AN880

AN880 STMICROELECTRONICS | Alldatasheet

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Typical industrial TL ballasts requires complex control with dimming or automation interface. Here the L6569 is a driver between the power and control blocks. To use it with an external os- cillator, pin CF is used as an 0-12V logic input, and the L6569 becomes a high voltage buffer. Applications with power above 150W require a full bridge inverter. Figure 6 shows how two L6569 drive such a MOSFET bridge. If no external con- trol is required, the first L6569 master can control the switching with its oscillator, and synchronizes the other driver as (slave). The L6569 start up Two versions of the L6569 are available with dif- ferent start up characteristics. The L6569 drives the lower MOSFET ON at power-up until the sup- ply voltage reaches the Under Voltage Lock Out. The bootstrap capacitor is precharged to 4.6V and both the lower and the upper MOSFETs will switch immediately with the oscillator. This is in- tended for inverters which use only one DC block- ing capacitor connected to the power ground, as shown on figure 4 for CFL ballast. CHARGE PUMP CIRCUIT LOGIC L6569 600V 120 Ω ON

15.6 V ON

Figure 5: Bootstrap capacitor charge. L6569 EXTERNAL OSCILLATOR STB9NK50Z 100nF VS VS HV RF CF GND D02IN1386 L6569 100nF VS BOOT HVG OUT LVG BOOT HVG OUT LVG RF CF GND Figure 6: Basic diagram for 2x105 W lamp ballast in full bridge configuration. AN880 APPLICATION NOTE

Several ways can achieve the protection task. First it can be done by sensing the resonant cur- rent through a MOSFET source resistor or a sec- ondary winding on the choke. The switching is stopped when a large current reduction is de- tected by analog means. A logic circuit can also detect the presence of the lamp filaments. One end of a filament is always connected to a fixed voltage. If the other end of the filament is connected through a high imped- ance resistor to another voltage, the absence of the filament can be easily detected by monitoring the resistor voltage change as shown on figure 22. CONCLUSION The foregoing note shows how high voltage driv- ers, like the L6569, simplify the design of the lamp ballast. These devices includes all the cir- cuitry to drive MOSFETs in half bridge inverter. Since the optimized switching frequency in- creases above 50 kHz with a low tolerance, the size of the passive resonant components is re- duced, and the ballast becomes cheaper.With its supply and its oscillator the L6569 is versatile, and its flexibility permits to design any improved power control. BIBLIOGRAPHY [1]: AN 527 "Electronic fluorescent lamp ballast" A.Vitanza, R.Scollo, SGS-THOMSON [2]: Smart Power ICs, Chapter 8, High voltage in- tegrated circuits for off-line power applications. C.Diazzi, SGS-THOMSON [3]: AN 512 "Characteristics of power semicon- ductors" JM Peter, SGS-THOMSON [4] : "The L6569 half bridge driver: the shutdown function" [5] : "Compatibility test of dimming electronic bal- lasts used in daylighting and environment con- trols", A.Buddenberg, Rensselaer Polytechnic In- stitute [6]: "PSpice High Frequency Dynamic Fluores- cent Lamp Model", Bryce Hesterman, APEC’96, p.641 4_GND 3_CF 18V 10.R 100.R 10.R LAMP L6569 11.R Figure 22: Open load detection example. AN880 APPLICATION NOTE

APPENDIX B: Rating of the capacitive supply with the L6569 driver The supply is made with the snubber and a start up resistor RS. A snubber circuit is used to minimize the MOS- FETs dissipation. It also achieves a non dissipa- tive supply as shown on figure 10. The MOSFETs gate charge, the driver consump- tion, the oscillator, and the shunt regulator, define the circuit consumption. We can estimate this current is IS AV: ISAV > 2 ⋅ IG + IQS + IOSC + IREG = = 2 ⋅ QG ⋅ fsw + IQS + VS RF ⋅ 2 + IREG Where QG the MOSFET gate charge IQS the driver supply current VS the supply voltage RF the oscillator resistor and VS the driver supply voltage IREG the shunt regulator current. When V S is lower than the UVLO threshold UUVLO , the driver is only consuming. Its current must be mini- mal to reduce the dissipation of the resistor RS. The L6569 has a 150 µA start up current, and the maximum resistance is 2MΩ for a 230Vac line ap- plication. We can also reduce the resistor value to get a faster start up time TS. TS = R S ⋅ CS ⋅ UUVLO VDC Where C S is the supply capacitor, and VDC the line voltage. When the timer oscillates, the capacitor C sup- plies the lamp current during the lower MOS turn off. To avoid any cross conduction its capacitance is limited by the driver dead time TD (see figure 26). Hence the capacitive supply current IC is also limited. C < TD ⋅ IL VDC ICAV = C ⋅ VDC ⋅ FSW < IL ⋅ TD ⋅ FSW Where IL is the peak lamp current, and FSW the switching frequency. For a ballast such as a CFL one this circuit sup- plies easily the required current. For instance with a CF18DT lamp ( I L > 230 mA) the capacitor is 1nF on 120Vac line, 470 pF on 230 Vac line. At 50 kHz the average capacitive current is 6 mA in both cases. TD VHVG + VOUT GND GND GND RF IC 200 ns/dv ; 50 V/dv ; 0.1 A/dv Figure 26: Cross conduction of the snubber capacitor with the upper MOSFET: capacitor current and voltage waveforms. AN880 APPLICATION NOTE

Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specification mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a registered trademark of STMicroelectronics © 2003 STMicroelectronics – Printed in Italy – All Rights Reserved STMicroelectronics GROUP OF COMPANIES Australia - Brazil - Canada - China - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - United States. http://www.st.com AN880 APPLICATION NOTE