Abstract:
An electronic ballast for a high intensity discharge lamp combines the functions of a boost converter and a buck converter and combines the line voltage rectification with the load commutation. With such a configuration, only one controller is required, and the control signal for the controller is taken off a resistor in series with the bus capacitor. As such, the lamp power is indirectly controlled by regulating the input buck current to the buck converter portion, instead of by measuring the lamp current.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of The Invention 
     The subject invention relates to electronic ballasts for driving a high intensity discharge lamp, and more particularly, to controlling the output power in such electronic ballasts. 
     2. Description of The Related Art 
     It is typical to drive a high intensity discharge (HID) lamp with a low frequency square current waveform. An electronic ballast is used to generate the required drive waveform for the lamp and to provide power factor correction for the utility line. The present state of the art for electronic ballasts is the three-stage topology shown in FIG.  1 . This topology consists of a boost converter for power factor correction, a buck converter for regulating lamp power/current, and a commutator for converting the regulated DC voltage into an AC square waveform. In this topology, two controllers are required. One controller is used in the boost converter to regulate the bus voltage, Vbus and to shape the line voltage waveform to follow the input voltage sinusoidal waveform for power factor correction. The other controller is used in the buck converter for regulating the lamp current and power. This topology is well known to those skilled in the art of power electronics. It is simple and straightforward to sense its control variables, because the line voltage is rectified and the sensed parameters can be referenced to the same potential throughout all phases of circuit operation. For example, a simple current sense resistor, R 1  placed in series between ground and switch Q 2 , can be used to monitor the DC lamp current. However, the primary drawbacks for this topology are high parts count and low efficiency due to its multi-stage cascading nature. As shown in FIG. 1, the known three-stage converter ballast includes two controller switches, one for the boost converter and one for the buck converter, as well as four switches Q 1 -Q 4  for controlling the commutation of the high intensity discharge lamp, for a total of six switches. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a combined buck-boost function type electronic ballast in which the parts count is significantly reduced 
     It is a further object of the invention to provide a combined buck-boost function type electronic ballast in which the number of switches is reduced. 
     It is a still further object of the invention to provide a combined buck-boost function type electronic ballast which exhibits a high efficiency. 
     These and other objects are achieved in an electronic ballast for high intensity discharge lamps having combined boost and buck converters and a simple current sensing technique, wherein said electronic ballast comprises an AC voltage source having a first terminal and a second terminal; a series arrangement of a first inductance, a second inductance and a high intensity discharge lamp connected to said first terminal of said AC voltage source; a first capacitor arranged in parallel with said high intensity discharge lamp; a first series arrangement of a first high frequency switch having a body diode, and a first low frequency switch arranged in parallel with said second inductance and said high intensity discharge lamp; a second series arrangement of a second high frequency switch also having a body diode, and a. second low frequency switch arranged in parallel with said second inductance and said high intensity discharge lamp, a junction between said second high frequency switch and said second low frequency switch being connected to ground; a series arrangement of two diodes, a cathode of one of said two diodes being connected to a junction of said first high frequency switch and said first low frequency switch, and an anode of the other of said two diodes being connected to the junction of said second high frequency switch and said second low frequency switch, a junction between said two diodes being connected to a second terminal of said AC voltage source; and a series arrangement of a capacitor and a current sensing resistor arranged in parallel with said series arrangement of two diodes, wherein a current through said sensing resistor is used to detect an input buck current of said electronic ballast for indirectly controlling an output power generated by said electronic ballast. 
     The electronic ballast of the subject invention combines the buck and boost functions and combines the line voltage rectification as well as the load commutation. This circuit arrangement is suitable for driving HID lamps with power requirements less than 200 watts. When the boost converter is running in discontinuous conduction mode (DCM), the AC current waveform will naturally follow the AC voltage waveform without any feedback control. Based on this DCM boost property, the circuit can be rearranged into a form where both the boost converter and the buck converter can share the same active switch. Such an arrangement not only offers reduction in parts count over the conventional approach, but also allows for simplification in control. Only one controller is needed in the electronic ballast of the subject invention. The controller regulates the lamp power by modulating the duty cycle of the high frequency switches. However, if the conventional control philosophy is used in this circuit where lamp power is regulated directly by sensing and controlling the lamp current, then the implementation of sensing the lamp current can be complicated and expensive. 
     The concept behind the present invention is to control the lamp power indirectly by regulating the input power to the buck converter instead of the conventional lamp current. The buck input current can be easily sensed via a current sense resistor that is strategically placed in series with the bus capacitor. This method works because of two reasons: 
     (1) The voltage polarity across the bus capacitor is fixed throughout the whole circuit operation; and 
     (2) There is a unique current flowing pattern through the bus capacitor. 
     In general, it is not easy to separate the boost and the buck currents flowing in and out of the bus capacitor as in the three-stage topology of the conventional buck-boost converter power supply. However, when the buck and boost functions are combined, the boost current will only flow into the capacitor during the off time of the active switch (the second high frequency switch during the positive half line cycle, and the first high frequency switch during the negative half line cycle), and the buck current will only flow out of the capacitor during the on time of the active switch. Hence, the input current to the buck converter can be easily filtered out by simple circuitry. With this control method, the electronic ballast of the subject invention not only offers a reduction of parts count and an improvement in efficiency in its power stage, but also has a simple resistive current sensing method comparable to the three-stage approach, with the reduction of one controller. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     With the above and additional objects and advantages in mind as will hereinafter appear, the invention will be described with reference to the accompanying drawings, in which: 
     FIG. 1 is a schematic block diagram of a known three-stage converter electronic ballast; 
     FIG. 2 is a schematic block diagram of the buck-boost function type electronic ballast of the subject invention with an exemplary controller connected thereto; 
     FIGS. 3A and 3B show waveform diagrams of the bus capacitor current; and 
     FIG. 4 is a timing diagram showing the switching of the low and high frequency switches in relation to the AC input voltage. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a schematic block diagram of a known three-stage converter electronic ballast. An AC supply voltage  10  is supplied to an electro-magnetic interference (EMI) filter  12  and then is applied across a diode rectifier bridge D 1 -D 4 . The output from the diode rectifier bridge is applied to a boost converter  14  which includes the series arrangement of an inductor L 1  and a first controller switch S 1 . The output from the boost converter  14  is taken across the first controller switch S 1  and is shunted by a capacitor C 1  connected to ground. The voltage across the capacitor C 1  is applied to a buck converter  16  which consists of a series arrangement of a second controller switch S 2  and a diode D 5 , with an inductor L 2  connected to the junction between the second controller switch  52  and the diode D 5 . The inductor L 2  and the grounded end of the diode D 5  form the outputs from the buck converter  16  and are shunted by a capacitor C 2 . The inductor L 2  output from the buck converter  16  is applied directly to a first terminal of a commutator  18  while the grounded output from the buck converter  16  is applied through a resistor R 1  to a second terminal of the commutator  18 . 
     The commutator  18  includes, connected to the first and second terminals, the parallel arrangement of two serially-arranged commutator switches Q 1 /Q 2  and Q 3 /Q 4  in which a high intensity discharge lamp  20  is connected between the junctions of the serially-arranged commutator switches. 
     In this three-stage converter electronic ballast, the output current of the converter is measured across the resistor R 1  and the output voltage of the converter is measured across the capacitor C 2 . These sensed output current and voltage are used to control the first and second controllable switches S 1  and S 2 . 
     The buck-boost function type electronic ballast of the subject invention is schematically shown in FIG.  2 . In particular, as in the three-stage converter electronic ballast of the prior art, an input AC (line) voltage source  10  applies its output to an electromagnetic interference (EMI) filter  12 . One terminal (a) of the EMI filter  12  is applied to the series arrangement of a first inductor L 3 , a second inductor L 4  and an HID lamp  20 , the HID lamp  20  being shunted by a capacitor C 3 . A first series arrangement of a first high frequency switch HF 1 , along with its body diode BD 1 , and a first low frequency switch LF 1  are arranged in parallel with the second inductor L 4  and the HID lamp  20 . A second series arrangement of a second high frequency switch HF 2 , along with its body diode BD 2 , and a second low frequency switch LF 2  is also arranged in parallel with the second inductor L 4  and the HID lamp  20 , in which the junction between the second high frequency switch HF 2  and the second low frequency switch LF 2  is connected to ground. The series arrangement of two diodes D 6  and D 7  connect the junction of the first high frequency switch HF 1  and the first low frequency switch LF 1  to the junction of the second high frequency switch HF 2  and the second low frequency switch LF 2 . Finally, a series arrangement of a capacitor C 4  and a current sensing resistor Rs is arranged in parallel with the series arrangement of the two diodes D 6  and D 7 . 
     The first and second low frequency switches LF 1  and LF 2  are synchronized with the input AC voltage source  10 . When the AC voltage is positive, the first low frequency switch LF 1  is closed, while the second low frequency switch LF 2  is open. The second high frequency switch HF 2  operates as an active transistor in both the boost and buck functions. The body diode BD 1  of the first high frequency switch HF 1  acts as a boost diode as well as a buck diode. When the second high frequency switch HF 2  is closed, the boost function of the converter transfers energy from the line voltage source to the boost inductor L 3 . The current path of the boost inductor L 3  is confined between the line voltage source  10 , L 3 , the second high frequency switch HF 2  and the diode D 7 . The buck function of the converter delivers energy to the HID lamp  20  and to the buck inductor L 4  from the bus capacitor C 4  via the second high frequency switch HF 2  and the first low frequency switch LF 1 . When the second high frequency switch is open, the boost function transfers energy stored in the inductor L 3  to the bus capacitor C 4  via the body diode BD 1  of the first high frequency switch HF 1 . The buck function delivers the energy stored in the inductor L 4  during the off cycle of the second high frequency switch HF 2  to the HID lamp  20  via the first low frequency switch LF 1  and the body diode BD 1  of the first high frequency switch HF 1 . 
     When the input voltage is negative, the roles of the first and second high frequency switches HF 1  and HF 2  are reversed. The second low frequency switch LF 2  is closed and the first low frequency switch LF 1  is open. Similar to the positive case, when the first high frequency switch HF 1  is closed, bus capacitor C 4  delivers energy to the load and to the buck inductor L 4  through the buck function of the converter. When the second high frequency switch HF 2  is open, the boost inductor L 3  delivers energy to the bus capacitor C 4  through the boost function of the converter. 
     Hence, in both the positive and negative input voltage cases, the positive current of the bus capacitor C 4 , as shown in FIG. 2, is always contributed by the boost function, while the negative current of the bus capacitor C 4  is always contributed by the buck function. 
     The current waveform of the bus capacitor C 4  is shown in FIGS. 3A and 3B. In particular, FIG. 3A shows the overall current waveform over time, while FIG. 3B shows the portion BBC of FIG. 3A in detail. This shows the separation of the buck current and boost currents. This clear separation of the buck and boost currents flowing through the bus capacitor C 4  enables the use of a simple resistive current sensing technique to extract input buck current information for lamp power control. 
     FIG. 2 also shows an example of circuitry for controlling the buck-boost function type electronic ballast of the subject invention. In particular, the voltage across the resistor Rs is applied to a differential amplifier  22  with polarity inversion, which is connected to a half-wave rectifier  24 . The output from the half-wave rectifier  24 , which is now representative of only the buck current as shown in FIG. 3B, is applied to one input of a multiplier  26 . The other input of the multiplier  26  receives the output signal from a gain scaler  29 , whose input is connected to the junction between the first high frequency switch HF 1  and the first low frequency switch LF 1 . The output from the multiplier  26 , representing the buck converter input power, is applied to one input of an error amplifier  30  which compares this output signal to a reference voltage. The output from the error amplifier  30  is applied to a pulse-width modulator  32 , and the modulated signal is applied to a control input of a HF 1 /HF 2  select logic circuit  34 . The input line voltage from AC voltage source  10  is applied to a polarity detector  36 , the outputs therefrom forming the drive signals for the first and second low frequency switches LF 1  and LF 2 . These outputs are also applied to the HF 1 /HF 2  select logic circuit  34 . The outputs from this logic circuit  34  are applied as drive signals for the first and second high frequency switches HF 1  and HF 2 . 
     FIG. 4 shows timing diagrams for the switching of the first and second low frequency switches LF 1  and LF 2 , and the first and second high frequency switches HF 1  and HF 2 , in relation to the AC input voltage. 
     Numerous alterations and modifications of the structure herein disclosed will present themselves to those skilled in the art. However, it is to be understood that the above described embodiment is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.