A New Full-Protected Control Mode to Drive Piezoelectric Transformers in DC-DC Converters

1. A New Full-Protected Control Mode to Drive Piezoelectric Transformers in DC-DC Converters J.A.M. Ramos, M.A.J. Prieto, F.N. Garica, J.D. Gonzalez, F.M.F. Linera IEEE Transactions on Power Electronics, Vol. 17, No. 6 November 2002 발표자 : 장성수

2. Abstract • Piezoelectric Transformers (PTs) • High Power Density • Low EMI Generation • Frequency-Dependent / Capacitive Feature : • Changes in the Power Topology and the Control Strategy • To control the output voltage • Frequency Modulation • Frequency Modulation + PWM (Pulse Width Modulation) • Summary : • New control method for PT-based converter • Control method • Simple to design • Requires few components • Regulate the output voltage maintaining constant the frequency and PWM

3. Flyback DC/DC Converter (90 Watts)

4. AC/DC Converter using PT

5. Introduction • Piezoelectric Transformers (PTS) • Can transfer electric energy ensuring galvanic isolation • Energy transfer is only possible in certain frequency range • Usually, three main resonances (Fig.1) • The selection of the optimum one to obtain good efficiency and high energy density • The PT Impedance (Fig.1) • No load condition • Fig.1(a) : The Impedance evolution for a PT • Fig.1(b) : The Impedance among 400KHz and 500KHz (430KHz) • Equivalent circuit for a PT (Fig.2) • The magnetic ideal transformer • The resonance model (RLC)

6. Introduction (cont.) Fig.1 (a) Impedance versus frequency plot for a PT under no load condition. (b) Detail around the main resonance

7. Introduction (cont.) Fig.2 Lumped equivalent circuit for a PT based on RLC components

8. Introduction (cont.) • Constraints for PT to construct a converter • That the selected topology could cope with all the parasitic described in the equivalent circuit • A PT, as every resonant device, exhibits a variable gain with frequency. In this case, a very narrow optimum operating range • To attain the soft switching conditioning in order to minimize losses in the transitions of power switches, since the operating frequency is high • To carefully control the no-load an the short circuit situations, since they could cause dangerous over-voltage at the PT input • Self-Protected Control Method • No load condition • Short-Circuit condition

9. Converter Power Topology • Converter Power Topology (Fig.3) • A half bridge inverter with only the two switch • An auxiliary inductor between the inverter and the PT • A ring-shaped PT operating in thickness-mode. • Multilayer design to obtain a suitable conversion ratio (3.8:1) • High operating frequency to reduce the size • The simplest output stage in secondary side : a rectifier and a filter capacitor • External inductance : • Zero voltage switching (ZVS) to reduce switching loss • PT Converter Operation (Fig.3) • Min. number of switches • Additional devices to transfer the energy and min. losses

10. Converter Power Topology (cont.) Fig.3. Proposed topology and main waveforms

11. Converter Power Topology (cont.) • External inductor • Act as a filter and as a mean to obtain soft-switching • Provide enough circulating current to smoothly charge/discharge the Cp1 capacitance during the switching transition (ZVS) • Fig.4 shows the imaginary part of Zin versus frequency • The frequency ranges that might provide ZVS are those where this plot becomes positive Fig.4 Imaginary part of Zin versus frequency

12. Converter Power Topology (cont.) • Enough voltage gain • Fig. 5 shows the frequency and load dependence of that gain for two particular cases : no external inductor and 180uH • Overlapping the possible frequency ranges for gain and ZVS, the minimum required inductor is estimated. • The converter output voltage can be controlled by modifying the switching frequency (Resonant Converter) • In quantum-resonant mode : PT operates at a fixed frequency- the optimum mode Fig.5 Gain plots for several loads versus frequency Rac = 8*Rload/phi^2

13. Frequency Control • Frequency control • Conventional way to drive dc/dc converter • To obtain load and line regulation, the frequency range is very short (20KHz) • Control circuit to adjust with high frequency

14. Quantum Mode Control • Quantum Mode Control (Operation Step) • Inverter operation : Switching with constant frequency and constant duty cycle • Constant square voltage is applied to the series inductor and PT, and converter output voltage is increased until max. allowable value. • Control circuit detects this situation, and commands to stop driving the switching of the inverter • When the converter output voltage is reached the min. allowable value, the control circuit activates the inverter switches again. • The most important feature of Quantum Mode Control • An oscillator fixes the frequency of the voltage • A Schmidt-trigger inverter detects if the output voltage has reached its max. or min. permitted value

15. Quantum Mode Control (cont.) • The typical waveform in a quantum mode control (Fig.6 & Fig.7) Fig.6 Detailed voltage waveforms. (1us/div), CH2 PT input voltage. CH3 Inverter output voltage

16. Quantum Mode Control (cont.) • Fig.7 shows the converter output, PT input, and inverter output voltage • Switching Frequency : 470KHz (enough voltage gain at any load and good efficiency) • Quantum mode control • No steady-state condition in the converter (a sequence of transient stages that alternate each other as certain voltage level are reached) • A Composition of two frequencies • High switching frequency used to drive the inverter MOSFETs • Lower frequency determined by the periods during which energy is supplied to the PT and those during which it isn’t • Larger output filter will be needed in the output stage • Increasing the size of the output filter will reduce the low frequency value, and also the high frequency ripple

17. Quantum Mode Control (cont.) Fig.7 Converter output voltage (CH1, top waveform) and PT input voltage (CH2) printed over the inverter output voltage (CH3), (bottom waveforms) when the output current was 0.1A

18. Quantum Mode Control (cont.) • To avoid undesired voltage peaks • The PT input capacitor must be completely discharged at the end of each low frequency cycle • Fig.8 shows high voltage peaks in the PT input • A discharge path provided by the lower transistor, to avoid the interruption of the converter Fig.8 High voltage peaks at the PT input with the input capacitor not fully discharged. CH1. Converter output voltage. CH3. PT input voltage. CH4. Inverter output

19. A Comparison of the Converter Protections in Frequency and Quantum Mode Control • Short Circuit (Open-Loop) • PT are far more sensitive to no-load condition than short circuit • Test • Open loop condition in the converter, Load : 33 ohms • Under this circumstances, the output was shorted (Fig.9) • PT is self protected against short-circuit condition Fig.9 Waveforms before (up) and during (down) a short circuit in open loop. CH1 PT input current, CH2 PT input voltage, CH4 converter output voltage

20. A Comparison of the Converter Protections in Frequency and Quantum Mode Control (cont.) • Short Circuit (Feedback-Loop) • The PT input voltage rises to not allowable values in a few cycles • The control circuit detects a voltage lower than the reference, moves the frequency, trying to raise that low output level • The PT input voltage increases dramatically (Fig.10 & Fig.11) • Protection methods • Shutdown the converter when the PT input voltage reaches a dangerous value (Fig.10 was destructive) • Fix the frequency until short-circuit disappears. • The validity and sign of this mechanism depends on the value of the additional inductor • The frequency should be limited above 453KHz, maintaining PT input voltage under 100Vac • In quantum mode control • The frequency is invariable, the PT is self protected under this condition (Fig.9) • In Fig.11, the frequency is maintained in 470KHz(Fig.11)

21. A Comparison of the Converter Protections in Frequency and Quantum Mode Control (cont.) Fig. 10 Waveforms in short-circuit mode in case of variable frequency control and no protection. CH1 driven pulses. CH2 ac input voltage at the PT. CH4 (0.5A/div PT input current)

22. A Comparison of the Converter Protections in Frequency and Quantum Mode Control (cont.) Fig. 11 Switching frequency and PT input voltage at full load. Evolution of PT input voltage under short-circuit and minimum safe frequency

23. A Comparison of the Converter Protections in Frequency and Quantum Mode Control (cont.) • No Load Condition • The PT input voltage arises to non-acceptable values (The converter mayl be destroyed in a few cycles) • When working at the variable frequency - Implement an over-voltage protection and stop the converter, using an Schmidt-trigger comparator • In the quantum mode control • Over-voltage protection can also be skipped, since a Schmidt-trigger inverter is implemented in the main feedback loop • Test (No load condition) - The feedback loop : acts as minimum load, and the Schmidt-trigger inverter will shutdown the converter when necessary • Quantum mode control : Self-protected by the main loop • Fig.12 shows the transition from full load to no load condition • Fig.13 shows the no-load to full load transition

24. A Comparison of the Converter Protections in Frequency and Quantum Mode Control (cont.) Fig. 12 Full-load to no load transition. CH1 converter output voltage. CH2 PT input voltage Fig. 13 No load to full load transition. CH1 Converter output voltage. CH2 PT input voltage

25. Experimental Results • Line and Load Regulation Fig. 14 Converter output voltage versus output current (dc input voltage fixed) Fig. 15 Converter output voltage versus converter input voltage (output current fixed)

26. Experimental Results (cont.) • Converter Efficiency Fig. 16 Output regulation boundary (converter dc input voltage versus converter output current) Fig. 17 Efficiency versus output (fixed dc input voltage)

27. Experimental Results (cont.) • Dynamic Test Fig. 18 Output voltage and PT input voltage [top:200ms/div, bottom:20ms/div]

28. Conclusion • Quantum mode control for DC/DC converter using PT • Very simple feedback loop • Using a Schmidt-trigger inverter to perform the full control • Safe operation mode for the converter (PT) • The main drawback - Filter capacitance sizing to reduce ripple • The control method was tested and verified - 8 watts, AC/DC adapter (110Vac,12 Vdc) operating at 470 KHz