Patent Publication Number: US-6218788-B1

Title: Floating IC driven dimming ballast

Description:
FIELD OF INVENTION 
     The present invention relates to a ballast, or power supply circuit, for gas discharge lamps of the type using regenerative gate-drive circuitry to control a pair of serially connected, complementary conduction-type switches of a d.c.-a.c. inverter. More particularly, the invention relates to directly driving switches using a single low-voltage integrated circuit configured with a floating ground design. 
     BACKGROUND OF THE INVENTION 
     Phase-controlled dimmable ballasts have gained a growing popularity in industry due to their capability for use with photo cells, motion detectors and standard wall dimmers. 
     Dimming of fluorescent lamps with class D converters is accomplished by either regulating the lamp current, or regulating the average current feeding the inverter. For cold cathode fluorescent lamps (CCFLs), the pulse width modulating (PWM) technique is commonly used to expand a dimming range. The technique pulses the CCFLs at full rated lamp current thereby modulating intensity by varying the percentage of time the lamp is operating at full-rated current. Such a system can operate with a closed loop or an open loop system. The technique is simple, low cost, and a fixed frequency operation, however, it is not easily adapted to hot cathode fluorescent lamps. For proper dimming of hot cathode lamps, the cathode heating needs to be increased, as light intensity is reduced. If inadequate heating exists, cathode sputtering increases as the lamp is dimmed. Also, the lamp arc crest factor should be less than 1.7 for most dimming ranges, in order to maintain the rated lamp life. The higher the crest factor, the shorter will be the life of the lamp. The PWM method does not address these problems, and therefore so far has been limited to CCFL applications. 
     Class D inverter topology with variable frequency dimming has been widely accepted by lighting industry for use as preheat, ignition and dimming of a lamp. The benefits of such a topology include, but is not limited to (i) ease of implementing programmable starting sequences which extend lamp life; (ii) simplification of lamp network design; (iii) low cost to increase lamp cathode heating as the lamp is dimmed; (iv) obtainable low lamp arc crest factor; (v) ease of regulating the lamp power by either regulating the lamp current or the average current feeding the inverter; and (vi) zero voltage switching can be maintained by operating the switching frequency above the resonant frequency of the inverter. 
     Conventional class D circuits which are used for d.c.—to—d.c. converters or electronic ballasts, implement a two-pole active switch via two, n-channel devices or n-p-channel complementary pairs. A gate is voltage controllable from a control-integrated circuit (IC), which is normally referenced to ground, thus, the control signals have to be level shifted to the source of the high-side power device, which, in class D applications, swings between two rails of the circuit. The techniques presently used to perform this function are by either, transformer coupling or a high-voltage integrated circuit (HVIC) with a boot-strapped, high side driver. Either solution imposes a severe cost and performance penalty. 
     For transformer coupling, the transformer needs to have at least three isolated windings wound on a single core, adding to cost and space considerations. The windings need to be properly isolated to prevent breakdown due to the presence of high potential. Also, the gate&#39;s drive circuit needs to be damped and clamped to prevent ringing between leakage inductors of the transformer and parasitic capacitors of switching MOSFETs. 
     In the case of high-voltage integrated circuits (HVIC), the HVIC has two isolated output buffers and logic circuitry which is sensitive to negative transients. The high-voltage process for the IC increases the size of the silicon die, and the boot-strap components add to the part count and costs. Such a system is also severely limited as to the switching frequency obtainable, which commonly is less than 100K Hz. Consequently, it uses the large sizes of EMI filters and resonant components and requires larger space for implementation. 
     In incandescent lamp dimming systems, dimming is controlled by a phase dimmer, also known as a triac dimmer. A common type of phase dimmer, blocks a portion of each positive or negative half cycle immediately after the zero crossing of the voltage. The clipped waveform carries both the power and dimming signal to the loads. The dimmer replaces a wall switch which is installed in series with a power line. 
     It would be desirable to use existing phase dimmer signals for dimming of compact fluorescent lamps (CFL). It would also be desirable to have such a system use a single-stage design for dimming and interfacing with a phase dimmer, provided at a low cost, with a direct gate drive for both high and low side MOSFET switches, with minimal voltage and current stresses on a resonant circuit. Still a further desirable aspect is to have a circuit which would allow programmable starting sequences to extend a lamp life, allow for low lamp arc crest factors and zero voltage switching over wide ranges. Such a system should also include compact size with low component counts and be easily adapted for different line input voltage and powers and provide for adequate protection for abnormal operations. The present invention provides the foregoing advantages, as well as others. 
     SUMMARY OF THE INVENTION 
     In an embodiment of the present invention, a dimmable ballast circuit is designed to receive a phase dimmer signal to control output of a fluorescent lamp, the dimming ballast includes an input section configured to receive the phase dimmer signal. The system includes a low cost integrated chip having an internal operational amplifier with a non-inverting input tied to a steady-state input within the integrated chip, a totem pole output. The IC is also configured in a floating ground arrangement. A coupling capacitor is connected at one end of the output of the controller IC. A switching network is designed with a pair of complementary connected switches, and is also connected to receive the output from the IC through a second end of the coupling capacitor. A current-sensing resistor is used to sense the switching current of a power switch in order to generate a feedback signal. A level shifter is designed to receive a signal from the input section, and to shift the received signal from a level of the reference ground to a level of the floating ground, the level shifted signal and the feedback signal are summed, and the summed signal is supplied to the inverting input of the integrated chip. In this manner, an error signal, the difference between two signals, is supplied to the inverting input, thereby adjusting output of the operational amplifier to regulate the output of the light level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a somewhat simplified depiction of a ballast incorporating the concepts of the present invention; 
     FIG. 2 is a block diagram of a IC of the type used in one embodiment of the present invention; 
     FIG. 3 depicts the level shifted signal and feedback signal which are summed; 
     FIG. 4 is a waveform illustrating the concept of the floating ground of the present invention; and 
     FIG. 5 is a more detailed schematic of one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a floating IC-driven ballast  10  of the present invention. An input voltage source  12  generates a bus voltage  14 , and a phase dimmer signal  16 . Input voltage source  12  has a circuit ground reference  18 . Bus voltage  14  is provided to a switching network  20 , and phase dimmer signal  16  is provided to a level shifting circuit  22  having a floating ground reference  24 . A controller integrated circuit (IC)  26 , such as a current mode pulse width modulated (PWM) controller IC, delivers a gate drive  28  to switches  30  and  32  through the coupling capacitor  34 . In the present embodiment switches  30 ,  32  may be configured as a complementary pair of MOSFETs, with switch  30  being a n-channel MOSFET and switch  32  being a p-channel MOSFET. Controller IC  26  is configured with a floating ground  36 , and is supplied with a compensation network  38  and a reference voltage  40 . The IC  26  is powered by a signal from a voltage source  42 . Phase dimmer signal  16  is therefore a chopped input voltage which is shifted from circuit ground to a floating signal ground. 
     Switching network  20  delivers signals to a load circuit  44  having a series resonant configuration including resonant inductor  46  in series with resonant capacitor  48 . Matching capacitor  50  is provided for low bus applications in order to maintain sufficient voltage as lamp  52  is dimmed, with the lamp cathodes heating being powered through windings  54  and  56 . Lamp  52  may, in one embodiment, be a compact fluorescent lamp. 
     Resistors  58  and  60 ,  138  and  142  work in conjunction with voltage source  42  in order to ensure proper start-up of controller IC  26 . The parallel combinations of diode  62 , resistor  64  and diode  66 , resistor  68  provides sufficient dead time to complementary switches  30  and  32 , respectively. Resistor  70  works in conjunction with capacitor  34  to convert the pulse DC output of the IC  26  to an AC square waveform through diode  62 , resistors  64 ,  68 , and diode  66  in order to drive the switches  30  and  32 . The network of capacitor  74  and resistors  76  and  78  function as a low pass filter to provide an average current feedback signal  79 , based on the output of current sense resistor  72 , so as to provide current feedback signal  79  to summing node  80 . 
     Switching network  20  has a common to ground  82 , and the center point between switch  30  and switch  32  is at a floating ground  84 . 
     Further in the preferred embodiment of the present invention, controller IC  26  is designed, as shown in FIG. 2, with an internal operational amplifier or error amplifier  86  having a non-inverting output tied to a bias voltage of 2.5 volts, although other bias voltages are possible. Therefore, only the inverting input of amplifier  86  is available for use. Additionally, output  6  of controller IC  26  has two transistors  90 ,  91  arranged in a totem pole configuration. Therefore, the totem pole, “class D”, output configuration of controller IC  26  drives a class D switching network  20 , whereby the present invention is designed as a cascaded driving configuration. 
     A controller IC as shown in FIG. 2 is well known in the art and therefore only selected portions of its structure and operation will be discussed. An IC of the type which may be used in this embodiment includes the UC3844A, as well as the UC1842A/3A/4A/5A and UC3842/314/5 families of controller ICs, sold by Unitrode Integrated Circuits of Merrimack, N.H. 
     Whereas the potential of circuit ground such as circuit grounds  18  and  82  are unchanging, the potential of a floating ground, such as  24 ,  36  and  84 , are constantly changing with reference to the circuit grounds. Thus, when switch  32  is turned on, floating ground  84  will be moved to circuit ground. However, when switch  30  is turned on, floating ground  84  will become substantially equivalent to the bus voltage value  14 . Further, since  24  is tied to  36  and floating ground  84 , controller IC  26  also varies between these levels. 
     Use of the floating ground configuration allows the use of a low voltage IC, such as a 35-volt IC instead of a more expensive high-voltage IC. Also, by implementing the low-voltage IC, a transformer coupling the gate drives is not necessary. Further, using the floating ground IC technique, it is possible to drive the ballast circuit into the megahertz range since power dissipation on the IC is extremely low compared to high-voltage techniques. 
     A challenge faced when implementing the present design of using a floating ground reference for controller IC  26 , is a manner of desirably delivering dimming signal  16  to controller IC  26 . This is a challenge since the floating ground value swings from ground reference to substantially the bus voltage input. In the present invention, dimming signal  16  is provided to controller IC  26  through level shifter circuit  22 , which is provided with a floating ground  24 , tied to the floating ground  36  of controller IC  26 . By this arrangement, a signal provided from the rectified input voltage source  12 , which is tied to circuit ground  18 , may be shifted through resistors  122 ,  126 ,  128 ,  130 , diode  124 , Zener diode  137  and switch  32 . 
     It is also a feature of the present invention to use controller IC  26 , which has its non-inverting input of operational amplifier  86  (FIG. 2) internally connected to a DC voltage bias. Therefore, only inverting pin  2  of operational amplifier  86  is available for use. The inventors are aware that another manner of controlling lamp output would be to have a lamp current feedback signal provided to a non-inverting input and a dimming signal provided to an inverting input of an operational amplifier. In this manner, the dimming signal could be controlled and controlling of the lamp lumen output would be possible. However, as previously noted, the low-voltage chip is a mass-produced, low-cost device the use of which has economic benefits. To require a building of a specific chip would increase the economic cost of a system of configuring a ballast to drive a compact fluorescent lamp. 
     Therefore, the present invention further uses a technique to provide the desired output under the constraints of controller IC  26  as described herein. In particular, current sensing resistor  72  is used to obtain actual lamp system power. Capacitor  74  and resistor  76  provides the average value of the switching current when the bus voltage is fixed. 
     Using an average value of the bus voltage times the average value of switching current, the system power can be controlled and therefore also, the lamp lumen output. It is noted that the average current of the system is that detected through transistor  72 , and obtaining the average current of the bus voltage may be achieved by various known techniques. By lowering system power, light output of lamp  52  will be lowered and by increasing system power light output of lamp  52  is increased. 
     Using the floating ground system configuration of the present embodiment means feedback signal  79  will be a negative signal. Negative feedback signal  79  is summed with level shifted signal  92  from level shifter  22  at summing node  80 . The output of summing node  80  is then provided to input pin  2  of controller IC  26 . Input pin  2  is the inverting input of operational amplifier  86 . By obtaining and summing the above-noted signals, the present embodiment controls the received signals such that the negative feedback signal  79  and positive level shifted signal  92  are of opposite polarities. Thus the system is controlled by the magnitude of signals which differ only by the error between the set point and the feedback signal. This operation adjusts the output of operational amplifier  86  and maintains the lumen output at a given dimming level. By providing the signals, in such a manner it is possible to use a single non-inverting input for control of the output from the controller IC  26 . 
     The present invention uses a complimentary pair of MOSFETs driven by controller IC  26  through a.c.-coupling capacitor  34  to operate lamp  52 . The driving scheme eliminates the need for a high-side driver or a pulse transformer and/or generating a negative bias gate or other driving scheme. 
     The present embodiment uses a sensed negative average value of the switching current from sense resistor  72  to generate feedback signal  79 , to allow the use of an internal operational amplifier  86  of controller IC  26 . 
     A further mentioned concept of the present invention is the use of level-shifting circuit  22  which shifts a chopped dimming signal  16 , from a ground reference level of voltage source  12  to a floating ground signal. The shifting of this dimming signal  16  allows the input signal from level shifter  22  to be used by controller IC  26 . 
     FIG. 3 illustrates the negative feedback signal  79  obtained from sense resistor  72 , and the positive level shifted signal  92  from level shifter  22 . As shown, these signals are generated in a manner that they are intended to substantially cancel each other upon being summed at summing point  80  of FIG.  1 . Thereby, the error between the signals is amplified and compensated. By this design, the output of operational amplifier  86  is adjusted so as to adjust the switching frequency to maintain a constant lumen level. Thus, whereas feedback signal  79  is negative going, positive going signal  92  has been level shifted for appropriate cancellation of the summed signals. The present embodiment may obtain the negative feedback signal  79  by use of a negative RMS switching current sensing procedure. 
     The operation of the floating ground configuration of the present embodiment is depicted in connection with FIG. 4, through quasi-square wave  93  which illustrates the potential between circuit ground to the source of switch  30 . It is noted that this quasi-square wave  93  shows the source potential is flowing between a ground reference to the d.c. bus voltage. When switch  32  is turning on, the potential goes down to the circuit ground. When, on the other hand, switch  32  is turning off, the waveform reaches its upper level Vbus. This waveform illustrates the concept of floating ground of controller IC  26 , since the controller IC floating ground  36  is tied to the floating ground  84 . 
     Turning attention to FIG. 5, depicted is a more detailed schematic of the floating IC-driven dimming ballast  10  according to a preferred embodiment. For components previously mentioned, like numerals have been used for identification. 
     With attention to input section  12 , phase dimmer output  16  is connected at inputs  100 ,  102 . The input section includes a fusing element  104  and resistive inductive components  106  and  108 , respectively. An RC network comprised of capacitor  110  and resistor element  112  are placed across the inputs of full-bridge rectifier  114 . Phase dimmer input signal  16  is rectified through full-bridge rectifier  114 , which may be a rectifier package, or four appropriately sized diodes. Capacitors  116  and  118  are placed on either side of blocking diode  120 . The rectified phase dimmer signal  16  is supplied to level shifter circuit  22  via resistor  122  and positive-going diode  124 . Use of positive-going diode  124  ensures that the signal supplied to level shifter circuit  22  is a positive signal. Level shifter circuit  22  is comprised of RC networks including resistors  126 - 132  in connection with capacitors  134  and  136  and switch  32 . Zener diode  137  is supplied to ensure appropriate voltage levels. 
     Turning attention to the voltage source  42  which supplies voltage to controller IC  26 , a network including parallel resistors  138  and  140 , resistor  142 , diodes  144 ,  146 , capacitor  148  and resistors  58  and  60 , generate the necessary voltage for starting of controller IC  26 . It is noted that once controller IC  26  is charged up to an operating voltage, controller IC  26  will consume more power than can be supplied by the described start-up circuit  42 . Therefore, further provided is a charge pump circuit consisting of capacitor  150 , and diodes  144  and  152 . Capacitor  154  is provided in connection with the VCC input, pin  7  of controller IC  26 . 
     Remaining circuitry provides compensation for error amplifier output pin  1 , the oscillation signal input pin  4 , and the reference voltage pin  8 , of controller IC  26 . In particular, a compensation network is connected to pin  1  of controller IC  26  via a resistor  160 , capacitor  162  parallel network. Capacitor  162  is connected to resistor  164  which in turn is connected to capacitor  166  tied to floating ground  168 . An input side of the resistor  160 , capacitor  162  parallel combination is also tied to resistor  170  which in turn is provided to a collector of NPN transistor  172 , paired with FET transistor  174 . It is noted a biasing resistor  176  is interconnected between the base of transistor  172  and a drain of transistor  174 . The emitter of NPN transistor  172  is tied to floating ground  168 . Voltage reference, pin  8  of controller IC  26  is supplied through a resistor  178 , which at one end is interconnected to resistor  176  and at another end to resistor  180 , input pin  4 , and capacitor&#39;s  181  and  182 . 
     Switching FET  183  has its drain connected to capacitor  182  and its source to floating ground  168 . The gate of transistor  183  has a gate to source resistor  184  and oppositely positioned diodes  186  and  188 . Interconnected between the oppositely positioned diodes  186  and  188  is capacitor  190 , which on one end is further connected to resistor  192 , while the opposite end of capacitor  190  is connected to floating ground  168 . The circuit further provides connection between diode  188  and the gate of FET  174 . 
     The above-described circuit provides a voltage-fed series resonant class D system with variable frequency, which is particularly applicable for use in compact fluorescent lamps. This topology allows easily operating in zero-voltage switching (ZVS) resonant mode, reduces the MOSFET switching losses and electrical magnetic interference. Further, by varying the switching frequency, it is possible to modulate the average current in the switching MOSFETs and therefore the output power. 
     The complementary pair of MOSFETs  30 ,  32  of the present embodiment are driven by a low-cost, single totem pole, class D, buffer output, such as a UC3844A or equivalent controller IC  26 , through a.c. coupling capacitor  34 . The cascade class D driving scheme eliminates the need for a high-voltage integrated chip (HVIC) or a pulse transformer and/or generating a negative gate bias. The technique is capable of providing switching frequency up to the megahertz range. 
     It was noted that in this application resonant feedback techniques used as an interface circuit to the phase dimmer are not included. For phase dimming, if an input EMI filter is not properly damped, it would resonate and cause misfiring of the triac dimmer, causing the lamp to flicker. 
     Exemplary component values and/or designations for the circuit of FIGS. 1 and 5 are as follows for a compact fluorescent lamp rated at 28 watts with a d.c. bus voltage of at least 120 volts: 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Inductor 46 
                 520 
                 micro-henries 
               
               
                   
                 Capacitor 48 
                 .0033 
                 farads 
               
               
                   
                 Capacitor 50 
                 .0047 
                 micro-farads 
               
               
                   
                 Resistors 58, 60 
                 200K 
                 ohms 
               
               
                   
                 Resistors 64, 68 
                 1K 
                 ohm 
               
               
                   
                 Resistor 70 
                 10K 
                 ohms 
               
               
                   
                 Resistor 72 
                 5.1 
                 ohms 
               
               
                   
                 Capacitor 74 
                 .01 
                 micro-farads 
               
               
                   
                 Resistor 76 
                 10K 
                 ohms 
               
               
                   
                 Resistor 78 
                 500K 
                 ohms 
               
               
                   
                 Fuse 104 
                 120 
                 volts 
               
               
                   
                 Resistor 106 
                 5.1 
                 ohms 
               
               
                   
                 Inductor 108 
                 2.5 
                 micro-henries 
               
               
                   
                 Capacitor 110 
                 .1 
                 micro-farads 
               
               
                   
                 Resistor 112 
                 330 
                 ohms 
               
               
                   
                 Capacitor 116 
                 .047 
                 micro-farads 
               
               
                   
                 Capacitor 118 
                 47 
                 micro-henries 
               
               
                   
                 Resistor 122 
                 500K 
                 ohms 
               
               
                   
                 Resistor 126 
                 5.5K 
                 ohms 
               
               
                   
                 Resistors 128, 130 
                 10K 
                 ohms 
               
               
                   
                 Resistor 132 
                 500K 
                 ohms 
               
               
                   
                 Capacitor 134 
                 .47 
                 micro-farads 
               
               
                   
                 Capacitor 136 
                 .01 
                 micro-farads 
               
               
                   
                 Resistors 138, 140 
                 200K 
                 ohms 
               
               
                   
                 Resistor 142 
                 100 
                 ohms 
               
               
                   
                 Capacitor 148 
                 22 
                 micro-farads 
               
               
                   
                 Capacitor 150 
                 1 
                 nano-farad 
               
               
                   
                 Capacitor 154 
                 .1 
                 micro-farads 
               
               
                   
                 Resistors 160, 164 
                 240K 
                 ohms 
               
               
                   
                 Capacitor 162 
                 10 
                 nano-farads 
               
               
                   
                 Capacitor 166 
                 .1 
                 micro-farads 
               
               
                   
                 Resistor 170 
                 10K 
                 ohms 
               
               
                   
                 Resistors 176, 178 
                 20K 
                 ohms 
               
               
                   
                 Resistor 180 
                 40K 
                 ohms 
               
               
                   
                 Resistor 184 
                 20K 
                 ohms 
               
               
                   
                 Capacitors 181 and 182 
                 1 
                 nano-farad 
               
               
                   
                 Capacitor 190 
                 4.7 
                 micro-farads 
               
               
                   
                 Resistor 192 
                 500K 
                 ohms 
               
               
                   
                   
               
            
           
         
       
     
     In addition, MOSFET  30  is sold under the designation IRF240, MOSFET  32  under designation IRF9240, transistor  172  under designation N2222, and MOSFETS  174  and  183  under designation IRL5020. Diodes  62 ,  66 ,  144  and  152  are sold under designation 1N4148, diodes  120  and  124  under designation 1N4005, Zener diodes  137 ,  186 ,  188  under designation 1N4702 and Zener diode  146  under designation 1N4617, which are well known in the industry. 
     While the invention has been described with respect to specific embodiments by way of illustration, many modifications and changes will occur to those skilled in the art. It is therefore, to be understood that the appended claims are intended to cover all such modifications and changes which fall within the true spirit and scope of the invention.