Transcript Power diodes. - Universidad de Oviedo

Electrical Energy
Conversion and
Power Systems
Universidad
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
Universidad
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)
• Axial leaded through-hole packages
(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.
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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
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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
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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
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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
Load line
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).
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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.
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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
made up of silicon.
• 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).
• More information given by manufacturers (example).
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