NXP Semiconductors
AN10971
TL applications with NXP ballast controllers
1. Introduction
This application note describes the use of an NXP Semiconductors half-bridge driver IC
for High Frequency (HF) Tube Light (TL) single tube, fixed DC bus voltage applications.
2. Scope
This application note is organized as follows:
•
•
•
•
•
•
Section 3
describes the basic operation of a half-bridge ballast for fluorescent tubes.
Section 4
describes how to select
MOSFET
s for the half-bridge.
Section 5
describes how to select a resonant coil and capacitor.
Section 6
describes how to design magnetic components.
Section 7
describes the feedback control loop used in the dimmable controllers.
Section 8
describes system performance.
3. Lamp characteristics and half-bridge principles
3.1 Introduction
Electronic ballasts must preheat, ignite, control and monitor the condition of the TL
according to the TL specification. In cases of no ignition or lamp end of life, the ballast
must shut down to avoid damage to the following:
•
the ballast
•
or the overheating of the lamp electrodes
Depending on the lamp type, tube ignition occurs at different voltage levels. For example,
a Compact Fluorescent Lamp (CFL) ignition occurs at around 500 V and up to 1200 V for
a TL. Following ignition a lamp is in its operating state. During operation lamp voltage is
dependent on the shape and content of the tube. For TL lamps, voltages range from 80 V
to 220 V (RMS).
A fluorescent lamp has a so called "negative incremental impedance". The more current
flowing through a fluorescent lamp the lower the voltage (unlike a resistor). Driving a
fluorescent lamp with a Constant Voltage (CV) would result in an unstable system. In
addition, a breakdown of the supply and/or tube within a few milliseconds also occurs.
To stabilize the current through the lamp, a series impedance is needed typically a
resistor. However, in practice a coil is used to reduce losses as shown in
Figure 1.
In
magnetic 50 Hz to 60 Hz Low Frequency (LF) systems, see
Figure 2
coils are large.
However, in HF TL 40 kHz to 80 kHz systems coils are smaller as the impedance of a coil
increases linearly with the frequency.
In steady state, at frequencies higher than 10 kHz a fluorescent lamp can be considered
to be a resistor. At high frequencies Ohms law V = I
R is applicable to fluorescent tubes.
The resistance (R) varies with the power supplied to the lamp. For example, a 36 W T8
lamp with an operating voltage of 100 V can be considered as a 277
resistor. This
condition is true when operated at its nominal power.
AN10971
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2011. All rights reserved.
Application note
Rev. 1 — 15 September 2011
3 of 41
NXP Semiconductors
AN10971
TL applications with NXP ballast controllers
+
I
La
lamp
V
AB
lamp
-
I
La
-
V
i
lamp potential (V)
+ V
R
-
+ V
La
200
lamp potential (V)
120
110
100
100
dn
e
<0
dt
200
(a)
+ V
La
-
V
AB
starting voltage V
i
V
m
dn
e
>0
dt
dn
e
>0
dt
dn
e
<0
dt
V
R
V
La
400 500
I (mA)
V
La
iss
I (A)
(b)
019aaa652
300
a. Discharge potential drop versus
current
b. Effect of series resistance in
stabilizing lamp current
(1) dn
e
= delta in the number of free electrons in th plasma
Fig 1.
Lamp characteristics
+ I
b
+
B
-
+
V
i
La
-
Vm
0
S
I
l
019aaa654
-
Fig 2.
Magnetic 50/60 Hz TL ballast
3.2 Lamp filaments
Each lamp has two filaments (or electrodes), each consisting of a coated tungsten wire as
shown in
Figure 3.
The resistance of the tungsten wire is directly related to its
temperature.
Figure 4
identifies the relationship between electrode temperature and the
ratio of Resistance hot (R
h
) and Resistance cold (R
c
) of a typical tungsten wire.
filament
length D
019aaa657
Fig 3.
Lamp filament
AN10971
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2011. All rights reserved.
Application note
Rev. 1 — 15 September 2011
4 of 41
NXP Semiconductors
AN10971
TL applications with NXP ballast controllers
12
ratio
Rh/Rc
8
019aaa658
4
0
0
500
1000
1500
T (°K)
2000
Fig 4.
R
h
/R
c
ratio versus temperature for 100 % wolfram filaments
Lamp electrodes are preheated before the lamp is ignited or lamp life time is considerably
reduced. During preheat, electrode resistance increases typically by a factor of between
four to six. To generate sufficient and an even spread of heat before ignition, preheat time
must not be lower than 0.5 s. Preheat times longer than 1.5 s to 1.7 s are considered
undesirable as heat is lost to the gasses in the lamp tube.
Figure 5
identifies the operating area of fixed preheat current for a T8 36 W burner with
two current levels.
Figure 5
also shows that if the current is too high the optimal ignition
temperature is reached before 0.5 s. As a result, either ignition takes place too early or the
filament is overheated before ignition.
019aaa661
705.5 mA
R
hc
6.25
operating
area
4.25
467.0 mA
1.0
0
0
0.5
1.0
1.5
t (s)
2.0
Fig 5.
Fixed preheat current window
AN10971
All information provided in this document is subject to legal disclaimers.
© NXP B.V. 2011. All rights reserved.
Application note
Rev. 1 — 15 September 2011
5 of 41
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