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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
Abstract

Piezoelectric Transformers (PTs)
1.
High Power Density
2.
Low EMI Generation
 Frequency-Dependent / Capacitive Feature :
-
Changes in the Power Topology and the Control
Strategy

To control the output voltage
1.
Frequency Modulation
2.
Frequency Modulation + PWM (Pulse Width Modulation)
 Summary :
1.
New control method for PT-based converter
2.
Control method
-
Simple to design
Requires few components
Regulate the output voltage maintaining constant the
frequency and PWM
Flyback DC/DC Converter (90 Watts)
AC/DC Converter using PT
Introduction

Piezoelectric Transformers (PTS)
1.
Can transfer electric energy ensuring galvanic isolation
2.
Energy transfer is only possible in certain frequency range
3.
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.
2.
3.

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)
1.
The magnetic ideal transformer
2.
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 Method
1.
No load condition
2.
Short-Circuit condition
Converter Power Topology

Converter Power Topology (Fig.3)
1.
A half bridge inverter with only the two switch
2.
An auxiliary inductor between the inverter and the PT
3.
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 :
4.

-

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 softswitching
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
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 frequencythe optimum mode
Fig.5 Gain plots for several loads
versus frequency Rac =
8*Rload/phi^2
Frequency Control

Frequency control
1.
Conventional way to drive dc/dc converter
2.
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 cycle
2.
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 Control
1.
An oscillator fixes the frequency of the voltage
2.
A Schmidt-trigger inverter detects if the output voltage has
reached its max. or min. permitted value
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
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
1.
2.
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
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.)

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
A Comparison of the Converter Protections in
Frequency and Quantum Mode Control

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
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
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
3.
In the quantum mode control
-
4.
Implement an over-voltage protection and stop the converter, using
an Schmidt-trigger comparator
Over-voltage protection can also be skipped, since a Schmidttrigger 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
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.)

Converter Efficiency
Fig. 16 Output regulation boundary (converter
dc input voltage versus converter output
current)
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