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Power Supply Systems. Electrical Energy Conversion and Power Systems . Universidad de Oviedo. Power Electronic Devices. Semester 1 . Lecturer: Javier Sebastián. Outline. Review of the physical principles of operation of semiconductor devices.

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Power electronic devices

Power Supply Systems

Electrical Energy Conversion and Power Systems

Universidad

de Oviedo

Power Electronic Devices

Semester 1

Lecturer: Javier Sebastián


Outline

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).


Lesson 3 power diodes

Electrical Energy Conversion and Power Systems

Universidad

de Oviedo

Lesson 3 - Power diodes.

Semester 1 - Power Electronics Devices


Outline1

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.


Power electronic devices

i [mA]

100

Vext [V]

0

- 0.25

0.25

0.5

Vext

i »IS·e

i [nA]

VT

Vext[V]

0

-0.5

-10

  • 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·(evext/VT- 1),where VT = kT/q and Is is thereverse-bias saturation current(a very small value).

i » -IS

(exponential)

(constant)

i

+

vext

-


Power electronic devices

According to Shockley equation

i [A]

3

Vext[V]

0

1

-4

  • 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.

Actual I-V characteristic

According to Shockley equation

Actual I-V characteristic

i [A]

Vext[V]

-VBR

-600

0

i

+

vext

-

10


Power electronic devices

Model

i [A]

ideal

0

Vext[V]

rd = 1/tga

V

  • Actual (asymptotic)

  • Review of PN-diode operation (III)

  • Static model for a diode (asymptotic):

Actual I-V characteristic

Slope = 1/rd

a

V= Knee voltage

rd = Dynamicresistance

V

  • Equivalent circuit:

i

+

vext

-


Power electronic devices

Ideal diode

i [A]

0

Vext[V]

  • Review of PN-diode operation (IV)

  • Ideal diode:

Whatever the forward current is, the forward voltage drop is always zero.

i

+

  • The ideal diode behaves as a short-circuit in forward bias.

  • The ideal diode behaves as a open-circuit in reverse bias.

Whatever the reverse voltage is, the reverse current is always zero.

vext

-


Power electronic devices

Terminal

Package

(glassor epoxi resin)

Metal-semiconductor contact

P

Semiconductor die

N

  • Metal-semiconductor contact

Marking stripe on the cathode end

Terminal

  • Review of PN-diode operation (V)

  • Low-power diode.

Anode

Anode

Cathode

Cathode


Power electronic devices

DO 35

DO 41

DO 15

DO 201

  • Packages for diodes (I)

  • Axial leaded through-hole packages

  • (low power).


Power electronic devices

  • Packages for diodes (II)

  • Packages to be used with heat sinks.


Power electronic devices

DO 5

B 44

  • Packages for diodes (III)

  • Packages to be used with heat sinks

  • (higher power levels).


Power electronic devices

Common cathode

(Dual center tap Diodes)

Doubler

(2 diodes in series)

  • Packages for diodes (IV)

  • Assembly of 2 diodes (I).


Power electronic devices

  • Packages for diodes (V)

  • Assembly of 2 diodes (II).


Power electronic devices

  • Packages for diodes (VI)

  • 2 diodes in the same package, but without electrical connection between them.


Power electronic devices

Name

Package

  • Packages for diodes (VII)

  • Manufacturers frequently offer a given diode in different packages.


Power electronic devices

Dual in line

  • Packages for diodes (VIII)

  • Assembly of 4 diodes (low-power bridge rectifiers).


Power electronic devices

  • Packages for diodes (IX)

  • Assembly of 4 diodes

  • (medium-power bridge rectifiers).


Power electronic devices

  • Packages for diodes (X)

  • Assembly of 4 diodes

  • (high-power bridge rectifiers).


Power electronic devices

  • Packages for diodes (XI)

  • Assembly of 6 diodes

  • (Three-phase bridge rectifiers).


Power electronic devices

  • Packages for diodes (XII)

  • Example of a company portfolio regarding single-phase bridge rectifiers.


Power electronic devices

Internal structure of PN power diodes (I)

  • Basic internal structure of a PN power diode.

Aluminum contact

Aluminum contact

Anode

NA = 1019 cm-3

P+

10 mm

100 mm

(for VBR=1000V)

N-(epitaxial layer)

ND1 = 1014 cm-3

N+(substrate)

250 mm

ND2 = 1019 cm-3

Cathode


Power electronic devices

Internal structure of PN power diodes (II)

  • Problems due to the nonuniformity of the electric field.

Depletion region in reverse bias

Anode

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.


Power electronic devices

Internal structure of PN power diodes (III)

Depletion region in reverse bias

  • Use of guard rings to get a more uniform electric field.

Aluminum contact

Anode

SiO2

Aluminum contact

SiO2

P+

Guard ring

N-

N+

Cathode

P

P

  • 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.


Power electronic devices

Depletion region in reverse bias

Internal structure of PN power diodes (IV)

  • Case where the metallurgical junction extends to the silicon surface (I).

Anode

High electric field intensity in these regions

P+

N-

N+

Cathode


Power electronic devices

Internal structure of PN power diodes (V)

Depletion region in reverse bias

  • Case where the metallurgical junction extends to the silicon surface (II).

Anode

SiO2

SiO2

P+

N-

N+

Cathode

  • 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.


Power electronic devices

Internal structure of Schottky power diodes (I)

  • Problems due to the nonuniformity of the electric field.

Aluminum contact

(N+MÞohmic)

Aluminum contact

(N-M Þ rectifying)

Depletion region in reverse bias

Anode

SiO2

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.


Power electronic devices

Depletion region in reverse bias

Internal structure of Schottky power diodes (II)

  • Use of guard rings to get a more uniform electric field.

Aluminum contact

(N+MÞohmic)

Aluminum contact

(N-M Þ rectifying)

Anode

SiO2

SiO2

Guard ring

N-

N+

Cathode

P

P

  • 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.


Power electronic devices

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.


Power electronic devices

Maximum peak reverse voltage.

  • Sometimes, manufacturers provide two values:

    - Maximum repetitive peak reverse voltage, VRRM.

  • - Maximum non repetitive peak reverse voltage, VRSM.


Power electronic devices

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.


Power electronic devices

i

ID

Vext

VD

ideal

5 A

rd

V

Forward voltage drop, VF (I).

  • The forward voltage drop increases when the forward current increases.

  • It increases linearly at high current level.

Operating point

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.


Power electronic devices

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).


Power electronic devices

Forward voltage drop, VF (III).

  • It can be directly obtained from the I-V characteristic, for any possible current.

IF(AV) = 5A,

VRRM = 1200V

IF(AV) = 4A, VRRM = 200V

1.25V @ 25A

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.


Power electronic devices

Forward voltage drop, VF (IV).

  • In log scales.

IF(AV) = 25A, VRRM = 200V

IF(AV) = 22A, VRRM = 600V

0.84V @ 20A

1.6V @ 20A


Power electronic devices

Forward voltage drop, VF (V).

  • Schottky diodes exhibit better forward voltage drop, at least for VRRM < 200 (for silicon devices).

0.5V @ 10A


Power electronic devices

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


Power electronic devices

Schottky

Schottky

PN

Forward voltage drop, VF (VII).

  • Comparing silicon Schottky and PN diodes, taking into account their VRRM.

  • 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.


Power electronic devices

IF(AV) = 8A, VRRM = 200V

IF(AV) = 4A, VRRM = 200V

IF(AV) = 5A, VRRM = 1200V

Reverse current, IR (I).

  • It is measuredat 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.


Power electronic devices

IF(AV) = 10A, VRRM = 40V

IF(AV) = 10A, VRRM = 170V

Reverse current, IR (II).

  • IR increases when IF(AV) and Tj increase.

  • IR decreases when VRRM increases.

  • Case of Schottkydiodes:


Power electronic devices

i

i

trr

t

ts

tf

v

t

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).

t

  • Reverse recovery peak

td

tr

tfr

v

  • Forward recovery peak

t

ts = storage time.

  • tf = fall time.

  • trr = ts + tf = reverse recovery.

td = delay time.

tr =rise time.

tfr = td + tr = forward recovery time.


Power electronic devices

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


Power electronic devices

Dynamic characteristic of power diodes (III).

  • More information given by manufacturers (example).


Power electronic devices

-

-

+

+

-

-

-

+

+

N

-

N-type

-

Metal

+

+

-

+

+

·q·ND

Cj= A·

2·(V0+ Vrev)

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

  • Cj

Cj

Vrev

0


Power electronic devices

Dynamic characteristic of power diodes (V).

  • Information given by manufacturers (example).


Power electronic devices

iD

Example

Ideal

(lossless)

rd

V

Losses in power diodes (I).

  • Static losses:

  • Reverse losses Þ negligible in practice due to the low value of IR.

  • Conduction losses Þ They must be taken into account.

  • Switching (dynamic) losses:

  • Turn-on losses.

  • Turn-off losses Þ 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:

iD

  • Þ

+

PD_cond = V·Iavg + rd·IRMS2

Iavg: averagevalue of iD(t)

IRMS: RMSvalue ofiD(t)

vD

-


Power electronic devices

Losses in power diodes (II).

iD

  • Turn-off losses: actual waveforms.

+

Power losses in a transistor

Power losses in the diode

vD

iD

  • Instantaneous value:

  • pD_s_off(t) = vD(t)·iD(t)

-

10 A

trr = 30ns

t

  • Average power in a period:

ts

tf

3 A

VD

0.8 V

t

  • Turn-off losses in the diode take place during tf.

  • Moreover, remarkable losses take place in other devices (transistors) during ts.

-200 V


Power electronic devices

Losses in power diodes (III).

  • Information given by manufacturers (example).

  • (Diode STTA506 datasheet)


Power electronic devices

Losses in power diodes (IV).

  • (Diode STTA506 datasheet)


Power electronic devices

Losses in power diodes (IV).

  • (Diode STTA506 datasheet)


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