Abstract:
A ballast which controls the open-circuit voltage of the ballast. The ballast includes a power factor corrector (PFC) for receiving an AC input voltage and converting the AC input voltage into a power factor corrected DC voltage; a DC/DC converter connected to the PFC and having a switch placed at a low-voltage side of the DC/DC converter for converting the DC voltage of the PFC into a DC output voltage according to the switching operation of the switch; a controller connected to a control terminal of the switch of the DC/DC converter for sending a switching control signal to control the switch; and an open-circuit voltage controller for detecting a voltage associated with the open-circuit voltage of the ballast and regulating the duty ratio or pulse density or switching frequency of the switching control signal in response to the results of the detection, thereby controlling the open-circuit voltage.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a ballast for gas discharge lamp, and more particularly to a ballast for gas discharge lamps with a control device for controlling the open-circuit voltage of the ballast. 
       BACKGROUND OF THE INVENTION 
       [0002]    Generally, open-circuit voltage is the voltage difference of electrical potentials between two terminals of a device when there is no external load connected. In the applications of gas discharge lamps, ballasts are used to limit the current flowing through the gas discharge lamps. Therefore, electrical ballasts have to provide sufficient open-circuit voltage to allow the gas discharge lamps to have enough energy to maintain the glow discharge state and transits from the glow discharge state to the arc discharge state. 
         [0003]      FIG. 1  shows a block diagram of a ballast for gas discharge lamps according to the prior art. As shown in  FIG. 1 , a ballast includes a PFC (power factor corrector)  102 , a DC/DC converter  104 , an input filtering capacitor C 1 , and an inverter  106 . The ballast is used to receive an AC input voltage Vin and provide the energy required to ignite the gas discharge lamp Lp 1 . The gas discharge lamp Lp 1  is a high intensity discharge lamp (HID) lamp. The PFC  102  is used to rectify the AC input voltage Vin into a DC voltage with a corrected power factor for suppressing the harmonic noises in the input current. The input filtering capacitor C 1  is connected to the output end of the PFC  102  for removing the noises and interferences of the DC voltage outputted from the PFC  102 . The DC/DC converter  104  is connected to the input filtering capacitor C 1  and is made up of a buck converter, which converts the DC voltage outputted from the PFC  102  into a lower DC voltage. The inverter  106  is connected to the output end of the DC/DC converter  104  for converting the output DC voltage of the DC/DC converter  104  into an AC voltage, thereby driving the gas discharge lamp Lp 1 . It is noteworthy that if the gas discharge lamp Lp 1  is a DC lamp, the inverter  106  may be eliminated. The DC/DC converter  104  is used to control the operating state of the gas discharge lamp Lp 1 , thereby allowing the gas discharge lamp Lp 1  to operate under the steady state with constant current mode or constant power mode. 
         [0004]      FIG. 2  shows a block diagram of a ballast for gas discharge lamps according to the prior art, in which the detailed circuitry of the DC/DC converter  104  of  FIG. 1 . It should be noted that similar circuit elements are labeled with the same reference numeral. In  FIG. 2 , the DC/DC converter  104  is a buck converter and includes a switch Q 1 , a rectifying diode D 1 , an inductor L 1 , and a capacitor C 2 . The rectifying diode D 1  is connected in parallel with the input filtering capacitor C 1 , and the capacitor C 2  is connected in parallel with the rectifying diode D 1 . The inductor L 1  is connected between the anode of the rectifying diode D 1  and one end of the capacitor C 2 . The switch Q 1  is driven by a driver (not shown) to conduct switching operations. The switch Q 1  has a first terminal (or positive terminal), a second terminal (or negative terminal), and a third terminal (or control terminal). There are two possible ways to locate the switch Q 1 . The first way of locating the switch Q 1  is to place the switch Q 1  between the capacitor C 1  and the cathode of the rectifying diode D 1 , i.e. the switch Q 1  can be located at the high-voltage side of the DC/DC converter. However, such configuration requires an additional isolation device such as a photo coupler to drive the switch Q 1 . Hence, such configuration will increase the cost and make the switch Q 1  to be driven in a difficult manner. The second way of locating the switch Q 1  is to place the switch Q 1  between the capacitor C 1  and the anode of the rectifying diode D 1 , i.e. the switch Q 1  can be located at the low-voltage side of the DC/DC converter, as shown in  FIG. 2 . Such configuration allows the second terminal of the switch Q 1  to share the same ground point with the input filtering capacitor C 1 , thereby eliminating the isolation device and rendering the switch Q 1  easy to be driven. Hence, the configuration of locating the switch Q 1  is to place the switch Q 1  between the capacitor C 1  and the anode of the rectifying diode D 1  is widely used. The operation of the DC/DC converter  104  is described as follows. The energy of the voltage on the input filtering capacitor C 1  is transferred to the inductor L 1  and the capacitor C 2  by the switch operation of the switch Q 1 . The rectifying diode D 1  provides a current path for the inductor L 1  when the switch Q 1  is turned off. The inductor L 1  and the capacitor C 2  form an output filter for removing the noises of the output voltage derived from the switching operation of the switch Q 1 . The output voltage of the DC/DC converter  104  is established on the capacitor C 2 . 
         [0005]    Referring to  FIGS. 1 and 8 , in which  FIG. 8  shows the waveform diagram of the lamp voltage and the lamp current of the ballast for gas discharge lamps. Before the gas discharge lamp Lp 1  is ignited, the ballast of the gas discharge lamp Lp 1  needs to provide an appropriate open-circuit voltage having a voltage level of 300V, for example, to supply sufficient energy for the gas discharge lamp Lp 1  to transit from the glow discharge state to the arc discharge state. Hence, before the gas discharge lamp Lp 1  is ignited, the lamp voltage Vlamp is the open-circuit voltage of the ballast. Also, before the gas discharge lamp Lp 1  is ignited, the lamp current Ilamp flowing through the gas discharge lamp Lp 1  is zero. After the gas discharge lamp Lp 1  is ignited, the lamp current Ilamp is ascending and the impedance of the gas discharge lamp Lp 1  is descending rapidly, thereby lowering the lamp voltage Vlamp of the gas discharge lamp Lp 1 . When the gas discharge lamp Lp 1  enters the steady state, the impedance of the gas discharge lamp Lp 1  is maintained at a stable value (the stable value is the impedance of the gas discharge lamp Lp 1  which enters the steady state). In this way, the waveform of the lamp voltage Vlamp and the waveform of the lamp current Ilamp will be periodically fluctuating in positive half-cycles and negative half-cycles. 
         [0006]    In prior art ballasts, a compensation circuit is required to substantially control the open-circuit voltage of the ballast within a predetermined range. Nonetheless, the circuit structure of the compensation circuit is quite complex and the efficiency of the ballast is reduced by using the compensation circuit. More disadvantageously, the cost of the ballast will increase due to the incorporation of the compensation circuit. The applicants propose a control device for controlling the open-circuit voltage of the ballast with a simplified circuit structure, thereby limiting open-circuit voltage of the ballast within a predetermined range. 
       SUMMARY OF THE INVENTION 
       [0007]    The primary object of the invention is to provide a ballast for gas discharge lamps and having a control device with a simplified circuit structure for limiting the open-circuit of the ballast within a predetermined range. 
         [0008]    According to a broad aspect of the invention, the invention provides a ballast for gas discharge lamps. The inventive ballast includes a power factor corrector receiving an AC input voltage and converting the AC input voltage into a DC voltage with a corrected power factor; a DC/DC converter connected to the power factor corrector and having a switch located at a low-voltage side of the DC/DC converter for converting the DC voltage with a corrected power factor into a DC output voltage according to switching operations of the switch; a controller connected to a control terminal of the switch in the DC/DC converter for sending a switching control signal to control the switching operations of the switch; and a control device for open-circuit voltage for detecting a voltage associated with an open-circuit voltage of the ballast and regulating a duty ratio or a pulse density or a switching frequency of the switching control signal according to results of detection, thereby controlling the open-circuit voltage of the ballast. 
         [0009]    Now the foregoing and other features and advantages of the invention will be best understood through the following descriptions with reference to the accompanying drawings, wherein: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  shows a block diagram of a ballast for gas discharge lamps according to the prior art; 
           [0011]      FIG. 2  shows a block diagram of a ballast for gas discharge lamps according to the prior art, in which the detailed circuitry of the DC/DC converter  104  of  FIG. 1 ; 
           [0012]      FIG. 3  shows a representative block diagram of the ballast and the control device for open-circuit voltage according to the invention; 
           [0013]      FIG. 4  shows a circuit block diagram of the ballast and the control device for open-circuit voltage according to a first exemplary embodiment of the invention; 
           [0014]      FIG. 5  shows a circuit block diagram of the ballast and the control device for open-circuit voltage according to a second exemplary embodiment of the invention; 
           [0015]      FIG. 6  shows a circuit block diagram of the ballast and the control device for open-circuit voltage according to a third exemplary embodiment of the invention; 
           [0016]      FIG. 7  shows a circuit block diagram of the ballast and the control device for open-circuit voltage according to a fourth exemplary embodiment of the invention; and 
           [0017]      FIG. 8  shows the waveform diagram of the lamp voltage and the lamp current of the ballast for gas discharge lamps. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0018]    Several exemplary embodiments embodying the features and advantages of the invention will be expounded in following paragraphs of descriptions. It is to be realized that the present invention is allowed to have various modification in different respects, all of which are without departing from the scope of the present invention, and the description herein and the drawings are to be taken as illustrative in nature, but not to be taken as a confinement for the invention. 
         [0019]      FIG. 3  shows a representative block diagram of the ballast and the control device for open-circuit voltage according to the invention. Compared to the ballast for gas discharge lamps of  FIG. 2 , the ballast of  FIG. 3  includes a control device for open-circuit voltage  300  which is connected to a controller  302  for the ballast. The controller  302  is connected to the control terminal of the switch Q 1  and configured to generate a switching control signal Sc to control the switching operation of the switch Q 1 . The low-voltage side of the capacitor C 1  is defined as a first reference potential, and the low-voltage side of the capacitor C 2  is defined as a second reference potential. The control device for open-circuit voltage  300  is configured to detect a voltage Voc′ associated with the open-circuit voltage of the ballast, such as the output voltage of the DC/DC converter  104  or the voltage difference between the output voltage of the PFC  102  and the output voltage of the DC/DC converter  104  (including the switch Q 1 , the rectifying diode D 1 , the inductor L 1 , and the capacitor C 2 ). In response to the results of detection of the voltage Voc′, a detecting signal is generated to control the duty ratio or pulse density or switching frequency of the switching control signal Sc. By sampling the voltage Voc′ associated with the open-circuit voltage of the ballast, a detecting signal is generated and the duty ratio or pulse density or switching frequency of the switching control signal Sc which is used to control the switching operation of the switch Q 1  is regulated accordingly. In this embodiment, if the lamp Lp 1  is a DC lamp, the inverter  106  can be eliminated. In this embodiment, the open-circuit voltage of the ballast may be limited within the predetermined range. And when the DC/DC converter  104  is generating the open-circuit voltage, the DC/DC converter  104  may be controlled to operate in burst mode. 
         [0020]      FIG. 4  shows a circuit block diagram of the ballast and the control device for open-circuit voltage according to a first exemplary embodiment of the invention. As shown in  FIG. 4 , the control device  300  includes a detecting circuit consisted of a first resistor R 1  and a second resistor R 2 . The control device  300  further includes a switch control device consisted of a control switch S 1  and a third resistor R 3 . The controller  302  includes a first control unit  402 , a second control unit  404 , and an isolation device Ph 1 . In this embodiment, the voltage Voc′ associated with the open-circuit voltage of the ballast is the output voltage Vo of the DC/DC converter  104  (i.e. the voltage on the capacitor C 2 ). The first resistor R 1  and the second resistor R 2  are connected in series between the voltage output terminal of the DC/DC converter and a signal ground terminal to form a voltage divider for dividing the output voltage Vo of the DC/DC converter  104 , thereby generating a first sampled voltage Vs 1 . The control switch S 1  is made up of a bipolar junction transistor having a base connected to an intermediate node between the first resistor and the second resistor, an emitter connected to the signal ground terminal, and a collector connected to one end of the third resistor R 3 . The other end of the third resistor R 3  is connected to an intermediate node between the isolation device Ph 1  and the first control unit  402 . The second control unit  404  is connected to the control terminal of the switch Q 1 . The first control unit  402  and the second control unit  404  are separated by the isolation device Ph 1 . In this embodiment, the isolation device Ph 1  is made up of a photo coupler. As the ground potential of the first control unit  402  is the second reference potential and the ground potential of the second control unit  404  is the first reference potential, the isolation device Ph 1  is essential as the ground potential of the first control unit  402  is different from the ground potential of the second control unit  404 . The control terminal of the control switch S 1  (base) receives the first sampled voltage Vs 1  from the voltage divider consisted of the first resistor R 1  and the second resistor R 2 , and is configured to turn on when the output voltage Vo is higher than a predetermined value, such as 300V. When the output voltage Vo is higher than the predetermined value so as to turn on the control switch S 1 , the collector voltage of the control switch S 1  is low, and the isolation device Ph 1  is turned on accordingly. Under this condition, the second control unit  404  will turn off and the switch Q 1  is turned off accordingly. The first control unit  402  is configured to sample the lamp voltage and the lamp current of the gas discharge lamp Lp 1 , and send a power limiting signal (not shown) to the switch Q 1  in response to the sampled lamp voltage and the sampled lamp current. Thus, the output power of the gas discharge lamp Lp 1  is fixed at a constant value as the gas discharge lamp Lp 1  enters the steady state. In this embodiment, the open-circuit voltage of the ballast can be controlled within a predetermined range. And when the DC/DC converter  104  is generating the open-circuit voltage, the DC/DC converter  104  may be controlled to operate in burst mode. Besides, the control switch S 1  may be made up of a triode, a MOSFET, or an IGBT. 
         [0021]      FIG. 5  shows a circuit block diagram of the ballast and the control device for open-circuit voltage according to a second exemplary embodiment of the invention. In this embodiment, the voltage Voc′ associated with the open-circuit voltage of the ballast is the output voltage Vo of the DC/DC converter  104  (i.e. the voltage on the capacitor C 2 ). Compared to  FIG. 4 , the control device for open-circuit voltage  300  of  FIG. 5  additionally includes a zener diode Z 1  having an anode connected to the first resistor R 1  and a cathode connected to the positive voltage output terminal of the DC/DC converter. The zener diode Z 1  is used to overcome the tolerance of the base-emitter voltage (Vbe) of the control switch S 1 . Another advantage of the zener diode Z 1  is that the zener diode Z 1  is turned off to decrease the loss of the resistors R 1  and R 2  to zero when the gas discharge lamp Lp 1  is operating normally. In this embodiment, the open-circuit voltage of the ballast may be limited within the predetermined range. And when the DC/DC converter  104  is generating the open-circuit voltage, the DC/DC converter  104  may be controlled to operate in burst mode. 
         [0022]      FIG. 6  shows a circuit block diagram of the ballast and the control device for open-circuit voltage according to a third exemplary embodiment of the invention. In this embodiment, the voltage Voc′ associated with the open-circuit voltage of the ballast is the voltage of the output voltage of the PFC  102  (i.e. the voltage on the capacitor C 1 ) minus the output voltage Vo of the DC/DC converter  104  (i.e. the voltage on the capacitor C 2 ). Compared to  FIG. 4 , the ground voltage of the control device for open-circuit voltage  300  is the second reference potential, and the open-circuit voltage of the electric ballast is detected indirectly. In  FIG. 6 , the third resistor R 3  is connected to an auxiliary voltage Vcc, and the control device for open-circuit voltage  300  further includes a diode D 2  connected between the collector of the control switch S 1  and the second control unit  404 . In this embodiment, the controller  302  may include the first control unit and the isolative phone coupler (not shown). Or otherwise, the control functions of the first control unit may be integrated into the second control unit. In this manner, the first control unit and the isolative photo coupler are unnecessary. The voltage divider consisted of the first resistor R 1  and the second resistor R 2  divides the voltage difference between the voltage on the capacitor C 1  and the voltage on the capacitor C 2  and thus generates a second sampled voltage Vs 2 . If the output voltage Vo (i.e. the voltage on the capacitor C 2 ) is higher than a predetermined value, for example, 300V, the second sampled voltage Vs 2  will not be sufficient to turn on the control switch S 1 . Therefore, the control switch Si is turned off and the diode D 2  is turned on. Under this condition, the second control unit  404  will alter the output signal Sc as a result of the conduction of the diode D 2 , thereby turning off the switch Q 1 . After the gas discharge lamp Lp 1  is operating normally, the output voltage Vo (i.e. the voltage on the capacitor C 2 ) will be lower than a predetermined value, for example, 300V. Under this condition, the second sampled voltage Vs 2  will be sufficient to turn on the control switch S 1 . Thus, the diode D 2  is turned off, and the second control unit  404  will not be controlled by the control device for open-circuit voltage  300  as a result of the OFF state of the diode D 2 , such that the second control unit  404  is able to generate the switching control signal Sc to control the switching operation of the switch Q 1 . It is noteworthy that the diode D 2  is optional. In this embodiment, the open-circuit voltage of the ballast may be limited within the predetermined range. And when the DC/DC converter  104  is generating the open-circuit voltage, the DC/DC converter  104  may be controlled to operate in burst mode. 
         [0023]      FIG. 7  shows a circuit block diagram of the ballast and the control device for open-circuit voltage according to a fourth exemplary embodiment of the invention. In this embodiment, the voltage Voc′ associated with the open-circuit voltage of the ballast is the voltage of the output voltage of the PFC  102  (i.e. the voltage on the capacitor C 1 ) minus the output voltage Vo of the DC/DC converter  104  (i.e. the voltage on the capacitor C 2 ). Compared to  FIG. 6 , the control device for open-circuit voltage  300  of  FIG. 7  additionally includes a zener diode Z 1  having an anode connected to the first resistor R 1  and a cathode connected to the negative voltage output terminal of the DC/DC converter. The zener diode Z 1  is used to overcome the tolerance of the base-emitter voltage (Vbe) of the control switch S 1 . In this embodiment, the open-circuit voltage of the ballast may be limited within the predetermined range. And when the DC/DC converter  104  is generating the open-circuit voltage, the DC/DC converter  104  may be controlled to operate in burst mode. 
         [0024]    Another aspect of the invention is directed to a method for controlling the open-circuit voltage of a ballast for gas discharge lamps. The method disclosed herein includes the following steps. First of all, sampling a voltage in association with the open-circuit voltage of the ballast and generating a sampled voltage accordingly. Next, determining if the sampled voltage is higher than a predetermined value and in response to the results of the determination generating a detecting voltage. Finally, controlling the duty ratio or pulse density of switching frequency of the switching control signal used to control the switching operation of the ballast according to the detecting voltage, thereby limiting the open-circuit voltage of the ballast within a predetermined range. 
         [0025]    In conclusion, the invention proposes a ballast for gas discharge lamps and a control device for the open-circuit voltage of the ballast. The inventive control device for the open-circuit voltage of the ballast includes a detecting circuit and a switching control device consisted of a control switch. The ballast includes a controller for outputting a switching control signal to control the switching operation of the DC/DC converter in the ballast. The detecting circuit is configured to sample a voltage associated with the open-circuit voltage of the ballast, and determining if the result of the sampling is higher than a predetermined voltage value, thereby generating a detecting signal. The switching control device will alter the switching control signal in response to the detecting signal, thereby limiting the open-circuit voltage of the ballast within a predetermined range. And when the DC/DC converter  104  is generating the open-circuit voltage, the DC/DC converter  104  may be controlled to operate in burst mode. 
         [0026]    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 be restricted 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. Therefore, the above description and illustration should not be taken as limiting the scope of the invention which is defined by the appended claims.