| Abstract |
During the last years, a clear trend to miniaturise AC/DC adapters for portable terminals has been
detected world-wide. This could be possible by replacing the classical transformer, built with
commercial magnetic cores that not only limit the miniaturisation of the converter but are also EMI
sources, by an alternative piezoelectric transformer, which features low profile, high power density
(several times higher than high frequency magnetic ones) and low EMI generation.
Due to the PT behavior, very frequency-dependent, previous works presented frequency modulation
to control the output voltage like the most natural way. Also PWM control techniques can be
employed in combination with the frequency control to achieve both line and load regulation. In this
paper, a new control strategy is introduced that can be used with PT-based converters. It can simplify
the control design process and only a few components are necessary. The switches in the power stage
are so operated that a constant-frequency, constant duty-cycle voltage waveform is applied to the
input of the PT. The response of the component to this waveform originates an output voltage that
builds up to a maximum allowable value. When this value is reached, the control circuit detects this
situation and stops driving the switches. The PT input is zero and the output voltage is pulled down
until a minimum allowable value is reached. At this point the control circuit activates the switches
again and the cycle starts anew. This hysteretic control is well-known and employed in low standbypower
SMPS but never for a PT control. The main features of this control are presented in the paper.
In any case, Piezoelectric Transformers are not yet usual devices in power electronics converters,
only a few companies develop this kind of elements and several limitations have been detected along
this work. |
