### Power diodes. - Universidad de Oviedo

```Electrical Energy
Conversion and
Power Systems
de Oviedo
Power Electronic Devices
Semester 1
Power Supply
Systems
Lecturer: Javier Sebastián
Outline

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
Electrical Energy
Conversion and
Power Systems
de Oviedo
Lesson 3 - Power diodes.
Semester 1 - Power Electronics Devices
3
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.
4
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 = IS·(ev /V - 1), where VT = kT/q and Is is the reverse-bias
saturation current (a very small value).
ext
i  IS
100
Vext
·e VT
T
(exponential)
i  -IS
(constant)
+
vext
i [mA]
i [nA]
Vext [V]
0
0.25
-
0
-0.5
- 0.25
i
Vext [V]
0.5
-10
5
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.
According to
Shockley equation
i [A]
Actual I-V
characteristic
3
According to
Shockley equation
+
vext
-
i [A]
-VBR
-600
i
Vext [V]
0
Actual I-V
characteristic
Vext [V]
-4
0
1
10
6
Review of PN-diode operation (III)
• Static model for a diode (asymptotic):
i
i [A]
Actual I-V
characteristic
+
vext
Slope = 1/rd
-
Model
a
0
Vext [V]
V
V = Knee voltage
rd = Dynamic resistance
• Equivalent circuit:
ideal
rd = 1/tga
Actual (asymptotic)
V
7
Review of PN-diode operation (IV)
• Ideal diode:
i
+
i [A]
vext
-
Whatever the forward
current is, the forward
voltage drop is always
zero.
Ideal diode
0
Whatever the reverse
voltage is, the reverse
current is always zero.
Vext [V]
• The ideal diode behaves as a shortcircuit in forward bias.
• The ideal diode behaves as a opencircuit in reverse bias.
8
Review of PN-diode operation (V)
• Low-power diode.
Terminal
Anode
Anode
Metal-semiconductor
contact
Cathode
P
N
Package
(glass or epoxi resin)
Semiconductor die
Metal-semiconductor
contact
Marking stripe on
the cathode end
Cathode
Terminal
9
Packages for diodes (I)
(low power).
DO 35
DO 41
DO 15
DO 201
10
Packages for diodes (II)
• Packages to be used with heat sinks.
11
Packages for diodes (III)
• Packages to be used with heat sinks
(higher power levels).
DO 5
B 44
12
Packages for diodes (IV)
• Assembly of 2 diodes (I).
Common cathode
(Dual center tap Diodes)
Doubler
(2 diodes in series)
13
Packages for diodes (V)
• Assembly of 2 diodes (II).
14
Packages for diodes (VI)
• 2 diodes in the same package, but without
electrical connection between them.
15
Packages for diodes (VII)
• Manufacturers frequently offer a given diode
in different packages.
Name
Package
16
Packages for diodes (VIII)
• Assembly of 4 diodes (low-power bridge rectifiers).
Dual in line
17
Packages for diodes (IX)
• Assembly of 4 diodes
(medium-power bridge rectifiers).
18
Packages for diodes (X)
• Assembly of 4 diodes
(high-power bridge rectifiers).
19
Packages for diodes (XI)
• Assembly of 6 diodes
(Three-phase bridge rectifiers).
20
Packages for diodes (XII)
• Example of a company portfolio regarding single-phase bridge rectifiers.
21
Internal structure of PN power diodes (I)
• Basic internal structure of a PN power diode.
Anode
Aluminum contact
NA = 1019 cm-3
10 mm
P+
ND1 =
1014
cm-3
ND2 = 1019 cm-3
Cathode
N-
(epitaxial layer)
N+ (substrate)
100 mm
(for VBR=1000V)
250 mm
Aluminum contact
22
Internal structure of PN power diodes (II)
• Problems due to the nonuniformity of the electric field.
Anode
Depletion region in reverse bias
P+
High electric
field intensity
N-
N+
Cathode
• Breakdown electric field intensity can be reached in these regions.
• Regions with local high electric-field should be avoided when the device is
designed.
23
Internal structure of PN power diodes (III)
• Use of guard rings to get a more uniform electric field.
SiO2
Anode
P
Aluminum contact SiO
2
P+
Guard ring
P
N-
Depletion region
in reverse bias
N+
Cathode
Aluminum contact
• 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
24
electric field reaches very high local values.
Internal structure of PN power diodes (IV)
• Case where the metallurgical junction extends to the silicon surface (I).
Anode
Depletion region in reverse bias
P+
High electric field
intensity in these
regions
NN+
Cathode
25
Internal structure of PN power diodes (V)
• Case where the metallurgical junction extends to the silicon surface (II).
SiO2
Anode
SiO2
P+
N-
Depletion region
in reverse bias
N+
Cathode
• The use of beveling minimizes the electric field intensity.
• Coating the surface with appropriate materials such as silicon dioxide helps
26
control the electric field at the surface.
Internal structure of Schottky power diodes (I)
• Problems due to the nonuniformity of the electric field.
Aluminum contact
(N-M
 rectifying)
Anode
Depletion region in reverse bias
SiO2
High electric
field intensity
N-
N+
Cathode
Aluminum contact
(N+M  ohmic)
• Breakdown electric field intensity can be reached in these regions.
• Regions with local high electric-field should be avoided when the device is
27
designed.
Internal structure of Schottky power diodes (II)
• Use of guard rings to get a more uniform electric field.
SiO2
Anode
Aluminum contact
(N-M  rectifying)
SiO2
P
P
Guard ring
N-
Depletion region
in reverse bias
N+
Cathode
Aluminum contact
(N+M  ohmic)
• The depletion layers of the guard ring merge with the growing
depletion layer of the N-M region, which prevents the radius of
28
curvature from getting too small.
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.
29
Maximum peak reverse voltage.
• Sometimes, manufacturers provide two values:
- Maximum repetitive peak reverse voltage, VRRM.
- Maximum non repetitive peak reverse voltage, VRSM.
30
Maximum forward current.
• Manufacturers provide two or three different values:
- Maximum RMS forward current, IF(RMS).
- Maximum repetitive peak forward current, IFRM.
- Maximum surge non repetitive forward current, IFSM.
IF(RMS) depends on the package.
31
Forward voltage drop, VF (I).
• The forward voltage drop increases when the forward current increases.
• It increases linearly at high current level.
ideal
rd
V
Operating
point
i
Operating
point
ID
5A
• Actual I-V characteristic given by the manufacturer
(in this case is a V-I curve). Many times, current is in
a log scale.
Vext
VD
32
Forward voltage drop, VF (II).
• The higher the value of the maximum peak reverse voltage VRRM, the
higher the forward voltage drop VF at IF(RMS).
33
Forward voltage drop, VF (III).
• It can be directly obtained from the I-V characteristic, for any
possible current.
IF(AV) = 4A,
VRRM = 200V
1.25V @ 25A
IF(AV) = 5A,
VRRM = 1200V
2.2V @ 25A
• As the values of IF(RMS), IFRM and IFSM are quite different, the scale corresponding
to current must be quite large.
• Due to this, forward voltage drop corresponding to currents well below IF(RMS)
cannot be observed properly. Therefore, log scales are frequently used.
34
Forward voltage drop, VF (IV).
• In log scales.
IF(AV) = 25A,
VRRM = 200V
0.84V @ 20A
IF(AV) = 22A,
VRRM = 600V
1.6V @ 20A
35
Forward voltage drop, VF (V).
• Schottky diodes exhibit better forward voltage
drop, at least for VRRM < 200 (for silicon devices).
0.5V @ 10A
36
Forward voltage drop, VF (VI).
• Silicon Schottky diode with high VRRM.
• The forward voltage drop is quite similar
to the one corresponding to a PN diode.
0.69V @ 10A
37
Forward voltage drop, VF (VII).
• Comparing silicon Schottky and PN diodes,
taking into account their VRRM.
Schottky
• In case of diodes with similar
values of VRRM, the forward
voltage drop is quite similar in PN
and Schottky diodes, in both cases
• However, Schottky diodes always
have superior performances from
the dynamic point of view.
Schottky
PN
38
Reverse current, IR (I).
• It is measured at VRRM.
• It depends on the values of IF(AV) and VRRM (the higher IF(AV) and VRRM , the higher IR).
• It increases when the reverse voltage and the temperature increase.
IF(AV) = 8A, VRRM = 200V
IF(AV) = 4A, VRRM = 200V
IF(AV) = 5A, VRRM = 1200V
39
Reverse current, IR (II).
• Case of Schottky diodes:
 IR increases when IF(AV) and Tj increase.
 IR decreases when VRRM increases.
IF(AV) = 10A, VRRM = 40V
IF(AV) = 10A, VRRM = 170V
40
Dynamic characteristic of power diodes (I).
• In the case of PN diodes, manufacturers give information about switching times,
reverse recovery current and forward recovery voltage (slides 108-111, Lesson 1).
i
i
trr
t
t
ts
Reverse
recovery peak
tf
v
t
Forward
recovery peak
v
td tr
tfr
t
ts = storage time.
tf = fall time.
trr = ts + tf = reverse recovery.
td = delay time.
tr = rise time.
tfr = td + tr = forward recovery time.
41
Dynamic characteristic of power diodes (II).
• 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
Switch-off
IF(AV) = 2x8A,
VRRM = 200V
42
Dynamic characteristic of power diodes (III).
43
Dynamic characteristic of power diodes (IV).
• In the case of Schottky diodes, manufacturers give information about the depletion
layer capacitance (or junction capacitance, slides 103-106 and 116, Lesson 1).
ND
-- + +
-- +
+
Metal
-- + +
- + +
Cj
N
N-type
0
Cj
V
Cj = A·
UV
Vrev
·q·ND
T
p  e2·(V
PN
0+
Vrev)
44
Dynamic characteristic of power diodes (V).
• Information given by manufacturers (example).
45
Losses in power diodes (I).
• Static losses:
- Reverse losses  negligible in practice due to the low value of IR.
- Conduction losses  They must be taken into account.
• Switching (dynamic) losses:
- Turn-on losses.
- Turn-off losses  higher switching losses.
• Conduction power losses:
Instantaneous value: pD_cond(t) = vD(t)·iD(t) = [V + rd·iD(t)]·iD(t)
Average power in a period: PD _ cond 
Ideal iD
(lossless)
+
V
-
TS
TS
p
D _ cond
( t )· dt 
0
PD_cond = V·Iavg + rd·IRMS2
vD
rd
1
iD
Iavg: average value of iD(t)
Example
IRMS: RMS value of iD(t)
46
Losses in power diodes (II).
Power losses
in a transistor
Power losses
in the diode
iD
trr = 30ns
ts
3A
VD
t
-
• Average power in a period:
PD _ s _ off 
tf
+
vD
• Instantaneous value:
pD_s_off(t) = vD(t)·iD(t)
10 A
0.8 V
iD
• Turn-off losses: actual waveforms.
1
TS
t rr
p
0
D _ s _ off
( t )· dt 
1
TS
tf
p
D _ s _ off
( t )· dt
0
t
• Turn-off losses in the diode take
place during tf.
-200 V
• Moreover, remarkable losses take
place in other devices (transistors)
during ts.
47
Losses in power diodes (III).
• Information given by manufacturers (example).
(Diode STTA506 datasheet)
48
Losses in power diodes (IV).
(Diode STTA506 datasheet)
49
Losses in power diodes (IV).
(Diode STTA506 datasheet)
50
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