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. 6November 2002

발표자 : 장성수

Abstract Piezoelectric Transformers (PTs)

1. High Power Density2. Low EMI Generation

Frequency-Dependent / Capacitive Feature :- Changes in the Power Topology and the Control Strategy

To control the output voltage1. Frequency Modulation2. Frequency Modulation + PWM (Pulse Width Modulation)

Summary :1. New control method for PT-based converter2. Control method

- Simple to design- Requires few components- Regulate the output voltage maintaining constant the frequ

ency and PWM

Flyback DC/DC Converter (90 Watts)

AC/DC Converter using PT

Introduction

Piezoelectric Transformers (PTS)1. Can transfer electric energy ensuring galvanic isolation2. Energy transfer is only possible in certain frequency

range3. Usually, three main resonances (Fig.1)4. The selection of the optimum one to obtain good

efficiency and high energy density The PT Impedance (Fig.1)

1. No load condition2. Fig.1(a) : The Impedance evolution for a PT3. Fig.1(b) : The Impedance among 400KHz and 500KHz

(430KHz) Equivalent circuit for a PT (Fig.2)

1. The magnetic ideal transformer2. The resonance model (RLC)

Introduction (cont.)

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

Introduction (cont.)

Fig.2 Lumped equivalent circuit for a PT based on RLC components

Introduction (cont.) Constraints for PT to construct a converter

1. That the selected topology could cope with all the parasitic described in the equivalent circuit

2. A PT, as every resonant device, exhibits a variable gain with frequency. In this case, a very narrow optimum operating range

3. To attain the soft switching conditioning in order to minimize losses in the transitions of power switches, since the operating frequency is high

4. 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 Method1. No load condition2. Short-Circuit condition

Converter Power Topology

Converter Power Topology (Fig.3)1. A half bridge inverter with only the two switch2. An auxiliary inductor between the inverter and the PT3. 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 size4. 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

Converter Power Topology (cont.)

Fig.3. Proposed topology and main waveforms

Converter Power Topology (cont.)

External inductor- Act as a filter and as a mean

to obtain soft-switching- Provide enough circulating cur

rent 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

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

Frequency Control

Frequency control1. Conventional way to drive dc/dc converter2. To obtain load and line regulation, the frequency range is

very short (20KHz)3. Control circuit to adjust with high frequency

Quantum Mode Control

Quantum Mode Control (Operation Step)1. Inverter operation : Switching with constant frequency and

constant duty cycle2. Constant square voltage is applied to the series inductor

and PT, and converter output voltage is increased until max. allowable value.

3. Control circuit detects this situation, and commands to stop driving the switching of the inverter

4. 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 Control1. An oscillator fixes the frequency of the voltage2. A Schmidt-trigger inverter detects if the output voltage

has reached its max. or min. permitted value

Quantum Mode Control (cont.)

Fig.6 Detailed voltage waveforms. (1us/div), CH2 PT input voltage. CH3 Inverter output voltage

The typical waveform in a quantum mode control (Fig.6 & Fig.7)

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 efficien

cy) Quantum mode control

1. No steady-state condition in the converter (a sequence of transient stages that alternate each other as certain voltage level are reached)

2. A Composition of two frequencies- High switching frequency used to drive the inverter MOSFETs- Lower frequency determined by the periods during which energy is suppli

ed 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 valu

e, and also the high frequency ripple

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

Quantum Mode Control (cont.)

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

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

A Comparison of the Converter Protections in Frequency and

Quantum Mode Control

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

Short Circuit (Open-Loop)1. PT are far more sensitive to

no-load condition than short circuit

2. 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

A Comparison of the Converter Protections in Frequency and Quantum

Mode Control (cont.) Short Circuit (Feedback-Loop)

1. The PT input voltage rises to not allowable values in a few cycles

2. The control circuit detects a voltage lower than the reference, moves the frequency, trying to raise that low output level

3. The PT input voltage increases dramatically (Fig.10 & Fig.11) 4. 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

1. The frequency is invariable, the PT is self protected under this condition (Fig.9)

2. In Fig.11, the frequency is maintained in 470KHz(Fig.11)

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)

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

A Comparison of the Converter Protections in Frequency and Quantum

Mode Control (cont.) No Load Condition

1. The PT input voltage arises to non-acceptable values (The converter mayl be destroyed in a few cycles)

2. When working at the variable frequency- Implement an over-voltage protection and stop the converter, using an Sch

midt-trigger comparator

3. In the quantum mode control- Over-voltage protection can also be skipped, since a Schmidt-trigger invert

er is implemented in the main feedback loop

4. Test (No load condition)- The feedback loop : acts as minimum load, and the Schmidt-trigger inverte

r 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

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

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)

Experimental Results (cont.)

Fig. 16 Output regulation boundary (converter dc input voltage versus converter output current)

Converter Efficiency

Fig. 17 Efficiency versus output (fixed dc input voltage)

Experimental Results (cont.)

Dynamic Test

Fig. 18 Output voltage and PT input voltage [top:200ms/div, bottom:20ms/div]

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