The post explains a simple 20 watt fluorescent ballast circuit using a single chip FAN7711 from Fairchild semiconductor.
The proposed mains 220V operated, 20 watt fluorescent ballast circuit is built around an LCC resonant tank and a half-bridge network.
Implementing Zero Voltage Switching
The main features and operations may be understood with the following explanation:
In order to execute a zero-voltage switching (ZVS) through the half-bridge inverter circuit, the LCC undergoes a high frequency operation beyond the resonant frequency, fixed by the components L, Cs, Cp and RL where RL is the equivalent to the lamps impedance, and it also crucial with the transfer operation of the LCC resonant tank circuit.
The in-built oscillator stage inside the IC FAN7711 is capable of generating optimal driving pulses in order to implement effective lamp ignition and enhance lamp life.
During the procedures, the oscillation frequency goes through the following transitions:
Preheating frequency > Ignition frequency> normal running frequency.
Initially, the lamp impedance is relatively high, but once it’s ignited the impedance is brought down substantially. Due to this high initial impedance, the resonant peak could be very high too, owing to this the lamp is fired at a higher frequency than the resonant frequency.
During the operations, the current fundamentally flows via Cp which becomes responsible for linking up the two filaments of the tube and producing ground path for the passing current.
In the course, the passing current preheats the filament for a quick striking.
The amount of amps that may be required for this is adjustable and could be done by setting up the capacitance of Cp.
Calculating Preheating Frequency
The preheating frequency which forms the driving frequency may be expressed as:
fPRE = 1.6 x fOSC
As soon as the above preheating is concluded, the IC pulls down the frequency, increasing the voltage to the lamp for executing the intended ignition of the lamp.
The ignition frequency which is also the function of CPH voltage may be written as:
fIG = [0.3 x (5 – VCPH) + 1] x fOSC
Where VCPH is the voltage rating of the capacitor used
Once the above operations are implemented, the lamp witnesses a constant frequency drive via an external resistor Rt for the required sustained illumination.