JPH0646575A - Dc-ac converter - Google Patents

Abstract

PURPOSE:To provide a highly efficient DC-AC converter for driving the two switching elements of a half bridge circuit efficiently. CONSTITUTION:A DC-AC converter is composed of main switching FETs 2a and 2b, capacitors 3a and 3b for dividing the voltage of a DC power source, auxiliary windings 5a and 5b being magnetically coupled with a main winding 4 and being connected so that positive feedback may be applied to the FETs 2a and 2b, shunt regulators 6a and 6b for controlling the on time of the switching element with high accuracy, using the induced voltage from the auxiliary winding, and time constant circuits 1a and 1b, and it can perform highly efficient power converter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a DC-AC converter, and more particularly to the structure of a switching element drive circuit suitable for improving efficiency.

[0002]

2. Description of the Related Art FIG. 4 shows an example of a conventional DC-AC converter, which is a simplified version of the circuit diagram of FIG. 3 disclosed in Japanese Patent Publication No. 3-1914. DC-A here
The C converter refers to a type of power supply device that receives direct current, converts it into different voltages, and outputs alternating current. In FIG.
The transistors 41a and 41b are main switching transistors that switch the current supplied from the DC power supply, and the capacitors 3a and 3b divide the voltage of the DC power supply to form a midpoint. The transformer 50 has a main winding 4 and an auxiliary winding 5
The transformer is composed of four windings a, 5b and a load winding 40. The main winding 4 is a winding in which the direction of the flowing current changes when the transistors 41a and 41b are alternately turned on, and electric power is taken out from the load winding 40 of the transformer. The auxiliary windings 5a and 5b of the transformer, which are magnetically coupled to the main winding, are connected to the bases of the transistors 41a and 41b via resistors 7a and 7b so as to provide positive feedback. Resistance 23
a and 23b form a time constant circuit together with capacitors 24a and 24b, respectively. The resistors 7a and 7b are resistors that limit the base currents of the transistors 41a and 41b, respectively. The auxiliary transistors 32a and 32b are alternately turned on after a certain time by the time constant circuit, and the main switching transistor is turned off alternately by drawing the base current of the main switching transistor.

The circuit of FIG. 4 operates as follows. If the transistor 41a is on, electromagnetic induction is caused from the main winding 4 and a current flows in the auxiliary winding 5a in the direction of positive feedback, so that the transistor 41a remains on. However, with the time constant determined by the resistor 23a and the capacitor 24a, a current starts to flow in the base of the transistor 32a, and the auxiliary transistor 32a is turned on. Therefore transistor 4
The base current of 1a is extracted and the transistor 41a is turned off. When the transistor 41a is turned off, the voltage applied to both ends of the main winding is inverted by the electromagnetic induction of the main winding 4, and the main transistor 41b is turned on this time. Similarly, the main transistor 41b is turned off after a fixed time, and the transistors 41a and 41b are alternately turned on.

[0004]

As described above, in the prior art, the bipolar transistor is used to draw the base current. The bipolar transistor has a problem that the characteristics and the storage time vary depending on the element, and the on-time of the main switching element is not uniform. That is, there is a problem in that the output contains frequency components other than the set frequency, which eventually leads to a decrease in efficiency.

Therefore, the present invention is intended to solve such a problem, and an object thereof is to evenly distribute the on-time of the main switching elements to improve the efficiency of the DC-AC converter. .

[0006]

Means for Solving the Problems First and second main switching FETs connected in series to a DC power supply, a connection point between the first and second main switching FETs, and an intermediate potential of the DC power supply. The main winding of the transformer connected between
The first or second main switching FET is connected between the load winding of a transformer magnetically coupled to the main winding for extracting electric power to the load and the gate and source of the first and second main switching FETs. Two auxiliary windings of a transformer that generate an electromotive force induced by the main winding so as to be positively fed back to the first or second main switching FET when a current flows from the second main switching FET to the main winding. A wire, a time constant circuit connected in series at both ends of the auxiliary winding and driven by an induced electromotive force of the auxiliary winding, and comprising a resistor and a capacitor, and an anode and a cathode for the first and second main electrodes. It is characterized in that it is composed of first and second shunt regulators which are respectively connected between the gate and the source of the switching FET to turn off the first or second main switching FET.

[0007]

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a diagram showing a concrete circuit configuration in an embodiment of the DC-AC converter of the present invention. Here, the same numbers as those in FIG. 4 indicate the same elements and means. Circuit 1a,
Reference numeral 1b is a time constant circuit, and FETs 2a and 2b are main switching elements. The capacitors 3a and 3b generate a midpoint potential from the DC power supply. The main winding 4 of the transformer sends a current to the secondary side by passing a current whose direction is changed by a main switching element which is alternately turned on and off. The auxiliary windings 5a and 5b magnetically coupled to the main winding 4 and the load winding 40 are the same as those described in the related art.
The shunt regulators 6a and 6b have the property of conducting between the cathode and the anode when a voltage higher than a certain level is applied to the reference terminal. Resistors 7a and 7b are FET2
It is a resistor for limiting the gate currents of a and 2b. The diodes 8a and 8b are diodes that block a current when a reverse bias is applied between the gates and sources of the FETs 2a and 2b. The resistors 9a, 10a, 9b and 10b are start-up bias resistors. The resistors 20a and 20b are resistors that limit the Zener diodes 21a, 21b, 22a, and 22b so that an overcurrent does not flow. Zener diode 2
Reference numerals 1a, 21b, 22a and 22b denote Zener diodes that clip the voltage induced in the auxiliary windings 5a and 5b in both positive and negative directions. The resistor 23a and the capacitor 24a and the pair of 23b and 24b are connected to the shunt regulator 6 respectively.
The time constant for turning on a and 6b is determined. Diode 25
Reference numerals a and 25b are diodes for protecting the shunt regulators when reverse bias is applied to the reference terminals of the shunt regulators 6a and 6b. The capacitors 30a and 30b are the capacitance between the gate and the source of the FET,
The diodes 31a and 31b are so-called body diodes that are structurally added to the FET.

Now, the FET2a is on and the FET2
Assuming that b is off, a current flows through the main winding from the beginning of winding of the coil (the one marked "." in FIG. 1) to the end of winding. Then, an induced voltage having a positive winding start is generated in the auxiliary winding 5a, and this voltage is generated by the resistor 7a,
It is applied to the gate of FET 2a via diode 8a. Since this is a positive feedback, the FET 2a holds the ON state, while the FET 2b holds the OFF state. The voltage induced from the auxiliary winding 5a is the Zener diode 21.
Since it is also applied to both ends of a and 22a, the voltage of the reference terminal of the shunt regulator 6a also rises with a time constant determined by the resistor 23a and the capacitor 24a. When the voltage of the reference reaches the threshold of the shunt regulator after a certain period of time, the cathode and anode of the shunt regulator 6a become conductive, and the gate voltage of the FET 2a becomes lower than the on-voltage and is turned off. Then, the current flowing through the main winding 4 stops flowing, so that the main winding 4
The voltage applied to both ends of the FET is inverted, and the FET 2b side is turned on this time. It takes a certain amount of time from when the FET 2a is turned off until when the FET 2b is turned on, but this is secured by the time constant determined by the resistor 7b and the gate-source capacitance 30b of the FET. Therefore FET2
A through current that flows when a and 2b are turned on at the same time is prevented, and safe switching can be realized. FET2
When a is off and FET2b is on, the same operation as described above is performed. After all, it is the shunt regulators 6a and 6b that determine the on-time of the two switching elements, but this shunt regulator has much less variation in characteristics than the bipolar transistor and is highly accurate. Therefore, the switching times of the two switching elements can be equalized, and high efficiency can be achieved.

FIG. 2 shows waveforms at various parts of the converter shown in FIG. (A) shows the voltage V1 between the cathode of the Zener diode 21a and the cathode of 22a. Since it is clipped by the Zener voltage Vz, it has two states of + Vz and -Vz. (B) shows the potential V2 of the reference terminal viewed from the anode of the shunt regulator 6a.
Assuming that the threshold of the shunt regulator is Vth1 and the forward voltage of the diode 25a is Vth2, there are two states of + Vth1 and -Vth2 (for example, + 2.5V and -0.6V). (C) is the gate-source voltage V3 of the FET 2a. (D) shows the voltage V4 of the reference terminal seen from the anode of the shunt regulator 6b.
(E) is the gate-source voltage V5 of the FET 2b. If it is just after the FET 2a is turned on, the voltage V1 is constant at + Vz, and V2 rises from -Vth2.
The voltage of V3 reaches the ON voltage of the FET after a dead time determined by the resistance 7a and the gate-source capacitance 30a, and the FET 2a is turned on. The voltage of V4 is the opposite of V2,
Since a reverse bias is applied to the reference terminal of the shunt regulator 6b, it drops from + Vth1 to -Vth2. After a predetermined time determined by the resistor 23a and the capacitor 24a, the reference terminal voltage V2 of the shunt regulator 6a reaches + Vth1 and the shunt regulator 6a becomes conductive. Then, the gate voltage V3 of the FET2a drops to Vth1 and the FET2a is turned off. The voltage of V1 is inverted to -Vz, the reference terminal voltage V2 of the shunt regulator 6a begins to drop to -Vth2, and the reference terminal voltage V4 of the shunt regulator 6b is increased.
Starts rising to + Vth1. At this time, the gate voltage of the FET 2b reaches the ON voltage after a dead time determined by the resistor 7b and the gate-source capacitance 30b as shown by the waveform of V5, and the FET 2b is turned on.

FIG. 3 shows a concrete circuit in another embodiment of the DC-AC converter of the present invention.
The same numbers as in FIG. 1 indicate the same elements and means. Reference numeral 1 is a time constant circuit, and FETs 2a and 2b are main switching elements. Three
Reference numerals a and 3b are capacitors, and 4 is a main winding of a transformer.
Reference numerals 5a and 5b are auxiliary windings of the transformer, 40 is a load winding of the transformer, and 6a and 6b are shunt regulators. The resistors 7a and 7b are for limiting the gate current of the FETs 2a and 2b. The diodes 8a and 8b are FET2
The current is blocked when a reverse bias is applied between the gate and the source of a and 2b. The resistor 20 is a Zener diode 2
It is restricted so that an overcurrent does not flow in Nos. 1 and 22. Resistance 23
a and a capacitor 24a, and a pair of a resistor 23b and a capacitor 24b are shunt regulators 6a and 6b, respectively.
Determines the time constant for turning on. Diodes 25a, 25
b protects the shunt regulator. Capacitor 30
Reference numerals a and 30b are gate-source capacitances of the FETs 2a and 2b, respectively. The diodes 31a and 31b are FETs
Is a so-called body diode that is structurally added to.

As described with reference to FIG. 1, the shunt regulators 6a and 6b alternately set the gate voltages of the FETs 2a and 2b below the threshold, thereby switching from ON to OFF, and switching continues alternately. The waveform of each part at this time is the same as that shown in FIG.
The voltage between the cathode of the Zener diode 21 and the cathode of the Zener diode 21 is the same as V1 in FIG. The reference voltage seen from the anode of the shunt regulator 6a is V2 in FIG. 2 (b). The gate voltage seen from the source of the FET 2a is V3 in FIG. 2 (c). On the other hand, the reference voltage seen from the anode of the shunt regulator 6b is V4 in FIG. 2 (d). The gate voltage seen from the source of the FET 2b is V5 in FIG. 2 (e). As shown by the waveforms of V3 and V5, the ON times of the two FETs are equal. Further, when it is turned on, a dead time is secured, and when it is turned off, the shunt regulator causes the gate voltage to rapidly fall and the state changes from on to off. Furthermore, by using a shunt regulator, the on-time of two switching elements can be determined with high accuracy. Therefore, by making the ON times equal, it is possible to perform highly efficient DC-AC conversion.

[0013]

As described above, in the DC-AC converter of the present invention, in order to turn off the main switching element,
Since a shunt regulator with less variation in characteristics is used, the on times of the two switching elements can be made equal, and a highly efficient DC-DC converter can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a specific circuit configuration in an embodiment of a DC-AC converter of the present invention.

FIG. 2 is a diagram showing a waveform of each part of the DC-AC converter of the present invention.

FIG. 3 is a diagram showing a specific circuit configuration in another embodiment of the DC-AC converter of the present invention.

FIG. 4 is a diagram showing an embodiment of a DC-AC converter according to the prior art.

[Explanation of symbols]

 1, 1a, 1b ... Time constant circuit 2a, 2b ... Main switching element (FET) 3a, 3b ... Capacitor 4 ... Transformer main winding 5a, 5b ... Transformer auxiliary winding 6a, 6b ... Shunt regulator 21a, 21b, 22a, 22b ... Zener diode 23a, 23b ... Resistor 24a, 24b ... Capacitor 30a, 30b ... FET gate-source capacitance 31a, 31b ... FET body diode 40 ... Transformer load winding + V, -V ... DC power source positive / negative terminals

Claims (1)

[Claims] 1. A first and a second main switching FETs connected in series to a DC power source, and a connection point between a connection point of the first and second main switching FETs and an intermediate potential of the DC power source. A main winding of the transformer, a load winding of the transformer magnetically coupled to the main winding for extracting electric power to a load, and a gate and a source of the first and second main switching FETs. When an electric current flows from the first or second main switching FET to the main winding, the induced electromotive force generated by the main winding is connected to the first or second main switching FE.
It consists of two auxiliary windings of a transformer that generate positive feedback with respect to T, and a resistor and a capacitor that are connected in series at both ends of the auxiliary winding and that are driven by the induced electromotive force of this auxiliary winding. A time constant circuit, first and second anodes and cathodes respectively connected between the gate and source of the first and second main switching FETs to turn off the first or second main switching element,
And a shunt regulator of the present invention.