Efficiency and better quality of lighting

Efficiency and better quality of lighting


ELECTORONIC ballast is key elements in the search for better efficiency and better quality of lighting [1]-[4]. The substitution of the conventional ballast for electronic ballast has come to confirm that these are effectively an efficient way of saving energy in lighting systems. Electronic ballast with additional control circuitry can provide dimming capability [5]-[9]. This important feature allows the ballast with this type of specification have wide acceptance in diverse residential, commercial, and industrial applications. Except in the very low lamp power region, the ON-state lamp voltage does not change significantly, and the lamp power can thus be varied by controlling the lamp current.

During the electronic ballast designing process, important factors like high power factor, low total harmonic distortion, low electromagnetic interference (EMI), and low lamp current crest factor and low flickering must be considered. In general, there are three controlled parameters that can be used to adjust lamp current: the dc-link voltage, the switching frequency, or the duty ratio of the inverter [9], .

We can provide a smooth and desirable dimmer control for fluorescent-lamp systems using an additional power stage to provide variable dc voltage. It uses constant duty cycle and frequency for the switching of the half-bridge inverter, thus ensuring a wide power range of continuous inductor current operation for soft-switching operation [12]. Due to the cost penalty of an additional power stage and to the difficulty in achieving good lamp soft starting .

Vary in the duty ratio of the switches allows one to control the pulse width of the inverter, which means controlling its rms value and consequently providing a variable rms current through the lamp. However, this results in asymmetrical lamp current waveform, which causes the premature aging of the lamp which is considered as a severe drawback [13], [14]. Furthermore, if the duty cycle is too small, then the inductor current becomes discontinuous, zero voltage switching (ZVS) conditions will be lost, and the switches will suffer high switching stress due to the high value of the dc-link voltage. This could lead to reduced reliability and increased EMI emission. The practical minimum duty cycle also restricts the dimming range of the fluorescent lamps. Therefore, duty-cycle control is not used in commercial electronic ballasts.

Among these control methods, the frequency control has a wide dimming range with a simple circuit configuration and, thus, is popularly adopted in practical implementation. If the switching frequency of the inverter is increased, the inductor's impedance, in series with the lamp, is also increased, and therefore, the inductor current can be controlled, which in turn means controlling lamp power [10].

2. Dynamic Fluorescent-Lamp Model

Model Theory

The model that was chosen for implementation consists in a simple equation capable of describing the electrical characteristics of the lamp at high frequency. The fluorescent-lamp model presented is based on equation (1) which represents a curve fitting to the experimental data of equivalent resistance versus average power, using only monotonically decreasing functions based on exponentials [11].

Considering the future implementation of the model in Matlab/Simulink, a simple adaptation can be made if, instead of considering the lamp resistance, we consider the lamp conductance. So, for dynamic studies lamp voltage and lamp current can be related as follows

Fig. 1represents the curve fitting for the Rlamp-PI vs the average lamp power, PAVG. This model implies a monotonically decreasing lamp equivalent resistance with a maximum value at zero

Supposing Vdc constant, if the quality factor of the load is sufficiently high, the current through the resonant circuit is sinusoidal and the currents through the switches are half-wave sinusoids. The voltages across the switches are square waves [15]. If the fluorescent lamp is off, it behaves as an open circuit, presenting almost infinite impedance.

If the fluorescent lamp is on, its impedance presents finite values. Fig. 2 represents the equivalent simplified circuit of the series resonant parallel-loaded ballast for the lamp off state.

4. Analysis ofthe Control Method

4.1 Lamp Characteristics and Modeling

Some authors have used a fixed resistance model for simulating fluorescent-lamp systems with electronic control gear [6], [11]. This is one of the simplest ways to model the lamp, but it can only be used to design electronic control gear at specific conditions, for example, at nominal power. In fact, in the normal operating range, the effective resistance of a fluorescent lamp Rlamp decreases with increasing lamp current. Therefore, specific models which vary the lamp resistance with the average lamp power are, in fact, the most suitable for the majority of studies. The selected lamp is a TLD 36 W from Philips, but the obtained results would be similar for any other manufacturer. The lamp was tested at high frequency, using frequency-controlled electronic ballast. For this type of ballasts, different operating frequencies correspond to different levels of average lamp power. For each level, the rms values of the lamp voltage, arc current and resonant current were measured, as well as the average arc power. The resulting function will be used later on in order to obtain the dimming characteristics of the proposed method.

Resonant tank is the most common topology used as high-frequency inverter for electronic ballasts. With this topology, safe operation for the lamp can be provided Even so, a comparative analysis between some other resonant tank topologies, considering this specific dimming method.

The LC topologies, operating close to the natural frequency, behave as a current source to the load. This behavior is very adequate to supply discharge lamps because it ensures good discharge stability, keeping the lamp transitory power fluctuations. The series capacitance into the resonant tank, this allows one to limit the ignition voltage by means of avoiding immediate damage of lamp electrodes. The dc-blocking capacitance is high enough so that its ac voltage ripple is negligible, avoiding dc current flow through the lamp.

The dimming characteristics of the proposed control method should provide lamp power as a function of the resonant inductance, which is the control parameter. In order to use the real behavior of the fluorescent lamp, the characteristics of the lamp resistance versus average lamp power must also be considered in the deduction of the dimming characteristics.

In the LC resonant tank case, the roots of must be found, considering a specific variable inductance range. This range is chosen considering that the inductance should have a variation of 2:1, with the minimum value being the one calculated considering that the lamp is working at nominal power, for a resonant frequency equal to the switching frequency of 50 Hz [6].

The values for the LC tank parameters can be calculated, resonant tank parameters for an operating frequency.

Table: 1

Simulink model parameters for fluorescent-lamp

Lamp Model

Electronic ballast

a=8147; b=-0.2113;

c=1433; d=-0.05353

Vdc=150 V

Time constant = 3*10-4

L=2.2358*10-3 H


R= 20 ohms

L1=50*10-3 H


5. Matlab-Simulink Implementation

5.1 Model Description

The main objective is to try to simulate the behavior of a fluorescent lighting system with wide dimming range similar to the one obtained in the laboratory. Fig.3 shows the schematic of the electronic ballast that was implemented. Fig. 3 a) shows the schematic of the electronic ballast that was implemented [11].

The electronic ballast parameters and the fluorescent-lamp model parameters are shown in Table 1. The fluorescent-lamp model was implemented in Matlab/Simulink as shown in Fig.3 (b). Lamp current and lamp voltage are sensed and multiplied. The resulting Instantaneous power is then filtered in order to estimate the low-pass filtered lamp power. The time constant of the filter is related to the ionization constant of the arc discharge. Lamp current is generated by a controlled current source, controlled by the results of equation (2), and obtained using Simulink blocks.

The fluorescent-lamp model is shown in Fig. 3(b). The electronic ballast parameters and the fluorescent-lamp model Parameters are shown in Table 1. The schematic of the electronic ballast circuit, for the LC tank, is shown in Fig.4; the scope block displays its input with respect to simulation time. It displays the lamp voltage and the lamp arc current, the filtered lamp arc power, and the inverter voltage waveforms. The rms values of the lamp voltage, lamp arc current, and finally, the lamp arc average power are also displayed in the table 2(a) &2(b)

Arc current and lamp voltage are sensed and multiplied. The resulting instantaneous power is then filtered in order to estimate the low-pass filtered lamp arc power. The time constant of the filter is related to the ionization constant of the arc discharge. With this filtered lamp arc power, a value for the lamp arc resistance is obtained. Lamp arc current is then generated by a controlled current source, controlled by the multiplication of lamp voltage and lamp conductance.

5.2 Simulation Results

Lamp current and lamp voltage waveforms 5(a) &5(b) obtained with simulation results at different dimming levels. The simulation results for lamp current and lamp voltage waveforms, considering the lamp model, for the same dimming levels.

We can observe that the lamp voltage is slightly different form a sine wave, showing a tendency to a triangular-like form. So it is natural to observe some discrepancies between with tank and without tank results, particularly on the rms values of lamp voltage and lamp current.

RMS Values

Waveform Values

Iarc = 1.68 e-006 A

Input voltage = 150V

Vlamp= 0.0161 V

Lamp current = 2.4*10-6 A


Lamp voltage = 0.0228 V

Tabel.2: a) With tank output (50Hz)

RMS Values

Waveform Values

Iarc = 2.138 e-006 A

Input voltage = 150V

Vlamp= 0.02048 V

Lamp current = 3.023*10-6 A


Lamp voltage = 0.0313 V

Tabel.2: b) Without tank output (50Hz)

As the lamp power level decreases, the lamp voltage waveform becomes increasingly sinusoidal, which it is observed from both with tank and without tank simulation results 5(a) &5(b).

The lamp current waveform 5(a) &5(b) shows a different behavior, with a tendency to flatten the peaks as the lamp power level decreases. As can be seen from waveforms, only the lamp voltage waveform can be considered as a sine wave. The lamp current waveform also shows asymmetric behavior during the switching period, which in turn reflects the different aging effect of the electrodes.


This paper has presented for a fluorescent-lamp dimming range using resonant tank. Considering LC resonant tank circuits applied to the high-frequency ballast resonant inverter. Theoretical predictions were performed using Matlab-Simulink tools. These predictions were verified with the experimental results for a TLD 36W. Analysis showed that instabilities appear when the lamp power is decreased below a minimum value. This effectively limits the dimming range of the ballast. However, if the switching frequency and dimming range are adequately chosen and if the resonant tank is selected and designed properly, this dimming method is able to provide a linear and smooth control of lamp power.

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