EVAL6520-1421

EVAL6520-1421 14 W / 21 W T5 miniature ballast driven by L6520 and STT13005D bipolar transistors Data brief Features ■ Drives either T5-14W-HE or T5-21W-HE lamps ■ Standard form factor (19 mm x 120 mm) ■ Compliance with IEC61347-2-3, IEC61000-2-3 and EN55022 Class-C Description The EVAL6520-1421 is a demonstration board able to drive either a 14 W or 21 W linear T5 fluorescent lamp with the L6520 low voltage ballast controller. EVAL6520-1421 The half bridge consists of NPN high voltage power transistors driven by a suitable pulse transformer. November 2011 Doc ID 018537 Rev 2 For further information contact your local STMicroelectronics sales office. 1/16 www.st.com 16 Contents EVAL6520-1421 Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Board description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3 Board performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 4 Application specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5 Bill of material and board schematics . . . . . . . . . . . . . . . . . . . . . . . . . . 10 5.1 Board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Appendix A Magnetic components data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 2/16 Doc ID 018537 Rev 2 EVAL6520-1421 1 Introduction Introduction The L6520 low voltage ballast controller is intended to drive extremely compact applications based on either MOSFETs or bipolar transistors. The EVAL6520-1421 is capable of driving either a T5-14W-HE or T5-21W-HE lamp, with the same miniatured (16 mm wide) ballast. The selection of both the resonant components and the bipolar transistors, together with the design of the suitable pulse transformer, and IC power supply is also described. Doc ID 018537 Rev 2 3/16 Board description 2 EVAL6520-1421 Board description The board is supplied by any AC voltage in the European mains range and does not need any power factor correction having an input power of less than 25 W. The half bridge voltage is obtained by filtering the rectified input voltage. This allows the use of a cheaper bulk capacitor and bipolar transistors. The selection of a target condition is required by the range of the input voltage together with the necessity to drive two different kinds of lamps. In particular, the best driving condition and the best efficiency is obtained at 240 Vac with a 14 W lamp connected. An EMI filter is placed at the board’s input to meet IEC61000 standards. The lamp's cathodes are current preheated to make the ballast choke more compact thanks to the absence of auxiliary windings. The resonant network design starts from the selection of the resonant capacitor (C10) that corresponds to the desired ballast efficiency. The inductance (L1) can be obtained by the following equation: Equation 1 1 Lamp R = 1 R Lamp + j ωC V HB ⋅ j ωL RES RES 1 + 1 Lamp R + j ωC RES Lamp VHB is the effective voltage obtained across the half bridge along one mains cycle. The lower voltage is obtained at 50 Hz and can be approximately computed as: Equation 2 VHB ⎛ 2 ⎞ = ⎜⎜ ⋅ VIN − VF ⎟⎟ + ⎝ 2 ⎠ 2 ⎛ 2 ⎞ 1.54 ⋅ 10 5 ⋅ P LAMP ⎜ ⎟ ⋅ V − V ⎜ 2 IN F⎟ − fMAINS ⋅ COUT ⎝ ⎠ where: ● VF is the forward voltage drop of the rectifier bridge (1 V typ.) ● VIN is the RMS value of the input voltage ● COUT is the value of the bulk capacitor in µF (In this case C3 = 4.7 µF has been selected) A resonant capacitor equal to 3.9 nF has been selected and resonant inductor equal to 3 mH has been calculated. 4/16 Doc ID 018537 Rev 2 EVAL6520-1421 Board description It is now possible to estimate the ignition frequency, which must be higher than 46 kHz (minimum programmable value), and the minimum preheating frequency that guarantees a preheating voltage higher than 130 Vrms, as required by the lamp specifications. Equation 3 VPH−IGN VPFC ⋅ 2 1 ⋅ π jωCRES = 1 + jωLRES jωCRES A preheating frequency equal to 70 kHz has been selected by connecting a 2.49 kΩ resistor (R5) between the FPRE pin and GND. The half bridge is based on two STT13005D power bipolar transistors (Q1 and Q2). Both the high side and low side transistor are driven by a pulse transformer (T2). To design this pulse transformer, the following parameters must be taken into account: ● Maximum available spacing on the PCB: this determines the core dimension. ● Maximum magnetizing current (Imag,rms) on the primary side of the transformer: this current causes core losses to not be transferred as a useful signal on the secondary side of the transformer. To minimize it, a higher primary inductance should be adopted. Typical inductances are between 6 mH and 40 mH, depending on the core dimension and the core permeability. ● Primary to secondary transfer ratio (n): the output voltage of a step-down transformer is lower than the input voltage, whereas the output current is higher than the input current. This helps to obtain higher DC currents with lower IC power dissipation. The minimum Vbe(sat) must be guaranteed in any condition as well as the minimum IB that guarantees the saturation condition of the BJT. The Imag,rms(MAX) is selected lower than 10 mA when Vcc is equal to 13 V and a typical storage time of 1.2 µs is considered, therefore the primary inductance must be: Equation 4 1 − Tdt − Tsto − 200ns Ton,max 2 ⋅ frun 9.42us - Tsto Lpri ≥ Vcc ⋅ = 3 ⋅ Vcc = 1.732 ⋅ Vcc = 6.18mH Imag,rms Imag,rms Imag,rms 3 The Vbe of the bipolar transistor can be calculated as follows (see Figure 2): Equation 5 ⎡ ⎤ Ib Vbe = Vpri ⋅ n − Ib ⋅ Rb = ⎢ Vcc − ⋅ (Rds, on_h + Rpri + Rds, on_l)⎥ ⋅ n − Ib ⋅ Rb ⎣ ⎦ n The Rds,on_h and the Rds,on_l are the ON resistances of the L6520 drivers and can be considered equal to 10 Ω each. Ib is equal to Ic/hfe, and can be considered equal to Ic/6. Ib times Rb can be set between 0.7 V and 1 V, during run mode: in this design Rb (R6 and R8) can be selected between 10 Ω and 13 Ω. Doc ID 018537 Rev 2 5/16 Board description EVAL6520-1421 With these constraints the following is obtained: Equation 6 ⎡ ⎤ 0.46 Vbe = ⎢13 − ⋅ (20 + Rpri)⎥ ⋅ n − 0.76 = 13 ⋅ n - 9.2 - 0.46 ⋅ Rpri - 0.76 > 1.1 → 13 ⋅ n - 0.46 ⋅ Rpri > 11.6 ⎣ ⎦ n Selecting an Rpri (R7) equal to 47 Ω, the minimum transfer ratio should be equal to 2.55. n = 5.6 has been selected. The PWM_det pin network is composed of 3x220 kΩ resistors (R11 to R13) together with a 47 pF speed-up capacitor (C11). The value of the speed-up capacitor also avoids a misdetection of the hard switching. During normal operation the IC absorbs the following currents from the Vcc: 1. Effective base currents of the BJTs divided by n (5.6). A 39 mA(MAX) is estimated. 2. Magnetizing current = 10 mA(MAX) 3. L6520 power consumption: 8 mA(MAX) A maximum current of 57 mArms must be foreseen. For this reason, the IC power supply has been connected in series with the resonant network (D4 and D5). This connection does not interfere with the optimum preheating of the lamp’s cathodes, but introduces a little offset (7.5 V typ.) into the lamp voltage. This offset affects the EOL detection, but a different choice of values of the Zener diodes (D6 and D7) makes the detection symmetrical. The two values, together with the resistance values (R14 and R15), can be calculated through the following system of equations (VLamp,MAX = 30 V and VLamp,min = -16 V): Equation 7 ⎧ VLamp,MAX = VEOL + VZ,D7 + IBIAS ⋅ (R14 + R15) + VF,D6 ⎨ ⎩ VLamp,min = VEOL − VZ,D6 − IBIAS ⋅ (R14 + R15) − VF,D7 Finally, a 4.7 µF is used as the Vcc bulk capacitor (C4) and two 100 nF ceramic capacitors (C5) are placed close to the Vcc pins of the two ICs. By allowing the startup network (R2 to R4) to pass through the upper cathode of the lamp, the automatic re-lamp feature is easily obtained. 6/16 Doc ID 018537 Rev 2 EVAL6520-1421 3 Board performance Board performance Figure 1. EMI spectrum at nominal input voltage (230 Vac) Figure 2. Lamp voltage and current (T5 14 W HE) Doc ID 018537 Rev 2 7/16 Board performance Figure 3. 8/16 EVAL6520-1421 Lamp voltage and current (T5 21 W HE) Doc ID 018537 Rev 2 EVAL6520-1421 4 Application specifications Application specifications Table 1 and 2 show the application specifications for the input and lamp requirements. Table 1. Table 2. Input requirements Parameter Value Unit Input voltage 198 to 264 Vrms Mains freq. 50 to 60 Hz Input power 25 W max Lamp requirements Parameter T5 - 14 W Lamp current Lamp voltage T5 - 21 W 170 ± 30% 82 ± 6% Unit mArms 123 ± 6% Vrms Max. ignition voltage 1000 Vpk Max. preheating voltage 130 Vrms Doc ID 018537 Rev 2 9/16 Bill of material and board schematics 5 Bill of material and board schematics Table 3. 10/16 EVAL6520-1421 Bill of material Reference Value / part number Rating C1 100 nF 275 Vac C2 100 nF 275 Vac C3 4.7 µF 400 Vdc - 105 °C C4 4.7 µF 50 Vdc C5 100 nF 25 Vdc C6 1 nF 25 Vdc C7 100 nF 400 Vdc C8 100 nF 25 Vdc C9 1 nF 25 Vdc C10 3.9 nF 1000 Vdc C11 10 pF 500 Vdc C12 100 nF 25 Vdc C13 Not mounted C14 22 nF 50 Vdc R1 PCB fuse 6A-1s R2 330 kΩ R3 330 kΩ R4 270 kΩ R5 2.94 kΩ R6 10 Ω R7 47 Ω R8 10 Ω R9 470 Ω R10 1.2 Ω R11 220 kΩ R12 220 kΩ R13 220 kΩ R14 560 kΩ R15 560 kΩ T1 2 x 33 mH CM-filter Notes Panasonic ECQ P6392JU 0.1% 1% 440 mA / 250 Vac Doc ID 018537 Rev 2 SCLE16333-ITACOIL EVAL6520-1421 Bill of material and board schematics Table 3. Bill of material (continued) Reference Value / part number Rating Notes T2 5.6 : 1:1 12 mH E0802-ITACOIL (Figure 5) L1 3 mH 0.9 A E16113-ITACOIL (Figure 6) U1 L6520 Q1 STT13005D Q2 STT13005D D1 B6S-E3/80 D2 RB751V40T1 D3 RB751V40T1 D4 MMSD4148T1G D5 BZT03C15 3W D6 MM3Z6V8ST1 6.8 V Zener D7 MM3Z6V8ST1 16 V Zener J1 VIN connector 198-264 Vac J2 Lamp connector Doc ID 018537 Rev 2 11/16 * 2  # Doc ID 018537 Rev 2   07-?DET ("#3 %/, &02% 5 # # $ 5     2   $ & '.$ ,3$ (3$ 6##     # # # 6CC # 2 2  2 # 2  2 2   4      $  $ 2 2 $  /54 # 2 (6 1 1 # # 2  , #  2 2  $     # * 7 74 $ Figure 4.  6AC   # 4   12/16 5.1  2 Bill of material and board schematics EVAL6520-1421 Board schematic Board schematic !-V EVAL6520-1421 ' ' Pulse transformer (T2) datasheet ' , 4 Figure 5. Magnetic components data # Appendix A Magnetic components data AM07774v1 Doc ID 018537 Rev 2 13/16 Magnetic components data Figure 6. EVAL6520-1421 Ballast choke (L1) datasheet AM07475v1 14/16 Doc ID 018537 Rev 2 EVAL6520-1421 Revision history Revision history Table 4. Revision history Date Revision Changes 07-Mar-2011 1 Initial release. 28-Nov-2011 2 Updated Section 2 and Table 3. Doc ID 018537 Rev 2 15/16 EVAL6520-1421 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. Purchasers are solely responsible for the choice, selection and use of the ST products and services described herein, and ST assumes no liability whatsoever relating to the choice, selection or use of the ST products and services described herein. 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