Implementation and Validation of a Physics-based Circuit Model for IGCT with Full Temperature Dependencies X. Wang, A. Caiafa, J.L. Hudgins, and E. Santi P.R. Palmer Department of Electrical Engineering Department of Engineering University of South Carolina Columbia, SC 29208, USA Wangx@engr.sc.edu Abstract -- This paper presents a physics-based Fourier solution IGCT model for circuit simulation with full temperature dependencies. Besides the external electrical characteristics, the model can also provide internal physical and electrical information of the device, such as the junction temperature, and the charge distribution. The model is shown to give good agreement with experimental waveforms and accurately predicts the device behavior under changing temperatures. I. INTRODUCTION The Integrated Gate Commutated Thyristor (IGCT) is an advanced GTO-based semiconductor device with its gate drive unit coaxially integrated with a Gate Commutated Thyristor, resulting in an extremely low gate-cathode parasitic inductance ranging from 5nH to 15 nH [1]. Since it was commercially introduced, the IGCT has rapidly gained acceptance in major areas of high power electronics, such as propulsion inverters for mass transit and locomotives, high power industrial drives for steel and paper mills, and utility power conditioning, including static VAR compensation and flexible AC transmission [2]. Due to the rapidly increasing application of IGCT in high power applications, an IGCT model for circuit simulation is needed. However, a bibliography search shows only one IGCT model reported in [3], in which all partial derivatives were approximated by finite differences in order to solve the ambipolar diffusion equation. That model is implemented by Saber MAST and does not give detailed description of the temperature dependent behavior of IGCT. For the circuit simulation purpose, models with full thermal response and implemented by widely used circuit simulator such as PSpice are desired. This paper presents a new physics-based circuit model for IGCT with full temperature dependent features. This IGCT model was developed by employing Fourier-based solution for ambipolar diffusion equation, and was implemented in circuit simulator PSpice™. Besides the external electrical characteristics, the model can also provide information on physical quantities inside the device, such as the junction temperature, hole and electron currents at different junctions inside the device, and the charge dynamic distribution in the buffer layer and base regions. To validate the model, a chopper circuit with an inductive load was built to test the IGCT switching behavior. The switching characteristics of IGCTs at ambient temperatures ranging from –40 to 50 °C University of Cambridge Trumpington Street Cambridge CB2 1PZ, UK were tested. Good agreement has been obtained between the simulation results and experimental data. II. MODEL DEVELOPMENT Fig.1 shows the IGCT semiconductor structure, charge profile and current components inside the device. The dashed lines represent the charge profile and depletion boundary while junction J2 is reverse biased. The proposed physicsbased IGCT model employs the Fourier expansion to solve the ambipolar diffusion equation for the N-base region, uses a charge control approach to describe the charge behavior in the P-base region, applies quasi-static equations for the buffer layer, and considers the N+ emitter as the minority sink. All these regions interact through their current components and charge densities at junction boundaries. G K A + N buffer IA In1 Ip1 P PH0 - N base N IG Px1 In2 Ip2 Idis Px2 (Px2) x1 J1 (x2) Ip3 nb1 PHW (Px1) J0 + P base IK In3 nb2 (nb1) (nb2) x2 b2 J2 b1 J3 Fig.1 The IGCT semiconductor structure, charge profile and current components A. Modeling the N-base region During the IGCT operation process, the thick and lowdoped N-base can be divided into two regions — the undepleted region (storage charge zone) and the space charge layer. The undepleted region contains the storage charge, and the space charge layer sustains most of the voltage drop applied to the device. The total width of the two regions is equal to WN (N-base width) and each region can vary from zero to WN depending on the applied external voltage. 1) The undepleted N-base region Under the assumption of one-dimensionality and high-level injection, the charge dynamics in the undepleted N-base region can be described by the well-known ambipolar diffusion equation: D p ( x , t ) ∂p ( x , t ) ∂ 2 p( x, t) = + τ ∂t ∂x 2 (1) where p(x, t) is the excess carrier concentration, τ the carrier lifetime, and D the ambipolar diffusion coefficient. Many attempts have been made to solve this equation for device modeling purposes, such as algorithmic solutions [4], Hefner’s solution [5], and Fourier-based solutions [6] [7]. An algorithmic solution is practical but cumbersome in the context of use with a circuit simulation while Hefner’s solution introduced mathematical oversimplifications to the diffusion equation itself. The Fourier-based solution solves the ambipolar diffusion equation quite exactly by an electrical circuit analog, without extra-assumptions concerning the drive or the boundary conditions. Diode [8] [9] and IGBT [7] [10] [11] models have been successfully created using this Fourierbased solution and have been validated. The Fourier solution modeling technique provides a good trade-off between simulation accuracy and speed. Based on the Fourier solution modeling technique, the excess carrier concentration p(x,t) can be represented as a discrete cosine Fourier expansion: ∞ kπ ( x − x1 ) p( x, t ) = P0 (t ) + ∑ Pn (t ) cos n =1 x 2 − x1 Vk dV + Ck k + I k (k = 0, 1, 2, 3, 4, 5…) Rk dt (3) (6) C0 = x2 − x1 (7) ∞ dx n dx 2 I 0 = ∑ Pn (t ) 1 − (− 1) dt dt n =1 (8) For k ≠ 0: kπ 2 Rk = D x 2 − x 1 x 2 − x 1 Ck = Ik = 2 1 + τ −1 (9) x 2 − x1 2 (10) Pk (t ) d ( x 2 − x1 ) ∞ n 2 Pk (t ) dx1 dx +∑ 2 − ( −1) k + n 2 2 4 dt dt n =1 n − k dt V2 I even V2n R0 R2 R2n C0 C2 C2n I2 I0 ∂p ( x , t ) = D ∂x (11) Reven I2n ∂p ( x , t ) x2 − x1 ∂x V1 V3 V2n+1 R1 R3 R2n+1 C1 C3 C2n+1 I1 I3 I2n+1 x2 + Rodd ∂p ( x , t ) x1 ∂x Fig.3 Equivalent RC circuit describing the Fourier coefficients Ik Fig.2 The RC cell used to calculate Pk (k=0,1,2,3…) The force current Iforce is function of the boundary conditions: D I n2 − I n1 I p2 −I p1 (for k = 0, 2, 4…) − 2qA Dn Dp τ x2 − x1 ∂p ( x , t ) I odd = − D ∂x Rk Ck I force = I even = R0 = V0 Vk Iforce D I n 2 + I n1 I p 2 + I p1 (for k = 1, 3, 5…) (5) − 2qA Dn Dp The values of capacitor and resistor are determined by the carrier lifetime, undepleted base width and the harmonic number k, and the parallel current sources, Ik (k=0,1,2,3…), represent the moving boundary of the depletion region. For k = 0: (2) By doing some mathematic derivation it is found that the Fourier coefficients P0(t) and Pn(t) can be obtained by solving a series of ordinary differential equations, which can be represented by the analogue electric equation shown in (3), and can be modeled by equivalent RC cell as shown in Fig.2. I force = I force = Iodd = − (4) Therefore, the ambipolar diffusion equation can be represented in the form of two RC networks, shown in Fig.3, which correspond to the even and odd values of k. The voltages across the successive cells represent the Fourier series coefficients P0(t) and Pn(t). The implementation of the entire Fourier series expansion would require an infinite number of RC-cells in the series string. Practically, less than ten terms are required to achieve accuracy within the limits of precision related to process parameters of the IGCT. Therefore, after a small but suitable number of terms (RCcells) have been used the remaining resistive components of the series are added together to form the equivalent resistances, Reven and Rodd. The value of Reven and Rodd ranges from 10-4 to 10-5 s/cm. 2) The space charge layer The space charge layer behaves as either a quasi-ohmic region, if the junction J2 is forward-biased, or a depletion region, supporting most of the applied terminal voltage, if the junction J2 is reverse-biased. The space-charge behavior can be described by the Poisson’s equation. Assuming a constant space charge density qNeff, the relation between the voltage and the depletion width is: (12) 2ε ×Vsc 2ε ×Vsc x2 − x1 = Wn − qNeff = Wn − q nB + I A qAVSat Here, the space-charge includes the ionized impurities and the density of free carriers. The second term is most important if the device operates in an inductive-load circuit, in which the device conducts high currents even at high bias voltage. The voltage across the space charge layer, Vsc, is calculated by: (13) Vsc = − K × Px 2 a & Vsc⊆[0,Vbreak] Where, K is a very big positive constant; Px2a is the carrier concentration at x2 calculated using the Fourier expansion formula (2). It can be seen that, if Px2a is positive or zero, junction J2 is forward biased and Vsc is equal to zero; on the other hand, if Px2a is negative, J2 is reverse biased and Vsc can have any value smaller than the junction breakdown voltageVbreak. B. Modeling the P-base region Due to the comparatively narrow width and high doping of the P-base, the charge control approach can be applied to model the charge behavior in the p-base region. Consider the built-in NPN transistor on the cathode side of the IGCT in Fig.4. Since the total base region charge QB is approximately linear, it can be broken into two parts QF and QR. QF is the total injected charge from the N+ emitter to the base, and QR is the total injected charge from the N- collector into the base. N- Idis Ip2 In2 J3 IG G J2 Ip3 In3 P QB QR b2 QF K N+ b1 Fig.4 Charge and boundary current components for the base of the cathode NPN transistor Applying the continuity equation to the P-base region, the charge transient can be described by the charge control relation as: dQB Q (14) = I p2 + I G + I dis − I p3 − B dt τ BHL Where, τBHL is the high-level lifetime in the P-base region; Idis is the displacement current due to the changing depletion width at J2, and can be obtained by: dV J 2 dV J 2 1 (15) I =C = εA dis J2 W N − ( x 2 − x1 ) dt dt The hole current at J3 can be obtained by considering the N+ emitter as a hole sink: (16) I p3 = qAhn n 2b1 The parameter hn is the recombination coefficient and nb1 is the charge concentration at the boundary of J3. Since the QF and QR are assumed to be linearly distributed, the charge concentrations at the boundaries are related to the forwardand reverse-injection charges by the following equations: 2Q F (17) n b1 = qAW P 1 (18) Q R = qAWP nb 2 2 Wp is the P-base width. The charge concentration at the boundary of J2 can be obtained by the Boltzmann’s relation: n2 qV (19) nb2 = i exp J 2 PB kT where PB is the background doping of P-base region. Combining (15) to (17) and considering that QB is the sum of QF and QR, the charge concentration nb1 can be determined. Substituting the value of nb1 into (14), Ip3 can be obtained. The collector electron current In2 can be expressed by the forward and reverse charge transportation described in equation (20), in which the τF and τR are the normal and reverse base transit times. Q Q (20) I n2 = F − R τF τR The hole current Ip2 can easily be determined as: (21) I p 2 = I A − I n 2 − I dis Therefore, the above equations link together the P-base region to the N-base region, gate region and cathode. C. Modeling the buffer layer Since the buffer layer is highly doped and very thin (few µm), low-level injection and quasi-static assumptions can be applied. The continuity equation of holes in the buffer layer can then be expressed by: d 2 p( x) p( x) p(x) (22) = = Dτ BF L2pH dt2 Where τBF is the minority lifetime in the buffer layer, and LpH is the diffusion length of holes. The general solution for equation (22) is: p (x ) = C 1 e x L pH + C2e − x L pH (23) J1 J0 P+ - N base N buffer layer PH0 Px1 p(x) PHW IA Ip0 Ip1 In0 In1 x1 x H WH 0 Fig.5 Buffer layer carrier distribution and variables definition If the hole concentrations at the two edges of the buffer layer are PH0 and PHW, respectively (as shown in Fig.5), the solution of this equation is: W − x (24) 1 + PHW sinh x p( x ) = PH 0 sinh H W LpH LpH sinh H L pH where, WH is the width of buffer layer. Under low-level injection, the minority hole current is mainly diffusion current. Thus, the hole currents at the buffer layer boundary of J0 can be obtained from the derivative of p(x) (equation (24)): (25) qADp WH dp( x) I p0 = qADp x =0 dt = On the other hand, the hole concentration at the boundary of J1, - PWH, can be obtained by applying the Boltzmann’s relation to J1 under the assumption of high-level injection in the N-base (e.g. px1 >> nB). p ( p + n B ) p x21 (31) PHW = x1 x1 ≈ nH nH The hole current at the boundary of J1 can be easily obtained by taking the derivative of equation (24): W (32) qADp dp(x) I p1 = qADp PH0 − PHW cosh H x=W = dt W LpH LpH sinh H L pH Therefore, equations (29) to (32) link the buffer layer to the N-base and to the anode. − PHW PH 0 cosh LpH WH LpH sinh L pH D. The total voltage across the device The total voltage across the device consists of the voltage drop in the undepleted N-base region, the voltage supported by the space charge layer, and the voltages across the four junctions. Here, the Ohmic voltage drops in the P-base and emitter regions are ignored due to the high background doping concentrations. The voltage drop across the undepleted N-base region (charge storage zone) is: I x2 (33) Vbn (t ) = ∫ ρ ( x, t )dx A x1 Applying the Boltzmann’s relation to J0 with the quasiequilibrium assumption, results in equation (26), which relates the hole and electron concentrations, with NH as the background doping concentration of the buffer layer. VJ 0 (26) N H PH 0 = n i2 e Vt In addition, under low-level injection, the electron current at J0 can be expressed by (27), where Jsne is the emitter electron saturation current density. I n 0 = AJ sne e VJ 0 Vt (27) Combining (26) and (27), the electron current can be represented by: I n 0 = AJ sne N H PH 0 n i2 (28) The sum of electron and hole currents at J0 and J1 should be equal to the anode current: I A = I p0 + I n0 = I p1 + I n1 (29) Combining equations (25), (28) and (29), the hole concentration at J0 can be obtained: −1 J sne AN H (30) qAD P PHW qAD P PH 0 = ni2 + × I A + L pH tanh( W H / L pH ) L pH sinh(W H / L pH ) Fig.6 Discretized carrier profile for simulation of charge storage region voltage drop A typical effective way to realize the integration calculation in simulation is calculating the sum of discretized values. As shown in Fig.6, the charge profile is assumed to be linear between two adjacent points. The charge concentrations at every node along x1 to x2, i.e. p(x1,t), p(m1,t),… p(x2,t), can be calculated by the cosine Fourier expansion formula (2). Considering the trade-off between simulation speed and accuracy, seven sampling points uniformly distributed along the charge storage zone are typically used. Therefore, the total voltage drop across the undepleted N-base region can be calculated by: V bn (µ n + µ p ) p (m 1 , t ) + n B µ n ln ( ) (µ n + µ p )p ( x 1 , t ) + n B µ n (t ) = I × x 2 − x 1 6 Aq (µ n + µ p ) p (m 1 , t ) − p ( x 1 , t ) + ... (µ n + µ p ) p ( x 2 , t ) + n B µ n ln (µ + µ ) p (m 5 , t ) + n µ n p B n + p ( x 2 , t ) − p (m 5 , t ) (34) Parameter Intrinsic Carrier Concentration On the other hand, the voltage drops across the junctions can be obtained by Boltznmann’s relation: P n (35) VJ 0 = Vt ln H 02 H ni n (36) V J 1 = Vt ln B p x1 p V J 2 = 2V t ln x 2 ni for p x 2 ≥ ni Table 1. Temperature Dependence of the IGCT Model Parameters [12] [13] Electron Mobility Hole Mobility Lifetime E. The thermal sub-model Assuming double-sided cooling, the junction temperature of the device is estimated by the coupled thermal equivalent circuit shown in Fig.7. The power input source (current source) represents the heat dissipation in the device and can be expressed by equation (40). The R and C values are based on data sheet parameters or on package physical parameters and constants (e.g. thermal conductivity and heat capacity of the materials). All the temperature dependent parameters for the IGCT model are listed in Table 1. Therefore, the heat dissipation of the device can be instantaneously fed back to adjust the temperature dependent parameters and affect the model behavior, as shown in Fig.8. (40) Pin = I AV AK + I G × VGK µ III. p = 495 × (T 300 (T )1 . 5 τ =τ Recombination coefficient (37) n b1 (n b1 + n p ) (38) V J 3 = Vt ln ni2 where high-level injection is assumed in the N-base region to obtain (36) and (37). In addition, the value of px2 is limited to be equal to or higher than ni, since equation (37) is only used to calculate the forward voltage drop of J2. If px2 is less than ni, then J2 is reverse biased, and the voltage across the J2 is represented by the voltage drop across the space charge layer. Therefore, the total voltage drop across the device can be obtained by: (39) VD = Vbn + V J 0 + V J 1 + Vsc − V J 2 + V J 3 where Vbn can be calculated using (34) and Vsc is expressed by (13). Temperature Dependence Equation − 7.02×103 ni = 3.87×1016T1.5 exp( ) T µ n = 1360 × (T 300 )− 2 . 42 300 300 h n = J spo (T 300 ) 0 .5 )− 2 . 2 −2 q −1 n i e 14000 ( 1 1 − ) 300 T MODEL IMPLEMENTATION in PSpice™ The proposed IGCT model is implemented using the widely used circuit simulator– PSpice. Every mathematical equation describing the IGCT model is represented by one or more circuits in PSpice. For instance, behavioral model voltage controlled voltage sources (E) and voltage controlled current sources (G) are frequently used to realize the functions of calculating changing parameters. Differentiation is normally performed by measuring the current through a unity-valued capacitor to which a voltage equal to the quantity to be differentiated is applied. Pdiss = I AV AK + I GVGK A IA EJ0 Ebn EJ1 Tj Thermal model EJ3 -EJ2 Esc IK IG p x1 IA PH0 G px1 Ieven Ip1 In1 Buffer layer K Ip2 Fourier RC Iodd ± d ( x 2 − x1 ) dt In2 network Idis In2 IA : : ∑ Ln(..) px2 px2 Idis Ip2 pm1 nb1 P-base IG (x2-x1) f Vsc Ip3 Ip2 nb1 Space charge layer Ip3 qAh n n 2b1 Regulate temperature dependent parameters Fig.8 The block diagram describing the IGCT model implementation in PSpice Fig.7 Coupled thermal equivalent circuit used to calculate the junction temperature The block diagram describing the IGCT model implementation in PSpice is shown in Fig.8. The IGCT model, MODEL VALIDATION A. Experimental circuit and measured results As part of the validation process, experimental characterization of the snubberless switching behavior of a 4500V/340A IGCT was carried out. Fig.9 shows the testing circuit. The Lload represents the inductive load, and Lcl the stray inductance. The IGCT is placed in an environmental chamber, so that the ambient temperature can be controlled. The IGCT switching characteristics were tested at ambient temperature from –40 to 50 °C, with different conduction currents and clamping voltages. Fig.10 and Fig.11 are the measured waveforms with conduction current of 300 A and clamping voltage of 2500 V. D0 Lc1 LLoad VDC 3000 2500 -40 C 0C 25 C 50 C 2000 1500 Temperature increases 1000 500 0 -500 32 33 B. Some simulation results Corresponding simulation was performed using the proposed IGCT model. The simulation circuit is similar to the experimental one, except that the gate drive is simply represented by a piecewise linear voltage source in series with a resistor and an inductor. Figures 12 to 14 present some of the simulation results. The anode and gate currents during turn-off in Fig.12 indicate that the IGCT model has a unity turn-off gain, an important feature of the IGCT. Fig.13 shows that the junction temperature is approaching dynamic equilibrium after several switching cycles. This result proves that the thermal sub-model presents a proper thermal response for the semiconductor device. Fig.14 shows the evolution of the charge concentration profile during turn-off. It shows that the storage charge is swept out during turn-off process, and the storage time is less than 0.9 µs. This storage duration is close to the experimental data reported in [15]. 3500 350 Va Control Signal 250 Ia (A), Ig(A) & Vg(V) 20V 35 Fig.11 Experimental anode voltages during turn-off at temperatures from –40 to 50 °C IGCT C 34 Time (us) 2500 Ia 1500 150 500 50 -5 0 4 0 50 60 70 Ig 80 90 1 0 0-5 0 0 Vg -1 5 0 0 -1 5 0 Fig. 9 Circuit for testing IGCT switching Va (V) IV. 3500 Voltage (Volts) including its thermal sub-model, is wrapped in one sub-circuit, and can be called by any external circuit. The IGCT model consists of five sub-models described previously. They are: the buffer layer sub-model, the Fourier RC networks representing the undepleted N-base region, the space charge layer sub-model, the P-base region sub-model, and the thermal sub-model. The thermal sub-model links with other submodels through the junction temperature and the other four sub-models are linked together by their boundary charge densities and current components. All the sub-models are linked together with the external circuits and can be solved by the solver. -2 5 0 -2 5 0 0 T im e (u s) Fig.12 Anode and gate currents and voltages 350 320 300 200 Junction Temperature (Kelvin) Current (A) 250 150 100 Temperature increases 50 316 314 312 310 308 306 304 302 0 -50 Tj 318 -40C 0C 25C 50C 32 33 34 35 36 Time( us) Fig.10 Experimental anode currents during turn-off at temperatures from –40 to 50 °C 300 0 100 200 300 Time (us) 400 500 600 Fig.13 Junction temperature corresponding to several switching cycles V. 3.5E+16 t=0 t=0.7us t=0.9us t=1.3us t=3us t=5us t=8us Charge Density (cm-3) 3.0E+16 2.5E+16 2.0E+16 1.5E+16 1.0E+16 5.0E+15 0.0E+00 Buffer Layer 0 50 100 150 200 250 N- base region 300 350 400 450 CONCLUSION This paper presents a newly developed physics-based IGCT model based on a Fourier-series solution modeling technique. This IGCT model consists of five sub-models describing the buffer layer, storage zone of the N-base region, the space charge layer, P-base region and a thermal sub-model. All submodels were explained and the PSpice implementation was discussed. The proposed physics-based circuit model for IGCT works properly in the PSpice simulator, and is shown to give good agreement with experimental waveforms and correctly models the device behavior at various temperatures. P base ACKNOWLEDGMENT Fig.14 Charge distributions in the base regions during turn-off C. Comparison between experimental and simulation results Fig.15 and Fig.16 are the comparisons of experimental and simulation waveforms during turn-off with conduction current of 300 A, clamping voltage of 2500 V, and under ambient temperature of 25°C and 50°C. Good agreement has been obtained. Similar comparison results have been obtained at other temperatures with different conduction currents and clamping voltages. Due to the space limit, they are not presented here. 400 3000 Va 350 2500 250 2000 Ia_exp_25C Ia_sim_25C Va_exp_25C Va_sim_25C Ia 200 150 1500 1000 100 Voltage (V) Current (A) 300 500 50 0 0 -50 31 33 35 37 39 Time (us) 41 -500 Fig.15 The simulated and measured anode current and voltage during turn-off @25°C 400 3500 350 3000 Va 300 Current (A) 200 Ia_Exp_50C Ia_sim_50C Va_exp_50C Va_sim_50C Ia 150 2000 1500 1000 100 500 50 0 0 -50 Voltage (V) 2500 250 32 34 36 38 40 42 -500 Time(us) Fig.16 The simulated and measured anode current and voltage during turn-off @50°C This work was supported by the U.S. Office of Naval Research under contract numbers of N00014-02-1-0623 and N00014-03-1-0434. REFERENCES [1] Bjørn Ødegård, and Rene Ernst,” Applying IGCT Gate Units”, ABB Doc. No. 5SYA 2031-01, Dec. 02 [2] Eric Carroll, Sven Klaka, Stefan Linder, “INTEGRATED GATECOMMUTATED THYRISTORS: A New Approach to High Power Electronics”, IEMDC Milwaukee, IGCT Press Conference, May 20, 1997. [3] H. 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