The circuit is built around the IR2520D Ballast Control IC. The IR2520D provides adjustable preheat time,
adjustable run frequency to set the lamp power, high starting frequency for soft start and to avoid lamp flash,
fault protection for open filament condition and failure to strike, low AC line protection and auto-restart after
line brownout conditions. The IR2520D is a low-cost solution with only 8 pins and allows the component
count for the complete solution to be reduced down to 19 components.
In the CFL market, the self-oscillating bipolar transistor solution is still more popular than a ballast control IC plus FETs solution due to lower cost. This approach is very simple in nature but has disadvantages including: ! DIAC or additional circuit required for starting, additional free-wheeling diodes required ! operating frequencies determined by bi-polar tran- sistor storage time and toroid saturation (not easy to design, very dependent on tolerances in production and difficult to set the frequencies precisely), ! unreliable “always hot” PTC thermistor used for preheat that often fails in the field, ! no protection against lamp non-strike or open filaments conditions, ! no smooth frequency ramping during ignition, ! capacitive mode operations, ! high crest factor in the lamp current.
These can result in high susceptibility to components and load tolerances and/or catastrophic failure of ballast output stage components and a short lamp life. The IR2520D includes adaptive zero-voltage switching (ZV, adaptive run frequency for zero-voltage switching), internal crest factor and non-zero voltage switching (ZVS) protection, as well as an integrated bootstrap diode.
The heart of this IC is a voltage controlled oscillator (VCO) with externally programmable minimum frequency and a 0-5VDC analog voltage input. One of the biggest advantages of the IR2520 is that it uses the VS pin and the...