Back-light inverter faults for LCD monitors

LCD Monitor Back-light Inverter – Common faults

For a newer LCD Monitor design, the inverter board is joining together with the power board. Older LCD Monitor has the inverter board separated from the power board.

There are four types of inverter designs 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.

1. Buck Royer Type Inverter

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.

The common Buck P-channel FET is FU9024N, J598 and etc. The SMD FET IC’s are 4431, BE3V1J and etc. The common push pull transistors part numbers are C5706, C5707 and etc.

2. Push Pull Inverter (Direct Drive)

The push pull inverter shown above when Q1 switches on, current flows through the 'upper' half of T1's primary and the magnetic field in T1 expands. The expanding magnetic field in T1 induces a voltage across T1 secondary. When Q1 turns off, the magnetic field in T1 collapses and after a period of dead time (dependent on the duty cycle of the PWM drive signal), Q2 conducts, current flows through the 'lower' half of T1's primary and the magnetic field in T1 expand. Now the direction of the magnetic flux is opposite to that produced when Q1 conducted. The expanding magnetic field induces a voltage across T1 secondary. After a period of dead time, Q1 conducts and the cycle repeats. The above diagram only showing a single channel IC that is driving the Q1 and Q2. Some inverter IC can have two channels in order to drive two high voltage transformers. Each output from the transformer can drive more than one lamp.

3. Half Bridge Inverter (Direct Drive)

The half bridge inverter is similar to the push pull inverter, but a centre tapped primary is not required. The reversal of the magnetic field is achieved by reversing the direction of the primary winding current flow. This type of inverter is found in many LCD Monitor too. The control circuit of a half bridge inverter is similar to that of a push-pull inverter. This design has optimal utilization of transformer core and primary winding (one vs. two for push pull). The above diagram only showing a single channel IC that is driving the Q1 and Q2. Some inverter IC can have two channels in order to drive two high voltage transformers.

Each output from the transformer can drive more than one lamp

4. Full Bridge Inverter (Direct Drive)

The full bridge inverter is similar to the push pull inverter, but a centre tapped primary is not required. The reversal of the magnetic field is achieved by reversing the direction of the primary winding current flow.

This type of inverter is found in many latest LCD Monitors. Diagonal pairs of transistors will alternately conduct, thus achieving current reversal in the transformer primary. This can be illustrated as follows - with Q1 and Q4 conducting, current flow will be 'downwards' through the transformer primary and with Q2 and Q3 conducting, and current flow will be 'upwards' through the transformer primary. The control circuit monitors V out and controls the duty cycle of the drive waveform to Q1, Q2, Q3 and Q4. The control circuit operates in the same manner as for the push-pull inverter and half-bridge inverter, except that four transistors (FET) are being driven rather than two. In some LCD Monitor like the HP1703 that uses the OZ960 inverter IC, the output from inverter IC can parallel out to drive another high voltage transformer.

The full bridge inverter design has 4 IC’s (each IC have two FET (N and P channel)) in it. Two IC’s were used to drive each high voltage transformer.

The dual N and P channel PowerTrench Mosfet IC can be in SMD type or Dual in Line package

Common Faults 

1) Dry Joints (Very common in the buck choke and high voltage transformer pins)

2) Shorted or burnt high voltage transformers
3) Shorted or leaky push pull transistors
4) Capacitance value problems
5) Shorted buck P-channel FET
6) Inverter Pico fuse open circuit or turned high ohm
7) Ballast capacitors value out causing shutdown and brightness fluctuate
8) Burnt pins or loose connection in the backlights connector
Some common part numbers for inverter IC’s are TL1451ACN, 0Z960, 0Z962, 0Z965, BIT3105, BIT3106, TL5001 and etc.