25W mini ballast designed for small fluorescent lamps

Until today, fluorescent lamps are the cheapest way to produce white light with the least amount of power (lumens/watt). Today, hundreds of millions of compact fluorescent lamps are sold each year, and the demands on their reliability continue to increase. Today's lighting systems require ballast controls to drive compact fluorescent lamps, but this increases cost and design time, and these ballast controls must be re-adjusted for each different type of fluorescent lamp.

Therefore, design engineers need a solution that integrates all control functions so that they can focus more on the output stage design of the lamp and shorten the time to market. This article describes how to design a 25W small fluorescent lamp ballast using a single IC that integrates multiple control functions. It also discusses the output stage design, the selection of programmable devices, the schematic diagram, the ballast measurement waveform, and the comparison of simulation results and measured results.

The output stage of the lamp can be designed using a simplified model based on a standard resonant circuit topology (Figure 1). The lamp requires current to preheat the filament and high voltage to ignite the filament within a given time before it can start working. These requirements can be met by selecting the appropriate inductor and capacitor and changing the input voltage frequency. To preheat and ignite the filament, the lamp is not conducting at this time, and the circuit is in the form of an inductor-capacitor series connection. After the lamp is lit, the lamp is in the conducting state, and the circuit becomes an inductor parallel resistor-capacitor string.

The output stage operating points in the three states of lamp preheating, lighting and normal lighting can be obtained according to the transfer function of the circuit (Figure 2). The frequency decreases smoothly from the starting frequency to the final operating frequency within the determined preheating time. During the frequency decrease process, the filament is preheated, and the voltage across the lamp increases as the frequency approaches the resonance point of the high-Q LC circuit. When the voltage is high enough, the lamp is lit and the operating point is transferred to the low-Q LC curve. The frequency continues to decrease and finally reaches the operating frequency.

Figure 1: The simplified model of a lamp output stage is a basic RLC circuit.

The transfer function of a high-Q series LC circuit is:

Where Vin _is the amplitude of the input square wave voltage (V), _Vign is the amplitude of the filament lighting voltage (V), L is equal to the output stage inductance (H), C is equal to the output stage capacitance (F), and fign _is the frequency when the lamp is lit (Hz). According to formula 1, the expression of _fign is :

Equation 2 gives the frequency of the operating point on the high-Q LC transfer curve when the lamp is lit. It should be noted that the fundamental frequency of the input square wave (V _in ) is used for linear analysis. Once the lamp is lit, its resistance can no longer be ignored and the system becomes a low-Q series-parallel RCL circuit with the following transfer function:

From Equation 3, we can get the operating frequency on the low-Q RLC transmission curve:

Here R is the resistance of the fluorescent lamp which is determined by the operating power and voltage of the fluorescent lamp.

Among them, P _run is equal to the operating power (watts), and V _run is the operating voltage of the lamp tube (volts). Finally, the starting frequency operating point on the high-Q value LC curve is obtained based on the maximum frequency of the VCO of the IC used.

Using these equations and the parameters of the fluorescent lamp and ballast, the fluorescent lamp output stage can be designed. The parameters of a 25W CFL fluorescent lamp and ballast (powered by a 230V AC power supply) are: V _in =280V, V _ign =380V (peak), P _run =25W, V _run =175V (peak), f _run =45kHz.

Select C=6.8nF and use Formula 4 to gradually increase L from 0.1mH until the expected operating frequency is obtained. After determining L and C, use Formula 2 and Formula 6 to calculate the lighting and starting frequencies. The IC used is IR2520, whose frequency can be swept from the starting frequency to the operating frequency (Figure 3). To ensure that the fluorescent lamp can be lit normally, the lighting frequency must be greater than the operating frequency. The above design method can determine the inductance value to be 2.3mH.

Figure 2: These curves describe the transfer function of the output stage at different operating points.

Ballast design

This article designs a 25W mini ballast demonstration board and tests its performance. The input stage is designed for 230V AC power supply, and the ballast control IC is used to program the frequency and preheating time, perform frequency sweeps, and drive high-side and low-side half-bridge MOSFETs. The IC also provides a light-off reset function and protection functions in the event of inrush failure, non-zero voltage on/off, open filament, and lamp removal.

Use the above design method to calculate L, C and the frequency of the lamp output stage, and select the programmable IC based on the calculation results (Figure 4). Select the inductance and capacitance as 2.3mH and 6.8nF respectively, substitute the capacitance, inductance and calculated operating frequency into the following formula to calculate the input parameters of the programmable IC:

Where RFMIN sets the desired operating frequency (f _run ), C _VCO sets the desired preheat/ignition time (t _ph ), and components R _SUPPLY , C _VCC , D _CP1 , D _CP2 , and C _SNUB are used to provide the supply voltage for the IC. Initially, as the AC voltage increases, R _SUPPLY charges capacitor C _VCC until the voltage rises to equal the IC's internal turn-on voltage. Before turn-on, the IC draws only a few microamps of current, so R _SUPPLY can be high to minimize power consumption. When VCC exceeds the IC's internal turn-on voltage, the gate-driven outputs LO and HO begin to oscillate at a frequency equal to the starting frequency and a duty cycle of 50%. The charge pump supply formed by C _SNUB , D _CP1 , and D _CP2 is the main power supply for the IC, which keeps VCC at the internal clamping voltage level of 15.6V. Capacitor C _SNUB also provides buffering at the half-bridge output to increase rise and fall times, thereby reducing electromagnetic radiation (EMI).

Figure 3: IR2520D lamp control sequence as voltage (top) and frequency (bottom) vary.

Capacitor CDC provides DC isolation for the oscillating circuit to maintain the AC operating current and voltage of the fluorescent lamp. This can prevent the migration of mercury in the fluorescent lamp, which will cause the ends of the lamp to blacken and shorten the life of the lamp. Test the test board and compare the test values ​​with the expected simulation values. During the startup and lighting process, the voltage and current waveforms are sinusoidal.

The deviation between the measured frequency and the expected frequency is less than 5%, while other types of lamps and component configurations can produce deviations of up to 10%. Such deviations are expected because the above design method ignores harmonics, nonlinear resistance, filament resistance, inductor losses and component tolerances. Therefore, the components need to be screened again.

In the process of building a fully functional mini ballast reference design using the above method, all factors such as temperature, lamp life, performance margin, packaging, layout, manufacturability and cost are considered. The above method has shown good results in predicting the operating point of the lamp for a variety of different geometries (streamlined and compact) and different powers (various powers).

This design method can not only significantly shorten the time of designing ballasts for different types of lamps on the market, but it is also an effective tool for optimizing ballast size and cost. In addition, this method can also help reduce the number of ballast product families, thereby improving manufacturability.

Figure 4: Circuit diagram of a 25W mini ballast demonstration board with 230V AC input.