JP2893466B2 - Push-pull inverter - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a push-pull inverter used as a driver of, for example, a cold cathode discharge tube or a hot cathode discharge tube.

[Prior Art] Push-pull inverters having various circuit configurations are known, one example of which is shown in FIG.

FIG. 5 shows an inverter circuit configured as a driver for the fluorescent tube 11, which is a push-pull circuit including a step-up transformer 12, transistors 13 and 14 for switching operation, a choke coil 15, and the like.

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

From this, through the resistor 19 to the transistor 13,
A base current flows into each of the transistors 14 through the resistor 20. For this reason, the transistors 13 and 14 move in a direction in which both transistors are turned on, but one of the transistors greatly advances in a conductive state in terms of transistor characteristics and circuit configuration.
This transistor turns on first.

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

At this time, an induced voltage is generated in the secondary coil 12S in the direction indicated by the solid line in the drawing, and the lighting of the fluorescent tube 11 is started.

Further, the transistor 13 and the
Since the biasing capacitors 21 and 22 connected between the base and the collector of 14 are charged to the polarity shown, the transistors
13 receives positive feedback and the collector current increases rapidly.

Since the increase in the current of the transistor 13 is suppressed when the saturation point determined by the base current and the amplification degree is reached, the primary coil of the transformer 12 decreases as the increase in the current decreases.
12P generates a voltage in the direction of the dotted line in FIG.
Switches from ON to OFF, and the transistor 14 switches from OFF to ON.

As a result, an induced voltage is generated in the secondary coil 12S in the direction indicated by the dotted line, and the lighting of the fluorescent tube 11 is continued.

In addition, the capacitors 21 and 22 are charged to polarities opposite to the illustrated polarities, and positive feedback is applied to the base of the transistor 14.

Thereafter, the transistors 13 and 14 are alternately turned ON in the same manner as described above, and generate a high alternating voltage in the secondary coil 12S.

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

[Problem to be Solved by the Invention] Although the output voltage of the inverter circuit described above is a distorted wave close to a sine wave AC voltage, there is no practical problem in lighting the fluorescent tube 11, and furthermore, this inverter 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 with the primary coil 12P. As a result, an extremely efficient inverter circuit that generates less heat from the step-up transformer 12 is obtained.

However, when the above-described inverter circuit is operated under no load with the fluorescent tube 11 removed, unstable oscillation occurs.

That is, when the fluorescent tube 11 is removed, the transistor
Since normal feedback is not applied to the transistors 13 and 14, the operations of the transistors 13 and 14 become 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 to develop an inverter circuit that operates stably even under no load.

[Means for Solving the Problems] In order to achieve the above-described object, the present invention provides a primary coil having an intermediate tap, a step-up transformer having a secondary coil connected to a load, and a primary coil on one side of the intermediate tap. A first and a second switching element with a control pole for alternately intermitting a current flowing through the coil and the primary coil on the other side; a bias capacitor connected between the control poles of the first and second switching elements; In a push-pull inverter having a feedback circuit configured to allow a load current to flow around a capacitor, a primary coil voltage generated when the first switching element cuts off the primary coil current on one side is applied to a control pole of the second switching element. The primary coil voltage, which is generated when the second switching element returns and cuts off the primary coil current on the other side, is supplied to the first switch. A push-pull inverter characterized by having an auxiliary feedback circuit for feeding back to the control pole of the switching element.

[Operation] 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 alternately by the feedback action of the auxiliary feedback circuit. To continue the oscillating operation.

When the load is removed and no load is applied, the voltage generated in the primary coil is fed back to the control pole of the second switching element when the first switching element cuts off the primary coil current on one side by the operation of the auxiliary feedback circuit. When the second switching element interrupts the primary coil current on the other side, the voltage generated in the primary coil is fed back to the control pole of the first switching element. As a result, the first and second switching elements are repeatedly turned ON and OFF to continue the oscillating operation.

Next, an embodiment of the present invention will be described with reference to the drawings.

In the push-pull inverter circuit shown in FIG. 1, reference numeral 23 denotes a capacitor connecting the base of the transistor 13 and the collector of the transistor 14;
Of the transistor 13 and the collector of the transistor 13.

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

That is, when the transistor 14 of the second switching element is switched to OFF from ON, the voltage due to the counter electromotive force generated from the intermediate tap a primary coil 12P ₂ on the other side through the capacitor 23 back to the base of the transistor 13 Then, the transistor 13 is reliably switched from OFF to ON.

Similarly, a transistor as a first switching element
When 13 is switched from ON to OFF, the primary coil 12P on one side
_The voltage generated by the back electromotive force generated in ₁ is fed back to the base of the transistor 14 via the capacitor 24, and the transistor 14 is reliably switched from OFF to ON.

Other configurations are the same as those 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.

That is, when the load current flows through the bias capacitors 21 and 22, the transistors 13 and 14 are turned on alternately and the oscillation is repeated.

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

When the fluorescent tube 11 is removed and no load is applied, no load current flows through the bias capacitors 21 and 22.

However, when the transistor 13 switches from OFF to ON and the transistor 14 switches from ON to OFF, the voltage generated in the primary coil 12P ₂ is fed back to the base of the transistor 13 via the capacitor 23, and this transistor 13 turns ON promptly.

Vice versa, when the transistor 14 is switched from OFF to ON, a voltage generated in the primary coil 12P ₁ capacitor 24
Is fed back to the base of the transistor 14 and the transistor 14 is quickly turned on.

As a result, even in the no-load state, the transistors 13 and 14 are reliably turned on alternately and continue to oscillate.

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

Actually, Vce has a significantly higher peak value than the illustrated Vbe waveform.

Although the output voltage of the inverter circuit described above is an alternating voltage including a distorted wave, it can be considerably improved 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 becomes a factor of increasing the temperature of the step-up transformer 12, and therefore, it is preferable to use a capacitor having an appropriate capacity in consideration of the improvement of the output voltage waveform and the transformer temperature. .

[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 by using a load current, the first switching element or the second switching element is turned ON. An auxiliary feedback circuit is provided to feed back the primary coil voltage of the step-up transformer generated at the time of transition to OFF to the control pole of the first switching element or the second switching element. Regardless, the push-pull inverter operates stably.

[Brief description of the drawings]

1 is a push-pull inverter circuit diagram showing an embodiment of the present invention, FIG. 2 is a load voltage waveform diagram, FIG. 3 is a load current waveform diagram, and FIG. FIG. 5 is a waveform diagram showing a base-emitter voltage and an emitter-collector voltage of an oscillation transistor. FIG. 5 is a push-pull inverter circuit diagram shown as a conventional example. 11 Fluorescent tube 12 Boost transformer 13, 14 Transistor 21, 22 Bias capacitor 23, 24 Capacitor for auxiliary feedback circuit

Claims (1)

(57) [Claims] 1. A step-up transformer including a primary coil having an intermediate tap, a secondary coil connected to a load, and a current flowing through a primary coil on one side and a primary coil on the other side of the intermediate tap are alternately intermittently intermittently interposed. A push switch including first and second switching elements having a control electrode, a bias capacitor connected between the control electrodes of the first and second switching elements, and a feedback circuit configured to allow a load current to flow into the capacitors; In the pull inverter, the primary coil voltage generated when the first switching element interrupts the primary coil current on one side is fed back to the control pole of the second switching element, and the second switching element interrupts the primary coil current on the other side. An auxiliary feedback circuit is provided for feeding back the primary coil voltage generated at the time of switching to the control pole of the first switching element. Push-pull inverter.