A SINGLE STAGE ELECTRONIC BALLAST FAMILY FOR HIGH PRESSURE SODIUM LAMPS
Dos Reis, F. S., IEEE Member; Clima, J. C. M.; Tonkoski Jr., R., IEEE Student Member;
Maizonave, G. B., Ceccon, G. B., Bombardieri, A., Dos Reis, R. W.
Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, Brazil.
FENG – DEE – LEPUC
Abstract – In the recent years many authors [1-4] are working to obtain single stage HPF electronic ballast for fluorescent and HID lamps to achieve cost reduction and to comply with international standard requirements. Usually to obtain HPF in electronic ballast for high pressure sodium lamps a Power Factor Corrector (PFC) is used between the mains and the electronic ballast [5]. In this paper will be reported the study and implementation of two single stage high power factor (HPF) electronic ballasts for high pressure sodium (HPS) lamps using a LCC filter, one using a half-bridge inverter and the second one using a full-bridge inverter. The main idea in this work is to present two simple electronic ballasts topologies with HPF for HPS lamps working with a 220 VRMS mains voltage. Design criteria, simulation and experimental results will be also presented. These topologies present some drawbacks like moderate THD, lamp power limitation once the converter works as a Buck inverter and it is not an ideal PFC. PF around .95 are easily obtained. This paper intends to warn other researchers about these structures limitation, but may be an interesting option for some applications.
I. introduction
Nowadays, an important topic of awareness is the importance of environment preservation. In this direction, important efforts have been made in the diverse areas of knowledge. In electrical engineering field, this phenomenon has reflected in searching for alternatives energy systems, higher efficiency on available resources utilization, losses reduction in equipments and to increase power quality.
In the last few years the market was flooded by a great number of electronic ballasts for fluorescent lamps operating in high frequency, especially by compact fluorescent lamps. Its utilization was widely stimulated by Brazilian media for energy economy, due the fact that luminous efficiency increases with the frequency for this kind of lamp. Brazil faced a serious energy crisis in 2001. Many corrective actions were taken to mitigate this serious problem. One of them was the energy rationing which consisted in overtaxing or even cutting energy supply from consumers which exceeds the prefixed energy quotes. Also many electric energy concessionaires had distributed gratuitously compact fluorescent lamps for residential consumers, showing the importance of illumination’s segment inside the global energy consumption, estimated to be about thirty percent of total consumption of electrical energy in the country. Because of these, innumerable research groups around the world, like [1-8], have dedicated their efforts to the development of new topologies and new control techniques for different kinds of discharge lamps.
Most of magnetic ballast manufacturers had to develop electronic ballasts for discharge lamps to guarantee their survival in business because the consumers started to demand more and more this type of product. It also simplifies the production line, which has expressive physical reduction and productivity increase in relation the line that produces the conventional ballasts. Now, the challenges for industries are the reduction of production costs, the reduction of converter size, unitary power factor and null harmonic distortion which implies in a substantial improvement of energy quality consumed by ballasts. In Brazil, the development of electronic ballasts for HID lamps is being made by a few groups of researchers. However in a close future, these ballasts will be in the production lines of main national manufacturers.
In the power electronics group of PUCRS (LEPUC) an smart public illumination system was developed, where an electronic ballast with dimming capability is controlled by a central using SMS technology, this ballast, called Master, controls a whole neighborhood of slave ballasts thru a power line communication system. This system is presented on figure 1.
Figure 1. Smart Ilumination System proposed by LEPUC.
The purpose of this paper is to report the development of two low cost single stage HPF electronic ballasts for HPS lamps for a 220 VRMS mains voltage developed for utilization with this system. Each ballast was implemented using a different converter topology. The design criteria will be presented in this work for the proposed circuits.
There are many kind of high intensity discharge lamps; however, this work will focus only the high-pressure sodium lamps (HPS), widely used in public illumination. The HPS lamps radiate energy on a great part of the visible spectrum. Those lamps provide a reasonable color reproduction (it has IRC 23 color reproduction index). They are available up to 130 lm/W of luminous efficiency and color temperature of 2100 K, approximately.
The HPS lamps, as any other HID lamps, need ballast to operate correctly. The ballast is an additional equipment connected between the power line and the discharge lamp. The ballast has two main functions: to guarantee lamps ignition through the application of a high voltage pulse between the lamp electrodes and to limit the current that will circulate through it. The lamp would be quickly destroyed without current limitation, due the negative resistance characteristic of the lamp, as can be observed in figure 2.
The HPS lamps have many particularities when they operate in high frequency, such as:
- Can be modeled by a resistance in steady state;
- Can have luminous intensity controlled;
- The spectrum color reproduction can be modified;
- Presents the acoustic resonance phenomenon, which can result in the arc extinguishing until the lamp destruction;
Figure2. - Typical voltage versus current curve for HID lamps.
In order to achieve low cost electronic ballast for HPS lamps with HPF a single stage converter was conceived. The idea is very simple: Once, in high frequency, the HPS lamps have a resistive behavior, why the electronic ballast (inverter and LCC filter) can not be connected directly to the full bridge rectifier? This idea will be discussed in this paper.
In [9], it was studied the possibility of using this concept applied to a half bridge (HB) inverter. Unfortunately, in this arrangement the half bridge inverter reduces the available RMS lamp voltage and therefore, restricts the maximum output power about 70 W for HPS lamps. The full bridge-inverter was also explored by the same authors in [10], in order to increase the available RMS lamp voltage. The utilization of these topologies will be debated in this paper.
II. Studied Electronic Ballasts
Both studied single stage high power factor electronic ballast for high pressure sodium lamps structure incorporates a bridge rectifier and an input LC filter to minimize the EMI generated by the electronic ballast. Two inverters topologies were proposed to supply the HPS lamp, using this simple concept to avoid the utilization of an external PFC.
Figure 3 shows the electrical diagram of the first proposed circuit. In this circuit it was proposed the utilization of a half bridge inverter connected to the LCC filter. The capacitor CF in this figure has the main function of receive the reactive current from the electronic ballast. This arrangement provides high power factor to the electronic ballast because in this case the capacitor CF is not a bulk capacitor. Actually this is a small capacitor in the range of nano Faradays.
Fig. 3. Half Bridge HPF Electronic Ballast.
The second electronic ballast studied used a full bridge inverter topology as showed in figure 4. The input stage of this topology has the same characteristic as the topology presented on figure 3. The main difference between HB and FB inverters topologies is the available RMS voltage applied to the LCC filter; this crucial difference will affect the application of the topology.
Fig. 4. Full Bridge HPF Electronic Ballast.
III. Ballast Design EXAMPLE
To verify the performance of the proposed systems LCC two electronic ballasts, one for a 250 W HPS lamp and another for a 70 W HPS Lamp were designed. The nominal lamp voltage (Vlamp) was obtained from the lamp’s manufacturer datasheet and its value is 100 VRMS for the 250 W HPS Lamp and 70 VRMS for the 70W HPS Lamp. The electronic ballast input power voltage comes from the output of an input bridge rectifier; consequently, this input voltage is mains dependent. In the present design example the mains voltage adopted was considered to be equal to Vmains= 220 VRMS. The switching frequency chosen was 68 kHz. Assuming the resistive comportment of the lamp, we can estimate the value of its resistance (Rx), where x represents thenominal power of the lamp, after ignition using equation 1.
(1)
where P is the lamps power.
As it was indicated in [5], the best relationship between the switching frequency and the tank resonance frequency before the lamp turn on is 0/s = 3, guaranteeing the high voltage generation for the lamp ignition and limiting the peak current at the MOSFET to acceptable levels. If it was adopted to work at resonance 0=s in theory we would have the possibility of an infinite voltage generation over the lamp which could be good for a quickly lamp turn on. On the other hand current would also rise to infinite because the impedance of the circuit formed by L, CS and CP is null just before the lamp is turned on. This operation mode will result in the MOSFET’s and driver’s destruction.
Figure5. Simplified circuit for the LCC Ballast.
For the circuit showed in figure 5, in the case of a half bridge inverter, the voltage Ve is an asymmetrical wave (from 0 to Vpksin (t)V). In the case of a full bridge inverter, the voltage Ve is a symmetrical wave (from Vpksin (t) to - Vpksin (t)V). A good simplification to study the system behavior comes from the frequency domain approach. To use this approach the first harmonic component for this wave must be knew. Bum & Hee [5] presented the first harmonic peak amplitude for a half bridge inverter considering an ideal fixed DC bus voltage (E). In the present case, the first harmonic peak amplitude was obtained using the same method but the DC bus voltage (E) was replaced by the mains voltage resulting in expression (2) for a half bridge inverter and in expression (3) for a full bridge inverter. The experimental results validate this procedure:
(2)
(3)
If we compare the available RMS voltage for each topology, for systems with same mains voltages, the full bridge topology will generate a higher RMS voltage value than the half bridge inverter as it was expected. As it can be observed in equation 2, the half bridge topology will never generate the necessary voltage to achieve the lamp full rated power, witch impossibilities the design of the HB ballast for most of 250 W HPS lamps with a 220 VRMS mains, so the half bridge topology is indicated for low power HPS lamps, where the necessary lamp RMS voltage is lower than 100 VRMS. Hence the topology chosen for the 70 W HPS lamp was the half bridge (HB) one and for the 250 W HPS lamp was the Full Bridge (FB) one.
In this study an expression was obtained to determine the peak voltage across the capacitor CP. This expression, shown in equation (4), is valid before the lamp start up for the HB topology. For the FB topology the VCp would be twice the value obtained for the HB converter. In this design criteria sample the parameters would be only obtained for the HB topology, considering that for the full bridge topology there are no difference on this procedure, only some equations must be adequated for this case.
(4)
Where, RESR is the circuit equivalent series resistance and F is the switching frequency. Preliminary tests demonstrated that necessary peak voltage (Vopk) to guarantee the lamp ignition is about 3.8 kV. A typical RESR value is 6.5 Ω. Manipulating equation (4) maybe obtained the value of inductor L in equation (5) for the HB topology.
(5)
The resonance frequency may be calculated using equation (6).
(6)
Considering the fact that the switching frequency is estimated to be three times lesser then the resonance frequency and, usually, capacitor CP is, at least, 10 times smaller then capacitor CS, equation 6 may be simplified into equation 7, because the effect of the capacitance CS is almost null.
(7)
Manipulating equation (7), it can be obtained the value for the capacitor CP as it is shown in equation (8).
(8)
To determinate the real value of the RESR, an experimental circuit using a 220 µH inductor L and a 2,7 nF capacitor CP was stimulated with a 60 V peak-to-peak square wave signal, which generated a 660 V signal over the lamp terminals, allowing the determination of RESR using equation 4. This RESR was obtained experimentally and its value was 6.5 Ω. Before the lamp startup a leakage current flows into the lamp. To determine the equivalent lamp resistance before the startup, the following measurement was made: a 10 Ω resistor was placed in series with the lamp. The obtained equivalent lamp resistance was 100kΩ. If this resistance is taken to account a new RESR = 5.7 Ω could be easily obtained.
The reference [2] and our experimental results allow us to consider that after lamps ignition, the lamp resistance is too low considering the CP reactance. Therefore, it can be deduced the equation 9:
(9)
Consequently, after lamp ignition, the equivalent circuit is showed in figure 6.
Figure6. – Ballast equivalent circuit after ignition.
After lamp ignition the ballast must guaranty that RMS voltage over the lamp do not overcome the nominal value. The RMS lamp voltage Vlamp can be obtained using the well known voltage divider, equation 10 presents this result:
(10)
The modulus of the impedance of the circuit can be calculated with equation 10, and is presented in equation 11. To simplify the design of the LCC filter the parameterized LCSR circuit transfer function was obtained and the result is shown in figure 7.
(11)
Where Kc is the capacitor relationship factor defined as Kc = CS/CP, the relationship of switching frequency and resonance frequency of the circuit of figure 4, R is the lamp resistance after startup and τ is the parameterized time constant .
Figure 7 presents the relationship between the RMS lamp voltage and the RMS first harmonic voltage, Vlamp/V1stRMS, called as parameterized lamp voltage gainGLCC(GLCC=70/99 =0.707), for different values of Kc as design parameter. Using the graphic of figure 5, a Kc =1/32 was adopted. This relationship will allow achieving the desired GLCC relationship in the frequency where the lamp is turned on, hence after the start up the HID lamp is driven at rated power.
Figure 7. LCC transfer function for different values of Kc.
With the Kc relationship, the value of CS may be obtained using equation 11 for a GLCC=0.707.
(12)
IV. Experimental Results
Two prototypes, one using the HB topology and another using the FB topology for 70 W and 250 W HPS lamps respectively were built. The main components and parameters used in these implementations are shown in table 1.
Table 1. Main components value and design parameters.
FulL Bridge / Half BridgeVmains / 220 V / 220 V
Fs / 68 kHz / 68 kHz
L / 220 uH / 220 uH
Cp / 2,7 nF / 2,7 nF
Cs / 55 nF / 90 nF
Fmains / 60 Hz / 60 Hz
A conventional SMPS power line filter with differential and common mode mitigation paths was used. The EMI Filter topology used is presented on Figure 8 and very good results were obtained. Usually a capacitor between the line and the filter is used, in this project the input capacitor was suppressed once it will block the PLC communications between the other units as schematized in Fig. 1. The experimental results are presented for the FB topology, since for the half bridge topology the results obtained are similar.
Figure 8. Topology of the EMI Filter used.
In figure 9, voltage and current in the mains are shown for the FB ballast. It may be observed that the voltage and current in these waveforms have almost null displacement factor. The current waveform presented would be similar to a usual converter operating in the discontinuous conduction mode if no EMI filter was used. With the waveforms presented in figure 7, the power factor was calculated and the value found is PF=0.98, which would be in conformity with all known ballast standards.
Figure. 9. Voltage (200 V/Div.) and Current (2 A/Div.) in the mains.
In figure 10 is showed the lamp’s voltage and current. It may be observed that nearby each zero crossing, the lamp is turned off and on, as well as it happens with regular electromagnetic ballasts. This phenomenon is attributed to the non-linear lamps characteristic. During this period the lamp is reigniting because the voltage available was too low to keep the arc in the lamp, so the lamp is turned off. This phenomenon reflects into the input current waveform, where in each beginning of semi cycle the current goes nearby zero causing a flattening in the waveform. The crest factor obtained was around 2. In Figure 11 is shown the voltage in the DC bus, where it may be observed that voltage goes close to zero what causes the extinction of the lamp’s arc. In the same figure it is observed the current in the output of the rectifier.