IEEE Power Electronics Magazine - June 2018 - 80

Drain
SiC JFET
Si MOSFET
Gate

Rg
Source

FIG 2 The SiC cascodes encompass
other allowable gate-drive voltage ranges.

IGBTs, significant gate-drive power.
The cascode, with its typical drive of
+12 V/0 V and maximum of +/−25 V,
drops in to an existing Si MOSFET slot
with little or no change to the surrounding circuitry.
The first tests were encouraging,
and the engineers realized that, with
simple adjustments to the gate resistor
values and driver dead time, keeping
the existing gate-drive voltages and
operating frequency, the product worked.
Detail tests showed that efficiency had
actually increased by 1% at operating
loads and by an impressive 10% at light

loads, offering real energy savings to
the company's customers to the order
of 750 kWh in five years/u in real applications. The voltage rating of the cascode is 1,200 V, against the obsolete
Si MOSFET at 950 V, for increased
reliability with the voltage overstress
that one might see with inductive loads;
its on-resistance is better by a third, at
100 milliohms. The engineers found that
even snubber network sizes could be
reduced and electromagnetic interference performance maintained with just
two extra Y capacitors. With two cascodes replacing sets of three Si MOSFETs, the overall cost was no greater,
and Micropower Group became excited
about the prospect of achieving similar
gains in other products.

The Final Push
With a tentative sigh of relief, Micropower Group embarked on a thorough
qualification program to ensure that
the results could be duplicated in production and, most important, that the
devices would survive the reality of

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short circuits and overvoltages. In the
12-month test sequence, full functional
and environmental tests were performed,
including a six-month program of thermal cycling. Abnormals included output short circuits; load disconnections
and bouncing phase errors, each performed 20,000 times; voltage surges,
according to IEC 61000-4-5; and thermal overloads, simulating a loss of
cooling. All of the tests were passed in
the time planned, with no failures. As a
final precaution, prior to full production, 20 chargers using cascodes were
shipped to selected global customers,
followed by 100 more, for real-life field
testing, again with no issues arising.
From the dark days of obsolescence
notices, Micropower Group now has a
product that is better and that likely has
a long production life ahead, with close
support from the cascode manufacturer and its local distributor. The company now sees SiC cascodes as the future
in its designs and has already started a
new 13-kW charger project using paralleled cascode devices. The tantalizing
possibility of even higher efficiency,
with a hugely reduced size of the magnetics and other passive components, is
there for the taking in new designs,
with consequent cost, size, and energy
savings for customers. For Micropower
Group, the future is lithium-ion battery
charging, higher battery voltages of
400 V and more for electric vehicle
applications, and wireless charging
at >1 kW, all supported by SiC cascode technology.

About the Author
Christopher Rocneanu (crocneanu@
unitedsic.com) received his post-graduate degree in electrical engineering and
business from the University of Kiel,
Germany, in 2011 following his graduate
studies at the University of California,
Santa Barbara, in 2009. He began his
career as a field applications engineer at
MEV Elektronik Service GmbH, where
he developed an interest in silicon carbide (SiC) devices. He then moved to
ROHM Semiconductor, where he did
business development for the SiC device
product line, before joining UnitedSiC.

http://EP.MERSEN.COM

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2018