AN4242
Application note
New generation of 650 V SiC diodes
Introduction
For many years ST has been a worldwide leader in high voltage rectifiers dedicated to
energy conversion. During the last decade, electronic systems have followed a continuous
trend towards higher power density and more energy savings driven by governments’
environmental awareness. Power-supply designers are permanently confronted with
stringent efficiency regulations (Energy Star, 80Plus, European Efficiency…). They are
forced to consider the use of new power converter topologies and more efficient electronic
components such as high-voltage silicon-carbide (SiC) Schottky rectifiers. To help them face
this challenge, ST developed in 2008 a first family of 600 V SiC diodes. After having sold
millions of pieces, ST’s reliability and know-how is confirmed on these new components
using wide band gap materials.
In hard-switching applications such as high end server and telecom power supplies, SiC
Schottky diodes show significant power losses reduction and are commonly used. A
growing use of those rectifiers is also recorded in solar inverters, motor drives, USP and
HEV applications.
However, the high cost of this technology tends to drive designers to use it at high
current-density levels (3 to 5 times higher than standard Si diodes), inducing more
constraints on the diode. Indeed, the Silicon-carbide material features a positive thermal
coefficient potentially leading to some instability and lower current-surge robustness than
silicon diodes. ST decided to review the design and develop a second generation of SiC
diodes offering an enhanced current capability while still featuring an attractive switching-off
behavior. The peak reverse voltage was also increased to 650 V in order to ensure a safer
operation in certain designs.
Typical applications (non-exhaustive list)
Charging station
ATX power supply
AC/DC power management unit, high voltage, and other topologies
Desktop and PC power supply
Server power supply
Uninterruptible power supply
Photovoltaic string and central inverter architecture
Photovoltaic power optimizer architecture
Photovoltaic microinverter grid-connected architecture
Photovoltaic off-grid architecture
Telecom power
May 2013
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www.st.com
Contents
AN4242
Contents
1
Features of the SiC diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
Turn off behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1
1.1.2
Comparison with Si bipolar diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Capacitive charge (QC) measurement . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2
1.3
Forward characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Other characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1
1.3.2
Low leakage current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
“C” thermal coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2
Forward thermal runaway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.1
2.2
Thermal runaway risk in regular working mode . . . . . . . . . . . . . . . . . . . . . 8
Thermal runaway risk in transient phase . . . . . . . . . . . . . . . . . . . . . . . . . .11
3
New 650 V JBS SiC diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
3.1
3.2
Device structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Comparison between first and second generation of SiC diodes . . . . . . . 13
3.2.1
3.2.2
3.2.3
Forward voltage comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
IFSM PSpice simulation: comparison between 1st and 2nd generation 15
IFSM datasheet comparison between SiC G2 and SiC G1 . . . . . . . . . . 16
3.3
JBS structure trade-off: current surge capability versus Qrr . . . . . . . . . . 17
3.3.1
3.3.2
3.3.3
Forward characteristics comparison between ST’S SiC 2nd generation
and other JBS designs 17
No recovery charge area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
PSpice electro-thermal simulation result . . . . . . . . . . . . . . . . . . . . . . . . 19
4
Efficiency measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.1
4.2
dI/dt optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Example of efficiency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
5
6
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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AN4242
Features of the SiC diodes
1
1.1
1.1.1
Features of the SiC diodes
Turn off behavior
Comparison with Si bipolar diode
The benefits brought by silicon-carbide diodes on the switching losses in the applications
working in continuous-conduction mode (such as PFC applications) are already well known.
The capacitive nature of the recovery current allows constant turn-off characteristics when
the temperature increases. In contrast the turn-off behavior of bipolar diodes is
characterized by a strong dependency on junction temperature, dI/dt slope and forward
current level (see
Figure 1).
Thanks to their properties, SiC diodes allow significant reduction of power losses in the
associated MOSFETs when switched-on. They also permit new optimization options for the
power converter (for example, increasing the switching frequency and speed, lowering the
size of passive components, snubber-circuits and EMI filters).
Figure 1. Switching behavior comparison between Si and SiC diodes
for T
j
=75 °C and T
j
=125 °C
V
R
=380 V, I
F
=8 A, dI/dt=200 A/μs, Tj=75 °C
20 ns/div
2 A/div
V
R
=380 V, I
F
=8 A, dI/dt=200 A/μs, Tj=125 °C
20 ns/div
2 A/div
8 A SiC
diode
8 A tandem (2 x
300 V diodes in
series)
8 A SiC
diode
8 A bipolar
diode
8 A tandem (2 x
300 V diodes in
series)
8 A bipolar
diode
The capacitive recovery current is generated by the charge of the junction capacitance C
j
under a certain reverse voltage and corresponds to a quantity of stored charges Qc.
1.1.2
Capacitive charge (Q
C
) measurement
Some confusion exists about the measurement conditions of Qc. A comparison between the
switch-off behavior and the integral of the current used to estimate Qc is shown in
Figure 2.
Figure 2A
and
Figure 2B
show measurements at low forward current (I
F
=1 A) and low dI/dt
slope (50 A/µs), with and without reverse voltage across the diode. A certain inaccuracy of
the measurement of Qc can be observed. It is linked to the probe, which features its own
equivalent capacitance.
Figure 2C
shows a measurement at I
F
=2 A and a high dI/dt slope
(200 A/µs) without any probe voltage. With such a high value of current-slope some
oscillations appear. Taking into account the total capacitive current until t
0
when the reverse
voltage reaches V
R
, Qc measured by the integral of the current is similar to the one in
Figure 2B.
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Features of the SiC diodes
AN4242
Figure 2. Qc measurement of a 6 A SiC diode at I
F
= 1 A, T
j
= 25 °C, V
R
= 400 V,
dI/dt = 50 A/µs
Voltage probe
0
Qc = 24.2 nC
t
0
Qc = 19.6 nc
A. With voltage probe
B. Without voltage probe
Qc=20.4nc
C. I
F
= 2 A, dI/dt = 200 A/µs
To avoid false readings due to some measurement inaccuracy, a theoretical approach is
preferred. The quantity of charge Q during a certain period of time [0-t
0
] is delimited by the
reverse voltage variation V across the junction capacitance C
j
between 0 and V
R
and is
given by the following formulas:
Equation 1
ò
Qc
dQ =
0
ò
t0
i(t) dt
0
with
Equation 2
i(t) = C
j
dV(t)
dt
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AN4242
Features of the SiC diodes
After simplification and introduction of the junction capacitance variation versus the reverse
voltage C
j
(V), Qc is defined by the following formula:
Equation 3
Qc(V
R
) =
V
R
Cj(V) dV
ò
0
This relation demonstrates that Qc is defined by the integral of the junction capacitance C
j
between 0 and V
R
, the voltage reapplied on the diode. This theoretical approach allows the
direct and accurate evaluation of Qc, avoiding the inaccuracy introduced by potential
measurement problems.
The strict expression of the energy stocked in the junction capacitor for a given reverse
voltage can be determined by:
Equation 4
Qc(V
R
) =
ò
V
R
Cj(V) · V dV
0
Due to the non-linearity of the junction capacitance versus the reverse voltage, this relation
is different from the traditional energy formula ½ · C · V² (or ½ · Q · V), which is valid only
when considering a constant capacitance.
1.2
Forward characteristics
Another main feature of SiC diodes is the variation of the forward voltage drop (V
F
) with the
junction temperature.
Figure 3
shows the forward current versus forward voltage drop characteristics for 3
different junction temperature levels. A crossing-point can be observed at a certain level of
current I
C
. When the current is lower than this level, the temperature coefficient of the
forward voltage drop (V
F
) is negative. When the current is higher, it becomes positive. The
same crossing point exists for traditional silicon diodes, but it appears at a much higher
current level (>10 times the nominal current). This is linked to the higher forward current
density of SiC diodes.
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