Power Electronic Devices

Power Electronic Devices

Semester 1

Lecturer: Javier Sebastián

Electrical Energy Conversion and Power Systems

Universidadde Oviedo

Power Supply Systems

2

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).

Outline

Lesson 3 - Power diodes.

Semester 1 - Power Electronics Devices

Electrical Energy Conversion and Power Systems

Universidadde Oviedo

3

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.

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

ivext

+

-

Vext [V]0

100

0.25- 0.25

i [mA]

0.5-10

-0.5 0

i [nA]

Vext [V]

i » IS·eVext

VT (exponential) i » -IS(constant)

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.

ivext

+

-

0 1-4

3i [A]

Vext [V]

According to Shockley equation

Actual I-V characteristic

According to Shockley equation


0

-VBR

10

i [A] Vext [V]

-600

7

Review of PN-diode operation (III)

• Static model for a diode (asymptotic):i

vext

+

-

0

i [A]

Vext [V]


V

Slope = 1/rd

• Equivalent circuit:

Model

V

rd = 1/tgaActual (asymptotic)

ideal

V = Knee voltagerd = Dynamic resistance

a

8

Review of PN-diode operation (IV)• Ideal diode:

ivext

+

-

0

i [A]

Vext [V]

Ideal diodeWhatever 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.

9

Review of PN-diode operation (V)• Low-power diode.

Anode

Cathode

Package (glass or epoxi resin)

Terminal

Terminal

PN

Marking stripe on the cathode end

Metal-semiconductor contact

Semiconductor die

Anode

Cathode

Metal-semiconductor contact

10

Packages for diodes (I)• Axial leaded through-hole packages

(low power).

DO 35 DO 41 DO 15 DO 201

11

Packages for diodes (II)

• Packages to be used with heat sinks.

12

Packages for diodes (III)

• Packages to be used with heat sinks(higher power levels).

B 44

DO 5

13

Packages for diodes (IV)

• Assembly of 2 diodes (I).

Doubler(2 diodes in series)

Common cathode(Dual center tap Diodes)

14

Packages for diodes (V)

• Assembly of 2 diodes (II).

15

Packages for diodes (VI)

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

16

Packages for diodes (VII)• Manufacturers frequently offer a given diode

in different packages.

Name Package

17

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

Dual in line

18

Packages for diodes (IX)• Assembly of 4 diodes

(medium-power bridge rectifiers).

19

Packages for diodes (X)• Assembly of 4 diodes

(high-power bridge rectifiers).

20

Packages for diodes (XI)• Assembly of 6 diodes

(Three-phase bridge rectifiers).

21

Packages for diodes (XII)• Example of a company portfolio regarding single-phase bridge rectifiers.

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 mm

250 mm

100 mm (for VBR=1000V)ND1 = 1014 cm-3

ND2 = 1019 cm-3

NA = 1019 cm-3 Anode

Cathode

23

N+

N-

Cathode

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.

24

N+

N-

Cathode

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+ PP

Aluminum contact

Aluminum contact

SiO2SiO2

Guard ringDepletion region

in reverse bias

25

N+

N-

Cathode

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


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


SiO2

SiO2

27

N+

N-

Cathode

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)

SiO2

Depletion region in reverse biasAluminum contact(N-M Þ rectifying)

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N+

N-

Cathode

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

PP

Aluminum contact(N-M Þ rectifying)

Aluminum contact(N+M Þ ohmic)

SiO2

SiO2

Guard ring Depletion region in reverse bias

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.

30

Maximum peak reverse voltage.

• Sometimes, manufacturers provide two values:

- Maximum repetitive peak reverse voltage, VRRM.

- Maximum non repetitive peak reverse voltage, VRSM.

31

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.

32

Forward voltage drop, VF (I). • The forward voltage drop increases when the forward current increases.• It increases linearly at high current level.

i

Vext

ID

VD

5 A

V

rd

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 point

33

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).

34

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 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.


35

Forward voltage drop, VF (IV). • In log scales.

0.84V @ 20A1.6V @ 20A



36

Forward voltage drop, VF (V).

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

0.5V @ 10A

37

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

38

Forward voltage drop, VF (VII).

Schottky

Schottky

PN

• 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.

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

39

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.




Reverse current, IR (II).

IR increases when IF(AV) and Tj increase.

IR decreases when VRRM increases.



• Case of Schottky diodes:

40

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).

ts = storage time.tf = fall time.trr = ts + tf = reverse recovery.

i

v

t

t

trr

ts

tfReverse recovery peak

td = delay time. tr = rise time.tfr = td + tr = forward recovery time.

v

t

Forward recovery peak

i

trtd

tfr

t

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

IF(AV) = 2x8A, VRRM = 200V

Switch-off

Dynamic characteristic of power diodes (III).

43

• More information given by manufacturers (example).

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).

Cj = A· p P p N e

VU V

T 2·(V0 + Vrev)

·q·ND

MetalN+

++

+++

+ +-------

-N-type

ND

0 Vrev

Cj

Cj

Dynamic characteristic of power diodes (V).

45

• Information given by manufacturers (example).

Losses in power diodes (I).

46

• 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.

iD Example

• Conduction power losses:

Instantaneous value: pD_cond(t) = vD(t)·iD(t) = [V + rd·iD(t)]·iD(t)

Average power in a period: ST

0cond_D

Scond_D dt)·t(p

T1

P

V

rd

Ideal(lossless)

iD

vD

+

-

PD_cond = V·Iavg + rd·IRMS2

Iavg: average value of iD(t)

IRMS: RMS value of iD(t)

Þ

Losses in power diodes (II).

47

• Turn-off losses: actual waveforms.

»frr t

0off_s_D

S

t

0off_s_D

Soff_s_D dt)·t(p

T1

dt)·t(pT1

P

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

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

trr = 30ns

iD

t

VD t0.8 V

-200 V

10 A

3 A tf

ts

Power losses in the diode

Power losses in a transistor

• Instantaneous value: pD_s_off(t) = vD(t)·iD(t)

iD

vD

+

-

• Average power in a period:

Losses in power diodes (III).

48

• Information given by manufacturers (example).

(Diode STTA506 datasheet)

Losses in power diodes (IV).

49


Losses in power diodes (IV).

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Power Electronic Devices