Preheating circuit for detecting the filament temperature of fluorescent lamps

[0002] The present in vent ion relates to a preheating circuit for detecting the filament temperature of fluorescent lamps, and more particularly to a circuit indirectly detecting a filament temperature to ensure that filaments operate at a thermionic emission temperature.

[0003] Properly preheating filaments becomes considerably necessary to avoid deteriorating the lamp life. Igniting a lamp at a low filament temperature requires a relatively high ignition voltage, causing bombardment and resulting in extremely sputtering on filaments. On the other hand, overheating the filaments will cause their coating material over evaporating and thermal shock. Both of the two improper preheating conditions engender sputtering and shorting the life of the lamp. Lamp filaments must reach their emission temperature at starting stage to minimize electrode sputtering. The preheating ratio (γ=R_h/R_c) of the hot resistance (R_h) of the electrodes to their cold resistance (R_c) is an index in knowing a n approximate emission temperature, and the electrodes with such a ratio means that it reaches a temperature high enough for thermionic emission.

[0004] FIGS. 1(a)˜(c) show three typical preheating circuits for fluorescent lamps. Please refer to FIG. 1(a). The preheating circuit is implemented by using the characteristic that the resistance of the positive temperature coefficient (PTC) of the resistor R₁is increased with increasing temperature to preheat the filaments. When the resistance of the resistor R₁is low at a low temperature, most of the preheating current flows through the capacitor C₁and the resistor R₁. At this time, the circuit operates at a preheating frequency to preheat the filaments. When the resistance of the resistor R₁increases with the increasing temperature, more current flows from the capacitor C₁to the capacitor C₂. The disadvantage of the preheating circuit is that the filaments are hard to operate at a thermionic emission temperature because the preheating time depends on the variation of the positive temperature coefficient resistance.

[0005] Referring to FIG. 1(b), the resistors R₃and R₄in series form a voltage divider. The voltage V₁in the voltage divider turns on the switching element Q₂and the switching element Q₂is in parallel with the capacitor C₄. Therefore, the voltage across the lamp is low. When the current flows through the resistor R₂to charge the capacitor C₃until the capacitor voltage of the capacitor C₃reaches the breakdown voltage of the diode D₁, the switching element Q₁is turned on and the switching element Q₂is forced to be turned off. The capacitance of the capacitor C₃is adjusted to determine the charging time of the capacitor C₃to control the preheating time so as to let the filament temperature is high enough. Therefore the preheating time is determined by the amount of the charges on the capacitor C₃. If the initial voltage of the capacitor C₃is high, the charging time for reaching the breakdown voltage of the diode D₁is shorter. On the other hand, the initial voltage of the capacitor C₃is zero, the charging time for reaching the breakdown voltage of the diode D₁is the longest. Therefore, the phenomenon of overheating the filaments or igniting a lamp at a low filament temperature also exists because the preheating time depends on the amount of the charges on the capacitor C₃but does not depend on the filament temperature.

[0006] As shown in FIG. 1(c), the charging time of the RC circuit is used to control the preheating time. When the voltage of the capacitor C₅is not charged to the breakdown voltage of the diode D₂, the circuit operates in higher frequency and the lamp voltage is not high enough to ignite the lamp. And the resonant current is used to preheat the filament. The drawback is same as described in FIG. 1(b). The phenomenon of overheating the filaments or igniting a lamp at a low filament temperature also exists because the preheating time depends on the amount of the charges on the capacitor C₅but does not depend on the filament temperature.

[0007] Otherwise, U.S. Pat. No. 5,920,155 discloses an electronic ballast for discharge lamps which sets a filament current and a voltage across a discharge lamp at their suitable operational levels according to respective operational states of the discharge lamps, and which also provides a sufficient dimming function even when the lamp is of a slim type. However, it is not mentioned how to dynamically adjust the preheating time. Therefore, the filaments are not sure to operate at a thermionic emission temperature. Thus, the preheating circuit needs to be improved to overcome the above problem.

[0008] It is therefore an object of the present invention to propose a preheating circuit for a fluorescent lamp. The preheating circuit includes a filament detecting circuit for indirectly detecting a filament resistance in a fluorescent lamp by measuring a filament voltage and a filament current, a pulse generation circuit for providing pulses of one of a first frequency and a second frequency determined by the detected filament resistance and a specific filament resistance, and a filament resonance circuit operating the fluorescent lamp at an operating frequency determined by the pulse generation circuit so that the filament resonance circuit operates at the first frequency to preheat the fluorescent lamp when the detected filament resistance is smaller than the specific resistance and the filament resonance circuit operates at the second frequency to operate the fluorescent lamp when the detected filament resistance is one of a first value being larger than and a second value being equal to that of the specific resistance.

[0009] According to an aspect of the present invention, the first frequency is a preheating frequency ω_s(ph).

[0010] Preferably, the second frequency is a switching frequency ω_s(fl)at full load.

[0011] Preferably, the specific resistance is a hot filament resistance R_hwhich is an index to preheat the fluorescent lamp when the detected filament resistance R_fis smaller than the hot filament resistance R_h.

[0012] Preferably, the hot filament resistance R_his γ times a cold filament resistance R_Cwhere γ is a preheating ratio and γ>1.

[0013] Preferably, the filament detecting circuit includes a first series circuit of a secondary winding of a transformer and a first diode electrically connected in parallel to a first smoothing capacitor and a first resistor for generating a first DC output voltage, a second series circuit of a filament resistor and a second diode connected in parallel to a second smoothing capacitor and a second resistor for generating a second DC output voltage, and a comparator having an inverting input electrically connected in parallel to the first smoothing capacitor, and a noninverting input electrically connected in parallel to the second smoothing capacitor for providing a switching signal to the pulse generation circuit for generating the operating frequency.

[0014] Preferably, the first DC output voltage is in proportion to a secondary voltage V′_Lrof the secondary winding of the transformer and the second DC output voltage is in proportion to a filament voltage V_Racross the filament resistor.

[0015] Preferably, the secondary voltage V′_Lrequals to γR_c*V_Lr/ω_s(ph)L_rwhere V_Lris a primary voltage of the primary winding of the transformer, and L_ris an inductance of the primary winding of the transformer.

[0016] Preferably, the filament voltage V_Rfequals to R_f*V_Lr/ω_s(Ph)L_r.

[0017] Preferably, the filament resonance circuit operates at the preheating frequency ω_s(ph)to preheat the fluorescent lamp when the detected filament resistance R_fis smaller than the hot filament resistance R_hwhile the filament resonance circuit operates at the switching frequency ω^s(fl)to operate the fluorescent lamp when the detected filament resistance R_fis one of a first value being larger than and a second value being equal to that of the hot filament resistance R_h.

[0018] Preferably, the filament resonance circuit operates at the preheating frequency ω_s(ph)to preheat the fluorescent lamp when the filament voltage V_Ris smaller than the secondary voltage V′_Lrwhile the filament resonance circuit operates at the switching frequency ω_s(fl)to operate the fluorescent lamp when the filament voltage V_Ris one of a first value being larger than and a second value being equal to that of the secondary voltage V′_Lr.

[0019] The present invention may best be understood through the following description with reference to the accompanying drawings, in which:

[0020] FIGS. 1(a)˜(c) illustrate three preheating circuits according to prior art;

[0021] FIG. 2 is a schematic diagram illustrating a preheating circuit for detecting the filament temperature of a fluorescent lamp according to the first preferred embodiment of the present invention; and

[0022] FIG. 3 is a schematic diagram illustrating the equivalent circuit of the resonant circuit according to the present invention.

[0023] FIG. 2 is a schematic diagram illustrating a preheating circuit for detecting the filament temperature of a fluorescent lamp according to the preferred embodiment of the present invention. As shown in FIG. 2, the preheating circuit for a fluorescent lamp includes a filament detecting circuit 2, a pulse generation circuit 3, and a filament resonant circuit 1. The filament detecting circuit 2 indirectly detects a filament resistance R_fin a fluorescent lamp 25 by measuring a filament voltage V_Rand a filament current I_R. The pulse generation circuit 3 provides pulses of one of a first frequency and a second frequency determined by the detected filament resistance R_fand a specific filament resistance. And the filament resonant circuit 1 operates the fluorescent lamp 25 at an operating frequency determined by the pulse generation circuit 3 so that the filament resonant circuit 1 operates at the first frequency to preheat the fluorescent lamp 25 when the detected filament resistance R_fis smaller than the specific resistance. On the other hand, the filament resonant circuit 1 operates at the second frequency to operate the fluorescent lamp 25 when the detected filament resistance R_fis one of a first value being larger than and a second value being equal to that of the specific resistance.

[0024] Meanwhile, the first frequency is a preheating frequency ω_s(ph). The second frequency is a switching frequency ω_s(fl)at full load. The specific resistance is a hot filament resistance R_hwhich is an index to preheat the fluorescent lamp 25 when the detected filament resistance R_fis smaller than the hot filament resistance R_h.

[0025] However, the hot filament resistance R_his γ times a cold filament resistance R_c, where γ is a preheating ratio and γ>1.

[0026] The filament detecting circuit 2 includes a first series circuit, a second series circuit, and a comparator 29. The first series circuit including a secondary winding 211 of a transformer 21 and a first diode 22 is electrically connected in parallel to with a first smoothing capacitor 23 and a first resistor 24 for generating a first DC output voltage. The second series circuit of a filament resistor 251 and a second diode 26 is electrically connected in parallel with a second smoothing capacitor 27 and a second resistor 28 for generating a second DC output voltage. And the comparator 29 has an inverting input 291 electrically connected to one end of a third resistor 293 and a noninverting input 292 electrically connected to one end of a fourth resistor 294. The other end of the third resistor 293 is electrically connected to the first smoothing capacitor 23, the first resistor 24, and the first diode 22. The other end of the fourth resistor 294 is electrically connected to the first smoothing capacitor 27, the second resistor 28, and the second diode 26. The output terminal of the comparator 29 is electrically connected to the pulse generation circuit 3 to provide a switching signal to the pulse generation circuit 3.

[0027] The pulse generation circuit 3 includes a pulse generator 32, and a switching element. The switching element is a bipolar transistor 31. The output terminal of the comparator 29 is electrically connected to the base of the bipolar transistor 31 and one end of a fifth resistor 35. The other end of the fifth resistor 35 is electrically connected to a voltage source 36. The collector of the bipolar transistor 31 is electrically connected to one end of a first timing capacitor 341. The other end of the first timing capacitor 341 is electrically connected to a timing capacitor terminal C_Tof the pulse generator 32 and one end of a second timing capacitor 342. The other end of the second timing capacitor 342 is ground. One end of a sixth resistor 33 is electrically connected to a timing resistor terminal R_Tof the pulse generator 32 and the other end of the sixth resistor 33 is ground. The voltage source 36 provides a voltage V_CCto turn on the bipolar transistor 31 when the output signal of the comparator 29 is High. On the other hand, the bipolar transistor 31 is turned off when the output signal of the comparator 29 is Low.

[0028] The first DC output voltage is in proportion to a secondary voltage V′_Lrof the secondary winding 211 of the transformer 21, and the second DC output voltage is in proportion to a filament voltage V_Racross the filament resistor 251 where a turn ratio of the transformer 21 is ω_s(ph)L_r/γR_c, and L_ris an inductance of the primary winding 212 of the transformer 21. The secondary voltage V′_Lrequals to γR_c*V_Lr/ω_s(ph)L_rwhere V_Lris a primary voltage of the primary winding 212 of the transformer 21. And, the filament voltage V_Requals to R_f*V_Lr/ω_s(ph)L_r.

[0029] The filament resonant circuit 1 operates at the preheating frequency ω_s(ph)to preheat the fluorescent lamp 25 when the detected filament resistance R_fis smaller than the hot filament resistance R_hwhile the filament resonant circuit 1 operates at the switching frequency ω_s(fl)to operate the fluorescent lamp 25 when the detected filament resistance R_fis one of a first value being larger than and a second value being equal to that of said hot filament resistance R_h. The filament resistance R_fof the filament resistor 251 can be obtained from the filament voltage V_Rso that the filament resonance circuit 1 operates at the preheating frequency ω_s(ph)to preheat the fluorescent lamp 25 when the filament voltage V_Ris smaller than the secondary voltage V′_Lr. Nevertheless, the filament resonant circuit 1 operates at the switching frequency ω_s(fl)to operate the fluorescent lamp 25 when the filament voltage V_Rfis one of a first value being larger than and a second value being equal to that of the secondary voltage V^′_Lr.

[0030] FIG. 3 is a schematic diagram illustrating the equivalent circuit of the resonant circuit according to the present invention. As shown in FIG. 3, the filament resistance R_fis obtained from the filament voltage V_Rfand the filament current I_Racross the filament 25, which is given as follows: $R_f=\frac{V_{R_f}}{I_{R_f}}.$

[0031] In practice, sensing voltage is much easier than sensing current. In the present invention, the filament voltage V_Ris measured directly from a voltage across the filament resistor 251, while the filament current I_Ris measured by way of an inductor voltage V_Lracross the primary winding 212 of the transformer 21 for the convenience of implementation. The filament current is given as follows: $I_{R_f}=\frac{V_{L_r}}{{\omega }_{s\left(ph\right)}{L}_r}.$

[0032] Thus, $R_f=\frac{V_{R_f}}{\frac{V_{L_r}}{{\omega }_{s\left(ph\right)}{L}_r}}.$

[0033] FIG. 2 shows the circuit implementation of detecting R_h=γR_c, in which the turns ratio n=ω_s(ph)L_r/γR_cand γ>1. At the beginning, the filament resistance R_f=R_c, so that V_Rfequals to R_cV_Lr/ω_s(ph)L_r. Because V_Rfis smaller than γR_cV_Lr/ω_s(ph)L_r, the output of the comparator 29 is close to ground level. Thus, the switching element is in the off state and the preheating frequency ω_s(ph)is determined by the capacitance of the second timing capacitor 342 and the resistance of the timing resistor 33. When the filament resistance R_fof the filament resistor 251 reaches R_h=γR_c, the output of the comparator 29 is pulled to the voltage V_cc, which turns on the switching element and causes operating frequency changing from the preheating frequency ω_s(ph)to the switching frequency ω_s(fl). This switching frequency ω_s(fl)is determined by the resistance of the timing resistor 33 and the summation of the capacitance of the first timing capacitor 341 and the capacitance of the second timing capacitor 342. When filament resistance reaches R_h=γR_c, the lamp 25 is ready to be ignited.

[0034] In sum, the preheating circuit of the present invention can ensure that the filament always operates at a proper thermionic emission temperature, which results in reducing sputtering significantly.

[0035] While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not to be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.