ST AN1902 Application note

AN1902

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- APPLICATION NOTE

VIPower: HF CO NVERTER BASED ON VK 06TL

DEVICES TO DRIVE 58W TL TUBES

N. AIELLO - S. MESSINA

This document des cribes a re ference desi gn for Lightin g Ballast dedicate d to drive 58 W T8 tubes. The board accepts DC in put vo l tage (u p to 430V ) reali zing the c athod es pr ehea ti ng, t he E oL prot ecti on a nd the maximum current limitation. It is based on the new VK06 device that integrates the controller and the Power stage on the same chip. It is housed in SO-16 and SIP-9 packages.

INTRODUCTION

The European Comm unity has agreed on a new direct ive for banning electro magnetic control gea r for fluorescent lamps. The aim is to improve the system efficiency (EEI-Energy Efficiency Index) reducing the environmen tal impact. This new di recti ve div ide s the b all ast in different classe s fr om A1 to D . A1 is the most efficient system, D the least efficient.

■ A1 → Dimmable electronic

■ A2 → Low-loss electronic

■ A3 → Standard electronic

■ B1 → Extra low-loss magnetic

■ B2 → Low-loss magnetic

■ C → Normal-loss magnetic

■ D → High-loss magnetic

Since 1998, the energy classi fication has be come compulsory and it has be en inserted in a Cenelec standard. It means:

- since April 2002, all ballasts with an EEI of D are banned;

- starting from October 2005, all ballasts with an EEI of C will be banned. Thus the market is aski ng for cost effectivene ss, good per formance, l ow noise and com pact ballasts to

feed this kind of applications. The VK06 is a very suitable device, satisfying all the requirements with few external components.

The proposed reference design can supply 58W T8 FL tube with preheating function and EoL protection. Being the design reference focused on the converter realization (we don ’t co ver the PFC stage) it has been set to give out the right output power when 400V dc voltage is applied.

1. VK06 DESCRIPTION

The VK06 is a monolithic device made by using the VIPower® M3-3 STMicroelectronics proprietary technology that integrates in the same chip a vertical flow Power stage and a BCD based control circuit. The Power stage is made by a high v oltage Bipolar transistor together with a low voltage n-channel MOS transistor in emitter switching configuration Its performances are a good trade-off betwee n the Bipolar transisto r low drop/hig h breakdow n voltages and the MOS transi stor hi gh switching s peed. The block diagram is shown in figure 1.

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In the control part the following sections can be analyzed:

1) Supply

2) Oscillator/Trigg er

3) Diac

4) Protections

Figur e 1: VK06 Internal Block diagram

VCC

SEC

DIAC

CapPREH

Vdd

CLAMP

Vref1

DIAC

Reset Preheating

Vcc

diac on/off

COLL

sec on/off

protection

Vref3

Vcc

Reset CAP1

Vdd

Vref2

Vdd

Supply

Vref4

CAP2 CAP1 GND

Delay on

protection

Gate Driver

latch

Vcc

Vcc charge

VddVcc

Vref6

CapEOL

Bipolar Driver

Over Curr ent Detector

Over Temperature Detector

Vcc

Vref5

COLL

sense

1.1 SUPPLY (Figure 2) The device is supplied from the VCC pin connected to an R-C network. From VCC both the control and the power stage are supplied. A t start up the supply capacitor is charged through a resistor and only few

hundreds µ A are ne eded. Duri ng the opera ti on the devic e is s elf-supplied recovering o n VCC the charge taken from the Power Bipolar base at turn-off. The voltage on VCC is internally clamped at ~6.8V.

Figur e 2: Internal Supply Block

Bipolar Driver

Vcc

COLL

sense

DC BUS

VCC

CLAMP

Vcc charge

Gate Driver

GND

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1.2 OSCILLATOR/TRIGGER (Figure 3)

It fixes the conver ter working frequenci es (preheating, ign ition, and steady-state ). The tON (conducti on time) is set using S EC, CAP1, CAP2 and C apPREH pins. T he device is tri ggered ON when the voltage

on SEC reache s ~2.2V. When this condition is detected the Pow er stage is switched ON and internal current gene rators start to give co nstant currents to C AP1 and CapPREH. T he device will be switched OFF when one of the two following conditions is present: the voltage across CAP1 is equal to the internal voltage referenc e (~2.3V), the voltage on S EC is lower than 0.9V. Using a capacitor on CapPREH and the two frequency capacitors on CAP1 and CAP2 it is possible to have both preheating and steady state frequencies. Until the voltage on CapPR EH is lower than 4.2V only the Cfpreh (capaci tor connected to CAP1) will be cha rged setting the prehea ting frequency. When 4.2V on CapPRE H pin is overcome, an internal switch puts in parallel Cfpreh with the Cfst capacitors (connected between CAP1 and CA P2) lowering th e fre quency to the steady-state one. The value of CapPREH fixes t he pr ehea ting dur ati on. In all the operative conditions the frequency capacitors will be discharged when the voltage on SEC becomes lower than 0.9V.

During the la mp igni tion the frequency cont rol is realized throug h the secondary wi ndin gs wou nd o n the primary choke and connected to the SEC pins. In this phase the voltage on SEC reaches 0.9V before the tON is set by the f requency capacitors. The system oscillate at its resonance freq uency (higher than

steady state one ) allowing the tube igniti on. After the tube ignition the tON will be set by the frequenc y capacitors. An internal delay at Power turn-on avoids the hard switching condition.

Figur e 3: Internal oscillator/trigger block

Vcc

Reset CAP1

CAP2 CAP1

Cfst

Vdd

2.3V

Delay o n

Cfpreh

Gate Driver

SEC

2.2V

Vdd

CapPREH

4.2V

Cappreh

1.3 DIAC (Figure 4) Through the DIAC pin two functions are achieved: start of oscillations and reset of the preheating

capacitor CapPREH.

1) St art of oscillation: in OFF condition (voltage on the SEC pin lower than 2.2V) the device can be turned ON when t he voltage across DIAC ov erco mes ~ 28V. An HV diode keeps the DIAC low when the Po wer stage is ON.

2) Reset of preheating capacitor: in order to guarantee the right preheating timing the preheating capacitor must be discharg ed before starti ng oscilla tions. To realize this function a switch on CapPREH

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pin is activated when the voltage across DIAC pin overcomes ~12V. On the other side the diac can activate the circuit only when the voltage on CapPREH becomes lower than ~0.6V.

Figur e 4: Internal diac block

DC BUS

DIAC

CapPREH

Cappreh

SEC

DIAC

Reset Preheating

Vref1

diac on/off

COLL

Vcc

sec on/of f

Vcc

0.6V

1.4 PROTECTIONS (see figure 5) The device is protected against over-current and over-temperature. Both protections are activated

connecting on the CapEOL p in a capacitor that fixes the timing. T he over-curren t protection works as follows: an internal Rsense checks the cur rent through the Power stage and if it exceeds ~1.5A, an internal generator gives current to CapEOL pin. When the voltage across CapEOL pin reaches ~4.3V the Power stage is kep t OFF, the diac is deactivated and the current consum ption from VCC is lowered. At

the same time another current generator is activated latching the device in OFF state.

The thermal prote ction is activated when the junction temperatu re exceeds ~150°C. This block, wh en activated, acts on the same EoL circuit latching the device.

Figur e 5:. Internal protections block

COLL

Vcc

Capeol

diac on/off

Vdd

4.3V

Gate Driver

Over Current Detector

Vcc

Over Temperature Detector

CapEOL

Rsense

Vref

DIAC

latch

protection

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2. PACKAGES

The VK06 is ass embled in two different packages in order to cover both the s urface mounting and the through-hole PCB. The packages are the SO-16 narrow and the SIP-9 (see figure 6).

Figur e 6: Package outline and pin configuration

SO16 PACKAGE SIP9 PACKAGE

N° pin Name N° pin Name

1 CAP1 1 CAP1 2 CapPREH 2 CapPREH 3 GND 3 GND 4DIAC 4DIAC 5 SEC 5 COLL. 6V

7CAP2 7V

6SEC

8 CapEOL 8 CAP2

9÷16 COLL 9 CapEOL

In figure 7 the SO-16 thermal characterization is reported. In this package eight pins are connected to the tab to reduce the junction-pin thermal resistance whereas the case-ambient thermal resistance is related to the copper a rea on the P CB (device h eat-s ink). The devi ce has bee n charac terized at th ree differe nt

copper ar eas: 0.5, 1 and 2 cm2; at three different po wer dissipatio ns: 0.25, 0.5 and 1W an d measur ing the devices case temperatures.

Figur e 7: SO-16 R

th case-ambient

°C/W

Vs. PCB Copper Area

160 150 140 130 120 110 100

90 80 70 60 50

0 0.5 1 1.5 2

Rth case-ambient

1W 0.5W 0.25W

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In Figure 8 the SIP-9 thermal characterization (no heat-sink) is reported.

Figur e 8: SIP-9 R

th case-ambient

[°C/W]

(no heat-sink)

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2

Rth case-ambient

[W]

3. CONVERTER DESCRIPTION

In figure 9 th e elec t ric al sc hem e, re ali zing a v oltage-fe ed c on verte r based on VK06 is repo rted. The two windings Ls connected to the S EC p ins are wou nd on th e chok e Lp. Four different o perative phases can

be described:

- start-up

- preheat

- steady state

- ignition

Figur e 9: Typical application circuit

R14

C12

C15

C16

C10

VCC CAP2

CAP1

CapPREH

VCC CAP2

CAP1

CapPREH

CapEOL

COLL

DIAC

SEC

GND

COLL

DIAC

SEC

GND

C8 C7

R15

C11

Dz2

C14

R10

Tube

DC BUS

Cbulk

D1 Dz1 R6

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3.1 START-UP

As soon as the system is supplied, the VCC capacitors C3 and C1 2 start to be ch arged from the DC bus respectively by means of the resistors R1 and R14-R10. At the same time, the low side diac capacitor C4

starts to be charged by the resistor R3. The network R15-C11 biases the high side DIAC. In normal ope rations the l ow side device is the one to go o n the fir st time, wh ile in the h igh side d evice

the DIAC pin, clamped by Dz2, is used only to reset the preheating capacitor C9. As soon as the voltage on C4 reaches about 28.5V the diac block switch ON the device. At that time both

devices must b e biased with the minim um requested VCC voltage (~ 5V) in order to make the system oscillate properly. In figure 10 low side (LS) diac and VCC typical wave forms are shown. In the picture the VCC voltage reaches 6.8V after 4.2 m sec rema ini ng constant at th is value (it is internally clampe d) until the converter

starts to oscillate. At that time the storage charge recovery will be responsible f or the device supply, charging the VCC capacitor at the final voltage value (~6.8V).

Figure 10: Typical start-up operation

LS DIAC pin voltage

For the diac and VCC biasing networks the following choices can be done:

R14=R10; R14+R10=R1; C3=C

R15>>R

With this setting the converter midpoint will stay at V the same for any DC bus voltage.

For a proper start up phase the following condition has to be satisfied:

dcbus/2

LS V

pin voltage

and the voltages across C3 and C12 will be

VC3=V

≥ 5V when V

C12

=28.5V

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Where:

Vc3=R1

The networ k R15-C11 must be chosen to have the co mplete discha rge of C9 when th e low side diac strikes the converter. The zener diode Dz2 (~18V) clamps the DIAC pin below the diac activation threshold.

During oscillations the diac capacitors C4 and C11 will be discharged b y the in ternal diod es connect ed between DIAC and Power collector while the VCC capacitor will be charged by the charge recovered from

the Power stage (see figure 11).

Figur e 11: Typical waveforms after the diac strike

Mid point voltage

• C

Vc4=R3

= (R

• C

14 + R10

(low side)

) • C

LS VCC pin voltage

LS DIAC pin voltage

3.2 PRE-HEAT

Still referring to the figure 9, the capacitors C7-C9 (with C7=C9) fix the cathodes preheating time duration. The preheating frequency is set by the capacitors C10-C6 (with C10=C6).

3.2.1 PRE-HEATING TIME

CapPREH pin supplies a constant current I

capPREH

55µA. This current is supplied only during the t

The preheating ends when 4.2V is reached on CapPREH. Assu ming that

∆ 4.2V=

CapPREH

the preheating time t

55µA≅

will be:

preh

∆

CapPREH

preh

-------------------------- -

• 0.15s==

CapPREH

---------------------

/µF

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3.2.2 PRE-HEATING FREQUENCY

For the pre-heating frequency calculation the following considerations can be done:

-- -T t 2

++=

storage

dVdt

storage

const 300n≈= sec

(storage duration of the device Power stage)

tdV/dt is the duration of the snubber capacitor (C14) charge during the half-bridge mid-point commutation between ground and VDC bus.

It can be calculated using the following relationship:

dV/dt

= C

snubber

∆V/i

peak

where:

■ ∆V (DC bus voltage);

■ i

(peak current);

peak

■ C

(snubber capacitance).

snubber

tON is the conduction time fixed by the preheating frequency capacitor (C6) and device characteristics. It can be calculated according to the following formula:

tON=K • C6

Where K=6.7µsec/nF (fixed by the VK06)

3.3 STEADY STATE

The steady stat e fre quen cy is se t by the par all el of the capac itors C 6- C16 for the l ow si de a nd C10- C 15 for the high side, where C16=C15. The same formulae of the preheat can be applied:

-- -T t 2

storage

++=

const 300n≈=

storage

dVdt

sec

KC6C16+()=

3.4 IGNITION

The converter of figur e 9 has two different resonan ce frequencies, the firs t one before the tube igni tion the second o ne after the tube ignition . Th e con verte r operation from preh eat to st e ady state is shown in figure 12.

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Figure 12: Load typical resonance curves

Resonance frequency after the tube ignition (f2).

Where the two frequencies are:

------------------------------------------------

π L

-----------------------

π L

seriesC

()

PC1

PC2

Resonance frequency before the tub e ignition(f1).

fpreh

fign fsteady

(before the tube ignition)

(after the tube ignition)

In good b allast design the cathode prehe ating is requeste d in order to increase the tub e lifetime. I t is obtained making high curren t flow through the cathodes for a fixed time. A simple rule for the prehea ting efficiency check is reported:

1) measure the cathode resistance at the beginning of the preheating;

2) measure this resistance at the end of the preheating;

3) if its value is increased 3-4 times, the cathodes will work at the right temperature during ignition.

During the preheating the current level has to be able to heat the cathodes without generating the ignition voltage on the start-up capacitor C1. Still referring to figure 12, the converter will operate as follows: it will start working at the pre heat in g frequ ency (do t 1) that mu st be higher than the re sonance freque nc y f1. It will remain in this condition for the time fixe d by the preheating capacitor. After the preheating the device frequency control is taken by the two secondar y windings mov ing the working fre quency up to the dot 2 where the tube is supposed to ignite. Once the tube is ignited the converter resonance frequency is lowered to f2 and the conver ter can work at the steady state frequency (dot 3) fixed by the o scillato r capacitors.

In figure 13 the complete start-up sequence is shown.

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