10970
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 34, NO. 11, NOVEMBER 2019
LLC Synchronous Rectification Using Resonant
Capacitor Voltage
Jhih-Da Hsu , Student Member, IEEE, Martin Ordonez , Member, IEEE, Wilson Eberle , Member, IEEE,
Marian Craciun, Member, IEEE, and Chris Botting , Member, IEEE
Abstract—Synchronous rectification (SR) for LLC resonant con-
verters has been developed to enhance the power conversion effi-
ciency and achieve high-power-density design. Conventional SR
driving strategies can be categorized as current-driven methods,
vDS-ON sensing methods, and alternative approaches. The current-
driven methods widely adopt current transformers (CTs) to sense
the rectifier current and generate the SR driving signals. The CTs
are lossy and bulky, which is unfavorable to high-power-density
design. The vDS-ON sensing methods remove the current sensors by
sensing the voltage across the ON-state resistor of the SR MOSFET,
yet the sensed signal is small and prone to be offset by the induc-
tive voltage induced from parasitic components. Most alternative
approaches avoid sensing noise-sensitive signals; however, the op-
erating range is narrow due to the limited information from the
converter. This paper proposes an SR driving strategy based on
the resonant capacitor voltage (RCV) to address those issues. The
RCV SR driving strategy does not require current sensors. The
sensed RCV is insensitive to the parasitic effects. In addition,
the RCV strategy controls the SR ON-time effectively over a wide
range of operating frequency and loading conditions. Simulation
and experimental results of a 650-W/24-V LLC converter are pre-
sented to validate the effectiveness of the proposed RCV strat-
egy. Compared with the conventional vDS-ON sensing method, the
ON-time error caused by the parasitic effect is greatly reduced,
which improves the power conversion efficiency and reduces the SR
MOSFET temperature.
because of the ability to achieve wide output regulation range
and perform soft switching even at no-load conditions [4]–[6].
Also, soft switching allows high-switching-frequency opera-
tion, which shrinks the size of magnetic components and enables
high-power-density design. LLC resonant converters require no
output filter inductor, which not only reduces its volume, but
also lowers the voltage stress of the output rectifiers. To further
boost the power conversion efficiency, synchronous rectifiers
(SRs) are favorable choices to reduce the conduction losses in
the rectifier stage. In particular, for high-power and high-current
applications such as server power supplies [7]–[9] and electric
vehicle battery chargers [10]–[13], there is a need for employing
an SR to achieve high efficiency and better thermal performance.
In recent years, vast amounts of research have been conducted in
the field of SRs for LLC converters. Based on the SR control sig-
nal sensing strategies, these SR techniques can be categorized as
follows: current-driven methods, vDS-ON sensing methods, and
alternatives.
Regarding the first category, the current-driven methods, cur-
rent transformers (CTs) are widely applied as the current sensor.
The CT monitors the rectifier current and generates SR driving
signals, as the CT1 illustrated in Fig. 1. Mounting the CT on the
secondary side in series with the SR switches facilitates the gate
driving, for the SR driving signal is naturally synchronous with
the rectifier current [14]. Yet, the CT introduces extra conduc-
tion losses due to the high rectifier current. In the research [15],
an energy recovery current-driven SR is proposed. A set of en-
ergy recovery winding is integrated with the CT for recycling
the current sensing energy and hence improves the efficiency of
the CT. However, each SR requires one CT for gate driving; two
CTs for center-tapped full-wave rectification are needed. For
full-bridge rectification, the transformer structure is even more
complicated. The high volume occupied by the CT is unfavor-
able for high-power-density design. Guo et al. [16] propose a
current-driven SR for center-tapped full-wave rectifiers that re-
quires only one CT. The CTs are integrated into one, and the
utilization of the CT is improved. However, each SR needs one
set of winding to detect the current, and hence, the conduction
losses are similar. In [17]–[19], the application targets toward
the voltage-doubler and full-bridge rectifier topology, and the
current sensor count for each topology is reduced to one as well.
Another idea to reduce the space occupied by the CTs is to in-
tegrate them with the main transformer, as investigated in the
literature [20]. Intrinsically, placing the CT in series with the
power loop introduces power losses. The peak rectifier current
Index Terms—Current sensorless, LLC resonant converter,
resonant capacitor voltage (RCV), synchronous rectification (SR).
I. INTRODUCTION
ESONANT converters are advantageous for high-
switching-frequency design for their capability of reduc-
R
ing the switching losses of active components by performing soft
switching [1]–[3]. Among those various topologies of resonant
converters, the LLC resonant converter is attractive primarily
Manuscript received April 10, 2018; revised September 6, 2018 and Decem-
ber 23, 2018; accepted February 5, 2019. Date of publication February 20,
2019; date of current version August 29, 2019. This work was supported by
the Natural Sciences and Engineering Research Council of Canada (NSERC).
Recommended for publication by Associate Editor T. Qian. (Corresponding
author: Martin Ordonez.)
J. Hsu, M. Ordonez, and W. Eberle are with the Department of Electrical
and Computer Engineering, The University of British Columbia, Vancouver,
BC V6T 1Z4, Canada (e-mail:,
M. Craciun and C. Botting are with Delta-Q Technologies Corpora-
tion, Burnaby, BC V5G 3H3, Canada (e-mail:,
Color versions of one or more of the figures in this paper are available online
Digital Object Identifier 10.1109/TPEL.2019.2900459
0885-8993 © 2019 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.
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