To a large extent, the position of the sensor within the module affects its ability to protect its temperature. In fact, the position of the sensor is more important than the error of the sensor in this respect, especially if the hardware disconnect level is set by the driver or control circuit.
A study was conducted on the effects of sensors at different locations. A model of the power module is shown in Figure 2. The module has no copper backplane and is mounted on an air-cooled aluminum heat sink. Different sensors have different thermal couplings, from sensor A directly connected to the power semiconductor on the same copper layer, to sensors B and C isolated at different locations within the module, to sensor D placed next to the module on the heat sink. Each sensor has a different junction (j) to sensor (r) thermal resistance Rth(jr) due to different thermal coupling.
Figure 2 Case study of different temperature sensor locations within the power module; model and temperature simulation
The trip level for overheat protection can be set for each sensor in quasi-static conditions. For example, if Tj cannot exceed 140 °C, the "thermal shutdown" trip level of the case study system will be 120 ° C (sensor A), 110 ° C (sensor B), 100 ° C (sensor C) to 70 ° C (sensor D) Don't wait. The better the coupling between the source and the sensor, the lower the impact of the cooling system. This is a big advantage of an integrated solution.
However, for other cooling conditions (heat sink material and base thickness, cooling medium, thermal grease thickness), the trip level has to be set to a new value. This makes it difficult for IPM manufacturers to set the thermal shutdown level to an appropriate value for any given application. For this purpose, the sensor signal should be monitored by an external host controller and, if required, the thermal protection level should match the cooling system.
In order to show the effect of the cooling system, the thickness of the thermal grease layer is increased from the original 50 μm to 100 μm. Since sensor A has the best thermal coupling with the power semiconductor, it can be seen that the effect on Rth(jr) is the lowest, and its value is only increased by 3%. The Rth(jr) values ​​of sensors B and C increased by 7% to 8%. The cooling system has the greatest influence on the Rth(jr) of sensor D, and its value increases by more than 25%.
Another issue is whether the temperature sensor can protect the power semiconductor in the event of a short-term overload. There is a delay in the time each sensor reacts to an increase in junction temperature, which is related to the position of the sensor. This characteristic is described by the thermal impedance Zth(jr). Its performance is inconsistent with expectations (see Figure 3). A comparison of Zth(jr) with the thermal impedance Zth(jr) of the junction to the heat sink (directly under the chip) shows that after one second the junction heat dissipation of the system has reached a steady state condition, and the junction of the system - The sensor takes 100 seconds to reach steady state. The reason for this is the thermal diffusion inside the heat sink.
Figure 3 junction (j) to different position sensor (rX) and heat sink thermal impedance
For each power semiconductor, the maximum value of its static power consumption Ptot is specified. For an overload jump from 50% Ptot to 200% Ptot in the example, the semiconductor will overheat after a period of time. Sensor A will reach its 120°C trip level after 0.19s, providing reliable device protection and maintaining the junction temperature at approximately 150°C. The junction temperature of the device protected by sensors B and C will be in the critical range of 160 ° C to 170 ° C; in these cases, the sensor requires 0.3 to 0.4 s to reach the trip level. Depending on the nature of the device, this may mean that the limits specified in the data sheet have been exceeded. Sensor D has a reaction time of more than 1 second, so the device cannot be protected. For situations where the overload is very high and the starting temperature is low, the temperature sensor does not provide any suitable protection.
An overview of the advantages and disadvantages of different temperature sensor locations is listed in Table 1. Due to isolation, sensors located at the B position are now the preferred solution. If the driver has a protection circuit in the future and the signal is changed on the secondary side of the driver, it may mean that sensor position A may be a better solution.
Table 1 Comparison of whether different temperature sensors are suitable for protecting power semiconductors
Sensor A | Sensor B | Sensor C | Sensor D |
With power semiconductors Excellent thermal coupling | With diodes and IGBTs Acceptable thermal coupling | Thermal coupling with the IGBT is acceptable, Insufficient thermal coupling to the diode | Low thermal coupling |
Fast reaction time | Medium reaction time | Medium reaction time, faster than sensor B | |
External cooling system pair The influence of R th ( jr ) is small | External cooling system pair R th ( jr ) has an effect | External cooling system for R th ( jr ) The impact is greater than sensor B | External cooling system pair The influence of R th ( jr ) is large |
No isolation, additional measures on the drive side | Basic isolation, requiring additional security measures | Basic isolation, need extra Safety isolation measures | Safety isolation |
Integrated protection
If a short-term overload occurs, there will be a gap in the device protection. The current sensor's trip value is set to a higher value to allow for a short-term overload, such as when the motor is starting. Long-term operation at this current level will cause the device to overheat. In most cases, the temperature protection element has a reaction time that is too long to detect such overheating.
One possible way to fill this gap is to use software shutdown of current and temperature signals. The inverter controller calculates the junction temperature based on the temperature and electrical operating conditions of the sensor. The junction temperature at tp can be calculated by:
Tj(tp)=Tr+P0*Rth(jr)+(Pover-P0)·Zth(jr)(tp)
P0 is the power consumption at t=0s, and Pover is the power consumption at the time of overload. Here, the thermal impedance Zth(jr) is also required to simulate the temperature signal Tr as described in the data sheet.
to sum up
The integrated sensor in the IPM protects power modules like the SKiiP under a wide range of operating conditions. Equipped with a suitable evaluation circuit that provides high quality information for process control as a synergistic effect. This saves space, cost and development time. With an external observer, the combination of available sensor signals fills the gaps that are specifically protected in the application.
Previous page
Special-Shaped Needle Rollers,High Precision Needle Rollers,Spherical Head Needle Roller Pins,Flat End Cylindrical Rollers
Qingdao Yukun Bearing Needle Roller Co.,Ltd. , https://www.ykneedleroller.com