ST AN1731 Application note

AN1731

APPLICATION NOTE

EMERGENCY LIGHTING APPLICATIONS

G. Consentino

1. ABSTRACT

This application note shows the topologies implemented in the Emergency Lighting Applications and the STMicroelectronics’s 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)Vceo ³ 60 V;

2)Vces ³ 150 V;

3)Vebo ³ 7 V;

4)Ic = 5 A (continuous current);

5)Ib = 1 A (continuous current);

6)Vce(sat) = 140 mV (typ) @ Ib = 50 mA @ Ic = 2 A (typical conditions);

7)Hfe = 270 (typ) @ Ic = 2 A @ Vce = 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 DCAC 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

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The power bipolar transistor collector current Ic depends on the load, turns rapport

K =	N2	(3.1)
	N
	1
	2

where N2 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

N2

N1 (3.2)

2

is about 30, the current Ic 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 Vdc so that the typical Vce_max is around 10-20 Vdc. The voltage and current values ranges, Vce_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

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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

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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.

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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 T1 has three windings. The primary winding vices are connected to the collectors of the NPN power bipolar transistors Q1 and Q2. The same primary winding has a central vice where the inductor L1 is connected. The secondary winding vices are connected to the load.

The third winding vices are connected to the base of the transistors Q1 and Q2 so that when the first is on, the second is off and vice versa. During the Q2 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 T1 is much lower compared to the inductance L1. The resonance frequency of the PUSH-PULL converter is also due to LT. N2 (secondary winding turns) and N1/2 (half primary winding turns) rapport is around 60, while N1/2 and N3 (third winding turns) rapport is around 5. Considering a 6 Vdc input voltage, the voltage

v1max (the max voltage across the vice of the primary winding central point and the reference) can be written as:

v		=	p	× V =	3.14	6 @ 9V	(6.1)
	1max
		2		dc	2

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v2max (the max voltage across the secondary winding vices) can be written as:

v2 max	=	p	× Vdc	N2		=	3.14	6 * 60 @ 560V	(6.2)
				(N1	)
	2					2
				2

v3max (the max voltage across the vices of the third winding) can be written as:

v3 max	=	p	× Vdc	N3	=	3.14	6 *	1	@ 2V	(6.3)
				N1
	2				2		5
				2

As exposed above, it is highlighted N1/2 and not N1. In order to understand the reason of it, it is necessary to consider the graph below.

Figure 6: Particular of T1

When Q2 is on, Q1 is off and vice versa. Now, considering fig. 6 where T2 is on; the current ‘I’ flows through the half primary winding 'b' and it generates a magnetic force (Hopkinson law):

	N1	×I = Â × F	(6.4)
	2

Φ is the magnetic flux and is the magnetic reluctance of the T1 core;Φ can be written as:

		N1	×I	(6.5)
	F =	2
		Â
	can be written as:
	Â =	l		(6.6)

μ × A

µ is the core permeability, A is the core section and l is the core length. When T2 switches off, T1 switches on, the current flows through the other half primary winding 'a' and the flux Φ inverts its direction. Such flux flows into the transformer core creating a link with N2, N3 and also with the other

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turns N1/2, generating the voltages v2 and v3 (magnetic law-Lenz law):

v2 = −N2 ΔΦ ; t

v

= −N

ΔΦ

;

(6.7)

3

3

t

v1/ 2 = −

N1 ΔΦ

2

t

v2

=

N2

,

v3

=

N3

,

vc 2

= 2

(6.8)

v1

N1

v1

N1

v1

2

2

Furthermore, i2 (the current that flows through the lamp) can be written as:

	N1		1	(6.9)
i2 = I	2	= I
	N2		K

In fact, the apparent input power can be written as:

			Ain		= v1I				(6.10)
The output power can be written as:
			Aout		= v2i2				(6.11)
Considering an ideal transformer:
			v2i2 = v1I						(6.12)
	i2		v1			N1		1	(6.13)
		=		=		2	=
	I		v2			N2		k

Before the lamp strike, or when the lamp is disconnected, the operation frequency (about 60 KHz) is due to the resonance between C2 and the primary transformer winding inductance LT (see fig. 7).

Figure 7: Resonant Schematic Circuit Before the Lamp Strike

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f =		1			(6.14)
	2 ×π
			LTC2

When the lamp is connected, the transformer circuit can be showed as in the graph below.

Figure 8: Ideal Schematic Circuit of the Transformer After the Lamp Strike

The input apparent power can be written as:

Ain = v1i1

(6.15)

Now it is possible to consider an equivalent circuit to fig. 8, as in fig. 9, where the apparent input power is equal.

Figure 9: Equivalent Schematic Circuit of the Transformer After the Lamp Strike

Furthermore, after the lamp strike, the resonant schematic circuit can be represented as in figure 10 where, usually, the operation frequency is due to LT, C2 and C1K2 (25-30 KHz).

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Figure 10: Resonant Schematic Circuit of the Transformer After the Lamp Strike

In this transformer equivalent circuit the output impedance has been transferred from the secondary winding to the primary winding.

		v1I = v			2i2 = i2			2	(RLamp					- j					1			)						(6.16)
																	ω ×C1
v I					1				V I						v	1				N	2	2			v	1	k 2
1	= (RLamp			- j			)	=		1				=										=				(6.17)
2					ω ×C1					i2 i2					I (N1								)2
i2																					2				I
		v1		= zeq1 =			1		(RLamp					- j					1			)						(6.18)
		I				k 2												ω ×C
																			1

Where:

	RLAMP	(6.19)
	k 2
is the primary equivalent resistance and where:

C1K2

is the primary equivalent capacitance.

Now, the equivalent primary admittance (Yeq1) can be written as:

Yeq1	=	- j	+ jω ×C2	+		k 2 jωC
						1
		ω ×LT				+ jCRLampω )
				(1

and where:

k 2 jω ×C1

(1+ jCRLampω )

is the admittance of the series net

RLamp -C k 2
k 2	1

(6.20)

(6.21)

(6.22)

(6.23)

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Considering
RLamp	(6.24)

k 2

negligible compared to

(6.25)

1

ω ×C1k 2

deriving Yeq1 compared to the pulsation and equal to zero, it is possible to achieve the frequency that maximizes, the Yeq1 (such frequency is the resonance frequency of the application during the lamps on state).

ω	2	@			1				(6.26)
			LT (C		2	+ k 2C )
								1
f @					1					(6.27)
	2 ×π LT (C						2	+ k 2C )
								1

When the board is powered, R1 and R2 enable Q1 and Q2 and the lamp turns on. After the lamp start-up, during the Q2 on state, the current flows through L1, the half primary winding transformer T1 and Q2, and it increases as:

tgα =	vL1 * Dt	(6.28)
	L1

angular coefficient but, after a while, the current curves and it becomes flat. However, in the permanent state, even if the current oscillates around its average value, there is a ripple of this same value. The current ripple decreases increasing the inductance value L1.

Figure 11 shows the PUSH-PULL current FED converter schematic circuit with the theoretical waveform of ‘I’.

Figure 11: PUSH-PULL Current FED Converter Schematic Circuit with the Theoretical Waveform

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After the strike, ‘I’ generates the current i2 and, at the beginning, the same i2 can be written as:

i2	=	v	2	(6.29)
		RLamp

because the capacitor C1 is discharged. Immediately after, C1 gets charged and i2 decreases to zero until the voltage across C1 reaches the maximum value. A this time, the current i2 inverts its direction and the capacitors C1 start discharging until the charge inside it becomes zero and the current i2 reaches its maximum negative value. Furthermore, when i2 inverts itself, also the voltage across the third winding inverts its direction so that Q2 switches off and Q1 switches on and ‘I’ flows through the other half primary winding of the transformer T1 (see fig. 11).

Figure 12: PUSH-PULL Current FED Converter Schematic Circuit with vc1, vc2, i2, and vt1b2 Theoretical Waveforms

In the above graph, vc2 is the voltage between the vices of the Q1 and Q2 collectors. The maximum value of such voltage is twice v1, where v1 is the voltage between the vices of the central point of the primary winding of T1 and the reference. The voltage v1 is a half positive sine wave and this reaches the maximum value when Q1 or Q2 are on, while it drops to zero during the turn-off and the turn-on of the same transistors (see fig. 13).

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