Improved zero-current transition converters for high power applications*

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Improved Zero-Current Transition Converters For High Power Applications*
Hengchun Mao, Fred C. Lee, Xunwei Zbou, and Dushan Boroyevich
Virginia Power Electronics Center
The Bradley Department of Electrical Engineering
Virginia Polytechnic Institute and State University
Blacksburg, VA 2406 1-0 111
Fax: (540) 23 1-6390, Email: hmao@vtvml

Abstract - Existing Zero-CurrenitTransition (ZCT) converters
do not solve main switch turn-on problems and require
auxiliary switches to be turned off with high current, and
therefore are not suitable for high power applications. Novel
ZCT schemes proposed in this paper enable all main switches
and auxiliavy switches to be turned on and off under zero
current conditions. The zero-current switching at both turnon and turn-off notonly reduces switching losses
sign@cantly, but also eliminaires the necessity of passive
snubbers due to the much reduced switch stress. The cost o
the auxiliary switches can be reduced compared to the
existing ZCT schemes due to their zero-current turn-ofl The
proposed technology is well suited for dc-de and three-phase
converters with IGBTs, MCTY and GTOs. Theoretical
analysis, computersimulation and experimental results are
presented to explain the proposed schemes.

Power semiconductor switches in high power
applications are subject to high switching stresses and
switching losses.
In conventional designs, significant
derating of device voltage and current ratings and elaborate
passive snubbers have to be used due to the switching
stresses, and theswitching firequency is limited to low
frequency ranges. In recent years, various soft-switching
techniques have been proposed to alleviate these problems.
Significant performance improvements as well as cost, size
and weight reduction can be achieved with the help of soft
switching. A successful s oft swlltching scheme for high power
applications should reduce the switching losses, diodereverse
recovery, and switching stress for all main and auxiliary
switches without increasing the device voltage rating, because
the device voltage margin is usually small and the thermal
management very difficult. The recently developed zerovoltage transition (ZVT) [7] and zero-current transition
(ZCT) [ I] PWM techniques incorporate soft switching into
PWM converters, so that the switchinglosses can be reduced

with minimum voltagelcurrent stresses and circulating energy.
The ZVT technique forces the voltage of a switch to zero
before its turn-on, to practically eliminate switch turn-on loss.
The switch turn-off loss, which is usually the dominating
Switching loss in high power applications, cannot be
alleviated effectively with ZVT technique, because a
capacitor snubber sizedfor high frequency operation has only
marginal effect on the turn-off loss. The ZCT technique can
significantly reduce the switch turn-off loss by forcing the
switch current to zero prior to its turn-off. A ZCT boost
converter proposed in [ l] is shown in Fig. l(a), where the
auxiliary circuit is shown within the dotted frame. The key
waveforms of the circuit operation are shown in Fig.l(b). As
can be seen from Fig. l(b), the current of the main switch is
reduced to zero prior to its turn-off. However, the turn-on of
the main switch is not affected by the auxiliary circuit, and
severe diode reverse recovery causes high turn-on loss in the
switch. Moreover, the auxiliary switch turn-off current (at the
moment t3) is the same as the inductor current I, i.e. the same
as themain switch turn-off current in the hard-switched
converter. Therefore, this scheme can achieve efficiency
improvement only if Sx has much lower turn-off loss than S
(such as in the low power and low voltage applications where
MOSFETs with low turn-off loss can be used to implement
the auxiliary switch), and is not suitable for high power
applications. Additionally, high current devices ( >...
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