# Implementation of a Series Resonant Inverter to Improve Fluorescent Lamp Efficiency/Implementation de un Inversor Resonante en Serie para Mejorar la Eficiencia en Lamparas Fluorescentes.

1. IntroductionIn recent years, fluorescent lamps are used in a high percentage, due to the demand of artificial lighting in the world it has been increasing [1] [2] [3] [4].

Bad or imperfect switching is the main cause of power loss in converters; this is due to losses in the switching transition where the voltage and current are nonzero. With increasing switching frequency these transitions occur more often increasing the average power loss in the device. On the other hand the high switching frequencies are desirable, with this parameter filtering components and transformers are reduced, which in turn reduces the size and weight of converters.

Resonant circuits occurs when the voltage switching is zero (ZVS) and the current switching is zero (ZCS), avoiding simultaneous transitions of voltage and current eliminating switching losses, this type of switching is known as soft switching [1]. When working with fluorescent lamps as load, the electronic ballasts are implemented with resonant converters in order to achieve ignition voltage and used another converter to achieve its steady state voltage [2] [3] [5]. One of the topologies used for electronic ballasts is the resonant inverter series-parallel (SPRI) which combines different topologies such as class E resonant, half-bridge and full-bridge configuration, within these configurations the most used is the topology SPRI half-bridge due to the reduction of costs in manufacturing [6] [7] [8], Figure 1 shows a typical configuration. This inverter circuit provides the voltage ignition and steady state of the lamp, however during steady state operation, the current through capacitor connected to the electrodes is very high and does not contribute to the power absorbed by the lamp, which means a low efficiency of the ballast.

This work shows in detail a circuit SRI designed to reduce power losses and improve the efficiency of electronic ballast using the first harmonic approximation, additionally some aspects of design are related in order to facilitate its implementation.

2. Resistance model lamp

The lamp can be approximated to a resistor that changes with light intensity, for example the RMS voltage of lamp increases almost linearly when the power of the lamp decreases, therefore the lamp behaves as a negative resistance, equally the equivalent resistance of the lamp is in function of its power [9] [10] [11]. Figure 2 shows the equivalent resistance of the lamp decreases as the power increases and increases when the power decreases.

Equations (1) and (2) show a lamp fluorescent model to calculate the equivalent resistive where the parameters a and b can be determined experimentally with the measured data of the lamp [12] [13].

[V.sub.la]([P.sub.la]) = a + b[P.sub.la] (1)

[R.sub.la]([P.sub.la]) = [V.sub.la][([P.sub.la]).sup.2]/[P.sub.la] (2)

[mathematical expression not reproducible] (3)

3. Inverter resonant series SRI

In this converter a capacitor and an inductor are connected in series to a load or equivalent resistance of the lamp [R.sub.la]. It is a DC-AC converter in half-bridge configuration, as shown in Figure 3a the function of this converter is supplying the steady state current of the lamp, Q1 and Q2 are switched alternately ,then a square wave is produced at the midpoint of them. A capacitor Cs and an inductance Ls are calculated so that the switching frequency equals the resonance frequency.

Figure 3b shows a representation a wave at the resonant frequency where the combination LC works as a filter where attenuation of harmonics reflect one sine wave with the same fundamental frequency of the square wave, this can be expressed as the following Fourier series:

[[upsilon].sub.y] = Vcc/2 + [[summation].sub.n=1,3,5, ...] 2Vcc/n[pi] sin (n[[omega].sub.s]t) (4)

The fundamental component of Eq. (1) is:

[[upsilon].sub.y1] = 2Vcc/[pi] sin([[omega].sub.s]t) Where [[omega].sub.s] = 2[pi][f.sub.s] = 2[pi]/[T.sub.s] (5)

To analyze the SRI you can use the approximation of first harmonic which has been studied in detail by several authors [5] [6], SRI proposed in this paper operates in continuous conduction mode (CCM) with a high Q which allows operation SRI through linear AC circuit, as illustrated in Figure 4.

The normalized effective voltage of the lamp depends of Q on the design chosen and it analyzes considering the reflected values on the Figure 5, this parameter being a determining factor in the design of SRI.

This voltage is defined by the following equation:

V = [V.sub.la] rms/[V.sub.y1] rms = 1/[square root of [((F - [F.sup.-1])Q).sup.2] + 1] (6)

Where the normalized frequency (F) and quality factor (Q) also known as normalized load are given by:

F = [[omega].sub.s]/[[omega].sub.o] = [[omega].sub.s] [square root of [L.sub.s][C.sub.s]] (7)

Q = [Z.sub.s]/[R.sub.la] = [square root of [L.sub.s]/[C.sub.s]] 1/[R.sub.la] (8)

Other parameters that must be present for the design of SRI are:

First, the current LC tank circuit ([i.sub.L]) which is equal to the current of the lamp ([i.sub.la]), and flows through the Mosfet in the SRI, can be obtained considering (3).

Second, the angle ([alpha]) that defines the type of switching in Mosfet of SRI is part of the current LC tank circuit and is calculated by the following equation:

[alpha] = -arctan((F - [F.sup.-1])Q) (9)

As seen in Figures 6 and 7 the angle ([alpha]) relates current and voltage to consider in the switching modes.

4. Implementation of SRI, SPRI and the ignition circuit

To provide the ignition voltage of the lamp a converter DC-AC is designed in configuration full-bridge as is shows in Figure 8 is powered by a source Vcc and is composed of: Four MOSFETs [Q.sub.3], [Q.sub.4], [Q.sub.5] and [Q.sub.6] alternately that ignite for SRI, working the same frequency fs, a transformer [T.sub.1] that providing galvanic isolation and increases the voltage to ignite the lamp and a capacitor [C.sub.x] which removes the DC component produced by the full bridge, to analyze efficiency parameters are measured for SRI and SPRI considering the condenser [C.sub.p] that shows in this circuit.

Experimental measurements of the SRI, SPRI and the ignition circuit were performed using a fluorescent tube of 32 W brand OSRAM to verify the theoretical analysis. In Figure 9a shown the waveform of the lamp current for a power (Pla) of 32 W, when SRI is used, you can see that waveform is almost sinusoidal, this means that the factor peak is approximately 0.41, which increases the life the lamp. For a power (Pla) of 28 W was measured the power of the source Vcc as it is shown in Figure 9b, first using the SRI the power was of 29.6 W, then with SPRI the power was 31.7 W, this means that SRI proposed has a better efficiency compared to traditional SPRI.

5. Conclusions

Experimental results developed on a fluorescent lamp 32W showed that SRI proposed improved efficiency by 6% compared to the classic SPRI, this is because the SRI does not use a parallel capacitor to the lamp, reducing losses in the cooper of inductance in the tank circuit LC, the proposed circuit SRI also shows a decrease in the switching or conduction losses on MOSFETs.

DOI: http://dx.doi.org/10.18180/tecciencia.2016.21.2

References

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Jhon Fredy Bayona Navarro (1) *, Jhon Edwin Vera Vera (2), Nancy Yaneth Gelvez Garcia (3)

(1) Universidad Sergio Arboleda, Bogota, Colombia

(2) Universidad ECCI, Bogota, Colombia

(3) Universidad Distrital Francisco Jose de Caldas, Bogota, Colombia

Received: 23 May 2016

Accepted: 22 Jul 2016

Available Online: 19 Aug 2016

* Corresponding Author.

E-mail: jhon.bayona@usa.edu.co

Caption: Figure 1 Resonant inverter SPRI.

Caption: Figure 2 Lamp resistance vs. power.

Caption: Figure 3 Proposed circuit and switching signal, a) SRI half-bridge., b) Waveform of [v.sub.y] y [v.sub.y1].

Caption: Figure 4 Equivalent circuit AC.

Caption: Figure 5 Normalized lamp voltages.

Caption: Figure 6 Waveform [v.sub.y1] and [i.sub.L].

Caption: Figure 8 Proposed prototypes.

Caption: Figure 9 Waveforms performed in the implemented circuit a. Waveform current with SRI b. Power measurement at source 28 W.