AN609 Vishay Siliconix Thermal Simulation of Power MOSFETs on the P-Spice Platform Author: Kandarp Pandya INTRODUCTION R-C thermal model parameters for Vishay power MOSFETs available under the product information menu offer a simple means to evaluate thermal behavior of the MOSFET under a defined transient operating condition. Steady state values of thermal impedance, Rth(j-a) and Rth(j-c) / Rth(j-f), along with normalized thermal transient impedance characteristics published in a power MOSFET datasheet, are adequate to analyze the thermal behavior of a part under a regular wave-shaped, single pulse or the periodic power dissipation of known duty cycle. However, thermal analysis for transient or irregular wave-shaped power profiles requires extending the thermal characterization offered in the datasheet. What is really needed is a thermal model emulating the thermal transient behavior of the power MOSFET on a suitable software platform. The thermal transient impedance characteristics published in a datasheet are a net effect of the thermal resistance and thermal capacitance of the physical structure of a device. Hence the latter can be used for developing a thermal model for the part, but it is necessary to parameterize the thermal characteristics. Incidentally, there exists a direct behavioral analogy between thermal components / parameters and electrical components / parameters; see Table 1. Table 1 Description Electrical Thermal R=V/I Rth = °C / W R - Electrical Resistance in Ohms Rth - Thermal Resistance in °C / W Potential V - Electrical Potential Difference in Volts °C - Temperature Difference in Celsius Energy flow I - Electrical Current in Ampere W - Power Dissipation in Watts Capacitance C - Electrical Capacitance Cth - Thermal Capacitance Ohm’s law analogy Resistance Using the analogy described above, we can use electrical simulation software like P-Spice to analyze thermal behavior by using the corresponding parameters. This requires a means to obtain the electrical parameters equivalent to the corresponding thermal parameters. Typical thermal characteristics as represented in a datasheet of a power MOSFET are shown in Figure 1. Document Number 73554 07-Oct-05 Curve-fitting techniques can be applied to such time varying thermal characteristics while using a combination of electrical resistances (R) and capacitances (C) in pairs as variable parameters. The nature of transient thermal characteristics for power MOSFETs necessitates at least four R-C pairs to obtain the best curve fit. These R-C pairs in turn represent thermal model parameters.[1] These pairs can be used on the same OrCAD Capture and P-Spice platform to run electrical simulations and carry out thermal analyses by correctly employing the analogy discussed earlier. www.vishay.com 1 AN609 Vishay Siliconix NEW PRODUCT SI7390DP N-CHANNEL 30-V (D-S) FAST SWITCHING WFET® THERMAL RESISTANCE RATINGS Parameter Symbol t ≤ 10 sec Maximum Junction-to Ambient (MOSFET)a Steady State Maximum Junction-to-Case (Drain) Steady State RthJA RthJC Typical Maximum 20 25 53 70 2.1 3.2 Unit *C / W Notes a. Surface Mounted on 1“ x 1“ FR4 Board Normalized Thermal Transient Impedance, Junction-to-Ambient Normalized Effective Transient Thermal Impedance 2 1 Duty Cycle = 0.5 0.2 Notes: 0.1 PDM 0.1 0.05 t1 t2 1. Duty Cycle, D = t1 t2 2. Per Unit Base = RthJA = 125• C/W 0.02 3. TJM – TA = PDMZthJA(t) Single Pulse 4. Surface Mounted 0.01 10–4 10–3 10–2 10–1 1 Square Wave Pulse Duration (sec) 10 100 600 Normalized Thermal Transient Impedance, Junction-to-Case Normalized Effective Transient Thermal Impedance 2 1 Duty Cycle = 0.5 0.2 0.1 0.1 0.05 0.02 Single Pulse 0.01 10–4 10–3 10–2 Square Wave Pulse Duration (sec) 10–1 1 Figure: 1: Datasheet Information R-C THERMAL MODELS Two configurations of R-C thermal models offered on the Vishay Web site are a "tank" circuit, Figure 2, and a "filter" circuit, Figure 3. These configurations are also www.vishay.com 2 known as Cauer and Foster. Both models are developed using the same database of transient thermal characteristics with curve -fitting techniques, hence closely matching thermal analysis results are obtained. Document Number 73554 07-Oct-05 AN609 Vishay Siliconix Table 2 T(Junction) RT1 CT1 RT2 CT2 RT3 CT3 RT4 T(Ambient) Time (sec) CT4 Figure: 2: Tank Circuit Configurations T(Junction) RF1 CF1 RF2 CF2 RF3 CF3 RF4 T(Ambient) CF4 Power (Watts) 0 0 0.0001 100 0.00099 100 0.001 0.1 0.002 0.1 0.0021 20 0.005 20 0.0051 0 0.1 0 GND Figure: 3: Filter Circuit Configurations ⎛− t ⎞ ⎛− t ⎞ ⎛ ⎛ ⎜ τ ⎟⎞ ⎜ τ ⎟⎞ RT (t ) = R1 (t ) * ⎜⎜1 − e ⎝ 1 ⎠ ⎟⎟ + R2 (t ) * ⎜⎜1 − e⎝ 2 ⎠ ⎟⎟ ⎝ ⎠ ⎝ ⎠ ⎛− t ⎞ ⎞ ⎛− t ⎞ ⎞ ⎛ ⎛ ⎜ τ ⎟ ⎜ τ ⎟ + R3 (t ) * ⎜1 − e ⎝ 3 ⎠ ⎟ + R4 (t ) * ⎜1 − e ⎝ 4 ⎠ ⎟ ⎜ ⎟ ⎜ ⎟ ⎝ ⎠ ⎝ ⎠ τx Cx = Rx However, the "filter" configuration requires a complex approach. That is where a linear square integration routine is employed in the curve fitting to obtain R-C values in both configurations. Refer to Appendix A for an example of R-C models developed using this approach. Appendix A is also an illustration of the R-C thermal models offered under the product information menu on the Vishay Web site. The model validation can be observed from close-fitting curves: one curve for the raw data obtained during part characterization and another curve that is produced by a curve-fitting routine using model R-C values. Accordingly, this thermal model represented by R-C electrical parameters can be used for a thermal analysis of a power MOSFET as discussed in the following example. THERMAL SIMULATION EXAMPLES (a) Example 1 Aim: Demonstrate the use of a junction to ambient (j-a) model and verify that the junction temperatures obtained by each model configuration are comparable. We shall estimate the junction temperature of Vishay MOSFET part number Si7390DP with the dissipating power profile described in Table 2. Document Number 73554 07-Oct-05 Here are the steps to set up and run simulations: Step 1 Obtain R-C thermal model file Si7390DP_RC from the Vishay Web site; see Appendix A. Step 2 Start a new project on the OrCAD Capture/P-Spice platform, create a new design folder Tank j-a, and add a schematic page as shown in Figure 4, Tank Configuration. Junction Temperature V We can observe from the following expression that, mathematically, the "tank" configuration is very easy to develop. Repeating after every 500 milliseconds. Ambient temperature = 25 °C The power profile described in Table 2 can produce high temperature excursions during each cycle and a cumulative temperature rise over a period of 500 milliseconds, which is long enough to produce a temperature rise beyond the part package. Hence, a junction-to-ambient model is useful for this analysis. R1 R2 R3 R4 2.0964 9.0489 8.0177 50.6167 C1 C2 C3 24.2346m 558.4598m 70.7118m I2 C:\Si7390DP_RC\100W50mSReverse.txt Power Profile I C4 1.4427 25Vdc V1 Ambient 0 Figure 4: Tank Configuration Use R-C thermal model values for ambient temperature from the Table 3 R-C values for the tank configuration from Appendix A. www.vishay.com 3 AN609 Vishay Siliconix Table 3 Temperature: 27.0 (A) bias.dat (active) R-C VALUES FOR TANK CONFIGURATION 150c 120W Thermal Resistance (°C/W) RT1 2.0964 18.1348 u N/A RT2 9.0489 713.7923 m N/A 100W Foot RT3 8.0177 1.3126 N/A RT4 50.6167 1.1896 N/A Thermal Capacitance (Joules/°C) Junction to Ambient Case Foot CT1 24.2346 m 81.2387 u N/A CT2 558.4598 m 673.3554 u N/A CT3 70.7118 m 15.6345 m N/A CT4 1.4427 m 6.7185 m N/A Step 3 Set the property value file of the current source 12 part name IPWL_F_RE_N_TIMES to point to text file C:\Si7390DP_RC\100W50mSReverse.txt, which contains the power profile described earlier. 100c 80W 60W 50c 40W 20W >> 0W 0s 1 100ms 2 I(I2) Temperature: 27.0 (A) bias.dat (active) 45c 2 Power Profile in Watts (B) 100W 40c 80W 60W 35c 40W 30c 20W >> 0W 0s 1 4ms 2 I(I2) Step 7 Run the simulations. The simulation result in Figure 5 shows that the junction reaches the maximum operating temperature limit of 150 °C in 500 milliseconds. Time Figure 6 shows detailed power profile and corresponding temperature excursion for first two cycles Step 4 Set the repeat value to 50 in the part property editor. Step 6 Create a new simulation profile named Tank j-a and set the run time to 600 milliseconds. V(R1:1) 0c 500ms 400ms Figure: 5: j-a Tank Simulations Red: Junction temperature Blue: Power profile 120W Step 5 Set the value of voltage source V1 to 25, the value of the ambient temperature. 300ms 200ms 1 Junction Temp Deg.C (Red) Case 1 Junction Temp Deg.C (Red) Ambient 2 Power Profile in Watts (B) Junction to 8ms 12ms Time V(R1:1) 16ms 25c 20ms Figure: 6 First Two Cycles Step 8 Create a new design folder Filter j-a and add a schematic page as shown in Figure 7, Filter Configuration. Junction Temperature R1 6.2277 R2 7.9322 R3 7.6428 V C1 C2 C3 16.9231m 156.4465m 2.8690m I1 C:\Si7390DP_RC\100W50mSReverse.txt Power Profile I R4 47.9015 25Vdc C4 1.2888 V1 Ambient 0 Figure: 7: Filter Configuration Use R-C thermal model values for the ambient temperature from Table 4, and the R-C values for the filter configuration from Appendix A. www.vishay.com 4 Document Number 73554 07-Oct-05 AN609 Vishay Siliconix The transient power profile for high-power, short-duration or even single incidence may exhibit a high temperature rise at the junction before the heat is conducted away from the package. In such cases, j-c or j-f thermal models are useful. Table 4 R-C VALUES FOR FILTER Thermal Resistance (°C/W) Junction to Ambient Case Foot RF1 6.2277 164.8787 n N/A RF2 7.9322 888.8177 m N/A RF3 7.6428 1.2671 N/A RF4 47.9015 1.0331 N/A Table 5 describes the single pulse profile to be analyzed. Thermal Capacitance (Joules/°C) Junction to Ambient Case Foot CF1 16.9231 m 166.0609 u N/A CF2 156.4465 m 357.5156 u Table 5: Time (sec) N/A Power (Watts) 0 0 900 CF3 2.8690 m 4.6473 m N/A 10 µs CF4 1.2888 304.1798 u N/A 90 µs 900 100 µs 0 1s 0 Step 9 Repeat steps 3 through 7 described above and compare the simulation results obtained by using the j-a filter configuration shown in Figure 8 with the simulation results obtained by using the j-a tank configuration in Figure 5. Temperature: 27.0 (A) j-a Filter.dat (active) 150c 120W Duration = 1 second Ambient temperature = 25 °C Follow the same steps as those described in Example 1, except that the R-C values are obtained from the j-c part of the Table 3 and Table 4. The schematic diagram for the j-c tank configuration is shown in Figure 9. Junction Temperature R1 100c 80W 60W 50c 40W 2 Junction Temp (Red) V 1 Power Profile (Purple) 100W 20W >> 0W 0s 1 100ms I(I1) 200ms 2 V(R1:1) 300ms 400ms 0c 500ms 600ms Time 18.1348u R2 713.7923m C1 R3 R4 1.3126 1.1896 C2 81.2397u 673.3554u C3 15.6345m I2 C:\Si7390DP_RC\900w100uS.txt Power Profile I C4 6.7185m V1 25Vdc Ambient 0 Figure 9: j-c Tank Configuration Figure 8: j-a Filter Simulations Red: Junction temperature Purple: Power profile We can observe that the junction temperature value obtained by either configuration is within couple of degrees Centigrade. These results are acceptable for all practical purposes. (b) Example 2 Aim: Demonstrate the use of the junction-to-ambient (jc) model and verify that the junction temperatures obtained by each model configuration are comparable. Document Number 73554 07-Oct-05 www.vishay.com 5 AN609 Vishay Siliconix We can observe that the junction temperature value obtained by either configuration is within 5 to 7 degrees Centigrade. These results are acceptable for all practical purposes. Temperature: 27.0 (A) j-c tank.dat (active) 1.0KW 160c 0.8W 2 Junction Temp (Red) 1 Power Profile (Blue) 120c 0.6W 80c 0.4W 40c 0.2W >> 0W 0s 1 40us 2 I(I2) 80us 120us Time V(R1:1) 160us 0c 200us Figure 10: j-c Tank Simulations Red: Junction temperature Blue: Power profile The simulation result shows that the junction temperature rises to 150 °C with just one power pulse. Next, create a new design folder Filter j-c and add a schematic page as shown in Figure 11, Filter Configuration. Junction Temperature R1 V R2 164.8787n R3 888.7177m C1 C2 166.0609u 357.5156u I2 C:\Si7390DP_RC\900w100uS.txt Power Profile I R4 1.2671 1.0331 V1 25Vdc C4 304.1798u C3 4.6473m Ambient Figure 11: j-c Filter Configuration (A) Filter j-c.dat (active) Temperature: 27.0 180c 1 Power Profile (Blue) [4] "Rigorous Model and Network for Transient Thermal Problems, "Y.C. Gerstenmaier and G. Wachutka, 7th Therminic Workshop, September 2001. x 150c x 0.8W [2] "Thermal Modeling for Power MOSFETs in DC/Dc Applications, "Yalcin Bulut and Kandarp Pandya, May 2004 Euro-Sime Proceedings. x 0.6W 100c x 0.4W 2 Junction Temp (Red) x REFERENCES: [1] "A Simplified Method of Generating Thermal Models for Power MOSFETs, "Kandarp I. Pandya and Wharton McDaniel, March 2002 IEEE SEMI-THERM Proceedings. [3] "Thermal Analysis of Power MOSFETs Using Rebeca-3D Thermal Modeling Software (From Epsilon Ingenierie) versus Physical Measurements and Possible Extractions, "Kandarp Pandya and Serge Jaunay, April 2005 Euro-Sime Proceedings. 0 1.0KW SUMMARY R-C thermal model parameters are available for Vishay power MOSFETs under the product information menu. These models can be used on the P-Spice platform to estimate the junction temperature of the MOSFET that is dissipating transient power. The j-a model parameters are employed for repetitive, high peak power and longer duty cycle pulses. All these cases result in residual junction temperatures at the end of each period. On the other hand, the j-c model parameters are employed for single, very high power transients. However, in these cases the junction temperature returns to ambient before the subsequent power pulse is applied. The estimation of junction temperature falls within a practically acceptable range of +/- 5 °C to 7 °C. This approach offers a quick, simple, and very useful alternative to high-end complex thermal modeling tools. 50c x 0.2W >> 0W 0s 1 40us 2 I(I2) 80us 120us Time V(R1:1) x 160us 0c 200us Figure 12: j-c Filter Simulation Results www.vishay.com 6 Document Number 73554 07-Oct-05 AN609 Vishay Siliconix Appendix A SI7390DP_RC R-C THERMAL MODEL PARAMETERS DESCRIPTION The parametric values in the R-C thermal model have been derived using curve-fitting techniques. These techniques are described in "A Simple Method of Generating Thermal Models for a Power MOSFETs"[1]. When implemented in P-Spice, these values have matching characteristic curves to the Single Pulse Transient Thermal Impedance curves for the MOSFET. R-C values for the electrical circuit in the Foster/Tank configuration are included. The corresponding values for the Cauer/Filter configuration are available upon request. R-C THERMAL MODEL FOR TANK CONFIGURATION T(Junction) RT1 CT1 RT2 CT2 RT3 RT4 CT3 T(Ambient) CT4 R-C VALUES FOR TANK CONFIGURATION Thermal Resistance (°C/W) Junction to Ambient Case Foot RT1 2.0964 18.1348 u N/A RT2 9.0489 713.7923 m N/A RT3 8.0177 1.3126 N/A RT4 50.6167 1.1896 N/A Junction to Ambient Thermal Capacitance (Joules/°C) Case Foot CT1 24.2346 m 81.2397 u N/A CT2 558.4598 m 673.3554 u N/A CT3 70.7118 m 15.6345 m N/A CT4 1.4427 6.7185 m N/A This document is intended as a SPICE modeling guideline and does not constitute a commercial product data sheet. Designers should refer to the appropriate data sheet of the same number for guaranteed specification limits. Document Number 73554 07-Oct-05 www.vishay.com 7 AN609 Vishay Siliconix R-C THERMAL MODEL FOR FILTER CONFIGURATION RF1 T(Junction) CF1 RF2 CF2 RF3 RF4 CF3 T(Ambient) CF4 GND R-C VALUES FOR FILTER CONFIGURATION Thermal Resistance (°C/W) Junction to Ambient Case Foot RF1 6.2277 164.8787 n N/A RF2 7.9322 888.8177 m N/A RF3 7.6428 1.2671 N/A RF4 47.9015 1.0331 N/A Junction to Ambient Case Foot CF1 16.9231 m 166.0609 u N/A CF2 156.4465 m 357.5156 u N/A CF3 2.8690 m 4.6473 m N/A CF4 1.2888 304.1798 u N/A Thermal Capacitance (Joules/°C) Note: NA indicates not applicable For a detailed explanation of implementing these values in P-Spice, refer to Application Note #AN8xx http://www.vishay.com/doc?7xxxxx Reference: [1] "A Simple Method of Generating Thermal Models for a Power MOSFET" by Wharton McDaniel and Kandarp Pandya, IEEE / SEMITHERM 2002 www.vishay.com 8 Document Number 73554 07-Oct-05 AN609 Vishay Siliconix (A) j-c.opt, SI7390DP j-c txt (A) j-a.opt, SI7390DP j-a txt 4.0 80 3.0 60 2.0 40 1.0 120 x 0 100us V(CF1:1) 10ms 1.0ms 100ms 300ms Time “j-c“ 0 1.0ms 10ms 100us V(RF1:1) “j-a“ (A) j-c.opt, SI7390DP j-c txt 1.0s 100ms Time 10s 100s 1.0Ks 10s 100s 1.0Ks (A) j-a.opt, SI7390DP j-a txt 4.0 80 x 3.0 60 2.0 40 x 1.0 0 100us V(CF1:1) 120 10ms 1.0ms “j-c“ Document Number 73554 07-Oct-05 Time 100ms 300ms 0 1.0ms 10ms 100us V(RT1:1) “j-a“ 1.0s 100ms Time www.vishay.com 9