Download I. Introduction

Single Stage Electronic Ballast for High Pressure Sodium Lamps
With Low Crest Factor
Abstract – In this paper will be reported the study
and implementation of a single stage High Power Factor
(HPF) electronic ballast for High Pressure Sodium
(HPS) lamps using a Half-Bridge Boost Rectifier
integrated with an Electronic Ballast based in a LCC
filter. In the recent years many authors are working to
obtain single stage HPF electronic ballast for fluorescent
lamps [1-3]. Normally to obtain HPF in electronic
ballast for high pressure sodium lamps a Power Factor
Preregulator (PFP) is used between the mains and the
electronic ballast [3]. The main idea in this work is to
present a simple and cheap electronic ballast with HPF
for HPS lamps. This simple solution also presents low
crest factor. Design criteria and experimental results
will be also presented in the final version of this paper.
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 electric energy 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], [2], [3] and [4], 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. Here 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.
The porpoise of this paper is to report the
development of a low cost single stage HPF electronic
ballast for HPS lamps. The design criteria will be
presented in this work for the proposed circuit. There
are many kind of high-pressure 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 [5]. These 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 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 1.
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
1
the lamp destruction;
III. BALLAST DESIGN CRITERIA
Positive Resistance
Lamp
Current
Negative Resistance
Lamp
Voltage
Breakdown Voltage
Fig. 1. - Typical voltage x current curve for HID lamps.
In order to obtain low cost electronic ballast for
HPS lamps with HPF a single stage converter was
conceived. The idea is to build the PFC using the
half-bridge boost rectifier working in discontinuous
conducting mode (DCM) integrated with the
electronic ballast. Using this simple idea high power
factor and low crest factor can be obtained in a single
stage converter.
II. STUDIED ELECTRONIC BALLAST
The studied single stage high power factor
electronic ballast for high pressure sodium lamps
structure incorporates a bridge rectifier, a half-bridge
boost rectifier and an input LC filter to minimize the
EMI generated by the DCM input current. Figure 2
shows an electrical diagram of the proposed circuit.
Similar circuits have been proposed by other authors
using fluorescent lamps. but any paper using this
topology for HPS lamps was not found. The capacitor
CF in this figure is responsible by the DC bus voltage
once the voltage in this capacitor has a low ripple the
crest factor in the HPS is minimized. The inductor
LPFC is the boost inductor, this inductor must be
designed to work in DCM once the MOSFET duty
cycle is constant to guarantee the perfect operation of
the electronic ballast. The resistor ESR represents the
equivalent series resistance of the capacitor Cs,
inductor Ls and MOSFETs on resistance.
To verify the performance of the proposed system a
LCC electronic ballast (figure 3) for a 250 W HPS
lamp was designed. The nominal lamp voltage (Vlamp)
was obtained from the lamp’s manufacturer datasheet
and its value is 100 VRMS. To design the LCC ballast
it was added 10 % to consider loses effect. 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 Vmains=
127 VRMS. The switching frequency chosen was 68
kHz. Assuming the resistive comportment of the
lamp, we can estimate the value of its resistance (R)
after ignition using equation 1.
Vlamp 2
(1)
R
 40
P
Where P is the lamps power.
As it was indicated in [1], the best relationship
between the switching frequency and the resonance
frequency before the lamp turn on is ω0/ ωs = 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 ω0 = ωs 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.
L
Ve
Cp
R
Fig. 3. LCC Ballast.
For the circuit showed in figure 3, considering the
voltage Ve an asymmetrical square wave (from E to 0
V). It is easy to obtain the peak value Vm of the first
harmonic from Fourier series. The equation 4 has
shown this value:
2E
(2)
Vm 

In this study an expression was obtained to
determine the peak voltage across the capacitor Cp.
This expression, shown in equation 3, is valid before
the lamp start up.
Vcp 
Fig. 2. Studied HPF Electronic Ballast.
Cs
2E
1 e
 RESR
4 L F
(3)
2
Where, F is the switching frequency. Preliminary
tests demonstrated that necessary peak voltage (Vopk)
to guarantee the lamp ignition is 3.8 kV. A typical
RESR value is 6.5 Ω. Manipulating equation (3) we can
obtain the value of inductor L in equation (4).
 RESR
4 F
L
 219.5 H

2E 
ln 1 


Vopk 

2   L 
1
1
1

C p Cs
1
6   L  Cp
Ve
R
Fig. 4. – 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 for the
circuit shown in figure 4, the equation 9 presents this
result:
R
(9)
Vlamp   Vm
Z
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 5 may be
simplified into equation 6, because the effect of the
capacitance Cs is almost null.
F
Cs
(4)
The resonance frequency may be calculated using
equation (5).
(5)
1
Fo 
L
(6)
Manipulating equation (6), it can be obtained the
value for the capacitor Cp as it is shown in equation
(7).
1
Cp 
 2, 767nF
2
6   F   L
(7)
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 peakto-peak square wave signal, which generated a 660 V
signal over the lamp terminals, allowing the
determination of RESR using equation 3. 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
100 kΩ. 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
8:
1
(8)
// R  R
  CP
Consequently, after lamp ignition, the equivalent
circuit is showed in figure 4.
The modulus of the impedance of the circuit can be
calculated with equation 10. To facilitate the design
of the LCC filter the parameterized LCsR circuit
transfer function was obtained and the result is shown
in figure 13.
(10)
Vlamp
1
Vm
  ,  
1
 2


  2
Where η is the capacitor relationship factor defined
as η = Cs/Cp, κ 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   L .
R 2C p
Figure 5 presents the relationship between the
RMS lamp voltage and the RMS first harmonic
voltage, Vlamp/V1stRMS, called as parameterized lamp
voltage V, for different values of η as design
parameter. Using the graphic of figure 5, a η =180
was adopted. This relationship will allow us to
achieve the desired V relationship shifting the
frequency after the lamp is turned on.
Fig. 5. Transfer Function varying κ for different
values of η.
With the η relationship, the value of Cs may be
obtained using equation 11 for a V=0.78 relationship.
3

Cs
 180  Cs    C p  498nF
Cp
(11)
IV. SIMULATION RESULTS
To validate the proposed system, a half-bridge
electronic ballast with the following specification:
250 W HPS lamp, input voltage (127 VAC), DC bus
voltage (360 VDC) and operation frequency of (68
kHz), was simulated using the software PSIM® 6.0.
Figure 6 shows the current and voltage in the mains
without the EMI filter. The voltage in the lamp is
showed in figure 7. The crest factor was measured.
Tests indicate that ballasts with higher crest factors
may result in depreciation of lumen output or reduced
lamp life. It was found a crest factor of 1.37 using this
ballast. HID lamp recommendations suggest a
maximum crest factor of 1.8 for HPS lamps.
V. CONCLUSION
This paper described single stage high power factor
electronic ballast for high pressure sodium lamps.
This ballast presents a very low cost because it avoids
an external PFP. A high power factor was obtained.
The crest factor found was very low because the
Electronic Ballast works with low DC bus ripple. The
main drawback of this topology is to work in DCM,
but for this power level is possible to use this solution
with an input EMI filter.
VI. ACKNOWLEDGEMENTS
The authors wants to thanks CEEE “Companhia
Estadual de Energia Elétrica” to the financial support
given which made possible the realization of this
project.
VII. REFERENCES
Fig. 6. –Above, voltage and current in the mains,
and voltage in the lamp with 250 W below.
Fig. 7. –Voltage in the lamp.
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