User Guide for onsemi models including: AND90344, AND90344-D, AND90344-D, AND90344 EV Traction Modules, AND90344, EV Traction Modules, Traction Modules, Modules
15 apr 2025 — The DSC module shown in Figure 1 (b) has a single IGBT chip per functional switch where the temperature sensing diode is integrated at the corner of the ...
15 Apr 2025 — The junction temperature of power semiconductors is one of the critical parameters limiting the output power of the traction inverter.
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DocumentDocumentOver-temperature Detection Guide for the Traction Power Modules AND90344/D Introduction The junction temperature of power semiconductors is one of the critical parameters limiting the output power of the traction inverter. The output power of an inverter can be controlled based on the estimated junction temperature at the given operating condition of the inverter. However, there are multiple factors that affect the junction temperature such as degraded thermal impedance of the power semiconductor, insufficient coolant flow, excessive current flowing due to partial malfunction of the inverter part, and so forth. Hence, proactive protection based on temperature monitoring is required to improve the safety of whole vehicles. Temperature Sensors in the Power Module All EV traction modules from onsemi have a built-in temperature sensor which is either an NTC thermistor or an on-chip temperature sensing diode. If a traction power module is composed of multiple chips per functional switch, an NTC thermistor is integrated in the power module close to the power switch shown in Figure 1 (a). If the functional switch consists of a single large chip based on Silicon technology, a diode-type temperature sensor is fabricated on the chip as shown in Figure 1 (b). Characteristics of NTC Thermistor An NTC thermistor is characterized by its resistance and corresponding temperature. There are several parameters of NTC thermistors that define their characteristics such as B- and thermal dissipation constants. The B-constant characterizes the resistance with respect to temperature which is needed to design the temperature readout circuit parameters such as bias voltage and potential divider resistance. The thermal dissipation constant defines the self-heating characteristic of the thermistor required in the voltage-divider network for the thermistor. Such parameters required in the design of the sensing network are described in the datasheet for each product. A table and graph of the temperature and corresponding resistance of the NTC thermistor (shown in Figure 2) with fine resolution are also available upon request. Characteristics of On-chip Temperature Sensor The DSC module shown in Figure 1 (b) has a single IGBT chip per functional switch where the temperature sensing diode is integrated at the corner of the chip. The voltage drop of the temperature sensing diode is 2.5 V (typ) at Tvj of 25 °C and 1.7 V (typ) at 150 °C with a forward bias current of 1 mA and has a good linearity with the sensed temperature as shown in Figure 3. To ensure tight tolerance in the inverter's temperature sensing circuit, the measured voltages across temperature sensors at 25 °C and 150 °C for both high- and low-side switches can be marked as a QR code on the package upon separate request for the product. APPLICATION NOTE www.onsemi.com NTC thermistor a) On-chip temperature sensor b) Figure 1. Built-in Temperature Sensors of onsemi Traction Modules (a) NTC Type in Sigle-side Direct Cooling (SSDC) Module (b) On-chip Sensor type in Dual-side Cooling (DSC) Module © Semiconductor Components Industries, LLC, 2025 1 April, 2025 - Rev. 0 Publication Order Number: AND90344/D AND90344/D Table 1. NTC THERMISTOR CHARACTERISTICS OF SSDC MODULE FAMILY (NVH***S75L4S***, NVXR**S90M****) Symbol R25 DR/R P25 B25/50 B25/80 B25/100 Parameter Rated Resistance Deviation of R100 Power Dissipation B-Value B-Value B-Value Conditions TNTC = 25 °C TNTC = 25 °C, R25 = 5 kW TNTC = 25 °C R = R25 exp [B25/50 (1/T-1/298)] R = R25 exp [B25/80 (1/T-1/298)] R = R25 exp [B25/100 (1/T-1/298)] Min Typ Max Unit - 5 - kW -5 - 5 % - - 20 mW - 3375 - K - 3411 - K - 3433 - K Table 2. NTC THERMISTOR CHARACTERISTICS OF A TRANSFER-MOLDED SiC MODULE (NVVR26A120M1WS*) Symbol R25 DR/R P25 B25/85 Parameter Rated Resistance Deviation of R100 Power Dissipation B-Value Conditions TNTC = 25 °C TNTC = 100 °C, R100 = 877 W TNTC = 25 °C R = R25 exp [B25/85 (1/T-1/298)] Min Typ Max Unit - 10 - kW -3 - 3 % - - 125 mW - 3610 - K Figure 2. Table and Graph of the NTCT thermistor in SSDC Modules Figure 3. Temperature Sensor Characteristics of DSC Modules www.onsemi.com 2 AND90344/D Temperature Correlation Between a Switch and a Sensor Once the characteristics of the thermal sensor are defined, temperature at the sensor can be measured using an additional bias circuit. However, the measured temperature at the sensor does not represent the junction temperature of the switch. Even if an on-chip sensor is used, there is a temperature differences between the sensor and the junction of the power semiconductor. Furthermore, because the inverter does not always operate in steady-state mode, time delay between junction temperature of the chip and the sensor temperature must be considered for safe protection against an over-temperature event. To determine the relationship between junction temperature and the sensor temperature, power loss vs. junction temperature and sensor temperature is characterized at specific cooling conditions. Figure 4 shows the hardware setup for thermal characterization of the SiC SSDC module. DC current is applied to the D-S path of the SiC device to heat it up and the voltage drops across the SiC switch and the NTC thermistor are measured over temperature. Measured voltage responses can be correlated with temperatures by applying calibration K-factors shown in Figure 5, where K-factor is characterized under steady-state condition. Figure 4. Thermal Characterization Hardware Setup Figure 5. K-factors for SiC Chip and NTC www.onsemi.com 3 AND90344/D Once K-factors are applied to the measured voltage during cooling-down phase, the corresponding temperatures of the chip and the NTC thermistor can be represented in Figure 6. Because the thermistor is located next to the high-side switch only in the SiC SSDC module, the temperature of the thermistor is more closely correlated to the temperature of the high-side switch. By using the same temperature cool-down curve and the applied power loss on the chip, thermal impedance can be plotted as shown in Figure 7. From the thermal impedance curve, it is easy to see how much the temperature of the NTC thermistor is delayed. Figure 6. Temperature Measurement During Cooling Down Figure 7. Thermal Impedance of the SiC Switch and the NTC Thermistor Consideration for Temperature Sensing Delay If the sensed temperature at the NTC thermistor represents the variation of chip temperature, thermal impedance of the thermistor can be scaled to that of the chip in terms of the steady-state value. In Figure 8, the blue and orange lines indicate the thermal impedance of the chip and the scaled thermal impedance of the thermistor. The dashed line in red shows the difference in the thermal impedances of the chip and the scaled thermal impedance of the thermistor. www.onsemi.com 4 AND90344/D Assuming that temperature is sequentially monitored at t1 and t2, and the corresponding change of the power loss of the switch is DPLOSS, actual junction temperature Tj can be expressed as follows: Tj(t2) + Tj, est (t2) ) Tdelay, error + Tj, est (t2) ) DPLoss @ DZth, max (eq. 1) where Tj, est(t2) is the estimated junction temperature by the monitored temperature at t2, DZth, max is the maximum discrepancy of thermal impedances of the chip and the scaled thermal impedance of the thermistor. Because Tj(t2) should be less than the target temperature Tj, target, (eq. 1) can be expressed in terms of DPLOSS. Ttarget * Tj, est(t2) DPLoss t DZth, max (eq. 2) Hence, to prevent over-temperature damage, the output power must be controlled so that the power loss (DPLOSS) of the switch meets the requirements of (eq. 2). Figure 8. Thermal Impedance of the SiC Switch and the NTC Thermistor The following case study will use the 1200 V/1.5 mW SiC SSDC module as an example to illustrate the power loss limitation described above. Since the magnitude of output current is the dominant factor responsible for power loss variation while the inverter is running, DPLOSS/switch can be quantified in terms of the output current variation. Figure 9 shows the graph of DPLOSS/switch /DIout when the 1200 V / 1.5 mW SiC SSDC module operates under the condition described in the table. When this module delivers an output current of 500 Arms, the average junction temperature of the chip is known to reach 132 °C. If Ttarget is set at 140 °C for over-temperature protection, 8 °C of temperature margin is allowed until the over-temperature protection is activated. If the output current suddenly increases at this condition and the temperature sensor is unable to respond to this transient change and capture the temperature variation due to the sensing delay, the estimated temperature of the chip will still be 132 °C. Considering DZth, max of 0.07 due to the delay of the temperature reading in Figure 8, allowable DPLOSS from (2) is (140-132) / 0.07 = 114 W. Since DPLOSS / DIout at an Iout of 500 Arms is 1.85 as per Figure 9 graph, the allowed increase in the output current is 114 / 1.85 = 61 Arms. Thus, the inverter should not increase the output current greater than 61 Arms at this condition. Consideration for Temperature Ripple When the temperature detection level is used for protection, the additional rise in chip temperature should be considered due to the transient ripple in the junction temperature. This temperature ripple occurs within the modulation period of the sinusoidal output current, which is too short to be detected by the thermistor. The ripple in chip temperature can be simulated using the thermal network model and the power loss during the inverter's steady state operation. Figure 10 (a) shows an example of the temperature ripple for the 1200 V / 1.5 mW SiC SSDC module at an output current level and frequency of 550 Arms and 40 Hz, respectively, and (b) represents the difference between peak and average junction temperature as a function of the output modulation frequency. In the example of Figure 10, if the thermistor indicates the steady-state temperature, it can be correlated to the average junction temperature of 132 °C. If the output frequency of the inverter current is 120 Hz, the peak junction temperature of the SiC device could be higher than the average temperature by ~6 °C due to the temperature ripple. www.onsemi.com 5 AND90344/D Figure 9. Ration of Power Loss Variation to the Output Current Variation as a Function of the Output Current for 1200 V/1.5 mW SiC SSDC Module Figure 10. (a) Transient Temperature Response at the Steady State Condition of the Inverter at the Output Frequency of 40 Hz (b) Temperature Rise for the Output Frequency Consideration for Chip-to-chip Temperature Distribution AQG-324 guidelines for power modules recommend characterizing Tvj of the power semiconductor switch by measuring an equivalent voltage drop associated with temperature. However, if a functional switch comprises of multiple SiC chips in parallel, Tvj as defined in AQG-324 only shows the average temperature across the chips, as temperature is not uniformly distributed over the switch area. Figure 11 shows the layout of the SSDC SiC module, an FEM simulation result for the thermal distribution, an experimental setup, and the corresponding result. Because thermal impedance is characterized by means of the Tvj, that is, average temperature across the chips, a practical thermal impedance value can be defined using the Tj,max across the chips. Tj, max can either be estimated by performing an FEM simulation or measured using a thermal camera if the chip surface is visible. Avg.: 133.75 5C Max.: 139.55 5C Figure 11. Temperature Distributions of the SiC SSDC Module - Simulation and Measurement Results www.onsemi.com 6 AND90344/D By applying the average and the maximum temperature over the chips, transient thermal impedance can be characterized as shown in Figure 12 (a). By applying thermal impedances in the inverter simulation, time-averaged temperatures can be estimated as shown in Figure 12 (b), where the same module and inverter operating conditions are applied as in the previous section. At an output current of 550 Arms, the difference between both temperatures is ~8.4 °C, which indicates the temperature at the hottest spot across the chips is higher than the average temperature by ~8.4 °C. (a) (b) Figure 12. (a) Zth,avg, from the Average Junction Temperature, and Zth,max, from the Maximum Temperature (b) The Average Temperature of the SiC Switch from the Inverter Simulation with Zth,avg and Zth,max Conclusion When a built-in temperature sensor monitors the chip temperature of a power module, the monitored temperature may be lower than the highest temperature on the chip. This discrepancy can occur due to the sensor's time delay, the dimensional distribution of temperature across the entire area of chips connected in parallel, and temperature variations resulting from the output modulation frequency. The discrepancy in monitored temperature resulting from measurement delay can be estimated using the scaled thermal impedance of the temperature sensor, which can be a reference value to limit the increase in the output current. The temperature difference due to non-uniform thermal distribution across the entire area of multiple chips can be estimated by applying the thermal impedance for the hottest spot, which can be simulated using the FEM method. Additionally, a temperature gap can occur due to fluctuations in the output modulation frequency, which can be estimated using inverter simulation for steady-state operation. 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