JOINT-STOCK COMPANY «ELECTROVIPRYAMITEL» www.elvpr.ru Development and Investigations of Light Triggered Thyristors for Pulse Application V.V. Chibirkin 1, V.A. Martynenko 1, A.A. Khapugin 1, A.V. Konuchov 2, S.A. Tundykov 1, A.V. Grishanin 1, R.Sh. Enikeev 3, R.A. Serebrov 3 1 Electrovypriamitel JSC, Proletarskaya str., 126, 430001, Saransk, Russia, Tel: +7-8342-480-733 Fax: +7-8342-480-733, Email: martin@moris.ru 2 The V.I. Lenin All-Russian Electrotechnical Institute, Krasnokazarmennaya str., 12, 111250, Moscow, Russia, Tel. +7-495-3619567, Fax: +7-495-3619407, Email: cont@vei.ru 3 The D.V. Efremov Scientific Research Institute of Electrophysical Apparatus, Metallostroy str., 3, 196641, Saint Petersburg, Russia, Tel. +7-812-4644470, Fax: +7-812-4644623, Email: enikeevrustam@rambler.ru Abstract This report presents the results of development and investigations of power LTT (Light Triggered Thyristor) for pulse applications. LTTs are often used in high voltage applications where a series connection of devices is required. Light triggered thyristors should be preferred as solid-state switches because of the advantage of high voltage blocking and the easy way of light triggering. LLTs allow simple and reliable circuits in power transmissions and super-high power pulse commutation units [1,2]. This work continues power LTT developments in JSC Electrovypriamitel and All-Russian Electrotechnical Institute [3]. The design of power fast LTT with 4 basic unit for pulse commutation up to 100 ka 700 μs is presented. 1. Introduction The reliable switches for commutation of short pulses more than 10 2 ka are highly demanded in modern power pulse technique. Solid state switches are very attractive for this purpose because of long term reliability, low operating costs and ecological safety. These devices may be used in equipment for ultra-high electromagnetic fields creation, in power laser supply and other pulse circuits. Our tests have shown that power light triggered thyristors are suitable as solid-state switches for super-high power commutation in sub-millisecond range and work reliably without degradations of electrical parameters. A thyristor in pulse application should commutate the single or seldom current pulses with near maximal permissible surge amplitude. Very large power dissipation restricts the commutation capability as well as on-state spreading velocity. The total turn-on time depends on silicon structure electro-physical properties, cathode emitter shorts density, gate structure layout, current amplitude and other factors. Simulation and experimental studies were carried out for increasing maximum current amplitude and total turn-on time minimization. The LTT cathode layout is similar to traditional electrically-triggered thyristors (ETT). The main difference between ETTs and LTTs is the light sensitive gate area, which is used for triggering of LTT by the optical signal delivered from the fiber optical cable. The optimized design of LTT gate permits high di T /dt-capability, low turn-on power losses and high optical sensibility. Special care in power light triggered thyristors optimization is taken with respect to switching ruggedness. 2. LTT layout Fig.1 shows the basic unit layout of LTT for pulse applications. A light-sensible region is at the centre of semiconductor structure. The characteristic features of LTT layout are high degree interdigitated gate and four stage amplifying gate. The interdigitated gate improves a turn-on spreading time and losses. The auxiliary thyristors amplify low gate current, arising due to charge carrier generation by optical irradiation of the light-sensible region.
Fig. 1: Silicon structure layout of LTT The silicon structure layout and the diffusion layer parameters have been optimally designed for optical sensibility improving, turn-on losses minimization and high di T /dt capability. A high di T /dt capability was reached owing to the special amplifying gate design and two integrated in LTT structure resistors for current limitation at turn-on beginning. 3. Simulations and design Performance and reliability of thyristors for pulse commutation with high di T /dt depend on the dynamic temperature distribution in a semiconductor structure. The high current density during turn-on leads to the overheating in amplifying gate region and at inner rand of main cathode. The simulation was carried out to define overheating locations in the LTT central region. The simulation results are shown on fig. 2. The red curve is the temperature distribution in the optimized current limiting resistors. The blue curve corresponds to LTT structure without current limiting resistors. Fig. 2: Simulation of LTT central region and radial temperature distribution at maximum power dissipation moment As can be seen from the simulation results, the main di T /dt limiting factor is a significant overheating of gate region at turn-on beginning instant. The first amplifying gate stage is the most overheated. The temperature of this region can rise at high di T /dt up to the critical value, leading to current filaments and thyristor destruction. A current filament arises when the thermal generation becomes main conductivity modulation mechanism. The critical temperature for a current filament arising is 400-600ºC. Overheating of the first amplifying stage depends on the integrated current limiting resistors. The resistance values can be adjusted by means of trench mesa-etching in p-base. From fig. 2 can be seen that the resistor R1 is warmed up to near 120ºC. The increase of current limiting resistor value allows 30ºC temperature reducing. Fig. 3 shows experimental trade-off between resistance value and etching depth. Fig. 4 shows the experimental trade-off between minimum anode voltage and current limiting resistor.
Fig. 3: Trade-off between value of current limiting resistors and etching depth in p-base Fig. 4: Experimental trade-off between minimum anode voltage and current limiting resistor As a result of investigations the optimum resistance values have been defined for high di/dt capability and stable turn-on with low anode voltage (V A min 20V). Fig. 5 shows the current and voltage curves during LTT turn-on for unoptimazed and optimized LTT layout. It can be seen that the turn-on delay time increases slightly. Fig. 5: Turn-on simulation of LTT central region Fig. 6: Calculated trade-off between LTT turnon delay time and laser diode wave length LTTs are triggered with IR light pulses. The correct choice of wave lenght is very important for proper LTT operation. It depends on a blocking p-n junction depth. Fig. 6 shows calculated the trade-off between LTT turn-on delay time and laser diode wave length (pulse power 200 mw). The simulation results show that the optimum wave length for LTT triggering lays between 1.0 and 1.05μm. The wave length more than 1.1μm is uneffective because photons energy is too small for electron-hole pairs generation. The light effectiveness decreases also because of low penetration when the wave length is under 1.0μm. As a result electron-hole pairs recombine near surface. The radiation spectrum of commercial laser diodes corresponds to this optimum. Very important problem by LTT development is the high light sensitivity of the triggering region because output optical power of a light cable system is much lower as a gate input electric power. The high light sensitivity thyristor becomes very sensible to interferences and has low off-state dv D /dt capability. For dv D /dt increasing the light sensible region must be small. At the same time the small size of this region limits initial turn-on square and decreases di T /dt capability. This problem was successfully overcame by the LTT development. LTT must have high optical sensibility and low turn-on power losses on the one hand, and high dv D /dt capability and positive temperature dependence of blocking voltage on the other hand. The developed LTTs are really triggered with optical power no more as 20mW and maintain dv D /dt = 5000V/μs. These demands were fulfilled owing to optimum diffusion profiles, light sensible layout, cathode regions, photocurrent amplifying elements and amplifying gate.
4. Experimental results The LTT was tested in pulse conditions for commutation limits definition. Fig. 7 shows the anode current curve and the light triggering pulse during critical on-state di/dt value measuring. The laser diode with wave length λ = 0.88 μm and pulse power 150mW was applied for LTT triggering in the test circuit. It can be seen from fig.7 that the developed thyristor allows the pulse commutation with current rate of rise up to 5000A/μs. The test with greater di/dt was impossible because of circuit restrictions. Fig. 7: LTT anode current and triggering light pulse The developed LTTs were tested in high voltage stack from 3 LTT in series (fig. 8). The stack was designed for the capacitive storage device with voltage 5kV and storage energy 50kJ. Fig. 9 shows the electric circuit of the capacitive storage device. It was found during LTT tests in the condenser battery that a thyristor turn-off overvoltage was twice much as a charging voltage 5kV. This dangerous overvoltage can destroy LTT. It arises because of a current fall in the inductor L during LTT reverse recovery. Snubber RC-circuits were used for LTTs protection from overvoltage. Fig. 10 shows the LTT protection circuit for reverse recovery. RC-circuit components and reverse recovery charge were estimated by computer simulation with 3 LTT in series. RC-components were modified (C = 0 4 μf, R = 2 5 Ohm) and two LTT versions were used: Version 1 structure without reverse recovery charge controlling (Q RR = 20 mc, V TM = 2.0 V) Version 2 structure with reverse recovery charge controlling (Q RR = 10 mc, V TM = 2.4 V) As a result of simulation (fig. 11) the optimum version was chosen: LTT version 2, snubber components C = 3 4 μf, R = 2 Ohm, overvoltage V O 7.5 kv. Fig. 8: Power stack for capacitor battery discharge Fig. 9: Equivalent test circuit for LTT
Fig. 10: Equivalent protection circuit for LTT reverse recovery Fig. 11: Calculated reverse recovery current and voltage waves of high voltage LTT The tests in practical circuits show that the increased RC-circuit have weak influence on commutation overvoltage. Main method of overvoltage decreasing is LTT reverse recovery charge lowering. A reverse recovery charge of thyristor is proportionate to the carriers life time. The modern practical methods of life time controlling in silicon are based on high energy particle irradiation. The demanded reverse recovery charge values were obtained by electron irradiation of LTT silicon structures. This method gives narrow Q RR dispersion and decreases commutation overvoltage. The optimum reverse recovery charge provides safely LTT operation without dangerous overvoltage. The power stack with 3 LTT was tested in a capacitive storage device circuit for determination of maximum allowable pulse currents in sub-millisecond range. The LTT stack withstood 500 storage device discharges with pulse current magnitude 106 ka. Fig. 12 shows corresponding current and voltage curves across LTT. Fig. 12: Current and voltage curves during 100 ka current commutation Table 1 shows key parameters of the developed LTT. Table 1: Key parameters of LTT V DRM, V RRM, V 4200 @ T J = -40..+125 C I T(AV), A 2500 @ T C = 70 C, sin, t p = 10 ms I peak-pulse, ka 100 @ T J = 25 C, sin, t p up 700 µs I TSM, ka 50 @ T J = 125 C, tp = 10 ms V TM, V 2.4 @ I T = 7850 А, T j = 125 C P LM, mw 40 @ V D = 100 V (dv D /dt) crit, V/µs 2000 @ V D = 0.67V DRM, T j = 125 C (di T /dt) crit, A/µs 5000 @ V D = 0.67V DRM, I T = 8000 А, P LM = 40 mw, t L = 10 µs, t rise = 0.5 µs, T j = 25 C
t q, µs 320 @ I T = 2500 A, di T /dt = - 5 A/µs, V R 100 V, V D = 0.67V DRM, (dv D /dt) = 50 V/µs, T j = 125 C Q RR, µc 2400 @ I T = 2500 А, di T /dt = - 5 A/ µs, V R 100 V, T j = 125 C R thjc, C/W 0.065 @ DC, double side cooled 5. Conclusion This report presents the development and investigation results of power LTTs for pulse applications. The tests confirm high reliability and prospects of LTTs. The LTTs development is now in progress. Further LTTs optimization will increase the maximum commutation currents up to 1.5-2 times. 6. Literature [1] J. Przybilla, R. Keller, U. Kellner, H.-J. Schulze, F.-J. Niedernostheide, T. Peppel, Direct lighttriggered solid-state switches for pulsed power applications, Digest of Technical Papers 14th IEEE International PPC-2003, Vol.1, 2003, pp. 150-154. [2] J. Dorn, U. Kellner, F.-J. Niedernostheide, H.-J.Schulze, State of the Art Light Triggered Thyristors with Integrated Protection Functions, Power Electronics Europe Issue2, 2002, pp. 29-35. [3] V. Martynenko, A. Khapugin, A. Grishanin, V. Chibirkin, A. Konuchov, I. Veselova, А. Surma, The development of power thyristors with direct control of lighting and protection functions, Power electronics, 5, 2009, pp. 8-14.