Portable equipment for efficient application CCFL Backlight converters

The portable device of backlight, usually in buck converters followed by a push-pull converter (Royer inverter), but its efficiency is low, the number of devices.

This article discusses an approach based on piezoelectric transformer power supply of high-efficiency backlight solution, using UCC3977 (push-pull controller) and piezoelectric transformer EFTU11RoMX50 and EFTU18R0Mx50.

Recently, PDA, Internet handhelds and notebooks and other portable devices development is very fast, which further enhances the market for small cold cathode fluorescent lamp (CCFL) backlight converters demand.

Typically, these applications use a buck converter is followed by a push-pull converter (Royer inverter) approach, but in portable applications in this approach is inefficient, and because it is essentially a two-stage scheme that uses the component count and more.

Traditionally, the need of high CCFL operating voltage and high voltage is excited by the high turns than wirewound magnetic transformers.

Piezoelectric transformer (PZT) of the latest application for backlight provides many potential advantages, including more efficient, smaller, electromagnetic noise is lower, the excitation voltage higher, non-combustible and sine run, etc.

This article discusses an approach based on piezoelectric transformer power supply of high-efficiency backlight solution that employs UCC3977 (push-pull controller) and Panasonic piezoelectric transformer EFTU11RoMX50 and EFTU18R0Mx50.

Backlight lamp

CCFL usually as laptops and portable electronic device color liquid crystal display (LCD) with background light, CCFL is by far the most efficient monitor background lighting.

As a result of running the CCFL need high AC voltage, so you will need high voltage DC/AC inverter. Pzt inverter with sinusoidal output voltage can be provided, this very helps reduce RF radiation. RF radiation may interfere with other electronic devices, and may reduce overall system efficiency. In addition, the inverter based PZT by sinusoidal excitation can also produce in CCFL optimum current into the light-converting efficiency. CCFL's work voltage voltage than other light sources is much higher, normally need 300 to 800 volts AC (depending on the length of the lamp), but the overall power consumption is very low.

Recently a very bright white light emission diode (LED).

In the LCD monitors, cell phones and PDA application, it is a strong CCFL competitors. However, when using white LEDs as the monitor's backlight requires attention to the following questions:

A. nearly all LED manufacturers to new candle (cd) of the light intensity.

Typically this value is very high, but it represents the brightest direction data. Angle and brightness of the LCD is usually inversely proportional to the total because the light output of LCD cover a larger area, in any given direction, angle and wider, the lower the brightness. The same luminous flux output is usually a series of LED by different cross-sections and light intensity. At 20 ° angle of light intensity may reach 6.4cd, 70 degrees viewing angle possible 0.48cd and 160 degrees viewing angle light intensity may almost to zero. Due to the lighting consistency on comfort and Visual performance is important, therefore, to provide a consistent on the monitor, you need multiple LCD in-line or parallel mode.

B. These white LED's life is short.

In the run for approximately 1000 hours later, they will turn yellow and somewhat darker, the color display is not acceptable. Of course, in more than the rated current conditions LED to run will accelerate this process.

C. because of the LED drive current is basically, if the LED is not good to share current, may cause display partial lighting does not balance.

In this case, you can each LED is combined with a balanced resistance, but this will reduce overall system efficiency.

D. There is also a major disadvantage is the high cost, the bulk price starts at $ per 2.3, than the same brightness CCFL expensive alone.

When you use these as photosource, you need to consider is that you want to use it to do.

If you care about is the light of point brightness, you should consider peak intensity; if you need to light diffusion effect, select the appropriate output section is very important; the need for larger display area provides lighting or background lighting of the LCD, and the need to have a long service life of lamps, small CCFL is preferred.

Used for background light converter transformers

Magnetic transformers and the development of piezoelectric transformer so that we can construct a high efficiency, small size converter.

Transformer selection depends on many factors, including cost, volume and efficiency. For example, compared with the PZT transformer, in a given power level, magnetic transformers may be thicker, heavier and less efficient, but it has a low cost, but also in the wider load conditions. PZT transformer has the inherent characteristics of the sine, high excitation voltage, non-combustible and non-electromagnetic noise advantages. Magnetic transformers and comparison of piezoelectric transformer as shown in table 1 below.

Ceramic piezoelectric transformer is in 1956 by C.A.

Rosen was first proposed. And dependent electromagnetic energy conversion of magnetic transformers, piezoelectric transformer converts the mechanical energy to EMF. Electrical energy to mechanical energy conversion is called inverse piezoelectric effect, and the conversion of mechanical energy into electrical energy is called as the piezoelectric effect.

PZT transformer of materials and technology determine their performance characteristics, each manufacturer has their unique materials and structure of the "formula", manufacturing of common materials are PZT zirconate and lead titanate.

Single-layer PZT lower-cost and easy-to-manufacturing but smaller voltage gain (typical value is 5 ~ 10), and may require a step-up transformer for the lamps and magnetic. Multiple layers of PZT design and manufacturing is more difficult, but with higher voltage gain (20 ~ 70).

Figure 1 a typical power supply for CCFL long mode PZT.

The piezoelectric transformer contains some for energy conversion of rectangular Piezoelectric ceramic layer with a beginner electrode (to enter) and a secondary electrode (for output). Input to the primary electrode signal to piezoelectric means converted to mechanical shock, mechanical shock delivered to ceramic layer of secondary, where the mechanical vibration to piezoelectric means converted to a high level of output. The entire conversion process consumes little energy.

To predict the PZT in system performance, it is necessary to establish its circuit model.

Figure 1 shows the circuit model is often used to describe long mode PZT in basic resonance frequency of performance in the vicinity. Many PZT manufacturers based on the various frequency and output load measurement results for the model element value, specific symbol value depends on the structure of PZT. Basic electrode materials for multilayer structure and dielectric constant form a large master-level input capacitance (Cinput). Due to the secondary structure of single-layer and master level electrode and the distance between the electrodes, the output capacitor to small lot.

Figure 3 shows the Panasonic 1.8W PZT (component model for EFTU11R8MX50) gain (Voutput/in) relative to the output load and frequency characteristic curve.

The PZT on Figure 2 equivalent circuit of the equivalent element values are: Cinput = 61.6nF, Coutput = 11.4pF, n = 35, RLC = (0.66 Ω, 0.934mH, 2.79nF). As shown in Figure 3, in the absence of load conditions ceramic transformer provides high Q value and gain, and produces high-excitation voltage. Once the fluorescent lamp is fired, the transformer with the load. Load caused by the decline of transformer gain and resonant frequency. To achieve in a unidirectional control circuit under excitation and run fluorescent lamps, usually working piezoelectric transformer at the right side of the resonant peaks.

Inverter control system

Figure 4 shows a background based on PZT simplifications optical converter.

PZT by a rate proportional to input voltage of resonant power-driven, it offers the driver a fluorescent lamp voltage gain needed. Around the error amplifier form a control loop, the average error amplifier with fluorescent lamp current reference signal (REF), so that the light intensity of the fluorescent lamps. Control voltage Vc driver used to determine the resonant power level of the operating frequency of the voltage-controlled oscillator (VCO).

VCO frequency range must include the stimulation of PZT and run frequently.

Reduce the programmable frequency range can improve the feedback loop control response. For example, in Figure 5 the PZT frequency range 100kHz. In order to ensure the control circuit has been on the right side of PZT resonant peaks, PZT gain must ensure minimum input voltage still has enough high voltage of fluorescent lamps.

Power supply topologies

A resonant-pull topology control of piezoelectric transformer electric circuit as shown in Figure 5.

This topology uses two standard inductance (L1 and L2), through UCC3977 controller and MOSFET S1 and S2, the two inductors at 50% duty cycle produces 180 degree phase shift. The push-pull circuit is available from the DC input voltage to the piezoelectric transformer primary voltage gain, and through these two primary inductance and capacitance between the PZT LC resonant relationship.

And magnetic transformers circuit, circuit based on frequency of use of PZT instead of duty to control fluorescent lamp current.

UCC3977 contains a COMP and OSC pin programmable formed between VCO, the VCO is used to set the system's operating frequency range (it must contain the stimulation of PZT and frequency), fluorescent lamp current can be measured in FB pin and PZT gain/frequency characteristics for control (see Figure 3). In order to ensure the control circuit has been working in the right side of the resonant peaks, PZT gain must be minimum input voltage to provide a sufficiently high voltage fluorescent lamps.

MOSFET S1 and S2 to 50% duty cycle is driven to produce phase shift.

Inductors L1 and L2 and PZT primary capacitor resonance, S1 and S2 the drain of drain formed half-sine waveform. Junior in PZT at both ends of the sine voltage waveform close. Due to the ceramic transformer, high q value is the sine of fluorescent lamps, in this application is approximately 300V.

To achieve zero voltage switching, drain voltage switch must be in the next cycle before back to zero, which require LC resonance frequency than switching frequency.

Meet the criteria of maximum inductance can be obtained from the following formula:

Formula 1

Which Cp is the principal stage capacitor PZT.

With push-pull topology, rated power 1W and 1.8W respectively of PZT driver a 300-Volt fluorescent lamps, in current 3mA, lower input voltage range efficiency more than 87%.

High input voltage range, because the gain reduction resulting in PZT inefficiency.

To decrease in a linear fashion to fluorescent light adjustment of current results in inefficiency.

Due to the system in less than optimal gain conditions, piezoelectric transformer circuits of light load efficiency. Using pulse dimming technology can improve efficiency, higher than the naked eye cannot perceive frequencies (& gt; 100Hz) through the modulation duty cycle of on-off control of the average current fluorescent lamps, so that the fluorescent lamp has been working at full current.

Figure 6 shows based on piezoelectric transformer circuit pulse dimming waveform.

An external drive signal (curve 4) for the given work cycle and pulse train frequency, in this case, at 50% duty cycle pulse train frequency is 100Hz. Curve 1 is feedback network COMP pin signals used to set the operating frequency. Fluorescent lamp voltage as shown in the curve 3. These pictures are using digital Oscilloscopes have, so there is overlap. Fluorescent lamp excitation voltage almost not detected because the light has a heat and from previous pulse train cycle run over.

Summary: this article

This article gives a piezoelectric transformer for backlight converters as a step-up transformer use features, discussed can meet the needs of CCFL fluorescent lamps and high-pressure piezoelectric transformer works, a use UCC3977 PZT based high background light power supply scheme.

Because background light of PZT based converter's resonant power circuit provides sinusoidal voltage, thereby improving the efficiency of the fluorescent lamps-electronic brightness can be linear or pulse dimming technology. Overall efficiency (more than 86% can be achieved) improve prolong battery power.