Dimming control of electronic ballasts

Details Dimming control of electronic ballasts Dimming control of electronic ballasts

1998-10-13

2002-11-26

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Current and/or voltage regulation

C315S2090SC

Reexamination Certificate

active

06486615

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an apparatus and method for the dimming control of an electronic ballast for a fluorescent lamp. In particular the invention relates to an apparatus and method for such dimming control that generates low electromagnetic interference and low switching stress.
BACKGROUND OF THE INVENTION
Electronic ballasts for the high-frequency operation of fluorescent lamps have been increasingly adopted as an energy efficient solution in residential, commercial and industrial lighting applications. Electronic ballasts have a number of advantages including improved efficiency of the overall system, higher lumen output per watt and longer lifetime of the fluorescent lamps. Electronic ballasts are in effect switched mode power electronic circuits, and most modem electronic ballast designs employ series resonant converters as the power circuits for driving the lamps.
PRIOR ART
FIG. 1
shows a conventional electronic ballast design. The basic concept of this design is to use the resonant voltage across the resonant capacitor C
r
to cause the lamp arc to strike at high frequency, typically from 25 kHz to 50 kHz. Because of the high frequency of the excitation voltage the lamp is essentially in a continuous on-state, which provides high-quality illumination without any unwanted flickering effect.
FIG. 2
shows a conventional implementation of a half-bridge series resonant inverter for an electronic ballast application. In this arrangement the two switches S
1
and S
2
are complementary switches (ie when S
1
is on S
2
is off, and vice versa). If the potential at point Y is taken as the zero voltage reference point, then voltage V
xy
will have the values ±V
dc
/2 where V
dc
is the DC voltage applied to the ballast circuit either by an AC-DC converter if the power source is AC or by a DC—DC converter if the power source is DC. The operation of this conventional circuit will now be described for the purposes of illustration.
The two capacitors C are much larger than the resonant capacitor C
r
and provide a stable DC voltage nominally at V
dc
/2 at the point Y. By operating the switching frequency f
sw
of S
1
and S
2
slightly higher than the resonant frequency f
r
of inductor L
r
and capacitor C
r
the resonant load becomes inductive. If the current (i
Lr
) in the inductor L
r
is continuous, S
1
and S
2
can be turned on under zero-voltage. This zero-voltage switching is desirable because it reduces turn-on switching loss and minimises the electromagnetic interference (EMI) from the power switches. If additional small capacitors Cs
1
and Cs
2
are added as shown in
FIG. 2
, switches S
1
and S
2
can also be turned off under zero-voltage as long as the inductor current (i
Lr
) is continuous.
Series resonant converter designs such as that shown in
FIG. 2
are very popular. One reason for this popularity, for example, is that a circuit of this design can be used for a multiple lamp system simply by connecting several sets of resonant tanks and lamps across points X and Y. This flexibility greatly reduces the ballast cost per lamp.
Difficulties arise with the circuit of
FIG. 2
, however, when it is desired to provide a method of dimming control. Most electronic ballasts employ a nominally constant converter DC voltage and in order to control the light intensity of the fluorescent lamp dimming control is provided. Two methods of providing dimming control are commonly used in this type of ballast arrangement: duty cycle control and variation of switching frequency and these will now both be described.
The first method of dimming control is by control of the duty cycle (d) of the two switches S
1
and S
2
. The ideal duty cycle is 0.5 but in practice the maximum d should be slightly less than 0.5 so that a small deadtime when both switches are off is provided to avoid shoot-through in S
1
and S
2
.
FIG. 3
shows typical waveforms of the gating signals of S
1
and S
2
. By controlling the turn-on and turn-off times of the two switches the voltage applied to the series resonant circuit can be controlled. This method is not without its drawbacks however, especially at low duty-cycles, ie at low applied voltage, as will be seen from the following.
A major advantage of the circuit of
FIG. 2
is that the switches can be turned on and off under zero-voltage conditions which substantially reduces EMI emission and switching stress in the power switches. However as will be seen below, if the duty cycle is too small the inductor current may become discontinuous and the zero-voltage switching conditions will be lost and the switches will suffer switching stress, leading to reduced reliability and increased EMI emission. This can be seen from the following explanation of the operating modes of the power converter which are described with reference to
FIG. 4
of the accompanying drawings which schematically highlight the main current paths.
FIG.
4
(
a
) shows a first stage in which switch S
1
is ON while switch S
2
is OFF and the main current path is highlighted in bold. In a second stage shown in FIG.
4
(
b
) the two switches are OFF while Cs
1
is charged up to V
DC
and Cs
2
is discharged. When Cs
2
is discharged the anti-parallel diode of S
2
will start to conduct. Again the main current path is highlighted in bold. FIG.
4
(
c
) shows this third stage in which the two switches S
1
and S
2
are both still OFF and the anti-parallel diode is conducting clamping the voltage across S
2
to almost 0V and when the switch S
2
is later turned on again it is turned on under this zero-voltage condition. However, this assumes that the inductor current is continuous. If the duty cycle is too small the inductor current may decay to zero before the switch S
2
is turned on again giving the condition shown in FIG.
4
(
d
). If the inductor current falls to zero before S
2
is switched on again, the voltage across S
2
is not clamped to near zero and as both switches are turned off the voltage across S
2
and thus Cs
2
will rise. When in the next stage S
2
is turned on again the energy stored in Cs
2
will be dissipated in S
2
causing high discharge current and high switching loss and stress in S
2
.
In the next stage shown in FIG.
4
(
e
) S
2
is ON while S
1
is OFF and the inductor current becomes negative. As both switches once more go to OFF, shown in FIG.
4
(
f
), the anti-parallel diode of S
1
starts to conduct clamping the voltage across S
1
to near zero (FIG.
4
(
g
)). Again, as with S
2
, if the duty cycle is not too small S
1
will be switched on again before the inductor current decays to zero and so will be switched on while still clamped to near zero voltage, with the advantages discussed above. If the duty cycle is too small, however, the inductor current will decay to zero before S
1
is switched on again causing the voltage across S
1
and Cs
1
to rise. When S
1
is finally turned on again the energy stored in Cs
1
is dissipated in S
1
as discussed above with regard to S
2
and with the same problems. This possibility is shown in FIG.
4
(
h
).
Thus if dimming control by variation of duty cycle is provided, soft switching is possible provided that the inductor current is continuous. However if the duty cycle is reduced too far then the inductor current may at points in the cycle decay to zero and non-zero-voltage switching takes place with its attendant disadvantages of higher EMI emission and higher switching stress.
As an alternative to dimming control by duty cycle variation, it is also known to provide dimming control by varying the switching frequency. If the switching frequency is increased, the inductor impedance is increased and thus the inductor current is reduced. This allows the output of a fluorescent lamp to be controlled by varying the switching frequency and
FIG. 5
shows the power of a 4-ft 40 W fluorescent lamp plotted against switching frequency. It can be seen that the lamp power, and therefore the intensity of the emitted light, decreases with increasing switching frequency.
Dimming control by varyin

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