32-inch high-voltage LIPS LCD TV power supply reference design introduction

Mainstream LCD TV power supplies for 32-inch and larger sizes require multiple voltage rails to power different system modules such as audio, backlight and signal processing. Local linear and DC-DC converters are used on the signal processing board to provide different low-voltage rails. For manufacturers, a universal power supply is usually used, supporting 90 to 265 Vac voltage. This enables a single power supply design based on a specific TV size to be used for a range of different models, meeting the needs of different regional markets, simplifying logistics and reducing development costs.

If a model of LCD TV is intended for the global market and its power is above 75 W, it needs to comply with the European IEC61000-3-2 standard for harmonic reduction, which requires the use of an active power factor corrector. Generally speaking, the most power-hungry subsystem in an LCD TV is the backlight module. Most LCD TVs today use an array of cold cathode fluorescent tubes (CCFL) as the backlight source. These tubes need to be driven by a high AC voltage and the lamp current needs to be regulated. Traditionally, the inverter is a separate module powered by a DC power supply with a nominal voltage of 24 V. In this approach, the backlight requirements are linked to the LCD panel, and one power supply design can be used for panels from multiple different suppliers, which simplifies the LCD TV design, but this approach is less efficient and adds an additional power supply section (referring to the 24V output). For example, the AC input voltage of an LCD TV is boosted to 400 Vdc in the PFC stage and then converted to 24 Vdc using a resonant (LLC) half-bridge. This 24 Vdc is then supplied to the inverter module, which converts the DC low voltage into an AC high voltage of more than 1,000 V to drive the CCFL lamp. This multi-stage conversion process generates large losses and increases system cost.

The high-voltage LIPS (HV-LIPS) architecture used in this reference design aims to improve the overall system efficiency by eliminating the 400 Vdc to 24 Vdc conversion stage and directly powering the inverter from the high-voltage power factor correction (PFC) input stage. This requires integrating the traditional power supply functions in the LCD TV in a high-voltage direct conversion manner to optimize the overall system solution.

Traditionally, the standards of many voluntary and regulatory agencies around the world have focused on reducing the standby power consumption of electronic products. Many international standards in the United States, the European Union and other places require 1 W for the standby power consumption of television products, while the China Certification and Accreditation Service (CSC) requires 3 W. As the size of LCD TVs increases, their energy consumption is also increasing. As the market share of LCD TVs increases, regulatory agencies are also paying more and more attention to the cumulative impact of the energy consumption of flat-panel TVs on the power grid when they are in operation. For example, "Energy Star" has released the third edition of operating energy consumption requirements for TV products (see Table 1).

Table 1: Energy Star 3.0 TV specification energy consumption requirements for working mode

Key Design Goals

Input Voltage: Universal Input 85-265 Vac, 47-63 Hz

System Power

* Active Power Factor Correction (Active PFC), in compliance with IEC61000-3-2 specification

* Maximum steady-state power consumption 50 W, peak 60 W

* 12 V / 4 A peak

*5 V / 2.5 A peak

* 24 V – MOSFET gate drive bias

* Flexible modification to support other voltage/current configurations

* Standby input power consumption (Pin) at 50 mW load < 400 mW (5 V@10 mA)

Inverter power supply

* Supports 100 W power

* Strike voltage> 1,500 Vac, working voltage> 800 Vac

* Fixed frequency inverter, adjustable in the range of 40-80 kHz

* Support digital and analog dimming

* Ability to sync to video clock source

PFC stage design

The heart of the high voltage LIPS architecture is the active PFC front-end boost stage. First, it allows the design to meet the harmonic content requirements of the IEC61000-3-2 specification for power supplies with input power above 75 W. Second, it provides a regulated 400 Vdc high voltage to the inverter stage. In addition to powering the backlight, the PFC stage also provides energy to the isolated flyback switching power converter, which provides all the power required to power the digital and analog circuits that perform functions such as control, interface, signal processing, and audio amplification within the LCD TV. Depending on the LCD TV feature set and functionality, this module can range from 30 to 60 W. To simplify the design and reduce the overall complexity of this power conversion stage, this reference design uses ON Semiconductor's proprietary high-efficiency flyback controller NCP1351, which eliminates the need for a dedicated standby power supply for most LCD TV applications. The NCP1351 was chosen because it uses a quasi-resonant fixed on-time (FON) control principle to reduce the switching frequency of the flyback converter as the load demand decreases. Two additional switches (placed on the secondary side) disconnect the main power load in standby mode, thus eliminating parasitic losses in standby mode.

The PFC controller used in the PFC stage is ON Semiconductor's NCP1606B, which is designed to operate in variable frequency critical conduction mode (CrM) and is the most suitable solution for 150 W (<180 W) power applications. For more information on the electromagnetic interference (EMI) design, specific control methods, detailed design process and test results of this PFC stage, please refer to reference [1].

Flyback converter stage for control, signal and audio functions

While a dedicated DC-AC converter is used to power the CCFL lamp, a flyback switcher is used to power analog and digital blocks for functions such as control, signal processing, and audio amplification. Due to the limited total power required (<60 W), it is possible to consider a flyback topology with a standby mode without the need for a dedicated standby switcher, which improves the overall cost-effectiveness of the solution. To achieve this goal, a controller architecture that can provide high efficiency under light load conditions is required.

This flyback converter uses the NCP1351 PWM controller, which is mainly used for offline flyback power supplies with power below 60 W. The NCP1351 uses quasi-fixed on-time technology, with different off-times corresponding to different loads and different input voltages. When the load becomes lighter, its switching frequency is reduced, while reducing switching losses. Therefore, the power supply using this solution naturally provides very small standby power and optimizes energy efficiency under other load conditions. As the frequency decreases, the peak current gradually decreases to about 30% of the maximum peak current, preventing the mechanical resonance of the transformer. The risk of audible noise is also greatly eliminated, while also achieving good standby energy consumption performance.

Since the PWM controller operates with a quasi-fixed on-time, the switching frequency varies with the load. Under light load conditions, this flyback converter operates in discontinuous conduction mode (DCM). As the load increases, the frequency increases until the controller enters continuous conduction mode (CCM), which is optimized to provide very high efficiency.

Additionally, achieving good cross regulation in LCD TV applications is a design challenge because the tolerances are very tight (typically ±5%) and the dynamic operation can vary widely due to the large dynamic range of the audio amplification and the signal processing power supply load that varies depending on the input video source. The typical output voltage and load range for the reference design (SMPS1) is:

* +5 V: 0 to 2.5 A

* +12 V: 0 to 4 A

To improve the overall cross regulation, the +5 V diode is connected to the ground (GND) of the winding, and the +12 V winding is above the 5 V winding. In standby mode, the switching power supply operates in the audio frequency range. Therefore, depending on the transformer construction and mechanical design, some audible noise may occur. Most people are most sensitive to the frequency range of 7 to 13 kHz. This specific reference design is used for a rated standby load of 50 to 75 mW, so the frequency in standby is less than 5 kHz.

This reference design provides sufficient flexibility to accommodate multiple output configurations with simple BOM adjustments. The NCP1351B flyback design is flexible enough to support up to four unique voltage outputs. The standard configuration (SMPS1) used in this reference design has 5 Vdc and 12 Vdc inputs and a 24 Vdc voltage output. Table 2 lists the various optional configurations that can be used to accommodate different power supply mechanisms.

Table 2: Flyback converter segment standard and optional output voltage configurations

High voltage backlight inverter power stage

1) Comparison between half-bridge and full-bridge topologies

The high voltage inverter can be implemented using a half-bridge or full-bridge topology. There are many factors to consider when deciding which topology to use. The full-bridge topology has many advantages over the half-bridge topology, such as zero voltage switching (ZVS) at a fixed operating frequency, reduced EMI and power losses, reduced MOSFET switching stress, and reduced heat dissipation. In addition, in the full-bridge topology, since the controller operates at a fixed frequency, it is possible to synchronize the switching frequency with the video frequency to avoid any possible interference from the backlight subsystem that could affect the video image.