This thesis introduces the ‘tunable’ piezoelectric transformers (TPT) which provide an extra control terminal, used in this case, to regulate the output voltage. A detailed mathematical analysis is done on the electrical equivalent circuit of the TPT to understand the effect of control terminal loading on the circuit performance. Based on this analysis, a variable capacitor connected across the control terminal is proposed to regulate the output voltage for line and load variations is suggested. The concept of ‘tunability’ in a TPT is introduced and mathematical conditions are derived to achieve the required ‘tunability’. This analysis can help a TPT designer to design the TPT for a specific application and predict the load and line regulations limits for a given design.
A circuit implementation of the variable capacitor, intended for control, is presented. With the proposed control circuit design, the effective value of a fixe capacitor can be controlled by controlling the duty cycle of a switch. Hence, this enables pulse width modulated (PWM) control for the TPT based converter operating at a constant frequency. Fixed frequency operation enables a high efficiency operation of TPT near its resonant frequency and the complete secondary control requires no isolation in the voltage feedback and control circuit. This prevents any ‘cross-talk’ between primary and secondary terminals and reduces the component count. The design of series input inductor for achieving zero voltage switching (ZVS) in the inverter switches for the new control is also discussed.
Experimental results for two different TPT designs are presented. Their differences in structure and its effect on the circuit performance has been discussed to support the mathematical analysis. / Master of Science / Piezoelectric transformers (PTs) are electromechanical devices which can transfer electrical energy by using acoustic coupling. Piezoelectricity is a phenomenon where certain crystalline materials develop electric potential across their surface when subjected to a mechanical stress (transducer). This can also occur in inverse i.e. an electric field inside such a material can produce a mechanical strain inside them (actuator). These direct and indirect piezoelectric effects are used to make a PT which has a transducer and an actuator coupled together to transfer electrical energy.
Power electronics is a rapidly growing field which relies heavily on conventional electromagnetics to store energy (inductors), step-down and step-up voltage (magnetic transformers) and to act as band pass circuits (resonant converter topologies) etc. to enable power conversion. Piezoelectric transformers behave as band pass circuits as such they resonate at a certain frequency and hence allow only a narrow range of frequencies to pass through them. Owing to their light weight, high power density and automated manufacturing capability, they are seen as a potential replacement for electromagnetic transformers in power converters.
This thesis introduces a new structure of PTs, namely the tunable piezoelectric transformers, which allow for better control techniques as compared to standard PTs. Using the extra ‘control’ terminal provided in such a structure the design of a DC-DC converter using TPT is discussed in detail. Mathematical analysis to support the design is presented and the two hardware prototypes, with distinctive designs, are developed to verify the results.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/86442 |
| Date | 26 June 2017 |
| Creators | Khanna, Mudit |
| Contributors | Electrical and Computer Engineering, Burgos, Rolando, Priya, Shashank, Ngo, Khai D. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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