JPH04249893A - Separately excited invertor lighting circuit - Google Patents

Abstract

PURPOSE:To enable stable driving operation while preventing thermal breakdown of a switching transistor as well as carrying out very careful lighting driving control on a fluorescent tube. CONSTITUTION:A control circuit D to generate on-off driving voltage having variable frequency supplied to a switching transistor Q to excite a LC serial resonance circuit is realized by means of a digital circuit instead of a conventional analog circuit. While keeping off-time of the switching transistor Q constant, the frequency of the driving voltage is changed so that only on-time can be extended or contracted.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter lighting circuit used for lighting control of fluorescent lamps.

0002

[Prior Art] Fluorescent lamps used as indoor lighting equipment, etc. are made of
1 than this instead of driving at Hz ~ 60Hz
By adopting a high-frequency drive method in which the light is driven by an AC power source of 40 KHz to 100 KHz, which is about 1,000 times higher frequency, good light emission characteristics with less flicker can be achieved.

A typical high frequency drive system is realized by an inverter lighting circuit as shown in FIG. That is, an LC series resonant circuit is formed by a capacitor C1 connected in parallel to the fluorescent tube LP and an inductance L1 connected in series to the fluorescent tube LP, and an LC series resonance circuit is formed through an inductance L2 to an input terminal IN to which a DC power supply voltage is supplied. By turning on/off the connected switching transistor Q, the series resonant circuit consisting of the inductance L1 and the capacitor C1 is excited near its resonant frequency, and a high voltage capable of being discharged is generated between the electrodes of the fluorescent lamp LP. ing. The current flowing through this LC series resonant circuit is fed back to the base terminal of the switching transistor Q via a transformer T including an inductance L1 and a capacitor C3, and high frequency drive is performed by self-oscillation. This self-oscillation frequency is determined by the capacitor C
It is adjusted by the capacitance value of 3.

That is, the waveform diagram in FIG.
As shown by the base voltage Vb and collector voltage Vc of the transistor Q, if the capacitance value of the capacitor C3 is large, the self-oscillation frequency decreases as shown in the same figure (A),
Conversely, if this capacitance value is small, the self-oscillation frequency increases as shown in FIG. Furthermore, the duty ratio of the base voltage Vb decreases as the self-oscillation frequency increases. This relationship between the self-oscillation frequency and the duty ratio is very advantageous for stable operation of the switching transistor Q in that the collector voltage Vc is kept low during the conduction period of the switching transistor Q.

Generally, when starting a fluorescent lamp, applying a high voltage to a low-temperature electrode (filament) and immediately starting a discharge will shorten the life of the electrode, so it is necessary to provide an appropriate preheating time before starting the discharge. is desirable. Setting the preheating time in this way is to avoid applying high voltage to the fluorescent tubes by setting the excitation frequency to a value that deviates from the series resonance frequency immediately after starting the inverter lighting circuit, and to gradually reduce the series resonance frequency after a certain period of time. This is achieved by starting lighting with an appropriate delay time after starting by bringing the frequency close to the starting frequency.

[0006]

[Problems to be Solved by the Invention] The self-excited inverter lighting circuit with the configuration shown in FIG. It is desirable to adopt a separately excited type that is supplied externally. However, when attempting to implement such a separately excited inverter lighting circuit using a conventional analog circuit, there is a problem in that highly accurate frequency control becomes difficult due to the temperature characteristics of element constants and changes over time.

[0007] Furthermore, in a separately excited inverter lighting circuit using an analog circuit, there is a problem in that it is difficult to control the duty ratio in order to stabilize the operation of the switching transistor, as described above.

[0008]

[Means for Solving the Problems] A separately excited inverter lighting circuit of the present invention includes a series resonant circuit including a capacitor connected in parallel to a fluorescent tube and a first inductance connected in series to the fluorescent tube, and a DC power source. a second inductance connected between the input terminal and the first inductance; a switching transistor connected between the connection point of the first and second inductances and a reference potential;
The switching transistor is provided with a digital control circuit that supplies a variable frequency drive signal that changes to extend or shorten only the on time while keeping the off time substantially constant to the switching transistor.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing the configuration of a separately excited inverter lighting circuit according to an embodiment of the present invention, together with the relationship with a fluorescent tube to be driven.

In the separately excited inverter lighting circuit of this embodiment, the capacitor C1 is connected in parallel to the fluorescent lamp LP to be driven, and the first inductance L1 is connected in series to the fluorescent lamp LP. A resonant circuit is formed. The resonant frequency of this series resonant circuit is
Typically it is set in the range of 40KHz to 50KHz. Further, a second inductance L2 is connected between the input terminal IN of the DC power supply +B and the first inductance L1, and these first and second inductances L1,
A MOS type switching transistor Q is connected between the connection point of L2 and the ground potential.

Further, a digital control circuit D is installed for supplying a control signal Vg for turning on/off to the gate terminal of the switching transistor Q. The control signal Vg supplied from this digital control circuit D is a binary signal that transitions between a high level and a low level. When this signal goes high level, the switching transistor Q is turned on (conducting), and when it goes low level, the switching transistor Q is turned on (conducting). Switching transistor Q is turned off (non-conductive). The LC series resonant circuit is excited by turning on/off the switching transistor Q.

The digital control circuit D changes the frequency of the control signal Vg supplied to the switching transistor Q according to specifications such as an initial value, steady value, dimming value, etc. received from the outside. This change in frequency is performed by expanding or contracting only the on time of the switching transistor Q while keeping the off time of the switching transistor Q substantially constant. That is, as shown in FIGS. 2 and 3, the period T of the control signal Vg is
High level on period τ1 and low level off period τ0
The frequency (1/T) is changed by expanding or contracting only the on-period τ1 while keeping the off-period τ0 at a constant value.

When the control signal Vg changes from a low level to a high level, the switching transistor Q changes from an off state to an on state, and a current flows from the input terminal IN through the inductance L2 and the switching transistor Q to the ground point. In this on state, the drain voltage Vd of the switching transistor Q is kept at a low voltage of approximately zero. Thereafter, when the control signal Vg changes to a low level, the switching transistor Q changes from an on state to an off state, and the current flowing through the inductance L2 flows into the series resonant circuit, and this series resonant circuit is excited. Along with this, the switching transistor Q
The drain voltage Vd starts to rise from zero voltage, reaches a peak value, starts to fall, crosses zero voltage again and becomes a negative voltage. The time from when the drain voltage Vd starts rising until it crosses zero voltage again during its fall is governed by the element constants in the lighting circuit and the load impedance of the fluorescent lamp LP, and is almost independent of the excitation frequency. In this embodiment, the drain voltage V determined by the above element constants etc.
A constant off time τ0 is set to be approximately equal to the period from zero voltage of d to the next zero voltage.

According to the separately excited inverter lighting circuit of this embodiment, the off-period τ0 of the switching transistor is maintained at a constant value regardless of the frequency, so that the drain voltage Vd is always maintained regardless of the excitation frequency. When the zero-crossing point is approached, the switching transistor Q enters the next on state, and stable switching operation is performed while avoiding thermal destruction.

Next, an example of frequency control of the control voltage Vd by the digital control circuit D will be explained with reference to FIG. Upon receiving the lighting start command, the digital control circuit D first preheats the fluorescent lamp LP by keeping the frequency of the control voltage Vd at a constant value f0 that is higher than the series resonance frequency over a period of t1. Subsequently, while keeping the off period τ0 of the control voltage Vd constant, only the on period τ1 is linearly increased over time, thereby gradually reducing the frequency of the control voltage Vd to approach the series resonance frequency. As the series resonant frequency approaches, the drive voltage of the fluorescent tube LP exceeds the discharge start voltage and discharge is started. After the discharge starts,
The discharge sustaining voltage of the fluorescent tube LP decreases to a value lower than the discharge starting voltage. After this discharge starts, the frequency of the control voltage Vd is maintained at a steady value fc near the series resonance frequency.

FIG. 5 is a block diagram showing an example of the configuration of the digital control circuit D of FIG. 1, in which 1 is an oscillator, 2 is a load counter, 3 is a load value holding register, and 4 is a decoder.

The load counter 2 counts the several MHz clock signal output from the oscillator 1, and maintains the output at a high level or a low level depending on the counted value. Decoder 4
decodes a predetermined combination of digits of the count value of the load counter 2, and issues a load command to the load counter 2 when this matches a predetermined value. The load counter 2 that receives this load command loads the load value held in the load value holding register 3 as an initial value. By controlling the load value held in the load value holding register 3, the frequency division ratio of the variable frequency divider circuit mainly composed of the load counter 2 is changed, and the off time of the control voltage Vd is kept constant. Only time is controlled. For more details on this variable frequency divider circuit using a load counter, please refer to
If necessary, please refer to the specification of Japanese Patent Application No. 1-96303 entitled "Inverter Lighting Circuit" related to the applicant's earlier application.

[0018] In the above, a configuration has been exemplified in which the frequency of the control voltage Vg is controlled by a variable frequency divider circuit mainly consisting of a load counter. However, instead of such a hardware control method, a software control method using a microcomputer or the like may be adopted.

[0019]

[Effects of the Invention] As explained above in detail, the separately excited inverter lighting circuit of the present invention has a configuration in which a digital control circuit generates a control voltage for turning on/off the switching transistor that excites the series resonant circuit. , the effect that fine frequency control becomes possible is achieved.

Furthermore, the separately excited inverter lighting circuit of the present invention is configured to vary the frequency of the drive signal so that only the on time of the switching transistor is expanded or contracted while keeping the off time of the switching transistor almost constant. It becomes possible to make the transistor conductive at all times near the zero cross point regardless of the excitation frequency, and the effect is achieved that stable operation can be realized while preventing thermal destruction of the transistor.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of a separately excited inverter lighting circuit according to an embodiment of the present invention together with a fluorescent tube to be driven.

FIG. 2 is a waveform diagram illustrating the waveform of the control voltage supplied to the switching transistor of the separately excited inverter circuit of the above embodiment.

FIG. 3 is a waveform diagram illustrating the waveform of the control voltage supplied to the switching transistor of the separately excited inverter circuit of the above embodiment.

FIG. 4 is a conceptual diagram illustrating a frequency change in the separately excited inverter circuit of the above embodiment.

FIG. 5 is a block diagram showing an example of the configuration of a digital control circuit of the separately excited inverter circuit of the above embodiment.

FIG. 6 is a block diagram showing the configuration of a conventional self-excited inverter lighting circuit.

FIG. 7 is a waveform diagram for explaining the operation of the self-excited inverter lighting circuit.

[Explanation of symbols]

Fluorescent tube C1 for LP drive
Capacitor L1 forming a series resonant circuit
First inductance L2 and second inductance Q constituting the series resonant circuit
Switching transistor IN DC power supply input terminal D Digital control circuit Vg
Control voltage Vd Drain voltage

Claims (3)

[Claims] Claims: 1. A series resonant circuit including a capacitor connected in parallel to a fluorescent tube and a first inductance connected in series with the fluorescent tube, and a series resonant circuit connected between an input terminal of a DC power source and the first inductance. a second inductance, a switching transistor connected between the connection point of the first and second inductances, and a reference potential; 1. A separately excited inverter lighting circuit comprising: a digital control circuit that supplies a variable frequency drive signal that changes to expand or contract. 2. The separately excited inverter lighting circuit according to claim 1, wherein said digital control circuit is comprised of a variable frequency dividing circuit including a load counter with a variable load value. 3. The separately excited type according to claim 1, wherein the digital control circuit maintains the frequency of the control signal constant for a predetermined period of time after receiving a lighting command, and then gradually decreases the frequency to a predetermined value. Inverter lighting circuit.