Design of high-efficiency CFL driver based on UBA2211

Compared with traditional incandescent lamps, compact fluorescent lamps have the advantages of high luminous efficiency, good energy-saving effect, long life, small size and easy use. In terms of the market, compact fluorescent lamps have broad prospects. According to statistics from Datapoint Research, 20%-25% of the world's electricity is used for lighting. By 2012, the global sales of energy-saving bulbs will exceed 5 billion. At the same time, China's National Development and Reform Commission has issued relevant policies and is expected to ban (ban the sale of) incandescent lamps within 10 years. As incandescent lamps gradually withdraw from the lighting market, people are in urgent need of a new energy-saving lighting solution to meet the various requirements of consumers. Under this situation, CFL and LED will surely usher in a new lighting era. However, for ordinary consumers, due to its cost and lifespan, LED cannot be widely popularized in a short time. Under the huge market demand, it can be expected that CFL will dominate the current lighting market.

But how to maximize the advantages of fluorescent lamps? This leads to the concept of electronic ballasts. Early CFL electronic ballasts were mostly composed of separate components such as transformers and transistors. This driving method has many disadvantages. With the development of semiconductor technology, dedicated driver ICs have gradually emerged. This article takes the non-dimmable driver IC UBA2211 of NXP Semiconductors (NXP) as an example to introduce the working principle and circuit design of CFL driver ICs, and gives the test results.

CFL driver design

The typical CFL driver circuit composed of UBA2211 is shown in Figure 1. The ballast input voltage is AC 220V and the input power is 12W.

The entire system consists of three parts: rectifier filter circuit, control circuit and lamp resonant circuit.

The rectifier filter circuit consists of four diodes 1N4007, CBUF and LFILT. It can rectify and filter the 220V AC input into a DC voltage of about 310V (). The RFUSE at the input end is a surge suppression resistor, which is mainly used to suppress the surge current generated during startup. The control circuit is composed of UBA2211, which mainly realizes the control of the current during the preheating, ignition and stable operation of the filament. The resonant circuit is composed of CLA and LLA, which mainly provides high-voltage pulses for the filament ignition.

1. Internal structure of the chip

UBA2211 is the core device of the entire circuit. To facilitate the analysis in the following text, this article first introduces some structures and characteristics of UBA2211. UBA2211 is a CFL dedicated driver IC, which uses two packages: dual in-line package (DIP8) and 14-pin plastic small package (SO14). .

2. Startup phase

After power-on, the circuit enters the startup state, at which time the upper bridge HS is turned off and the lower bridge LS is turned on. The internal current source (1mA) charges the capacitor CVDD on VDD and the capacitor on the FS pin, and the internal circuit is reset. At this time, RC and SW will be grounded. Once VDD>VDD(start) (typically 11V), and the over-temperature protection (OTP) is not activated at this time, the circuit ends the startup state. Note: Once VDD

3. Preheating stage

When VDD>VDD(start) and OTP is not activated, the circuit enters the preheating state. In the preheating state, the VDD power supply is provided by the dvdt charge pump (see Figure 2). At this time, the capacitor on the SW pin is charged by Isweep, and the half-bridge circuit begins to oscillate. Its oscillation signal is shown in Figure 3. The initial oscillation frequency is 2.5 times the operating frequency, and it will gradually drop to the normal operating frequency during the preheating process. In the preheating state, the circuit preheats the filament to reduce the loss of the filament when it starts and extend the service life of the lamp. The external resistor RSENSE is used to detect the preheating current so that the peak voltage of RSENSE is equal to the internal reference voltage Vref(prht) (0.5V). The preheating time is set by the external capacitor CSW. Once the voltage on CSW drops to VSW(prht), the circuit ends the preheating, as shown in Figure 4.

4. Ignition stage

After the filament preheating is completed, the frequency begins to decrease. Once the frequency reaches the resonant frequency, series resonance occurs on the LC at both ends of the lamp, generating a high voltage of 600-1200V to break down the lamp tube and complete the ignition of the filament. Under normal operation, RMS control is activated. By controlling the RMS value of the filament current, the IC loss and IC temperature can be controlled at a constant value.

During the operation of the circuit, UBA2211 provides a dead time (tno in Figure 3) to prevent the upper and lower bridge power switches from turning on at the same time. UBA2211 also provides over-temperature protection (OTP), minimum glow discharge control under cold start, anti-saturation protection and capacitive mode protection.