Evaluation of close loop digital control based in a microcontroller and used to eliminate acoustic resonances in HID lamps

Abstract

A closed loop digital control to eliminate acoustic resonances (AR) in electronic ballast for high intensity discharge lamps is proposed in this paper. The system senses the AR intensity and a microcontroller compares this signal with a reference level in order to apply different frequency modulation patterns to reduce the AR intensity until a smaller level to the reference value. The use of different frequency modulation patterns allows avoiding the AR intensity due to changes in the lamp eigenfrequencies, ambient temperature and the lamp aging.

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HAIT Journal of Science and Engineering B, Volume 2, Issues 5-6, pp. 655-672
Copyright C ° 2005 Holon Academic Institute of Technology
Close loop digital control implemented in
a microcontroller and used to eliminate
acoustic resonances in HID lamps
Javier Correa1,∗, Mario Ponce2, Jaime Arau2,
Miriam Sánchez1, and Elías Rodríguez3
1Departamento de Ingeniería Eléctrica y Electrónica
Instituto Tecnológico de Morelia, Morelia, C.P. 58120, Mexico
2Departamento de Electrónica,
Centro Nacional de Investigación y Desarrollo Tecnológico-CENIDET
P.O. Box 5-164, C.P. 62050 Cuernavaca, Mexico
3Departamento de Ingeniería Electrónica
Instituto Tecnológico de Celaya, Celaya, C.P. 58120, Mexico
∗Corresponding author: jcorrea@itmorelia.edu.mx
Received 1 March 2005, accepted 7 June 2005
Abstract
A closed loop digital control to eliminate acoustic resonances (AR)
in the electronic ballast for high intensity discharge lamps is proposed
in this paper. The system senses the AR intensity and a microcon-
troller compares this signal with a reference level in order to apply
different frequency modulation patterns to reduce the AR intensity
below the reference level value. The use of different frequency modu-
lation patterns allows avoiding the AR intensity due to changes in the
lamp eigenfrequencies, ambient temperature and the lamp aging.
1Introduction
Acoustic resonance (AR) is one of the main problems to be solved in the
electronic ballast design for high intensity discharge (HID) lamps. It is char-
acteristic when the HID lamps are operated at frequencies above 10 kHz and
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appears when the lamp power at an eigenfrequency exceeds a threshold value
[1]. Some problems caused by this phenomenon are: emitted light fluctua-
tions (flicker), color temperature changes, lamp voltage variation and, in the
worst case, a discharge tube cracking and ballast damage. This phenomenon
depends on several parameters like pressure and type of the lamp filling gas,
geometric dimensions discharge tube, ambient temperature, etc. As conse-
quence, the lamp eigenfrequencies vary with operation temperature, aging,
and the lamp type.
Several solutions have been proposed to solve this problem, including:
1) lamp operation at non-eigenfrequencies [2], 2) lamp feeding with volt-
age and current square wave forms [3,4], 3) voltage and lamp current with
frequency modulation [5], and (4) superimposing the third harmonic to the
lamp voltage [6]. Lamp eigenfrequencies depend on the lamp type, the am-
bient temperature, and the lamp aging, so that the first solution is not
reliable. The same problem is presented in the third solution, because if the
modulated frequency reaches an eigenfrequency, AR will not be eliminated.
The second solution implies application of a low frequency DC power to
the lamp so that any acoustic resonance could be presented. This requires,
however, a two stage circuit: the first one for lamp stabilization, the second
one an inverter for generating lamp supply voltage, furthermore electronic
ballast operation is in a hard switching mode. The fourth solution also tries
to apply a DC lamp power and operates with sinusoidal waveforms in a soft
switching mode, but the circuit is complex because two resonant tanks are
needed: the first one for the fundamental and the second one for the third
harmonic.
In this paper, frequency modulation is used to avoid AR caused by the
uncertainty in the lamp eigenfrequencies at which acoustic resonance may
appear. Different modulation frequency patterns with several modulation
index values are applied to the lamp until the AR intensity is reduced to non-
critical values avoiding flicker or lamp damage. In this way, the resultant
circuit is a conventional resonant electronic ballast with soft switching mode
operation, where no extra resonant tanks are necessary. In order to simplify
the electronic ballast and to reduce the components, a microcontroller is
used to generate different switching patterns according to the AR intensity
signal.
This paper is organized as follows: Section 2 presents the proposed
method for the closed loop digital control, the algorithm description, and its
implementation in a microcontroller. Experimental results for several HID
lamps are included in Section 3, and, finally, conclusions are presented in
Section 4.
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2Closed loop method description
Figure 1: Proposed closed loop control method.
Figure 1 shows the closed loop circuit. The detailed description of each stage
given below.
2.1Electronic ballast
The electronic ballast consists of a resonant inverter (class D amplifier plus a
LCC resonant tank) working in soft switching mode. The circuit is designed
for operating at eigenfrequency at which AR is presented in the lamp. LCC
resonant tank is designed for the maximum gain voltage condition [7].
AR is detected in the lamp voltage using a transformer, this signal is
filtered by a fourth order Chebyshev filter in order to eliminate all compo-
nents with frequencies above 100 Hz, in this way only the AR components
are obtained. Then AR signal is adjusted by an op-amp and applied to the
microcontroller A/D converter. The digital signal is compared with an in-
ternal reference level. If the AR intensity is greater than the reference level,
a first frequency modulation pattern is applied to eliminate the acoustic
resonance; if the AR intensity remains high, the modulation index (m) is
increased. If the AR remains above the reference level even after several
modulating increases, the frequency modulation pattern is changed until
AR intensity becomes lower than the reference value. Different frequency
modulating patterns are generated by the microcontroller in order to control
the class D amplifier by means of the IR2104 driver.
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2.2Acoustic resonance sensing circuit
In a typical electronic ballast, when AR occurs, the lamp current has three
frequency components: a) a high frequency one caused by the resonant
inverter frequency operation, b) a 100 — 120 Hz frequency component due to
the power factor correction output voltage stage and c) a very low frequency
component due to the AR in the lamp (1 — 10 Hz). If higher frequency
components are eliminated, it is possible to detect the AR effect in the
lamp current [8]. In this way, AR is detected in the lamp voltage using a
transformer, this signal is filtered by a fourth order Chebyshev filter in order
to eliminate all components with frequencies above 100 Hz, in this way only
AR components are obtained. Then AR signal is rectified by diodes and
then is adjusted by an op-amp to be applied to the microcontroller A/D
converter.
2.3Frequency modulation patterns
Figure 2: Sinusoidal frequency modulated waveform.
The control method objective is to generate a PWM signal with 50% of
duty cycle in which the frequency could be modulated by a modulating
signal, that can be sinusoidal, saw-tooth or another one. This process can
be explained using a sinusoidal waveform shown in Fig. 2. Fs is the central
frequency and corresponds to the operation frequency when no modulation
is applied. Dev represents the frequency deviation form the central to the
maximum and minimum values; Tmod represents the time period in which
one cycle of the frequency variation will be made. The signal shown in Fig.
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2 can be expressed by the equation:
Fout = Devsin
·π
2
µt
90− 1
¶¸
+ Fs.(1)
Due to necessity to generate this waveform with a microcontroller, the
expression must be digitized:
hπ
where
Fout is the frequency generated by the microcontroller,
Fs is the central frequency with no modulation,
Dev is the maximum frequency deviation.
Fout = Devsin
2
³n
90− 1
´i
+ Fs (2)
Figure 3: Discrete sinusoidal frequency modulated waveform.
Discreet waveform for Fs = 35 kHz with Dev = 2 kHz is showed in
Fig. 3. When the Fout frequency values are known, then it is necessary to
calculate the value that must be loaded in the PIC16F876 PR2 register in
order to generate this frequency value. Expression (3) relates Fout with the
value of PR2.
PR2 =Fclock
4 Fout− 1(3)
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