2952 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 9, SEPTEMBER 2015
Analysis of Low- and High-Frequency Oscillationsin IGBTs During Turn-ON Short Circuit
Carmine Abbate, Giovanni Busatto, Annunziata Sanseverino, Francesco Velardi, and Cesare Ronsisvalle
Abstract— The purpose of this paper is to present a detailedexperimental and numerical study on the mechanisms involvedin Insulated Gate Bipolar Transistor (IGBT) instability duringturn-ON short circuit that can compromise its robustness inparticular load and driving conditions. It is shown that theIGBT may exhibit oscillations at both low and high frequenciesdepending on different physical mechanisms. Furthermore, a newmethodology that allows determining the stability limits of thedevice in relation with the parameters of the external circuit isproposed. The experimental measurements confirm the obtainedresults.
Index Terms— Oscillations, power semiconductor devices,semiconductor device modeling.
I. INTRODUCTION
OPERATIONS Insulated Gate Bipolar Transistor (IGBT)in short circuit (SC) are limited by the occurrence
of instabilities which may cause electromagnetic interfer-ence (EMI) problems and in some cases the failure of thedevice [1], [2].
These instabilities are accompanied by the oscillations thatappear on the current and voltage waveforms in specificconditions and for appropriate parameters of the externalcircuit. In the literature, the oscillations are always associatedwith the presence of a negative capacitance at the inputleads of the IGBT under test. The origin of this negativecapacitance was first attributed to the hole current coming fromthe collector at high collector voltage conditions that wouldproduce a positive charge accumulation under the gate [3], [4].Such a charge recalls an equivalent negative charge in thegate electrode resulting in the decrease of the gate chargewhen the gate voltage is increased. More recently, the negativecapacitance was also attributed to the Miller capacitanceof the device, which, in particular frequency ranges, maycause a positive feedback between the output (collector) andinput (gate) terminal thus causing the circuit instability [5]–[7].
These works focused on the device behavior during SCinstability and did not correlate this behavior with thefrequency range where the instabilities take place. In [5],
Manuscript received September 18, 2014; revised June 19, 2015; acceptedJuly 16, 2015. Date of publication August 4, 2015; date of current versionAugust 19, 2015. The review of this paper was arranged by Editor F. Udrea.
C. Ronsisvalle is with Fairchild Semiconductor GmbH, Aschheim 85609,Germany (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TED.2015.2459135
a fixed frequency (higher than tens of kilohertz) was used todetermine the input capacitance as a function of the biasingconditions and to identify the instability regions of the device.An attempt to extract stability conditions to be used for thedevice design was done in [6]. However, these conditionswere extracted by observing the behavior of the device atlow frequency and extending the results to the high-frequencyrange.
The objective of this paper is to perform an experimentaland numerical investigation in a wide range of frequencyin order to understand the physical phenomena that underliethe occurrence of oscillations in SC condition. It is finallydemonstrated that the physical mechanisms involved inlow- and high-frequency oscillations are much different fromeach other, and that the external circuit plays a relevant rolein the device instability. This paper deepens the preliminaryresults presented on PCIM Conference evidencing this doublebehavior for the first time [8].
II. DETERMINATION OF THE INSTABILITY CONDITIONS
Let us consider, in Fig. 1, a real circuit that includes thedriver, the load circuit, and the parasitic elements connectedto the emitter lead of the IGBT.
The oscillations are always associated with the resonanceof the circuit, which can be either a series or a parallel oneaccording to the physical behavior of the circuit.
In a series resonance, the circuit has a capacitive behaviorfor the frequencies lower than the resonance one and inductivefor higher. The circuit behaves in the opposite way in a parallelresonance.
In the following equations, we assume a series resonanceso we can schematize the elements of the circuit by means oftheir impedances. This schematization is suitable to describethe high-frequency oscillations [9].
From the theory of linear oscillators, with reference to thecircuit of Fig. 1, we can write that the condition for havingpersistent oscillations is
Zdr = −Z in (1)
where Zdr is the output impedance of the driver circuit andZ in is the input impedance of the device, including theload circuit and the parasitic elements, connected to theemitter lead.
To compute Z in, we refer to the well-known transformationrelationship
ABBATE et al.: ANALYSIS OF LOW- AND HIGH-FREQUENCY OSCILLATIONS IN IGBTs 2953
Fig. 1. Block diagram of the analyzed circuit.
whereas Z E and Zout are the emitter and the load impedancesand Z11, Z12, Z21, and Z22 are the elements of matriximpedance of the device seen as a dual dipole in isolatedconditions which correspond to the open circuit both at theinput and output side.
Equation (1) can be written in terms of real and imaginarypart of the two impedances
Imag(Zdr) + Imag(Z in) = 0 (3)
Real(Zdr) + Real(Z in) = 0. (4)
Equation (3), which involves the imaginary part of Zprovides the frequency at which the circuit oscillate.We can use (3) either to calculate the oscillation frequency ofa circuit with assigned parameters or to identify the inductanceof the driver circuit, which leads to the oscillations at a givenfrequency.
On the other side, (4) is the condition to have stableand persistent oscillations. It is worth to note that whenReal(Zdr) + Real(Z in) > 0, the circuit has a damped behavior,whilst the oscillations are diverging when Real(Zdr) +Real(Z in) < 0. This latter case is the most interesting in areal circuit. In fact, the above analysis refers to a linearcircuit and applies only for the small signal operations. In thereal circuit, the IGBT operates in a nonlinear mode, and thecondition Real(Zdr) + Real(Z in) < 0 is needed to triggerdiverging oscillations, as it happens in all the high-frequencyexperimental observations for which the stable oscillationsare always preceded by a diverging phase. The oscillationsstabilize when the nonlinearity of the circuit limits furtherincreases in the oscillation amplitude.
III. PHYSICS OF THE SHORT CIRCUIT INSTABILITIES
The possibility of being able to impose the conditionsfor oscillation provides us the opportunity to investigate thephysical phenomena that are the basis of this behavior.
For this purpose, we have simulated the behavior ofa 1200 V trench gate field stop IGBT inserted in a suitablecircuit.
A detail of the simulated elementary cell, to which weattribute an area of ∼11 mm2, and the schematic of thecircuit used for the simulations, are reported in Figs. 2 and 3,respectively. The device presents a dummy cell for each
Fig. 2. Sketch of the gate region of the simulated elementary cell.
Fig. 3. Schematic of the simulated circuit.
active cell. Simulations were carried out using the SynopsysTCAD software [11].
A preliminary ac analysis of the device at fixed gate andcollector bias conditions Vge = 10 V and Vce = 400 Vwas performed in order to evaluate the impedance matrix Z11,Z12, Z21, and Z22. Successively, Z in was calculated by (2)including the effects of the load and parasitic elements.
At this stage for emphasizing the behavior of the devicewith respect to the influence of the external circuit, a loadthat approximates an ideal SC was simulated. Successively,the same procedure will be used to assess the stability of thedevice under realistic conditions.
The turn-ON in SC condition was simulated imposingLc = 10 nH and Rc = 0.1 �, in addition, a parasiticinductance Le = 1 nH and a parasitic resistance Re = 0.1 �at the emitter terminal were considered.
Once Z in has been identified, being for a real driverReal(Zdr) > 0 and Imag(Zdr) > 0 due to its inductive nature,we can recognize the frequencies, where the deviceshows stable and persistent oscillations by identifying theregion where both Imag(Z in) and Real(Z in) are negative.Fig. 4 reports the plot of the simulated real and imaginary partof the input impedance Z in as a function of the frequency.
As one can see, this condition can be achieved at both lowand high frequency.
First, we have calculated the input circuit parameterscausing low-frequency instabilities. Setting as an example afrequency f = 30 kHz, we obtain for the driving circuita gate resistance Rg = 38.8 � and a gate inductanceLg = 5.25 mH. In this condition, a time-domain analysis wascarried out.
2954 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 9, SEPTEMBER 2015
Fig. 4. Simulated real and imaginary components of the input impedanceversus frequency at Vge = 10 V and Vce = 400 V. Other parameters areLc = 10 nH, Rc = 0.1 �, Le = 1 nH, and Re = 0.1 �.
Fig. 5. Simulated current and voltage waveforms during turn-ON SC at(a) gate terminal and (b) collector terminal. Circuit parameters areRg = 38.8 �, Lg = 5.25 mH, Lc = 10 nH, and Rc = 0.1 �.
Fig. 5 reports the current and voltage waveforms at gate andcollector terminals.
It is interesting to note that there is a direct proportionalitybetween the collector current and the gate voltage when the
Fig. 6. Time evolution of the hole concentration under the gate and voltagedrop across the gate oxide compared with the waveform of the collectorcurrent. The results refer to a cut line at the middle of the trench gate.
latter is positive. This proportionality is even more evidentif we report (see Fig. 6) the voltage drop across the gateoxide, the collector current, and the hole concentration underthe gate in the first 20 μs of the simulation in arbitraryunits. This is a clear indication that the displacement currentnecessary to sustain oscillations is caused by the holes injectedfrom the anode as expected from the physical mechanismdescribed in [6], where the equivalent circuit, not reportedhere for brevity, includes a voltage-controlled current generatoraccounting for the holes reaching the gate oxide coming fromthe collector.
Second, let us analyze the instability condition at highfrequency. The frequency of the oscillation was set atf = 40 MHz for which we obtained: Rg = 0.2 � andLg = 1 nH.
Fig. 7 shows the simulated waveforms at the gate andcollector. The first difference observed in comparison withthe previous case is that the collector voltage exhibits a largevariation. The gate voltage is proportional to the derivativeof the collector voltage thus indicating that it is associatedwith a displacement current involving the output capacitance.The regenerative effect necessary to sustain oscillations is,therefore, due to the Miller capacitance, which causes apositive feedback, and then a regenerative effect betweencollector and gate voltages. The circuit model reported in [6]cannot predict this behavior to describe which a capacitancein parallel with the current generator is required.
This capacitance, in series with the gate oxide capacitance,is influenced by the space charge induced by the electronsand holes current flowing through the base region of theIGBT p-n-p transistor, where a high electric field is observed.Consequently, the results presented in [6] are applicable toevaluate the IGBT stability in low-frequency ranges but theycannot be applied to identify strategies for preventing thehigh-frequency oscillations.
IV. IGBT STABILITY MAP
As demonstrated by the above analysis, the IGBT duringturn-ON SC can oscillate at both low and high frequency.
ABBATE et al.: ANALYSIS OF LOW- AND HIGH-FREQUENCY OSCILLATIONS IN IGBTs 2955
Fig. 7. Simulated current and voltage waveforms during the SC at(a) gate terminal and (b) collector terminal. Circuit parameters areRg = 0.2 �, Lg = 1 nH, Lc = 10 nH, and Rc = 0.1 �.
In real applications, the high-frequency oscillations are muchmore dangerous both because they cause EMI emission and,much more, because they can be stimulated in circuit withstray inductances in the range of few tens of nanohenry whichare typical of a real circuit. On the contrary, the low-frequencyoscillations require much larger and unpractical values ofthe stray inductance to be triggered. For this reason, in thefollowing, we focus on the occurrence of high-frequencyinstabilities considering the device inserting in a realisticcircuit.
We consider a parasitic inductance Le = 40 nH and parasiticresistance Re = 0.1 � at the emitter terminal of the deviceand a stray SC inductance Lc = 56 nH and Rc = 0.1 � atthe collector side. The gate voltage was fixed to Vge = 10 Vand Vce = 400 V.
First, we calculate the input impedance of the device,including the load circuit and the parasitic elements.Fig. 8 shows the real and imaginary components of Z in.As one can see, there is a frequency range that extends upto 50 MHz, where the two terms are negative. Therefore,there are resistance and inductance values that bring the deviceto oscillate. These can be plotted on a graph to obtain aninstability map of the device as a function of bias conditionsand load circuit.
Fig. 9(a) and (b) shows the simulated instability maps ofthe device. They have been obtained for different collector
Fig. 8. Simulated real and imaginary components of the input impedanceversus frequency at Vge = 10 V and Vce = 400 V. Other parameters areLc = 56 nH, Rc = 0.1 �, Le = 40 nH, and Re = 0.1 �.
Fig. 9. Simulated stability map at Vge = 10 V and Vce = 400, 600,and 800 V. Stray inductances (a) Lc = 56 nH and (b) Lc = 100 nH. Forsome points, the oscillation frequency is also reported.
voltages and for two different values of stray SC inductanceLc = 56 nH and Lc = 100 nH. In the figure, the dotsrepresent the combinations of the values of Rg and Lg forwhich the device exhibits the stable and persisting oscillations.The points on the right side of the lines result stable, whereas,on the left side, the circuit exhibits diverging oscillations.
2956 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 9, SEPTEMBER 2015
Fig. 10. Simulated gate capacitance versus gate voltage varying the collectorvoltage for a fixed frequency f = 4 MHz.
Fig. 11. Measured I–V characteristic at Vge = 8 and 10 V.
It is worth to emphasize that our approach has alloweddetermining a region of instability, which would not haveresulted from the mere analysis of the input capacitance ofthe device. Fig. 10 shows the gate capacitance of the devicevarying the gate voltage for a fixed frequency of 4 MHz andfor different collector voltages. The fact that the capacity isalways positive does not prevent that the coupling with thecircuit favors an unstable behavior of the device.
V. EXPERIMENTAL RESULTS
To confirm the simulation results, the behavior duringturn-ON SC of a trench gate 1200 V, 30-A IGBT havinga structure very similar to that of the simulated one hasbeen analyzed. Fig. 11 reports the device I–V characteristicsmeasured at Vge = 8 and 10 V.
For the transient measurements, an appropriate experimentalsetup was used [10], whose schematic is reported in Fig. 12.The gate voltage Vge was supplied by a driver with theON voltage variable in the range of 8–16 V and a fixed negativevoltage of −15 V.
Although not of practical interest, the first set ofmeasurements demonstrates the occurrence of low-frequencyoscillations. In Fig. 13(a), the gate voltage and collectorvoltage and current for Rg = 100 � are reported. The testconditions are the same as those ones used in the simulations:Vge = 10 V and Vce = 400 V. The gate inductance was
Fig. 12. Schematic of experimental setup used for SC characterization.
Fig. 13. Measured waveforms during the SC at Vge = 10 V and Vce = 400 V.Circuit parameters are Lg = 330 μH and Lc = 56 nH. (a) Rg = 100 � and(b) Rg = 30 �.
set to Lg = 330 μH and the measured SC inductancewas Lc = 56 nH. The duration of SC was set to ∼10 μs.We observe diverging oscillations at the expected frequencyof ∼330 kHz. As predicted by the theory, in the low-frequencyrange, the device tends to become more stable to decrease thegate resistance, as shown in Fig. 13(b), where the measuredwaveforms for Rg = 30 � are reported.
Fig. 14(a) reports an example of the measuredhigh-frequency oscillations. The test conditions wereVge = 10 V, Vce = 400 V, Rg = 10 �, Lg = 27 nH, andLc = 56 nH. The oscillation frequency is ∼70 MHz.
ABBATE et al.: ANALYSIS OF LOW- AND HIGH-FREQUENCY OSCILLATIONS IN IGBTs 2957
Fig. 14. Measured waveforms during SC at Vge = 10 V,Vce = 400 V, Lg = 27 nH, Lc = 56 nH, and a) Rg = 10 � andb) Rg = 50 �.
Fig. 15. Stability map of the device as a function of Rg and Lg at Vge = 10 Vand Vce = 400 V. Measured Lc = 56 nH.
The device becomes more stable increasing Rg .Fig. 14(b) shows that using Rg = 50 �, the amplitudeof the oscillations decreases.
Following the same criteria as in the simulations, it waspossible to obtain the stability map of the device reportedin Fig. 15. It was experimentally achieved by varying bothRg and Lg in a wide range of values. The test conditions wereVge = 10 V and Vce = 400 V. Stable and unstable points aremarked with O and X, respectively. The agreement between
the experiment and the simulation is enough good consideringthat we are comparing the results of a numerical small signalanalysis with the large signal experimental results.
VI. CONCLUSION
A comprehensive study about the oscillations occurringduring IGBTs SC operation has been presented. It was foundthat the physical mechanisms involved in the low-frequencyoscillations are much different from those observed in thehigh-frequency oscillations. It was demonstrated that the linearmodel presented in the literature to identify the strategiesto prevent oscillations does not apply to the high-frequencyoscillations. A procedure, which allows identifying theparameters of the external circuit for which the device becomesstable, was proposed. The procedure was used to determine thestability map of a 1200 V, 30-A trench gate IGBT.
REFERENCES
[1] R. Pagano, Y. Chen, K. Smedley, S. Musumeci, and A. Raciti, “Shortcircuit analysis and protection of power module IGBTs,” in Proc. Appl.Power Electron. Conf. Expo. (APEC), vol. 2. Mar. 2005, pp. 777–783.
[2] M. Ershov, H. C. Liu, L. Li, M. Buchanan, Z. R. Wasilewski, andA. K. Jonscher, “Negative capacitance effect in semiconductor devices,”IEEE Trans. Electron Devices, vol. 45, no. 10, pp. 2196–2206,Oct. 1998.
[3] I. Omura, H. Ohashi, and W. Fichtner, “IGBT negative gate capacitanceand related instability effects,” IEEE Electron Device Lett., vol. 18,no. 12, pp. 622–624, Dec. 1997.
[4] I. Omura, W. Fichtner, and H. Ohashi, “Oscillation effects inIGBT’s related to negative capacitance phenomena,” IEEE Trans.Electron Devices, vol. 46, no. 1, pp. 237–244, Jan. 1999.
[5] S. T. Kong, L. Ngwendson, M. Sweet, and E. M. S. Narayanan,“A study of the influence of technology on the negative gate capacitancein 1.2kV IGBTs,” in Proc. 20th Int. Symp. Power Semicond. DevicesIC’s, May 2008, pp. 177–180.
[6] S. Milady, D. Silber, F. Pfirsch, and F.-J. Niedernostheide, “Simulationstudies and modeling of short circuit current oscillations in IGBTs,”in Proc. IEEE 21st Int. Symp. Power Semicond. Devices IC’s, Jun. 2009,pp. 37–40.
[7] J. Bohmer, J. Schumann, and H.-G. Eckel, “Negative differential millercapacitance during switching transients of IGBTs,” in Proc. 14th Eur.Conf. Power Electron. Appl., Aug./Sep. 2011, pp. 1–9.
[8] C. Abbate, G. Busatto, A. Sanseverino, and C. Ronsisvalle,“Experimental and numerical study of low and high frequency oscilla-tions in IGBTs during short circuit,” in Proc. PCIM Europe, Nuremberg,Germany, May 2014, pp. 1–8.
[9] D. M. Pozar, Microwave Engineering, 4th ed. New York, NY, USA:Wiley, 2012.
[10] C. Ronsisvalle et al., “High frequency capacitive behavior of field stoptrench gate IGBTs operating in short circuit,” in Proc. 28th Annu. IEEEAppl. Power Electron. Conf. Expo. (APEC), Mar. 2013, pp. 183–188.
[11] Sentaurus Device User Guide, Version J-2014.09, Synopsys, MountainView, CA, USA, Sep. 2014.
Carmine Abbate was born in Italy in 1976.He received the degree in telecommunication engi-neering from the University of Cassino, Cassino,Italy, in 2001, and the Ph.D. degree in electrical andinformation engineering in 2006.
He is currently a Researcher with the Universityof Cassino and Southern Lazio, Cassino. His currentresearch interests include power devices reliability,power devices analysis and characterization, innov-ative converter, and driving topologies.
2958 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 62, NO. 9, SEPTEMBER 2015
Giovanni Busatto received the Laurea degreein electronic engineering from the University ofNaples, Naples, Italy, in 1983.
He is currently a Full Professor of Power Elec-tronics and Telecommunication Electronics with theUniversity of Cassino, Cassino, Italy. His currentresearch interests include power system integration,modeling, simulation, and non-destructive character-ization of power devices.
Annunziata Sanseverino was born in Naples, Italy,in 1965. She received the Degree in electronic engi-neering in 1991, and the Ph.D. degree in electronicengineering and information in 1995.
She has been a Professor of Electronics with theUniversity of Cassino, Cassino, Italy, since 2003.Her current research interests include electrical char-acterization of semiconductor materials, modeling ofpower devices, and solar cells technology.
Francesco Velardi received the Laurea (cum laude)degree in electronic engineering with a thesis on theidentification of complex nonlinear dynamic systemsfrom the University of Naples Federico II, Naples,Italy, in 1999, and the Ph.D. degree in electricaland information engineering from the University ofCassino and Southern Lazio, Cassino, Italy, in 2003.
He is currently a Researcher with the Universityof Cassino and Southern Lazio.
Cesare Ronsisvalle was born in Rome, Italy, in1956. He received the degree in physics from theUniversity of Catania, Catania, Italy.
He joined the Secowest Semiconductor, Turin,Italy, in 1981. In 1983, he moved to STMicro-electronics, Catania. In 2011, he joined Infineon(Villach), as a Process Engineer. In 2012, he joinedFairchild Semiconductor, Munich, Germany, to con-tinue to develop the latest generation of IGBTs.
