AN4242 Application note New generation of 650 V SiC diodes Introduction For many years ST has been a worldwide leader in high voltage rectifiers dedicated to energy conversion. During the last decade, electronic systems have followed a continuous trend towards higher power density and more energy savings driven by governments’ environmental awareness. Power-supply designers are permanently confronted with stringent efficiency regulations (Energy Star, 80Plus, European Efficiency…). They are forced to consider the use of new power converter topologies and more efficient electronic components such as high-voltage silicon-carbide (SiC) Schottky rectifiers. To help them face this challenge, ST developed in 2008 a first family of 600 V SiC diodes. After having sold millions of pieces, ST’s reliability and know-how is confirmed on these new components using wide band gap materials. In hard-switching applications such as high end server and telecom power supplies, SiC Schottky diodes show significant power losses reduction and are commonly used. A growing use of those rectifiers is also recorded in solar inverters, motor drives, USP and HEV applications. However, the high cost of this technology tends to drive designers to use it at high current-density levels (3 to 5 times higher than standard Si diodes), inducing more constraints on the diode. Indeed, the Silicon-carbide material features a positive thermal coefficient potentially leading to some instability and lower current-surge robustness than silicon diodes. ST decided to review the design and develop a second generation of SiC diodes offering an enhanced current capability while still featuring an attractive switching-off behavior. The peak reverse voltage was also increased to 650 V in order to ensure a safer operation in certain designs. Typical applications (non-exhaustive list) Charging station ATX power supply AC/DC power management unit, high voltage, and other topologies Desktop and PC power supply Server power supply Uninterruptible power supply Photovoltaic string and central inverter architecture Photovoltaic power optimizer architecture Photovoltaic microinverter grid-connected architecture Photovoltaic off-grid architecture Telecom power May 2013 DocID024196 Rev 1 1/26 www.st.com Contents AN4242 Contents 1 Features of the SiC diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 2 3 1.1.1 Comparison with Si bipolar diode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1.2 Capacitive charge (QC) measurement . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Forward characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3 Other characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.1 Low leakage current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3.2 “C” thermal coefficient . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Forward thermal runaway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.1 Thermal runaway risk in regular working mode . . . . . . . . . . . . . . . . . . . . . 8 2.2 Thermal runaway risk in transient phase . . . . . . . . . . . . . . . . . . . . . . . . . .11 New 650 V JBS SiC diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.1 Device structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2 Comparison between first and second generation of SiC diodes . . . . . . . 13 3.3 4 Turn off behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3.2.1 Forward voltage comparison . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.2.2 IFSM PSpice simulation: comparison between 1st and 2nd generation 15 3.2.3 IFSM datasheet comparison between SiC G2 and SiC G1 . . . . . . . . . . 16 JBS structure trade-off: current surge capability versus Qrr . . . . . . . . . . 17 3.3.1 Forward characteristics comparison between ST’S SiC 2nd generation and other JBS designs 17 3.3.2 No recovery charge area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 3.3.3 PSpice electro-thermal simulation result . . . . . . . . . . . . . . . . . . . . . . . . 19 Efficiency measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 4.1 dI/dt optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 4.2 Example of efficiency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 6 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 2/26 DocID024196 Rev 1 AN4242 Features of the SiC diodes 1 Features of the SiC diodes 1.1 Turn off behavior 1.1.1 Comparison with Si bipolar diode The benefits brought by silicon-carbide diodes on the switching losses in the applications working in continuous-conduction mode (such as PFC applications) are already well known. The capacitive nature of the recovery current allows constant turn-off characteristics when the temperature increases. In contrast the turn-off behavior of bipolar diodes is characterized by a strong dependency on junction temperature, dI/dt slope and forward current level (see Figure 1). Thanks to their properties, SiC diodes allow significant reduction of power losses in the associated MOSFETs when switched-on. They also permit new optimization options for the power converter (for example, increasing the switching frequency and speed, lowering the size of passive components, snubber-circuits and EMI filters). Figure 1. Switching behavior comparison between Si and SiC diodes for Tj=75 °C and Tj=125 °C VR=380 V, IF=8 A, dI/dt=200 A/μs, Tj=125 °C 20 ns/div 2 A/div VR=380 V, IF=8 A, dI/dt=200 A/μs, Tj=75 °C 20 ns/div 2 A/div 8 A SiC diode 8 A SiC diode 8 A tandem (2 x 300 V diodes in series) 8 A bipolar diode 8 A tandem (2 x 300 V diodes in series) 8 A bipolar diode The capacitive recovery current is generated by the charge of the junction capacitance Cj under a certain reverse voltage and corresponds to a quantity of stored charges Qc. 1.1.2 Capacitive charge (QC) measurement Some confusion exists about the measurement conditions of Qc. A comparison between the switch-off behavior and the integral of the current used to estimate Qc is shown in Figure 2. Figure 2A and Figure 2B show measurements at low forward current (IF=1 A) and low dI/dt slope (50 A/µs), with and without reverse voltage across the diode. A certain inaccuracy of the measurement of Qc can be observed. It is linked to the probe, which features its own equivalent capacitance. Figure 2C shows a measurement at IF=2 A and a high dI/dt slope (200 A/µs) without any probe voltage. With such a high value of current-slope some oscillations appear. Taking into account the total capacitive current until t0 when the reverse voltage reaches VR, Qc measured by the integral of the current is similar to the one in Figure 2B. DocID024196 Rev 1 3/26 Features of the SiC diodes AN4242 Figure 2. Qc measurement of a 6 A SiC diode at IF = 1 A, Tj = 25 °C, VR = 400 V, dI/dt = 50 A/µs Voltage probe t0 0 Qc = 24.2 nC Qc = 19.6 nc A. With voltage probe B. Without voltage probe Qc=20.4nc C. IF = 2 A, dI/dt = 200 A/µs To avoid false readings due to some measurement inaccuracy, a theoretical approach is preferred. The quantity of charge Q during a certain period of time [0-t0] is delimited by the reverse voltage variation V across the junction capacitance Cj between 0 and VR and is given by the following formulas: Equation 1 Qc ò dQ = 0 ò t0 i(t) dt 0 with Equation 2 i(t) = Cj 4/26 dV(t) dt DocID024196 Rev 1 AN4242 Features of the SiC diodes After simplification and introduction of the junction capacitance variation versus the reverse voltage Cj(V), Qc is defined by the following formula: Equation 3 VR Qc(VR) = ò Cj(V) dV 0 This relation demonstrates that Qc is defined by the integral of the junction capacitance Cj between 0 and VR, the voltage reapplied on the diode. This theoretical approach allows the direct and accurate evaluation of Qc, avoiding the inaccuracy introduced by potential measurement problems. The strict expression of the energy stocked in the junction capacitor for a given reverse voltage can be determined by: Equation 4 VR Qc(VR) = ò Cj(V) · V dV 0 Due to the non-linearity of the junction capacitance versus the reverse voltage, this relation is different from the traditional energy formula ½ · C · V² (or ½ · Q · V), which is valid only when considering a constant capacitance. 1.2 Forward characteristics Another main feature of SiC diodes is the variation of the forward voltage drop (VF) with the junction temperature. Figure 3 shows the forward current versus forward voltage drop characteristics for 3 different junction temperature levels. A crossing-point can be observed at a certain level of current IC. When the current is lower than this level, the temperature coefficient of the forward voltage drop (VF) is negative. When the current is higher, it becomes positive. The same crossing point exists for traditional silicon diodes, but it appears at a much higher current level (>10 times the nominal current). This is linked to the higher forward current density of SiC diodes. DocID024196 Rev 1 5/26 Features of the SiC diodes AN4242 Figure 3. ST’s STPSC806 first generation: typical forward voltage drop versus forward current 16 IFM (A) 14 12 Tj==25 25 °C °C Tj =150 °C 10 8 Tj =175 °C 6 4 2 0 0.0 VFM (V) 0.5 1.0 1.5 2.0 2.5 3.0 As a consequence, the working area of SiC rectifiers usually corresponds to VF > 0, which leads to an increase of the forward voltage drop with the junction temperature, meaning an increase of the conduction power losses, hence an increase of the temperature and so on. This electro-thermal mechanism results in a thermal runaway loop. The effect is explained in Section 2. 1.3 Other characteristics 1.3.1 Low leakage current The new generation of 650 V SiC diode offers some low leakage current values similar to the 600 V Si counterparts. Therefore the reverse power losses defined in PFC by the Equation 5 stays negligible as showed in Table 1. Equation 5 PREV (Tj) = dav · VR · IR (VR, Tj) with Equation 6 dav = 1 - 2 · Ö2 · Vin p · Vout Table 1. Leakage current and reverse losses comparison in PFC @90Vac between Si and SiC diode IR @ VR = VRRM Product @ Vin = 90 V, Vout = 400 V Typical / Maximum Typical STTH8R06D 35 / 400 µA @ 125 °C 0.011 STPSC806D 150 / 1000 µA @ 125 °C 0.047 65 / 335 µA @ 150 °C 0.02 STPSC8H06D 6/26 Prev in PFC DocID024196 Rev 1 AN4242 1.3.2 Features of the SiC diodes “C” thermal coefficient The “C” thermal coefficient represents the leakage current dependence on the junction temperature. The leakage current increases by an exponential law with the junction temperature. Knowing a reference point IR(VR,TjRef) and the value of the thermal coefficient “C”, one can easily calculate the leakage current at a given temperature Tj using the following formula: Equation 7 IR(VR,Tj) = IR(VR,TjRef) · ec(Tj - TjRef) where VR is the reverse voltage applied across the diode. Each diode has its own coefficient that can be calculated using two points as follows: Equation 8 c= æ IR(VR,TjRef2) ö 1 · ln ç ÷ TjRef2 - TjRef1 è IR(VR,TjRef1) ø If the SiC Schottky diodes have low leakage currents and they have also a smaller temperature dependence compared to the Si counterparts. As illustrated in Table 2, typically the “C” thermal coefficient should be around 2 times lower than the Si diodes. Table 2. “C” thermal coefficient comparison between Si and SiC diode Product IR1 @ VR = 400 V, Tj1 IR2 @ VR = 400 V, Tj2 C coefficient STTH8R06 8 µA @ 125 °C 50 µA @ 150 °C 0.070 STPSC8H065D 4 µA @ 150 °C 8.5 µA @ 175 °C 0.030 The feature of low dependence with the Tj is interesting to push back the limit of thermal runaway due to the power reverse losses. Regarding the stability criterion formula linked to PPREV given by Equation 9, the interest for the use at high Tj of small packages with high thermal resistance becomes certain. Equation 9 dPPREV(Tj) 1 £ dTj Rth(j-a) DocID024196 Rev 1 7/26 Forward thermal runaway 2 AN4242 Forward thermal runaway In some particular application conditions a thermal runaway loop can be triggered (see Figure 4) and the thermal system of the diode may become unstable. Figure 4. Thermal runaway loop Tj VF(Tj ) Losses P(Tj) Positive loop Two kinds of application conditions can be linked to the thermal runaway risk: 2.1 the stationary regime during the regular working mode the critical transient phases. Thermal runaway risk in regular working mode During the regular operating mode, the average current in the diode can modeled with a constant current generator as shown in Figure 5. Figure 5. Simple electrical model Io Rd(Tj) Vt0 (T j) 0 The electrical model given by Equation 10, simulates the variation of the forward voltage drop versus the junction temperature for a given current I0. Equation 10 VF(I0,Tj) = Vt0150° + aVt0(Tj - 150) + [Rd + aRd · (Tj - 150)] · I0 Equation 11 Vt0(Tj) = Vt0150° + aVt0(Tj - 150) Equation 12 Rd(Tj) = Rd150° + aRd · (Tj - 150) Vt0(Tj) is the VF value for a fixed Tj when IF is null. The inverse function of Rd(Tj) represents the straight slope between 2 forward current levels and the threshold voltage Vt0 for a fixed 8/26 DocID024196 Rev 1 AN4242 Forward thermal runaway Tj. Vt0 and Rd are thermal coefficients. They represent the junction temperature impact on Vt0 and Rd. Using the electrical model previously defined, the conduction power losses P(Tj) can be estimated. Using the analogy between thermal and electrical units, a simple electro-thermal model is described in Figure 6. The thermal model is defined by the thermal resistance Rth(j-a) and the thermal capacitance Cth(j-a) junction to ambient. Figure 6. Simple electro-thermal model Tj C th(j-a) R th(j-a) P(Tj)=I0·VF(Tj,I 0) T amb The resolution of the above electro-thermal system gives the Tj(t) expression used to find the thermal runaway limit and to highlight the stability condition of the diode. The equation giving the conduction power losses versus Tj is the following: Equation 13 P(Tj) = [Vt0150° + aVt0(Tj - 150)] · I0 + [Rd150° + aRd · (Tj - 150)] · I02 with Equation 14 A = I0 · (Vt0150° - 150 · aVt0) + I02 · (Rd150° - 150 · aRd) and Equation 15 I0 · aVt0 · Tj + I02 · aRd · Tj = (I0 · aVt0 + I02 · aRd) · Tj = B · Tj A simplified version is: Equation 16 P(Tj) = A + B · Tj The global system equation is defined by: Equation 17 æ dTj ö Tj = Tamb + Rth · çP(Tj) - Cth · dt ÷ è ø Or again Equation 18 Tj · (1 - B · Rth) + Rth · Cth · dTj = Tamb + A · Rth dt Solving the differential equation gives the stability condition on the junction temperature: DocID024196 Rev 1 9/26 Forward thermal runaway AN4242 Equation 19 æB · Rth - 1 ö Rth · (A + Tamb · B) çè Cth · Rth ·e Tj (t) = B · Rth - 1 ÷· ø t - Tamb + A · Rth B · Rth - 1 Due to the exponential function in the expression of Tj(t), if B · Rth - 1 > 0 then the limit lim Tj(t) ® ¥ t®¥ leads to the diode destruction if the current I0 is not interrupted. Thus, the stability condition is given by: Equation 20 B · Rth - 1 < 0 so Equation 21 Rth < 1 B The detailed expression gives: Equation 22 Rth < 1 aVt0 · I0 + aRd · I02 Note that the limit of thermal runaway can be directly found by: Equation 23 dPT(Tj) 1 £ Rth(j-a) dTj Numerical application: An application with an average current I0 = 6 A using the STPSC6H065 is considered here. The typical forward voltage curve versus forward current of the STPS6H065 datasheet is calculated between 3 A and 9 A and between 25 °C and 150 °C: Vt0150°C = 0.85 V, Rd150°C = 0.175 , Vt0 = -800 µV/°C, Rd = 600 µ/°C. So, the “B coefficient” is equal at 0.017 W/°C, and the critical Rth, beyond which the thermal runaway is reached, is Rth > 59.5 °C/W. Considering the Rth(j-a) of the TO-220 package in air to be around 60 °C/W, the stability condition will not be respected since B · Rth(j-a) - 1 > 0 and thus the diode cannot be used without a heatsink for this value of current. The diode must be mounted on its own heatsink, in choosing the Rth value checking B · Rth(j-a) - 1 < 0 and respecting Tj < Tjmax. In this case of a stable condition, lim Tj(t) ® t®¥ Tamb + A · Rth B · Rth - 1 if Tj targeted is 125 °C with Tamb = 40 °C, the heatsink should be chosen for an Rth(j-a) value equal to 7.45 °C/W. 10/26 DocID024196 Rev 1 AN4242 2.2 Forward thermal runaway Thermal runaway risk in transient phase The thermal runaway phenomenon can easily be observed with the short IFSM test waveform by sensing the forward voltage drop. IFSM is defined by a sine-wave of 10 ms shown in Figure 7 and is described in the datasheet as the non-repetitive maximum surge forward current. Figure 7. Thermal runaway phenomenon during an IFSM-test waveform VF = 11 V STPSC606D 2 ms/div 2 V/div 10 A/div IFSM = 38 A In standard applications, the current waveforms are either shorter or longer and more complex due to the switching frequency. However the IFSM parameter stays a reference that reflects the capability of the diode to sustain a surge current. In an SMPS, during the transient phases such as the start-up phase, a power line drop-out, a lightning surge or a short circuit, experience shows that some high surge current stresses are applied to the diode. Examples are shown in Figure 8. Figure 8. Inrush current proportional to dVout/dt during a start-up phase and a power line drop-out 200 ms / Div 5 A / Div 100V / Div 5 ms / Div 5 A / Div 100 V / Div Inrush current VOUT Idbypass IdSiC VOUT Inrush current IdSiC ILine 20 ms / Div IdSiC VOUT INTC=Idbypass Unlike the regular operating mode, the current stress duration is generally lower than 1 s. In this case a thermal runaway phenomenon can be triggered and the advised limit to avoid the destruction of the diode is the Tjmax given in the datasheet. The estimation of Tj during these transient conditions involves the transient thermal impedance: DocID024196 Rev 1 11/26 Forward thermal runaway AN4242 Equation 24 t DT(t) = P(t) · Zth¢(t) = ò P(t)Zth¢(t - t)dt 0 In most cases, the complexity of the current waveform implies that the above equation is solved with help of an electro-thermal model as shown in Figure 12. To push back the thermal runaway limit and improve the capability of SiC diodes to withstand high current surges, a second generation of SiC diode has been developed by STMicroelectronics using a combination of a Schottky diode and a PN diode. This new technology is usually called Junction Barrier Schottky (JBS). 12/26 DocID024196 Rev 1 AN4242 New 650 V JBS SiC diodes 3 New 650 V JBS SiC diodes 3.1 Device structure Figure 9. Comparison between a pure SiC Schottky structure with the JBS SiC structure Schottky barrier 4H - SiC epitaxy Guard ring 4H - SiC substrate Schematic cross section of conventional SiC Schottky diode (1st generation device) Schottky on N-type Ohmic contact on P+ P+ PN junction P+ N-type 4H - SiC epitaxy Schottky on P-type Passivation Top metal Schottky barrier Metal P area Schottky area N - epitaxy N+ 4H - SiC substrate Ohmic backside contact N+ substrate Schematic cross section of a JBS SiC diode (2nd generation device) The 2nd generation SiC device is based on JBS (junction barrier Schottky) concept. At high forward voltage drops, this structure benefits from the injection of minority carriers by the PN junctions inserted within the main Schottky contact. Thus in case of surges current, modulation of resistivity induces a lower VF and a smaller increase of Tj. Moreover, the PN grid supports the decrease the leakage current IR and to increase the breakdown voltage Vbr of the device. So, thanks to this new design, the robustness of device is drastically increased compared to standard Schottky diode. 3.2 Comparison between first and second generation of SiC diodes 3.2.1 Forward voltage comparison The forward voltage characteristics of the first and the second generation are compared in Figure 10. The dotted line network corresponds to the linear characteristics of the pure SiC Schottky diode. The positive thermal coefficient is evident. Those curves highlight the difficulties in characterizing the pure SiC Schottky diode at high current with constant junction temperature due to the overheating linked to the measurement. To limit this thermal effect, the tests are made with a short duration pulse tp = 50 µs. The second generation of SiC diodes also presents a linear characteristic up to a certain level of current. A clamping effect linked to the JBS structure then appears at higher current. This effect happens when DocID024196 Rev 1 13/26 New 650 V JBS SiC diodes AN4242 there is a bias of the merged PN junctions, roughly beyond 3 V, 10 A @ 175 °C; 3.5 V, 15 A @ 125 °C; 4 V, 25 A @ 75 °C…. Figure 10. Forward voltage comparison between pure Schottky SiC diode and JBS SiC diode 8 VF (V) 175 °C VF =f(I F) versus Tj (tp=50 µs) 7 Thermal effect 6 125 °C 75 °C 5 4 25 °C 3 2 STPSC606D 1G ( SiC Schottky) STPSC6H065D 2G ( SiC JBS) 1 IF (A) 0 0 5 10 15 20 25 30 35 40 Figure 11 shows the characterization of the 2nd generation SiC diode up to 100 A with a pulse duration tp = 1 µs. This network of curves highlights two crossing points. First, VF is negative below 1.5 A, then positive up to around 42 A, then once again negative. When the merged PN junction is biased, at high Tj (>125 °C) all the curves converge to one straight line giving a forward characteristics almost independent of the temperature. Figure 11. Forward voltage characteristic of JBS SiC diode up to 100 A with tp = 1 µS 9 VF (V) VF =f(I F) versus Tj (tp = 1 µs) 8 7 6 aVF>0 5 aVF<0 225 °C 4 STPSC6H065D 2G 25 °C 3 2 1 aVF<0 IF (A) a VF>0 0 0 14/26 10 20 30 40 DocID024196 Rev 1 50 60 70 80 90 100 AN4242 New 650 V JBS SiC diodes The JBS structure clamps the forward voltage at high current and high Tj. This new technology thus avoids the thermal runaway phenomenon and the IFSM value can go up to 9 or 10 times the nominal current rating. 3.2.2 IFSM PSpice simulation: comparison between 1st and 2nd generation The electro-thermal model simulates the variation of the forward voltage drop during a current spike and gives an estimate of the junction temperature. The electro-thermal model of a 2nd generation 6 A SiC diode is given in Figure 12. The model is composed of an electrical model based on the typical forward characteristics shown in Figure 11 and a thermal model based on the typical transient thermal impedance junction-to-case curve given in the datasheet. Figure 12. Electro-thermal model of the 6 A /650 V SiC G2 (STPSC6H065D) from STMicroelectronics VF1 IN PARAMETERS: OUT+ Vt1 = 9.7788E-01 Rd1 = 9.1267E-02 aVt1 = -6.4005E-04 aRd1 = 2.1542E-04 bVt1 = -1.1968E-06 bRd1 = 2.5860E-06 OUT- IN1 IN1 IN2 OUTVF2 IN2 OUT VF11 PARAMETERS: PARAMETERS: a = 8.8736E-02 a1 = 1.3166E+00 b = -4.7201E-04 b1 = 2.6601E-02 c = 3.0955E-06 d1 = 9.0534E-07 c1 = -2.2788E-04 d = -8.5670E-09 e1 = -1.3932E-09 e = 8.4095E-12 IN1 VF3 IN2 OUT Tj VF_tj_IF (V(%IN1) *V(%IN2)) TJ 2 U1 TOPEN = 0.01 PARAMETERS: 1 IF I3 R77 1 R81 1 R80 1 R82 1 R83 1 Vt3 = 2.5132E+00 aVt3 = 1.4256E-02 bVt3 = -7.8925E-05 cVt3 = 1.3556E-07 Rd3 = 7.2948E-02 aRd3 = -4.3599E-04 bRd3 = 2.3970E-06 cRd3 = -4.1511E-09 Electrical model R79 R76 0.01547 0.52881 C74 0.00118 C75 0.00053 R74 0.32484 C76 0.00311 0 R84 R85 R86 0.27507 0.19657 0.10262 R87 0.02207 25 C77 C78 C79 C80 0.00962 0.03716 0.20431 0.74923 V25 Tc Thermal model Figure 13 shows the result of a PSpice simulation for an IFSM value of 42 A. With such a surge current, thermal instability is reached with the 1st generation device. The forward voltage drop and the junction temperature increase exponentially until the diode is destroyed. With the 2nd generation device, the JBS effect clamps the forward voltage drop and limits the increase of junction temperature. DocID024196 Rev 1 15/26 New 650 V JBS SiC diodes AN4242 Figure 13. Result of IFSM PSpice simulation: comparison between 6 A / 650 V SiC 2nd generation (STPSC6H065D) and 6 A /600 V SiC 1st generation (STPSC606D) VF (V) IFSM (A) 15 Tj (°C) 50 400 Tj 1G > 400 °C thermal runaway phenomenon IFSM = 42 A @ Tc = 25 °C 40 300 10 VF typ 1G 30 200 °C Tj 2G 20 200 VF typ 2G 5 100 10 Time (ms) 0 3.2.3 0 0 1 2 3 4 5 6 7 8 9 10 0 IFSM datasheet comparison between SiC G2 and SiC G1 The non-repetitive IFSM curves versus pulse duration presented in Figure 14 come from the datasheet of the STPSC606 and the STPSC6H065. The graph, based on measurements, shows the improvement of the surge current capability with the second generation. Thanks to the JBS structure, the IFSM values are more than doubled. Figure 14. Non repetitive IFSM versus tp comparison between 6 A SiC 1st generation and 6 A SiC 2nd generation 1·E+03 IFSM (A) STPSC6H065 2nd generation 1·E+02 STPSC606 1st generation tp (s) 1·E+01 1·E-05 16/26 1·E-04 DocID024196 Rev 1 1·E-03 1·E-02 AN4242 New 650 V JBS SiC diodes Current stresses in the range of tens of microseconds are usually linked to the switching period. Such a surge current can also happen during lightning surge tests. Stresses in the range of tens of milliseconds are usually related to line-dropout tests. Table 3. IFSM with tp = 10 ms and Tj = 25°C: comparison between first and second generation IFSM, sinusoidal, 10 ms, @ 25 °C 3.3 IF (A) 4 6 8 10 SiC 1st generation 14 27 30 40 SiC 2nd generation 38 60 75 90 JBS structure trade-off: current surge capability versus Qrr The efficiency of the JBS structure to sustain a current spike is linked to the bias current level of the merged PN junction. This level characterizes “the JBS positioning”. Designing the diode with a higher bias current level leads to a higher forward voltage drop, and hence a higher Tj for the same surge. Likewise, the lower the bias current level, the lower the forward voltage drop at high current. However, the conduction of a PN junction implies some recovery charges (Qrr) when the diode switches and turns off. This is linked to the minority carriers’ recombination, which does not happen in a conventional SiC Schottky structure. As a consequence, a trade off between IFSM and Qrr should be considered. 3.3.1 Forward characteristics comparison between ST’S SiC 2nd generation and other JBS designs Figure 15 illustrates in dotted lines another dimensioning of the merged PN junction compared with ST’s design. The dotted lines present forward voltage drops around 2 volts higher than ST’s diode between 30 A and 70 A at 225 °C. On the other hand, this characteristic indicates that the carrier injection phenomenon (Qrr) should appear at higher forward current levels. Figure 15. Forward voltage drop between STPSC6H065D and another JBS structure 11 VF (V) VF=f(IF) versus Tj with (tp=50 µs) 10 9 8 7 6 5 STPSC6H065D 2G Other JBS technology 25 °C 4 75 °C 125 °C 3 225 °C 175 °C 2 IF (A) 1 0 0 10 20 30 40 50 DocID024196 Rev 1 60 70 80 90 100 17/26 New 650 V JBS SiC diodes AN4242 The characterization of the recovery charges for a given junction temperature versus the forward current allows the determination of the no recovery charges area. 3.3.2 No recovery charge area Figure 16 illustrates the no-recovery-charges area for a 6 A SiC 2nd generation diode in the reference plan of forward current versus junction temperature. ST’s STPSC6H065D SiC G2 was designed to be used without any recovery charges up to 2 · IF(AV) at 150 °C or again 3 · IF(AV) at 100°C. The 2 oscilloscope traces illustrate the switch-off behavior of the diodes at IF = 12 A and IF = 18 A for Tj = 150 °C. At IF = 12 A, the behavior at 25°C and 150°C is stable confirming the absence of recovery charges. At IF = 18 A, recovery charges start to appear between 100 °C and 125 °C and become more significant at 150 °C. The same characterization was made on a sample of diodes featuring another JBS technology (corresponding to the dotted lines in Figure 15). This other JBS trade-off presents a larger no-recovery-charge area but compromises on the forward voltage drop, that is higher at high current levels. Figure 16. Comparison of no recovery charge area between ST’s 6 A SiC 2nd generation diode and another JBS technology 35 IF (A) STPSC6H065D 30 Other JBS technology 25 No recovery charge area 20 15 10 6 A SiC 2nd generation diode working area 5 Tj (°C) 0 0 25 50 75 100 125 150 175 200 In a PFC, the peak current flowing through the diode can be estimated by: Equation 25 Ipeakdiode = Ö2 · Pout Vin(min)rms · h For example, in an 800 W server application with a PFC working at Vin(min) = 90 V AC and an efficiency of 90%, the peak current reaches 14 A (it’s the same for a 1600 W PFC application working at 180 V AC). In this application the choice of a 6 A SiC G2 working with a Tj around 125 °C is adapted. 18/26 DocID024196 Rev 1 AN4242 3.3.3 New 650 V JBS SiC diodes PSpice electro-thermal simulation result The interest of the dimensioning of the ST’s JBS structure compared to another JBS positioning can clearly be highlighted with the electro-thermal simulation. Figure 17 and Figure 18 present the result of the PSpice simulation respectively for an IFSM waveform and a startup phase in a PFC application. The electrical model of the STPSC6H065D is compared to one of the other 6 A JBS technologies coupled with a thermal model similar to the one of the STPSC6H065D. Figure 17 shows that the higher values of the forward characteristic of the other JBS technology in Figure 15 lead to a much higher Tj (+ 100°C) compared to ST’s product during a 42 A IFSM spike. Figure 17. IFSM electro-thermal simulation with Tj comparison between ST’s 6 A SiC G2 and another 6 A JBS technology VF (V) IF (A) 8.0 Tj (°C) 350 50 VF typ6A other JBS techno Tj other JBS techno =300 °C 300 IFSM [email protected]=25 °C 40 6.0 250 VF typSTPSC6H065 30 200 4.0 150 TjSTPSC6H065 =200 °C 20 100 2.0 10 50 Time (ms) 0 0 0 0 1 2 3 4 5 DocID024196 Rev 1 6 7 8 9 10 19/26 New 650 V JBS SiC diodes AN4242 A second electro-thermal simulation was done during an SMPS start-up phase (Figure 18). It demonstrates once again the interest of correctly dimensioning the JBS structure. Figure 18. Electro-thermal simulation of a PFC start-up phase with Tj comparison between ST’s 6 A SiC 2nd generation and another 6 A JBS technology Tj (°C) 240 60 A 1000 W PFC start-up PSpice simulation 90 V, 70 kHz, Cout = 600 µF, L = 270 µH, Tc = 125 °C IdSiC Tjother 650 V SiC JBS technology 215 °C 200 40 A Tj STPSC6H065 175°C 160 20 A 120 0A 5 10 15 20 25 30 Time (ms) The lower Tj observed through the electro-thermal simulation on ST’s JBS structure contributes to the robustness of the ST’s product in the application. 20/26 DocID024196 Rev 1 AN4242 4 Efficiency measurement Efficiency measurement Table 4 summarizes the key parameters for the 1st and 2nd generation of SiC diodes. If the JBS structure improves the surge current capability, it degrades somewhat the values of forward voltage drop at low current level. Table 4. Comparison of key parameters between first generation and second generation of SiC diodes Product 6 A SiC, 1st gen STPSC606D 6 A SiC, 2nd gen STPSC6H065D IFSM, (A) VRRM (V) VF (V) @ 6 A, 25 °C VF (V) @ 6 A, 150 °C sinusoidal, 10 ms typical / maximum typical / maximum 600 27 1.4 / 1.7 1.6 / 2.1 650 60 1.5 / TBD 1.9 / TBD In a typical PFC application, the efficiency will be affected by less than 0.1% between the 1st and 2nd generation. A first approximation demonstrated by the following equations shows that the efficiency difference in a PFC could be estimated by VF/VOUT. Equation 26 Pcond Vt0 · Iav + Rd · Irms2 = Pout Vout · Iout with Equation 27 Iav = Pout Vout and Equation 28 Pout · Irms2 = V inpk · h Ö3 · p · V 16 · Vinpk out then Equation 29 Pcond Vt0 + k · Iav · Rd VF(k·Iav) = = Pout Vout · Iout Vout with Equation 30 k= Vout · 16 3 · Vinpk · h2 DocID024196 Rev 1 21/26 Efficiency measurement 4.1 AN4242 dI/dt optimization The contribution of the SiC diode in the switching cell is essential. Its switching performance leads to new optimizations that can help to go a step forward in increasing the efficiency. It is well known that the MOSFET switching speed (dI/dt) is an important parameter to optimize the efficiency. The dI/dt slope (when the transistor turns on and when the diode turns off) can be easily changed by tuning the value of the gate resistor Rg of the transistor. Figure 19 shows, the efficiency drop between SiC diodes and silicon diodes for different dI/dt slopes. This efficiency drop is defined by the total power losses due to the diode divided by POUT. The conduction power losses, the switch-off power losses in the diode and the switch-on power losses in the transistor due to the Qrr of the silicon diodes are taken into account. Figure 19. Comparison of efficiency drop in a 500 W PFC with VIN = 90 V F = 100 kHz, Tj = 125 °C Efficiency (%) 6 5 4 8A Ultrafast diode 3 8A Tandem G1 2 8A Tandem G2 6A SiC G2 1 dI/dt (A/µs) 0 0 100 200 300 400 500 600 6A SiC G1 700 800 900 1000 1100 With silicon bipolar rectifiers, there is an optimized dI/dt slope to reduce the power losses. When the slope increases, the switching time decreases but the reverse recovery current increases. For low dI/dt values, the impact of the switching time dominates, and for higher dI/dt the impact of Qrr may become more important. Hence, the switching power losses due to the recovery charges (Qrr) decrease with the increase of dI/dt until a certain point from which they start to increase again due to Qrr. For those silicon diodes, the slope choice must also be made taking into account electromagnetic considerations (EMC) which sometimes impose a limitation of that slope. With SiC diodes, the power losses continue to decrease whenever dI/dt increases. Being naturally soft (due to capacitive nature of the recovery current), they offer the possibility to switch the transistor more quickly and thus increase the efficiency of the converter. 22/26 DocID024196 Rev 1 AN4242 4.2 Efficiency measurement Example of efficiency measurements Compared to the conventional ultrafast diode, using SiC diodes we can expect, an efficiency gain of between 1% and 2%. An example of efficiency measurements in a 480 W PFC at VIN = 115 V AC is presented in Figure 20. The efficiency gain with the SiC diode compared to the conventional 600 V silicon diodes reaches 1.2%. Figure 20. Typical efficiency measurement in a 480 W PFC at VIN = 115 V, Fsw = 100 kHz, dI/dt = 600 A/µs 95 Efficiency (%) 6 A, 600 V SiC G1 New 6A 650V SiC G2 New 8A 600V Tandem G2 94 8 A, 600 V Tandem G1 8 A, 600 V Turbo 2 diode 93 Tjdiode»120 °C DCM mode 92 CCM mode Tjdiode»50 °C Load (%) 91 0 10 20 30 40 50 DocID024196 Rev 1 60 70 80 90 100 23/26 Conclusion 5 AN4242 Conclusion To keep its leadership in power rectifiers, ST developed a complete portfolio of silicon carbide diodes that are more and more popular in power converters thanks to their very high switching performance. To help designers in their quest for more current density and helping them to reduce cost, STMicroelectronics developed a second generation of SiC Schottky rectifiers. The design of these new diodes provides increased robustness while not impacting their performance and blocks the effect of the positive thermal coefficient of the silicon carbide material. These new diodes have already proven to be very efficient in high-power SMPS. To help designers reduce their time to market, STMicroelectronics developed a complete electro-thermal model of the diode. Combined with a model of the electrical circuit in which the SiC rectifier is used, the model can simulate all the worst case conditions of the transient phases of the power-supply. This way, power-supply designers can verify that the diode is completely safe in all conditions. Supporting a wide range of applications, ST’s SiC rectifiers are available in a variety of supported currents and packages, giving more flexibility on the power density/power dissipation trade-off. 24/26 DocID024196 Rev 1 AN4242 6 Revision history Revision history Table 5. 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