"Keywords:
TV power supply
Mainstream LCD TV power supplies of 32 inches and larger require multiple voltage rails to power different system modules such as audio, backlight and signal processing. Among them, local linear and DC-DC converters are used on the signal processing board to provide different low-voltage rails. For manufacturers, a general-purpose power supply is usually used to support a voltage of 90 to 265 vac. This enables a single power supply design based on a specific TV size to be used in a series of different models to meet the market needs of different regions, simplify logistics and reduce development costs.
If a certain type of LCD TV faces the global market and its power is higher than 75 W, it needs to comply with the European iec61000-3-2 standard on harmonic reduction, so it needs to use active power factor corrector. Generally speaking, the most power consuming subsystem in LCD TV is backlight module. Most LCD TVs today use cold cathode fluorescent tube (CCFL) arrays as backlight. These lamps need to be driven by AC high voltage, and the lamp current needs to be stabilized. Traditionally, the inverter is a separate module, which is powered by a DC power supply with a rated voltage of 24 v. In this way, the backlight requirements are associated with the LCD panel, and one power supply design can be used for panels provided by multiple different suppliers, which simplifies the LCD TV design, but this way is inefficient and adds additional power supply (24V output). For example, the AC input voltage of LCD TV is boosted to 400 VDC in PFC section, and then converted to 24 VDC by resonant (LLC) half bridge. This 24 VDC is then supplied to the inverter module to convert from DC low voltage to AC high voltage higher than 1000 V to drive the CCFL lamp. This multi-stage conversion process will cause large loss and increase the system cost.
The high voltage lips (hv-lips) architecture used in this reference design aims to eliminate the 400 VDC to 24 VDC conversion section and supply power to the inverter directly from the high voltage power factor correction (PFC) input section, so as to improve the overall efficiency of the system. This requires the integration of traditional power functions in LCD TV by high voltage direct conversion to optimize the overall system scheme.
Traditionally, the standards of many voluntary and regulatory institutions around the world focus on reducing the standby energy consumption of electronic products. Many international norms in the United States, the European Union and other places require the standby energy consumption of TV products to be 1 W, while the Chinese bid winning Certification Center (CSC) requires 3 W. With the increasing size of LCD TV, its energy consumption is also higher and higher. Due to the increasing market share of LCD TV, regulatory agencies pay more and more attention to the cumulative impact of energy consumption on power grid under the working state of flat-panel TV. For example, "Energy Star" released the 3rd edition of working energy consumption requirements for TV products (see Table 1).
Table 1 energy consumption requirements of "Energy Star" version 3.0 TV specification for working mode
Key design objectives
Input voltage: general input 85-265 VAC, 47-63 Hz
System power supply
Active power factor correction (PFC) in accordance with iec61000-3-2
The maximum steady-state energy consumption is 50 W, and the peak value is 60 W
12 V / 4 a peak
5 V / 2.5 a peak
24 V – mosfe gate drive offset
It can be flexibly modified to support other voltage / current configurations
Standby input energy consumption (PIN) inverter power supply at 50 MW load
Support 100 W power
Strike voltage > 1500 VAC, working voltage > 800 VAC
Fixed frequency inverter, adjustable in the range of 40-80 kHz
Support digital and analog dimming
It can be synchronized to the video clock source
PFC section design
The core of high voltage lips architecture is the front-end boost section of active PFC. Firstly, for the power supply with input power higher than 75 W, it makes the design meet the harmonic content requirements of iec61000-3-2 specification. Secondly, it provides regulated 400 VDC high voltage for the inverter section. In addition to supplying power for the backlight, the PFC section also provides electric energy to the isolated flyback switching power converter, which provides all required power supply for the digital and analog circuits that perform the functions of control, interface, signal processing and audio amplification in the LCD TV. Depending on the characteristic set and function of LCD TV, the power of this module may be between 30 and 60 W. In order to simplify the design and reduce the overall complexity of this power conversion section, this reference design selects ansenmey semiconductor's proprietary high-energy efficiency flyback controller NCP1351, which eliminates the need for special standby power supply for most LCD TV applications. NCP1351 is selected because it adopts the quasi resonant fixed on time (fon) control principle to reduce the switching frequency of flyback converter when the load requirements become smaller. Two additional switches (placed at the secondary end) disconnect the main power load in standby mode, thus eliminating parasitic losses in standby mode.
The PFC controller used in PFC section is ncp1606b of ansenmey semiconductor. This controller is designed to work in variable frequency critical conduction mode (CRM) and is a flyback converter section with 150 W power (for control, signal and audio functions)
Since a special DC-AC converter is used to power the CCFL lamp, the flyback switching power supply is used to power the analog and digital modules with functions such as control, signal processing and audio amplification. Due to the limited total power required (this flyback converter uses NCP1351 PWM controller, which is mainly used for off-line flyback power supply with power less than 60 W. NCP1351 adopts quasi fixed conduction time technology. Different loads and different input voltages correspond to different closing times. When the load becomes lighter, the switching frequency is reduced and the switching loss is reduced. Therefore, the power supply using this scheme naturally provides minimal standby power and optimizes energy efficiency under other load conditions. With the decrease of frequency, the peak current gradually decreases to about 30% of the maximum peak current to prevent the mechanical resonance of the transformer. The risk of audible noise is also greatly eliminated, and good standby energy consumption performance is obtained.
Because the PWM controller works in quasi fixed on time, the switching frequency changes with the load. Under light load, the 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 extremely high efficiency.
In addition, in LCD TV applications, achieving good cross voltage stabilization is a design challenge, because the tolerance is very strict (usually ± 5%), and due to the large dynamic range of audio amplification and the change of signal processing power load according to different input video sources, the dynamic operation may change in a large range. The typical output voltage and load range of this reference design basis (smps1) are:
+5 V: 0 to 2.5 A
+12 V: 0 to 4 A
In order to improve the overall cross voltage regulation performance, 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 frequency of switching power supply works in the audio range. Therefore, according to the different transformer structure and mechanical design, there may be some audible noise. The most sensitive frequency range for most people is 7 to 13 kHz. This special reference design is applied to the rated standby load of 50 to 75 MW, so the standby frequency is less than 5 kHz.
This reference design provides sufficient flexibility to cooperate with a variety of output configurations, and only needs to make simple adjustments to the BOM. The ncp1351b flyback design can flexibly support up to 4 unique voltage outputs. The standard configuration (smps1) used in this reference design has 5 VDC and 12 VDC inputs and 24 VDC voltage outputs. Table 2 lists a variety of optional configurations that can be used with different power mechanisms.
Table 2: standard and optional output voltage configuration of flyback converter section
High voltage backlight inverter power supply section
1) Comparison of half bridge and full bridge topology
High voltage inverter can be realized by half bridge or full bridge topology. There are many factors to consider when deciding which topology to adopt. Compared with the half bridge structure, the full bridge topology has many advantages, such as zero voltage switching (ZVS) at fixed operating frequency, reducing EMI and power loss, reducing MOSFET switching stress and reducing heat dissipation. In addition, in the full bridge topology, because the controller works at a fixed frequency, it is possible to synchronize the switching frequency with the video frequency to avoid any possible backlight subsystem interference affecting the video image.
2) Lx6503 backlight controller
This reference design adopts lx6503 backlight controller of MICROSEMI. Lx6503 is a high-performance CCFL controller designed for LCD TV and other multi lamp LCD display systems. It has been specially optimized and is a cost-effective solution for high-voltage inverter architecture. The controller provides a pair of Push-Pull PWM driving signals, and has sufficient ability to drive push-pull half bridge or full bridge CCFL inverter with the addition of simple external circuit.
3) CCFL drive and current balance
CCFL lamp start-up and current balance must be carefully considered to have a reliable backlight system. The Jin balancer solution used in this reference design can provide excellent lamp current balance function. At the same time, it also combines frequency sweeping J start technology to ensure reliable lamp start. Based on the electromagnetic coupling principle of balance transformer, Jin balancer generates additional correction voltage to the lamp, so as to balance the lamp current. The basic configuration of this balancer network is shown in Figure 1. The serial loop of the secondary winding of the balancer equalizes the primary terminal current and provides a coupling mechanism between the lamp circuits. With such a coupling mechanism, if one of the lamps is not started, the energy of the started lamp will be automatically coupled to the primary winding of the non started lamp circuit, so as to increase the lamp voltage and help it start. As shown in Figure 1, the winding configuration of the balancer network is consistent regardless of the number of lights. In addition, one type of balancer transformer can accommodate almost all lamp sizes. These characteristics make Jin balancer solution very flexible for CCFL inverter applications.
Figure 1: basic configuration and waveform diagram of Jin balancer network.
The typical configuration of the 32 inch LCD TV backlight subsystem is:
12 lights
All lights are connected together to a common ground
The system current is sensed on the ground
All lamps are driven by single output high voltage transformer "in phase"
Overall energy efficiency performance and application advantages
The focus of this reference design is to provide excellent parameter performance with an energy-efficient architecture, which has low working loss in all power conversion sections. Some typical performance data are introduced in Table 3 below, in which the loads of flyback and PFC section are the loads under different test load conditions. The energy efficiency of the inverter is an estimated value, because it is very difficult to accurately measure the output power directly on the high-voltage lamp, and the accuracy is not high enough. Under typical load conditions, the energy efficiency of PFC over the full line input range is higher than 95%, and the peak energy efficiency of flyback converter under 37 w typical load configuration is 78%. This value is quite good considering the additional losses on the 5 V and 12 V outputs depending on the cross regulator technology used. The energy efficiency of the inverter is optimized, which is due to the full bridge zero voltage switching topology to minimize the switching loss. One argument supporting it is the fact that full bridge MOSFETs use surface mount DPAK devices without any additional heat sink.
Table 3: overall energy efficiency performance of ansenmey semiconductor high voltage lips LCD TV power supply reference design.
In general, the reference design of ansenmey semiconductor's 32 inch high-voltage lips LCD TV power supply adopts an optimized architecture, which has high energy efficiency and very low standby energy consumption, and meets the working energy efficiency, maximum working energy efficiency and standby energy consumption requirements of various energy efficiency specifications. The functional block diagram of this reference design is shown in Figure 2:
Figure 2: functional block diagram of reference design of ansenmey semiconductor 32 inch high voltage lips LCD TV power supply
Summary:
Ansenmey semiconductor's Greenpoint? The reference design supports the emerging high-voltage lips architecture, which directly supplies power to the inverter from the PFC section rather than the traditional 24 V, thus saving a complete power conversion part from the PFC section to 24 v. In addition, the reference design adopts the NCP1351 flyback controller with energy-efficient proprietary architecture, which eliminates the requirements and overhead related to the special standby power supply, thus further simplifying the solution. The architecture shown in this reference design is highly flexible and supports a variety of voltage / current configurations with minimal changes to the schematic and components used. Finally, due to the use of an advanced backlight controller with a zero voltage switching full bridge topology, the inverter power supply can be easily expanded to support a variety of LCD TV sizes up to 42 inches. Customers can shorten the development cycle and speed up the product launch process by using this energy-efficient advanced reference design“
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