ST AN2018 Application note

		AN2018
®		- APPLICATION NOTE
	VIPower: 18 - 23W ELECTRONIC BALLAST FOR
		REMOVABLE TUBES

ABSTRACT

This application note describes electronic high frequency ballast based on VK05CFL designed to drive from 18 to 23W TC DEL and T5 removable fluorescent tubes.

The design was performed for 185÷265 Vac main voltage.

1. DESCRIPTION

The VK05CFL is a dedicated device for realizing low power electronic ballast. In a monolithic structure it integrates the power stage and the logic part for the converter control. Using two VK05CFL and few of external components it is possible to realize a high frequency converter in a very simple way reducing the complexity and the cost of the application. The topology is the standard half bridge in voltage fed operating in zero voltage switching (ZVS) resonant mode, in order to reduce transistor switching losses and electromagnetic interference generated by the output wiring and the lamp. In the proposed ballast the preheating and the End of Life (EoL) function are realized without the use of PTC and high voltage components with system reliability increasing. SMD passive components are used in order to reduce PCB dimensions.

2. ELECTRICAL SCHEME

In Figure 1 the electrical scheme of 23W ballast is reported. The demo-board can be adapted to different needs by connecting or not three jumpers.

Following the obtainable configurations:

1)Pre-heating function requested: J1, J2 closed, and J3 open;

2)Pre-heating not requested: J1, J2 open, J3 closed, C11, R11, D2, R7, R12 not mounted.

2.1 Circuit description

About the circuit description, the used topology was already described in the AN1546 and AN1694, therefore in this paragraph only the main functions will be described.

The input section is composed by a fuse resistor R0 a full bridge diode rectifier D1, a bulk capacitor C0 and an input filter L1C2 that provide DC voltage and improve EMI performances according to IEC 61000- 3-2 standard.

The net R13-R13_1-C13 connected to diac pin realizes the start-up while R7 (R8), D2 (D3), C11 (C12), R11 (R12) perform the preheating function. The ballast working frequency is set by C7 and C8 capacitors. C9 is the snubber capacitor and R9, R9_1 are the pull-up resistor.

The net Q1, Q2, R14, R15, R16, C15, C16, D4, Dz1, detect the lamp failure (EoL) latching the converter. The devices are synchronized and supplied by two secondary winding turned on ballast choke connect with Sec pins by means input filter, R5, R5_1, C5 and R6, R6_1, C6, that provides a proper supply voltage delaying the Sec pins voltage compared to the secondary winding voltage in order to avoid hard switching condition.

September 2004

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			L1			R5	R5_1
							D2		R9_1
							D5
							R7
						TR	C5		C9
									VK05CF L
							R11	C11
							Dz2	J1	R9
	220V				C3	TR-CFL			C7
				T UBE
			+			R6	R6_1
		C0
			C2
		D1
							D3
				C4
							D6
			R13	R13_1			R8
		R0							VK05CF L
	N			Dz1
				R14
				R17
					D4
NOTE				C15	R15		R12	C12
			Q1				Dz3
								J2
- APPLICATION
	Electrical scheme.
					Q2	C13	C6
			R16				J3
									C8
				C16
AN2018	Figure 1:								2/11

AN2018 - APPLICATION NOTE

2.2 Component list

In table 1 the material list for both circuits is reported.

Table 1: Component list 18W.

Reference	Value	Description
C0	4.7µF, 450V	Electrolytic Capacitor
C2, C3	100nF, 400V	Capacitor
C4	2.7nF, 400V 5%	Start Up Capacitor
C5, C6, C7, C8	1,2nF, 63V	SMD Capacitor
C9	470pF, 630V	Snubber Capacitor
C11, C12	22µF, 35V	Capacitor
C13, C15, C16	22nF, 63V	SMD Capacitor
D1		SMD Bridge
D2, D3, D4, D5, D6		SMD Diode 1N4148
Dz1	12V	Zener Diode
Dz2, Dz3	20V	Zener Diode
IC1, IC2		STMicroelectronics VK05CFL
L1	820µH	Inductor
Q1		Transistor PNP MPSA92A
Q2		Transistor NPN MPSA42A
R0	22Ω 1W	Fuse Resistor
R5, R5_1, R6, R6_1	1KΩ 1/4W 5%	SMD Resistor 1206
R7, R8	10KΩ	SMD Resistor
R14, R17	27KΩ	SMD Resistor
R11, R12	1MΩ	SMD Resistor
R9, R9_1, R13, R13_1	510KΩ 200V	SMD Resistor 1206
R15, R16	33KΩ	SMD Resistor
T1	2.1 mH N1/N2=10:1 5%	Resonant Inductor VOGT (Drawing: LL001 023 21)
Table 2: Component list 23W.
Reference	Value	Description
C0	6.8µF, 350V	Electrolytic Capacitor
C2, C3	100nF, 400V	Capacitor
C4	2.7nF, 400V 5%	Start Up Capacitor
C5, C6	1nF, 63V	SMD Capacitor
C7, C9	1nF	SMD Capacitor
C9	470pF, 630V	Snubber Capacitor
C11, C12	10µF, 35V	Capacitor
C13, C15, C16	22nF, 63V	SMD Capacitor
D1		SMD Bridge
D2, D3, D4, D5, D6		SMD Diode 1N4148
Dz1	13V	Zener Diode
Dz2, Dz3	20V	Zener Diode
IC1, IC2		STMicroelectronics VK05CFL
L1	1000µH	Inductor
Q1		Transistor PNP MPSA92A
Q2		Transistor NPN MPSA42A
R0	22Ω 1W	Fuse Resistor
R5, R5_1, R6, R6_1	1KΩ 1/4W 5%	SMD Resistor 1206
R7, R8	9.1KΩ	SMD Resistor
R14, R17	27KΩ	SMD Resistor
R9, R9_1, R13, R13_1	510KΩ 200V	SMD Resistor 1206
R11, R12	1MΩ	SMD Resistor
R15, R16	33KΩ	SMD Resistor
T1	2.1 mH N1/N2=10:1 5%	Resonant Inductor VOGT (Drawing: LL001 023 21)

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AN2018 - APPLICATION NOTE

2.3 Start-up description

When a fluorescent lamp is switched on, the main voltage is not sufficient to cause the initial ionisation and an element is needed to provide high voltage across the tube. Ionised vapour radiates light in the ultra-violet spectrum and is converted to visible light by a fluorescent coating inside the tube.

There are two methods to ignite the tube: cool ignition and warm ignition. Our ballast can perform both methods.

Warm ignition is performed with the cathodes preheating; this method reduces the ignition voltage improving the lamp life. During preheating time the tube presents high impedance and the current flows through the filaments growing their resistance value. There is a simple way to determine the right preheating current/time value: the ratio between the cathode resistance before and after the preheating has to be in the range 3 ÷ 5.

In our circuit the start-up network R13, R13_1, C13 is linked to DC bus by the lamp cathode in order to guarantee automatic restart after lamp replacement. At start-up the ballast is in OFF state, when the voltage on C13 reaches the internal diac threshold (~30V) the Low side device is turned ON making the current flow; the voltage drop on the main choke is transferred to secondary windings confirming the Low side ON state and the High side OFF state respectively. In this phase the tube is an open circuit and Lres-C4 fix the system resonance frequency.

2.4 Preheating description

If the cathodes' preheating is requested, our proposed solution performs this function in the following way:

As soon as the sec pin voltage becomes positive, C11-C12 capacitors start to be charged adding current into C7-C8 capacitors with Ton reduction and consequent increasing in the working frequency. Since the preheating network is applied on both devices the fifty percent of duty-cycle is guaranteed.

At the end of preheating time, after ignition occurs, the capacitors C11, C12, thanks to the presence of diodes D2-D3, remain charged at the sec pin voltage stopping the current injection in C7 and C8. The resistors R11, R12 discharge the C11, C12 preheating capacitors allowing a preheating phase when the converter is switched-on again.

During preheating phase the working frequency is higher than the resonance one and the voltage across the tube is lower than ignition one.

Adopting our preheating circuit, the current and frequency are variable during preheating phase, this is due to the fact the injected current is:

Ipre=(Vsec-Vcappreh-Vfdiode)/R

thus, this current decreases as the preheating capacitor voltage increases; in this way the working frequency moves towards the resonance frequency in order to guarantee the tube ignition.

After this phase (Ipre=0) the preheating circuit and the ballast frequency become steady state frequency fixed by C7, C8.

2.5 EoL description

The ballast shall not impair safety when abnormal and fault conditions happen. Abnormal conditions are classified (European standard) as:

-lamp not inserted

-the lamp does not start because one of the two cathodes are broken;

-the lamp does not start although the cathodes are intact (EoL);

-the lamp operates, but a single cathode is de-activated or broken (rectifying effect).

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