Hardware Design
4.2.2 Inverter
The power inverter generates the proper voltage for the fluorescent tubes. The power inverter consists of two MOSFET transistors driven by a
The
The
The tube voltage difference circuit consist of coils L3A, L3B, and L3D, resistors R21, R24, R22, and R151, diode D8, and capacitor C17, and is used to sensing voltage differences between lamps. It helps to recognize aging of the lamps.
Coils L4 and L5 and diodes D4 and D5 are for filament preheating. Diodes maintain a small offset to remove the flickering effect. Coils L3A and L3D balance possible differences in current flow into tubes, mainly at ignition stage. Devices R19, D7, R152, D6, C16, and R20 sense the current flow in tube 1 (and similarly for tube 2).
For different tubes parameters, the tube currents are different. The ignition circuit formed by L3D, L3A, and R17 balance the current. The compensation current flows through R17 until the tube currents are equal, at which time the ignition of both tubes can be done reliably.
Tubes preheating heats the tubes to the desired temperature before startup. It decreases the
Each tube has its own preheating circuity. Tube 1 uses L4. Tube 2 uses L5. The preheating voltage and time is
The flickering effect is caused by reducing the voltage to zero during the light ballast operation. To avoid this, the Zener diodes D4 and D5 maintain a small voltage on both tubes at all times.
The output inductance circuity performs several important functions in the light ballast application. It helps to ignite lamps, when tube parameters differ (due, for example, to different aging of the lamps). It removes the flickering effect and provides filament preheating.
Dimmable Light Ballast with Power Factor Correction, Rev. 1
28 | Freescale Semiconductor |