A, In rectification. Four diodes can be used to fully rectify an ac signal as shown in Fig. 2.16. Apart from other rectifier circuits, this topology does not require an input transformer. However, transformers are used for isolation and protection. The direction of the current is decided by two diodes conducting at any given time. The direction of the current through the load is always the same. This rectifier topology is known as the bridge rectifier.

The average rectifier output voltage

where is the peak input voltage. The rms rectifier output voltage

V =-л1 V2

This rectifier is twice as efficient as a single-phase rectifier.

B, For Voltage Clamping, Figure 2.17 shows a voltage clamper. The negative pulse of the input voltage charges the capacitor to its max. value in the direction shown. After

FIGURE 2.17 Voltage clamping with diode.

FIGURE 2.18 Voltage doubler and quadmpler circuit.

charging, the capacitor cannot discharge because it is open circuited by the diode. Hence the output voltage.

V = V

V, = Vjl+sin(a;0)

The output voltage is clamped between zero and 2

0ms 7 0ms 8 0ms 9 0ms

FIGURE 2.16 Full bridge rectifier and its output dc voltage.

C. As Voltage Multiplier By connecting diodes in a predetermined manner, an ac signal can be doubled, tripled, and even quadrupled. This is shown in Fig. 2.18. The circuit will yield a dc vohage equal to 2 V. The capacitors are alternately charged to the maximum value of the input voltage.

2.8 Standard Datasheet for Diode Selection

In order for a designer to select a diode switch for specific applications, the following Tables and standard test results can be used. A power diode is chosen primarily based on forward current (Ip) and the peak inverse (Vrm) voltage [5]. For example, the designer chose the diode type V30 from Table 2.1 because it closely matches his/her calculated values of Ip and rrm without exceeding those values. However, if for some reason only the Vrm matches but the calculated value of Ip

2.7 Typical Applications of Diodes

comes in at a higher figure, one should select diode H14, and so on. A similar concept is used for Vrrm-

In addition to the forementioned diode parameters, one should also calculate parameters such as the peak forward voltage, reverse recovery time, case and junction temperatures etc. and check them against the datasheet values. Some of these datasheet values are provided in Table 2.2 for the selected diode У30. Figures 2.19-2.21 give the standard experimental relationships between voltages, currents, and power and case temperatures for our selected У30 diode. These characteristics help a designer to understand the safe operating range for the diode, and to make a decision as to whether or not a snubber or a heatsink should be used. If one is particularly interested in the actual reverse recovery time measurement, the circuit given in Fig. 2.22 can be constructed and experimented upon.

table 2.3 Characteristics (Tl = 25°C)

Forward characteristic

Used with permission, [7].

table 2.2 Details of diode for diode V30 selected from Table 2.1

Absolute maximum ratings

Lead mounting: lead temperature 300°C max. to 3.2mm from body for 5s. max.

Mechanical strength: bending 90° x 2 cycles or 180° x 1 cycle. Tensile 2kg, Twist 90° X 1 cycle.

2.8.1 General-Use Rectifier Diodes

table 2.1 Diode election based on average forward current /F(avg)> d peak inverse voltage Frrm [4]

с

CD CL

0 1 2 3 4 5

Peak forward voltage drop (V) figure 2.19 Variation of peak forward voltage drop with peak forward current.

Max. average forward power dissipation (Resistive or inductive load)

с 0.8 о

0.6

& 0.4

CD CO

О 0.2 CO

0.1 0.2 0.3 0.4 0.5 Average forward current (A)

figure 2.20 Variation of maximum forward power dissipation with average forward current.

Max. allowable ambient temperature (Resistive to inductive load)

о

о' 160 a5

I 120 с

ъ

S 80

Single-phase half sine wave 180° conduction (50Hz)

PC board (100x180x1.6t) Copper foil (Пб.б)

0.1 0.2 0.3 0.4 0.5 Average forward current (A)

, 2mA zb 15V

figure 2.21 Maximum allowable case temperature with variation of average forward current.

figure 2.22 Reverse recovery time () measurement.

References

1. N. Lurch, Fundamenals of Electronics, 3rd ed., John Wiley & Sons Ltd., 1981.

2. R. Tartar, Solid-State Power Conversion Handbook, John Wiley & Sons Ltd., 1993.

3. R. M. Marston, Power Control Circuits Manual, Newness circuits manual series. Butterworth Heinemann Ltd., 1995.

4. Internet information on Hitachi semiconductor devices, http: semi-conductor.hitachi.com.

5. International rectifier. Power Semiconductors Product Digest, 1992/93.

6. Internet information on, Electronic devices & SMPS books, http: www.smpstech.com/books/booklist.htm

7. Internal information on, Interactive power electronics , http: www.ee.uts.edu.au

Thyristors

Jerry Hudgins, Ph.D. Enrico Santi, Ph.D. Antonio Caiafa, Ph.D. Katherine Lengel, Ph.D.

Department of Electrical Engineering

University of South Carolina Columbia, South Carolina 29208 USA

Patrick R. Palmer, Ph.D.

Department of Engineering Cambridge University Trumpington Street Cambridge CB2 IPZ, United Kingdom

3.1 Introduction........................................................................................ 27

3.2 Basic Structure and Operation................................................................ 28

3.3 Static Characteristics............................................................................. 30

3.3.1 Current-Voltage Curves for Thyristors 3.3.2 Edge and Surface Terminations

3.3.3 Packaging

3.4 Dynamic Switching Characteristics.......................................................... 33

3.4.1 Cathode Shorts 3.4.2 Anode Shorts 3.4.3 Amplifying Gate 3.4.4 Temperature Dependencies

3.5 Thyristor Parameters............................................................................. 37

3.6 Types of Thyristors............................................................................... 38

3.6.1 SCRs and GTOs 3.6.2 MOS-ControUed Thyristors, MCT 3.6.3 Static Induction Thyristors 3.6.4 Optically Triggered Thyristors 3.6.5 Bidirectional Control Thyristor

3.7 Gate Drive Requirements....................................................................... 45

3.7.1 Snubber Circuits 3.7.2 Gate Circuits

3.8 P-Spice Model..................................................................................... 47

3.9 Applications........................................................................................ 50

3.9.1 Direct current-Alternating current Utility Inverters 3.9.2 Motor Control

3.9.3 VAR Compensators and Static Switching Systems

References........................................................................................... 53

3.1 Introduction

Thyristors are usually three-terminal devices with four layers of alternating p- and n-type material (i.e. three p-n junctions) in their main power handling section. In contrast to the hnear relation that exists between load and control currents in a transistor, the thyristor is bistable. The control terminal of the thyristor, called the gate (G) electrode, may be connected to an integrated and complex structure as part of the device. The other two terminals, anode (A) and cathode (iC), handle the large applied potentials (often of both polarities) and conduct the major current through the thyristor. The anode and cathode terminals are connected in series with the load to which power is to be controlled.

Thyristors are used to approximate ideal closed (no voltage drop between anode and cathode) or open (no anode current flow) switches for control of power flow in a circuit. This differs from low-level digital switching circuits that are designed to deliver two distinct small voltage levels while conducting small currents (ideally zero). Power electronic circuits must have the capability of delivering large currents and be able to withstand large externally applied voltages. All

thyristor types are controllable in switching from a forward-blocking state (positive potential applied to the anode with respect to the cathode with correspondingly little anode current flow) into a forward-conduction state (large forward anode current flowing with a small anode-cathode potential drop). After switching from a forward-blocking state into the forward-conduction state, most thyristors have the characteristic that the gate signal can be removed and the thyristor will remain in its forward-conduction mode. This property, termed latching, is an important distinction between thyristors and other types of power electronic devices. Some thyristors are also controllable in switching from forward-conduction back to a forward-blocking state. The particular design of a thyristor will determine its controllability and often its application.

Thyristors are typically used at the highest energy levels in power conditioning circuits because they are designed to handle the largest currents and voltages of any device technology (systems with voltages approximately greater than 1 kV or currents higher than 100A). Many medium-power circuits (systems operating at <lkVor 100 A) and particularly low-power circuits (systems operating <100Vor several amperes)

Copyright © 2001 by Academic Press.

All rights of reproduction in any form reserved.

generally make use of power bipolar transistors, power MOSFETs, or insulated gate bipolar transistors (IGBTs) as the main switching elements because of the relative ease in controlling them. The IGBT technology, however, continues to improve and multiple silicon die are commonly packaged together in a module. These modules are now replacing thyristors in 1-3 kV applications because of easier gate-drive requirements. Power diodes are used throughout all levels of power conditioning circuits and systems for component protection and wave-shaping.

A thyristor used in some ac power circuits (50 or 60 Hz in commercial utilities or 400 Hz in aircraft) to control ac power flow can be made to optimize internal power loss at the expense of switching speed. These thyristors are called phase-control devices because they are generally turned from a forward-blocking into a forward-conducting state at some specified phase angle of the applied sinusoidal anode-cathode voltage waveform. A second class of thyristors is used in association with dc sources or in converting ac power at one amplitude and frequency into ac power at another amplitude and frequency, and must generally switch on and off relatively quickly. A typical application for this second class of thyristors is that of converting a dc voltage or current into an ac voltage or current. A circuit that performs this operation is often called an inverter, and the associated thyristors used are referred to as inverter thyristors.

There are four major types of thyristors: i) silicon-controlled rectifier (SCR); ii) gate turn-off thyristor (GTO); in) MOS-controUed thyristor (MCT) and its various forms; and iv) static induction thyristor (SITh). The MCTs are so-named because many parallel enhancement-mode MOSFET structures of one charge type are integrated into the thyristor for turn-on and many more MOSFETs of the other charge type are integrated into the thyristor for turn-off. These MCTs are currently limited to operation at medium power levels. Other types of integrated MOS-thyristor structures can be operated at high power levels, but these devices are not commonly available or are produced for specific applications. A static induction thyristor (SITh), or field-controlled thyristor (FCTh), has essentially the same construction as a power diode with a gate structure that can pinch-off anode current flow. High-power SIThs have a subsurface gate (buried-gate) structure to allow larger cathode areas to be utilized, and hence larger current densities are possible. The advantage of using MCTs, derivative forms of the MCT, or SIThs is that they are essentially voltage-controlled devices, (e.g., little control current is required for turn-on or turn-off) and, therefore, require simplified control circuits attached to the gate electrode. Detailed discussion of variations of MCTs and SIThs as well as additional references on these devices are discussed by Hudgins [1]. Less important types of thyristors include the Triac (a pair of antiparallel SCRs integrated together to form a bidirectional current switch) and the programmable unijunction transistor (PUT).

Both SCRs and GTOs are designed to operate at all power levels. These devices are primarily controlled using electrical signals (current), although some types are made to be controlled using optical (photons) energy for turn-on. Subclasses of SCRs and GTOs are reverse conducting types and symmetric structures that block applied potentials in the reverse and forward polarities. Other variations of GTOs are the gate-commutated turn-off thyristor (GCT) and the bidirectional controlled thyristor (BCT). Most power converter circuits that incorporate thyristors make use of either SCRs or GTOs, and hence this chapter will focus on these two devices, although the basics of operation are applicable to all thyristor types.

All power electronic devices must be derated (e.g., power dissipation levels, current conduction, voltage blocking, and switching frequency must be reduced) when operating above room temperature (defined as 25 °C). Bipolar-type devices have thermal runaway problems, in that if allowed to conduct unlimited current, these devices will heat up internally, causing more current to flow, thus generating more heat, and so forth until destruction. Devices that exhibit this behavior are pin diodes, bipolar transistors, and thyristors.

Almost all power semiconductor devices are made from silicon (Si), but some limited commercial devices are available using gallium-arsenide (GaAs), and silicon-carbide SiC. The latter two semiconductor material systems will not be directly discussed because of the lack of availability and usage. The physical description and general behavior of thyristors are unimportant to the semiconductor material system used although the discussion and any numbers cited in the chapter will be associated with Si devices.

3.2 Basic Structure and Operation

Figure 3.1 shows a conceptual view of a typical thyristor with the three p-n junctions and the external electrodes labeled. Also shown in the figure is the thyristor circuit symbol used in electrical schematics.

Anode (A)

Л

J2 J3

к

Cathode (K) Gate (G)

figure 3.1 Simple cross section of a typical thyristor and the associated electrical schematic symbols.

A high-resistivity region, n-base, is present in all thyristors. It is this region, the n-base and associated junction /2 of Fig. 3.1, which must support the large apphed forward voltages that occur when the switch is in its off- or forward-blocking state (nonconducting). The n-base is typically doped with impurity phosphorus atoms at a concentration of 10 cm~. The n-base can be 10s to 100s of im thick to support large voltages. High-voltage thyristors are generally made by diffusing aluminum or gallium into both surfaces to obtain deep junctions with the n-base. The doping profile of the p-regions ranges from about 10 to 10 cm~. These p-regions can be up to 10s of im thick. The cathode region (typically only a few im thick) is formed by using phosphorus atoms at a doping density of 10 to 10 cm~.

The higher the forward-blocking voltage rating of the thyristor, the thicker the n-base region must be. However increasing the thickness of this high-resistivity region, results in slower turn-on and turn-off (i.e., longer switching times and/or lower frequency of switching cycles because of more stored charge during conduction). For example, a device rated for a forward-blocking voltage of IkV will, by its physical construction, switch much more slowly than one rated for 100 V. In addition, the thicker high-resistivity region of the 1 kV device will cause a larger forward voltage drop during conduction than the 100 V device carrying the same current. Impurity atoms, such as platinum or gold, or electron irradiation are used to create charge-carrier recombination sites in the thyristor. The large number of recombination sites reduces the mean carrier lifetime (average time that an electron or hole moves through the Si before recombining with its opposite charge-carrier type). A reduced carrier hfetime shortens the switching times (in particular the turn-off or recovery time) at the expense of increasing the forward conduction drop. There are other effects associated with the relative thickness and layout of the various regions that make up modern thyristors, but the major trade-off between forward-blocking voltage rating and switching times, and between forward-blocking voltage rating and forward-voltage drop during conduction should be kept in mind. In signal-level electronics the analogous trade-off appears as a lowering of amplification (gain) to achieve higher operating frequencies, and is often referred to as the gain-bandwidth product.

The operation of thyristors is as follows. When a positive voltage is apphed to the anode (with respect to a cathode), the thyristor is in its forward-blocking state. The center junction /2 (see Fig. 3.1) is reverse-biased. In this operating mode the gate current is held to zero (open-circuit). In practice, the gate electrode is biased to a small negative voltage (with respect to the cathode) to reverse-bias the GK-junction /3 and prevent charge-carriers from being injected into the p-base. In this condition only thermally generated leakage current flows through the device and can often be approximated as zero in value (the actual value of the leakage current is typically many orders of magnitude lower than the conducted current

in the on-state). As long as the forward applied voltage does not exceed the value necessary to cause excessive carrier multiplication in the depletion region around /2 (avalanche breakdown), the thyristor remains in an off-state (forward-blocking). If the applied voltage exceeds the maximum forward blocking voltage of the thyristor, it will switch to its on-state. However, this mode of turn-on causes nonunifor-mity in the current flow, is generally destructive, and should be avoided.

When a positive gate current is injected into the device /3 becomes forward-biased and electrons are injected from the n-emitter into the p-base. Some of these electrons diffuse across the p-base and are collected in the n-base. This collected charge causes a change in the bias condition of /j. The change in bias of causes holes to be injected from the p-emitter into the n-base. These holes diffuse across the n-base and are collected in the p-base. The addition of these collected holes in the p-base acts the same as gate current. The entire process is regenerative and will cause the increase in charge carriers until /2 also becomes forward biased and the thyristor is latched in its on-state (forward-conduction). The regenerative action will take place as long as the gate current is applied in sufficient amount and for a sufficient length of time. This mode of turn-on is considered to be the desired one as it is controlled by the gate signal.

This switching behavior can also be explained in terms of the two-transistor analog shown in Fig. 3.2. The two transistors are regeneratively coupled so that if the sum of their forward current gains (as) exceeds unity, each drives the other into saturation. Equation 3.1 describes the condition necessary for the thyristor to move from a forward-blocking state into the forward-conduction state. The forward current gain (expressed as the ratio of collector current to emitter current) of the pnp transistor is denoted by a, and that of the npn as a . The as are current dependent and increase slightly as the current increases. The center junction /2 is reverse-biased under forward applied voltage (positive v). The associated electric field in the depletion region around the junction can

к

к

figure 3.2 Two-transistor behavioral model of a thyristor.

result in significant carrier multiplication, denoted as a multiplying factor M on the current components I and Iq.

1 - M(a + a J

(3.1)

In the forward-blocking state, the leakage current I is small, both as are small, and their sum is < unity. Gate current increases the current in both transistors, increasing their as. Collector current in the npn transistor acts as base current for the pnp, and analogously, the collector current of the pnp acts as base current driving the npn transistor. When the sum of the two as equals unity, the thyristor switches to its on-state (latches). This condition can also be reached, without any gate current, by increasing the forward applied voltage so that carrier multiplication (M 1) at /2 increases the internal leakage current, thus increasing the two as. A third way to increase the as is by increasing the device (junction) temperature. Increasing the temperature causes a corresponding increase in the leakage current I to the point where latching can occur. The typical manifestation of this temperature dependence is an effective lowering of the maximum blocking voltage that can be sustained by the thyristor.

Another way to cause a thyristor to switch from forward-blocking to forward-conduction exists. Under a forward-applied voltage, /2 is reverse-biased while the other two junctions are forward-biased in the blocking mode. The reverse-biased junction of /2 is the dominant capacitance of the three and determines the displacement current that flows. If the rate of increase in the applied v is sufficient (dVjj/dt), it will cause a significant displacement current through the /2 capacitance. This displacement current can initiate switching similar to that of an externally applied gate current. This dynamic phenomenon is inherent in all thyristors and causes there to be a limit (dv/dt) to the time rate of applied v that can be placed on the device to avoid uncontrolled switching. Alterations to the basic thyristor structure can be produced that increase the dv/dt limit and will be discussed in Section 3.4.

Once the thyristor has moved into forward conduction, any applied gate current is superfluous. The thyristor is latched, and for SCRs, cannot be returned to a blocking mode by using the gate terminal. Anode current must be commutated away from the SCR for a sufficient time to allow stored charge in the device to recombine. Only after this recovery time has occurred can a forward voltage be reapplied (below the dv/dt limit of course) and the SCR again be operated in a forward-blocking mode. If the forward voltage is reapplied before sufficient recovery time has elapsed, the SCR will move back into forward-conduction. For GTOs, a large applied reverse gate current (typically in the range of 10-50% of the anode current) applied for a sufficient time can remove enough charge near the GK junction to cause it to turn off.

3.3 Static Characteristics

3.3.1 Current-Voltage Curves for Thyristors

A plot of the anode current (i) as a function of anode-cathode voltage (Vjj) is shown in Fig. 3.3. The forward-blocking mode is shown as the low-current portion of the graph (solid curve around operating point 1 ). With zero gate current and positive v the forward characteristic in the off- or blocking-state is determined by the center junction /2, which is reverse-biased. At operating point 1, very little current flows {I only) through the device. However, if the applied voltage exceeds the forward-blocking voltage, the thyristor switches to its on- or conducting-state (shown as operating point 2 ) because of carrier multiplication (M in Eq. 1). The effect of gate current is to lower the blocking voltage at which switching takes place. The thyristor moves rapidly along the negatively sloped portion of the curve until it reaches a stable operating point determined by the external circuit (point 2 ). The portion of the graph indicating forward conduction shows the large values of that may be conducted at relatively low values of v, similar to a power diode.

As the thyristor moves from forward-blocking to forward-conduction, the external circuit must allow sufficient anode current to flow to keep the device latched. The minimum anode current that will cause the device to remain in forward-conduction as it switches from forward-blocking is called the

II Ih

FIGURE 3.3 Static characteristic i-v curve typical of thyristors.

thus interrupting base current to the pnp transistor and causing thyristor turn-off. This is similar in principle to using negative base current to quickly turn off a traditional transistor.

latching current I p. If the thyristor is already in forward-conduction and the anode current is reduced, the device can move its operating mode from forward-conduction back to forward-blocking. The minimum value of anode current necessary to keep the device in forward-conduction after it has been operating at a high anode current value is called the holding current 4r. The holding current value is lower than the latching current value as indicated in Fig. 3.3.

The reverse thyristor characteristic, quadrant III of Fig. 3.3, is determined by the outer two junctions (/ and /3), which are reverse-biased in this operating mode (applied v is negative). Symmetric thyristors are designed so that will reach reverse breakdown due to carrier multiphcation at an applied reverse potential near the forward breakdown value (operating point 3 in Fig. 3.3). The forward- and reverse-bioeking junctions are usually fabricated at the same time with a very long diffusion process (10 to 50 h) at high temperatures (> 1200 °C). This process produces symmetric blocking properties. Wafer-edge termination processing causes the forward-blocking capability to be reduced to 90% of the reverse-blocking capability. Edge termination is discussed in what follows. Asymmetric devices are made to optimize forward-conduction and turn-off properties, and as such reach reverse breakdown at much lower voltages than those applied in the forward direction. This is accomplished by designing the asymmetric thyristor with a much thinner n-base than is used in symmetric structures. The thin n-base leads to improved properties such as lower forward drop and shorter switching times. Asymmetric devices are generally used in apphcations when only forward voltages (positive v) are to be apphed (including many inverter designs).

The form of the gate-to-cathode VI characteristic of SCRs and GTOs is similar to that of a diode. With positive gate bias, the gate-cathode junction is forward-biased and permits the flow of a large current in the presence of a low voltage drop. When negative gate voltage is applied to an SCR, the gate-cathode junction is reverse-biased and prevents the flow of current until avalanche breakdown voltage is reached. In a СТО, a negative gate voltage is applied to provide a low impedance path for anode current to flow out of the device instead of out of the cathode. In this way the cathode region (base-emitter junction of the equivalent npn transistor) turns off, thus pulling the equivalent npn transistor out of conduction. This causes the entire thyristor to return to its blocking state. The problem with the СТО is that the gate-drive circuitry is typically required to sink 10% of the anode current in order to achieve turn-off.

3.3.2 Edge and Surface Terminations

Thyristors are often made with planar diffusion technology to create the anode region. Formation of these regions creates cylindrical curvature of the metallurgical gate-cathode junction. Under reverse bias, the curvature of the associated

figure 3.4 Cross section showing a floating field ring to decrease the electric field intensity near the curved portion of the main anode region (leftmost -region).

depletion region results in electric field crowding along the curved section of the p+-diffused region. The field crowding seriously reduces the breakdown potential below that expected for the bulk semiconductor. A floating field ring, an extra p+-diffused region with no electrical connection at the surface, is often added to modif) the electric field profile and thus reduce it to a value below or at the field strength in the bulk. An illustration of a single floating field ring is shown in Fig. 3.4. The spacing W between the main anode region and the field ring is critical. Multiple rings can also be employed to further modif) the electric field in high-voltage rated thyristors.

Another common method for altering the electric field at the surface is to use a field plate as shown in cross section in Fig. 3.5. By forcing the potential over the oxide to be the same as at the surface of the p+-region, the depletion region can be extended so that the electric field intensity is reduced near the curved portion of the diffused p+-region. A common practice is to use field plates with floating field rings to obtain optimum breakdown performance.

High-voltage thyristors are made from single wafers of Si and must have edge terminations other than floating field rings or field plates to promote bulk breakdown and hmit

figure 3.5 Cross section showing a field plate used to reduce the electric field intensity near the curved portion of the -region (anode).

figure 3.6 Cross section of a thyristor showing the negative bevel (upper pn~- and -junctions) and positive bevel (lower -junction) used for edge termination of large-area devices.

leakage current at the surface. Controlled bevel angles can be created using lapping and polishing techniques during production of large-area thyristors. Two types of bevel junctions can be created: i) a positive bevel defined as one in which the junction area decreases when moving from the highly doped to the lightly doped side of the depletion region; and ii) a negative bevel defined as one in which the junction area increases when moving from the highly doped to the lightly doped side of the depletion region. In practice, the negative bevel must be lapped at an extremely shallow angle to reduce the surface field below the field intensity in the bulk. All positive bevel angles between 0 and 90° result in a lower surface field than in the bulk. Figure 3.6 shows the use of a positive bevel for the junction and a shallow negative bevel for the /2 and /3 junctions on a thyristor cross section to make maximum use of the Si area for conduction and still reduce the surface electric field. Further details of the use of beveling, field plates, and field rings can be found in Ghandi [2] and Baliga [3].

3.3.3 Packaging

Thyristors are available in a wide variety of packages, from small plastic ones for low-power (i.e., TO-247), to stud-mount packages for medium-power, to press-pack (also called flat-pack) for the highest power devices. The press-packs must be mounted under pressure to obtain proper electrical and thermal contact between the device and the external metal electrodes. Special force-calibrated clamps are made for this purpose. Large-area thyristors cannot be directly attached to the large copper pole-piece of the press-pack because of the difference in the coefficient of thermal expansion (CTE), hence the use of a pressure contact for both anode and cathode.

Many medium-power thyristors are appearing in modules where a half- or full-bridge (and associated anti-parallel diodes) is put together in one package.

A power module package should have five characteristics:

i) electrical isolation of the baseplate from the semiconductor;

ii) good thermal performance;

iii) good electrical performance;

iv) long life/high reliability; and

v) low cost.

Electrical isolation of the baseplate from the semiconductor is necessary in order to contain both halves of a phase leg in one package as well as for convenience (modules switching to different phases can be mounted on one heat sink) and safety (heat sinks can be held at ground potential).

Thermal performance is measured by the maximum temperature rise in the Si die at a given power dissipation level with a fixed heat sink temperature. The lower the die temperature, the better the package. A package with a low thermal resistance from junction-to-sink can operate at higher power densities for the same temperature rise or lower temperatures for the same power dissipation than a more thermally resistive package. While maintaining low device temperature is generally preferable, temperature variation affects majority carrier and bipolar devices differently. Roughly speaking, in a bipolar device such as a thyristor, switching losses increase and conduction losses decrease with increasing temperature. In a majority carrier device, conduction losses increase with increasing temperature. The thermal conductivity of typical materials used in thyristor packages is shown in Table 3.1.

Electrical performance refers primarily to the stray inductance in series with the die, as well as the capability of mounting a low-inductance bus to the terminals. Another problem is the minimization of capacitive crosstalk from one switch to another, which can cause an abnormal on-state condition by charging the gate of an off-state switch, or from a switch to any circuitry in the package - as would be found in a hybrid power module. Capacitive coupling is a major cause of electromagnetic interference (EMI). As the stray inductance of the module and the bus sets a minimum switching loss for the device because the switch must absorb the stored inductive energy, it is very important to minimize

table 3.1 Thermal conductivity of thyristor package materials

Thermal Conductivity Material (W/m K) at 300 К