Brief Analysis of Power Supply Improvement Solutions in LCD TV

Liquid crystal display, referred to as LCD. The world's first liquid crystal display device appeared in the early 1970s and was called a twisted nematic liquid crystal display. Although it was a monochrome display, it was still promoted to the fields of electronic watches and calculators. In the 1980s, super twisted nematic liquid crystal displays appeared, and thin-film transistor liquid crystal display technology was developed at the same time, but liquid crystal technology was still immature and difficult to popularize. In the late 1980s and early 1990s, Japan mastered the production technology of STN-LCD and TFT-LCD, and the LCD industry began to develop rapidly. Liquid crystal is a substance between solid and liquid. It is an organic compound with regular molecular arrangement. If it is heated, it will appear in a transparent liquid state, and if it is cooled, a turbid solid state with crystalline particles will appear.

1 Introduction to LCD TV power supply architecture

Figure 1 shows the power supply architecture of LCD TV. The input voltage of the architecture in the figure is 90-265V, and the input frequency ranges from 47Hz to 63Hz. After debugging and rectification, it will pass through the PFC architecture. Since this article is mainly aimed at MOS, the debugging and rectification part is not shown in the figure. PFC uses DCM or CCM according to the IC used. There is a PWM after coming out of PFC. If the output power is less than 65W, the PWM end uses Flyback or QR mode. LCD TVs now commonly use half-bridge resonance and LLC architectures. There is a backup power supply on the 5V side. Currently, the backup power supply of LCD TVs usually uses IC-bound MOS.

Figure 2 shows an actual TV board. From the perspective of the power architecture of Figure 1, there are two SteP EMI cores in this board. The red part in the figure has a PFC MOSFET. Currently, Vishay mainly promotes IRFP27N60KPBF. The PWM MOSFET adopts a half-bridge LLC architecture. This part can use a 500V MOS. This board uses IRFB840APBF. In addition, there are 3 groups of outputs, namely 5V, 12V and 24V, and a 5V backup power supply.

2 Topology and working principle

PFC, or Power Factor Correction, is a boost architecture. When the power supply of the PFC controller is greater than 70W and less than 200W, a DCM structure is usually used. This structure has a relatively high voltage and usually requires a 600 or 650V MOS. When the power supply is greater than 200W, a CCM structure is usually used. For the primary side PWM topology architecture, as shown in Figure 3, the Flyback architecture is generally used. Adapters with a power supply of 65W or less than 90W will use the Flyback architecture, and 26" and 32" TVs may also use the Flyback architecture. The half-bridge structure requires two MOS, while the full-bridge structure requires four MOS. In fact, the half-bridge and full-bridge architectures are more suitable for large power supplies. The currently widely accepted TV power supply is mainly the LLC architecture, which consists of two MOS Q1 and Q2 in series with Cr, Lr and Lm, and a transformer, as shown in the lower right figure in Figure 3.

3 Zero voltage switching

Since the voltage-current crossover area is smaller in the resonant switching mode, the relative switching loss is smaller, the efficiency is higher, and the temperature is lower, the resonant switching mode is usually adopted. Currently, the most commonly used is zero voltage switching. Due to the use of the LC structure, its resonant frequency is f_{r}=frac{1}{2πsqrt{L_{r}C_{r}}}. When the operating frequency is greater than the resonant frequency, it operates on the ZVS (zero voltage switching) structure. As can be seen from Figure 4, when the current increases, the voltage is zero, and when the voltage increases, the current is zero.

4 LLC resonant converter

FIG5 shows a half-bridge resonant LLC resonant converter. This architecture calculates the first and second points of the resonant frequency according to f_{r}=frac{1}{2πsqrt{L_{r}C_{r}}}. Generally, it is desirable to design the LLC resonance within the ZVS region. If the loss is relatively large within the ZCS region, it can be achieved by simply designing the switching frequency to be greater than the resonant frequency. The graph in the upper right corner of FIG5 shows the relationship between the switching frequency and the voltage gain transformation. The advantages of the resonant LLC converter when applied to LCD TVs are:

1) High efficiency: The primary MOS zero voltage switching has almost no loss, and the secondary rectifier diode is ZCS switching with less loss, so the overall efficiency is improved;

2) High power density;

3) Good EMI (low dV/dt and dI/dt);

4) Better cross adjustment rate;

5) Lower output ripple noise;

6) Low thermal disturbance

7) Cost savings

5 Working Principle of LLC Topology

Figure 6 shows the working principle of LLC topology. It can be seen that on the left are two MOS (high-voltage MOS and low-voltage MOS) connected in series with two inductors, a capacitor and a transformer. There are two rectifier diodes on the secondary side, which is a non-synchronous rectification method. The right side of the figure is a segmented schematic diagram of current and voltage changes. In the T0-T1 stage, the two MOS are not turned on, and the current flows back through the diodes of the MOS; in the T1-T2 stage, the high-voltage side MOS is turned on, the current flows from VDC to the ground, and the diode on the secondary side is turned on; the only change in T2-T3 is that the diode on the secondary side is turned off, the current flows out from Vout, the two sides are isolated, the high-voltage side MOS is still turned on, and the low-voltage side MOS is still turned off; in the T3-T4 stage, the high-voltage MOS is turned off, the low-voltage MOS is not turned on, but the current passes through the diode of the low-voltage side MOS, and the diode on the secondary side is also turned on; in the T4-T5 stage, the low-voltage MOS is fully turned on, and the diode on the secondary side is also fully turned on; in the T5-T6 stage, the secondary side diode is turned off, and the low-voltage MOS is turned on.

6 LCD TV power supply products currently available from Vishay

Vishay's current main LCD TV high-voltage MOS is Gen6, a relatively new technology manufactured with 6" wafers. It includes K-Series and L-Series, which can be seen from the last letter of the device name. L-Series is very suitable for the primary switch in the AC/DC SMPS ZVS full bridge. Figures 7 and 8 list the 1M voltage products.

7 Application Examples

Figure 9 shows an LCD TV application using LLC, where the red circled parts are the recommended models for PFC MOS and half-bridge resonance MOS. In the half-bridge resonance part, a 500V MOS can be selected in actual applications.