Semester 1 Lecturer Javier Sebastián Electrical Energy Conversion and Power Systems Universidad de Oviedo Power Supply Systems Review of the physical principles of operation of semiconductor devices ID: 265628
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Slide1
Power Electronic Devices
Semester 1
Lecturer: Javier
Sebastián
Electrical Energy Conversion and Power Systems
Universidad
de Oviedo
Power Supply SystemsSlide2
Review of the physical principles of operation of semiconductor devices.
Thermal management in power semiconductor devices.
Power diodes.Power MOSFETs.The IGBT
.High-power, low-frequency semiconductor devices (
thyristors).2
OutlineSlide3
Lesson 3 - Power diodes.
Semester 1 -
Power Electronics Devices
Electrical Energy Conversion and Power Systems
Universidad
de Oviedo
3Slide4
4
Outline
The main topics to be addressed in this lesson are the following:
Review of diode operation.
Power diode packages.
Internal structure of PN and Schottky
power diodes. Static characteristic of power diodes.
Dynamic characteristic of power diodes.
Losses in power diodes. Slide5
5
Review of
PN-diode operation (I)
Modern diodes are based either on PN or Metal-semiconductor (MS) junctions.
Reverse bias and moderate forward bias are properly described by the following equation
(by Shockley): i = I
S·(evext
/VT - 1)
,
where VT
= kT/q and
I
s is the
reverse-bias saturation current
(a very small value).
i
v
ext
+
-
V
ext
[
V]
0
100
0
.
25
-
0
.
25
i
[mA
]
0
.
5
-10
-
0
.
5
0
i
[nA
]
V
ext
[
V]
i
»
I
S
·e
V
ext
V
T
(
exponential)
i
»
-
I
S
(
constant) Slide6
6
Review of
PN-diode operation (II)
When the diode has been heavily forward biased (high forward current),
the voltage drop is proportional to the current (it behaves as a resistor).
When the reverse voltage applied to a diode reaches the critical value VBR
, then the weak reverse current starts increasing a lot. The power dissipation usually becomes destructive for the device.
i
v
ext
+
-
0
1
-4
3
i
[A
]
V
ext
[
V]
According to Shockley equation
Actual I-V characteristic
According to Shockley equation
Actual I-V characteristic
0
-V
BR
10
i
[A
]
V
ext
[
V]
-600Slide7
7
Review of
PN-diode operation (III)
Static model for a diode (asymptotic):
i
v
ext
+
-
0
i
[A
]
V
ext
[
V]
Actual I-V characteristic
V
Slope
= 1/r
d
Equivalent circuit:
Model
V
r
d
= 1/tg
a
Actual
(
a
symptotic
)
ideal
V
=
K
nee voltage
r
d
=
Dynamic
resistance
aSlide8
8
Review of
PN-diode operation (IV)
Ideal diode:
i
v
ext
+
-
0
i
[A
]
V
ext
[
V]
Ideal diode
Whatever the forward current is, the forward voltage drop is always zero.
Whatever the reverse voltage is, the reverse current is always zero.
The ideal diode behaves as a short-circuit in forward bias.
The ideal diode behaves as a open-circuit in reverse bias. Slide9
9
Review of
PN-diode operation (V)
Low-power diode.
Anode
Cathode
P
ackage
(glass
or
epoxi
resin)
Terminal
Terminal
P
N
Marking
stripe on the cathode end
M
etal-semiconductor contact
Semiconductor
die
Anode
Cathode
Metal-semiconductor contactSlide10
10
Packages for diodes (I)
Axial leaded through-hole packages
(
low power).
DO 35
DO 41
DO 15
DO 201Slide11
11
Packages for diodes (II)
Packages to be used with heat sinks.Slide12
12
Packages for diodes (III)
Packages to be used with heat sinks
(higher power levels).
B 44
DO 5 Slide13
13
Packages for diodes (IV)
A
ssembly
of 2 diodes (I).
Doubler
(2 diodes in series)
Common cathode
(Dual center tap Diodes)Slide14
14
Packages for diodes (V)
A
ssembly
of 2 diodes (II).Slide15
15
Packages for diodes (VI)
2 diodes in the same package, but without electrical connection between them.Slide16
16
Packages for diodes (VII)
Manufacturers frequently offer a given diode in different packages.
Name
PackageSlide17
17
Packages for diodes (VIII)
A
ssembly
of 4 diodes (low-power bridge rectifiers).
Dual in lineSlide18
18
Packages for diodes (IX)
A
ssembly
of 4 diodes
(medium-power bridge rectifiers).Slide19
19
Packages for diodes (X)
A
ssembly
of 4 diodes (high-power bridge rectifiers).Slide20
20
Packages for diodes (XI)
A
ssembly
of 6 diodes (Three-phase bridge rectifiers).Slide21
21
Packages for diodes (XII)
Example of
a company portfolio regarding single-phase bridge rectifiers.Slide22
22
Internal structure of
PN power diodes (I)
Basic internal structure of a
PN power diode.
P
+
N
+
(substrate)
N
-
(epitaxial layer)
Aluminum contact
Aluminum contact
10
m
m
250
m
m
100
m
m
(for
V
BR
=
1000V
)
N
D1
= 10
14
cm
-3
N
D2
= 10
19
cm
-3
N
A
= 10
19
cm
-3
Anode
CathodeSlide23
N
+
N
-
Cathode
23
Internal structure of PN power diodes (II)
Problems due to the
nonuniformity
of the
electric field.
Anode
P
+
Depletion region in reverse bias
High electric field intensity
Breakdown electric field intensity can be reached in these regions.
Regions with local high electric-field should be avoided when the device is designed. Slide24
N
+
N
-
Cathode
24
Internal structure of PN power diodes (III)
Use of guard rings to get a more uniform electric field.
The depletion layers of the guard ring merge with the growing depletion layer of the P
+
N
-
region, which prevents the
radius of curvature
from getting too
small. Thus there are not places where the electric field reaches very high local values.
Anode
P
+
P
P
Aluminum contact
Aluminum contact
SiO
2
SiO
2
Guard ring
Depletion region in reverse biasSlide25
N
+
N
-
Cathode
25
Internal structure of PN power diodes (IV)
Case
where
the metallurgical junction extends to the silicon surface (I).
Anode
P
+
High electric field intensity in these regions
Depletion region in reverse biasSlide26
26
Internal structure of PN power diodes (V)
Case
where
the metallurgical junction extends to the silicon surface (II).
The use of beveling minimizes the electric field intensity.
Coating the surface with appropriate materials such as silicon dioxide helps control the electric field at the surface.
N
+
N
-
P
+
Cathode
Anode
Depletion region in reverse bias
SiO
2
SiO
2Slide27
N
+
N
-
Cathode
27
Internal structure of
Schottky
power diodes (I)
Problems due to the
nonuniformity
of the
electric field.
Anode
High electric field intensity
Breakdown electric field intensity can be reached in these regions.
Regions with local high electric-field should be avoided when the device is designed.
Aluminum contact
(
N
+
M
Þ
ohmic
)
SiO
2
Depletion region in reverse bias
Aluminum contact
(
N
-
M
Þ
rectifying
)Slide28
N
+
N
-
Cathode
28
Internal structure of
Schottky
power diodes (II)
Use of guard rings to get a more uniform electric field.
The depletion layers of the guard ring merge with the growing depletion layer of the N
-
M region, which prevents the
radius of curvature
from getting too
small.
Anode
P
P
Aluminum contact
(
N
-
M
Þ
rectifying
)
Aluminum contact
(
N
+
M
Þ
ohmic
)
SiO
2
SiO
2
Guard ring
Depletion region in reverse biasSlide29
29
Information given by the manufacturers
Static characteristic:
- Maximum peak reverse voltage.
- Maximum forward current.
- Forward voltage drop. - Reverse current.
Dynamic characteristics:- Switching times in PN diodes.
- Junction capacitance in Schottky diodes.Slide30
30
Maximum peak reverse voltage.
Sometimes,
m
anufacturers
provide two values:
- Maximum r
epetitive peak reverse voltage
, VRRM
.-
Maximum non repetitive peak reverse voltage, V
RSM
.Slide31
31
Maximum forward current.
M
anufacturers
provide two or three different values:
- Maximum RMS forward current
, IF(RMS)
.
- Maximum
repetitive peak forward current, IFRM
.
-
Maximum
surge non repetitive forward current
,
I
FSM
.
I
F(RMS)
depends on the package.Slide32
32
Forward voltage drop, V
F (I).
The forward voltage drop increases when the forward current increases.
It increases linearly at high current level.
i
V
ext
I
D
V
D
5 A
V
r
d
ideal
Load line
Operating point
Actual I-V characteristic given by the manufacturer (in this case is a V-I curve). Many times, current is in a log scale.
Operating pointSlide33
33
Forward voltage drop, V
F (II).
The higher the value of the maximum peak reverse voltage V
RRM, the higher the forward voltage drop VF at I
F(RMS).Slide34
34
Forward voltage drop, V
F (III).
I
t can be directly obtained from the I-V characteristic
, for any possible current.
I
F(AV)
= 4A, V
RRM
= 200V
1.25V @ 25A
2.2V @ 25A
As the values of
I
F(RMS)
, I
FRM
and I
FSM
are quite different, the scale corresponding to current must be quite large.
Due to this, forward voltage drop corresponding to currents well below
I
F(RMS)
cannot be observed properly.
Therefore, log scales are frequently used.
I
F(AV)
= 5A,
V
RRM
= 1200VSlide35
35
Forward voltage drop, V
F (IV).
In log scales.
0.84V @ 20A
1.6V @ 20A
I
F(AV)
= 25A, V
RRM
= 200V
I
F(AV)
= 22A, V
RRM
= 600VSlide36
36
Forward voltage drop, V
F (V).
Schottky diodes exhibit
better forward voltage drop, at least for
VRRM < 200 (for silicon devices).
0.5V @ 10ASlide37
37
Forward voltage drop, V
F (VI).
Silicon Schottky diode with high V
RRM
.
The forward
voltage drop is quite similar to the one corresponding to a PN diode.
0.69V @ 10ASlide38
38
Forward voltage drop, V
F (VII).
Schottky
Schottky
PN
In case of diodes with similar values of V
RRM
, the forward voltage drop is quite similar in PN and Schottky diodes, in both cases made up of silicon.
However, Schottky diodes always have superior performances from the dynamic point of view.
Comparing
s
ilicon Schottky and PN diodes, taking into account their V
RRM
.Slide39
39
Reverse current, I
R (I).
It is measured
at VRRM.
It depends on the values of I
F(AV) and VRRM (the higher I
F(AV) and VRRM
, the higher IR). It increases when the reverse voltage and the temperature increase.
I
F(AV)
= 4A, V
RRM
= 200V
I
F(AV)
= 5A, V
RRM
= 1200V
I
F(AV)
= 8A, V
RRM
= 200VSlide40
Reverse current, I
R
(II).
I
R
increases when
I
F(AV)
and
T
j
increase
.
I
R
decreases when V
RRM
increases.
I
F(AV)
= 10A, V
RRM
= 170V
I
F(AV)
= 10A, V
RRM
= 40V
Case of
Schottky
diodes
:
40Slide41
Dynamic characteristic of power diodes (I).
41
In the case of PN diodes, manufacturers give information about switching times, reverse recovery current and forward recovery voltage (slides 108-111, Lesson 1).
t
s
= storage time.
t
f = fall time.t
rr =
ts + t
f = reverse recovery.
i
v
t
t
t
rr
t
s
t
f
Reverse recovery peak
t
d
= delay time.
t
r
=
rise time.
t
fr
= t
d
+
t
r
= forward recovery time.
v
t
Forward recovery peak
i
t
r
t
d
t
fr
tSlide42
Dynamic characteristic of power diodes (II).
42
The waveforms given by manufacturers correspond
to switch-off and to switch-on inductive loads, because this is the actual case in most of the power converters.
Switch-on
I
F
(
AV
)
=
2x8A
,
V
RRM
=
200V
Switch-offSlide43
Dynamic characteristic of power diodes (III).
43
More information given by manufacturers (example). Slide44
Dynamic characteristic of power diodes (IV).
44
In the case of Schottky diodes, manufacturers give information about the depletion layer capacitance (or junction capacitance, slides 103-106 and 116, Lesson 1).
C
j
= A·
2·(
V
0
+
Vrev
)
·
q·N
D
Metal
N
+
+
+
+
+
+
+
+
-
-
-
-
-
-
-
-
N-type
N
D
0
V
rev
C
j
C
jSlide45
Dynamic characteristic of power diodes (V).
45
I
nformation
given by manufacturers (example).Slide46
Losses in power diodes (I).
46
Static losses:
Reverse losses
Þ negligible in practice due to the low value of I
R.
Conduction losses Þ They must be taken into account. Switching (dynamic) losses:
Turn-on losses. Turn-off losses Þ
higher switching losses.
i
D
Example
Conduction power losses
:
Instantaneous value:
p
D_cond
(t) =
v
D
(t)·
i
D
(t) = [V
+
r
d
·i
D
(t)
]
·
i
D
(t)
Average power in a period:
V
r
d
Ideal
(lossless)
i
D
v
D
+
-
P
D_cond
= V
·I
avg
+
r
d
·I
RMS
2
I
avg: average value
of
iD(t)
IRMS
: RMS
value of
iD
(t)ÞSlide47
Losses in power diodes (II).
47
Turn-off losses
:
a
ctual
waveforms.
Tu
rn
-off losses in the diode take place during
t
f.
Moreover, remarkable losses take place in other devices (transistors) during
t
s
.
t
rr
= 30ns
i
D
t
V
D
t
0.8 V
-200 V
10 A
3 A
t
f
t
s
Power losses in the diode
Power losses in a transistor
Instantaneous value:
p
D_s
_
off
(t) =
v
D
(t)·
i
D
(t)
i
D
v
D
+
-
Average power in a period:Slide48
Losses in power diodes (III).
48
I
nformation
given by manufacturers (example).
(Diode STTA506 datasheet
) Slide49
Losses in power diodes (IV).
49
(Diode STTA506 datasheet
) Slide50
Losses in power diodes (IV).
50
(Diode STTA506 datasheet
)