When designing distributed intelligence in automotive power management systems, it is critical to ensure that the protection mechanism is truly intelligent when it comes to smart power switches, especially in scenarios involving multi-channel drives, where even a slight run-out of balance or an unexpected load short circuit can affect the protection effectiveness.
Intelligent drives play a key role in managing and distributing automotive battery packs to various components (ECUs, motors, lights, sensors, etc.) that control different electrical loads simultaneously, such as resistive, inductive, and capacitive actuators. Equalizing the current in all channels is essential for the proper functioning of the drive and ensuring the proper and efficient operation of the vehicle. In circuit layout, any unexpected situation such as a slight imbalance that causes a concentration of current through a specific metal path, damage or failure of a load, and improper wiring can cause a local circuit to accumulate current. Electrical imbalance can cause the chip to overheat and hot spots to gather, eventually damaging or burning components.
Despite thermal simulations and precautionary measures, the implementation of intelligent protection mechanisms needs to be checked and verified, which can help to identify potential problems that may affect the timeliness of interventions.
Thermal detection in smart switches
High-side switches need to handle large currents in a compact package with very little space, and current equalization is an important factor in efficiently managing heat. Smart power switches are often installed in enclosed areas with poor ventilation and heat dissipation, which makes thermal management even more important.
Therefore, the intelligent performance of the protection mechanism depends on the embedded thermal diagnostics that are based on thermal detection and protection mechanisms to monitor the temperature of the drive and perform protection actions when the temperature exceeds a preset threshold. Accuracy is a challenge for temperature measurement technology, as the current equalization of a multi-channel driver has a significant impact on the accuracy of temperature measurement.
Sudden increases in local current density or short circuits are of great concern to designers, as these two phenomena can create scattered hot spots that can lead to sudden heat build-up effects that can cause sudden temperature increases. These conditions can lead to overheating and component failure, as well as costly repairs.
To prevent thermal shock from damaging the components, the protection circuitry is designed to limit the current and keep the power MOSFET within the safe operating area (SOA) until the thermal shutdown function is triggered, shutting down the driver. However, this type of protection can create physical stresses on the surface of the power device. To meet wave requirements and process tolerances, the current limit value needs to be set high, but when driving a short-circuit load, a higher current limit value can cause the temperature on the chip surface to rise rapidly. Sudden temperature changes can create large thermal gradients on the chip surface, resulting in thermomechanical stresses that affect device reliability.
The solution for the VIPower M0-9 is to integrate a temperature sensor in the low-temperature and high-temperature regions of the high-side driver (as shown in Figure 1).
Figure 1: Schematic diagram of a smart switch with different temperature sensors
The temperature sensor is manufactured using polysilicon diode technology because the temperature coefficient of the polysilicon diode remains very linear over the entire operating temperature range. The cryogenic sensor is placed in the cryogenic zone inside the drive near the controller side, while the high-temperature sensor is located in the power stage area, which is the hottest area inside the drive.
This dual-sensor technology limits the drive's temperature rise, as thermal protection is triggered when the temperature reaches an over-temperature threshold, or when the dynamic temperature difference between the two sensors reaches a threshold. Once the overheating fault is gone, the smart switch reactivates when the temperature drops to the recovery value.
This method helps to reduce thermal fatigue caused by thermomechanical stress on the switch. Thermomechanical stress can increase over time, resulting in reduced switching performance and reliability.
Thermal mapping
In addition to thermal simulation experiments and preventive methods, infrared (IR) thermography is an effective technique for obtaining thermal maps of drivers, allowing designers to gain a comprehensive understanding of the heat distribution within an integrated circuit, revealing all potential risk factors.
In order to evaluate the protection of intelligent protection circuits in harsh automotive environments, the heat distribution within the drive must be analyzed in two different application scenarios and under harsh short-circuit conditions:
· Short Terminal (TSC)
· Short Load Circuit (LSC)
A terminal short circuit is a condition in which there is a low-resistance connection between the terminals of a component or device, as shown in Figure 2.
Figure 2: Temperature measurement test circuit under TSC conditions
On the other hand, when there is an inductive path between the load and the power supply, a load short circuit occurs, resulting in a sudden surge in current (Figure 3).
Figure 3: Temperature measurement test circuit under LSC conditions
The test conditions are as follows:
· Tamb = 25 °C
· Bat= 14 V
· 当热成像时，Ton = 1 ms
· When capturing the temperature of the thermal sensor and hot spots, Ton = 300 ms
· TSC条件: RSUPPLY = 10 mΩ, RSHORT = 10 mΩ
· LSC 条件: RSUPPLY = 10 mΩ, LSHORT = 5 µH, RSHORT = 100 mΩ
thereinto
Tamb is the ambient temperature
Vbat DC battery voltage
Ton is the short circuit duration
RSUPPLY is the internal resistance of the battery
RSHORT is a short-circuit resistor
LSHORT is a short-circuit inductor
To generate a thermal map, we used an infrared camera to capture the infrared radiation at each location and then convert it into a temperature value. To ensure that a particular color is converted to the correct temperature value, calibration is an essential and important process. The process compares the different colors captured by the sensor with the known temperature values, analyzes specific thermal parameters and their tendency with increasing temperature. By analyzing these parameters, the calibration process ensures that the heat map accurately reflects the temperature distribution of the area being scanned.
To calibrate the infrared camera sensor, the forward voltage (VF) of the MOSFET body-drain diode is chosen because it is linear with temperature. However, the diode needs to be pre-calibrated to accurately determine its temperature coefficient. The temperature coefficient of the diode can be determined by measuring the voltage VF of a constant forward current (IF) while changing the temperature from 25°C to 100°C. To prevent temperature rise caused by the current and its associated power dissipation, the IF value should be in the range of 10mA to 20mA.
The temperature coefficient of the diode is calculated by linear interpolation and mathematical fitting using the VF values collected at different temperature conditions, as shown in Figure 4.
Figure 4: Pre-calibration of a MOSFET body drain diode
Calculated with the following formula (1):
Thereinto:
Dt is the amount of temperature change;
DVF is a forward voltage change;
K is the temperature coefficient of the diode.
To create a heat map, first take a 1ms interval of each temperature point with an infrared imaging sensor. After capturing all the points on the sensor (which takes about 3000 seconds), the dedicated software generates a heat map that depicts the temperature of each point according to the minimum spatial resolution of the infrared sensor. By placing the heat map on top of the chip line diagram, it is possible to identify the hottest hot spots in the work area, and the coordinates of those hot spots can be determined as the current flows through the device.
Figure 5 shows a heat map of the VND9012AJ dual-channel smart switch under TSC conditions.
Figure 5: Heatmap of the VND9012AJ channel under TSC conditions
Thermography is an important way to detect any overheated areas and ensure that the drive is operating at a safe temperature range using different colors over a temperature range of 25°C to 150°C. By providing a heat map of each channel under different operating conditions, the heat map test method can verify the reliability of the drive's operation without increasing the temperature to the maximum threshold.
In order to find hot spots and monitor the temperature changes of high-temperature sensors and cryo-sensors, and to verify the effect of the thermal shutdown mechanism, it is necessary to consider extending the short-circuit time to 300ms in the experiment.
Figure 6 shows the temperature change of the VND9012AJ observed at TSC.
Figure 6: Temperature change of two sensors under TSC conditions
The graph above shows that the high temperature sensor detected hot spots in both channels of the VND9012AJ, with a maximum temperature in the 150 °C range.
Figure 7 shows a heat map of VND9012AJ under LSC conditions.
Figure 7: Heat map of the VND9012AJ channel under LSC conditions
Figure 8 shows the temperature change of the VND9012AJ observed under LSC conditions.
Figure 8: Temperature change between two sensors under LSC conditions
In both cases, thermal protection mechanisms are triggered, limiting the current to a safe level. Therefore>> to conclude the full article, click on this "link" to read the full article.