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CCFL Inverters based on Piezoelectric Transformers: Analysis and Design Considerations

CCFL Inverters based on Piezoelectric Transformers: Analysis and Design Considerations. Prof. Giorgio Spiazzi. Dept. Of Information Engineering – DEI University of Padova. Outline. Characteristics of Cold Cathode Fluorescent Lamps (CCFL) Review of piezoelectric effect

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CCFL Inverters based on Piezoelectric Transformers: Analysis and Design Considerations

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  1. CCFL Inverters based on Piezoelectric Transformers: Analysis and Design Considerations Prof. Giorgio Spiazzi • Dept. Of Information Engineering – DEI • University of Padova

  2. Outline • Characteristics of Cold Cathode Fluorescent Lamps (CCFL) • Review of piezoelectric effect • CCFL inverters based on piezoelectric transformers • Design considerations • Modeling

  3. Cold Cathode Fluorescent Lamp (CCFL) • CCFL is a mercury vapor discharge light source which produces its output from a stimulated phosphor coating inside glass lamp envelope. • Closely related to “neon” sign lamps first introduced in 1910 by Georges Claude in Paris • Cold cathode refers to the type of electrodes used: they do not rely on additional means of thermoionic emission besides that created by electrical discharge through the tube

  4. Cold Cathode Fluorescent Lamp (CCFL)

  5. Ultraviolet light Visible light Cold Cathode Fluorescent Lamp (CCFL) • The phosphors coating the lamp tube inner surface are composed of Red-Green-Blue fluorescent compounds mixed in the appropriate ratio in order to obtain a good color rendering when used as an LCD display backlight Energy conversion efficiency:

  6. Lamp length Cold Cathode Fluorescent Lamp Lamp v-i characteristic: Lamp voltage primarily depends on length and is fairly constant with current, giving a non-linear characteristic. Lamp current is roughly proportional to brightness or intensity and is the controlled variable of the backlight supply.

  7. Cold Cathode Fluorescent Lamp • These lamps require a high ac voltage for ignition and operation. • A sinusoidal voltage provides the best electrical-to-optical conversion. • There are four important parameters in driving the CCFL: • strike voltage • maintaining voltage • frequency • lamp current

  8. Percentage of light output as a function of lamp temperature Cold Cathode Fluorescent Lamp • Operating a CCFL over time results in degradation of light output. Typical life rating is 20000 hours to 50% of the lamp initial output • The light output of a CCFL has a strong dependency on temperature

  9. Cold Cathode Fluorescent Lamp Lamp display housing: • Stray capacitances to ground cause a considerable loading effect that can easily degrade efficiency by 25%

  10. Current-fed Self-Resonant Royer Converter

  11. High voltage transformer

  12. Ballast capacitor

  13. Self resonant inverter

  14. Control of supply current

  15. Lamp current measurement

  16. Dimming

  17. Magnetic and Piezoelectric Transformer Comparison Magnetic transformer characteristics • Low cost • Multiple sources • Single-ended or balanced output • Wide range of load conditions (output power easily scaled) • Secondary side ballasting capacitor required • Reliability affected by the high-voltage secondary winding • EMI generation (stray high-frequency magnetic field)

  18. Magnetic and Piezoelectric Transformer Comparison Piezoelectric transformer characteristics • Inherent sinusoidal operation • High strike voltage (no need of ballasting capacitor) • No magnetic noise • Small size • High cost (but decreasing) • Must be matched with lamp characteristics • Reduced power capability • Single-ended output (balanced output are possible) • Few sources • Unsafe operation at no load (can be damaged)

  19. Magnetic and Piezoelectric Transformer Comparison Size comparison

  20. Piezoelectric Effect • The piezoelectric effect was discovered in 1880 by Jacques and Pierre Curie: • Tension and compression applied to certain crystalline materials generate voltages (piezoelectric effect) • Application to the same crystals of an electric field produces lengthening or shortening of the crystals according to the polarity of the field (inverse piezoelectric effect)

  21. Piezoelectric Effect • In the 20th century metal oxide-based piezoelectric ceramics have been developed. • Piezoelectric ceramics are prepared using fine powders of metal oxides in specific proportion mixed with an organic binder. Heating at specific temperature and time allows to attain a dense crystalline structure • Below the Curie point they exhibit a tetragonal or rhombohedral symmetry and a dipole moment • Adjoining dipoles form regions of local alignment called domains • The direction of polarization among neighboring domains is random, producing no overall polarization • A strong DC electric field gives a net permanent polarization (poling)

  22. Piezoelectric Effect Polarization Random orientation of polar domains Polarization using a DC electric field Residual polarization Polarization axis

  23. Piezoelectric Effect P Residual polarization Effect of electric field E on polarization P and corresponding elongation/contraction of the ceramic material E Residual polarization S Relative increase/decrease in dimension (strain S) in direction of polarization E

  24. Piezoelectric Effect Generator and motor actions of a piezoelectric element Poling voltage Disk stretched Applied voltage of same polarity as poling voltage Applied voltage of opposite polarity as poling voltage Disk compressed Disk after polarization (poling)

  25. Actuator behavior Transducer behavior Piezoelectric Effect Polarization S=sE.T+d.E D=d.T+T.E Where: S: Strain [ ] T: Stress [N/m2] E: Electric Field [V/m] s: elastic compliance [m2/N] D: Electric Displacement [C/m2] d: Piezoelectric constant [m/V]

  26. longitudinal mode: P is parallel to T Has a larger d33, along the thickness direction when compared to the planar direction transverse mode: P is perpendicular to T Has a larger d31, along the planar direction when compared to the thickness direction Piezoelectric Effect Based on the poling orientation, the piezoelectric ceramics can be design to function in:

  27. Piezoelectric Transformers (PT) • In Piezoelectric Transformers, energy is transformed from electrical form to electrical form via mechanical vibration.

  28. Piezoelectric Transformers (PT) Three main categories • Longitudinal vibration mode • Transverse actuator and Longitudinal transducer Rosen-type or High-Voltage PT

  29. Piezoelectric Transformers (PT) Three main categories • Thickness vibration mode • Longitudinal actuator and Longitudinal transducer Low-Voltage PT

  30. Piezoelectric Transformers (PT) Three main categories • Radial vibration mode • Transverse actuator and Transverse transducer (radial shape preferred)

  31. Rosen-type Thick. Vibr. mode Radial Vibr. mode R 0.756199  1.44  6.89991  L 2.464173mH 27H 7.93842mH C 3.57nF 254pF 269.171pF N 35.89 0.47 0.908 Ci 81.6216nF 2.305nF 4.60799nF Co 23.85pF 8.911nF 1.62414nF length=30mm length=20mm radius=10.5mm width=8mm width=20mm thickness1=0.76mm thickness=2mm thickness=2.2mm thickness2=2.28mm Equivalent Electric Model

  32. Load resistance: 1M, 100k, 10k, 5k, 1k, 500 Voltage Gain Resonance frequencies Rosen-type Piezoelectric Transformer sample

  33. Input Impedance Load resistance: 1M, 100k, 10k, 5k, 1k, 500 Rosen-type Piezoelectric Transformer sample

  34. Half-Bridge Inverter for PT C2 S2 iinv iL + PT + UDC S1 ui Lamp C1 - Half-Bridge inverter

  35. Soft-Switching Condition Half-bridge inverter ui tr UDC t T/2 j/ w Fundamental components U1 iinv t

  36. + C2 UA S2 C L io R 1:n21 + + + iL + Coupling network UA Uo uinv ui Co S1 C1 Ci - - - Half-Bridge inverter PT Rosen-type Model Lamp Coupling Networks Zg

  37. Ls CN1 Coupling Networks Series inductor • It is not always possible to find a value for input inductor that guarantees both power transfer and soft switching requirements • Less circulating energy as compared to parallel inductor • Non linear control characteristics can lead to large signal instabilities

  38. Effect of Coupling Inductor Ls on Voltage Conversion Ratio MPT = UoRMS/UiRMS, Mi = UiRMS/UinvRMS, Mg = Mi MPT f1 f2 70 [dB] |Mgd|Io=1mA Udc=13V, Ls=42mH |Mgd|Io=6mA Io=1mA |Mg|{ |MPT|{ Io=6mA |Mi|{ -10 45 50 55 60 65 70 75 80 fsw [kHz]

  39. Zg Effect of Coupling Inductor Ls on Input Impedance Positive input phase f2 f1 60 [dBW] Io=6mA |Zg| 0 Io=1mA 0 45 50 55 60 65 70 75 80 fsw [kHz]

  40. Effect of Coupling Inductor Ls on Voltage Conversion Ratio • It introduces an additional voltage gain (frequency dependent) between the RMS value of the inverter voltage fundamental component and the RMS value of the PT input voltage • It introduces more resonant peaks in the overall voltage gain Mg (limitation in switching frequency variation)

  41. Io [mARMS] Udc = 13V 5 4 3 2 1 60 61 62 63 65 64 fsw [kHz] Control Characteristics: Variable Frequency

  42. Io [mARMS] fsw = 65kHz 5 Ls = 38mH 4 3 Ls = 42mH 2 CN1 1 11 12 13 14 15 16 Udc [V] Control Characteristics: Variable dc Link Voltage Increasing LS value causes the gain curve Io = f(UA) to become non monotonic

  43. Large-Signal Instability Main converter waveforms when Udc is slowly approaching 21V (fsw = 65kHz, Ls = 42mH).

  44. CB Lp CN2 Coupling Networks Parallel inductor • It is always possible to find a value for input inductor that guarantees both power transfer and soft switching requirements • Higher circulating energy as compared to series inductor

  45. Effect of Coupling Network on Voltage Conversion Ratio CN2: Lp=20mH, CB=1mF, Udc=30V f2 f1 50 |Mgd|Io=1mA [dB] |Mgd|Io=6mA |Mg|{ }Io=1mA |MPT|{ }Io=6mA Io=1mA Io=6mA |Mi|{ 0 45 50 55 60 65 70 75 80 fsw [kHz]

  46. Zg Effect of Coupling Network on Input Impedance f2 f1 60 [dBW] Io=6mA Io=1mA 0 |Zg| 0 45 50 55 60 65 70 75 80 fsw [kHz]

  47. Effect of Coupling Network on Switch Commutations • Differently from the series inductor coupling network, now the inductor current iLp has to charge and discharge also the PZT input capacitance, that is much higher than the switch output capacitances, so that the positive impedance phase is a necessary but not sufficient condition to achieve soft commutations

  48. Experimental Measurements Trapezoidal PT input voltage Charge of input capacitance

  49. Control Characteristics: Variable Frequency Lp = 20mH, CB = 1mF Io [mARMS] CN2 5 4 3 2 Udc = 30V 1 64 66 68 70 72 fsw [kHz]

  50. Control Characteristics: variable dc link voltage Lp = 20mH, CB = 1mF Io [mARMS] CN2 5 4 3 2 fsw = 65kHz 1 10 15 20 25 30 35 Udc [V]

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