Patent Application
20080122387
This application claims the priority benefit of Taiwan application serial no. 95141130, filed Nov. 7, 2006. All disclosure of the Taiwan application is incorporated herein by reference.
1. Field of Invention
The present invention relates to a DC/AC inverter for fluorescent lamp application. More particularly, the present invention relates to a DC/AC inverter control circuit varying frequency according to lamp states.
2. Description of Related Art
In the conventional art, a typical DC/AC inverter for fluorescent lamp application, which includes a full-bridge topology and a resonant tank, converts an input DC voltage source into an AC of high voltage for driving a fluorescent lamp. To ensure the fluorescent lamp lighting stably, i.e., the luminance of the fluorescent lamp does not change with the change of the input voltage, a negative feedback control circuit is often applied in the design to stabilize an operating current of the fluorescent lamp.
As an operating voltage of the fluorescent lamp and frequency-gain characteristic curve of the resonant tank vary when the fluorescent lamp striking, the inverter controller provides a control signal according to a state of the lamp. The state of the lamp is determined conventional by detecting whether or not a lamp current exceeds a certain value.
As shown in
When the lamp voltage signal OV reaches a predetermined value, the controller 110 adjusts the output power, so as to maintain the voltage of the fluorescent lamp 140 near a predetermined driving voltage value corresponding to the predetermined value. The fluorescent lamp 140 is then struck after being maintained at the appropriate predetermined driving voltage for a period of time. At this time, the controller 110 further determines whether or not the lamp is struck by determining whether or not the lamp state indicating signal CS generated by the current detecting signal 150 is higher than a predetermined value. When the controller 110 determines that the lamp is struck, the frequency is varied, i.e., a higher frequency for striking the lamp is changed to a frequency suitable for the normal operation of the lamp.
However, referring to
The present invention is directed to a DC/AC inverter capable of determining a load state by detecting a terminal voltage of the load and adjusting an operating frequency of the inverter accordingly.
The present invention is also directed to a controller for controlling the DC/AC inverter. The controller uses voltage variation characteristics when the load state changes, determines the load state according to relationship between the terminal voltage of the load and a reference voltage, and adjusts the operating frequency of the inverter according to the determination.
In order to achieve the above objectives, the present invention provides a DC/AC inverter for driving a load. The DC/AC conversion driving circuit comprises a switch device, a resonant tank, a voltage detecting circuit, and a controller. The switch device is coupled to a DC power source. The resonant tank is coupled to the switch device and the load, and converts power of the DC power source transmitted from the switch device to an AC signal, so as to drive the load. The voltage detecting circuit is coupled to the load, and generates a voltage detecting signal according to a voltage of the load. The controller is coupled to the voltage detecting device and the switch device, and controls a magnitude of the power of the DC power source transmitted to the resonant tank through the switch device. The controller determines whether or not to change an operating frequency thereof according to the voltage detecting signal.
The present invention also provides a controller for controlling a DC/AC inverter to drive a load. The controller comprises a frequency generator, a pulse width modulated circuit, a driving circuit, and a lamp state determining circuit. The frequency generator is used to generate a reference signal. The pulse width modulated circuit is coupled to the frequency generator, and generates a pulse width modulated signal according to the reference signal. The driving circuit is coupled to the pulse width modulated circuit, and generates a plurality of driving signals according to the pulse width modulated signal. The lamp state determining circuit comprises a first determining circuit for determining whether or not to generate a first determining signal according to a voltage detecting signal and a first reference voltage. The frequency generator determines whether or not to adjust a frequency of the reference signal according to the first determining signal.
The present invention will be more comprehensible with reference to the accompanying drawings and the detailed illustration below.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
Curve c shows a situation where the lamp is relatively easy to be struck, for example, when the ambient temperature is high, when external light exists, or when the condition of the lamp is quite fine, etc., so the lamp is struck before the lamp voltage reaches Vov, and then the lamp voltage reduces to the normal operating voltage range. Curve d shows a situation when the condition is the most preferred, for example, when the lamp condition is perfect, when the lamp is just extinguished and the lamp temperature is still high, or when the external light is sufficient, etc. At this time, the lamp does not struck with a high voltage as shown in curves b and c, but is struck directly once the voltage increases to reach the normal operating voltage range. Curve e shows a situation that the lamp cannot be struck as the lamp cannot receive enough driving voltage, which may be caused by a short circuit of an output terminal of the resonant tank.
Based on the possible lamp conditions described above, the present invention sets two voltages V1 and V2. V1 is a lowest possible voltage when the lamp is in the normal operation (i.e., after the lamp is struck), and V2 is a highest possible voltage when the lamp is in the normal operation (i.e., when the lamp is struck). The two voltages V1, V2 are used to determine whether or not the lamp is struck. Thus, the problem of error determination according to the lamp current in the conventional art is prevented.
Several embodiments are given below to illustrate possible schemes of implementation of the present invention. However, the present invention is not limited by the following embodiments. A low level signal or a high level signal mentioned in the description below can be interpreted as that, the signal at the low level represents that the signal is not generated, and the signal at the high level represents that the signal is generated, which are illustrated here in advance.
A DC voltage source 301 is coupled to the switch device 302. An output terminal of the switch device 302 is coupled to an input terminal of the resonant tank 303. An output terminal of the resonant tank 303 is coupled to a terminal of the fluorescent lamp 304. The resonant tank of the present invention can be a transformer of any type, such as a magnetic transformer, or a piezoelectric ceramic transformer, but is not limited to be the above types. Two terminals of the fluorescent lamp 304 are coupled to the current detecting circuit 380 and the voltage detecting circuit 390 respectively. The current detecting circuit 380 is also coupled to the pulse width modulated circuit 310, and the pulse width modulated circuit 310 is coupled to the frequency generator 320 and the driving circuit 360. The driving circuit 360 is coupled to the switch device 302 to form a so-called control loop connection.
In this embodiment, the DC/AC inverter is a half-bridge DC/AC inverter. However, the present invention is also applicable to full-bridge and push-pull DC/AC inverters, and is not limited to the aforementioned circuits. In the embodiment, the switch device 302 includes two power switches 302A, 302B. The power switch 302A can be a P-type metal oxide semiconductor (PMOS) power switch, and the power switch 302B can be an N-type metal oxide semiconductor (NMOS) power switch. However, the two power switches 302A, 302B are not limited to metal oxide semiconductor (MOS) power switches, and can also be other transistor switches, such as BJTs of NPN type or PNP type.
The frequency generator 320 generates a triangular wave signal S1 and a pulse signal S2 of the same frequency. However, the present invention is limited to use the triangular wave signal, and all ramp signals or sawtooth wave signals are applicable to the present invention.
The current detecting circuit 380 is connected to the fluorescent lamp 304 in series, and provides a load current signal S4 to indicate the value of a current flowing through the fluorescent lamp. The voltage detecting circuit 390 detects a load voltage signal S5 to indicate a terminal voltage of the fluorescent lamp 304 with resonant capacitors 307, 308 in the resonant tank 303 connected to the fluorescent lamp 304 in parallel.
The pulse width modulated circuit 310 includes an inverting integrator composed of an error amplifier 311, a resistor 317 and a capacitor 313, and a comparator 312. Moreover, the pulse width modulated circuit 310 further includes a controlled current source 315, which is connected to an inverting input terminal of the error amplifier 311 via a switch 316.
The protection circuit 340 includes a logic control circuit 341. The protection circuit 340 receives an abnormal signal S13, the signal S5 indicating the terminal voltage of the fluorescent lamp 304, and an error amplifying signal S3 of the error amplifier 311 in the pulse width modulated circuit 310.
The dimming circuit 350 includes a dimming frequency generator 351. A triangular wave signal S7 generated by the dimming frequency generator 351 is sent to a non-inverting input terminal of a comparator 355, and a dimming control signal S8 is sent to an inverting input terminal of the comparator 355. After comparison, a dimming pulse signal S9 is generated, which controls the time that a dimming voltage S20 is sent to the pulse width modulated circuit 310 through controlling switches 352 and 353.
It should be noted that the structures of the protection circuit 340, the dimming circuit 350, and the pulse width modulated circuit 310 described in detail above are to illustrate the operational relationship between the lamp state determining circuit 370 and various circuits coupled there-to, but are not to limit the design of the circuits.
The driving circuit 360 receives an output S12 of the pulse width modulated circuit 310, the pulse signal S2 output from the frequency generator 320 and an output signal S18 output from the protection circuit 340 and accordingly generates two driving output signals POUT, NOUT, so as to control the transistor switches 302A, 302B in the switch device 302 respectively.
In this embodiment, the timer 330 generates a reset signal S11 and a time out signal S10. The reset signal S11 serves as a reset signal for analog or digital circuits (e.g., the pulse width modulated circuit 310 and the logic control circuit 341) in the entire circuit, such that the circuits is reset at an appropriate time and output signals of original states. Thus, improper operation of corresponding circuits caused by improper signals generated when the circuits are started or in other situations are prevented. The time out signal S10 is a time signal indicating the time that the lamp should be struck normally. When the time out signal S10 is output, it indicates that the lamp should be in the ON state normally. The circuits not suitable for operating before the lamp is struck, e.g., the protection circuit 340, start to operate after receiving the time out signal S10.
The frequency generator 320 is also controlled by a signal S15, which is generated by the lamp state determining circuit 370 for indicating whether or not the fluorescent lamp is struck. When the fluorescent lamp is not struck, the frequency generator 320 generates signals at a frequency for striking the fluorescent lamp to the driving circuit 360. When the fluorescent lamp 304 is struck, the frequency generator 320 generates signals at a fluorescent lamp operating frequency. The resonant frequencies of the resonant tank 303 when the fluorescent lamp is struck or not struck are different. Such design providing different operating frequencies according to different states for the lamp ensures that the system operates at an appropriate operating frequency (e.g., near the resonant frequency) no matter whether or not the fluorescent is struck, so the system operates more efficiently.
The lamp state determining circuit 370 includes a first determining circuit 370a and a second determining circuit 370b. The first determining circuit 370a and the second determining circuit 370b respectively include determiners 377b and 377a. The determiner 377a is used to determine whether or not the situation of the lamp belong to lamp striking curves a, b, or c in
The lamp state determining circuit 370 outputs the signals S13 and S15 respectively. The signal S13 is transmitted to the protection circuit 340, such that the protection circuit 340 determines whether or not to cease the operation of the driving circuit 360 according to the signal S13. The signal S15 is transmitted to the dimming circuit 350 and the frequency generator 320, such that the dimming circuit 350 determines whether or not to perform the dimming according to the signal S15, and the frequency generator 320 determines whether or not to perform the frequency variation according to the signal S15. Generally, the dimming circuit 350 starts the dimming (adjusting the power of the DC power source 301 transmitted to the resonant tank 303) at the same time as the frequency generator 320 varies the operating frequency.
In the normal state, more detailed operation of this embodiment is described as follows.
When the system is supplied with power and started, the reset signal S11 generated by the timer 330 turns on a switch 316 through an OR logic 314, such that the current source 315 is connected to the inverting input terminal of the error amplifier 311, and forces the voltage on the inverting input terminal to be higher than a reference voltage Vref1. Thus, the output of the error amplifier 311 is forced to be low, such that the duty cycle of the output S12 of the pulse width modulated circuit 310 is changed to 0%. After the 0% output signal S12 passes through the driving circuit 360 of this embodiment, the duty cycles of POUT and NOUT are both changed to 0%, such that the switches 302A, 302B are turned off.
The timer 330 then stops outputting the reset signal S11 after a predetermined period of time, and the current source switch 316 is turned off. At this time, the pulse width modulated circuit 310 starts to operate, the inverting input terminal of the error amplifier 311 turns into a state of being lower than the reference voltage Vref1 because the fluorescent lamp 304 is not struck. The signal S3 output from the error amplifier 311 gradually increases according to the negative feedback control principle. Then, after the comparison of the signal S3 and the triangular wave S1, the comparator 312 sends out a pulse width modulation signal S12. The driving circuit 360 receives the signal S12 and the pulse signal S2 to generate two signals POUT, NOUT, which are respectively used to control the ON or OFF of the transistor switches 302A, 302B of the switch device 302, so as to adjust the power output to the fluorescent lamp 304. The duty cycle of the pulse width modulation signal S12 gradually becomes wider and increases, so the driving voltage of the fluorescent lamp 304 will gradually increase accordingly.
As shown in curve b of
Next, the voltage of the fluorescent lamp 304 becomes higher than the comparison voltage V2, the determiner 377a also starts to output a high level signal. The latch circuit 371c continuously output a high level signal after receiving the high level signal from the determiner 377a. In principle, the delay circuit 374 will be in the reset state and not work after receiving the high level signal from the latch circuit 371c in the predetermined period of delay and so the delay circuit 374 did not output the high level signal.
Furthermore, the inverter 379a starts to output a low level signal to the latch circuit 371b so that the latch circuit 371b starts to work. At the same time, the high level signal generated by the determiner 377a is transferred into a low level signal by the inverter 379b, so the latch circuit 371b also outputs the low level signal at this time. As the two input signals of the OR logic 375a are both low level signals, the low level signals will be output. In addition, the time out signal S10 of the timer 330 is still at the low level, and the inverter 379c makes the latch circuit 371a to enter the reset state and not work. Therefore, two input signals of the OR logic 375b are also low logic signals, and the low logic signals will be output.
After the driving voltage of the fluorescent lamp 304 finally increases to a predetermined lamp striking voltage Vov, the comparator 343 in the protection circuit 340 detects that the signal S5 indicating the terminal voltage of the fluorescent lamp exceeds a predetermined reference voltage Vref2, and sends out a signal S16 indicating that the terminal voltage of the fluorescent lamp exceeds, and then turns on the switch 316 via the OR logic gate 314. The current of the current source 315 flows into the inverting input terminal of the error amplifier 311, and reduces the level of output S3 of the error amplifier 311. Next, the duty cycle of the pulse width modulation signal S12 is shortened, so as to reduce the power transmitted to the terminals of the fluorescent lamp. Next, when it is detected that the signal S5 indicating the terminal voltage of the fluorescent lamp is lower than the predetermined reference voltage Vref2, the current source switch 316 is turned off, such that the level of output S3 of the error amplifier 311 increases. Thus, the terminal voltage of the fluorescent lamp is stabilized under the negative feedback control, and maintains near the predetermined lamp striking voltage Vov continuously.
Once the fluorescent lamp is struck and conducted under the adequate driving voltage Vov and time period, according to the characteristics of the fluorescent lamp, the terminal voltage S16 of the fluorescent lamp will suddenly drop by over a half to an operating voltage, which is an ON voltage and is nearly fixed. The operating voltage is between the voltages V1 and V2, as shown in curve b of
The situation shown as curve c in
As shown in curve d of
If the striking process of the fluorescent lamp is abnormal, and resulting in the curves a, e as shown in
When the situation of curve a of
As shown in curve e of
The latch circuit 371a is in the reset state and does not work before the time out signal S10 is sent out. When the fluorescent lamp 304 is struck successfully before the time out signal S10 is sent out, after the time out signal S10 is sent out, the time out signal S10 enables the latch circuit 371a to start to work through the inverter 379c. At this time, the latch circuit 371a sends out and latches the high level signal after receiving the high level signal of the OR logic 375a. Thus, after the time out signal S10 is sent out, the lamp state determining circuit 370 sends out the signal S15 and latches the signal S15 at the high level signal indicating the striking of the lamp. However, if the fluorescent lamp 304 is not successfully struck before the time out signal S10 is sent out, the lamp state determining circuit 370 will output the low level signal indicating that the lamp is not struck after the time out signal S10 is sent out.
In addition, the lamp state determining circuit 370 sends out the signal S13, and informs the protection circuit 340 the state of the fluorescent lamp 304. The protection circuit 340 starts to perform the protection after receiving the time out signal S10. If the fluorescent lamp 304 is struck before the time out signal S10 is sent out, the terminal voltage of the fluorescent lamp 304 will definitely be higher than the voltage V1, such that the determiner 377b sends out the high level signal, and the OR logic 375a outputs the high level signal as well. Therefore, two input signals of an NAND logic 373 are both high level signals, and low level signals indicating that the fluorescent is in the normal state will be output. However, when the fluorescent lamp 304 is struck normally but extinguishes during the operation such that the terminal voltage is lower than the voltage V1, the NAND logic 373 outputs the high level signal S13 representing that the fluorescent lamp state is abnormal, such that the protection circuit 340 also outputs the high level protection signal S18. After the driving circuit 360 receives the high level protection signal S18, the switch device 302 is switched to stop transmitting the power of DC voltage source 301 to the resonant tank 303.
According to another aspect, if the fluorescent lamp 304 is not struck before the time out signal S10 is sent out, 375a outputs the low level signal, and so the NAND logic 373 outputs the high level signal S13 indicating that the fluorescent lamp state is abnormal, such that the switch device 302 stops transmitting the power to the resonant tank 303.
Moreover, if the lamp is struck and the dimming circuit 350 operates, the terminal voltage of the lamp is periodically switch among zero voltage and the normal operating voltage. In the present invention, the protection circuit 340 uses a digital count unit to count by a pulse signal S6 generated by the low-frequency dimming frequency generator 351. Counting function is started after the high level signal S13 indicating the fluorescent lamp state is abnormal is received. If the signal S13 still remains at the high level signal after a predetermined time according to the counting of the digital count unit, the logic control circuit 341 sends out the output stop signal S18 to the driving circuit 360. Thus, the misdetermination of the extinguishment of the fluorescent lamp 304 when the dimming circuit 350 operates is prevented.
In addition, in normal operations, if the transformer 305 is seriously damaged because of electric leakage, the system will have the problem of overload due to the additional loss caused by the electric leakage. Thus, the error amplifier 311 increases the level of output S3 continuously, so as to provide sufficient power to the load to stabilize the current of the fluorescent lamp. If the electric leakage exceeds the maximum power provided by the system, the output S3 of the error amplifier 311 is definitely higher than a peak of the triangular wave S1. The protection circuit 340 compares the output S3 of the error amplifier 311 and a reference voltage Vref3 slightly higher than the peak of the triangular wave S1, and obtains a signal S14 indicating whether or not the system is overloaded. Similarly, when the timer 330 starts the protection circuit 340 with S11, if the signal S14 indicates that the system is overloaded, and the logic control circuit 341 start to count time by the pulse signal S2 generated by the frequency generator 320. If the system is overloaded for a period exceeding the predetermined time, the logic control circuit 341 sends out the output stop signal S18 to the driving circuit 360.
This embodiment further includes a dimming circuit 350. The dimming principle is using a pulse signal S7 with a frequency lower than the operating frequency of the fluorescent lamp, so as to control to stop or restore transmitting power to the fluorescent lamp. Thus, by alternatively turning on and off the lamp, the luminance of the fluorescent lamp is adjusted. Moreover, to prevent the flicker caused by an over low frequency, generally, the dimming frequency is controlled to be over 200 Hz. The dimming circuit 350 of this embodiment is controlled by the signal S15 indicating whether or not the fluorescent lamp is struck. When the signal S15 indicates that the fluorescent lamp is struck, a switch 353 that controls the output of the dimming signal is turned on.
A dimming voltage S20 of the dimming circuit 350 is higher than the reference voltage Vref1. When the dimming voltage S20 of the dimming circuit is transmitted to the pulse width modulated circuit 310 via the control switches 353, 352, and a resistor 354, the level of output S3 generated by the error amplifier 311 of the pulse width modulated circuit 310 is reduced, so the system stops outputting power to the load. When the dimming pulse signal S9 turns off the switch 352, the dimming voltage S20 is stopped to transmit to the pulse width modulated circuit 310, and the system restores the power supply to the lamp. Thus, the effect of luminance adjustment is achieved by using a low frequency to control the proportion that the power supply is stopped or restored in each period. The dimming circuit 350 starts the dimming function when the fluorescent lamp is struck, so as to ensure adequate and continuous power for striking the fluorescent lamp, such that the fluorescent lamp is struck in a long enough period.
In this embodiment, the dimming circuit 350 is coupled to the inverting input terminal of the error amplifier 311 of the pulse width modulated circuit 310, so as to realize the dimming function. However, in practice, the dimming signal can be sent to other circuit, such as the driving circuit 360, the comparator 312, and so on, to realize the dimming function. These variations are well known to persons of ordinary skill in the art.
The terminal voltage during the striking of the fluorescent lamp changes according to the ambient temperature, lighting, fluorescent lamp types, and fluorescent lamp states. In order to prevent possible misdetermination of the lamp striking only by the lamp voltage, the lamp state indicating signal CS of the conventional art can be used to provide assistance in the determination of the fluorescent lamp state.
In this embodiment, the predetermined delay time of the delay circuit 374a is the time required for charging the level of the signal S3 from a trough level of the triangular wave signal S1 to the level of the reference voltage Vref3 when the capacitor 313 is charged by the error amplifier 311. After the predetermined delay time, the latch circuit 371d returns from the reset state to the normal operating state through the inverter 379d. At this time, once the fluorescent lamp 304 is struck to turn the signal S14 to the low level, the high level signal is output through the inverter 379e. Thus, the output signal of the latch circuit 371d is latched at the high level, such that the lamp state determining circuit 370 outputs the high level signal S15 representing that the fluorescent lamp is struck.
As known from the description of the above embodiments, of the present invention firstly determines whether the terminal voltage of the lamp exceeds the lowest possible voltage V1 of the normal operation of the lamp, and secondly determines whether or not the lamp is struck according to a predetermined time after the signal S5 exceeds the voltage V1 or the load current signal S4 exceeds a predetermined value (to reduce the level of the signal S3). In the other aspect, the present invention also takes advantage of the characteristic that the terminal voltage increases before striking and drops after striking during the striking process of the lamp, and uses one or more groups of determining circuits to determine whether or not the lamp is struck. Certainly, the present invention can also use the lamp current detecting to assist in determining the lamp state.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
| Number | Date | Country | Kind |
|---|---|---|---|
| 95141130 | Nov 2006 | TW | national |
