JPH031797B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a discharge lamp lighting device using an emitter current controlled step-down chopper circuit using a pulse width modulation method.

[Technical background of the invention and its problems]

An example of this type of discharge lamp lighting device is shown in FIG. A rectifier circuit 2 is connected to an AC power source 1, a series circuit of a discharge lamp 4 and a chiyoke coil 5 is connected to the rectifier circuit 2 via an NPN power transistor 3, and a flywheel diode 6 is connected to the series circuit. It is connected in parallel with the power transistor 3 with opposite polarity. An emitter resistor 7 is inserted in series on the emitter side of the power transistor 3, the voltage across the emitter resistor 7 is averaged and detected by a filter circuit 8, and the detected voltage is supplied to a transistor drive circuit 9. The transistor drive circuit 9 controls the duty ratio of the rectangular wave supplied to the power transistor 3 according to the input detection voltage. When the detection voltage is large, the duty ratio of the rectangular wave is controlled to be small, so that the detection voltage increases. When it is small, the duty ratio of the square wave is controlled to be large. As a result, the average value of the emitter current of the power transistor 3 is kept substantially constant, and the output rectangular wave of the power transistor 3 is converted into a direct current by the choke coil 4 and the flywheel diode 6 and supplied to the discharge lamp 4. In this way, substantially constant power is supplied to the discharge lamp 3 during normal lighting.

However, in such a device, when the power supply voltage fluctuates, the power supplied to the discharge lamp fluctuates, and if the power supply includes ripples, the output also includes ripples. Furthermore, when a discharge lamp is started, the lamp impedance is low, but at such times a large inrush current flows, leading to the risk of blackening of the lamp and shortening of its life, and in extreme cases, even the risk of explosion. Furthermore, when the lamp impedance is low or a load short circuit occurs, the circuit input power is held approximately constant while the output power decreases, which has the disadvantage of increasing the collector loss of the power transistor and generating heat. be.

Based on this, the inventor first inserted a current control transistor in series on the emitter side of the power transistor, and when the on/off duty ratio of the power transistor was small, the internal resistance value of the current control transistor was controlled to be large, and vice versa. By controlling the internal resistance value of the current control transistor to a small value when the on/off duty ratio of the power transistor is large, stable output power can be supplied to the discharge lamp against fluctuations and ripples in the power supply voltage, and the discharge lamp A discharge lamp lighting device is provided that can suppress inrush current and short-circuit current at the time of starting, and can also prevent heat generation of a switching-driven power transistor. (Japanese Patent Application No. 58-49941) However, in this case, h _FE of the current control transistor
It was insufficient in that it was greatly affected by variations in In other words, h _FE of the current control transistor
The larger the value, the less effective it is to narrow down the short-circuit current.
h If _EF is small, the output will be limited even if the load impedance is high to some extent, so there is a risk that the lamp input will be insufficient during normal lighting.

[Purpose of the invention]

This invention was made to solve these problems, and by inserting a current control transistor in series on the emitter side of the power transistor, it is possible to stabilize the output power against fluctuations and ripples in the power supply voltage. Provides a discharge lamp lighting device that can suppress inrush current and short-circuit current during startup, prevent heat generation of power transistors, etc., and can sufficiently stabilize output characteristics against variations in h _FE of current control transistors. The purpose is to

[Summary of the invention]

This invention connects a series circuit of a chiyoke coil and a discharge lamp to a rectifier circuit via a power transistor, and connects a flywheel diode in parallel with the series circuit with the polarity opposite to that of the power transistor, so that the emitter current of the power transistor is Detects and turns on the power transistor,
In those that variably control the off-duty ratio,
A current control transistor is inserted in series on the emitter side of the power transistor, and a resistor is inserted in series in the emitter circuit of the current control transistor.When the on/off duty ratio of the power transistor is small, the internal resistance of the current control transistor is Conversely, when the on/off duty ratio of the power transistor is large, the internal resistance value of the current control transistor is controlled to be small.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

As shown in Fig. 2, a rectifier circuit 1 is connected to an AC power supply 11.
2 are connected to form a DC power supply. A series circuit of a discharge lamp 14 and a choke coil 15 is connected to the rectifier circuit 12 via a power transistor 13. The discharge lamp 14 and the chiyoke coil 15
A flywheel diode 16 with opposite polarity to the power transistor 13 is connected in series with the power transistor 13.
are connected in parallel. A series circuit consisting of a resistor 17 and a current control transistor 18 whose collector is connected to the resistor 17 is inserted between the emitter of the power transistor 13 and the negative terminal of the rectifier circuit 12. The current control transistor 18
A resistor 19 is inserted in series in the emitter circuit, and a capacitor 20 is connected in parallel to the series circuit of the resistor 17, transistor 18, and resistor 19. Note that the negative terminal of the rectifier circuit 12 is grounded. Further, a filter circuit 21 is connected in parallel to the series circuit of the resistor 17 and the current control transistor 18. The filter circuit 21 includes a capacitor 22 and a capacitor 24 connected in parallel through a resistor 23, and averages the voltage across the series circuit of the resistor 17 and transistor 18. The output of the filter circuit 21 is supplied to a transistor drive circuit 25 which switches and drives the power transistor 3 with a rectangular wave output.
The transistor drive circuit 25 has a pulse modulation circuit that inputs the output of the filter circuit 21 and controls the duty ratio of the output rectangular wave, and when the input voltage is high, the duty ratio of the output rectangular wave is made small, and when the input voltage is low. Sometimes, the duty ratio of the output rectangular wave is increased. The output terminal of the transistor drive circuit 25 and the power transistor 1
A transistor control circuit 26 is connected between the connection point with the base of No. 3 and the ground. The transistor control circuit 26 includes a series circuit of a resistor 27 and a capacitor 28, and a connection point between the resistor 27 and the capacitor 28 is connected to the base of the current control transistor 18.

In such a configuration, the power transistor 1
3 is switched and driven by a transistor drive circuit 25, and the discharge lamp 14 has a chiyoke coil 1.
5 and a flywheel diode 16, a DC output is supplied. In this operation, the emitter current of the power transistor 13 is converted into voltage by a series circuit of a resistor 17, a transistor 18, and a resistor 19, and is averaged and detected by a filter circuit 21. When the level of the input voltage is high, the transistor drive circuit 25 sets the duty ratio of the rectangular wave to be output, that is, the power transistor 1
The on/off duty ratio of power transistor 13 is controlled to be small, and when the level of the input voltage is low, the duty ratio of the output rectangular wave, that is, the on/off duty ratio of power transistor 13 is controlled to be large. On the other hand, since the base voltage of the power transistor 13 is averaged by the capacitor 28, the voltage at the connection point A between the resistor 27 and the capacitor 28 in the transistor control circuit 26 is shown in FIG. 3 in relation to the duty ratio. When the duty ratio is small, it is small, and when the duty ratio is large, it is large. As a result, when the load impedance is reduced and the on/off duty ratio of the power transistor 13 is controlled to be small, the internal resistance value of the current control transistor 18 is increased and the amount of emitter current is controlled to be reduced. Further, when the load impedance is above a certain level and the on/off duty ratio of the power transistor 13 is controlled to be large, the internal resistance value of the current control transistor 18 becomes small, and at this time, the emitter current is limited by the resistor 17. Therefore, the output characteristics of this circuit are as shown by the solid line in FIG. That is, at the time of short circuit, the current is approximately 0.1 A (ampere) as shown at point a, and at the maximum point b, it is approximately 1.3 A, resulting in a lamp inrush current in the range of 1.0 to 1.3 A. On the other hand, in the circuit shown in Figure 1, as shown by the dotted line in the figure, the lamp inrush current is approximately 3.8A when short-circuited.
A current of 2.2A flows. In addition, the circuit loss is less than 5W from a load short circuit to 1K ohm, as shown by the solid line in Figure 5. On the other hand, the circuit shown in Figure 1 has a circuit loss when the load is shorted, as shown by the dotted line in the figure.
It can reach over 40W.

Therefore, there is no risk that the discharge lamp 14 will be blackened or its life will be shortened due to the lamp rush current. Furthermore, since circuit loss is kept low, there is no risk that the power transistor 13 will generate a large amount of heat.

Further, when the power supply voltage decreases, the on/off duty ratio of the power transistor 13 increases, the resistance value of the current control transistor 18 decreases, and the emitter current of the power transistor 13 increases. Therefore, the output power can be stabilized against fluctuations and ripples in the power supply voltage.

Furthermore, since a resistor 19 is inserted in series in the emitter circuit of the current control transistor 18, the emitter voltage of the transistor 18 is lower than the voltage at point A by the forward drop voltage between the base and emitter of the transistor 18 (approximately 0.65 V). ) is determined to be a lower voltage. When the emitter voltage of the transistor 18 is determined, the emitter current is determined, and thereby the collector current is approximately determined. Therefore, the collector current of current control transistor 18 is ultimately determined by the voltage at point A, and transistor 1
There is no possibility that the variation in h _FE of No. 8 will greatly affect the collector current. Therefore, the output characteristic is also the sixth
As shown by the solid line in the figure, variations in the magnitude of h _FE can be suppressed to a small level. In this case, in the case where the resistor 19 is not inserted, as shown by the dotted line in FIG. 6, the output characteristics fluctuate greatly in response to variations in the magnitude of _hFE .

〔Effect of the invention〕

As detailed above, according to the present invention, by inserting a current control transistor in series on the emitter side of a power transistor, output power can be stabilized against power supply voltage fluctuations and ripples, and inrush current and short circuits can be prevented at startup. It is possible to provide a discharge lamp lighting device that can suppress the current, prevent heat generation of the power transistor, etc., and further stabilize the output characteristics sufficiently against variations in h _FE of the current control transistor.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing a conventional example, Figs. 2 to 6 show an embodiment of the present invention, Fig. 2 is a circuit diagram, and Fig. 3 is a voltage characteristic between point A and ground with respect to duty ratio. Figure 4 is a graph showing output characteristics, Figure 5 is a graph showing circuit loss characteristics with respect to load resistance, and Figure 6 is a graph showing changes in output characteristics with respect to changes in h _FE of the current control transistor. It is. 12... Rectifier circuit, 13... Power transistor, 14... Discharge lamp, 15... Chi York coil,
16... Flywheel diode, 18... Current control transistor, 19... Resistor, 21...
Filter circuit, 25...transistor drive circuit, 26...transistor control circuit.

Claims (1)

[Claims] 1. A DC power supply, a discharge lamp connected to the DC power supply through a chiyoke coil and a power transistor in series, and a discharge lamp connected in parallel to the series circuit of the discharge lamp and the chiyoke coil with opposite polarity to the power transistor. a flywheel diode, a current control transistor inserted in series with the emitter circuit of the power transistor,
A resistor inserted in series with the emitter circuit of this current control transistor and the emitter current of the power transistor are detected, and when the average value of the current is large, the power transistor is moved in a direction that reduces its on/off duty ratio. a transistor drive circuit that switches and drives the power transistor in a direction that increases its on/off duty ratio when the average value of the current is small;
A transistor control circuit that controls the internal resistance value of the current control transistor to increase when the off-duty ratio is small, and controls the internal resistance value of the current control transistor to decrease when the on-off duty ratio is large. A discharge lamp lighting device comprising: