Beware Of Zero-Voltage Switching
by Sanjay Havanur, Vishay Siliconix, Santa Clara, Calif, How2Power Today, Apr 15 2016
Focus:
Zero-voltage switching (ZVS) is often viewed as the panacea for all the challenges posed by
high frequency and higher efficiency requirements in power conversion. While ZVS is indeed a
blessing, designers need to be aware of its limitations and watch out for a whole range of
traps in its implementation. And while much has been written on how to implement ZVS, very
little has been written from the perspective of the switching device. This article looks at ZVS
from the MOSFET’s point of view. It starts by defining related terms such as soft switching,
quasi-resonant switching, resonant switching and synchronous switching, and cites examples of
each. Then it discusses the MOSFET losses not eliminated by ZVS such as turn-off loss, turn-on
loss due to energy stored in Coss during charging and the inseparable nature of Qoss of the
MOSFET from the QRR of its body diode. Next it discusses the problem of body-diode recovery
during MOSFET turn-off and the limitations it imposes on circuit operation. The loss of ZVS
during light load conditions is noted. Finally, the last section notes factors that mitigate
the liabilities of ZVS such as the low effective reverse-recovery charge of the body diode,
which is only a fraction of its datasheet value, and the ongoing improvement in body diode
characteristics for high-voltage MOSFETs.
What you’ll learn:
- How to understand the pitfalls of zero-voltage switching and account for these potential problems in MOSFET selection and power converter design
- How to understand the meaning of terms such as soft switching, quasi-resonant switching, resonant switching and synchronous switching
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