JPH03207275A - Push-pull inverter - Google Patents

Abstract

PURPOSE:To enable stable oscillation by feeding back the primary coil voltage of a step-up transformer, which occurs when a switching element shifts from ON to OFF, to the control pole of the switching element. CONSTITUTION:When a transistor 14 as the second switching element changes over from ON to OFF, the voltage by counter electromotive force, which has occurred in the primary coil 12P2 on the opposite side from the middle tap (a) of a step-up transformer 12, is fed back to the base of a transistor 13 through a capacitor 23 to change over the transistor 13 from OFF to ON. Moreover, when the transistor 13 as the first switching element changes over from ON to OFF, the voltage by counter electromotive force, which has occurred in the primary coil 12P1 on one side of the transformer 12, is fed back to the base of the transistor 14 through a capacitor 24 to change over the transistor 14 from OFF to ON. Hereby, the transistors 13 and 14 surely repeat ON alternately.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a push-pull inverter used as a driver for, for example, cold cathode discharge tubes or hot cathode discharge tubes.

"Prior Art" Various circuit configurations are known as push-pull inverters, an example of which is shown in FIG.

FIG. 5 shows an inverter circuit configured as a driver for the fluorescent tube 11, including a step-up transformer 12, transistors 13 and 14 for switching operation, and a choke coil 1.
It is a push-pull circuit composed of 5 and the like.

In this inverter circuit, when the power switch 16 is closed, the transistor 17 acting as a power supply switch is turned ON.
,, DC power is supplied from a DC power supply 18.

This causes the base current to flow into the transistor 13 through the resistor 19 and into the transistor 14 through the resistor 20, respectively. Therefore, these transistors 13.14
However, depending on the transistor characteristics and circuit configuration, one of the transistors becomes more conductive, and this transistor turns on first.

For example, when the transistor 13 is turned on first, the current sent from the DC power supply 18 flows through the choke coil 15 to the primary coil 12P of the transformer 12, and a voltage in the direction of the solid line shown in the figure is generated in the primary coil 12P. The collector potential of the transistor 13 becomes lower than the collector potential of the transistor 14.

At this time, an induced voltage in the direction of the solid line shown in the figure is generated in the secondary coil 12s, causing the fluorescent tube 11 to start lighting.

Also, due to the load current of the fluorescent tube 11, the transistor 13
．． Since the bias capacitors 21 and 22 connected between the base and collector of transistor 14 are charged to the illustrated polarity, positive feedback is applied to transistor 13, and the collector current increases rapidly.

The increase in current in the transistor 13 is suppressed when it reaches the saturation point determined by the base current and the amplification degree, so as the increase in current decreases, a voltage in the direction of the dotted line in the figure is generated in the primary coil 12P of the transformer 12. , the transistor 13 is switched from ON to OFF, and the transistor 14 is switched from OFF to ON.

As a result, an induced voltage in the direction of the dotted line in the figure is generated in the secondary coil 12S, causing the fluorescent tube 11 to continue lighting.

Also, capacitors 21 and 22 are charged to polarities opposite to those shown, and positive feedback is applied to the base of transistor 14.

After that, transistors 13 and 14 are turned on alternately in the same way as above.
is repeated to generate a high alternating voltage in the secondary coil 128.

The above-mentioned inverter circuit was developed by the inventor of the present application, and a patent application has already been filed as Patent Application No. 274825 of 1999.

"Problems to be Solved by the Invention" Although the output voltage of the above-mentioned inverter circuit becomes a distorted wave close to a sine wave AC voltage, there is no practical problem in lighting the fluorescent tube 11. In the circuit, the feedback coil can be omitted from the step-up transformer 12, and there is no need for a resonance capacitor connected in parallel to the negative coil 12P.

As a result, the inverter circuit generates less heat in the step-up transformer 12 and has extremely high efficiency.

However, when the above-mentioned inverter circuit is operated without load with the fluorescent tube 11 removed, unstable oscillation occurs.

That is, when the fluorescent tube 11 is removed, normal feedback is not applied to the transistors 13 and 14, and the operation of these transistors 13 and 14 becomes unstable.

Such unstable oscillation is undesirable because it causes damage to the step-up transformer 12 and other circuit components.

An object of the present invention is to improve the above-described inverter circuit and develop an inverter circuit that operates stably even under no load.

"Means for Solving the Problem" In order to achieve the above object, the present invention provides a step-up transformer including a primary coil having an intermediate tap, a secondary coil connected to a load, and a primary coil on one side of the intermediate tap. First and second switching elements with control poles that alternately connect and disconnect the current flowing through the coil and the primary coil on the other side, a bias capacitor connected between the control poles of these first and second switching elements, and this In a push-pull inverter equipped with a feedback circuit in which the load current flows around the capacitor, the primary coil voltage generated when the first switching element cuts off the primary coil current on one side is applied to the control pole of the second switching element. A push-pull inverter is provided with an auxiliary feedback circuit that feeds back the primary coil voltage generated when the second switching element cuts off the primary coil current on the other side to the control pole of the first switching element. suggest.

"Function" When a load is connected to the push-pull inverter, the load current flows through the bias capacitor and the first and second switching elements are alternately turned on and off by the feedback action of the auxiliary feedback circuit. The oscillation operation continues repeatedly.

When the load is removed and there is no load, when the first switching element cuts off the primary coil current on one side due to the operation of the auxiliary feedback circuit, the voltage generated in this secondary coil is transferred to the control pole of the second switching element. Also, when the second switching element cuts off the primary coil current on the other side, the voltage generated in this secondary coil is fed back to the control pole of the first switching element. From this, the first and second switching elements repeatedly turn on and off to continue the oscillation operation.

``Example'' Next, an example of the present invention will be described with reference to the drawings.

In the push-pull inverter circuit shown in Figure 1,
A capacitor 23 connects the base of the transistor 13 and a collector of the transistor 14, and a capacitor 24 connects the base of the transistor 14 and the collector of the transistor 13.

These two capacitors 23, 24 constitute an auxiliary feedback circuit.

That is, when the transistor 14 as the second switching element is switched from ON to OFF, the voltage due to the back electromotive force generated in the primary coil 12P2 on the other side from the intermediate tap a is returned to the base of the transistor 13 via the capacitor 23. to ensure that this transistor 13 is turned off.
Switch from to ON.

Similarly, the transistor 1 as the first switching element
3 is switched from ON to OFF, the voltage due to the back electromotive force generated in the primary coil 12P1 is fed back to the base of the transistor 14 via the capacitor 24 to ensure that the transistor 14 is switched from OFF to ON. .

The rest of the structure is the same as that of the inverter circuit shown as a conventional example in FIG.

The inverter circuit of this embodiment operates in the same manner as the conventional inverter circuit shown in FIG. 5 as long as the fluorescent tube 11 is connected as a load.

In other words, the load current flows through the bias capacitors 21, 22, so that the transistors 13, 14
repeats ONL and oscillation alternately.

FIG. 2 is a waveform diagram of the load voltage applied to the fluorescent tube 11 by the inverter circuit oscillating as described above, and FIG. 3 is a waveform diagram of the load current flowing through the fluorescent tube 11.

In the case of no load with the fluorescent tube 11 removed, no load current flows through the bias capacitors 21 and 22.

However, when the transistor 13 changes from OFF to ON,
When the transistor 14 is switched from ON to OFF, the voltage generated in the negative coil 12P2 is applied to the capacitor 23.
The signal is fed back to the base of the transistor 13 via the transistor 13, and the transistor 13 is quickly turned on.

Conversely, when the transistor 14 is switched from OFF to ON, the voltage generated in the secondary coil 12P is fed back to the base of the transistor 14 via the capacitor 24, and the transistor 14 is quickly turned ON.

As a result, even under no-load conditions, transistors 13.14
are reliably turned ON alternately to continue oscillation.

FIG. 4 is a waveform diagram showing the base-emitter voltage Vbe and the emitter-collector voltage Vce applied to the transistor 13 or the transistor 14 in the case of no load.

Note that, in reality, V ce has a considerably higher peak value than the illustrated Vbe waveform.

Although the output voltage of the inverter circuit described above is an alternating voltage containing a distorted wave, it can be improved considerably by connecting the resonance capacitor 25 in parallel to the primary coil 12P, as shown by the dotted line in the figure.

However, if it is not desired to improve the output voltage waveform, this capacitor 25 is not necessary.

When the above-mentioned resonance capacitor 25 is provided, it is preferable to use one with an appropriate capacity in consideration of improving the output voltage waveform and the transformer temperature, since it causes the temperature of the step-up transformer 12 to rise. .

"Effects of the Invention" As described above, in the present invention, in a push-pull inverter in which the first and second switching elements are alternately turned ON using load current, the first switching element or the second switching element changes from ON to ON. An auxiliary feedback circuit is provided to feed back the voltage in the primary coil of the step-up transformer that occurs when the switch is turned OFF to the control pole of the first switching element or the second switching element, so that it can be adjusted between the connected state of the load and the no-load state. However, it becomes a push-pull inverter with stable oscillation operation.

[Brief explanation of drawings]

Figure 1 is a push-pull inverter circuit diagram showing an embodiment of the present invention, Figure 2 is a load voltage waveform diagram, Figure 3 is a load current waveform diagram, and Figure 4 is a diagram of the above inverter circuit with no load. A waveform diagram showing a base-emitter voltage and an emitter-collector voltage of an oscillation transistor, and FIG. 5 is a push-pull inverter circuit diagram as a conventional example. 11... Fluorescent tube 12... Step-up transformer 13.14... Transistor 21.22... Bias capacitor 23.24...
・Capacitor 121t for auxiliary feedback circuit Beat<-? I-...12-thief 4...he\nΔbeshi) 5 capitals 1 generation

Claims (1)

[Claims] A step-up transformer includes a primary coil with an intermediate tap, a secondary coil that is connected to a load, and a transformer with a control pole that alternately interrupts the current flowing between the primary coil on one side and the primary coil on the other side of the intermediate tap. 1. A push-pull inverter comprising a second switching element, a bias capacitor connected between the control poles of the first and second switching elements, and a feedback circuit in which a load current flows around the capacitor. The primary coil voltage generated when the first switching element cuts off the primary coil current on one side is fed back to the control pole of the second switching element,
A push-pull inverter comprising an auxiliary feedback circuit that feeds back the primary coil voltage generated when the second switching element cuts off the primary coil current on the other side to the control pole of the first switching element.