JP2812649B2 - Inverter circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter circuit for lighting a fluorescent tube such as a cold cathode discharge tube.

[0002]

2. Description of the Related Art A conventional inverter circuit as shown in FIG. 3 is known. This is an inverter circuit for lighting the fluorescent tube 1 and includes switching transistors 2 and 3 connected in a push-pull manner, and a step-up transformer 7 having an input winding 4, an output winding 5 and a feedback winding 6. . 8 is a bias resistor, 9 is a resonance capacitor connected in parallel to the input winding 4, 10 is a ballast capacitor connected in series to the fluorescent tube 1, and an intermediate tap of the input winding 4 is a DC power supply via a choke coil 11. Connected to 12. Then, self-excited oscillation is performed by the feedback winding 6, and the transistors 2 and 3 are alternately switched to generate a high AC voltage in the output winding 5.

[0003]

The power supply voltage is, for example, 3V.
In order to turn on the fluorescent tube 1 by the inverter circuit as shown in FIG. 3, it is necessary to increase the turn ratio of the transformer 7 to increase the step-up ratio. However, there is a problem that a transformer having a large turns ratio is liable to cause a failure such as a parasitic vibration, which makes the design difficult. In order to increase the turn ratio, the input winding 4
When the number of turns is reduced, the inductance decreases in proportion to the square of the number of turns, and the input current increases. Therefore, power loss and copper loss due to the resistance component of the input winding 4 increase, and the efficiency decreases. On the other hand, in order to increase the turns ratio, the output winding 5
Increasing the number of turns causes the transformer 7 to be large, and thus cannot be accommodated in a limited space.

[0004]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an inverter circuit capable of lighting a fluorescent tube having a relatively high discharge starting voltage while using a transformer having a relatively low step-up ratio even with an inverter of a low voltage power supply. With the goal.

[0005]

The fluorescent tube requires a high voltage of about 1 kV before the discharge starts, but only a voltage of about 200 to 500 V need be applied after the start of the discharge. The present invention utilizes this characteristic of the fluorescent tube, and generates a required high voltage only at the start of discharge by adding a simple circuit in an inverter circuit having a low input voltage specification.
After the fluorescent tube starts discharging, a relatively low output voltage is generated. That is, the inverter circuit of the present invention includes a pair of switching elements connected to one end of a DC power supply, an input winding having an intermediate tap, an output winding to which a load is connected, and a feedback winding for feedback oscillation. A step-up transformer and a resonance capacitor connected in parallel to the input winding, both ends of the input winding are connected to a pair of switching elements, and an intermediate tap is connected to the other end of the DC power supply via a choke coil; The two ends of the feedback winding are connected to switching elements, respectively, and the feedback winding oscillates self-excitedly so that the pair of switching elements alternately conducts. In an inverter circuit for supplying an AC voltage to an output winding by alternately interrupting a current flowing to the other side, a second capacitor having one end connected to an intermediate tap is provided. The capacitors provided, temporarily immediately after turning on the power switch, and wherein the configuration of the second capacitor is connected in parallel to the choke coil.

[0006]

FIG. 1 shows an embodiment of an inverter circuit according to the present invention. 1 and 2 described below, the same parts as those in the conventional example of FIG. In FIG. 1, 12 is a DC power supply, 2 and 3 are transistors, and 11 is a choke coil. Reference numeral 7 denotes a step-up transformer having an input winding 4, an output winding 5, and a feedback winding 6. A resonance capacitor 9 is connected to the input winding 4 in parallel.
Are connected to the collectors of the transistors 2 and 3, respectively. The intermediate tap is connected to the positive electrode of the DC power supply 12 via the choke coil 11 and the power switch 13. A ballast capacitor 10 and a fluorescent tube 1 are connected in series to the output winding 5, and both ends of the feedback winding 6 are connected to the bases of the transistors 2 and 3, respectively. The base of the transistor 2 is connected to the positive electrode of the DC power supply 12 via the bias resistor 8, and the emitters of the transistors 2 and 3 are both grounded. Then, the power switch 13 is closed and the pair of transistors 2 and 3 are self-oscillated by the feedback winding 5,
A current flowing from the intermediate tap to one side of the input winding 4 and a current flowing to the other side of the input winding 4 are alternately intermittently supplied to supply an AC voltage to the fluorescent tube 1 as a load through the output winding 5. .

The circuit of this embodiment is characterized in that a circuit 20 surrounded by a broken line is added to the above-described inverter circuit. The additional circuit 20 includes a transistor 23 having a base grounded via a capacitor 21 and connected to the DC power supply 12 via a bias resistor 22, and a capacitor 24 connected to the collector of the transistor 23. Are connected in parallel to the choke coil 11. In the inverter circuit thus configured, when the power switch 13 is closed, the transistor 23
The base voltage gradually increases in a curve determined by the time constant of the resistor 22 and the capacitor 21, and then the DC power supply 12
Voltage. For this reason, the transistor 23 is turned on for a certain time immediately after the switch 13 is closed, and at this time, the inverter circuit connects the capacitor 24
Are connected in parallel with each other as shown in FIG. Then, when the base voltage rises to a predetermined voltage, the transistor 23 is turned off.

The output voltage V _{O, the} oscillation frequency f, the output current I _O , and the efficiency η [(load power) when the additional circuit 20 with the capacitor 24 having a capacity of 0.047 μF is connected and when the additional circuit 20 is not provided. / Input power) × 100], and the results are as shown in Table 1. The output voltage V _{O with} the additional circuit 20 is a value when the transistor 23 is turned on and the capacitor 24 is connected to the choke coil 11 in parallel. In the parameters, L _IN is a choke coil
11 of inductance, one side of the inductance of L ₁ and L ₂ are input winding 4 in which the intermediate tap as a boundary, respectively, L ₅
Represents the inductance of the output winding 5, C ₉ and C ₁₀ represent the capacitances of the capacitors 9 and ₁₀ , respectively, and R _L represents the value of the equivalent resistance of the fluorescent tube 1.

[0009]

[Table 1]

However, L _IN = 47 μH, L ₁ = L ₂ =
2.92 μH, L ₅ = 35.4 mH C ₉ = 0.047 μF, C ₁₀ = 82 pF, R _L
= 3.5 kΩ From this table, it can be seen that the output voltage V
It can be seen that _O increased by nearly 20%. On the other hand, the circuit
There is almost no change in the oscillation frequency f, the output current I _O , and the efficiency η when there is or does not have 20. Therefore, if the time constant of the resistor 22 and the capacitor 21 is set so that the transistor 23 is turned on until the fluorescent tube 1 is turned on, a high output voltage V _O is generated only when the discharge of the fluorescent tube 1 starts, After lighting, an efficient inverter circuit in which the output voltage V _O becomes low can be obtained. Note that a resistor (not shown) may be connected to the capacitor 24 in parallel. Further, although a bipolar transistor is used as the switching element in the embodiment, a MOS transistor may be used instead. In addition, capacitor 21 and resistor
Instead of using 22 or the like, another logic circuit may be provided, and a pulse wave generated by this logic circuit may be applied to the control electrode of the transistor 23 to control the transistor 23 on and off.

[0011]

According to the present invention, even if the power supply voltage is low and the inverter circuit has a low voltage specification, the output voltage temporarily becomes high immediately after the power switch is closed without increasing the boost ratio of the boost transformer. _{Since O} can be generated, it is possible to obtain a small-sized inverter circuit which is easy to design a boosting transformer without a risk of lowering efficiency or generating parasitic oscillation.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing one embodiment of the present invention.

FIG. 2 is an equivalent circuit diagram of the inverter circuit of the present invention.

FIG. 3 is a circuit diagram showing a conventional example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Fluorescent tube 2.3 transistor 4 Input winding 5 Output winding 6 Feedback winding 7 Transformer 11 Choke coil 23 Transistor

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) H05B 41/24 H02M 7/538-7/5383

Claims (3)

(57) [Claims] A step-up transformer including a pair of switching elements connected to one end of a DC power supply, an output winding having an intermediate tap, an output winding connected to a load, and a feedback winding for feedback oscillation; A resonance capacitor connected in parallel with the input winding, both ends of the input winding are connected to a pair of switching elements, and the intermediate tap is connected to the other end of the DC power supply via a choke coil; The two ends of the input winding are connected to a switching element, respectively, and the feedback winding causes self-excited oscillation so that the pair of switching elements are alternately turned on.The current flowing from the intermediate tap to one side of the input winding and the other end of the input winding A second capacitor having one end connected to an intermediate tap is provided in an inverter circuit for supplying an AC voltage to an output winding by alternately intermitting a current flowing to one side of the power supply. Immediately after turning on the switch,
An inverter circuit comprising a second capacitor connected in parallel to the choke coil. 2. A transistor having a control electrode grounded via a third capacitor and connected to a DC power supply via a resistor is added, and a series circuit of the transistor and the second capacitor is connected in parallel to a choke coil. The inverter circuit according to claim 1, wherein: 3. The inverter circuit according to claim 1, wherein a logic circuit and a transistor which is turned on / off by a pulse wave generated by the logic circuit are provided, and a series circuit of the transistor and the second capacitor is connected in parallel to a choke coil. .