ST AN2018 Application note

AN2018

- APPLICATION NOTE

VIPower: 18 - 23W ELECTRONIC BALLAST FOR

REMOVABLE TUBES

ABSTRACT

This application note describes ele ctronic high frequency bal last based on VK05CF L 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 balla st. In a monolithic structure it integrates the power stage and the logic part for the converter control. Using two VK05C FL and few of external components it is pos sible to realize a h igh frequency c onverter 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) resonan t mode, in order t o reduce transistor switching loss es and electromagnetic interf erence generated by t he output wiring and the lamp. In t he proposed balla st 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, R1 1, 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 sect ion is composed b y a fuse resist or R and an input filter L1C2 that provide DC voltage and improve EMI performances according to IEC 610003-2 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 m eans input filter, R5, R5_1, C5 and R6, R6_1, C6, that provides a proper supply voltage delaying the Sec pins vol tage com pared to th e sec ondary win ding vol tage in order t o av oid hard switching condition.

a full bridge diode rectifier D1, a bulk capac itor C

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AN2018 - APPLICATION NOTE

Figure 1: Electrical scheme.

R16

C16

C13

C15 R15

Dz1

R14

R17

R13

R13_1

220V

TUBE

TR-CFL

R6_1

Dz3

R12 C12

VK05CFL

Dz2

R11 C11

VK05CFL

R5_1

R9_1

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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 Capacit or 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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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 hi gh 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 prehea ting time the tube p resents high impedanc e and the cu rrent flows through the filaments growing their resistance value. There is a simple way to determine the right preheating current/time valu e: the ratio b etween the cathode res istance before an d after t he preheat ing

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 rep lacement. At s tart-up the ballast is in OF F state, when the voltage on C13 reaches t he internal diac threshold (~ 30V ) the Low side device is turned ON mak ing 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 reduc tion and con sequent increasing in the working freque ncy. Since the preheating network is applied on both devices the fifty percent of duty-cycle is guaranteed.

At the end of preheating time, af ter 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 t he C11, C12 preheating capacitors allo wing a prehe ating 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 frequenc y are variable during preheating phase, t his is due to the fact the injected current is:

=(V

pre

sec-Vcappreh-Vfdiode

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 (I

=0) the preheating circuit and the ballas t frequency becom e steady state frequency

pre

fixed by C7, C8.

)/R

2.5 EoL description

The ballast shall not impair saf et y when abnormal and fault conditions happen. Abnorm al c ondition s 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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