designfeature Tom Ribarich Director, Lighting Systems, International Rectifier How To Control and Reap the Benefits of HID Lamps Electronic HID (high intensity discharge lamp) ballasts: how they work, how to control them, and develop an electronic ballast solution for outdoor lighting applications. Bulb Envelope Mercury and Metal Halide Atoms Arc Electrode Arc Tube H brightness metal halide lamps typically have an efficacy of greater than 100 lumens/watt and have a lifetime of 20,000 hours. HID lamps produce light using a technique similar to that used in fluorescent lamps where mercury atoms are excited by an electrical current resulting in the production of ultraviolet (UV) light. The UV light is then converted into visible light through a phosphor coating on the inside of the tube. In the case of HID lamps, they operate at a high-temperature and high-pressure, the arc length is very short, and visible light is produced directly without the need for a phosphor. Metal halide lamps (Figure 1) comprise an arc tube surrounded by an outer bulb envelope. The arc tube is made of quartz or ceramic glass, has tungsten electrodes located at each end, and contains a mixture of argon, mercury and metal halide salts. Metal halide is added to the lamp to increase lumens and improve the light color. The lamp is started by applying a high voltage pulse across the tube to ionize the argon gas. Once the gas is fully ionized, a sustained arc extends from one electrode to another and current (supplied by a ballast) flows across the tube. As the pressure and temperature inside the tube increase, the materials within the arc tube vaporize and light is emitted in the form of visible light and UV radiation. The outer bulb envelope provides a stable thermal environment for the arc tube, prevents oxidation of the arc tube, and reduces the amount of UV radiation emitted by the lamp. HID Lamp Characteristics Fig. 1. HID lamps comrpise an arc tube surrounded by an outer bulb envelope. V, I 07_Ribarich_F1 Warm-up Running VLAMP PLAMP ILAMP Ignition Current Limitation Constant Power 36 Power Electronics Technology | July 2010 07_Ribarich_F2 HID lamps require a high voltage for ignition (3 to 4KV typical, >20KV if the lamp is hot), current limitation during warm-up, and a constant power during running. It is important to have a tight regulation of lamp power to minimize lamp-to-lamp color and brightness variations. Also, HID lamps are driven with a low-frequency AC voltage (<200Hz typical) to avoid mercury migration and to prevent damage of the lamp due to acoustic resonance. A typical metal halide 250W HID lamp has the following characteristics: • Nominal Wattage (W): 250 t Fig. 2. Typical start-up profile for HID lamp ignition, warm-up and running modes. www.powerelectronics.com cONTROLLINGHIDLamps Full-Bridge Stage Buck Stage Fig. 3 HID lamp ballast circuit employs a buck stage to control the lamp current, followed by a full-bridge output stage for ac lamp operation. MBUCK DC Bus (+) LBUCK Gate Drive TIGN Lamp • Nominal Voltage (Vrms): 100 Gate Gate Drive Drive • Nominal Current (Arms): 2.5 DIGN DC Bus (–) RIGN • Warm-up Time (min): 2.0 Current Zero Crossing • Ignition Voltage (Vpk): 4000 MIGN VLAMP CIGN Detection Figure 2 shows the typical start-up Lamp Power Off-Time Feedback On-Time profile for HID lamps. Before igniControl Control tion, the lamp is open circuit. After Full-Bridge Ignition Control Control the lamp ignites, the lamp voltage ILAMP drops quickly from the open-circuit voltage to a very low value (20V typical) due to the low resistance of the lamp. This causes and power increase. Eventually, the lamp voltage reaches the lamp current to increase to a very high value and its nominal value (100V typical) and the power is regushould therefore be limited to a safe maximum level. As lated to the correct level. the lamp warms up, the current decreases as the voltage To satisfy the lamp requirements and different oper- 07_Ribarich_F3 Buck Circuit VBUS+ 400 V dc MBUCK IRF P 22 N60 K Heatsink RBCS 0.16 R/5W/1% RBUCK1 RBB 2 68k 0.25 W RBB 3 68k 0.25 W DBUCK1 1N4148 RBB 3 68k 0.25 W DVBB 1 600 V 1A + 14 V dc – CBUS 82 µF 450 V LBUCK 0.23 mH/10A pk/E F 42 DBUCK ST T H 12 R 06 Heatsink RCCS 1 1k CCS 1 470 pF DVBB 2 15 V/0.5 W CBUCK 3.3 µF/400 V CS 1 VB1 28 BUCK 2 HO1 27 VSB 3 VS1 26 CVBB 2.2 µF VBB 4 RVBB 1 10 R VCC LO1 25 5 LO2 24 COM 6 VB2 23 ZX 7 HO2 IRS2573D 22 TOFF 8 VS2 21 CICOMP 3.3 nF ICOMP 9 IGN 20 CPCOMP 3.3 nF PCOMP 10 OV 19 CVCC 1 0.1 µF CVCC2 10 µF Full-Bridge Circuit RZX CRZX 33 k 10 pF CT OFF 3.3 nF RIREF 20k IREF 11 17 TIGN 13 16 CTCLK 0.18 µF TCLK 14 15 CTIGN 1 µF RLO1 22R MHS2 IRGB 15 B 60 K D Heatsink LIGN 1 mH/EF 25 VOGT IC 0806203101 250 W Lamp MLS1 IRGB 15 B 60 K D Heatsink MLS2 IRGB 15 B 60 K D Heatsink RHO2 22R RVS 2 180k 1% RLO2 22R CVB2 2.2 µF 50 V DIGN K 1V 26 COV 0.1 µF ROV 120k ISENSE VSENSE N/C COUT 1 nF/630 V MHS1 IRGB 15 B 60 K D Heatsink RHO1 22R OC ROC 10k 18 CT 12 CT 0.068 µF CVB1 2.2µF 50 V RVS 1 180k 1% COC RISENSE 0.1 µF 1k CISENSE 0.1 µF CVS 10 nF RIGN 3 22R RIGN 1 22R RIGN 2 18 K/3W M1 IRLL 3303 RCS 0.12R 5W 1% DVS 1 36 V 0.5 W MIGN IRGB 10 B 60 K D Heatsink RVS 3 100k 1% CIGN 0.1 µF 630 V RVS 4 7.5k 1% VBUS– Programming Inputs Voltage/Current Sensing Ignition Circuit Fig. 4 250W HID ballast circuit uses a complete buck and full-bridge control circuit designed around the IRS2573D HID Control IC. 07_Ribarich_F4 www.powerelectronics.com July 2010 | Power Electronics Technology 37 cONTROLLINGHIDLamps (A) (B) Figure 5 HID ballast waveforms A) During lamp warm-up: buck switching node voltage (upper trace); buck current (lower trace). B) During normal lamp running: buck switching node voltage (upper trace; buck current (lower trace). C) During normal lamp running: half-bridge output voltage (upper and middle traces); ac lamp current (lower trace). ating modes, an electronic ballast circuit is needed that ignites the lamp, controls lamp power, and produces an AC lamp voltage. HID Ballasts The standard circuit for controlling HID lamps includes (Figure 3) a buck stage for controlling the lamp current, followed by a full-bridge output stage for ac operation of the lamp. The output stage also includes an ignition circuit for striking the lamp. A control circuit or IC is necessary to control the buck and full-bridge stages and properly managing the different lamp modes. The buck circuit is fed by a constant dc bus voltage (400Vdc, typical) that can be supplied by a commonly used active power factor correction circuit. The buck stage is the main control circuit of the ballast and is used to control the lamp current and power. The buck stage steps the dc bus voltage down to the lower lamp voltage at the full-bridge stage. The lamp voltage and current are measured and multiplied together to produce a lamp power measurement. The lamp power measurement is then fed back to control the on-time of th buck switch (MBUCK). The buck off-time each switching e cycle is determined by the zero-crossing of the buck inductor current and is sensed using a secondary winding from the buck inductor (LBUCK). The full-bridge stage is necessary to produce an ac lamp current and voltage during running. The full-bridge 38 Power Electronics Technology | July 2010 (C) typically operates at 200Hz with a 50% duty-cycle. The full-bridge also contains a pulse transformer circuit for producing 4KV pulses across the lamp necessary for ignition. The ignition circuit includes a diac circuit to produce the required ignition pulses. The ignition circuit is activated by turning on MIGN, causing the lower leg of the diac, DIGN, to discharge with a time constant determined by RIGN and CIGN. When the voltage across the diac reaches the diac threshold, the diac then diac breaks down and a voltage pulse is produced across the primary winding of the ignition transformer, TIGN. This produces 4KV pulses across the secondary winding of TIGN and across the lamp for ignition. The complete buck and full-bridge control circuit schematic is shown in Figure 4. The circuit is designed around the IRS2573D HID Control IC from International Rectifier. The IRS2573D includes control for the buck stage, the full-bridge, lamp current and voltage sensing, and feedback loops for controlling lamp current and lamp power. The IC includes an integrated 600V high-side www.powerelectronics.com cONTROLLINGHIDLamps This solution also allows for scalability of design so that the same basic circuit can be used as a platform to realize a family of electronic ballasts for many lamp types and power levels. driver for the buck gate drive (BUCK pin) with cycle-by-cycle over-current protection (CS pin). The on-time of the buck switch is controlled by the lamp power control loop (PCOMP pin) or lamp current limitation loop (ICOMP pin). The off-time of the buck switch is controlled by the inductor current zero-crossing detection input (ZX pin) during criticalconduction mode, or by the off-time timing input (TOFF pin) for continuous-conduction mode. The IC also includes a fully-integrated 600V high- and low-side full-bridge driver. The operating frequency of the fullbridge is controlled with an external timing pin (CT pin). The IC provides lamp power control by sensing the lamp voltage and current (VSENSE and ISENSE pins) and then multiplying them together internally to generate the lamp power measurement. The ignition control is performed using an ignition timing output (IGN pin) that drives an external ignition MOSFET (MIGN) on and off to enable the ignition circuit of the lamp (DIGN, CIGN, TIGN). The ignition timer is programmed externally (TIGN pin) to set the ignition circuit on and off times. Finally, the IC includes a programmable fault timer (TCLK pin) for programming the allowable fault duration times before shutting the IC off safely when various fault conditions occur. Such fault conditions include failure of the lamp www.powerelectronics.com to ignite, failure of the lamp to warmup, lamp end-of-life, arc instabilities, and open/short circuit of the output. Waveforms Figure 5 illustrates the experimental results. Figure 5A shows the buck switching node voltage (upper trace) and buck current (lower trace) during lamp warm-up. Figure 5B shows the buck switching node voltage (upper trace) and buck current during steadystate running conditions. The buck is working in critical-conduction mode during steady state running conditions and the on-time is controlled by the constant power feedback loop. Figure 5C shows each half-bridge output voltage (upper and middle traces) and ac lamp current (lower trace) during normal lamp running conditions. The design presented here is a standard approach that utilizes a highly-integrated control IC, the IRS2573D, to greatly simplify the design of the circuit. This solution also allows for scalability of design so that the same basic circuit can be used as a platform to realize a family of electronic ballasts for many lamp types and power levels. The new IRS2573D control IC contains the complete HID system-in-a-chip, including lamp control, lamp ignition, and all fault protection circuitry. This control IC makes the lighting solution very reliable and ideal for designers facing time-to-market pressures to introduce their products into the marketplace. High intensity discharge lighting is a growing market with many applications. Outdoor lighting is especially attractive for high-intensity lamps, due to the long life-time and highbrightness these lamps deliver and the enormous energy saving benefits that electronic ballasts offer. The lamp requirements are critical and the ballast requirements are challenging, making the design of the electronic ballast a difficult task. July 2010 | Power Electronics Technology 39