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6th Indo – German Winter Academy 2007 Piezo Transformers Holger Schwarzmann Holger Schwarzmann University Erlangen-Nuremberg Piezo Transformers - Outline 1. Introduction 2. Piezo Transformer Technology 3. Transfer Characteristic and Control Method 4. Applications 5. Summary Holger Schwarzmann University Erlangen-Nuremberg 2 Piezo Transformer - Introduction Magnetic Transformer Compared to Piezo Transformer (PT) Source:http://en.wikipedia.org Electromagnetic coupling Design parameter, e.g., Turns ratio (number of windings) Transformer core material Control method Amplitude of primary voltage VP Electromechanical coupling Design parameter, e.g., Turns ratio (aspect ratio, number of layers) Piezoelectric ceramic Control method Amplitude and/or frequency of primary voltage Vin Holger Schwarzmann University Erlangen-Nuremberg 3 Piezo Transformer - Introduction Properties of PT Advantages Small volume (small size, low weight) Less electromagnetic noise than ‘conventional’ transformer High efficiency (up to 98%) High power density (up to 50W/cm³) High isolation voltage Short-circuit-proof Disadvantages Limited temperature range (< 50% of Curie-temperature: typical between 150°C and 350°C) Heat dissipation only through contact area (restriction of power transfer) Increase of temperature or load cycle depend on frequency tracking Holger Schwarzmann University Erlangen-Nuremberg 4 Piezo Transformer - Technology Rosen Transformer (I) Single rectangular piece of piezoelectric ceramic One half polarized in thickness direction (input - piezoelectric actor) Other half polarized in length direction (output – piezoelectric transducer) Single or multilayer design possible Vibration direction transversal Holger Schwarzmann University Erlangen-Nuremberg 5 Piezo Transformer - Technology Rosen Transformer (II) 6 Fundamental T S Fundamental and harmonics For less damping, contact only at vibration nodes / planes 1/4 T: stress S: strain 1st Harmonic 1/2 3/4 L 2nd Harmonic L Holger Schwarzmann University Erlangen-Nuremberg Vibration node Piezo Transformer - Technology Rosen Transformer (III) 4 l (mm) w (mm) 2 0 1 0 t (mm) x-strain (nm) x-stress (MPa) x-strain (nm) x-strain x-stress 20 10 0 l (mm) Holger Schwarzmann University Erlangen-Nuremberg 7 Piezo Transformer - Technology Rosen Transformer (IV) Step-up topology High gain High output impedance High output voltages possible, but only low currents Resonant frequency: in order of 100 kHz Source: http://www.prestostore.com Output power: 5 to 8 W Power density: 5 to 10 W/cm³ Applications: drive cold cathode fluorescent lamps (CCFL), backlighting for liquid crystal displays (LCD) Holger Schwarzmann University Erlangen-Nuremberg 8 Piezo Transformer – Transfer Characteristic Equivalent Circuit of PT (I) Equivalent circuit only valid near resonant frequency Cin, Cout: capacitances of input respectively output layers Lm, Cm: set resonant frequency Rm: combine losses (dielectric, vibration, heat, etc.) Ideal transformer Holger Schwarzmann University Erlangen-Nuremberg 9 Piezo Transformer – Transfer Characteristic Equivalent Circuit of PT (II) Transformation equation R ' 'L = R 'L 2 1 + (ω ⋅ C 'out ⋅R 'L ) 1 + (ω ⋅ C 'out ⋅R 'L ) C ' 'out = C 'out ⋅ (ω ⋅ C 'out ⋅R 'L )2 V V ' 'out = out2 N 2 with ω = 2 ⋅π ⋅ f R 'L = RL N2 C 'out = N 2 ⋅ Cout Holger Schwarzmann University Erlangen-Nuremberg 10 Piezo Transformer – Transfer Characteristic PT Transfer Characteristic (I) Constant frequency k 21 = c= V ' 'out Vin C 'out Cm Q = ω ⋅ Cout ⋅ RL Qm = 1 ω ⋅ Cm ⋅ Rm Source: G. Ivensky, Generic Operational Characteristics of Piezoelectric Transformers, IEEE Transactions on Power Electronics 17, 1049 (2002) Holger Schwarzmann University Erlangen-Nuremberg 11 Piezo Transformer – Transfer Characteristic PT Transfer Characteristic (II) Constant load 2nd Harmonic 1st Harmonic 3rd Harmonic Fundamental Mode Holger Schwarzmann University Erlangen-Nuremberg 12 Piezo Transformer – Transfer Characteristic Operating Point Constant load OP 2 OP 3 OP 4 OP 1 Holger Schwarzmann University Erlangen-Nuremberg 13 Piezo Transformer – Control Method Input Waveforms Square waveform Easy to generate Simple circuits (commercial devices available) Only the fundamental is used for driving PT (rest losses) Sinusoidal waveform Driving the PT with fundamental, no extra losses Resonant driving concepts Complex driving circuit Holger Schwarzmann University Erlangen-Nuremberg 14 Piezo Transformer – Control Method Bridge Converter Full-bridge converter Half-bridge converter Q1, Q4: clk Q2, Q3: clk Q1: clk Q2: clk Holger Schwarzmann University Erlangen-Nuremberg 15 Piezo Transformer – Control Method 16 Resonant Push-Pull Converter Generate sinusoidal waveform to drive PT V+ L1 L2 Equivalent Ciruit of Piezo Transformer Lm Cin Rm Cm Cout Q1 Dual Low-Side Driver Cds1 Q2 Cds2 Holger Schwarzmann University Erlangen-Nuremberg RL Piezo Transformer – Control Method V I II III 17 I Resonant Push-Pull Converter Vg1 Interval I: Q1 on, Q2 off V V+ Vg2 IL1 L1 L2 Vout V1 Lm Vg1 Rm Cm Cin Dual Low- Q1 Side Driver A 1:N Cout Cds1 RL IL1 V Cds2 V1 Q2 Vg2 200n 400n 600n 800n Time (s) Holger Schwarzmann University Erlangen-Nuremberg Piezo Transformer – Control Method V I II III 18 I Resonant Push-Pull Converter Vg1 Interval II: Q1 off, Q2 on V Vg2 A IL1 V V1 200n 400n 600n 800n Time (s) Holger Schwarzmann University Erlangen-Nuremberg Piezo Transformer – Control Method V I II III 19 I Resonant Push-Pull Converter Vg1 Interval III: Q1 off, Q2 on V Vg2 A IL1 V V1 200n 400n 600n 800n Time (s) Holger Schwarzmann University Erlangen-Nuremberg Piezo Transformer – Control Method I V II III 20 I Resonant Push-Pull Converter Vg1 Interval I: Q1 on, Q2 off V Vg2 V+ V IL1 V1 L1 L2 Vout V1 Lm Vg1 V2 Rm Cm V V2 1:N Cin Cout RL V V1 - V2 Dual Low- Q1 Side Driver Cds1 V Cds2 Q2 Vout Vg2 200n 400n 600n 800n Time (s) Holger Schwarzmann University Erlangen-Nuremberg Piezo Transformer – Application 21 Main Application: Inverter for CCFL (Rosen) 4 Watt converter with conventional transformer dimension: LxWxT: 55 x 20.5 x 14mm³ 4 Watt converter with PT dimension: 95 x 15 x 5.5mm³ 4x4 Watt converter with PT dimension: 160 x 33 x 5.5mm³ Source: http://newyah.diytrade.com Holger Schwarzmann University Erlangen-Nuremberg Piezo Transformer – Application Prototype of an AC-DC Converter (Noliac) Input < 130 V AC Output 12 V DC Output Power Up to 15 W Efficiency Up to 98% Power density 40 W/cm³ Frequency 400 kHz Source: http://www.noliac.com Isolation Voltage Above 5 kV DC Holger Schwarzmann University Erlangen-Nuremberg 22 Piezo Transformer – Application PTs Manufactured at IISB Focused on thickness extension PT (high resonant frequency) Possibility to transfer energy and information with one PT High- and Low-Side driver for power applications e.g., frequency inverter Holger Schwarzmann University Erlangen-Nuremberg 23 Piezo Transformer – Summary Four technologies of PT described by their vibration mode Equivalent circuit describes electrical behavior of PT PT transfer characteristic mainly depends of the resonant frequency PTs are controlled by frequency Output voltage Load changes Temperature changes Resonant converter topologies Take parasitic devices into account Drive PT with sinusoidal input voltage Holger Schwarzmann University Erlangen-Nuremberg 24 25 Thank You for Your Attention Holger Schwarzmann University Erlangen-Nuremberg
