IEEE Power Electronics Magazine - March 2018 - 30

Stored Energy Eoss (µ J)

Output Capacitance Coss (pF)

close to the values of e-mode GaN HEMTs.
With this energy dissipated as heat in
10,000
every switching cycle under hard-switching
conditions, an improvement in this FoM
directly relates to efficiency improvements
Longer Delay Times
in major applications, such as continuous1,000
current mode power factor correction (CCM
Lower Switching Losses
Stronger Nonlinearity
PFC) stages.
Minimum turn-off loss can only be
100
achieved if the channel can be turned off
Lower Eoss
faster as the voltage rises across the deHigher dv/dt
vice. Figure 3 shows measured turn-off
10
losses as a function of load current with
1
10
100
1,000
the gate resistor value as a parameter. If the
VDS (V)
gate resistor is chosen at 5 Ω or lower, then
near lossless turn off can be achieved for
8-mΩ . cm2 Superjunction Technology
load currents that exceed 20 A. In this case,
24-mΩ . cm2 Superjunction Technology
the load current commutes entirely into
38.5-mΩ . cm2 Superjunction Technology
the output capacitance, charging it with
GaN HEMT
a dv/dt being defined by the shape of the
output capacitance and the respective load
FIG 1 The development of the characteristic output capacitance of three conseccurrent. Even though this type of switchutive technology nodes of a superjunction device in comparison to an e-mode
ing is most beneficial from an efficiency
GaN HEMT. VDS: drain-source voltage.
and loss perspective, it is challenging in
terms of di/dt and dv/dt. The design of the
switching cell, especially the loop inductance, and the cou7
pling capacitances from the switching node to the ground
FoM Ron. Eoss
Scales
with
Pitch
of
and dc+ potential are determining the extent to which the
6
Superjunction Device
potential toward loss reduction can be used. For turn-on
5
losses, the same rationale applies. In this context, please
4
refer to [4].
With wide-bandgap power devices, such as Trench Si3
carbide (SiC) MOSFETs and e-mode GaN HEMTs, even
2
higher switching speeds can be achieved. Figure 4(a) shows
Latest
Superjunction
a measured transient of a GaN HEMT at a turn-off rate
1
Technology Is Already
exceeding 500 V/ns. This ultrahigh switching speed was
Close to GaN
0
enabled by a carefully planned layout, as shown in Fig0
100
200
300
400
ure 4(b), which placed ceramic capacitors short distances
VDS (V)
apart from one another and provided an elaborate gate
drive with very high dv/dt immunity [5]. Bearing in mind
38-mΩ . cm2 Superjunction Technology
these capabilities of modern power semiconductor devices,
24-mΩ . cm2 Superjunction Technology
the heterogeneous integration of active and passive com8-mΩ . cm2 Superjunction Technology
ponents offering large-area thermal coupling to heat sinks,
GaN HEMT
and proper mechanical and electric interfaces will be a
must to fully benefit from the potential of the latest power
FIG 2 The trend for the energy stored in the output capacisemiconductor devices.
tance across three consecutive generations of superjunction
devices in comparison to GaN HEMTs.

Future Developments in Passive Components
columns in the device structure, which is the key factor in
reducing the on-state resistance, leads to a more pronounced
nonlinearity of the output capacitance and a proportional
reduction of the high-voltage range of the C oss curve [3].
The latter effect is especially beneficial for the energy
stored in the output capacitance, E oss, as shown in Figure 2.
The subsequent reduction of C oss from 50 V and onward
in the latest superjunction technology already brings E oss

IEEE PowEr ElEctronIcs MagazInE

z March 2018

Underlying advances in power electronics in recent decades
is the assumption that higher switching frequencies can be
used to reduce the size, cost, and loss of passive components, particularly magnetics. Moving from line frequencies
to kilohertz electronic switching allowed dramatic improvements, and advances in semiconductors and circuit
designs have enabled substantial progress. However, the
approaches that yield very low loss in the kilohertz range do

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