LCD Display Tragedy (Part 2) - Power Supply

Last time we talked about the light source of LCD TVs. Optics and light are inseparable, so this time let’s talk about the power supply!

 

CCFL LCD TVs are generally divided into two groups of power supplies. One group is the so-called power board, which is responsible for supplying power to the motherboard and other peripherals on the one hand, and to the backlight driver on the other hand; the other group is what we usually call the high-voltage board (scientific name is inverter), which is mainly used to drive the CCFL backlight source.

 

Let me briefly introduce the power board. Like other general power supplies, it has 220V AC input, full-bridge rectification, PFC power factor correction, and a flyback architecture. The required voltages are obtained through several sets of primary and secondary windings with different turns ratios. Usually, there are these voltages:

 

a)       24V mainly provides the power supply voltage for the backlight driver board inside the LCD screen;

b)       12V 1) Connect to the mainboard, and then directly power the logic module (Tcon) of the power supply screen through LVDS connection; 2) Power the sound board by stepping down to 8V, and then stepping down to 5V to power the tuner;

c)       5V    1 ) Powers the mainboard, and then reduces the voltage to 3.3V, 2.5V, and 1.8V, mainly to power the image processing module (scaler); 2) Provides standby voltage, and reduces the voltage to 3.3V, 2.5V, and 1.8V to power the main chip and DDR memory;

d)       14V    is mainly used to power the audio amplifier

 

Of course, now that TVs have more and more functions, the required voltages may also be different, so I won't give examples one by one. Because the power board design is relatively modular, I won't go into detail here, and I'll mainly talk about the backlight drive inverter.

 

The function of the inverter is to convert DC voltage into AC voltage. Why do we need AC voltage? In the previous article, we mentioned that CCFL loads need high voltage breakdown to emit light, and research shows that alternating sine wave voltage with a frequency between 30k-80kHz can achieve the best CCFL luminous efficiency and life. Because the normal operating voltage of CCFL is very high, the initial breakdown voltage is even higher, which needs to reach thousands of volts, while the current flowing through CCFL is very small, only milliamperes, so the inverter design needs to meet such a high voltage and low current load requirement.

 

This is an earlier CCFL inverter solution that I have come across. The 24VDC voltage is input through the center tap, and the self-excited oscillation makes the upper and lower transistors conduct alternately, forming a sine wave voltage, which is then amplified by a transformer with a certain turns ratio to drive the lamp. Because it uses a self-excited method, its frequency will change with the influence of external factors. In addition, the push-pull architecture has higher requirements for the switching transistor, and the switching stress needs to reach more than twice the input voltage, so this architecture has not been used on a large scale.

 

 

This is a driving architecture that has been used for a long time. Although the full-bridge architecture uses the largest number of switch tubes, it puts the least pressure on power devices compared to push-pull, half-bridge and other architectures. Under the premise of emphasizing safety first, the cost of adding two switch tubes can be basically ignored. This architecture also introduces chip control. The frequency of the switch tube is controlled by the chip and has nothing to do with the external environment. In addition, the chip also integrates various protection mechanisms to ensure the safety of product use, while also greatly simplifying the peripheral circuit design.

 

 

 This is a revolution to the traditional architecture. It integrates the power board and the inverter together. The inverter input no longer uses the 24V voltage of the power board, but is directly taken from the 400V output after PFC. In this way, the electric energy of the backlight part reduces the first-level energy conversion, and also saves the first-level efficiency loss, which makes the design power of the power board can be greatly reduced, and also solves the heat dissipation problem of the power board. Most importantly, it greatly reduces the manufacturing cost. In addition, in order to improve the negative resistance characteristics of CCFL and enable CCFL to be driven in parallel, the circuit also adds a capacitor balance board or an inductor balance board, which greatly reduces the number of transformers used in the inverter, which also further reduces the manufacturing cost.

 

Now, because of the use of LED backlight, the design is even simpler. Because LED is DC driven, the trouble of converting DC to AC is eliminated. The circuit can be directly boosted after 24V input. The key to circuit design is to keep the current between each string of LEDs equal. Now all IC manufacturers have management chips in this regard, and there are many types, so it is not cumbersome. You can see from the picture that the board at the back is quite streamlined!

 

Here I will present to you what I know. If there is anything else, I hope you can add to it. I also hope that experts can give me more guidance.