Understanding LCD Monitor Inverter Board

 

For a newer LCD Monitor design, the inverter board is joining together with the power board as shown in the above photo. Older LCD Monitor has the inverter board separated from the power board as shown below. 

There are four types of inverter designs (topologies) used in the LCD Monitors.

1.     Buck Royer inverter

2.     Push pull inverter (Direct Drive)

3.      Half bridge inverter and (Direct Drive)

4.      Full bridge inverter (Direct Drive)

Number 2, 3 and 4 are called Direct Drive because it eliminates the need for the inductor (buck choke) and resonant capacitors found in a conventional Royer Oscillator. In other words, Direct Drive architecture reduces component count, lower production cost and most importantly improved transformer designs that optimize performance.

In order to drive the Backlights (CCFL lamps) embedded in the panel module, an inverter circuit is required to convert the 12 volt DC up to hundreds or even a thousand plus AC voltage output. The inverter is formed by symmetric circuitry, in order to drive the separate lamp modules. The input stage (buck converter circuit) consists of Inverter IC (PWM IC), Buck P-channel FET, Buck Choke and Buck Diode. The Buck converter circuit converts a DC voltage to a lower DC voltage.

The other stage consists of a tuning capacitors, high voltage transformer, and push-pull transistor pair to boost ac output to hundreds of voltage.

The ballast capacitor controls current amplitude through the lamp negative impedance by dropping an approximately equal voltage across its positive impedance. The feedback circuit is for protection purposes and will shut down the inverter IC just in case if the high voltage produced by the high voltage transformer exceeded the normal value and also it can detect bad or a flicker backlights. The inverter IC also used to control the brightness of the CCFL lamps. The AC frequency of the high voltage transformer is typically run at 30 to 70 KHz. The higher the frequency, the greater is the light output.

Note: Some LCD Monitor design has the Buck type P-channel FET integrated into an IC thus in order to successfully testing them you can use the comparison method with another known good FET (comparing the ohms value between pins) or by using the Peak Atlas Analyzer test equipment. The IC can be in Dual in Line package or SMD type.