Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`
`1
`
`This application note describes the basic characteristics and operating performance
`of IGBTs. It is intended to give the reader a thorough background on the device
`technology behind IXYS IGBTs.
`
`IGBT Fundamentals
`The Insulated Gate Bipolar Transistor (IGBT) is a minority-carrier device with
`high input impedance and large bipolar current-carrying capability. Many designers view
`IGBT as a device with MOS input characteristics and bipolar output characteristic that is
`a voltage-controlled bipolar device. To make use of the advantages of both Power
`MOSFET and BJT, the IGBT has been introduced. It’s a functional integration of Power
`MOSFET and BJT devices in monolithic form. It combines the best attributes of both to
`achieve optimal device characteristics [2].
`
`The IGBT is suitable for many applications in power electronics, especially in Pulse
`Width Modulated (PWM) servo and three-phase drives requiring high dynamic range
`control and low noise. It also can be used in Uninterruptible Power Supplies (UPS),
`Switched-Mode Power Supplies (SMPS), and other power circuits requiring high switch
`repetition rates. IGBT improves dynamic performance and efficiency and reduced the
`level of audible noise. It is equally suitable in resonant-mode converter circuits.
`Optimized IGBT is available for both low conduction loss and low switching loss.
`
`The main advantages of IGBT over a Power MOSFET and a BJT are:
`1.
`It has a very low on-state voltage drop due to conductivity modulation and has
`superior on-state current density. So smaller chip size is possible and the cost
`can be reduced.
`2. Low driving power and a simple drive circuit due to the input MOS gate
`structure. It canbe easily controlled as compared to current controlled devices
`(thyristor, BJT) in high voltage and high current applications.
`3. Wide SOA. It has superior current conduction capability compared with the
`bipolar transistor. It also has excellent forward and reverse blocking
`capabilities.
`The main drawbacks are:
`1. Switching speed is inferior to that of a Power MOSFET and superior to that of
`a BJT. The collector current tailing due to the minority carrier causes the turn-
`off speed to be slow.
`2. There is a possibility of latchup due to the internal PNPN thyristor structure.
`
`The IGBT is suitable for scaling up the blocking voltage capability. In case of Power
`MOSFET, the on-resistance increases sharply with the breakdown voltage due to an
`increase in the resistively and thickness of the drift region required to support the high
`operating voltage. For this reason, the development of high current Power MOSFET with
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`high-blocking voltage rating is normally avoided. In contrast, for the IGBT, the drift
`region resistance is drastically reduced by the high concentration of injected minority
`carriers during on-state current conduction. The forward drop from the drift region
`becomes dependent upon its thickness and independent of its original resistivity.
`
`Basic Structure
`The basic schematic of a typical N-channel IGBT based upon the DMOS process
`is shown in Figure 1. This is one of several structures possible for this device. It is
`evident that the silicon cross-section of an IGBT is almost identical to that of a vertical
`Power MOSFET except for the P+ injecting layer. It shares similar MOS gate structure
`and P wells with N+ source regions. The N+ layer at the top is the source or emitter and
`the P+ layer at the bottom is the drain or collector. It is also feasible to make P-channel
`IGBTs and for which the doping profile in each layer will be reversed. IGBT has a
`parasitic thyristor comprising the four-layer NPNP structure. Turn-on of this thyristor is
`undesirable.
`
`
`
`Figure 1: Schematic view of a generic N-channel IGBT [2]
`
`Some IGBTs, manufactured without the N+ buffer layer, are called non-punch through
`(NPT) IGBTs whereas those with this layer are called punch-through (PT) IGBTs. The
`presence of this buffer layer can significantly improve the performance of the device if
`the doping level and thickness of this layer are chosen appropriately. Despite physical
`similarities, the operation of an IGBT is closer to that of a power BJT than a power
`MOSFET. It is due to the P+ drain layer (injecting layer) which is responsible for the
`minority carrier injection into the N--drift region and the resulting conductivity
`modulation.
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`
`3
`
`
`Figure 2: Equivalent circuit model of an IGBT [2]
`
`
`Based on the structure, a simple equivalent circuit model of an IGBT can be drawn as
`shown in Figure 2. It contains MOSFET, JFET, NPN and PNP transistors. The collector
`of the PNP is connected to the base of the NPN and the collector of the NPN is connected
`to the base of the PNP through the JFET. The NPN and PNP transistors represent the
`parasitic thyristor which constitutes a regenerative feedback loop. The resistor RB
`represents the shorting of the base-emitter of the NPN transistor to ensure that the
`thyristor does not latch up, which will lead to the IGBT latchup. The JFET represents the
`constriction of current between any two neighboring IGBT cells. It supports most of the
`voltage and allows the MOSFET to be a low voltage type and consequently have a low
`RDS(on) value. A circuit symbol for the IGBT is shown in Figure 3. It has three terminals
`called Collector (C), Gate (G) and Emitter (E).
`
`
`
`
`
`Figure 3: IGBT Circuit Symbol
`
`
`IXYS has developed both NPT and PT IGBTs. The physical constructions for both of
`them are shown in Figure 4. As mentioned earlier, the PT structure has an extra buffer
`layer which performs two main functions: (i) avoids failure by punch-through action
`because the depletion region expansion at applied high voltage is restricted by this layer,
`(ii) reduces the tail current during turn-off and shortens the fall time of the IGBT because
`the holes are injected by the P+ collector partially recombine in this layer. The NPT
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`IGBTs, which have equal forward and reverse breakdown voltage, are suitable for AC
`applications. The PT IGBTs, which have less reverse breakdown voltage than the forward
`breakdown voltage, are applicable for DC circuits where devices are not required to
`support voltage in the reverse direction.
`
`4
`
`Figure 4: Structure (a) NPT-IGBT and (b) PT-IGBT [2]
`
`Table 1: Characteristics Comparison of NPT and PT IGBTs:
`NPT
`PT
`
`
`Switching Loss
`
`
`
`Low
`Short tail current
`Significant increase in Eoff
`with temperature
`Low
`Flat to slight decrease with
`temperature
`Difficult
`Must sort on VCE(on)
`
`Limited
`High gain
`
`Medium
`Long, low amplitude tail current.
`Moderate increase in Eoff with
`temperature
`Medium
`Increases with temperature
`
`Easy
`Optional sorting
`Recommend share heat
`Yes
`
`Conduction Loss
`
`Paralleling
`
`Short-Circuit Rated
`
`
`
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`
`5
`
`Operation Modes
`Forward-Blocking and Conduction Modes
`When a positive voltage is applied across the collector-to-emitter terminal with
`gate shorted to emitter shown in Figure 1, the device enters into forward blocking mode
`with junctions J1 and J3 are forward-biased and junction J2 is reverse-biased. A depletion
`layer extends on both-sides of junction J2 partly into P-base and N-drift region.
`
`An IGBT in the forward-blocking state can be transferred to the forward conducting state
`by removing the gate-emitter shorting and applying a positive voltage of sufficient level
`to invert the Si below gate in the P base region. This forms a conducting channel which
`connects the N+ emitter to the N--drift region. Through this channel, electrons are
`transported from the N+ emitter to the N--drift. This flow of electrons into the N--drift
`lowers the potential of the N--drift region whereby the P+ collector/ N--drift becomes
`forward-biased. Under this forward-biased condition, a high density of minority carrier
`holes is injected into the N--drift from the P+ collector. When the injected carrier
`concentration is very much larger the background concentration, a condition defined as a
`plasma of holes builds up in the N--drift region. This plasma of holes attracts electrons
`from the emitter contact to maintain local charge neutrality. In this manner,
`approximately equal excess concentrations of holes and electrons are gathered in the N--
`drift region. This excess electron and hole concentrations drastically enhance the
`conductivity of N--drift region. This mechanism in rise in conductivity is referred to as
`the conductivity modulation of the N--drift region.
`
`Reverse-Blocking Mode
`When a negative voltage is applied across the collector-to-emitter terminal shown
`in Figure 1, the junction J1 becomes reverse-biased and its depletion layer extends into
`the N--drift region. The break down voltage during the reverse-blocking is determined by
`an open-base BJT formed by the P+ collector/ N--drift/P-base regions. The device is prone
`to punch-through if the N--drift region is very lightly-doped. The desired reverse voltage
`capability can be obtained by optimizing the resistivity and thickness of the N--drift
`region.
`
`The width of the N--drift region that determines the reverse voltage capability and the
`forward voltage drop which increases with increasing width can be determined by
`
`
`
`
`
`
`(1)
`
`
`
`
`
`
`
`d
`
`1
`
`=
`
`V
`εε2
`mso
`qN
`
`D
`
`+
`
`L
`P
`
`
`
`Where,
`LP = Minority carrier diffusion length
`Vm = Maximum blocking voltage
`=oε Permittivity of free space
`
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`
`6
`
`=sε Dielectric constant of Si
`q = Electronic charge
`ND = Doping concentration of N-drift region
`
`Note: Reverse blocking IGBT is rare and in most applications, an anti-parallel diode
`(FRED) is used.
`
`Output Characteristics
`The plot for forward output characteristics of an NPT-IGBT is shown in Figure 5. It has a
`family of curves, each of which corresponds to a different gate-to-emitter voltage (VGE).
`The collector current (IC) is measured as a function of collector-emitter voltage (VCE)
`with the gate-emitter voltage (VGE) constant.
`
`
`
`
`
`Figure 5: Output I-V characteristics of an NPT-IGBT [IXSH 30N60B2D1] [3]
`
`
`
`
`
` A
`
` distinguishing feature of the characteristics is the 0.7V offset from the origin. The
`entire family of curves is translated from the origin by this voltage magnitude. It may be
`recalled that with a P+ collector, an extra P-N junction has been incorporated in the IGBT
`structure. This P-N junction makes its function fundamentally different from the power
`MOSFET.
`
`
`
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`
`7
`
`V
`CE
`
`=
`
`Cons
`
`tan
`
`t
`
`
`
`
`
`
`
`(2)
`
`C
`
`GE
`
`VI
`
`∂
`∂
`
`
`
`
`
`g
`
`=
`
`fs
`
`
`
`
`
`Transfer Characteristics
`The transfer characteristic is defined as the variation of ICE with VGE values at
`different temperatures, namely, 25oC, 125oC, and -40oC. A typical transfer characteristic
`is shown in Figure 6. The gradient of transfer characteristic at a given temperature is a
`measure of the transconductance (gfs) of the device at that temperature.
`
`
`
`
`Figure 6: IGBT Transfer Characteristics [IXSH30N60B2]
`
`
`
` large gfs is desirable to obtain a high current handling capability with low gate drive
`voltage. The channel and gate structures dictate the gfs value. Both gfs and RDS(on) (on-
`resistance of IGBT) are controlled by the channel length which is determined by the
`difference in diffusion depths of the P base and N+ emitter. The point of intersection of
`the tangent to the transfer characteristic determines the threshold voltage (VGE(th)) of the
`device.
`
`
`
`
`
`
` A
`
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`
`8
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`
`
`
`
`
`
`
` Figure 7:
`
`
`
`Transconductance Characteristics of an IGBT [IXSH30N60B2]
`
`
`
`
`
`
` A
`
` typical transconductance (gfs) vs collector current (IC) is shown in Figure 7. The gfs
`increases with collector current, flattening out at a peak level slowly for a range of
`collector currents. The gfs flattens out because the saturation phenomenon in the parasitic
`MOSFET decreases the base current drive of the PNP transistor.
`
`
`
`Switching Characteristics
`The switching characteristics of an IGBT are very much similar to that of a Power
`MOSFET. The major difference from Power MOSFET is that it has a tailing collector
`current due to the stored charge in the N--drift region. The tail current increases the turn-
`off loss and requires an increase in the dead time between the conduction of two devices
`in a half-bridge circuit. The Figure 8 shows a test circuit for switching characteristics and
`the Figure 9 shows the corresponding current and voltage turn-on and turn-off
`waveforms. IXYS IGBTs are tested with a gate voltage switched from +15V to 0V. To
`reduce switching losses, it is recommended to switch off the gate with a negative voltage
`(-15V).
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`D
`L
`
`9
`
`vCL
`
`
`
`+-
`
`IC
`
`G
`
`C
`DUT
`E
`
`+ -
`
`vcc
`
`
`
`
`
`
`Figure 8: IGBT Switching Time Test Circuit
`
`The turn-off speed of an IGBT is limited by the lifetime of the stored charge or minority
`carriers in the N--drift region which is the base of the parasitic PNP transistor. The base is
`not accessible physically thus the external means can not be applied to sweep out the
`stored charge from the N--drift region to improve the switching time. The only way the
`stored charge can be removed is by recombination within the IGBT. Traditional lifetime
`killing techniques or an N+ buffer layer to collect the minority charges at turn-off are
`commonly used to speed-up recombination time.
`
`
`Figure 9: IGBT Current and Voltage Turn-on and Turn-off Waveforms
`
`
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`
`
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`10
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`The turn-on energy Eon is defined as the integral of IC .VCE within the limit of 10% ICE
`rise to 90% VCE fall. The amount of turn on energy depends on the reverse recovery
`behavior of the free wheeling diode, so special attention must be paid if there is a free
`wheeling diode within the package of the IGBT (Co-Pack).
`
`The turn-off energy Eoff is defined as the integral of IC .VCE within the limit of 10% VCE
`rise to 90% IC fall. Eoff plays the major part of total switching losses in IGBT.
`
`Latch-up
`During on-state, paths for current flow in an IGBT are shown in Figure 10. The
`holes are injected into the N--drift region from the P+ collector form two paths. Part of the
`holes disappear by recombination with electrons came from MOSFET channel. Other
`part of holes are attracted to the vicinity of the inversion layer by the negative charge of
`electrons, travel laterally through the P-body layer and develops a voltage drop in the
`ohmic resistance of the body. This voltage tends to forward bias the N+P junction and if it
`is large enough, substantial injection of electrons from the emitter into the body region
`will occur and the parasiric NPN transistor will be turned-on. If this happens, both NPN
`and PNP parasitic transistors will be turned-on and hence the thyristor composed of these
`two transistors will latch on and the latchup condition of IGBT will have occurred. Once
`in latchup, the gate has no control on the collector current and the only way to turn-off
`the IGBT is by forced commutation of the current, exactly the same as for a conventional
`thyristor.
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`
`11
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`
`
`
`
`
`
`Figure 10: ON-state current flow path of an IGBT [3]
`
`
`If latchup is not terminated quickly, the IGBT will be destroyed by the excessive power
`dissipation. IGBT has a maximum allowable peak drain current (ICM) that can flow
`without latchup. Device manufacturers specify this current level in the datasheet. Beyond
`this current level, a large enough lateral voltage drop will activate thyristor and the
`latchup of IGBT.
`
`Safe Operating Area (SOA)
`The safe operating area (SOA) is defined as the current-voltage boundary within
`which a power switching device can be operated without destructive failure. For IGBT,
`the area is defined by the maximum collector-emitter voltage VCE and collector current IC
`within which the IGBT operation must be confined to protect it from damage. The IGBT
`has the following types of SOA operations: forward-biased safe operating area (FBSOA),
`reverse-biased safe operating area (RBSOA) and short-circuit safe operating area
`(SCSOA).
`
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`12
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`Forward-Biased Safe Operating Area (FBSOA)
`The FBSOA is an important characteristic for applications with inductive loads. It
`is defined by the maximum collector-emitter voltage with saturated collector current. In
`this mode, both electrons and holes are transported through the drift region, which is
`supporting a high collector voltage. The electron and hole concentrations in the drift
`region are related to the corresponding current densities by:
`J
`n
`
`
`
`
`
`n
`=
`qV
`sat
`J
`qV
`sat
`p
`,
`satV , are the saturated drift velocities for electrons and holes, respectively. p
`satV , and n
`where
`
`
`The net positive charge in the drift region is given by,
`J
`J
`p
`N
`N
`qV
`qV
`sat
`p
`sat
`n
`,
`,
`This charge determines the electric field distribution in the drift region. In steady-state
`DN . In FBSOA, the net
`forward blocking condition, the drift region charge is equal to
`charge is much larger because the hole current density is significantly larger than the
`electron current density.
`
`The breakdown voltage limit in the FBSOA is defined by
`
`
`,
`
`n
`
`p
`
`
`
`
`
`p
`
`=
`
`
`
`
`
`=+
`
`+
`
`D
`
`−
`
`n
`
`
`
`(3)
`
`(4)
`
`(5)
`
`
`
`
`
`BVSOA
`
`=
`
`x
`10
`34.5
`13
`N
`)
`(
`4/3
`+
`
`
`
`
`
`
`
`
`
`(6)
`
`
`Reverse-Biased Safe Operating Area (RBSOA)
`The RBSOA is important during the turn-off transient. The current which can be
`turned-off is limited to twice the nominal current of the IGBT. This means a 1200A
`IGBT is able to turn-off a maximum current of 2400A. The maximum current is a
`function of the peak voltage which appears between collector and emitter during turn-off.
`dIL
`dt
`
`The peak value of VCE is the sum of the DC link voltage and the product of
`C /
`σ
`where σL is the stray inductance of the power circuit. The relation between maximum IC
`and VCE can be seen in the RBSOA diagram in Figure 11 for the IGBT [IXSH30N60B2].
`
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
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`13
`
`Figure 11: RBSOA of IGBT [IXSH30N60B2]
`
`
`
`
`In this mode, the gate bias is at zero or at a negative value thus the current transport in the
`drift region occurs exclusively via the holes for an n-channel IGBT. The presence of
`holes adds charge to the drift region, resulting to the increase in the electric field at the P-
`base/N drift region junction. The net charge in the space charge region under the RBSOA
`condition is given by:
`N
`
`=+
`
`N
`
`+
`
`D
`
`J
`qV
`
`sat
`
`,
`
`p
`
`C
`
`
`
`
`
`
`
`
`
`(7)
`
`J
`qV
`
`
`where Jc is the total collector current. The avalanche breakdown voltage for RBSOA is
`given by:
`
`
`BV
`
`SOA
`
`=
`
`34.5
`
`x
`10
`13
`
`(
`
`C
`
`)
`
`
`
`
`
`
`
`(8)
`
`sat
`
`,
`
`p
`
`
`Short-Circuit Safe Operating Area (SCSOA)
`A very important requirement imposed on the power switching device, when used
`in motor control applications is that be able to turn-off safely due to a load or equipment
`short circuit. When a current overload occurs, collector current rises rapidly until it
`exceeds that which the device can sustain with the applied gate voltage. The key to
`survivability for the power device is to limit the current amplitude to a safe level for a
`period of time that is sufficiently long to allow the control circuit to detect the fault and
`turn the device off.
`
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`14
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`The IGBT collector current IC is a function of the gate-emitter voltage VGE and the
`temperature T. The transfer characteristic of a 600V/55A IGBT in Figure 6 shows the
`maximum collector current IC vs. the gate-emitter voltage VGE. For VGE of 15V the
`current is limited to a value of 80A, which is about 1.5 times the nominal value. This is
`very low value compared to the short circuit current which is typically 6-7 times the
`nominal value.
`
`
`
`
`La
`
`
`
`
`
`
`
`+ VCC
`
`D
`
`Q
`
`VGE
`
`Short-
`Circuit
`
`LSC
`
`MOTOR
`
`
`
`Figure 12: SCSOA Test Circuit [3]
`
`
`
` A
`
` circuit diagram for SCSOA test is shown in Figure 12. The short-circuit inductance
`,uH the
`value determines the mode of operation of the circuit. When it is in the range of
`operation is similar to normal switching of inductive load. When IGBT is turned on, VCE
`drops to its saturation voltage. The IGBT is saturated and IC is increasing with a dIc/dt of
`Vcc/Lsc. It is not allowed to turn-off the IGBT from the saturation region at a collector
`current higher than 2 times rated current because this is an operation outside the RBSOA.
`In case of short-circuit; it is necessary to wait until the active region is reached. The
`IGBT must be turned-off within 10 us to prevent destruction due to overheating.
`
`
`
`
`
`
`
`
`References
`[1] B. Jayant Baliga, “Power Semiconductor Devices” PWS Publishing Company, ISBN:
`0-534-94098-6, 1996.
`[2] Vinod Kumar Khanna, “Insulated Gate Bipolar Transistor (IGBT): Theory and
`Design” IEEE Press, Wiley-Interscience
`[3] IXYS, “Power Semiconductors Application Notes, 2002” IXYS Corporation, 3540
`Bassett Street, Santa Clara CA 95054, and Phone: 408-982-0700
`
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`Insulated Gate Bipolar Transistor (IGBT) Basics
`Abdus Sattar, IXYS Corporation
`IXAN0063
`[4] Ned Mohan, Tore M. Undeland, William P. Robbins, “Power Electronics: Converters,
`Applications and Design” John Willey & Sons, Inc.
` [5] Ralph E. Locher, Abhijit D. Pathak, Senior Application Engineering, IXYS
`Corporation, “Use of BiMOSFETs in modern Radar Transmitters” IEEE PEDS 2001-
`Indonesia
`[6] Ralph Locher, “Introduction to Power MOSFETs and their Applications” Fairchild
`Semiconductor, Application Note 558, October 1998.
`
`15
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