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Effects of temperature on thyristor performance

Posted: 21 Aug 2001     Print Version  Bookmark and Share

Keywords:power 

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TGD-1XAN4870 Application Note 1/5 80 100 120 140 160 180 200 Thyristor junction temperature - (0C) 0 20 40 60 80 100 Percentageofvoltagegrade VDRM VRRM Fig.1 Thyristor de-rating curves The junction temperature ( Tj ) of a power semicon- ductor in any particular situation profoundly affects its performance and reliability. During its working life a thyristor can experience a wide range of temperatures. Operating at -400C is not damaging but allowance must be made by the user for increased gate trigger current, latching current and holding current as well as slow turn-on (see application note AN4840 Gate Triggering and Gate Characteristics). Working in the range between room temperature and 1250C gives the best compromise be- tween ease of operation and operational life. Tj = 1250C is chosen as the design maximum value since above this, blocking current starts to increase rapidly, thus degrading voltage rating, see Fig.1. The device becomes much more susceptible to over- voltage transients , high dv/dt, di/dt and surge current. In the case of the forward blocking junction there is an increasing chance of forward breakover triggering. For special appli- cations it is possible to select devices to operate continu- ously with low leakage at Tj = 1400C but such devices may need to be fully characterised and rated on other param- eters at 1400C. Many applications involve infrequent current over- loadsforshortperiodsanditispossibletoallowTj torisewell above 1250C in such situations. A typical situation is during a load short circuit when the device is protected by a fuse. In 50Hz circuits the thyristor may often have to carry short circuit current for up to 10ms. During this time Tj can rise transiently to 300 - 5000C without the junction being dam- aged. Peak temperature lags peak current by typically 2 or 3 milliseconds and, although falling, is still high at the end of the current pulse. If current is interrupted by a fuse, little or no reverse voltage appears across the device. However, the re-application of reverse voltage at such a high tem- perature can result in very high reverse recovery power dissipation. This escalates the junction temperture further and the subsequent high blocking current leads to reverse voltage failure by thermal runaway. Limit case surge currents are determined by experi- mental means using a 50Hz half sine of current and pub- lished in the data sheet. These ITSM limit values are used to determine the peak temperature ( Using ITSM for VR =0 ) and the temperature at the end of the current loop ( Using ITSM for VR = 50% VRRM ). These temperatures are then taken as the limit temperatures for the particular device. If temperatures in other applications are kept below these, then the condi- tion will be safe. The method of calculating overload Tj for the pub- lished ITSM currents and other overload conditions is dis- cussed below. The overload above assumed a high speed fuse or circuit breaker will interrupt the supply before forward block- ing voltage appears. Some overloads require that the de- vice survives with both reverse and then forward voltage AN4870 Effects Of Temperature On Thyristor Performance Application Note Replaces March 1998 version, AN4870-1.2 AN4870-2.0 January 2000 TGD-1X 2/5 AN4870 Application Note 100% 75% 50% 25% 1 0.1 0.01 0.001 Failurerate/1000hrs 40 50 60 70 80 90 100 Junction temperature - (0C) Fig.2 Thyristor failure rate vs applied voltage as a percentage of VDRM (rated) and junction temperature due to ion migration in junction passivation Fig.3 Thermal fatigue life expectancy 1.00E + 03 1.00E + 04 1.00E + 05 1.00E + 06 1.00E + 07 1.00E + 08 No. of cycles 250 200 150 100 50 0 Temperatureexcursion-(0C) 30mm 38mm 50mm 75mm 100mm being reapplied. For forward blocking two possible failure modes apply:- 1) Failure to turn-off because of the high turn-off time,tq value at elevated temperature. 2) Breakover due to high blocking current alone. The most likely is 1). Variation of tq with temperature for a range of other conditions must be determined experimen- tally. Other important temperatures are: Temperatures below Tj(max) where ion migration on the silicon surface under the passivation can lead to long term degradation. ( See Fig.2 ) Continuous Tj permitted before thermal runaway oc- curs. This is likely to be important only with high leakage thyristors and when very small heatsinks are used. Circa 2500C continuous: Rubber locaters and organic passivation material starts to disintegrate; some an- nealing-out of electron irradiation. Above 6000C. The metal of the surface contacts starts to penetrate into the silicon causing eventual short circuit. This is probably a factor in di/dt failure. 1100 to 13000C. This is the temperature reached at non-repetitive di/dt limits. The high local thermal stress causes cracking of the silicon. 14150C - Melting point of silicon. Another important temperature limit is the magnitude of temperature excursions ( Tj ) which relates strongly to the operating life of the device. Slow temperature changes stress the various mechanical parts of the device and cause the movement of one component relative to another due to differential expansion and contraction. TGD-1XAN4870 Application Note 3/5 Rapid temperature changes associated with high di/dt can cause micro cracking. It has been shown that, in silicon, micro cracking occurs with T between 300 and 3500C. Somos et al have shown how the value of T relates the expected life time of the device measured in numbers of operations and device diameter. (Fig.3). Although continuous operation at 250 to 3000C will destroy PN junction characteristics it is possible to operate transiently in this region if allowance is made for reduced device life. Such is the philosophy behind surge current protection when roughly 100 operations up to ITSM values are allowed in the life time of a device. When any overload current wave shape is more complex than a simple sine wave a method of calculating end-of-pulse temperature has to be used. Calculation of steady state Tj takes account of the device case temperature, average current/power loss and steady state thermal resistance. However, for short term overloads it is necessary to include variation of device thermal resistance with time and the device on-state volt drop with temperature. A method of calculating junction temperature using a computer program is described for overloads lasting 1 to about 100ms: The information on the overload current is inputted as a series of instantaneous current values with corresponding time points. The device transient thermal impedance curve is represented as a polynomial with 5 components, Fig.4 5 Z( t ) = A(i).exp[ -t / B ( i ) ] i=1 where B = 0.001,0.01,0.1,1.0 and 2 seconds. Associated with each component is a constant and each device type has its own unique set of 5 constants. The variation of on-state voltage with forward current is also represented by a polynomial with 5 components. V( I, Tj ) = V ( 1 + BT x Tj ) + R + I (1 + AL x Tj ) + E( 273 + Tj ) Log10 (i) + 2.3025 Thecurveisdeterminedexperimentallyusinga10ms half sine pulse which goes to currents which are almost 90% of ITSM . The resultant heating effect is noticeable by the VF increaseonthefallingedgeofthecurrentpulse.Anexample of such a "surge loop" is shown in Fig.5. Notice that the surge loop equation includes a tem- perature term which the normal data sheet VTM curve does not. In other words, the "surge loop" model calculates change in VTM due to junction temperature increase. These three input items are then used to calculate instantaneous power and temperature rise at specified time 0.1 0.01 0.001 ThermalImpedance-junctiontocase-(0C/W) 0.001 0.01 0.1 1.0 10 Time - (s) Anode side cooled Double side cooled Conduction d.c. Halfwave 3 phase 1200 6 phase 600 Effective thermal resistance Junction to case 0C/W Double side 0.022 0.024 0.026 0.027 Anode side 0.038 0.040 0.042 0.043 Fig.4 Maximim (limit) transient thermal impedance - junction to case Current-(A) Voltage - (V) Fig.5 Surge loop TGD-1X 4/5 AN4870 Application Note P1 P2 P3 P4 P5 T1 T2 T3 T4 T5 Time Tj(1) Tj(2) Tj(3) Tj(4) Tj(5) Tj(6) Junction temperature = O6 T6 Fig.6 intervals, e.g. 1ms. the procedure using the superposition thereom is as follows: 1. Take the initial Tj at the start of the first 1ms period as Tj (1). 2. Use this in the "surge loop" equation to calculate average power in the first interval. (P1). 3. From the average period power and transient thermal resistance at 1ms calculate temperature rise in the first period and hence starting temperature for second period, Tj (2) where Tj (2) = Tj (1) + T rise (1). 4. Proceed to the second time period and use Tj (2) to calculate appropriate volt drop values and power in this period. 5. Use the average power in period 2 (P2) and the change in thermal resistance between 1ms and 2ms to calculate the rise in the second interval. This then gives the temperature at the end of the second interval, Tj (3). 6. Continue this procedure for as many intervals as necessary. The procedure is more clearly explained by consider- ing a waveform with 5 intervals. Tj(6) = P1 [ Z (T6 -T1 ) - Z( T6 -T2 ) ] + P2 [ Z (T6 -T2 ) - Z( T6 -T3 ) ] + P3 [ Z (T6 -T3 ) - Z( T6 -T4 ) ] + P4 [ Z (T6 -T4 ) - Z( T6 -T5 ) ] + P5 [ Z (T6 -T5 ) ] We are using the calculated results as a measure of device survivability so how reliable are the results? The main assumption is that current flow is uniform across the device area so that temperature is also assumed uni- form. This means that current pulses must be wide enough to allow the thyristor to reach full conduction. For small thyristors of a few mm diameter this is easily achievable for pulses of less than 1ms. With larger diameter devices e.g. 30 to 100mm, pulses of several milliseconds are required. For most converter applications this presents no restriction. Another possible source of error is the potential inaccuracy of the transient thermal impedance curve, par- ticularly at times of 1 to 10ms. It is very difficult to measure this part of the curve so calculation is used. A transmission line model is assumed but since it is difficult to assign accurate values to the various contact thermal resistances between metallic parts conservative values are used. Val- ues depend on surface finishes and clamping forces. For times longer than about 100ms heat generated at the junction starts to pass into the cooling fin. This is not accounted for in this particular model. Calculation of temperature rise for short pulses requires more complex 2 and 3 dimensional analysis, possibly in- volving finite element analysis techniques. Device turn-on behaviour and its dependency on voltage, temperature, di/ dt and gate drive has to be taken into account. TGD-1XAN4870 Application Note 5/5 CUSTOMER SERVICE CENTRES France, Benelux, Italy and Spain Tel: +33 (0)1 69 18 90 00. Fax: +33 (0)1 64 46 54 50 North America Tel: 011-800-5554-5554. Fax: 011-800-5444-5444 UK, Germany, Scandinavia & Rest Of World Tel: +44 (0)1522 500500. Fax: +44 (0)1522 500020 SALES OFFICES France, Benelux, Italy and Spain Tel: +33 (0)1 69 18 90 00. Fax: +33 (0)1 64 46 54 50 Germany Tel: 07351 827723 North America Tel: (613) 723-7035. Fax: (613) 723-1518. Toll Free: 1.888.33.DYNEX (39639) / Tel: (831) 440-1988. Fax: (831) 440-1989 / Tel: (949) 733-3005. Fax: (949) 733-2986. UK, Germany, Scandinavia & Rest Of World Tel: +44 (0)1522 500500. Fax: +44 (0)1522 500020 These offices are supported by Representatives and Distributors in many countries world-wide. ) Dynex Semiconductor 2000 Publication No. AN4870-2 Issue No. 2.0 January 2000 TECHNICAL DOCUMENTATION - NOT FOR RESALE. PRINTED IN UNITED KINGDOM HEADQUARTERS OPERATIONS DYNEX SEMICONDUCTOR LTD Doddington Road, Lincoln. Lincolnshire. LN6 3LF. United Kingdom. Tel: 00-44-(0)1522-500500 Fax: 00-44-(0)1522-500550 DYNEX POWER INC. Unit 7 - 58 Antares Drive, Nepean, Ontario, Canada K2E 7W6. Tel: 613.723.7035 Fax: 613.723.1518 Toll Free: 1.888.33.DYNEX (39639) This publication is issued to provide information only which (unless agreed by the Company in writing) may not be used, applied or reproduced for any purpose nor form part of any order or contract nor to be regarded as a representation relating to the products or services concerned. No warranty or guarantee express or implied is made regarding the capability, performance or suitability of any product or service. The Company reserves the right to alter without prior notice the specification, design or price of any product or service. Information concerning possible methods of use is provided as a guide only and does not constitute any guarantee that such methods of use will be satisfactory in a specific piece of equipment. It is the user's responsibility to fully determine the performance and suitability of any equipment using such information and to ensure that any publication or data used is up to date and has not been superseded. These products are not suitable for use in any medical products whose failure to perform may result in significant injury or death to the user. All products and materials are sold and services provided subject to the Company's conditions of sale, which are available on request. All brand names and product names used in this publication are trademarks, registered trademarks or trade names of their respective owners. http://www.dynexsemi.com e-mail: power_solutions@dynexsemi.com Datasheet Annotations: Dynex Semiconductor annotate datasheets in the top right hard corner of the front page, to indicate product status. The annotations are as follows:- Target Information: This is the most tentative form of information and represents a very preliminary specification. No actual design work on the product has been started. Preliminary Information: The product is in design and development. The datasheet represents the product as it is understood but details may change. Advance Information: The product design is complete and final characterisation for volume production is well in hand. No Annotation: The product parameters are fixed and the product is available to datasheet specification.




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