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AN1543/D Electronic Lamp Ballast Design Prepared by: Michaël Bairanzade http://onsemi.com APPLICATION NOTE ABSTRACT Obviously, a high end electronic lamp ballast will With a continuous growth rate of 20% per year, electronic certainly include other features like dimming capability, lamp ballasts are widely spread over the world. Even though lamp wear out monitoring, and remote control, but these are the light out of a fluorescent tube has a discontinuous optional and will be analyzed separately. spectrum, the higher efficiency brought by the electronic control of these lamps make them the best choice to save the Fluorescent Lamp Operation energy absorbed by the lighting systems. When the lamp is off, no current flows and the apparent A few years ago, the lack of reliable and efficient power impedance is nearly infinite. When the voltage across the transistors made the design of such circuits difficult! Today, electrodes reaches the Vtrig value, the gas mixture is highly thanks to the technology improvements carried out by ON ionized and an arc is generated across the two terminals of Semiconductor, design engineers can handle all of the the lamp. This behavior is depicted by the typical operating problems linked with the power semiconductors without curve shown in Figure 1. sacrificing the global efficiency of their circuits. This Application Note reviews basic electronic lamp I ballast concepts and gives the design rules to build industrial circuits. SUMMARY Inom 1. MAIN PURPOSE − Fluorescent tube basic operation − Standard electromagnetic ballast V − Electronic circuits 2. HALF BRIDGE CIRCUIT DESIGN Von 3. DIMMABLE CIRCUIT 4. NEW POWER SEMICONDUCTORS Vstrike 5. CONCLUSIONS 6. APPENDIX ELECTRONIC LAMP BALLAST Main Purpose To generate the light out of a low pressure fluorescent lamp, the electronic circuit must perform four main Figure 1. Typical Low Pressure Fluorescent Tube I/V Characteristic functions: a. Provide a start−up voltage across the end electrodes of the lamp. The value of Vstrike is a function of several parameters: b. Maintain a constant current when the lamp is operating − gas filling mixture in the steady state. − gas pressure and temperature c. Assure that the circuit will remain stable, even under − tube length fault conditions. − tube diameter d. Comply with the applicable domestic and international − kind of electrodes: cold or hot regulations (PFC, THD, RFI, and safety). © Semiconductor Components Industries, LLC, 2009 1 Publication Order Number: January, 2009 − Rev. 1 AN1543/D AN1543/D Typical values of Vstrike range from 500 V to 1200 V. The most commonly used network is built around a large Once the tube is on, the voltage across it drops to the inductor, connected in series with the lamp, and associated on−state voltage (Von), its magnitude being dependent upon with a bi−metallic switch generally named “the starter”. the characteristics of the tube. Typical Von ranges from 40 V Figure 3 gives the typical electrical schematic diagram for to 110 V. the standard, line operated, fluorescent tube control. The value of Von will vary during the operation of the lamp but, in order to simplify the analysis, we will assume, in a L first approximation, that the on−state voltage is constant when the tube is running in steady state. Consequently, the equivalent steady state circuit can be S described by two back to back zener diodes as shown in TUBE T Figure 2, the start−up network being far more complex, C BI-METALLIC TRIGGER particularly during the gas ionization. This is a consequence MAINS 220 V−50 Hz of the negative impedance exhibited by the lamp when the voltage across its electrodes collapses from Vstrike to Von. Figure 3. Standard Ballast Circuit for Fluorescent Tube BALLAST The operation of a fluorescent tube requires several components around the tube, as shown in Figure 3. The gas mixture enclosed in the tube is ionized by means of a high voltage pulse applied between the two electrodes. Vac Von To make this start−up easy, the electrodes are actually made of filaments which are heated during the tube ionization start−up (i.e,. increasing the electron emission), their deconnection being automatic when the tube goes into the steady state mode. At this time, the tube impedance decreases toward its minimum value (depending upon the Figure 2. Typical Fluorescent Tube Equivalent tube internal characteristics), the current in the circuit being Circuit in Steady State limited by the inductance L in series with the power line. The starting element, commonly named “starter”, is an Up to now, there is no model available to describe the start essential part to ignite the fluorescent tube. It is made of a up sequence of these lamps. However, since most of the bi−metallic contact, enclosed in a glass envelope filled with phenomena are dependent upon the steady state a neon based gas mixture, and is normally in the OPEN state. characteristics of the lamp, one can simplify the analysis by When the line voltage is applied to the circuit, the assuming that the passive networks control the electrical fluorescent tube exhibits a high impedance, allowing the behavior of the circuit. voltage across the “starter” to be high enough to ionize the Obviously, this assumption is wrong during the time neon mixture. The bi−metallic contact gets hot, turning ON elapsed from Vstrike to Von, but since this time interval is the contacts which, in turn, will immediately de−ionize the very short, the results given by the proposed simple model “starter”. Therefore, the current can flow in the circuit, are accurate enough to design the converter. heating up the two filaments. When the bi−metallic contact When a fluorescent tube is aging, its electrical cools down, the electrical circuit is rapidly opened, giving a characteristics degrade from the original values, yielding current variation in the inductance L which, in turn, less light for the same input power, and different Vstrike and generates an overvoltage according to Lenz’s law. Von voltages. Since there is no synchronization with the line frequency A simple, low cost electronic lamp ballast cannot (the switch operates on a random basis), the circuit opens at optimize the overall efficiency along the lifetime of the tube, a current level anywhere between maximum and zero. but the circuit must be designed to guarantee the operation If the voltage pulse is too low, the tube doesn’t turn ON of the lamp even under the worst case “end of life” and the start−up sequence is automatically repeated until the conditions. fluorescent tube ionizes. At that time, the tube impedance As a consequence, the converter will be slightly oversized falls to its minimum value, yielding a low voltage drop to make sure that, after 8000 hours of operation, the system across its end electrodes and, hence, across the switch. Since will still drive the fluorescent tube. the starter can no longer be ionized, the electrical network of the filaments remains open until the next turn−on of the Controlling the Fluorescent Lamp circuit. As already stated, both the voltage and the current must be We must point out that the fluorescent tube turns off when accurately controlled to make sure that a given fluorescent the current is zero: this is the source of the 50 Hz flickering lamp operates within its specifications.