Turn-Off Characteristics of Power Transistors Using Emitter-Open Turn-Off

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  • I. Introduction

    Turn-Off Characteristics ofPower Transistors UsingEmitter-Open Turn-Off

    DAN Y. CHENVirginia Polytechnic Institute and State University

    BARRY JACKSONGeneral Electric

    As compared with conventional reverse-biased turn-off, emitter-

    open turn-off provides superior transistor turn-off characteristics.

    Not only are the storage time and the fall time of the power tran-

    sistors much reduced, but also the device reverse-biased second

    breakdown phenomenon, commonly associated with turn-off of in-

    ductive load, is eliminated. Furthermore the storage time tolerance

    due to device variation and temperature change is minimized.

    Manuscript received October 24, 1980.

    Authors' addresses: D.Y. Chen, Department of Electrical Engineer-ing, Virginia Polytechnic Institute and State University, Blacksburg,VA 24061; B. Jackson, Teletype Corporation, Skokies, IL.

    0018-9251/81/0500-0386 $00.75 i 1981 IEEE

    The turn-off characteristics of a power transistorare very important consideration in the design ofpower electronic circuits. They affect both the circuitenergy efficiency and overall system reliability. Thetransistor should be turned off rapidly and held offfirmly during the entire off period. The turn-offcharacteristics of a transistor depend not only on theintrinsic characteristics of the transistors, but alsovery much on the base-drive strategy used. Conven-tionally, a power transistor is turned off by applying areverse-biased voltage to the base-emitter junction ofthe transistor. In so doing, a reverse base current iswithdrawn from the transistor during turn-off. Themost common way of increasing the turn-off speed isby drawing a large reverse base current from the tran-sistor. However a danger arises, especially in the caseof inductive load, because the transistor may enterinto reverse bias second breakdown which may lead tothe destruction of the transistor and possibly the restof the circuit as well. Recently a new transistor circuitturn-off technique, referred to as emitter-open switch-ing, was reported, which promises to improve boththe device second breakdown capability and the turn-off speed at the same time [1, 2].

    The purpose of the present paper is to investigatethe turn-off characteristics of high voltage power tran-sistors under the condition of emitter turn-off andcompare the results with conventional reverse-biasedturn-off. Effort was focused upon the comparison ofthe transistor turn-off storage time, fall time, and thesecond breakdown ruggedness.

    II. Emitter-Open Turn-off of a Power Transistor

    Fig. 1(A) shows the circuit arrangement and thebase current waveform associated with conventionalreverse-biased turn-off, in which the reverse base cur-rent 'BR is always smaller than the collector current tobe turned off. Fig. 1(B) shows the circuit arrangementof the emitter-open turn-off, where Q, is the mainpower transistor and Q2 is a low voltage high speedtransistor, rated for the full emitter current of Q1. Toturn on Q1, Q2 must be turned on at the same time.The turn-off of Q, is accomplished by open circuit ofthe emitter terminal, i.e., by turning off Q2. Duringturn-off when Q2 is open, the reverse base current IBRof Q, immediately assumes a value equal to the collec-tor current and flows through the diode strings D,,D2, and D3 to ground. The turn-off gain is thereforeequal to unity by conventional deflnition.

    It is important to note that Q, is a high voltagetransistor and Q, is a low voltage one. In the presentinvestigation, the transistor collector-base breakdownvoltage rating BVCBO ranges from 600 V to 1500 Vfor Ql and is only about 50 V for Q2. Because ofmuch different voltage ratings, the device structure

    IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-17, NO. 3 MAY 1981386

  • TABLE IDevices Types Investigated

    BVCBO ICnmaX (Continuous)Device Type Manufacturer Catalog Number (v) (A)

    A Toshiba 2SC1892 1500 2.5B Toshiba 2SC1 172B 1500 7.0C Amperex BU208A 1500 5.0D AEG-Telefunken BU207 1300 5.0E AEG-Telefunken BU208 1500 5.0F AEG-Telefunken BU209 1700 4.0G Motorola BU108 1500 5.0H General Electric D56W2 1400 5.0I Texas Instruments TIP54 600 5.0J General Electric D44TE 600 4.0

    for Q, and Q2 are quite different and so are the turn-off speed. For the BVCBO ratings of Q, and Q2 men-tioned above, it is not unusual to have a ten-to-onedifference in turn-off speed in favor of low voltagedevice Q2. Q2 can be turned off rapidly by reversebiasing of the base-emitter junction or even by justgrounding the base terminal.

    The main advantages of using emitter-open turn-off, as is shown in the experimental data in the nextsection, are the much improved turn-off speed and theenhanced reverse-bias second breakdown capability ofQ1. On the surface it may seem that the total tran-sistor losses increase in the emitter-open turn-off cir-cuit, because Q2 contributes to additional losses.However, because Q2 is a low voltage device, both theswitching losses and the conduction loss of Q2 are in-significant as compared with the turn-off loss of Q,.This is especially true in the high voltage and high fre-quency system in which the turn-off loss of Q, ispredominant. Therefore in the emitter-open turn-off,the gain in reducing tne turn-off loss of Q, much off-sets the loss due to Q2.

    III. Comparisons of Experimental Results BetweenReverse-Biased Turn-Off and Emitter-Open Turn-Off

    For the purpose of comparison, two testing cir-cuits were constructed, one for the reverse-biasedturn-off test and the other for the emitter-open turn-off test. The efforts were focused on the measurementof transistor storage time, collector current fall time,and the device second breakdown capability. Secondbreakdown may not be destructive to the device pro-vided that the duration of the breakdown is keptshort. To accomplish such a feature, a crowbar shutdown circuit is implemented in both of the testing cir-cuits. The function of the crowbar circuit is to removethe collector current from the transistor under test asquickly as possible after the initiation of the secondbreakdown and therefore to prevent any permanentdamage to the device [2, 3].

    Ten different transistor types were chosen in thepresent investigation. Table I lists the 10 types with

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  • TABLE IIRepresentative Data Showing Storage Time (ts) and Fall Time (tf) as a Function of Col-lector Current and Reverse Base Current for Both the Reverse-Biased and the Emitter-Open Turn-Off

    Reverse-Biased Turn-Off Emitter-Open Turn-Off

    BR 0.5 A 'BR = 1.0 A 'BR = 2.5 ATurn-Off ( 5 Turn-Off ( 2.5 Turn-Off (3 1

    TestDevice t, (ps) tf (,us) fs (US) tf (PS) ts (Us) tf (Ps)A 4.0 0.3 2.4 0.225 1.2 0.125B 12.0 0.3 6.7 0.25 2.7 0.15C 9.0 0.35 6.0 0.28 2.2 0.2D 12.0 0.4 6.2 0.3 2.8 0.15E 6.0 0.25 3.4 0.2 1.7 0.15F 2.5 0.175 1.7 0.15 0.8 0.15G 2.5 0.14 1.6 0.1 0.75 0.09H 6.0 0.35 3.3 0.175 1.7 0.151 5.0 0.30 3.0 0.18 1.4 0.101 4.0 0.30 2.8 0.18 1.2 0.10

    Note: All the devices except I and J were tested at 2.5 A collector current with aforced ( of 10 and a collector-emitter voltage of 600 V. Devices I and J were tested alsoat 2.5 A collector current but with a forced ( of 15 and a collector-emitter voltage of450 V. All the tests were conducted at room temperature.

    TABLE IIIVariation of Transistor Turn-Off Time for Both the Reverse-Biased and the Emitter-Open Turn-Off

    Reverse-Biased Turn-Off Emitter-Open Turn-Off

    IBR 0 5 A IBR 1.0 A 'BR 2.5 ADevice Types

    andCase Temp

    (0 C) t4 (US) tf (Us) ts (Us) tf (Us) ts (Us) tf (US)

    A, (25 C) 4.0 0.3 2.4 0.225 1.2 0.125A2 (250 C) 6.0 0.4 3.4 0.18 1.7 0.15A,(700C) 5.0 0.5 3.0 0.17 1.6 0.15A2 (700 C) 8.0 0.45 4.7 0.27 2.0 0.18D, (250 C) 12.0 0.4 6.2 0.3 2.8 0.15D2 (250 C) 8.0 0.35 4.8 0.28 2.3 0.19DI (750 C) 17.0 0.55 8.0 0.4 3.2 0.17D2 (750 C) 11.0 0.4 6.0 0.3 2.7 0.1H, (25 C) 6.0 0.35 3.3 0.175 1.7 0.15H, (-200 C) 4.0 0.25 2.4 0.22 1.3 0.13

    voltage rating and were tested at 2.5 A collector cur-rent and turn-on gain of 15 and a collector-emittervoltage of 450 V. All the results shown in this tablewere obtained at room temperature. The reverse-biased turn-off tests were conducted for the two caseswhen the turn-off gain is 5 and 2.5. In other words,the reverse base current is 0.5 A in one case and is 1.0A in the other. Several remarks can be concludedfrom the results shown in the table. 1) Using theemitter-open turn-off, the storage time ts is reducedby a factor of 2-4.5 as compared with the case whenthe turn-off gain is 5. 2) Collector current fall time tfis reduced by a factor of 1.5-2.6 as compared withreverse-biased turn-off. 3) In the emitter-open turn-off, the reduction of storage time and fall time is

    more pronounced for slower devices such as types Band D. The results are not surprising because a largereverse base current, equal to collector current, iswithdrawn from the base during the emitter-openturn-off, and the excessive carriers in the base regionare swept out rapidly primarily by the large reversebase current rather than by recombination.

    B. Temperature Effect on Storage Time and Fall Time

    Both the storage time and the fall time of a tran-sistor vary significantly with temperature and device.However, the extent of variation depends on the turn-off strategy used. Table III summarizes the test resultsof the variation of transistor turn-off time for both

    IEEE TRANSACTIONS ON AEROSPACE AND ELECTRONIC SYSTEMS VOL. AES-17, NO. 3 MAY 1981388

  • the reverse-biased and the emitter-open turn-off. Inthe table a transistor is designated with a device typeand a subscript number which is used to distinguishdifferent devices within the same device type. For ex-ample, transistors A, and A2 are different devices butboth are type A devices. As can be seen from thetable, the range of tolerance of transistor storage timeand fall time, due to either temperature change ordevice variation, is minimized in the emitter-openturn-off. The effect is especially pronounced for aslow device such as transistor D,. The results shownare plausible because it is the reverse base current, notthe recombination, that plays the dominant role insweeping out the excessive carriers in the emitter-openturn-off.

    C. Reverse-Biased Second Breakdown Ruggedness

    In this paper the device second breakdown rug-gedness is characterized by Vc,P, the peak voltageblocking capability of the collect-emitter terminal dur-ing turn-off. For a given device, the value of V,CEdepends on the condition of IBR and collector currentI(. The tests of device second breakdown ruggednessare focused on the display of collector-emitter voltagewaveforms during turn-off and the measurement ofVCEP from the displayed waveforms. Figs. 2 to 5 showthe collector-emitter waveforms for different types ofdevices for both the reverse-biased and the emitter-open turn-off. As can be seen from Fig. 3(A), duringthe turn-off period of type A device, VCE rises fromthe conduction voltage drop VCESAT to about 680 V, atwhich second breakdown initiates and VCE dropsrapidly to second breakdown level of about 100 V andstays there for about 160 ns until the collector currentis rapidly removed by the testing circuit. During thesecond breakdown period, large power dissipation oc-curs in the device due to the coexistence of high cur-rent and high voltage. But 160 ns is short enough thatany permanent damage to the device is avoided. Forthe same device under emitter-open turn-off, however,second breakdown phenomenon has never beenobserved. As can be seen from Fig. 3(B), VCE risesfrom VCESAT all the way up to BVCBO without losingvoltage blocking capability. Figs. 3 and 4 show similarresults for device types D and F.

    Table IV summarizes the results obtained fromsecond breakdown tests for both turn-off conditions.Because of the nondestructive nature of the tests, thesame transistor can be tested under different condi-tions. As can be seen from the table, for device B, thevalue of VCE, depends on Ic and 'BR For the same Ic,the value of VCEP could increase or decrease with IBRin the case of reverse-biased turn-off, which is consis-tent with the results reported in [3]. However, in allthe tests conducted for the emitter-open turn-off, thesecond breakdown phenomenon has never beenobserved.

    (A) (B)Fig. 2. Waveforms associated with second breakdown tests for typeA device. (A) Reverse bias turn-off. Parameters are IC 2 A/cm;VCE = 200 V/cm; time = 400 ns/cm. (B) Emitter-open turn-off.Parameters are VcE - 400 V/cm, IE lA/cm, time I Ms/cm.Fig. 3. Waveforms associated with second breakdown tests for typeD device. (A) Reverse bias turn-off. Parameters are Ic = I A/cm,VcE = 400 V/cm, time = 200 ns/cm. (B) Emitter-open turn-off.Parameters are Y= 400 V/cm, IE 1 A/cm, time 1 Ms/cm.

    (A) (B)

    Fig. 4. Waveforms associated with second breakdown tests for typeF device. (A) Reverse bias turn-off. Parameters are IC 2A/cm,VCE 400 V/cm, time 400 ns/cm. (B) Emitter-open turn-off.Parameters are VCE = 400 V/cm, IE = I A/cm, time 400 Ms/cm.

    (A) (B)

    The results obtained are somewhat surprising butare explainable. According to theory, the secondbreakdown phenomenon in the reverse-biased turn-offof inductive load is attributed to the emitter currentconstriction during the turn-off process [4, 5]. Theconstriction of the current is caused by the potentialdrop laterally along the base-emitter junction as aresult of the base region lateral resistance and thereverse-base current. In the case of the emitter-openturn-off, the emitter terminal is open during turn-off,and the collector current is diverted out of base ter-minal. Therefore current constriction phenomenondoes not occur and the second breakdown is avoided.

    V. Conclusion

    While the power transistor manufacturers con-stantly try to fabricate faster and more ruggeddevices, the design parameters are normally such thatthe device ruggedness is achieved at the expense of

    CHEN/JACKSON: TURN-OFF CHARACTERISTICS OF POWER TRANSISTORS USING EMITTER-OPEN TURN-OFF 389

  • TABLE IVDevice Second Breakdown Ruggedness for Both the Reverse-Biased and theEmitter-Open Turn-Off

    Reverse Biased Emitter-OpenTurn-Off (,BR < IC) Turn-Off (IBR IC)

    Device IC 'BR VCEP IBR 'C VCEP = B VCBOType (A) (A) (V) (A) (V)

    A 2.5 1.1 680 2.5 1500B 2.5 1.0 800 2.5 1500B 2.0 1.0 910 2.0 1500B 2.5 0.8 760 2.5 1500B 2.5 0.3 840 2.5 1500C 2.5 1.3 900 2.5 1500D 2.5 1.6 870 2.5 1500E 2.5 0.9 820 2.5 1500E 2.5 0.6 720 2.5 1500E 2.0 0.9 900 2.0 1500F 2.5 0.7 860 2.5 1500G 2.5 1.5 1100 2.5 1500H 2.5 1.0 1000 2.5 14001 2.5 1.0 390 2.5 600J 2.5 1.0 400 2.5 600

    turn-off speed or vice versa. The test results reportedin this paper show that, by using the emitter-openturn-off technique, not only can the device turn-offspeed be significantly increased but also the reversebias second breakdown phenomena can be eliminated.The very same device can then be utilized to its fullpotential for higher frequency and higher voltage ap-plications.

    In addition to the advantages mentioned above,emitter-open turn-off also provides the designers prac-tical means of minimizing the variations of devicestorage time, fall...

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