AN2870
Application note
L6585DE combo IC
Introduction
The modern requirements for fluorescent lamp electronics ballast concerns both efficiency
of the drivers and safety aspects.
The L6585DE offers the designer a high performance PFC stage, high capability half bridge
high voltage drivers, a fully programmable control and an enhanced set of protections.
Figure 1.
Typical electronic ballast block diagram
March 2009
Rev 1
1/41
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Contents
AN2870
Contents
1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
1.2
Typical configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Lamp requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
3
L6585DE combo IC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Device blocks description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1
3.2
Start-up and shut-down . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
PFC section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2.1
3.2.2
3.2.3
3.2.4
3.2.5
3.2.6
Error amplifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Multiplier block and THD optimizer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Current comparator and choke saturation detection . . . . . . . . . . . . . . . 11
Zero current detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Driver . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
PFC protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
3.3
Ballast controller section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.3.1
3.3.2
3.3.3
3.3.4
Oscillator and timer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Overcurrent control and protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
End of life detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Shutdown . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
4
Designing with L6585DE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
4.1
4.2
4.3
PFC stage design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Ballast stage design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
PCB hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
5
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
2/41
AN2870
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 15.
Figure 16.
Figure 17.
Figure 17.
Figure 18.
Figure 19.
Figure 20.
Typical electronic ballast block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Start-up and shut-down waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
PFC section block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
PFCCS pin waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Protections block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Oscillator and starting sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Current control sequence during ignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
HBCS thresholds summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Window comparator for rectifying effect detection (Cblock to GND). . . . . . . . . . . . . . . . . . 17
Window comparator for rectifying effect detection (lamp to GND) . . . . . . . . . . . . . . . . . . . 19
Typical application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
PFC MOSFET losses (example) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Multiplier characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
(A) voltage frequency response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
(B) current frequency response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
Oscillator characteristic curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
(A) k parameter versus Cosc (pF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
(B) e parameter versus Cosc (pF) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
EOL - Cblock to ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
EOL - lamp to ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Current consumption vs PFC frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
3/41
Introduction
AN2870
1
1.1
Introduction
Typical configuration
Typical fluorescent lamp electronic ballasts are composed by (Figure
1):
●
An input PFC section, if input power is greater than 25 W, usually a TM PFC converter,
that generates a DC output voltage and absorbs power from mains with very high
Power Factor (typically 0.95 or grater) and very low THD (mandatory less than 10%).
A high frequency half bridge driver, fed by the PFC output, with internal or external
oscillator, a timer and various protections in order to drive correctly the lamp, to avoid to
deliver an excessive power to the lamp and to detect any malfunction of the lamp
(broken lamp, broken cathode or lamp absence)
An output resonant stage, realized by reactive components (capacitors and inductors),
that, together with the half bridge driver, optimizes the power delivered to the lamp (one
or more) during all working conditions (preheating, ignition and run mode).
●
●
1.2
Lamp requirements
Fluorescent lamp, during its normal operation, has to be supplied by means of alternative
and controlled current. In order to reduce the size of the ballast and increase the light
efficiency of the lamp a frequency greater than 20 kHz is typically used. A half bridge quasi
resonant inverter (series-parallel converter
Figure 1)
is used to obtain sinusoidal current into
the lamp and to reduce the power dissipation of the half bridge switches, in fact zero voltage
switching is achieved.
Lamp current and lamp voltage during normal operation are reported in lamp documentation
and are to be considered as design specification. Moreover, a well preheated lamp ignites at
a lower voltage; this implies a longer lamp life and a greater number of ignitions. The
efficiency of the preheating is mainly related with the total energy delivered to the cathode
(reported on lamp documentation), and then it depends on the time available for this
operation: keeping constant the preheating energy, longer is the preheating time, smaller is
the instantaneous power delivered to the cathode. During the preheating operation the
voltage across the lamp must be kept below a specified value in order to avoid unwanted
ignitions (when these happen, the lamp experiences multiple re-strike and dissipates large
amounts of power).
There are many ways to deliver power to the cathode, but the most used are two:
1.
Current controlled preheating: the cathodes are interposed between the choke and the
resonant capacitor so they experience the same current of the resonant LC circuitry. An
efficient preheating is obtained controlling this current and the time of preheating. The
advantages of this method are the cheapness and easiness of design; it has also some
disadvantages, namely the difficulty of keeping low the lamp voltage during preheating
and the fact that during steady state the cathode experiences the sum of the lamp
current and of the resonant capacitor current.
Voltage controlled preheating: the current into the cathodes is generated by auxiliary
windings coupled with ballast choke or driven by an auxiliary oscillator. This implies
that, in any case, the design of the preheating circuitry is somewhat independent from
the design of the LC circuitry, even if it requires a lot of external components. This
method is then more efficient, but is cheaper and more difficult to design.
2.
4/41
AN2870
Introduction
After a good preheating, the voltage across the lamp is suddenly increased in order to
generate a strike inside the tube and ignite the lamp. This phase should last between 10 ms
and 100 ms.
The strike voltage depends on various parameters, many of which cannot be exactly
evaluated: preheating energy, remaining lamp life, number and efficiency of the past
ignitions. An insufficient preheating causes greater ignition voltage and a subsequent stress
of the cathodes that lose small amounts of material that darken the region of the tube near
to the cathode itself (sputtering).
Lamp ageing is related with the symmetrical or, more often, asymmetrical increasing of the
cathodes resistance. A symmetrically aged lamp absorbs more power causing hard
switching and over-current. Asymmetrically aged lamps experience a current that is more
intense in one direction than in the other. This implies that the current flowing into the lamp
has positive or negative mean value (DC component). This effect can be detected
measuring the mean values of the lamp voltage that should be zero in normal lamps. The
worst case of rectifying effect causes the current flowing only in one direction: the voltage
across the resonant capacitor can reach very high values and heavy hard switching is
detected.
When symmetrical or asymmetrical ageing of the lamp reaches a value indicated in
international norms, the lamp reaches its end of life (EOL).
5/41
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