®
AN1902
-
APPLICATION NOTE
VIPower: HF CONVERTER BASED ON VK06TL
DEVICES TO DRIVE 58W TL TUBES
N. AIELLO - S. MESSINA
This document describes a reference design for Lighting Ballast dedicated to drive 58W T8 tubes. The
board accepts DC input voltage (up to 430V) realizing the cathodes preheating, the EoL protection and
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 Community has agreed on a new directive for banning electromagnetic control gear for
fluorescent lamps. The aim is to improve the system efficiency (EEI-Energy Efficiency Index) reducing
the environmental impact. This new directive divides the ballast in different classes from A1 to D. A1 is
the most efficient system, D the least efficient.
A1
→
Dimmable electronic
s
A2
→
Low-loss electronic
s
A3
→
Standard electronic
s
B1
→
Extra low-loss magnetic
s
B2
→
Low-loss magnetic
s
C
→
Normal-loss magnetic
s
D
→
High-loss magnetic
s
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b
Since 1998, the energy classification has become compulsory and it has been 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 asking for cost effectiveness, good performance, low noise and compact 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 cover the PFC
stage) it has been set to give out the right output power when 400V dc voltage is applied.
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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 voltage Bipolar transistor together with a low voltage n-channel
MOS transistor in emitter switching configuration Its performances are a good trade-off between the
Bipolar transistor low drop/high breakdown voltages and the MOS transistor high switching speed. The
block diagram is shown in figure 1.
March 2004
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AN1902 - APPLICATION NOTE
In the control part the following sections can be analyzed:
1) Supply
2) Oscillator/Trigger
3) Diac
4) Protections
Figure 1:
VK06 Internal Block diagram
VCC
CLAMP
Supply
Vcc
SEC
Vref1
diac on/off
DIAC
DIAC
Vcc
Vref4
Vdd
protection
COLL
Vcc
CapPREH
Reset
Preheating
Vref3
Vdd
protection
Vcc
Reset
CAP1
Vref2
sec on/off
Delay on
Vdd
Vcc charge
COLL
Vcc
Bipolar Driver
Gate Driver
Vcc
latch
Vdd
Over Current
Detector
Over Temperature
Detector
CAP2
1.1 SUPPLY
(Figure 2)
The device is supplied from the V
CC
pin connected to an R-C network. From V
CC
both the control and the
power stage are supplied. At start up the supply capacitor is charged through a resistor and only few
hundreds µA are needed. During the operation the device is self-supplied recovering on V
CC
the charge
taken from the Power Bipolar base at turn-off. The voltage on V
CC
is internally clamped at ~6.8V.
Figure 2:
Internal Supply Block
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Vcc charge
CAP1
GND
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CapEOL
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R
sense
Ic
CLAMP
COLL
Vcc
Bipolar Driver
Ic
Gate Driver
R
sense
GND
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AN1902 - APPLICATION NOTE
1.2 OSCILLATOR/TRIGGER
(Figure 3)
It fixes the converter working frequencies (preheating, ignition, and steady-state). The t
ON
(conduction
time) is set using SEC, CAP1, CAP2 and CapPREH pins. The device is triggered ON when the voltage
on SEC reaches ~2.2V. When this condition is detected the Power stage is switched ON and internal
current generators start to give constant currents to CAP1 and CapPREH. The device will be switched
OFF when one of the two following conditions is present: the voltage across CAP1 is equal to the internal
voltage reference (~2.3V), the voltage on SEC 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 CapPREH is lower than 4.2V only the Cfpreh (capacitor connected to
CAP1) will be charged setting the preheating frequency. When 4.2V on CapPREH pin is overcome, an
internal switch puts in parallel Cfpreh with the Cfst capacitors (connected between CAP1 and CAP2)
lowering the frequency to the steady-state one. The value of CapPREH fixes the preheating duration. In
all the operative conditions the frequency capacitors will be discharged when the voltage on SEC
becomes lower than 0.9V.
During the lamp ignition the frequency control is realized through the secondary windings wound on the
primary choke and connected to the SEC pins. In this phase the voltage on SEC reaches 0.9V before the
t
ON
is set by the frequency capacitors. The system oscillate at its resonance frequency (higher than
steady state one) allowing the tube ignition. After the tube ignition the t
ON
will be set by the frequency
capacitors.
An internal delay at Power turn-on avoids the hard switching condition.
Figure 3:
Internal oscillator/trigger block
SEC
2.2V
Vcc
Vdd
CapPREH
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4.2V
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Reset
CAP1
CAP2
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Gate Driver
2.3V
Vcc
CAP1
Cfst
Cfpreh
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) Start of oscillation: in OFF condition (voltage on the SEC pin lower than 2.2V) the device can be turned
ON when the voltage across DIAC overcomes ~28V. An HV diode keeps the DIAC low when the Power
stage is ON.
2) Reset of preheating capacitor: in order to guarantee the right preheating timing the preheating
capacitor must be discharged before starting oscillations. To realize this function a switch on CapPREH
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AN1902 - APPLICATION NOTE
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.
Figure 4:
Internal diac block
DC BUS
SEC
Vref1
DIAC
DIAC
diac on/off
Vcc
0.6V
Vcc
sec on/off
COLL
CapPREH
Cappreh
Reset
Preheating
1.4 PROTECTIONS
(see figure 5)
The device is protected against over-current and over-temperature. Both protections are activated
connecting on the CapEOL pin a capacitor that fixes the timing. The over-current protection works as
follows: an internal Rsense checks the current 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 kept OFF, the diac is deactivated and the current consumption from V
CC
is lowered. At
the same time another current generator is activated latching the device in OFF state.
The thermal protection is activated when the junction temperature exceeds ~150°C. This block, when
activated, acts on the same EoL circuit latching the device.
Figure 5:.
Internal protections block
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Vcc
latch
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diac on/off
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DIAC
Gate Driver
Vdd
Over Current
Detector
Vcc
Vcc
Over Temperature
Detector
R
sense
Vref
protection
4.3V
CapEOL
Capeol
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AN1902 - APPLICATION NOTE
2. PACKAGES
The VK06 is assembled in two different packages in order to cover both the surface mounting and the
through-hole PCB. The packages are the SO-16 narrow and the SIP-9 (see figure 6).
Figure 6:
Package outline and pin configuration
9
16
8
1
9
1
SO16 PACKAGE
N° pin
1
2
3
4
5
6
7
8
9÷16
Name
CAP1
CapPREH
GND
DIAC
SEC
V
CC
CAP2
CapEOL
COLL
N° pin
1
2
3
4
5
SIP9 PACKAGE
Name
CAP1
CapPREH
GND
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 area on the PCB (device heat-sink). The device has been characterized at three different
copper areas: 0.5, 1 and 2
at three different power dissipations: 0.25, 0.5 and 1W and measuring
the devices case temperatures.
Figure 7:
SO-16 R
th case-ambient
Vs. PCB Copper Area
cm
2
;
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160
°C/W
150
140
130
120
110
100
90
80
70
60
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8
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DIAC
COLL.
SEC
V
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CAP2
CapEOL
Rth case-ambient
0
0.5
1W
1
0.5W
0.25W
1.5
Cm
2
2
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