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A3.1 General

All a.c. and d.c. drives use power semiconductor devices to convert and control electrical power. This section reviews important characteristics of the most conven-tional power devices in drives applications.

It is common to operate semiconductor devices in switched mode operation. This mode of operation implies that the device is either fully on or fully off, and power dis-sipation (product of I and V in Figure A3.1) is therefore low compared to that encoun-tered in the linear mode of operation. It is this feature that makes switched mode operation the key to achieving high efficiency.

The practically important power semiconductor devices in relation to motor drives are as follows:

† the diode,

† the thyristor (also called the silicon controlled rectifier, SCR),

† the triode thyristor (Triac),

† the gate turn-off thyristor (GTO),

2 4 6 8 10 I_d

40 80 120 160 200 240

V_ds V_gs = 4V V_gs = 5V V_gs = 6V V_gs = 7V V_gs = 8V

On

Off Linear

Figure A3.1 Linear versus switched mode operation

TableA3.1Typicalpowerdevicecharacteristics PropertyThyristor/SCRTriacGTOIGCTMOSFETIGBT Self-commutation abilityNoNoYesYesYesYes Maximumrmscurrent rating(A)5000400200017003002400 Maximumvoltage rating(V)1200012006000550015006500 Maximumswitched VArating30MVA240kVA30MVA12MVA30kVA4MVA SurgecurrentabilityExcellent (15Irms)Good(10Irms)Excellent (15Irms)Excellent (15Irms)Limited(4Irms)Limited(2Irms) Operatingcurrent densityatrateddevice voltage 140Acm22 at 2kV85Acm22 at 1.2kV30Acm22 at 4.5kV30Acm22 at 4.5kV75Acm22 at 200V 15Acm22 at 800V

140Acm22 at 1200V 35Acm22 at3.3kV Maximumjunction temperature(8C)1258C1258C1258C1158C1758C1758C On-statelossesLowLowMediumLowHighMedium SwitchinglossesVeryhighHighVeryhighMediumVerylowLow Turn-onabilityMedium(di/dt limit)Medium(di/dt limit)Medium(di/dt limit)Medium(di/dt limit)VerygoodVerygood 110 The Control Techniques Drives and Controls Handbook

Turn-offabilityNoneviagateMediumPoor–slowand lossyGoodVerygoodVerygood Turn-offsafeoperating area(percentageof ratedvoltageatrated rmscurrent)

NAMedium(60%)Poor(50%)Medium(70%)Excellent(100%)Excellent(100%) Loadshort-circuit turn-offabilityNoneNonePoor(2Irms)Poor(2Irms)Medium(4Irms)Excellent(10Irms) Snubbersusually requiredYesYesYesNoNoNo Minimumonoroff time10–100ms10–50ms10–50ms10ms,100ns,1ms Maximumswitching frequency25025050050010000010000 Switchingtime controllablefromdrive circuit

NoNoNoNoYesYes DrivecircuitpowerLowMediumHighHighLowLow Drivecircuit complexityLowMediumHighHighLowLow Seriesandparallel operationDeviceselection andpassive components required Deviceselection andpassive components required Verydifficult seriesorparallelFairlysimplein series,more difficultin parallel Fairlysimple seriesandparallelFairlysimpleseries andparallel,selection maybeneededfor parallel Power semiconductor devices 111

TableA3.2Applicationofpowersemiconductordevices ApplicationSupplyvoltageandequipmentVArating Upto240Va.c.,400Vd.c.From240Va.c.,400Vd.c. upto690Va.c.,1200Vd.c.Above690Va.c.,1200Vd.c. Upto1kVAFrom1kVAupto1MVAAbove1MVA A.C.motordrives VoltagesourceinverterMOSFET,IGBTIGBT,BJTIGBT,IGCT,GTO CurrentsourceinverterThyristor/SCR,BJT,GTOThyristor/SCR,GTO,IGCT CycloconverterThyristor/SCR Soft-startersThyristor/SCR,TriacThyristor/SCRThyristor/SCR D.C.motordrives LinecommutatedThyristor/SCR,TriacThyristor/SCRThyristor/SCR ForcecommutatedMOSFET,IGBT,BJTIGBT,BJTIGBT,IGCT,GTO 112 The Control Techniques Drives and Controls Handbook

† the integrated gate commutated thyristor (IGCT),

† the metal-oxide semiconductor field-effect transistor (MOSFET),

† the insulated gate bipolar transistor (IGBT), and

† the bipolar junction transistor (BJT) (although this device has largely been super-seded by the other devices shown in the list).

Table A3.1 shows typical power device characteristics. Clearly there is no outstanding device that has high voltage or current ratings, great VA ratings, easy controllability and simple driver circuit complexity. For example the MOSFET is the fastest switching device but is limited in achieving high blocking voltage. The unique characteristic of each power device makes them suitable for some drive applications but not all.

Table A3.2 provides an overview of typical applications of power semiconductor devices.

All power devices come in different packages allowing different mounting methods. The type of packaging has an impact on the performance of the device.

Table A3.3 summarises power device performances for different packages.

Table A3.3 Availability of power device packages

Property Discrete Power module Pressure pack

Available devices Thyristor/SCR, GTO,

Open circuit Open circuit Short circuit

Power circuit

Mounting method Solder, screw or clip Screw Pressure plate Cooling method Convection to air,

conduction to PCB or

Power switching devices require electronic ‘gate drive’ circuits for turning the device on and off. These, called ‘driver circuits’, are in general complex and also include protection features such as over-current protection. For this reason, details of these circuits have for the most been part limited to a description of the requirements to gate the devices.

A3.2 Diode A3.2.1 PN diode

The diode is the simplest power device. A diode may be considered as an electronic switch with a conduction state depending on the polarity of an externally applied voltage. When a sufficiently high positive voltage is applied to the anode with respect to the cathode, current will flow in a forward direction, the device acting as a closed switch. During conduction a voltage drop of the order of 0.7 – 1 V applies across the device. Conversely, when a negative voltage is applied, current flow is prevented and the diode is able to block voltages up to a certain level.

The principles of operation of a diode can be obtained by studying the PN diode. A junction is formed at the interface between two dissimilar semiconductor types: one side is p-type (doped with acceptors – usually group III elements) and other side is n-type (doped with donors – usually group V elements). In silicon, at room tempera-ture, the acceptors and donors are ionised, giving rise to positively and negatively charged mobile carriers (holes and electrons). With no external bias applied across the junction, a redistribution of charge occurs at the interface. The affected region around the junction is known as the depletion region (see Figure A3.2).

In power devices, it is typical to find one side of the junction much more heavily doped than the other (a factor of more than 1 000 is not uncommon). Such structures are referred to as one-sided junctions and the depletion region will extend much further into the lightly doped region.

The voltage – current characteristic of the PN junction diode is shown in Figure A3.3, illustrating the two modes of operation (forward conducting and reverse breakdown).

The ideal diode characteristic obeys an exponential law:

Id¼ Isat exp qV_d where Isatis the saturation current, q is the electron charge (1.602 10²¹⁹C), Vd

the voltage across the diode, k the Boltzmann’s constant (1.380 10²²³J K²¹) and T is the temperature given in kelvin.

At negative voltages Idbecomes – Isat, which is known as the saturation current or leakage current. As the applied reverse bias is increased the diode will breakdown.

The breakdown is caused either by the breakdown of the silicon because of the high electric field, or the edge of the depletion reaches the edge of the lightly doped region.

This latter process is known as punch-through.

114 The Control Techniques Drives and Controls Handbook

– + + + Junction

n = N_D

p = N_A n = 0

Ionised donors Ionised acceptors

Depleted region Charge

Electric field

Potential

N_A N_D

E_max V_j

w_p w_n

– –

Figure A3.2 The PN junction under reverse bias

Forward conducting

V I

V_rrm

V I

Leaking current

Reverse breakdown

Anode

Cathode

Figure A3.3 Symbol, PN junction and I/V curve of a diode

Power semiconductor devices 115

A PN diode, however, is not able to operate at high reverse blocking voltage. To increase the reverse bias an additional layer is integrated between the p and n types;

this is called the intrinsic layer, and will be discussed in the next section.

A3.2.2 PIN diode

This device makes use of a wide, lightly doped (I stands for intrinsic) region sand-wiched between heavily doped n-type and p-type regions (hence PIN). Under reverse-bias conditions the lightly doped region punches through at relatively low voltages.

Breakdown does not, however, occur, because of the heavily doped buffer layer.

Instead a high value of electric field is able to build up across the entire width of the lightly doped region (the slope of the electric field is proportional to the doping density). In the limiting case the theoretical maximum voltage supported by the structure is almost twice that of a standard diode of the same width:

V_br¼ Ecritw_m (A3:2)

where Vbris the breakdown voltage, Ecritthe critical electric field and wmthe length of the intrinsic layer. Figure A3.4 shows the structure of a PIN diode, including the distribution of the electric field.

In a PIN diode the idealised exponential characteristic described earlier is only a good approximation at relatively small levels of forward bias. For high levels of forward bias, additional physical effects in the device serve to increase the on-state voltage for a particular current level. At very high current densities the diode takes on an almost resistive characteristic (Figure A3.5).

The mechanism of conduction in a PIN diode involves the injection of carriers into the lightly doped region from the heavily doped regions; this process is known as conductivity modulation. Figure A3.6 shows representative electron and hole density distributions for the on and off states of a PIN diode. Note that the resulting

n⁺ n^–

p⁺

Lightly doped Heavily doped

E

w_m

Figure A3.4 Electric field distribution in a PIN diode 116 The Control Techniques Drives and Controls Handbook

large quantity of carriers that have been injected into the lightly doped region of the device now appear as a stored charge. The electron and hole concentrations are almost equal, and both are several orders of magnitude greater than the background doping level. As a result, the resistivity of the conductivity-modulated region is much lower than that afforded by the background doping level. In operational terms this translates to a much reduced on-state voltage. An approximate relationship between the quantity of stored charge QTOTand the diode current density J may be derived:

QTOT¼ J tHL (A3:3)

where tHLis the lifetime of the electrons and holes in the lightly doped region. The quantity of charge clearly increases in proportion to the current.

Doping level Off-state

Electrons Depleted region

On-state

Holes

p⁺ n^– n⁺ p⁺ n^– n⁺

Stored charge

10⁹ 10¹⁴ 10¹⁹ n,p

Figure A3.6 Carrier distributions in a PIN diode under reverse and forward bias

PN diode (dashed) PIN diode (solid)

V I

~e

~1/R

V_f V_f V_br

V_br

~e

Figure A3.5 Practical diode characteristics

Power semiconductor devices 117

From equation (A3.3) one might assume that the quantity of charge would be the same for all diodes at a given level of current density. This is, however, only true if the carrier lifetime is the same for each device. In practice, higher-voltage diodes must use longer carrier lifetimes to limit the on-state voltage drop to a low level. It may be shown that

t_HL. w_m²=4D_a (A3:4)

Here Dais a material parameter known as the ambipolar diffusion coefficient. The quantity of stored charge is thus higher in high-voltage diodes, which have a wider voltage-blocking layer.

A3.2.3 Transient processes (reverse and forward recovery)

The transient process of a diode can be split into reverse recovery (when the diode turns off) and forward recovery (when the diode turns on). Sections A3.2.3.1 and A3.2.3.2 describe the two processes and show that very rapid changes in diode current during switching transitions can cause significant transient over-voltages and, in the case of reverse recovery, a considerable reverse current. This results in increased switching stresses in devices throughout the circuit, a possible increase in switching losses, and in extreme cases failure of the diode and associated switch.

The faster the imposed transitions the worse the situation becomes. All transitions should therefore be controlled, either by judicious choice of switch drive conditions or, where this is impracticable, by application of snubber circuits.

A3.2.3.1 Reverse recovery

From Figure A3.6 it is clear that transition between the on and off states requires the removal of a considerable quantity of stored charge from the lightly doped region of the device. If forward current is suddenly removed from a diode (e.g. by disconnecting it), the level of charge will not fall immediately but will decay with a well-defined time constant. During this period the diode will be unable to support any significant reverse bias. Any attempt to apply reverse bias before the charge has been reduced to a low level will result in substantial reverse current flow. Note that the amount of stored charge increases with the level of current and voltage rating of the diode.

The circuit shown in Figure A3.7 is used by device manufacturers in the character-isation of diodes under reverse recovery conditions. Switch S controls the path taken by a constant current i0, and diode D (the device under test, DUT), diode Dcland Vcl

form a clamp circuit for the switch voltage.

The analysis presented here will consider a simple case in which Lsalone deter-mines the rate of change of current (i.e. the switch can be considered ideal). In practice, switch S, Lsand diode D have a complex interaction.

At t ¼ 0 switch S is closed and the voltage across inductance Lsrises rapidly to Vdc. The current in D thus begins to fall linearly with time (Figure A3.8):

Ls(did=dt)¼ vdc (A3:5)

118 The Control Techniques Drives and Controls Handbook

At time t1the current in the diode reverses and conduction continues with little reverse voltage across the diode. The reason for this apparently anomalous behaviour lies in the presence of the on-state stored charge (Figure A3.6), which provides the carriers needed to carry the reverse current. As the reverse current flows, the level of stored charge is gradually reduced (by natural recombination and the action of the reverse current) until a high electric field can once again be supported within the diode. At this time (close to t2in Figure A3.8), a significant reverse voltage begins to be estab-lished. Note that the level of reverse current must continue to increase until the voltage across inductance Ls reverses (time t2). The peak current attained is known as the reverse recovery current Irr.

The period from t2to t3is known as the tail period. During this time the decaying inductor current produces a voltage spike across the diode. The height of this spike is determined by the detailed physical structure of the diode and the inductor value. The period from t1to t3is the reverse recovery time trrand is terminated when the diode reverse current falls to 25 per cent of its peak value. A final parameter used to describe the reverse recovery process is the reverse recovered charge Qrr. This is the total charge recovered during the reverse recovery time (t1to t3).

V_dc

Figure A3.7 Typical diode recovery test circuit

t = 0

Figure A3.8 Typical reverse recovery waveforms

Power semiconductor devices 119

An approximate expression linking the reverse recovery time, peak current and charge may be obtained by assuming a triangular form for the reverse current between t1and t3:

Ls(did=dt)¼ vdc (A3:6)

The energy dissipated in the diode during this event can be estimated from the product of the tail charge Qrrand the supply voltage Vdc:

E_off 1

2Q_rrV_dc with Q_rr1

2I_rrt_rr (A3:7)

In general, faster rates of fall of current lead to higher peak reverse currents and larger amounts of recovered charge. Diode designers typically aim for a low quantity of stored charge (and hence small reverse recovery current) coupled with a controlled tail period to limit the amplitude and sharpness of the voltage spike. A very fast reverse recovery is not desirable in many applications. Note that the desire to reduce the level of stored charge is in direct conflict with the requirement for a low conduction voltage drop.

The reverse recovery time may be reduced by careful design of the doping profile of the PIN junction and measures such as doping with particular elements or irradiating the junction with an electron beam. These features are designed to reduce the number of charge carriers in the diode and also reduce their lifetime so that Irrand trrare both reduced. A side effect of this is that the forward voltage drop increases so there is a trade-off between speed and forward voltage drop.

A3.2.3.2 Forward recovery

Transition from conditions of reverse bias to those of strong forward bias requires the injection of large quantities of charge into the lightly doped region of the diode.

Exactly how much charge depends on the current rating (device area), voltage rating (width and carrier lifetime) and the detailed design of the device. It will usually be of the same order of magnitude as the reverse recovered charge Qrr.

A typical forward recovery event is illustrated in Figure A3.9.

Utilising the circuit of Figure A3.7, switch S is opened and the voltage across the diode collapses. This requires the supply of a relatively small amount of depletion charge (the background doping level is several orders of magnitude less than the final on-state carrier concentrations). Next, a flow of forward current is established.

This does not happen instantaneously because of the limiting effect of inductance Lsand the turn-on speed of the switch. If, however, the rate of rise of current is fast enough, the level of charge in the diode, and hence the degree of conductivity modu-lation, will still be quite low. Thus the forward voltage of the diode will rise to a higher than usual level. A rough guide to the possible magnitude of the voltage spike can be estimated by comparing the steady on-state charge level to the quantity of injected charge at any time in the transient. Transient voltages over 10 V are not uncommon.

120 The Control Techniques Drives and Controls Handbook

Finally, the continued injection of charge results in a gradual fall in voltage to its steady-state level.

A3.2.4 Diode types

Commonly there are three diode types available. Converter rectifier diodes are used in low-frequency rectification of a.c. to d.c. power. They have a long trrand high Qrr

optimised for minimum forward voltage drop. Diodes of this type are available in ratings as high as 9 000 V and 6 000 A. Diodes that have fast characteristics, i.e.

short trr and low Qrr, are referred to as fast-recovery diodes. As these have been optimised for speed they tend to have higher forward volt drops, which restricts their current rating for a given chip size. Fast-recovery diodes find their main use in freewheel functions (in which they must quickly commutate current from and to primary switching devices) and high-frequency rectification. Schottky diodes are mainly used in low-voltage switched-mode power supplies (Table A3.4).

Converter rectifier diodes and fast/ultrafast recovery diodes are mostly made from PIN diodes. The Schottky diode has a different semiconductor structure. The junction in Schottky diodes is formed by metal – semiconductor contact and not by semiconductor – semiconductor contact. A metal – semiconductor combination is based on the field effect, which is different to the semiconductor process involved in the PN junction. Schottky diodes are classified as majority carrier devices. PIN

Time i_d

v_d V_fr i_o

Figure A3.9 Typical forward recovery waveforms

Power semiconductor devices 121

and PN diodes are classified as minority carrier devices. In general, majority carrier devices have very fast turn-on and turn-off capabilities, resulting in fast switching frequencies. Minority devices have slow switching frequencies.

Most commercially available diodes are silicon based. Some of the new IGBT standard three-phase bridge inverters replace the anti-parallel-connected silicon diodes with diodes made from silicon carbide. Silicon carbide diodes have a very low reverse recovery effect, which places them in the category fast/ultrafast recovery diodes. The cost of silicon carbide diodes are currently high, making them cost-effective in a few niche drives applications only. It is, however, expected that the cost will fall progressively in the future, making them an attractive component.

A3.3 Thyristor (SCR) A3.3.1 Device description

Thyristors are used at very high powers (ratings to 5 kV and 4 kA) and in applications where their latching characteristics or low cost are particularly advantageous. They fall into two categories: thyristors (or silicon-controlled rectifier, SCR) having no gate turn-off capability, and gate turn-off thyristors (GTOs). Thyristor applications include HVDC transmission systems, static VAR compensators and large industrial converters (typically .1 MW). In drives, thyristors are used in current-fed inverters and small d.c. drives. Applications of GTOs include high-power traction (inverter Table A3.4 Applications for the most common diode types

Voltage/current range

Principal features Relative cost Typical application 122 The Control Techniques Drives and Controls Handbook

motor drives and choppers) and high-power industrial converters for welding and induction heating. GTO ratings can go up to 12 kV and 5 kA.

The thyristor is a four-layer PNPN device, as shown in Figure A3.10. Thyristor action can be best explained in terms of the two-transistor model. The p-anode emitter, n-base and p-base regions form the emitter, base and collector of a p – n – p transistor, while the n^þ, p-base and n-base form the emitter, base and collector of an n – p – n transistor. Note that the collector region of one transistor forms the base of

The thyristor is a four-layer PNPN device, as shown in Figure A3.10. Thyristor action can be best explained in terms of the two-transistor model. The p-anode emitter, n-base and p-base regions form the emitter, base and collector of a p – n – p transistor, while the n^þ, p-base and n-base form the emitter, base and collector of an n – p – n transistor. Note that the collector region of one transistor forms the base of

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