JP3396645B2 - Fluorescent display tube drive circuit - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fluorescent display tube drive circuit for driving a filament of a fluorescent display tube with an AC voltage.

[0002]

2. Description of the Related Art In a fluorescent display tube driving circuit of this type, a bias voltage is applied to an intermediate potential of an AC voltage for a filament in order to reduce the potential gradient in the filament and reduce the brightness gradient of the fluorescent display tube. I have to. Since the AC voltage for filaments is generally assumed to have a duty of 50%, the middle point of the winding, that is, the center tap of the winding is conventionally selected as the intermediate potential point of the AC voltage.

In order to light a fluorescent display tube, it is necessary to apply a direct current voltage of several tens of volts to the anode, which is generally obtained by a flyback DC / DC converter. Therefore, in consideration of economical efficiency, an AC voltage for filament driving is often obtained by providing a secondary winding in this flyback DC / DC converter.

[0004]

For example, the duty of an AC voltage waveform for filament driving in an automobile-mounted device is usually 70% to 80%, and rarely in the vicinity of 50%. Nevertheless, the conventional tap position is at the arithmetic center position of the number of turns. In this case, the peak value is different between positive and negative with the AC zero potential as the center. Due to this, the display erasure of the fluorescent display tube is incomplete, and the non-selected anode of the fluorescent display tube sometimes causes a thin light-emission phenomenon.

[0005]

According to the present invention, the tap position of the winding to which the bias voltage is applied is not set to the midpoint of the arithmetic winding number, but is made as close as possible to the midpoint of the positive and negative peak values. Therefore, the leakage light emission phenomenon is prevented.
That is, the fluorescent display tube drive circuit of the present invention comprises a transformer having a primary winding and a secondary winding, filler from the secondary winding
The AC voltage supplied to the
In a fluorescent display tube drive circuit for obtaining a DC voltage to be supplied , 2
A tap is provided at the position at the midpoint of the positive and negative peak values of the AC voltage in the next winding, and DC voltage can be obtained at this tap.
A feature is that a bias voltage is applied.

[0006]

FIG. 1 shows an embodiment of a fluorescent display tube drive circuit of the present invention. A predetermined input voltage Vin is applied to the input terminal 1 and driven by the pulse width modulation control circuit 3 to turn on / off the switching transistor 5. The DC voltage V ₀₁ obtained by rectifying and smoothing the electromotive voltage generated from the primary winding N ₁ of the transformer 4 with the diode 6 and the capacitor 7 becomes the anode voltage and grid voltage of the fluorescent display tube (not shown).
An AC voltage V ₀₃ obtained from the secondary winding N ₂ of the transformer 4 is applied to the filament of the fluorescent display tube. DC voltage V ₀₁
The voltage divided by the two resistors 8 and 9 is fed back to the pulse width modulation control circuit 3.

A voltage obtained by dividing the DC voltage V ₀₁ by the resistor 10 and the Zener diode 11 is applied to the tap 12 of the secondary winding N ₂ , whereby the filament voltage, that is, the AC voltage V _03.
Is biased to a value higher than the ground level by the voltage of the Zener diode 11. The position of the tap 12 of the secondary winding N ₂ is set to the midpoint of the positive and negative peak values of the AC voltage V ₀₃ . Therefore, when the secondary winding N ₂ is divided into two windings a and b with the tap 12 as a boundary, the winding numbers of the respective windings a and b are different, and for example, the number of windings of the winding a is 3 turns. Winding b
The number of turns is 7 turns.

The position of the tap 12 of the secondary winding N ₂ can be set at the midpoint between the positive and negative peak values of the AC voltage V _{03 by} the following procedure. In the case of the circuit shown in FIG. 1, the duty D of the AC voltage V ₀₃ is almost uniquely determined by the relationship between the input voltage Vin and the DC voltage V _01, and D≈1− (Vin / V ₀₁ ). Therefore, for example, when the input voltage Vin = 12V and the DC voltage V ₀₁ = 40V, the duty D becomes 70%.

The AC zero potential in the waveform of the AC voltage V ₀₃ is at a ratio of the peak value divided by the duty, that is, at a position 70% from the upper part of the waveform. On the other hand, the center of the peak value is 5
Since it is 0%, the tap can be set to the center of the peak value by shifting the position of the tap by 20% of the difference (70-50) from the arithmetic center position of the number of turns of the secondary winding N ₂ . Therefore, when the total number of turns of the secondary winding N ₂ is 10 turns, it is sufficient to set the winding a to 3 turns and the winding b to 7 turns.

In this way, if the position of the tap 12 is set to the midpoint of the positive and negative peak values of the AC voltage V ₀₃ , one end and the other end from the physical center of the effective display length of the fluorescent display tube. Since the potential gradient is the same, the leakage emission phenomenon of the fluorescent display tube does not occur. Note that the separately-excited drive circuit has been described, but the self-excited drive circuit may be used, and the control method is not limited to the pulse width modulation method.

[0011]

According to the present invention, it is possible to effectively prevent the leakage light emission phenomenon of the fluorescent display tube by simply changing only the tap position.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a fluorescent display tube driving circuit of the present invention.

[Explanation of symbols]

N ₂ Secondary winding a, b Winding 4 Transformer 12 taps

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Claims (2)

(57) [Claims] 1. An AC voltage supplied from a secondary winding to a filament , comprising a transformer having a primary winding and a secondary winding.
And in the fluorescent display tube drive circuit that obtains a DC voltage supplied to the anode and grid, the tap position of the secondary winding is
A fluorescent display tube drive circuit, characterized in that positions are divided by a ratio according to the duty of the flowing voltage, and a bias voltage obtained by the DC voltage is applied to the tap. 2. The fluorescent substance according to claim 1, wherein the position of the tap is shifted by (A-50)% from the arithmetical center position of the number of turns of the secondary winding, where A% is the duty of the AC voltage for driving the filament. Display tube drive circuit.