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Emergency lighting applications 1. ABSTRACT
This application note shows the topologies implemented in the Emergency Lighting Applications and the
STMicroelectronicss power bipolar transistors used.
Today, the Emergency Lamps market has grown considerably due to the new improved safety rules. In
fact, the Emergency Lamps are used in all public places and, also, in private homes replacing the
traditional lighting applications.
2. STSA851 DESCRIPTION
The STMicroelectronics's power bipolar transistor STSA851 is housed in the TO-92 package. This
device is manufactured in NPN planar technology using a 'Base Island' layout that involves a very high
gain performance and a very low saturation voltage.
The main characteristics of the STSA851 device are:
1) V
ceo
60 V;
2) V
ces
150 V;
3) V
ebo
7 V;
4) I
c
= 5 A (continuous current);
5) I
b
= 1 A (continuous current);
6) V
ce(sat)
= 140 mV (typ) @ Ib = 50 mA @ Ic = 2 A (typical conditions);
7) H
fe
= 270 (typ) @ Ic = 2 A @ V
ce
= 1 V (typical conditions).
3. HIGH EFFICIENCY DC-AC CONVERTERS
The part of the circuit used to drive the emergency lamp is composed of DC-AC converters. The DC-AC
converters transform the low DC input voltage in high AC output voltage required by the fluorescent tube.
Fluorescent tubes are employed in these applications because they are much more efficient at
converting electrical energy into light than conventional incandescent bulbs increasing the battery life.
Usually, DC-AC converters used in these applications are the Push-Pull switching converter forced to run
in synchronized mode by the inclusion of a supply inductor, and the Forward converter. Mainly, the DC-
AC converters have suitable transformers that increase the output voltage and allow the electrical
isolation between the secondary and primary of the transformer, and suitable switches. Usually, the
switches are power bipolars driven by a third winding magnetically coupled to the transformer, like in the
PUSH-PULL current FED converter.
February 2004
1/35
AN1731
APPLICATION NOTE

G. Consentino

EMERGENCY LIGHTING APPLICATIONS AN1731 - APPLICATION NOTE
2/35
The power bipolar transistor collector current I
c
depends on the load, turns rapport
where N
2
is the secondary turn number and N1 is the primary turn number of the transformer, and it also
depends on the battery voltage.
Usually, in these applications the lamp power is in the range of 8-24W, and the turns rapport
is about 30, the current I
c
is in the range of 1.5-3.0A. Furthermore, usually, the emergency lighting
boards are powered with an input voltage in the range of 3.6-6.0 V
dc
so that the typical V
ce_max
is around
10-20 V
dc
. The voltage and current values ranges, V
ce_max
and Ic, are inside the SOA of the STSA851
so that these devices can be used in all emergency lighting applications. The emergency lamp
applications drive a lamp up to 58W. Usually, these emergency lighting applications do not supply such
output powers but only 10-20 % of the nominal lamp power. Sometimes, such applications are used to
power lamps that commonly light up rooms. When the net voltage disappears the emergency lamp
switches on supplying around 10-20 % of the nominal lamp power just to light up the room.
4. FLUORESCENT TUBE CHARACTERISTICS
Fluorescent lamps are generally made with tubes filled with a gas mixture at a low pressure. The inner
sides of the tubes are covered with fluorescent elements. When the net voltage disappears, before the
tube lights on, the lamp has a higher resistance. In this moment, the electrodes voltage increases up to
around 500V and the electrodes start to warm up and emit ions. Figures 1 and 2 show the V-time and I-
time waveforms and the V-I waveform respectively before the start-up of a 24W tube.
Figure 1: V-time and I-time Waveforms Before the Striking
2
N
N
K
1
2
=
2
N
N
1
2
(3.1)
(3.2) AN1731 - APPLICATION NOTE
3/35
Figure 2: V-I waveform before the striking
As shown above, to strike the fluorescent tube the electrodes voltage reaches up to 505V of peak.
Furthermore, the current that flows through the lamp is very low, 56 mA, because the resistance
before the striking is high (around 10 KOhm).
When the fluorescent lamp lights on, the gas mixture inside is fully ionized, and an arc across the
electrodes occurs. In this new condition, the lamp resistance drops to around 1 KOhm value (Figures 3
and 4 show the V-time and I-time waveforms and the V-I waveform after the striking.
Figure 3: V-time and I-time waveforms after the striking AN1731 - APPLICATION NOTE
4/35
Figure 4: V-I waveform after the striking
After the striking, the gas mixture emits radiations that excite the fluorescent elements inside the tube
producing the light in the visible spectrum. In this example, after the striking, the voltage across the
electrodes drops from 505V of peak to 220V of peak and the current increases from 56mA of peak to
158mA of peak.
Usually, after the striking, in order to increase the lamp efficiency up to 15%, the operation frequency is in
the range of 25-30KHz. Furthermore, as shown in Fig. 4, the waveform I-V has a linear behavior until the
established voltage value is kept. In fact, if the voltage across the electrodes overcomes this established
voltage value, the characteristic becomes flat because no ion can emit other radiations. AN1731 - APPLICATION NOTE
5/35
5. PUSH-PULL CURRENT FED CONVERTER TOPOLOGY INTRODUCTION
As previously exposed, a topology solution for emergency lighting applications is the PUSH-PULL current
FED converter topology. This topology solution has a Push-Pull switching converter forced to run in
synchronized mode by the inclusion of a supply inductor.
Figure 5: PUSH-PULL current FED converter schematic circuit
The components values of capacitors, resistors, and inductors are selected operation on the input voltage,
power lamp, and operation frequency.
6. TRANSFORMER DESCRIPTION OF PUSH-PULL TOPOLOGY
In figure 5 the transformer named T
1
has three windings. The primary winding vices are connected to the
collectors of the NPN power bipolar transistors Q
1
and Q
2
. The same primary winding has a central vice
where the inductor L
1
is connected. The secondary winding vices are connected to the load.
The third winding vices are connected to the base of the transistors Q
1
and Q
2
so that when the first is on,
the second is off and vice versa. During the Q
2
on state, the current flows through the same device and
the respective half primary winding and vice versa. Usually the primary inductance LT of the transformer
T
1
is much lower compared to the inductance L
1
. The resonance frequency of the PUSH-PULL converter
is also due to LT. N
2
(secondary winding turns) and N
1
/2 (half primary winding turns) rapport is around 60,
while N
1
/2 and N
3
(third winding turns) rapport is around 5. Considering a 6 V
dc
input voltage, the voltage
v
1max
(the max voltage across the vice of the primary winding central point and the reference) can be
written as:

V
9
6
2
14
.
3
V
2
v
dc
max
1 =
=
(6.1) AN1731 - APPLICATION NOTE
6/35
v
2max
(the max voltage across the secondary winding vices) can be written as:
v
3max
(the max voltage across the vices of the third winding) can be written as:
As exposed above, it is highlighted N
1
/2 and not N1. In order to understand the reason of it, it is
necessary to consider the graph below.
Figure 6: Particular of T
1

When Q
2
is on, Q
1
is off and vice versa. Now, considering fig. 6 where T
2
is on; the current I flows
through the half primary winding 'b' and it generates a magnetic force (Hopkinson law): is the magnetic flux and is the magnetic reluctance of the T1 core; can be written as:
can be written as:
µ is the core permeability, A is the core section and l is the core length. When T
2
switches off, T
1
switches on, the current flows through the other half primary winding 'a' and the flux inverts its
direction. Such flux flows into the transformer core creating a link with N
2
, N
3
and also with the other
V
560
60
6
2
14
.
3
)
2
N
(
N
V
2
v
1
2
dc
max
2
=
=
V
2
5
1
6
2
14
.
3
2
N
N
V
2
v
1
3
dc
max
3
=
= = I
2
N
1
= I
N
2
1
A
l =
µ
(6.2)
(6.3)
(6.4)
(6.5)
(6.6) AN1731 - APPLICATION NOTE
7/35
turns N
1
/2, generating the voltages v
2
and v
3
(magnetic law-Lenz law):
Furthermore, i
2
(the current that flows through the lamp) can be written as:
In fact, the apparent input power can be written as:
The output power can be written as:
Considering an ideal transformer:
Before the lamp strike, or when the lamp is disconnected, the operation frequency (about 60 KHz) is due
to the resonance between C
2
and the primary transformer winding inductance LT (see fig. 7).
Figure 7: Resonant Schematic Circuit Before the Lamp Strike
t
N
v
t
N
v
t
N
v = = =
2
;
;
1
2
/
1
3
3
2
2
2
,
2
,
2
1
2
1
3
1
3