IEEE Power Electronics Magazine - December 2022 - 64

(a)(b)
(c)
(d)(e)
(f)
FIG 3 SmartEgg switching waveforms at different input voltage phase angles and output loads. ZVS at turn-on ensured for both switches. (a) 110 VAC, 0°,
50 W. (b) 110 VAC, 45°, 50 W. (c) 110 VAC, 90°, 50 W. (d) 110 VAC, 0°, 50 W. (e) 110 VAC, 45°, 50 W. (f) 110 VAC, 90°, 50 W.
forward phases. The storage inductor normally required in a
forward converter is not necessary here as the leakage
inductance serves in its place.
The magnetizing current of the transformer in forward
mode must be reset and this is achieved by forcing a small
delay before Q1 turn-off and before Q2 turn-on, which allows
reset current to flow back out of C1 through Q1 to the transformer.
This current also discharges
the parasitic output capacitances of
the MOSFETs, so that subsequent Q2
turn-on is at zero voltage, achieving
ZVS at turn-on of both low- and highside
switches and consequent high efficiency
at any load. Figure 3 shows ZVS
in operation: channel 1, (dark blue) is
transformer current, channel 2, (light
blue) is the low-side MOSFET drive signal,
channel 3, (purple) is the high-side
drive signal and channel 4, (green) is
the switching node.
Any mismatch between instansecondary
windings which also facilitates the required
creepage and clearance for reinforced safety isolation.
Thanks to the control strategy described above,
SmartEgg is capable of providing both output voltage regulation
and input PFC: Figure 4 shows ac input voltage,
low-distortion (high power factor) ac input current, and
regulated dc output voltage.
The control algorithm compenThe
storage inductor
normally required in a
forward converter is
not necessary here as
the leakage
inductance serves in
its place.
taneous input and output power
is passed into or drawn out of capacitor C1, which is
arranged to charge to a high voltage by sensing and
feedback to the controller, so that good ride through is
achieved with a physically small component. Voltage
feedback from the dc output back to the controller for regulation
may be by optocoupler or via a digital interface.
The required leakage inductance in the transformer can
be formed and controlled by segmenting the primary and
64 IEEE POWER ELECTRONICS MAGAZINE z December 2022
sates the input-to-output power mismatch
by providing the appropriate
duty cycle and frequency, depending
on the instantaneous input voltage
and output load. This leads to a variable
switching frequency: the converter
described in this article has
been designed to operate mostly of
the time within the 50-150 kHz range.
The frequency is strongly dependent
on hardware parameters such as the
transformer primary side inductance.
The proprietary controller, in
the Eggtronic " EPIC " series, sets
duty cycle, delays, and switching frequency of the two
primary switches, to shape the input current to a nearsinusoid
and control the energy passed to the secondary.
The EPIC101AGSE01, a mixed-signal, scalable, 32-bit
controller with a RISC-V digital core, is in a 7 × 7 mm,
48-pin LQFP48 package and includes all functionality
for a complete converter up to 1 kW, including integrated
gate drivers and support for USB-C power delivery

IEEE Power Electronics Magazine - December 2022

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