FM2822 is an ASIC for dimmable fluorescent lamp electronic ballast with independent property rights. It adopts 16-pin DIP package and dimming adopts phase detection scheme. The lamp power can be adjusted by changing the pulse width and frequency, thereby changing the lamp brightness to achieve the purpose of energy saving.
The circuit is shown in the figure below. L1 and C1 and C2 form an EMI filter. The internal gate driver of IC ⑨ (G1) and (11) (G2) and external transformer T1 and half-bridge switches V1 and V2 form Bridge inverter circuit, T2 primary inductance L2-1 and C13 form LC series resonant circuit, R12, R13, R14 and D7, C12 form lamp voltage detection circuit, T3, R16, R17 and R18, R7 form lamp current phase respectively Sampling and lamp current sampling circuit, R1, R2 and R3, R4, RP1, C4 form a dimming circuit.
The working principle of the circuit is as follows:
1. Startup After the system is powered on, the full-wave rectified output charges C5 via R5, R6, and D6. When the voltage on C5 is charged above 11V, the oscillation circuit starts to oscillate. The half-bridge generates a high-frequency output. The high-frequency signal is coupled by C10, rectified by diode D6 and filtered by C5 and C6 to provide the operating current for the VDD pin of the IC. The VDD pin voltage is clamped at 15.6V by the internal Zener diode.
2. Preheat and trigger (Qihui)
As soon as the voltage controlled oscillator starts to oscillate, it outputs the preheating frequency FPH (= fmax). Since fPH is much higher than the resonant frequency of the load LC (that is, L2-1 and C13) circuit, the ballast only provides current to heat the filament, and there will be no high voltage on the lamp tube that can break the lamp tube.
The warm-up time tPH is determined by the capacitance CP on the foot. The preheating frequency fPH is determined by the resistance RP on the foot. When the warm-up is completed, the frequency sweep mode is entered, and the oscillator frequency shifts from the warm-up frequency fPH to the normal operating frequency fRUN (= fmin), as shown on the right.
The rate of frequency decrease is determined by CP. When the frequency is close to the resonant frequency of the load, a high-voltage pulse above 1000Vpp generated on C13 is applied to both ends of the lamp tube, causing the lamp tube to break down and be lit. The lamp trigger time (frequency sweep time) is 20% of tPH, which is 0.2s. Once the lamp is lit, it enters the operating mode. The operating mode frequency fRUN is determined by the resistance RF on the (13) foot and the capacitance CF on the ⑤ foot (about 40kHz).
3. The lamp current sampling signal input at the dimming (15) pin (IT1) and the lamp voltage sampling signal input at the pin (VL) are multiplied in an internal multiplier, output from the pin â‘ and filtered by an external resistance-capacitance filter circuit In addition to high frequency components. â‘ The output current on the foot depends on the product signal of the lamp current and the lamp voltage signal, thereby forming a power closed-loop feedback for dimming control. â‘¢ The effective input dimming DC voltage of the foot (DIM) can be changed from 0V to 4V, and the lamp power adjustment range is 2% to 100%.
4. Abnormal state protection 1) Over-voltage protection: If the lamp leaks or the cathode is deactivated so that the lamp cannot be lit, an over-voltage will be generated and detected by the IC's â‘¡ (VL) pin, which will stop the system from oscillating and go into low Power consumption standby state.
2) Capacitive mode protection: If the lamp fails to light or the lamp voltage is too high, it will cause a non-zero voltage switch on the half-bridge and work in the capacitive mode, resulting in a large loss of power MOSFET in the half-bridge, or even Will be damaged. Once this happens, the â‘£ (RIND) pin of the IC will detect that the current of the main circuit is ahead of the voltage, and the system will enter the vibration stop standby mode.
3) End of lamp life (EOL) protection: At the end of the lamp life, the lamp tube will have a rectifying effect, causing harm or damage to the ballast. (14) Pin (EOL) has built-in + 1V and -1V window comparators. As long as the detection input on (14) pin exceeds the window range, the system immediately enters the vibration stop standby mode. In Figure 1, the EOL function is not used, so pin (14) is grounded.
By appropriately changing the value of some components, FM2822 can drive different types of lamps.
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