Performance Comparison of SiC Schottky Diodes and Silicon Ultra Fast Recovery Diodes

Transcription

1 Performance Comparison of SiC Schottky Diodes and Silicon Ultra Fast Recovery Diodes Marek Adamowicz 1,2, Sebastian Giziewski 1, Jedrzej Pietryka 1, Zbigniew Krzeminski 1 1 Gdansk University of Technology 2 Gdynia Maritime University Narutowicza 11/12, Gdansk, POLAND Morska 81-87, , Gdynia, POLAND zkrzem@ely.pg.gda.pl, lider@ely.pg.gda.pl madamowi@am.gdynia.pl Abstract- Advanced control systems combined with high speed gate driver circuits enable extremely high rate of change of power devices voltages, up to hundreds of kv/us. Short rise times of power devices could cause significant EMC problems, which are unacceptable in majority of power electronics applications. It is known that voltage variations during diode switch-off depend on how long it takes for the charge stored near the p-n junction to be recovered during voltage reversing. In fast switching applications good forward recovery characteristics are needed. The silicon carbide (SiC) diodes characterize almost zero reverse recovery charge. However the lossless operation in connection with extremely high dv/dt could cause the SiC diodes less effective in damping the voltage ringing. The compromise between high efficiency and low EMI emission is therefore the actual aim of the research. The paper compares the static and dynamic characteristics of ultra fast silicon (Si) and SiC Schottky diodes and presents the study of the mechanism of parasitic high frequency oscillations during turn-off transient. I. INTRODUCTION The hard-switched insulated gate bipolar transistors (IGBTs) and metal-oxide semiconductor field-effect transistors (MOSFETs) integrated with free-wheeling diodes made in silicon (Si) technology have been commonly used in the power converters of electrical drives and renewable generation systems for last two decades. IGBTs, with breakdown voltages above 1kV, have been historically preferred in high-voltage, low-frequency applications (<20kHz) due to efficiency restrictions. MOSFETS have been mainly chosen for high-frequency (up to hundreds khz), low-voltage and low output power applications due to higher on-state losses. However, the recently introduced low on-resistance MOSFETs characterizing high blocking voltage (up to 900V) and high dv/dt capability can also be used in high-current high power applications. The significant part of the overall losses of Si-based power converter are the reverse-recovery switching losses of Si diodes [1]. The reverse recovery of Si diodes affects the transistor causing additional turn-on losses [2] and lead to a significant amount of noise (EMI) in the system. The recovery softness factor (RSF) is determined [3] for power diodes as the ratio of reverse recovery current fall time to reverse recovery charge Q RR removal time. Low RSF may indicate that the diode will produce large amplitude voltage spike due to snappy recovery [3]. The snap-off in the Si Ultrafast diode causes oscillations in the IGBT voltage, which generate EMI. Excessive voltage spikes during turn-off process even can destroy the diode. These phenomena can be attenuated by the use of ultra fast soft recovery Si diodes. The diode has a soft recovery characteristic for RSF equal 1 or greater. At present the performance improvement in power converters is also accomplished through the change of semiconductor material [1]-[9]. The application of silicon carbide (SiC) to Schottky diodes reduces their reverserecovery current almost to zero and greatly improves the efficiency of power converter [1], [9]. Power electronic devices made in SiC technology can operate at high temperatures, very high voltages and very high switching frequencies. However, the introduction of high frequency SiC devices involves also an area of scientific problems. The very high dv/dt in connection with parasitic parameters of the circuits also would cause possible ringing and noticeable radiated EMI in a high frequency range (30MHz - 2GHz). The paper compares the static and dynamic characteristics of ultra fast Si and SiC Schottky diodes operated as the free-wheeling diodes with fast-switching normally-off SiC JFETs. The performance of 100kHz SiC JFET-based inverter is shown in the paper. II. STATIC CHARACTERISTICS Three diodes: two SiC Schottky diodes S1 and S2 and one ultra fast recovery Si diode S3 have been investigated for their potential use in high frequency SiC JFET based inverter. Table I shows the ratings of investigated diodes and two transistors used in laboratory test circuits. The investigated SiC Schottky power diodes S1 and S2, capable of carrying currents of 20A, have been fabricated by parallel connection of two diode elements on a single die. The parallel operation results in slight decrease in the forward voltage. The SiC Schottky diodes characterize almost zero reverse recovery charge and has only a small capacitive charge Q c of the junction. The first SiC Schottky diode (S1) has total capacitive charge Q c1 =122nC and active area of 2*0.0489cm 2. The second investigated SiC diode (S2) has total capacitive charge Q c2 =129nC at typical conditions of V R =1200V and di/dt =500A/µs.

2 TABLE I DIODES UNDER TEST AND TRANSISTORS USED IN TEST CIRCUITS Abbreviation Voltage Ratings Current Ratings at 100 Parameters SiC Schottky Q S1 1200V 20A c=122nc at Diode 1 di/dt=500a/µs SiC Schottky Q S2 1200V 20A c=129nc at Diode 2 di/dt=500a/µs Si Ultra Fast Q S3 1200V 30A rr=2000nc at Recovery Diode di/dt=500a/µs SiC JFET T1 1200V 30A R DS(on)=0.063Ω E TS,typ=440µJ Si MOSFET T2 900V 25A RDS(on)=0.14Ω The reverse recovery charge Q rr of third investigated ultrafast Si diode (S3) varies depending on: - rate of change of current through zero crossing di F /dt, - value of forward current i F, - temperature. The Q rr of S3 starts from 500nC for junction temperature of 100 C and i F = 15A and reaches over 2000nC for i F = 30A and high di F /dt. Fig. 1 describes the test circuit for determination of diodes static characteristics. Unlike S3 the Q c of SiC Schottky diodes is almost independent on these boundary conditions. Measurements of the static characteristics were carried out in the test circuit shown in Fig. 1. The circuit was supplied from low voltage V DC = 60V. A Si MOSFET transistor was used to discharge the capacitors through the 2.2Ω resistor and the investigated diode (DUT -diode under test) during the test. Fig. 2. i-v characteristics of first investigated SiC Schottky diode S1 at different operating temperatures. Fig. 3. i-v characteristics of second investigated SiC Schottky diode S2 at different operating temperatures. Fig. 1. Low voltage test circuit; DUT device under test, R G =10Ω; SW is used to charge the capacitors to the voltage set on the DC supply. The i-v characteristics of three investigated diodes S1, S2 and S3 were obtained at different temperatures in the 25 C to 125 C ambient temperature range. These characteristics are shown in Fig. 2 - Fig. 4. The features of SiC diodes and ultra fast silicon diode differ significantly. The most important difference is the positive temperature coefficient of the forward voltage of the SiC diodes. The positive temperature coefficient allows to operate SiC diodes in parallel. Fig. 4. i-v characteristics of investigated ultra fast recovery diode S3 at different operating temperatures. That enables the increase of power rating of power converters using commercially available (<30A) SiC devices.

3 The i-v curves of S1 and S2 are almost linear and deviate from exponential behaviour seen for S3. S1 and S2 differ in forward voltage drop. S2 characterizes lower forward voltage than S1. III. DYNAMIC CHARACTERIZATION The introduction of high frequency SiC devices characterizing high dv/dt involves wide area of scientific problems connected with the impact of parasitic inductances and capacitances on the switching performance and EMI problems. The application of zero reverse-recovery current SiC Schottky diodes improves the efficiency of the power converter, however, the possible oscillations and resonances during high speed switching are the source of noticeable EMC/EMI emission in the converter system. The current oscillations of high magnitude, caused by high dv/dt and circuit parasitics decrease the converter performance and might cause the devices failure and breakdown. The locations of possible failure events and EMI problems connected witch applications of high speed SiC devices within the power converter are shown in Fig. 5. Fig. 6. Schematic of the high voltage test circuit. Fig. 7. Experimental test setup Fig. 5. The occurrence of failure events and EMI problems in SiC based power converter: (1)gate driver failures caused by oscillations/emi problems in gate driver signals; (2), (3) device failures caused by oscillations/emi problems during switching; (4) IM bearings failures caused high dv/dt and common mode voltages. To predict the scale of possible failure events and EMI problems, the dynamic characteristics of different combinations of Si and SiC diodes with SiC JFET were measured in the high voltage test circuit described in Fig. 6. The current and voltage waveforms were observed and recorded using Tektronix DPO4104 oscilloscope equipped with TCP MHz current probe, P5205 high voltage differential and P6931A 500MHz voltage probe. The experimental test setup is shown in Fig. 7. The FR4 double layer PCB was used for realization of the test circuit. From the experimental test setup the switching waveforms of S1 and S2 and T1 were obtained to understand the switching features of investigated SiC devices. The gate resistor R 1 was 2.7Ω for both investigated combinations: T1 - S1 and T1 - S2. The measured switching characteristics of the investigated diodes S1-S3 are shown in Fig. 8 - Fig. 10. The turn-off waveforms of Si diode S3 are shown in Fig. 8. Fig. 8. Turn-off waveforms of combination III (SiC transistor T1 and Si diode S3): Diode S2 anode to cathode voltage V A-K and diode current I K.

4 a). b) c) d) e) Fig. 9. Switching waveforms of combination I (SiC transistor T1 and SiC diode S1): Diode S1 anode to cathode voltage V A-K and diode current I F during turnoff (a) and turn-on (c); SiC JFET (T1) gate to source voltage V G-S, drain to source voltage V D-S and drain current I D during turn-on (d) and turn-off (e); schematic of directions of measured voltage and currents. a) b) c) d) Fig. 10. Switching waveforms of combination II (SiC transistor T1 and SiC diode S2): Diode S2 anode to cathode voltage V A-K and diode current I K during turn-off (a) and turn-on (b); SiC JFET (T1) gate to source voltage V G-S, drain to source voltage V D-S and drain current I D: during turn-on (c) and turn-off (d).

5 As it can be seen from Fig. 8, the combination of T1 and S3 diode was not able to carry currents higher than 2A due to high magnitude of reverse recovery current (>24A) during the S3 diode turn-off and considerable voltage oscillations (>1.5kV in peak) observed in the test circuit. The Q rr of the ultra fast Si diode at di/dt >1.2kA/µs was unacceptable. The high dv/dt (>100kV/µs) and di/dt (>1.6kA/µs for S1 and >1.9kA/µs) values can be observed during turn-off transients of both investigated SiC diodes. As it can be seen from Fig. 9c and Fig. 10b, the turn-on waveforms of two SiC diodes are almost identical. This is because the SiC JFET turn-off process is not so much influenced by diode performance and does not depend considerably on diode characteristics. It can be seen from Fig. 10a and Fig. 10c that noticeable oscillations appear during transistor switch-on and simultaneous diode turn-off in the investigated system of T1 and diode S2 (combination II). S1 and S2 characterize similar current and voltage ratings and similar Q C however differ in the damping of the ringing. The observed oscillations in main circuit influence gate voltage V G-S waveforms and the gate circuit should be protected with the use of additional Zener diode. It can be deduced that V G-S ringing is caused by high magnitude voltage variations on SiC JFET source lead parasitic inductance. The insufficient damping of voltage and current oscillations might be explained by a very small diode losses dissipated during switching and very fast run of switching process. These undesirable phenomena can be attenuated by slowing down the switching process [10]. a) b) Fig. 11. Hardware prototype of normally-off SiC JFET-based DC-AC converter (a) and ADSP based control card (b). Unlike IGBT-based inverters where switching frequencies of 2 to 10 khz in combination with long generator leads (>5m) involve the application of large passive output filters, the increased switching frequency (>100kHz) in SiC JFET-based DC-AC converter considerably reduces the passive filter size. Fig. 12 shows output currents and voltages of the investigated SiC-based inverter fed small induction machine and equipped with small size 60μH (15 turns) output inductors. IV. PERFORMANCE OF SIC DIODES IN HIGH-FREQUENCY SIC-BASED INVERTER The picture of designed and investigated three phase SiC JFET-based inverter with applied S2-type SiC Schottky diodes is shown in Fig. 11a [10]. Six pairs of transistors and free-wheeling diodes are mounted together with DC-link capacitors and current transducers on main round-shaped printed circuit board (PCB). A secondary round-shaped PCB containing six fast-switching two-stage DC-coupled gate drivers is fixed to the main PCB. The ADSP (333MHz) based universal control card containing programmable logic unit CYCLONE II EP2C8F256 (Fig. 11b), worked out at Gdansk University of Technology, is used to control SiC-based inverter. The control system hardware enables three independent interruptions: IRQ1, during which the analog to digital conversion is executed of voltage and currents measurements (every 5µs), IRQ2 executing PWM algorithm (every 10µs) and IRQ3 during which state observer differential equations are solved and power flow control with generator control procedures are executed (every 50µs). 2A/div; 250V/div; 2ms/div a) 2A/div; 250V/div; 2us/div b) Fig. 12. Phase output current and line-to-line output voltage of investigated high frequency inverter with applied S2-type SiC Schottky diodes.

6 V. CONCLUSIONS Three 1.2kV fast diodes: two SiC Schottky diodes and one ultra fast recovery Si diode were investigated for the purpose of potential application in high speed SiC-JFET based inverter. The SiC diodes characterize better performance during transients. While the first investigated SiC Schottky diode has larger voltage drop during conduction which could cause higher on-state losses, the second SiC diode has worse damping coefficient during switching. Unlike SiC diodes the ultra fast recovery Si diode, when applied to SiC JFET as e free-wheeling diode, was not capable to carry currents over 2A. The preliminary tests of SiC JFET based inverter equipped with SiC Schottky diodes and small passive filter were completed successfully. ACKNOWLEDGEMENT This scientific work is financed from resources for science in years as a LIDER research project of the National Centre for Research and Development. REFERENCES [1] Elasser A., Kheraluwala M.H., Ghezzo M., Steigerwald R.L., Evers N.A., Kretchmer J., Chow T.P., "A comparative evaluation of new silicon carbide diodes and state-of-the-art silicon diodes for power electronic applications" IEEE Transactions on Industry Applications, Vol. 39, No 4, 2003, pp [2] S. Bontemps, A. Calmels, S. D. Round, J. W. Kolar, Low profile power module combined with state of the art MOSFET switches and SiC diodes allows high frequency and very compact threephase sinusoidal input rectifiers., Proc. of the Conference for Power Electronics, Intelligent Motion, Power Quality (PCIM'07), Nuremberg, Germany, May 22-24, CD-ROM (2007). [3] Jovalusky J.," New low reverse recovery charge (QRR) highvoltage silicon diodes provide higher efficiency than presently available ultrafast rectifiers," Proc. Twenty-Third Annual IEEE Applied Power Electronics Conference and Exposition APEC 2008, pp [4] Johnson C.M., Rahimo M., Wright, N.G., Hinchley, D.A., Horsfall A.B., Morrison, D.J., Knights A., "Characterisation of 4H-SiC Schottky diodes for IGBT applications," Proc. IEEE Industry Applications Conference, 2000, Vol. 5, pp [5] Lu B., Dong W., Zhao Q., Lee F.C., "Performance evaluation of CoolMOS and SiC diode for single-phase power factor correction applications," Eighteenth Annual IEEE Applied Power Electronics Conference and Exposition APEC '03, 2003, Vol. 2, pp [6] Galigekere, V.P.; Kazimierczuk, M.K., " Performance of SiC Schottky diodes," 50th Symposium MWSCAS 2007, pp [7] Millan J., Banu V., Brosselard P., Jorda X., Perez-Tomas A., Godignon P., " Electrical performance at high temperature and surge current of 1.2 kv power rectifiers: Comparison between Si PiN, 4H-SiC Schottky and JBS diodes," Proc. IEEE Semiconductor Conference, CAS 2008, pp [8] B. Ozpineci, M. S. Chinthavali, L. M. Tolbert, A. Kashyap, H. A. Mantooth, "A 55 kw Three-Phase Inverter with Si IGBTs and SiC Schottky Diodes," Proc. IEEE Applied Power Electronics Conference and Exposition APEC '06, 2006, pp [9] J. Richmond, " Hard-Switched Silicon IGBTs? Cut Switching Losses in Half with Silicon Carbide Schottky Diodes," Application Note CPWR-AN03, 2006, [10] M. Adamowicz, S. Giziewski, J. Pietryka, M. Rutkowski, Z. Krzeminski, "Evaluation of SiC JFETs and SiC Schottky Diodes for Wind Generation Systems", accepted for publication at 20th IEEE International Symposium on Industrial Electronics ISIE'2011, June, 2011, Gdansk, Poland.