JPS6181174A - Switching circuit - Google Patents

Abstract

PURPOSE:To reduce the switching loss by connecting a series circuit of a parallel connection unit of a diode and an inductance element and a capacitor between the drain and the source of a field effect transistor. CONSTITUTION:A switching power source is composed of a field effect transistor (FET)Q1, and a DC voltage V0 is output from output terminals 1, 2 through rectifying and smoothing circuits D1-D2, C1, L1 from the winding N2 of the transformer T1. In this case, a diode D2 provided between the drain and the source of the FETQ1, the diode D3 of the capacitor C2, and an inductance element L2 are provided in parallel. Thus, when the FETQ1 is turned from ON to OFF, since the diode D3 and the capacitor C2 become a low impedance circuit, the rise of the voltage Vds between the drain and the source is smoothed. When the FETQ1 is turned from OFF to ON, the discharge of the capacitor C2 is prevented by the element L2 to reduce the ON time loss.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a metal oxide film semiconductor (hereinafter referred to as O8) 5
The switching circuit according to the present invention is applied to a switching power supply or the like in which a main switch is formed of a field effect transistor.

Conventional technology and its problems Conventionally, switching power supplies have generally used bipolar power transistors as the main switching elements connected to the conversion transformer.
Recently, switching power supplies that use MO9 field effect transistors as switching elements instead of bipolar transistors have been attracting attention. This is because switching power supplies using field effect transistors are superior in terms of faster switching, smaller size, and higher efficiency than those using bipolar transistors.

Figure 4 shows a conventional example of a switching power supply using field effect transistors, in which a DC voltage Win obtained by rectifying and smoothing a commercial AC power supply is applied between the drain D and source S of the field effect transistor Q1. While applying
A pulse Pl is supplied to the gate G from a signal source (not shown) to turn it on and off, and the transformer Tz is connected to the drain D side (or source S side) of the field effect transistor Ql, and is connected to the winding Nl to the winding Nz side. Then, the switching output associated with the on/off operation is taken out, and this switching output is further connected to the diode D1. D2, a rectifying and smoothing circuit composed of a choke coil L1, a capacitor C1, etc. converts the voltage into direct current, and outputs a direct current voltage Vo from an output terminal 1.2.

A parallel circuit of a diode D3 and a resistor R1, and a series circuit of a capacitor C2 are connected between the drain D and source S of the field effect transistor Ql, and these circuits control the switching operation of the field effect transistor Qz. This is to reduce the losses associated with this. That is, there are three types of losses in the field effect transistor Ql: OFF loss when transitioning from ON to OFF, ON loss when transitioning from OFF to ON, and loss during saturated operation. Among these losses, on-state loss and off-state loss are as follows:
Drain-source voltage V of field effect transistor Q1
dg and the drain current Ids. Therefore, in this conventional example, a diode D3 and a resistor R are connected between the drain D and the source S of the field effect transistor Q1.
Connect the parallel circuit of t and the series circuit of capacitor C2,
Drain-source voltage Vds and drain voltage 11rds
This is to reduce the phase difference between the two and the cross point time of the two, thereby reducing the loss.

In the conventional circuit described above, when the field effect transistor Qz shifts from on to off, the drain-source voltage Vds increases and the drain current Ids tends to decrease. Here, the drain D of the field effect transistor Q1
Since a series circuit of a diode D3 and a capacitor C2 is connected between the source S and the source S, the current ■1 flowing in the DC power supply line is sucked into the circuit consisting of the diode D3 and the capacitor C2, and charges the capacitor C2. Therefore, the attenuation of the drain current Idg is promoted. Moreover, since the diode D3 and the capacitor C2 form a low impedance circuit, the drain-source voltage Vds rises slowly. Therefore, the cross point time between the drain-source voltage Vds and the drain current Ids is shortened, and the off-state loss is reduced.

The charge accumulated in capacitor C2 when field effect transistor Q1 transitions from on to off is discharged through resistor R1 when field effect transistor Q1 transitions from off to on.

Problems with the Prior Art However, the conventional switching circuit described above has the following drawbacks.

(a) Since the configuration is such that the charge of the capacitor C2 inserted to improve the loss when the field effect transistor Ql is on is discharged through the resistor FLl when the field effect transistor Ql is off, the resistor R1
losses due to this cannot be avoided.

(b) By reducing the value of the resistor R1, the capacitance of the capacitor C2 can be increased, so that the off-state loss can be reduced. However, if the value of resistor R1 is decreased, the discharge current increases and the heat generated by resistor Rz increases, so a large resistor with a large power capacity must be used.
This results in an increase in the size of the circuit configuration, a decrease in packaging density, and an increase in cost.

(c) By increasing the value of the resistor R1, it is possible to reduce the size of the resistor l÷, but since the discharge time constant Ri and C2 increase, the capacitance of the capacitor C2 must be reduced, and the off-state loss decreases. cannot be reduced.

(d) The above-mentioned drawbacks become m″Ii as the switching wave number is increased and miniaturization is attempted. This becomes an obstacle to increasing the frequency of the switching power supply and thereby miniaturizing it.

OBJECTS OF THE INVENTION The present invention solves the above-mentioned conventional problems and solves the problems without impairing high-speed switching characteristics.
It is an object of the present invention to provide a switching circuit that can reduce switching loss without increasing costs or increasing heat generation.

Structure of the Invention In order to achieve the above object, the present invention provides a switching circuit that turns on/off a field effect transistor by supplying a pulse to the gate of a field effect transistor to which a direct current is applied between the drain and the source. The present invention is characterized in that a parallel circuit of a diode and an inductance element, and a series circuit of a capacitor are connected between the drain and source of the field effect transistor.

Embodiment FIG. 1 is an electrical circuit connection diagram of a switching circuit according to the present invention. In FIG. 1, the same reference numerals as in FIG. 4 indicate the same components. The difference from the conventional method is that an inductance element L2 is connected in parallel to the diode D3.

The circuit constant value of this inductance element L2 is set to a value that can prevent the discharge current of the capacitor C2 from flowing until the drain-source voltage Vds reaches the saturation voltage value, taking into account the switching frequency. do.

Next, the circuit operation of the switching circuit shown in this embodiment will be explained with reference to FIGS. 2 and 3. first,
When the field effect transistor Ql shifts from on to off, the circuit operates in the same way as the conventional one shown in FIG. That is, as shown in FIG. 2, the current 11 flowing in the DC power supply line is sucked into the circuit consisting of the diode D3 and the capacitor C2 and charges the capacitor C2, thereby promoting the attenuation of the drain current Ids. . Moreover,
Since the diode D3 and the capacitor C2 form a low impedance circuit, the drain-source voltage Vds rises slowly. Therefore, the cross point time T1 between the drain-source voltage Vda and the drain current Ids is shortened, and the off-state loss is reduced.

Next, when the field effect transistor Q1 shifts from off to on, the drain/source voltage Vds decreases,
Drain current Ida tends to increase. Here, the inductance element L2 prevents the charge accumulated in the capacitor C2 from discharging until the drain-source voltage Vds reaches the saturation voltage value. Therefore, as shown in FIG. 3, the drain of the field effect transistor Q1,
The difference between the source 1ltI voltage Vds and the drain current rds is controlled so that the a point time T2 is shortened, and the on-time loss is reduced.

Furthermore, since the inductance element L2 has a low resistance value, the loss is reduced compared to a conventional circuit using a resistor.

Furthermore, even if the capacitance fa of the capacitor C2 is increased, the discharge current blocking effect by the inductance element L2 is not affected. Therefore, it is possible to use a capacitor with a large capacitance as the capacitor C2, and the capacitor C2 is connected to the capacitor C2 through the diode D3. increasing the charging fIl flow into the
Off-state loss can be reduced.

Furthermore, even if the switching frequency is increased to increase the speed, the above-described loss improvement effect and high-speed switching characteristics are not impaired, and the inductance value required for the inductance element L2 can be reduced. Therefore, it becomes easy to increase the frequency of the switching power supply and thereby reduce its size.

11 Effects of Unexploded As described above, the present invention provides a switching circuit that turns on and off a field effect transistor to which a direct current is applied between the drain and the source by supplying a pulse to the gate of the field effect transistor. between the drain and source of the effect transistor.

Since it is characterized by connecting a circuit consisting of a parallel connection of a diode and an inductance element to a series circuit of a capacitor, it does not impair high-speed switching characteristics, and does not cause an increase in the size of one circuit, cost increase, or increase in heat generation. Therefore, it is possible to provide a switching circuit that can reduce switching loss.

[Brief explanation of drawings]

: Figure JJ1 is an electrical circuit diagram of a switching power supply to which the switching circuit according to the present invention is applied, and Figure 2 shows the relationship between the drain-source voltage and drain current when a field effect transistor transitions from on to off. Figure 3 is an enlarged diagram showing the relationship between drain and source voltage and drain current when a field effect transistor transitions from OFF to ON on an enlarged time scale.
FIG. 4 is an electrical circuit diagram of a conventional switching circuit. Qr...Field effect transistor D3...Diode L2...φ inductance element C2...φ capacitor Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] (1) In a switching circuit that supplies a pulse to the gate of a field effect transistor to which a direct current is applied between the drain and source to turn it on and off, a diode is connected between the drain and source of the field effect transistor. A switching circuit characterized in that a circuit consisting of a parallel connection of an inductance element and a series circuit of a capacitor are connected.