Power Electronic Devices 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.  Thermal management in power semiconductor devices.  Power diodes.  Power MOSFETs.  The IGBT.  High-power, low-frequency semiconductor devices (thyristors). 2 Outline

Lesson 3 - Power diodes. Semester 1 - Power Electronics Devices Electrical Energy Conversion and Power Systems Universidad de 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 = I S ·(e v ext /V T - 1), where V T = kT/q and I s is the reverse-bias saturation current (a very small value). i v ext + - V ext [V] i [mA] i [nA] V ext [V] i  I S ·e V ext VTVT (exponential) i  -I S  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 V BR, then the weak reverse current starts increasing a lot. The power dissipation usually becomes destructive for the device. i v ext 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] -600

7 Review of PN-diode operation (III) Static model for a diode (asymptotic): i v ext i [A] V ext [V] Actual I-V characteristic VV Slope = 1/r d Equivalent circuit: Model VV r d = 1/tg  Actual (asymptotic) ideal V  = Knee voltage r d = Dynamic resistance 

8 Review of PN-diode operation (IV) Ideal diode: i v ext 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.

9 Review of PN-diode operation (V) Low-power diode. Anode Cathode Package (glass or epoxi resin) Terminal P N 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+P+ N + (substrate) N - (epitaxial layer) Aluminum contact 10  m 250  m 100  m (for V BR =1000V) N D1 = cm -3 N D2 = cm -3 N A = cm -3 Anode Cathode

N+N+ N-N- 23 Internal structure of PN power diodes (II) Problems due to the nonuniformity of the electric field. Anode P+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.

N+N+ 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 P Aluminum contact SiO 2 Guard ring Depletion region in reverse bias

N+N+ N-N- Cathode 25 Internal structure of PN power diodes (IV) Case where the metallurgical junction extends to the silicon surface (I). Anode P+P+ High electric field intensity in these regions Depletion region in reverse bias

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+ N-N- P+P+ Cathode Anode Depletion region in reverse bias SiO 2

N+N+ 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  ohmic) SiO 2 Depletion region in reverse bias Aluminum contact (N - M  rectifying)

N+N+ 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  rectifying) Aluminum contact (N + M  ohmic) SiO 2 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, V RRM. - Maximum non repetitive peak reverse voltage, V RSM.

31 Maximum forward current. Manufacturers provide two or three different values: - Maximum RMS forward current, I F(RMS). - Maximum repetitive peak forward current, I FRM. - Maximum surge non repetitive forward current, I FSM. I F(RMS) depends on the package.

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 IDID VDVD 5 A VV rdrd 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, V F (II). The higher the value of the maximum peak reverse voltage V RRM, the higher the forward voltage drop V F at I F(RMS).

34 Forward voltage drop, V F (III). It can be directly obtained from the I-V characteristic, for any possible current. I F(AV) = 4A, V RRM = 200V 25A 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 = 1200V

35 Forward voltage drop, V F (IV). In log scales. 20A 20A I F(AV) = 25A, V RRM = 200V I F(AV) = 22A, V RRM = 600V

36 Forward voltage drop, V F (V). Schottky diodes exhibit better forward voltage drop, at least for V RRM < 200 (for silicon devices). 10A

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. 10A

38 Forward voltage drop, V F (VII). 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 silicon Schottky and PN diodes, taking into account their V RRM.

39 Reverse current, I R (I). It is measured at V RRM. It depends on the values of I F(AV) and V RRM (the higher I F(AV) and V RRM, the higher I R ). 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 = 200V

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: 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 , Lesson 1). t s = storage time. t f = fall time. t rr = t s + t f = reverse recovery. i v t t t rr tsts tftf 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 trtr tdtd t fr 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 I F(AV) = 2x8A, V RRM = 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 and 116, Lesson 1). C j = A· 2·(V 0 + V rev)  ·q·N D Metal N N-type NDND 0 V rev CjCj CjCj

Dynamic characteristic of power diodes (V). 45 Information given by manufacturers (example).

Losses in power diodes (I). 46 Static losses: - Reverse losses  negligible in practice due to the low value of I R. - Conduction losses  They must be taken into account. Switching (dynamic) losses: - Turn-on losses. - Turn-off losses  higher switching losses. iDiD 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V rdrd Ideal (lossless) iDiD vDvD + - P D_cond = V  ·I avg + r d ·I RMS 2 I avg : average value of i D (t) I RMS : RMS value of i D (t) 

Losses in power diodes (II). 47 Turn-off losses: actual waveforms. Turn-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 iDiD t VDVD t 0.8 V -200 V 10 A 3 A tftf tsts Power losses in the diode Power losses in a transistor Instantaneous value: p D_s_off (t) = v D (t)·i D (t) iDiD vDvD + - 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 (Diode STTA506 datasheet)

Losses in power diodes (IV). 50 (Diode STTA506 datasheet)