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AN1902 |
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- APPLICATION NOTE |
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VIPower: HF CONVERTER BASED ON VK06TL |
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DEVICES TO DRIVE 58W TL TUBES |
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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 |
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The European Community has agreed on a new directive for banning electromagnetic control gear for |
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fluorescent lamps. The aim is to improve the system efficiency (EEI-Energy Efficiency Index) reducing |
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the environmental impact. This new directive divides the ballast in different classes from A1 o D. A1 is |
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the most efficient system, D the least efficient. |
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■ A1 → Dimmable electronic |
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■ A2 → Low-loss electronic |
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■ D → High-loss magnetic |
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■ A3 → Standard electronic |
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■ B1 → Extra low-loss magnetic |
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■ B2 → Low-loss magnetic |
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■ C → Normal-loss magnetic |
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Since 1998, the energy classification has become compulsory and it has been inserted in a Cenelec |
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standard. It means: |
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- since April 2002, all ballasts with an EEI of D are banned; |
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- starting from October 2005, all ballasts with an EEI of C will be banned. |
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Thus the market is asking f |
r cost effectiveness, good performance, low noise and compact ballasts to |
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feed this kind of applications. The VK06 is a very suitable device, satisfying all the requirements with few |
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external compon nts. |
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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
tage) 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 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 |
1/32 |
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 |
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Vcc charge |
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CLAMP |
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Supply |
Vdd |
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Vcc |
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sec on/off |
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SEC |
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Bipolar Driver |
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Vref1 |
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Delay on |
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diac on/off |
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Gate Driver |
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DIAC |
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DIAC |
Vcc |
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Vcc |
Vdd |
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Vref2 |
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Vref4 |
latch |
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Over Current |
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Vdd |
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Detector |
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protection |
Reset |
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COLL |
CAP1 |
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Vcc |
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Vcc |
Rsense |
CapPREH |
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Vdd |
Vcc |
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Vref5 |
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protection |
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Vref3 |
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Over Temperature |
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Detector |
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Reset |
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Vref6 |
CAP2 CAP1 GND |
CapEOL |
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. 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 VCC the charge taken from the Power Bipolar base at turn-off. The voltage on VCC is internally clamped at ~6.8V.
Figure 2: Internal Supply Block
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VCC |
Ic |
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Vcc charge |
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DC BUS |
CLAMP |
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COLL |
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Vcc |
Ic |
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Bipolar Driver |
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Gate Driver |
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Rsense |
2/32
AN1902 - APPLICATION NOTE
1.2 OSCILLATOR/TRIGGER (Figure 3)
It fixes the converter working frequencies (preheating, ignition, and steady-state). The tON (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 onProduct(s)SEC reaches 0.9V before the
tON 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 tON 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 |
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SEC |
Vcc |
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Delay on Gate Driver |
2.2V |
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2.3V |
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Vdd |
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Reset |
Cfst |
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CAP1 |
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CapPREH |
Vcc |
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Vdd |
4.2V |
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CAP2 |
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CAP1 |
ObsoleteThrough the DIAC pin two functions are achieved: start of oscillations and reset of the preheating |
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Cappreh |
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Cfpreh |
1.3 DIAC (Figure 4)
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
3/32
diac on/off
DIAC
Vcc
0.6V
COLL
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
CapPREH |
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Cappreh |
Reset |
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Preheating |
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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 VCC 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. |
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Figure 5:. Internal protections block |
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COLL |
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DIAC |
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diac on/off |
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DIAC |
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Gate Driver |
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Vcc |
Vdd |
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latch |
Over Current |
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Detector |
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Vcc |
Vcc Rsense |
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Vref |
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Over Temperature |
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Detector |
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protection |
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4.3V |
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CapEOL |
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Capeol |
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4/32
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
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16 |
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8 |
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9 |
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1 |
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1 |
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SO16 PACKAGE |
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SIP9 PACKAGE |
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N° pin |
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1 |
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CAP1 |
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1 |
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CAP1 |
2 |
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CapPREH |
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2 |
Product(s)CC |
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CapPREH |
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GND |
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3 |
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GND |
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DIAC |
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4 |
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DIAC |
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SEC |
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5 |
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COLL. |
6 |
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VCC |
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6 |
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SEC |
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CAP2 |
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7 |
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V |
8 |
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CapEOL |
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8 |
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CAP2 |
9÷16 |
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COLL |
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CapEOL |
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9 |
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In figure 7 the SO-16 thermal characterization is reported. In this package eight pins are connected to the |
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tab to reduce the junction-pin thermal resistance whereas the case-ambient thermal resistance is related |
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Product(s)160 |
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to the copper area on the PCB (device heat-sink). The device has been characterized at three different |
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copper areas: 0.5, 1 and 2 cm2; at three different power dissipations: 0.25, 0.5 and 1W and measuring |
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the devices case temperatures. |
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Figure 7: SO-16 Rth case-ambient Vs. PCB Copper Area |
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Rth case-ambient |
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°C/W |
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150 |
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140 |
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130 |
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120 |
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110 |
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100 |
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90 |
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80 |
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70 |
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60 |
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50 |
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0 |
0.5 |
1 |
1.5 |
Cm2 2 |
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1W |
0.5W |
0.25W |
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5/32
AN1902 - APPLICATION NOTE
In Figure 8 the SIP-9 thermal characterization (no heat-sink) is reported.
Figure 8: SIP-9 Rth case-ambient (no heat-sink)
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Rth case-ambient |
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46 |
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[°C/W] |
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45 |
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44 |
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42 |
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41 |
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40 |
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0 |
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0.25 |
0.5 |
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0.75 |
1 |
1.25 |
1.5 |
1.75 [W] |
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3. CONVERTER DESCRIPTION |
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In figure 9 the electrical scheme, realizing a voltage-feed converter based on VK06 is reported. The two |
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windings Ls connected to the SEC pins are wound on the choke Lp. Four different operative phases can |
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be described: |
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Product(s) |
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- start-up |
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- preheat |
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- steady state |
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- ignition |
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Figure 9: Typical application circuit |
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DC BUS |
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R14 |
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COLL |
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R15 |
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VCC |
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CAP2 |
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DIAC |
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C14 |
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C15 |
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C1 |
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SEC |
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C12 |
CAP1 |
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Dz2 |
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CapPREH |
CapEOL |
GND |
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Tube |
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Ls |
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C11 |
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C10 |
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C9 |
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Lp |
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Cbulk |
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C2 |
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R1 |
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VCC |
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COLL |
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R3 |
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CAP2 |
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DIAC |
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R10 |
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Obsolete C16 |
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SEC |
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C3 |
CAP1 |
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CapPREH |
CapEOL |
GND |
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Ls |
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C4 |
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C6 |
C7 |
C8 |
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D1 |
Dz1 |
R6 |
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6/32
AN1902 - APPLICATION NOTE
3.1 START-UP
As soon as the system is supplied, the VCC capacitors C3 and C12 start to be charged 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 operations the low side device is the one to go on the first time, while in the high side device 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 be biased with the minimum requested VCC voltage (~ 5V) in order to make the system oscillate properly.
In figure 10 low side (LS) diac and VCC typical waveforms are shown. In the picture the VCC voltage reaches 6.8V after 4.2m sec remaining constant at this value (it is internally clamped) until the converter starts to oscillate. At that time the storage charge recovery will be responsible for the device supply, charging the VCC capacitor at the final voltage value (~6.8V).
Figure 10: Typical start-up operation
LS DIAC pin voltage
Product(s) LS VCC pin voltage
ObsoleteFor the diac and VCC biasing networks the following choices can be done:
With this setting the converter midpoint will stay at Vdcbus/2 and the voltages across C3 and C12 will be the same for any DC bus voltage.
For a proper start up phase the following condition has to be satisfied:
VC3=VC12 ³ 5V when VC4 =28.5V
7/32
AN1902 - APPLICATION NOTE
Where:
τVc3=R1 ∙ C3 = (R14 + R10) ∙ C12
τVc4=R3 ∙ C4 (low side)
The network R15-C11 must be chosen to have the complete discharge of C9 when the 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 by the internal diodes connected between DIAC and Power collector while the VCC capacitor will be charged by the charge recovered from the Power stage (see figure 11).
Figure 11: Typical waveforms after the diac strike
Mid point voltage
LS VCC pin voltage
Product(s)
LS DIAC pin voltage
Still referring to Product(s)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).
Obsolete3.2.1 PRE-HEATING TIME
CapPREH pin supplies a constant current IcapPREH~55µA. This current is supplied only during the tON. The preheating ends when 4.2V is reached on CapPREH.
Assuming that
VCapPREH = 4.2V
ICapPREH 55μA
the preheating time tpreh will be:
VCapPREH |
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tpreh = C7 ∙ ---I--CapPREH--------------------- |
= 0.15s /µF |
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8/32
AN1902 - APPLICATION NOTE
3.2.2 PRE-HEATING FREQUENCY
For the pre-heating frequency calculation the following considerations can be done:
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f = |
1 |
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-- |
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1 |
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T |
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t |
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+ t |
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--T |
ON |
storage |
dVdt |
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tstorage |
= const ≈ 300nsec (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:
tdV/dt= Csnubber x V/ipeak
where:
■ V (DC bus voltage);
■ ipeak (peak current); |
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■ Csnubber (snubber capacitance). |
Product(s) |
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tON is the conduction time fixed by the preheating frequency capacitor (C6) and device characteristics. |
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It can be calculated according to the following formula: |
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Obsolete |
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tON=K ∙ |
C6 |
Where K=6.7µsec/nF (fixed by the VK06)
The steady state frequency is set by the parallel of the capacitors C6-C16 for the low side and C10-C15
for the high side, where C16=C15. The same-formulae of the preheat can be applied:
+ tstProduct(s)orage + tdVdt
const ≈ 300nsec
tON =
Obsolete3.4 IGNITION
The converter of figure 9 has two different resonance frequencies, the first one before the tube ignition the second one after the tube ignition. The converter operation from preheat to steady state is shown in figure 12.
9/32
fsteady fign fpreh f
Where the two frequencies are:
f1 |
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1 |
(before the tube ignition) |
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π--------L---P----(-C----1---ser--------ies-------C---2---) |
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f2 |
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1 |
(after the tube ignition) |
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------------------- |
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2 |
π LPC2 |
Product(s) |
efficiency check is reported:
In good ballast design the cathode preheating isObsoleterequested in order to increase the tube lifetime. It is obtained making high current flow through the cathodes for a fixed time. A simple rule for the preheating
1) measure the cathode resistance at the beginning- of the preheating;
2) measure this resistanceProduct(s)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 preheating frequency (dot 1) that must be higher than the resonance frequency f1. It
will remain in this condition for the time fixed by the preheating capacitor. After the preheating the device Obsoletefrequency control is taken by the two secondary windings moving the working frequency 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 converter can work at the steady state frequency (dot 3) fixed by the oscillator capacitors.
In figure 13 the complete start-up sequence is shown.
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