Method for improving LCD TV performance based on high voltage backlight inverter

The global application of digital TV allows consumers to experience high resolution that was not available on CRT TVs. LCD TVs are the next generation of home appliances that take advantage of digital TVs. Therefore, consumers are constantly demanding TVs with larger and thinner screens, lower power consumption, higher resolution, and lower prices.

Using a high-voltage backlight inverter to replace the existing low-voltage backlight inverter is one of the means to improve the performance of LCD TVs and reduce the overall system cost.

The advantage of using a high-voltage backlight inverter is that the inverter is directly connected to the power factor correction (PFC) stage without a DC-DC converter, while a low-voltage backlight inverter requires a DC-DC converter after the PFC stage. In fact, in a high-voltage backlight inverter solution, the DC-DC converter used to power other loads only needs to handle about 30% of the power of the entire LCD TV, because the typical power consumption of the backlight unit in an LCD TV accounts for 70% of the total power consumption. Therefore, using a high-voltage backlight inverter can reduce the cost of the transformer and MOSFET in the DC-DC converter.

High-voltage inverters usually use a half-bridge circuit topology. However, since it is difficult for a half-bridge circuit to achieve zero voltage switching (ZVS) in every situation, a blocking diode is usually connected in series with the MOSFET, and a fast recovery diode (FRD) is connected in parallel.

Although the MOSFET itself has a built-in diode, if the half-bridge circuit does not operate in the ZVS state, the reverse recovery current of the built-in diode will flow into the other MOSFET when the MOSFET is turned on. This will generate a lot of heat on the on-resistance (RDS(ON)), causing the temperature of the second MOSFET to rise, thereby increasing the reverse recovery current of the MOSFET when it is turned off. The increase in the reverse recovery current in the second MOSFET will cause the power consumption and reverse recovery current of the first MOSFET to increase.

This positive feedback effect will cause the temperature of the MOSFET to continue to rise until the heat dissipated is equal to the heat generated.

Typically, the reverse recovery current of the diode built into a typical MOSFET is large. Therefore, LCD TV manufacturers use a half-bridge solution that uses a series blocking diode to prevent the built-in diode from conducting.

To solve the power consumption design problem of LCD TV, Fairchild Semiconductor's Ultra FRFETTM solution can be used. Its MOSFET adopts excellent life control process to significantly reduce reverse recovery current.

Ultra FRFET has the following advantages:

- Soft switching reverse recovery characteristics

- Small reverse recovery current, maintaining good LCD TV EMI performance

- Small gate charge (Qg) reduces switching losses during turn-on and turn-off

- Higher diode dv/dt immunity (up to 20V/ns), while typical MOSFETs only reach 4.5V/ns

- High reliability under high temperature and high frequency conditions

In LCD TV high voltage backlight inverter applications, Ultra FRFETs can work well without blocking diodes and fast recovery diodes, making them very suitable for dimming ballast applications.