AN APPLICATION NOTE

Transcription

1 AN APPLICATION NOTE VIPower: ELECTRONIC BALLAST FOR REMOVABLE CFL N. Aiello S. Messina ABSTRACT This technical note describes how a High Frequency ballast based on VK05CFL is able to drive removable fluorescent tubes. The design is intended for 5 to 13W fluorescent lamps and 110V or 230V main voltage. 1. INTRODUCTION To drive a fluorescent lamp, the electronic ballast, with respect to the magnetic ballast, presents the following features: reduced ballast loss (higher efficiency) and weight, facility on lamp power control, more efficient tube ignition, no flickering and operating conditions improving lamp life. Voltage fed series half bridge converters are used. This topology is operating in Zero Voltage Switching (ZVS) resonant mode, reducing the switching losses. The proposed design, based on VK05CFL device, realizes preheating function, End Of Life (EOL) protection, Lamp Presence Detection and automatic restart. 2. START-UP PHASE When a fluorescent lamp is turned on, the main voltage is not sufficient to cause the initial ionization. An element is needed to provide high voltage across the tube to start the process. There are two methods to ignite the tube: a) cool ignition, b) warm ignition. a) COOL IGNITION This method is carried out by mounting only one capacitor in parallel to the tube (see Fig.1). Figure 1: Cool ignition electric scheme Cres Rcathod Rcathod Before ignition, no current flows into the lamp, the only conductive paths are the electrodes that can be considered as two small resistors. March /18

2 The system, oscillating at the resonance frequency fixed by -Cres, generates a current increase and, after few cycles, the voltage on capacitor Cres reaches enough value to strike the tube. After that, the current will flow between the cathodes and the lamp can be considered like a resistor (see figure 2). Figure 2: Simplified Schematic after lamp ignition Cres Rcathod Rlamp Rcathod Using cool ignition, high current values (3, 4 times the nominal value) and voltage values are present in the lamp for a short time, consequently the tube life is reduced. b) WARM IGNITION To provide long life and to insure an efficient ignition of the lamps the cathodes must be preheated. The preheating of the filaments allows an easy strike of the lamp thanks to the ignition voltage reduction. During preheating time the tube presents high impedance so the current flows through the filaments. Its resistance value is strictly dependent on the lamp model. A simple rule to determine the right preheating current/time value is the following: the ratio between the cathode resistance before and after the preheating has to be in the range of 3 5. There are two methods to obtain the cathodes preheating: < voltage mode heating; < current mode heating. Voltage Mode Heating This function is achieved by heating the lamp cathodes by means of two auxiliary windings. These ones are magnetically coupled with the main inductance as shown in figure 3. Figure 3: Voltage Mode Heating lamp connection L res L res L res C Cres 2/18

3 During the preheating phase, the lamp can be considered an open circuit, the current flows through L res and Cres. The voltage across L res is transferred to the secondary windings L res, generating a current heating the cathodes. The primary current value is related to the half bridge working frequency, so the preheating frequency is chosen according to the tube specs (rms current/time). The capacitor Cres must be chosen considering that, during the preheating, the voltage across it must be lower than the ignition voltage. At the end of the preheating phase, when the system moves the working frequency towards the steady state frequency, the voltage on Cres will increase allowing an easier ignition. Current Mode Heating The Current mode heating is obtained similarly to the voltage mode heating but in this case the current that heats the cathodes is the same that flows in the resonant inductor. Some typical solutions are reported below. Current Mode Heating in ballast with IC dedicated driver In electronic ballast the IC driver provides the control functions to drive the external power mosfets during all the operation conditions (i.e. preheating, ignition, steady state, end of life). In figure 4 a typical scheme is shown. Figure 4: Current mode heating: circuit using IC driver HV Integrated driver Cres During the preheating phase the lamp is an open circuit and the current flows through and Cres heating the lamp cathodes. The current in this phase depends on the half bridge working frequency. At the end of the preheating time the ICs perform the lamp ignition sequence reducing the frequency towards the resonant frequency fixed by Cres and. In this way the current and the voltage on Cres increase causing the strike of the lamp. Once the lamp is successfully ignited, the ICs determine the steady state frequency for a given power lamp. Current Mode Heating in self oscillating ballast When discrete components (i.e. BJT MOSFET IGBT etc.) are used to make the DC/AC converter (half bridge), external circuits are necessary to perform all control functions of the power switches. The methodology to heat the lamp filaments is similar to the previous cases. One method to obtain the preheating cathodes is to adopt a PTC resistor. Figure 5 shows its typical connection. 3/18

4 Figure 5: Preheating circuit using PTC C res PTC C res Referring to figure 5 the following relationship between the capacitors has to be respected: Cres>C res>c res (1) At start up, when the PTC is cold, it can be considered as a short circuit (see figure 6), the circuit working frequency is determined by C res and (we can neglect Cres). During the preheating the current flows through the PTC and C res heats both the cathodes and the PTC at the same time. The value of C res must be chosen in order to avoid high voltage on the lamp and consequent switch on (the voltage across PTC is negligible). Figure 6: Equivalent circuit when PTC is a short circuit C res C res When the PTC is hot (end of preheating) its resistance increases until it can be assumed as an open circuit (see figure 7). Figure 7: Equivalent circuit when PTC is an open circuit C res C res 4/18

5 In this case the current flows through the series formed by C res and C res. According to (1) the equivalent capacitance (C res series C res ) across the lamp becomes lower than the initial value (C res ) increasing the capacitive reactance and allowing the tube ignition. After the ignition, C res C res and PTC can be considered a high impedance in parallel to the tube, thus its contribute can be neglected. 3 END OF LIFE (EOL) PROTECTION The ballast shall not impair safety when operating under abnormal and fault conditions. Abnormal conditions are classified (European Standard) as reported below: < lamp not inserted; < the lamp does not start because one of the two cathodes is broken; < the lamp does not start although the cathodes are intact (EOL); < the lamp operates, but one of the cathodes is de-activated or broken (rectifying effect). Third condition is a typical EOL situation and it is verified when the gas inside the tube is exhausted. During the start-up phase, if the lamp doesn t strike, very high current will flow in the circuit with dangerous high voltage appearing across the tube. This anomaly can be damaging on base material resulting also dangerous for the operator that must replace the tube. For these reasons a protection against faults is necessary. Since during the EOL condition high values of current and voltage are present in the circuit, two methods can be adopted in order to detect it: 1) current sensing detection; 2) voltage sensing detection. Current Sensing Detection: Dedicated driver solution Using IC integrated driver the EOL protection is included in the IC functions. It is achieved connecting a resistor between the source and gnd in the low side device. In figure 8 the typical scheme is shown. Figure 8: Current sensing method using dedicated IC drivers Integrated Driver Cres Rsense At the end of the preheating, during the ignition sequence, the ICs reduce the ballast operating frequency towards the resonance frequency, following a fixed law, up to the steady state frequency. If the tube doesn t strike, high current will continue to flow through Rsense repeating the IC the start-up procedure. After several times the Ic consider the condition like EOL interrupting the oscillations. 5/18

6 Current Sensing Method: Discrete solution Using a circuit with discrete components the current sensing is realized in the same way as the previous solution. In this case the voltage on Rsense drives an external circuit (timer plus latch). The latch, for example, can be realized connecting two transistors in SCR configuration (see figure 9). Figure 9: Current sensing method using discrete components HV driver sw1 SW1 HV driver sw2 SW2 Latch Rsense When the capacitor voltage reaches the latch threshold it is activated switching-off the low side device. The latch will keep in off state the half bridge until the main voltage is present. Voltage Sensing Method Another way to realize the EOL function is to detect the voltage across the lamp. In figure 10 a scheme with external components is shown. Figure 10: Voltage sensing method connection using discrete components HV driver SW1 Cres HV driver SW2 Latch 6/18

7 During the EOL phase, the protection circuit detects the over-voltage condition and activates the latch stopping the oscillation in the ballast. Using this method the rectifying effect can also be detected because no over-current is present in the circuit but only over-voltage is generated across the tube. 4 BALLAST CIRCUIT BASED ON VK05CFL The proposed ballast, based on VK05CFL, drives removable tubes realizing the preheating and the end of life functions without the use of PTC and high voltage components with system reliability increasing. 4.1 ELECTRICAL SCHEME In figure 11 the electrical scheme is reported. The demo-board can be adapted to different needs by simple modifications i.e.: USA input voltage, EU input voltage, preheating function or not. Figure11: VK05CFL demo-board electrical scheme L1 R5 + C0 D5 1N4148 R7 C5 VK05CFL C9 R9 D1 1N4007 D2 1N4007 R11 C11 J4 1 4 C3 jumper C7 230V N R0 CFL 2 3 C2 C4 R6 110V D6 1N4148 Dz1 R14 R13 R8 VK05CFL D3 1N4007 D4 1N4007 R17 D7 1N4148 J1 Jumper + C1 Q1 C15 R15 R12 C12 R16 C16 Q2 C13 C6 J5 jumper J6 jumper C8 EOL PROTECTION 7/18

8 In the following notes the modifications necessary to adapt the board to different needs are listed: Note 1): EU Line Input Voltage: Vac between 230 and N terminals; Input Section: Full Bridge Rectifier, J1 closed, C1 not mounted. Note 2): USA Line Input Voltage: Vac between 110 and N terminals; Input Section: Voltage Doubler, J1 open, D2 e D4 not mounted. Note 3): Preheating function requested: J4 and J5 closed, J6 open. Note 4): Preheat not requested: J4 and J5 open, J6 closed, C11 D5 R7 R11 R12 not mounted. The electrical scheme reported in figure 12, shows the configuration without preheating. Figure 12: VK05CFL demo-board without preheating function L1 R5 + C0 C5 VK05CFL C9 R9 D1 D2 1 4 C3 C7 230V N R0 CFL 2 3 C2 C4 R6 2.2K 110V D6 Dz1 R14 R13 R8 VK05CFL D3 D4 R17 D7 C12 J1 Jumper + C1 Q1 C15 R15 R16 C16 Q2 C13 C6 C8 4.2 BILL OF MATERIAL In the following table the material list for both circuits is reported: 8/18

9 Table 1: Component list 5 13W for USA market (Circuit figure 11) Reference Value Description C0, C1 10µF, 200V Electrolytic Capacitor C2, C3 100nF, 400V Capacitor C4 2.4nF 5% 400V Start Up Capacitor C5, C6 1.5nF, 63V Capacitor C7, C8 1.2nF 5% Capacitor C9 470pF, 630V Snubber Capacitor C11, C12 22µF, 35V Capacitor C13 22nF, 63V Capacitor C15, C16 27nF, 63V Capacitor D1, D3 Rectifier Diode 1N4007 D5, D6, D7 Diode 1N4148 Dz1 9.1V Zener Diode IC1, IC2ST STMicroelectronics VK05CFL L1 820 µh Inductor Q1 Transistor PNP BC177 Q2 Transistor NPN BC107 R0 47Ω 1/2W Fuse Resistor R5, R6 2.2KΩ, 1/4W Resistor R7, R8, R14 27KΩ, 1/4W Resistor R9, R11, R12 R13 1MΩ, 1/4W Resistor R15, R16, R17 33KΩ, 1/4W Resistor T1 3.1mH 5% N1/N2=10:1 VOGT electronic AG Resonant Inductor (Drawing: LL ) Table 2: Component list 5 13W for USA market (Circuit figure 12) Reference Value Description C0, C1 10µF, 200V Electrolytic Capacitor C2, C3 100nF, 400V Capacitor C4 2.4nF 5% 400V Start Up Capacitor C5, C6 1.5nF, 63V Capacitor C7, C8 1.2nF 5% Capacitor C9 470pF, 630V Snubber Capacitor C12, C13 22µF Capacitor C13 22nF, 63V Capacitor C15, C16 27nF, 63V Capacitor D1, D3 Rectifier Diode 1N4007 D6, D7 Diode 1N4148 Dz1 15V Zener Diode IC1, IC2ST STMicroelectronics VK05CFL L1 820 µh Inductor Q1 Transistor PNP BC177 Q2 Transistor NPN BC107 R0 47Ω 1/2W Fuse Resistor R5, R6 2.2KΩ, 1/4W Resistor R8 100KΩ, 1/4W Resistor R14 27KΩ, 1/4W Resistor R9, R13 1MΩ, 1/4W Resistor R15, R16, R17 33KΩ, 1/4W Resistor T1 3.1mH 5% N1/N2=10:1 VOGT electronic AG Resonant Inductor (Drawing: LL ) 9/18

10 Table 3: Component list 5 13W for European market (Circuit figure 11) Reference Value Description C0 3.3µF, 380V Electrolytic Capacitor C2, C3 100nF, 400V Capacitor C4 2.4nF 5% 400V Start Up Capacitor C5, C6 1.5nF, 63V Capacitor C7, C8 1.2nF 5% Capacitor C9 470pF, 630V Snubber Capacitor C11, C12 22µF, 35V Capacitor C13 22nF, 63V Capacitor C15, C16 27nF, 63V Capacitor D1, D2, D3, D4 Rectifier Diode 1N4007 D5, D6, D7 Diode 1N4148 Dz1 9.1V Zener Diode IC1, IC2ST STMicroelectronics VK05CFL L1 820 µh Inductor Q1 Transistor PNP BC177 Q2 Transistor NPN BC107 R0 47Ω 1/2W Fuse Resistor R5, R6 2.2KΩ, 1/4W Resistor R7, R8, R14 27KΩ, 1/4W Resistor R9, R11, R12, R13 1MΩ, 1/4W Resistor R15, R16, R17 33KΩ, 1/4W Resistor T1 3.1mH 5% N1/N2=10:1 VOGT electronic AG Resonant Inductor (Drawing: LL ) Table 4: Component list 5 13W for European market (Circuit figure 12) Reference Value Description C0 3.3µF, 380V Electrolytic Capacitor C2, C3 100nF, 400V Capacitor C4 2.4nF 5% 400V Start Up Capacitor C5, C6 1.5nF, 63V Capacitor C7, C8 1.2nF 5% Capacitor C9 470pF, 630V Snubber Capacitor C12 27µF Capacitor C13 22nF, 63V Capacitor C15, C16 27nF, 63V Capacitor D1, D2, D3, D4 Rectifier Diode 1N4007 D6, D7 Diode 1N4148 Dz1 15V Zener Diode IC1, IC2ST STMicroelectronics VK05CFL L1 820 µh Inductor Q1 Transistor PNP BC177 Q2 Transistor NPN BC107 R0 47Ω 1/2W Fuse Resistor R5, R6 2.2KΩ, 1/4W Resistor R8 150KΩ, 1/4W Resistor R9, R13 1MΩ, 1/4W Resistor R14 27KΩ, 1/4W Resistor R15, R16, R17 33KΩ, 1/4W Resistor T1 3.1mH 5% N1/N2=10:1 VOGT electronic AG Resonant Inductor (Drawing: LL ) 10/18

11 4.3 CIRCUIT DESCRIPTION About the circuit description the used topology is the same already described in the AN1546, therefore this paragraph will only deal with the procedure used to realize the preheating and the EOL functions. Preheating Description The preheating function is realized modifying the ballast operating frequency. In the VK05CFL device it is possible to set the working frequency by external capacitor (see datasheet). If, by external added components, the capacitor current changes, also the working frequency will change. It is important to highlight that the circuit has to be applied to both sides in order to guarantee the fifty percent duty cycle. This function is described referring to the electrical scheme shown in figure 11. At the beginning both VK05CFL are OFF. As soon as the voltage on C13, connected to DC bus by resistor R2, reaches the internal diac threshold (~30V) the low side device is switched ON starting the oscillations. The voltage drop on Lp is transferred to the secondary windings confirming the "ON" state for the low side device and the "OFF" state for the high side. In this phase the tube is an open circuit and the resonance frequency is fixed by -C4. The preheating circuit made by diode D5 (6), resistors R7 (8) - R11 (12) and capacitor C11 (12) injects current into the frequency capacitor any time the filtered secondary winding voltage becomes positive, this until the capacitor C11 (12) is charged at the maximum secondary voltage, in this way the preheating time is defined. The capacitor cannot lose its charge thanks the presence of the diode D5 (6). The resistor R7 (8) fix the value of the current add to the frequency capacitors, consequently the working frequency is fixed. Looking at the formulae (2) it is possible to notice that the frequency is not stable during the whole preheating phase due to the drop increasing on C11(12) with consequent injected current reduction. Ipreh=(Vsec-VC11 (12))/R7 (8) (2) Where: Ipreh = Injected current into the frequency capacitor during the preheating phase. Vsec = Filtered secondary input voltage. VC11 = Voltage across the preheating capacitor. In the preheating frequency calculation this current has to be added to the VK05 Iosc. The preheating conditions are: - Working frequency higher than the resonance frequency - Voltage across the tube lower than ignition voltage Ipreh decrease increasing the capacitor voltage, for this reason, to maintain the lamp current more constant possible for longer time, several microfarad are requested, in any case near the end of the preheating time this current variation appears moving the working frequency towards the resonance frequency guaranteeing the tube ignition. After this phase the preheating circuit has no effect (Ipreh=0A) and the ballast frequency become the steady-state frequency fixed by Iosc. The resistor R11 (12) is necessary to discharge the capacitor C11 (C12) when the lamp is switched off in order to have a new preheating at the successive switch on. The time constant factor can be chosen early enough to take into account the tube temperature, having different preheating times for different consecutive switchings ON/OFF. 11/18

12 Eol Description Referring to the electrical scheme, the EOL condition is detected monitoring the voltage across the capacitor C12. When the tube fails the ignition or it starts to work in rectification mode, the voltage on the primary inductance increases with respect to the steady state value, consequently by the secondary windings this variation is transferred on the preheating capacitor increasing its average voltage as to the same one reached during the preheating phase, therefore, when the Dz1 voltage is reached, the latch circuit is activated switching off the converter. The latch acts on the sec pin by means of D7 diode in order to maintain its voltage (~1.5V) lower than the ON threshold. The latch sustain current comes from the resistor R13, it is connected to the tube cathode to reset the circuit when the tube is substituted. To avoid diac restart during the latch off phase also the diac capacitor voltage is maintained lower than the on threshold. During the EOL phase, the preheating circuit controls, by Ton reduction, the current in the converter, in order to avoid reaching very high values AUTOMATIC RESTART Considering the application scheme this function is realized connecting the diac network (R13 C13) at DC bus by the lamp cathode (see figure 17). In this way, when the lamp is not present or the cathode is broken, the diac capacitor is not charged and the ballast remains in the off state. When the lamp is replaced the system will perform the start up sequence normally. Figure 17: Automatic Restart Circuit Connection (shaded area) R5 D5 R7 C5 VK05CFL C9 R9 R11 C11 J4 1 4 C3 jumper C7 2 C4 CFL 3 R6 D6 R13 R8 Dz1 R14 VK05CFL R17 D7 C15 R15 R12 C12 Q1 R16 C16 Q2 C13 C6 J5 jumper J6 jumper C8 12/18

13 4.5 PCB DEFINITION Figure 13 and 14 show the general PCB with the copper and components views, while in figure its photos are reported. Figure 13: Demo-board PCB: component side (not in scale) Figure 14: Demo-board PCB: copper side (not in scale) Figure 15: Top view (not in scale) Figure 16: Bottom view (not in scale) The components placement on the PCB board is extremely important when SMD powers ICs are used. Some simple rules are explained below. The first one is related to frequency capacitor placement. This component must be mounted as close as possible to the VK05CFL osc pin (see figure 18). 13/18

14 Figure 18: PCB Layout example (not in scale) Secondary Voltage Ground Trace Cosc Connection EOL Circuit Ground Trace Power Ground Trace The second one is to have two different ground paths (signal and power) in order to reduce the interference on the logic part. 4.6 EXPERIMENTAL RESULTS In this section the board evaluation is reported. The results have been obtained considering the following setting: a) Main voltage Vmain = 230Vrms b) Ambient temperature Ta = 25 C c) Lamp power Plamp=11W Start-up Figure 19 shows the start up phase when preheating function is used. Figure 19: Vin 230V=V; Ch1=Mid point voltage; Ch2=diac voltage; Ch3=Sec pin voltage; Ch4=Device current Ch1 Ch4 Ch2 Ch3 14/18

15 When the low side diac pin voltage reaches 30V the device is switched ON and the converter starts to oscillate. An integrated diode discharges the diac capacitor keeping its voltage low during the steady state. The preheating oscillation frequency is 67kHz, it is higher than the system resonance frequency fres (with =3.1mH, and Cres=2.4nF => fres=58khz). In figure 20 the current during the preheating and the voltage across the cathode is shown. The preheating time is lower than 1s and the cathode voltage at the end of pre-heating phase is about 3 times higher than initial value ensuring the cathodes heating. Figure 20: Pre-heating phase - Ch4=Cathodes current; Ch3=Cathode voltage Ch4 Ch3 In figure 21 the start up phase using cool ignition method is shown (Electric scheme figure 12). The device current (Ch4) increases rapidly and after few cycles its value is able to generate, across the start-up capacitor, an over-voltage sufficient to ignite the tube. Figure 21: Ch1=Mid point voltage; Ch4=Primary inductance current Ch1 Ch4 15/18

16 Steady State In Figure 22 the steady state waveforms are shown. Figure 22: Ch1=Midpoint voltage; Ch2=secondary winding voltage; Ch3=Vk05 sec pin; Ch4=Device current Ch1 Ch2 Ch4 Ch3 The working frequency is 36 khz with duty cycle of 50%. End Of Life In figure 23 choke current (Ch4) and the voltage across C12 are shown. It is possible to notice that, after the preheating phase (duration about 1 sec), the system goes in end of life condition and the latch shuts the converter down after 2.5s (Dz1=15V). It is possible to modify this time by changing the zener value. In any case the device current doesn t exceed 700mA. Figure 23: EOL - Ch1=Preheating capacitor (C12) Voltage; Ch4=Choke Current Preheating time Ch4 Ch1 16/18

17 Figure 24 shows the converter switch-off when the latch is activated. Figure 24: Ch1=Midpoint voltage; Ch2=Voltage on the base of Q2; Ch3=Vk05 sec pin; Ch4=Device current Ch4 Ch1 Ch2 Ch3 In figure 25 the main converter waveforms showing the EOL intervention in the circuit without preheating function are reported. Figure 25: Ch2= Voltage on the base of Q2, Ch3= Voltage on (C12) capacitor Ch3 Ch2 Lamp Absence When the lamp is not present in the circuit, the system can t oscillate because of the absence of the cathodes open the circuit. 17/18

18 Broken Cathodes At start up if only one of the two cathodes are broken the half bridge cannot oscillate because the current path is interrupted. Thermal Behavior The thermal board analysis has been performed measuring the real devices temperature in application. The copper area dedicated to both devices to sink the heat is 100 mm 2. The temperature has been measured using thermocouples K type soldered on top of the packages. The measurements have been performed driving a 13W tube at two different ambient temperatures: 55 C and 75 C. In the following table the results are summarized: Table 5 T a =55 C T a =75 C T device =68 C T device =93 C The device power dissipation is lower than 200mW. Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may results from its use. No license is granted by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics. The ST logo is a trademark of STMicroelectronics 2003 STMicroelectronics - Printed in ITALY- All Rights Reserved. STMicroelectronics GROUP OF COMPANIES Australia - Brazil - Canada - China - Finland - France - Germany - Hong Kong - India - Israel - Italy - Japan - Malaysia - Malta - Morocco - Singapore - Spain - Sweden - Switzerland - United Kingdom - U.S.A. 18/18