ST AN3032 APPLICATION NOTE

AN3032

Application note

STEVAL-ILB007V1, 2 x 58 W/T8 ballast based on the L6585DE suitable for 2 x 36 W/T8 lamp

Introduction

This application note describes a demonstration board able to drive 2 x 58 W linear T8 fluorescent tubes. The last section of the document describes the changes that need to be made to adapt the same board for 2 x 36 W linear T8 fluorescent tubes.

The ballast is controlled by the new L6585DE IC that integrates the PFC and half-bridge control circuits, relevant drivers, and the circuitry that manages all the operating phases (preheating, ignition and run mode) of the lamp. Protections against failures such as lamp disconnection, anti-capacitive mode and PFC overvoltage are guaranteed and obtained with a minimum number of external components. In addition to the description of the circuit and design criteria, this document provides a short overview of the ballast performances.

Fluorescent lamps are driven more and more by electronic ballasts rather than by electromagnetic ones, primarily because fluorescent lamps can produce around 20% more light for the same input power when driven above 20 kHz instead of 50/60 Hz. Operation at this frequency also eliminates both light flickering (the response time of the discharge is too slow for the lamp to have a chance to extinguish during each cycle) and audible noise. Electronic ballasts consume less power and therefore dissipate less heat than electromagnetic ballasts. The energy saved can be estimated in the range of 20-25% for a given lamp power. Finally, the electronic solution allows better control of the filament current and lamp voltage during preheating with the unquestionable benefit of increasing the average lamp life.

Figure 1. 2 x 58 W T8 ballast demonstration board

!-V

June 2010

Doc ID 16165 Rev 2

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www.st.com

Contents	AN3032

Contents

1	Basis of half-bridge inverter topology . . . . . . . . . . . . . . . . . . . . . . . . .		. 4
2	Main characteristic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
3	Ballast design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
	3.1	L6585DE pin-by-pin biasing circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	3.2	Design of the PFC power section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11

3.2.1 Input capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.2.2 Output capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2.3 Boost inductor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.2.4 Power MOSFET . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.5 Boost diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3 Design of the half bridge inverter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4	Experimental results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
	4.1	Start sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	4.2	Protections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
	4.3	Conducted emissions test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
	4.4	Guidelines for connecting the two lamps to the ballast . . . . . . . . . . . . . .	19
5	Adapting the design for a 2 x 36 W T8 electronic ballast . . . . . . . . . .		20
6	Automatic restart circuit for lamp replacement . . . . . . . . . . . . . . . . . .		21
7	Bill of materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		22
8	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		25

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AN3032	List of figures

List of figures

Figure 1. 2 x 58 W T8 ballast demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. Electronic lamp ballast capacitor-to-ground configuration . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 3. Electronic lamp ballast lamp-to-ground configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 4. Dual lamp ballast series configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 5. Dual lamp ballast parallel configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Figure 6. Electrical schematic 2 x 58 W T8 - main wide range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 7. EOL circuit for first lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 8. L6585DE start-up sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 9. One lamp ignition phase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 10. Low-side current in run mode condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 11. Lamp voltage and current in run mode condition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 12. Run mode, rectifying effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 13. Ignition phase with a broken lamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Figure 14. Conducted emissions at 110 Vac 50 Hz - line 1 peak detector . . . . . . . . . . . . . . . . . . . . . 17 Figure 15. Conducted emissions at 110 Vac 50 Hz - line 2 peak detector . . . . . . . . . . . . . . . . . . . . . 18 Figure 16. Conducted emissions at 230 Vac 50 Hz - line 1 peak detector . . . . . . . . . . . . . . . . . . . . . 18 Figure 17. Conducted emissions at 230 Vac 50 Hz - line 2 peak detector . . . . . . . . . . . . . . . . . . . . . 18 Figure 18. Connecting two lamps to the ballast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Figure 19. Automatic restart circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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Basis of half-bridge inverter topology	AN3032

1 Basis of half-bridge inverter topology

The half-bridge inverter operates in zero voltage switching (ZVS) resonant mode to reduce the switching losses and the electromagnetic interference generated by the output wiring and the lamp. Voltage-fed, series-resonant, half-bridge inverters are currently used for compact fluorescent lamp (CFL) ballasts and for many european tube lamp (TL) ballasts.

In general for lighting applications, and given the current preheating, it is possible to choose between two different resonant circuit topologies: capacitor-to-ground or lamp-to-ground.

Figure 2. Electronic lamp ballast capacitorFigure 3.	Electronic lamp ballast lamp-to-
to-ground configuration	ground configuration

	&5(6
	/5(6	/U5(6	&%ORFN
9GF		9GF	&5(6
			&5(6
	&%ORFN
B		B
	!-V		!-V

In the presented design, a lamp-to-ground configuration has been used.

For dual lamp ballasts, the lamps can be connected in series (Figure 4) or in parallel (Figure 5). The presented system uses a parallel configuration for the following reasons.

●Lower voltage stress on the ballast output stage components, wiring and fixture sockets.

●The resonant L and C associated with the lamps is less sensitive to component tolerances due to the lower-running lamp voltages compared to the series configuration.

●Better lamp control. Both lamps can be monitored independently.

Figure 4. Dual lamp ballast series	Figure 5. Dual lamp ballast parallel
configuration		configuration
&5(6
		&5(6

		/U5(6
/U5(6	/U35(
	/U35(

	&%ORFN		&%ORFN

	!-V		!-V

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AN3032	Main characteristic

2 Main characteristic

The electrical specifications of the lamp ballast are shown in Table 1.

Table 1.	Input and output parameters
		Input parameters

	VIN	Input voltage range	85 to 265 VRMS
	fline	Line frequency	50/60 Hz
		Tube lamp

	Number	2

	Type	T8 in parallel configuration

	Power	58 W

		Expected output parameters

	PF	Power factor	= 0.9

	THD%	Total harmonic distortion	= 10

	η %	Efficiency	˜ 90

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Main characteristic

Figure 6. Electrical schematic 2 x 58 W T8 - main wide range

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AN3032

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AN3032	Ballast design

3 Ballast design

This section describes the main components of the circuit.

3.1L6585DE pin-by-pin biasing circuitry

Designed in a high-voltage BCD offline technology, the L6585DE embeds a PFC controller, a half-bridge controller, the relevant drivers and the logic necessary to build an electronic ballast.

●Pin1 OSC is one of the two oscillator inputs. The value of the capacitor connected to

ground defines the half-bridge switching frequency in each operating state. C5 is set to 1 nF.

●Pin2 RF: the choice of component and oscillator capacitance defines the half-bridge

switching frequency in each operating state. A resistor R14 connected to ground sets the run frequency, while during the preheating phase the switching frequency is set by

the parallel of the above resistance with the R13 resistor connected between the RF and EOI pins (the EOI pin is pulled to ground during preheating).

With the following frequencies and ignition time:

frun = 40kHz fpre = 65kHz tign = 60ms

R14 can be calculated with the following formula.

Equation 1

e =1−

1.33

k =

499.6 103

1/ e

= 33kΩ

(C5 )0.581

)0.872

R14

= f

run

The value of R13 is therefore given by:

Equation 2

k 1/ e

//R

= 51kΩ

pre

●Pin3 EOI is a multi-function pin. During preheating, the pin is internally shorted to ground by the logic, so the resistor (Rpre//Rrun) connected between the RF pin and ground sets the preheating switching frequency. During ignition it goes into a high impedance state: the ignition time is the time necessary for the pin voltage to -

exponentially - rise from zero to 1.9 V. The growth is steered by the C6*R13 time constant; since the value of R13 has already been calculated and tign at the start is fixed, C6 is calculated with the following formula.

Equation 3

C6	=	tign	= 392nF
		R13
	3

Three capacitors in parallel have been mounted to obtain this value (C6 = 220//150//22 nF).

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Ballast design	AN3032

●Pin4 TCH is the time counter and is activated during the preheating phase as well as after a protection is triggered (HBCS crossing during ignition/run mode, window

comparator at EOL). To achieve this, an R15C7 parallel network is connected between this pin and ground. With a protection time tTch,reduced fixed at 0.27 seconds (needed for the startup sequence with old or damaged lamps), C7 can then be calculated.

Equation 4

tTch,reduced C7 0.26974 106 C7 =1μF

With tpre set to 1 second and considering the internal current generator ICH = 31 µA, R15 can be calculated.

Equation 5

	tpre	−	C7			4.63
			ICH
R15 =							= 755kΩ 750kΩ
	C7 ln			4.63


					1.5

● Pin5 EOLP is a 2 V reference and allows programming the window comparator of Pin6 (EOL) according to the values defined inTable 4 in the L6585DE datasheet. Working in a lamp-to-ground configuration, a fixed reference mode has been selected, and for a window voltage amplitude of ± 240 mV, R16 has been set to 75 kΩ.

● Pin6 EOL is the input of the window comparator. Concerning this comparator, the fixed reference configuration requires two Zener diodes to shift the mean value of the lamp voltage to 2.5 V. The values of the two Zener diodes relate to the symmetry of the protection intervention, and the best symmetry is obtained by choosing two values whose difference is equal to twice the reference voltage.

Referring to the first series lamp (Figure 7):

Equation 6

VKmax = 2.5 + VfD15 + VZD16 + W2

VK min = 2.5 −(VzD15 + VfD16 )− W2

2 2.5 = VzD15 − VzD16 VzD16 = 5.1V,VzD15 =10V

If we consider that VfD15 = VfD16 = 0.7 V and take into account that W/2 = 0.240 V, the maximum/minimum voltage on the low resistance of the voltage divider of the lamp is

VK = 8.2V .

With R56 equaling 1.8 MΩ, considering the current capability of EOL and fixing the maximum deviation voltage lamp Vlamp = 18V , the value of R57 can be calculated as 1.5 MΩ.

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