Prepared by: Michaël Bairanzade Power Semiconductor Applications Engineer Motorola SPS Toulouse

Michael Bairanzade

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Niveau: Supérieur, Doctorat, Bac+8
1MOTOROLA Prepared by: Michaël Bairanzade Power Semiconductor Applications Engineer Motorola SPS Toulouse ABSTRACT With a continuous growth rate of 20% per year, electronic lamp ballasts are widely spread over the world. Even though the light out of a fluorescent tube has a discontinuous spectrum, the higher efficiency brought by the electronic control of these lamps make them the best choice to save the energy absorbed by the lighting systems. A few years ago, the lack of reliable and efficient power transistors made the design of such circuits difficult! Today, thanks to the technology improvements carried out by MOTOROLA, design engineers can handle all of the problems linked with the power semiconductors without sacrificing the global efficiency of their circuits. This Application Note reviews basic electronic lamp ballast concepts and gives the design rules to build industrial circuits. SUMMARY 1. MAIN PURPOSE – Fluorescent tube basic operation – Standard electromagnetic ballast – Electronic circuits 2. HALF BRIDGE CIRCUIT DESIGN 3. DIMMABLE CIRCUIT 4. NEW POWER SEMICONDUCTORS 5. CONCLUSIONS 6. APPENDIX ELECTRONIC LAMP BALLAST Main Purpose To generate the light out of a low pressure fluorescent lamp, the electronic circuit must perform four main functions: a – b – c – d – provide a start–up voltage across the end electrodes of the lamp. maintain a constant current when the lamp is operating in the steady state.

• ac current


• basic operation


• standard device


• ballast


• fluorescent tube


• typical fluorescent


• voltage across


• fluorescent lamp

Sujets

• Europe
• Motorola
• Alternating current
• Ballast
• Fluorescent lamp

Informations

• Publié par: profil-zyak-2012
• Nombre de lectures: 141
• Langue: English

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SEMICONDUCTOR APPLICATION NOTE by AN1543/D
Prepared by: Michaël Bairanzade
Power Semiconductor
Applications Engineer
Motorola SPS Toulouse
Fluorescent Lamp OperationABSTRACT
When the lamp is off, no current flows and the apparent
With a continuous growth rate of 20% per year, electronic
impedance is nearly infinite. When the voltage across the
lamp ballasts are widely spread over the world. Even though
electrodes reaches the V value, the gas mixture is highlytrigthe light out of a fluorescent tube has a discontinuous
ionized and an arc is generated across the two terminals of the
spectrum, the higher efficiency brought by the electronic
lamp. This behavior is depicted by the typical operating curve
control of these lamps make them the best choice to save the
shown in Figure 1.
energy absorbed by the lighting systems.
A few years ago, the lack of reliable and efficient power
Itransistors made the design of such circuits difficult! Today,
thanks to the technology improvements carried out by
MOTOROLA, design engineers can handle all of the problems
linked with the power semiconductors without sacrificing the
global efficiency of their circuits.
IThis Application Note reviews basic electronic lamp ballast nom
concepts and gives the design rules to build industrial circuits.
SUMMARY V
1. MAIN PURPOSE Von
– Fluorescent tube basic operation
– Standard electromagnetic ballast
Vstrike– Electronic circuits
2. HALF BRIDGE CIRCUIT DESIGN
3. DIMMABLE CIRCUIT
4. NEW POWER SEMICONDUCTORS
5. CONCLUSIONS
6. APPENDIX
Figure 1. Typical Low Pressure Fluorescent
Tube I/V CharacteristicELECTRONIC LAMP BALLAST
Main Purpose
The value of V is a function of several parameters:strikeTo generate the light out of a low pressure fluorescent lamp,
– gas filling mixturethe electronic circuit must perform four main functions:
– gas pressure and temperature
a – provide a start–up voltage across the end electrodes – tube length
of the lamp. – tube diameter
b – maintain a constant current when the lamp is – kind of electrodes: cold or hot
operating in the steady state.
Typical values of V range from 500 V to 1200 V.strikec – assure that the circuit will remain stable, even under
Once the tube is on, the voltage across it drops to the
fault conditions.
on–state voltage (V ), its magnitude being dependent uponond – comply with the applicable domestic and international
the characteristics of the tube. Typical V ranges from 40 Vonregulations (PFC, THD, RFI, and safety).
to 110 V.
Obviously, a high end electronic lamp ballast will certainly The value of V will vary during the operation of the lampon
include other features like dimming capability, lamp weareout but, in order to simplify the analysis, we will assume, in a first
monitoring, and remote control, but these are optional and will approximation, that the on–state voltage is constant when the
be analyzed separately. tube is running in steady state.
MOTOROLA 1 Motorola, Inc. 1995

LConsequently, the equivalent steady state circuit can be
described by two back to back zener diodes as shown in
Figure 2, the start–up network being far more complex,
particularly during the gas ionization. This is a consequence
of the negative impedance exhibited by the lamp when the
S
TUBE Tvoltage across its electrodes collapses from V to V .strike on C BI–METALLIC
MAINS TRIGGER
220 V– 50 Hz
BALLAST
Figure 3. Standard Ballast Circuit for
Fluorescent Tube
The operation of a fluorescent tube requires severalVac Von
components around the tube, as shown in Figure 3. The gas
mixture enclosed in the tube is ionized by means of a high
voltage pulse applied between the two electrodes.
To make this start–up easy, the electrodes are actually
made of filaments which are heated during the tube ionization
start–up (i.e.: increasing the electron emission), their
deconnection being automatic when the tube goes into theFigure 2. Typical Fluorescent Tube Equivalent
steady state mode. At this time, the tube impedanceCircuit in Steady State
decreases toward its minimum value (depending upon the
tube internal characteristics), the current in the circuit being
Up to now, there is no model available to describe the start limited by the inductance L in series with the power line.
up sequence of these lamps. However, since most of the The starting element, commonly named starter”, is an
phenomena are dependent upon the steady state essential part to ignite the fluorescent tube. It is made of a
characteristics of the lamp, one can simplify the analysis by bi–metallic contact, enclosed in a glass envelope filled with a
assuming that the passive networks control the electrical neon based gas mixture, and is normally in the OPEN state.
behavior of the circuit. When the line voltage is applied to the circuit, the
Obviously, this assumption is wrong during the time elapsed fluorescent tube exhibits a high impedance, allowing the
from V to V , but since this time interval is very short, thestrike on voltage across the starter” to be high enough to ionize the
results given by the proposed simple model are accurate neon mixture. The bi–metallic contact gets hot, turning ON the
enough to design the converter. contacts which, in turn, will immediately de–ionize the
When a fluorescent tube is aging, its electrical starter”. Therefore, the current can flow in the circuit, heating
characteristics degrade from the original values, yielding less up the two filaments. When the bi–metallic contact cools
light for the same input power, and different V and Vstrike on down, the electrical circuit is rapidly opened, giving a current
voltages. variation in the inductance L which, in turn, generates an
A simple, low cost electronic lamp ballast cannot optimize overvoltage according to Lenz’s law.
the overall efficiency along the lifetime of the tube, but the Since there is no synchronization with the line frequency
circuit must be designed to guarantee the operation of the (the switch operates on a random basis), the circuit opens at
lamp even under the worst case end of life” conditions. a current level anywhere between maximum and zero.
As a consequence, the converter will be slightly oversized If the voltage pulse is too low, the tube doesn’t turn ON and
to make sure that, after 8000 hours of operation, the system the start–up sequence is automatically repeated until the
will still drive the fluorescent tube. fluorescent tube ionizes. At that time, the tube impedance falls
to its minimum value, yielding a low voltage drop across its end
Controlling the Fluorescent Lamp electrodes and, hence, across the switch. Since the starter
As already stated, both the voltage and the current must be can no longer be ionized, the electrical network of the
accurately controlled to make sure that a given fluorescent filaments remains open until the next turn–on of the circuit.
lamp operates within its specifications. We must point out that the fluorescent tube turns off when
The most commonly used network is built around a large the current is zero: this is the source of the 50 Hz flickering in
inductor, connected in series with the lamp, and associated a standard circuit. It’s an important problem which can lead to
with a bi–metallic switch generally named the starter”. visual problems due to the stroboscopic effect on any rotating
Figure 3 gives the typical electrical schematic diagram for the machines or computer terminals.
standard, line operated, fluorescent tube control.
2 MOTOROLA
To take care of this phenomena, the fluorescent tubes, at
%least those used in industrial plants, are always set on a dual
basis in a single light spreader, and are fed from two different
phases (real or virtual via a capacitor) in order to eliminate the
flickering.
The value of the inductor L is a function of the input line
frequency (50 Hz or 60 Hz), together with the characteristics
of the lamp.
The impedance of L is given by Equation 1:
Z = L* (1)L
with: = 2* *F
F = in Herz L = in Henry Z = in Ohm
F
50 Hz 10 kHz 1 MHz
Computing the value of L is straightforward. Assuming a
European line (230 V/50 Hz) and a 55 W tube (V = 100 V,on Figure 4. Typical Fluorescent Lamp Efficiency as a
V = 800 V), then:trig Function of the Operating Frequency
Ptube
I (2)RMS The electronic circuit one can use to build a fluorescentVon
lamp controller can be divided into two main groups:
I 55 100 0.55 ARMS A – Single switch topology, with unipolar AC current,
To limit the steady state current, the impedance must be (unless the circuit operates in the parallel resonant
equal to: mode).
B – Dual switch circuit, with a bipolar AC output current.
Line–Von (3)Z The manufacturers of the fluorescent lamps highlyIRMS
recommend operating the tubes with a bipolar AC current.
Z (230–100) 0.55 238 This avoids the constant bias of the electrodes as an
Anode–Cathode pair which, in turn, decreases the expectedTherefore, the inductor must have a value of (assuming the
lifetime of the lamp.pure Ohmic resistance of the total circuit being negligible):
In