Abstract:
The invention pertains to a switchmode power inverter, and particularly to a frequency controlled half-bridge inverter. More particularly, the invention pertains to high frequency ballast for gas discharge devices, especially for high intensity discharge lamps, completed by an internal high voltage ignition circuit and an external control apparatus for dimming.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to high frequency switchmode power inverters and specifically to high frequency electronic ballasts for gas discharge devices. More specifically, the present invention relates to a high frequency electronic ballast for high intensity discharge (HID) lamps. 
     2. Prior Art 
     The prior art is replete with many known circuits providing high frequency ballast for gas discharge lamps, especially for high intensity (HID) lamps. For instance, high efficient electronic ballasts which can be used with HID (HPS) lamps are U.S. Pat. No. 5,097,183, entitled “Master-Slave Half-Bridge DC-to-AC Switchmode Power Inverter” and U.S. Pat. No. 5,229,927, entitled “Self-Symmetrizing and Self-Oscillating Half-Bridge Power Inverter”, assigned to the assignee of the present invention. 
     An important application of switchmode power inverters is supplying gas discharge devices, especially high intensity discharge (HID) lamps in the range of 35W to 400W. In this case, the load impedance of the inverter is a HID lamp connected in series with an inductor. In the case of a high frequency powering of the HID lamp, the interaction between the high frequency ballast and the lamp is stronger than that of a conventional low frequency (50/60 Hz) ballast. The high frequency ballast can be significantly better than a conventional low frequency (50/60 Hz) ballast due to its lessened weight and essentially high efficiency. Additionally, the high frequency ballast, utilized with an HID lamp would have a simple, reliable dimming capability and an essentially more reliable light switch (day and night switch) than the conventional low frequency ballasts. 
     Therefore, the critical design targets for high frequency ballasts supplying HID lamps would be the following: 
     a) very high efficiency, &gt;94% (energy saving and low temperature rise of the critical components); 
     b) fully controlled lamp power eliminating the effects of input voltage fluctuation and the lamp aging; 
     c) dimming capability, from 100% to 50% (35%); 
     d) automatic light switching (day and night time switch); 
     e) over temperature protection; 
     f) automatic switch off in the case of lamp failure; 
     g) instant re-ignition of the warmed up lamp; and 
     h) isolated interface circuit providing the connection of several ballasts and therefore implementing fully controlled lighting system. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a high frequency switchmode half-bridge square wave inverter which has a substantially increased efficiency in a wide frequency and temperature range. 
     A second object of the present invention to provide an electronic ballast for gas discharge devices having substantially increased efficiency, stability and reliability. 
     A further object of the present invention to provide an electronic ballast for high intensity discharge lamps implementing the ideal ballast curve. 
     Another object of the present invention to provide a high frequency and high voltage ignition circuit for a reliable ignition of HID lamps, especially the instant re-ignition of warmed-up lamps, in a very wide temperature range. 
     A further object of the present invention to provide an electronic ballast for high intensity discharge lamps with built-in dimming capability. 
     Another object of the present invention to provide an electronic ballast for high intensity discharge lamps with built-in light switch controlled externally by a simple photoresistor. 
     These and other objects, features and advantages of the present invention will be more readily apparent from the following detailed description, wherein reference is made to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a schematic diagram of the preferred electronic ballast for HID lamps including a Power Unit with Ignitor, Control Unit, Logic Supply Unit and an Interface/Timer(I/T) Unit; 
     FIG. 2 shows the ballast curve and the corresponding lamp current realized by the preferred electronic ballast; 
     FIG. 3 illustrates the lamp current waveforms used for various for HPS and MH lamps; 
     FIG. 4 shows the preferred embodiment of the control unit used as a driver and frequency controller; 
     FIG. 5 illustrates the preferred embodiment of the logic supply unit providing stabilized supply voltages for the other units; 
     FIG. 6 shows the preferred embodiment of the I/T Unit implementing a programmable timer and external control for dimming and light switching; 
     FIG. 7 illustrates the timing diagram of the optional automatic dimming provided by the preferred programmable timer; and 
     FIG. 8 shows the implementation of a controlled lighting system provided by the preferred electronic ballast. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a schematic diagram of the preferred electronic ballast where the Power Unit (connected to an input unit) is illustrated in detail. The Input Unit includes filters, a bridge rectifier and optionally a power factor pre-regulator known to practitioners in the art and need not be described herein. Furthermore, the Control Unit, Logic Supply Unit and Interface &amp; Timer Unit (I/T Unit) which are shown as blocks and will be described hereinbelow. 
     The Power Unit includes the basic components of a halfbridge inverter: two electronically-controlled switches (MOSFET T 1  and T 2 ), two voltage divider capacitors (capacitor C 1  and C 2 ) and a load impedance (a HID lamp) connected in series with a winding N 1  of an inductor M 1 . 
     The Power Unit also includes a preferred embodiment of a high voltage ignition apparatus in which winding N 2  is connected in series with a capacitor C 3  and a transistor T 3 . When transistor T 3  is on, a high frequency damped sinusoidal voltage, repeated with the half period time of the inverter, occurs across the winding N 2 . This voltage is transformed by winding N 1  to approximately 1500V, providing sufficient ignition voltage for HID lamps, also achieving instant re-ignition of warmed-up lamps. The ON-time of transistor T 3  is controlled by the Control Unit connected to driver transistors T 8  and T 9  through opto-isolator O- 3 . 
     The Power Unit further includes a preferred embodiment of a MOSFET driver utilized by the present invention. As it is shown in FIG. 1, two identical MOSFET drivers MD 1  and MD 2  are provided. MOSFET driver MD 1  includes a transistor T 5 , as well as rectifiers D 2  and D 3  and resistors R 4  and R 5  connected to the main transistor switch T 2 . Inputs B 1  and B 2  are connected to one of the secondary windings N 5  of a control transformer M 2 . MOSFET driver MD 2  includes a transistor T 4 , as well as rectifiers D 4  and D 5  and resistor R 6  and R 7  connected to the main transistor switch T 4 . Inputs Al and A 2  are connected to one of the secondary windings N 4  of the control transformer M 2 . 
     The control transformer M 2  provides a square wave AC control signal. During the positive half-period, with respect to the point sign of the secondary winding N 4 , a positive voltage is connected across the resistor R 6  and rectifier D 4  to the gate of the N-channel MOSFET T 4  of MOSFET driver MD 2  providing the ON state. Therefore, the gate of MOSFET T 1  is short-circuited to its source by MOSFET T 4 , providing an excellent current sink capability and a very short switching time for the MOSFET T 1 . Obviously, the DC loss of the described MOSFET driver is very low. The most significant advantage of this driver description can be applied for the upper MOSFET drivers. A similar configuration will be described with respect to the MOSFET driver MD 1 . In this situation, during the positive half-period, with respect to the point sign of the secondary winding N 5 , a positive voltage is connected across the resistor R 4  and rectifier D 2  to the gate of the N-channel MOSFET T 2  providing the ON state. During the negative half-period, a positive voltage is connected across resistor R 5  and rectifier D 3  to the gate of MOSFET T 5  providing the ON state. Therefore, the gate of MOSFET T 2  is short-circuited to its source by MOSFET T 5 . Consequently, a very low power loss can be achieved with respect to the switching transistors T 1  and T 2 , resulting a very high efficiency for the global circuit. 
     The Power Unit also includes a power resistor R 1  in which the current is unidirectional but fluctuated with 120 Hz. The voltage across the resistor is filtered by capacitor C 6  and resistor R 2 , and therefore, the voltage across C 6  is nearly DC and proportional to the average load current. This voltage—supposing nearly constant supply voltage—is also proportional to the input power which is nearly equal to the lamp power. Therefore, the control of the lamp power can be easily implemented, as shall be further explained. 
     FIG. 2 illustrates the ballast curve as diagram of the functional relationship between the lamp power and the lamp voltage. Two different ranges can be distinguished depending on the lamp voltages, namely: 
     1. constant lamp current range in the warming up period; and 
     2. Constant lamp power range in a certain range of lamp voltage. 
     The constant lamp power range depends on the lamp type: 80V-160V for HPS lamps and 120V-150V for MH lamps. 
     FIG. 3 shows a normalized lamp current diagram where the wave forms are parametrized by typical lamp voltages, namely: old HPS (160V), MH (130V), new HPS (80V) and cold start (20V) which is practically equivalent to the short circuit condition. As it can be seen in FIG. 3, the lamp current is always continuous and piecewise exponential. 
     FIG. 4 shows a detailed schematic diagram of the Control Unit providing appropriate control signals for the Power Unit. Functionally, the Control Unit has four basic parts, namely: 
     1. a timer and oscillator; 
     2. a voltage controlled oscillator (VCO); 
     3. a logic driver; and 
     4. a low power signal transformer. 
     The timer and oscillator includes voltage comparators IC 1  and IC 3 , a 14 bit ripple counter IC 2 , and an oscillator based on the Schmitt trigger IC 5 . The output  5  provides the control signal for the ignitor. The counter is controlled via an output  3  in such a way that if ignition of the lamp failed, the power unit will be switched off within six minutes. The voltage controlled oscillator IC 6  provides square wave signal (D=0.5) for the driver. The frequency of the oscillator is controlled by the operational amplifier IC 4  in such a way that the lamp power remains constant in a predetermined lamp voltage range (80V-160V for HPS lamps and 120V-150V for MH lamps). The logic driver including logic gates IC 7  and IC 8  and provides appropriate signals for the MOSFETs T 6  and T 7  shown in FIG. 1, avoiding cross current conduction. The low power signal transformer M 2  also shown in FIG. 1, provides isolation between the Control Unit and the Power Unit. 
     FIG. 5 shows the preferred embodiment of a low power Logic Supply Unit based on a self-oscillating half-bridge configuration. The inputs  1  and  2  provide supply voltage for the logic supply. The transformer M 3  includes five windings: a primary winding N 2  connected between the common points of the main switching transistors and voltage divider capacitors of the half-bridge configuration, feedback windings N 1  and N 3 , secondary windings N 5  and N 6  providing unstabilized voltages for the linear regulators IC 9  and IC 10 , transistors T 8  and T 9  and diac V 1  provide a starter circuit for transistor T 10  and T 11 , which are the main controlled switch of the self-oscillating half-bridge configuration. The outputs  3  and  4  provide 12V stabilized logic supply for the I/T Unit. The output  5  and  6  provide 12V stabilized logic supply for the Control Unit. 
     FIG. 6 shows the preferred embodiment on the Interface &amp; Timer Unit (I/T) Unit) providing an isolated external control of the ballast. The Interface part includes the comparators IC 11  and IC 12  where an isolated (4000V) control connection to the Power Unit and the Control Unit is implemented by opto-isolators O- 1  and O- 2 . As it is shown in FIG. 1, a low power switch DS, functioning as dimming switch, can be connected to input  1  and  3  of the I/T Unit. Furthermore, for implementing automatic light switch (day/night switch) a photoconductive cell (photoresistor) PC can be connected to the inputs  2  and  3  of the I/T Unit. These inputs connected to a low power switch can be also utilized as a remote ON/OFF switch of the ballast. 
     An optional programmable Timer provides a timed dimming capability for the ballast essentially increasing energy saving. The corresponding timing diagram is shown in FIG.  7 . 
     FIG. 8 illustrates a schematic diagram for preferred implementation of a centrally-controlled lighting system provided by the isolated external control feature of a preferred individual electronic ballast. 
     Thus, while preferred embodiment of the present invention have been shown and described in details, it is to be understood that such adaptation and modifications, as may occur to those skilled in the art, may be employed departing from the spirit and scoping of the invention, as set forth in the claims.