the forward voltage V _F of the diode element.
If a transistor element is used as the rectifying element, the loss in the rectifying element can be reduced. Therefore, in the present invention, a MOS FET is used as the rectifying element of the switching power supply device, and the circuit having the configuration shown in FIG. 1 is used to drive the MOS FET as the rectifying element. In addition, in FIG.
The same components as those in FIG. 4 are designated by the same reference numerals.
The circuit configuration of the switching power supply device according to the present invention shown in FIG. 1 is as follows. Input terminal 1 is a capacitor C1
The input terminal 1 is connected to the drain of the switching transistor Q1 formed by the N-channel MOS FET via the choke coil L1.

The source of the switching transistor Q1 is connected to the ground, and the gate is connected to the output terminal of the control circuit 3. The connection point between the choke coil L1 and the drain of the switching transistor Q1 should be a P-channel MOS FET.
Is connected to the drain of the transistor Q2 as a rectifying element, and the source of the transistor Q2 is connected to the output terminal 2. A resistor R3 is provided between the source and gate of the transistor Q2.
To form a current path, and the gate of the transistor Q2 is further connected to the output terminal of the control circuit 3 to which the gate of the switching transistor Q1 is connected via the capacitor C3. A smoothing capacitor C2 is connected between the output terminal 2 and ground, and a series circuit of resistors R1 and R2 is connected. The connection point of the resistors R1 and R2 is connected to the voltage detection terminal of the control circuit 3.

In the switching power supply device according to the present invention shown in FIG. 1, since the switching transistor Q1 and the transistor Q2 as a rectifying element have opposite polarities, the switching transistor Q1 and the transistor Q2 receiving the same control signal perform complementary operations. Will be done. Here, a resistor R3 is provided between the source and gate of the transistor Q2.
Current path is formed, and the transistor Q2
Capacitor C3 between the gate of control circuit 3 and the output terminal of control circuit 3
Are connected, the potentials of the source and the gate of the transistor Q2 are statically the same unless a voltage is applied via the capacitor C3. However, capacitor C3
Since the DC component is blocked by the presence of the above, a backflow from the gate of the transistor Q2 to the output terminal of the control circuit 3 does not occur, and only a pulsed control signal from the control circuit 3 can pass through the capacitor C2.

Therefore, first, when the voltage of the control signal from the control circuit 3 falls, the switching transistor Q1 is turned off and a flyback voltage is generated in the choke coil L1. Further, when the voltage of the control signal falls, the voltage between the source and gate of the transistor Q2 becomes a positive bias state, and the transistor Q2 is turned on.
As a result, a high voltage in which the flyback voltage generated in the choke coil L1 is superimposed on the input voltage V _IN charges the smoothing capacitor C2, and the voltage appearing across the smoothing capacitor C2 is supplied to the outside as the output voltage V _O. Next, when the voltage of the control signal from the control circuit 3 rises, the switching transistor Q1 turns on, and a current flows from the input terminal 1 through the choke coil L1, the switching transistor Q1 and the ground. When the voltage of the control signal rises, the potential of the gate of the transistor Q2 becomes higher than the potential of its source, and the transistor Q2 is turned off. That is, in the present invention shown in FIG. 1, by using MOS FETs having opposite polarities for the switching transistor Q1 and the transistor Q2, the operations of the two transistors are complemented, and the switching transistor Q1 is driven by the resistor R3 and the capacitor C3. From the control signal for controlling, the voltage between the source and gate required for the on / off operation of the transistor Q2 can be obtained.

On the other hand, in the circuit shown in FIG. 2, the current path between the source and the gate of the transistor as the rectifying element is formed by the inductance element. In the circuit of FIG. 2, an inductance L2 is connected between the source and gate of the transistor Q2 instead of the resistor R3 in the circuit of FIG.
The other circuit configuration is exactly the same as in FIG. The inductance element has an impedance with respect to a current and a voltage whose value changes. Therefore, the inductance L2 of FIG.
Is a transistor Q, which is statically similar to the resistor R3 in FIG.
The source and gate of 2 have the same potential, but the control circuit 3
When the pulse-shaped control signal from is applied through the capacitor C2, the source-gate voltage required for the on / off operation of the transistor Q2 is generated. Also,
The use of an inductance in the current path has an advantage that a flyback voltage is generated when the voltage of the control signal falls to improve the turn-on speed of the transistor Q2.

Further, in the circuit shown in FIG. 3, the current path between the source and the gate of the transistor as the rectifying element is formed by a diode element. In the circuit of FIG. 3, instead of the resistor R3 of FIG. 1 and the inductance L2 of FIG. 2, a diode D2 having a forward direction from the source to the gate of the transistor Q2 is provided. 2 is the same as the circuit shown in FIG. 1 and 2, the current path provided between the source and the gate of the transistor rectifying element as the rectifying element is formed by only the resistance element or the inductance element, but the resistance element and the inductance element are connected. The current path may be formed by an impedance circuit. In the description of the embodiments of the present invention, the switching element is an N-channel MOS FET,
Although the explanation has been given using the P-channel MOS FET as the transistor rectifying element, the P-channel M-type as the switching element.
N-channel MO for OS FET and transistor rectifier
You may use SFET. However, in this case, the input terminal 1 and the output terminal 2 are on the low potential side with respect to the ground.

[0012]

As described above, the switching power supply device according to the present invention uses a MOS FET having a polarity opposite to that of the switching element as the rectifying element, and a current between the gate and the source of the transistor as the rectifying element. A path is provided and a control signal for driving the switching transistor is input to the gate of the transistor rectifying element via the capacitive element. Since the MOS FET is used as the rectifying element, the loss generated in the rectifying element is reduced as compared with the case where the conventional diode is used, and the switching power supply device with high efficiency can be obtained. Further, the transistor as the rectifying element can be driven by the control signal for driving the switching transistor simply by adding an element for forming a current path with the capacitive element,
The circuit configuration is simple.

[Brief description of drawings]

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

FIG. 2 is a circuit diagram of another embodiment of the switching power supply device according to the present invention.

FIG. 3 is a circuit diagram of still another embodiment of the switching power supply device according to the present invention.

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

[Explanation of symbols]

 1 Input Terminal 2 Output Terminal 3 Control Circuit Q1 Switching Transistor Q2 Transistor as Rectifier R3 Resistor Element Forming Current Path L2 Inductance Element Forming Current Path D2 Diode Element Forming Current Path

Claims (3)

[Claims] 1. The input voltage is higher than the input voltage by connecting a choke coil and a switching element formed by a field effect transistor in series, and rectifying and smoothing energy due to a flyback voltage generated in the choke coil when the switching element is turned off. In a step-up switching power supply device for obtaining an output voltage, a field effect transistor having a polarity opposite to that of the switching element is used as a rectifying element, and a passive element other than a capacitive element is provided between a gate and a source of the transistor rectifying element. Current path by
Further, a switching power supply device characterized in that a control signal applied to the gate of the switching element is applied to the gate of the transistor rectifying element via a capacitive element. 2. The switching power supply device according to claim 1, wherein the current path is formed by a resistance element or an inductance element or an impedance circuit including a resistance element and an inductance element. 3. The switching power supply device according to claim 1, wherein the current path is formed by a diode element having a forward direction from a source to a gate of a transistor as a rectifying element.