Today, operational parameter monitoring has become an integral part of power modules. In power modules, temperature sensors have become more or less standard, and even current sensors are being used more and more widely. In fact, integrated sensors are a more cost-effective solution than external sensor solutions, which provide additional protection for the user while reducing the size of the module.
current sensor
If a power module is equipped with a current sensor, its signal is mainly used for output current control (for example, in transmission applications) and can also serve as a protection device. The need for motor control determines the characteristics of the current sensor. In many cases, faults (including temperature drift) must be less than 1% to 2%. The requirements for temperature (-40 ° C ~ 125 ° C) and low current loss are set by the power module itself. The device protection function sets the overcurrent capability (the maximum short-circuit current is 5 times the rated current) and the upper cut-off frequency (>100 kHz).
For low and medium power devices, the use of current shunts is an accurate, cost effective solution. The current limit is approximately 30 to 40A. The downside is that there is additional power loss, and if the shunt is used to measure the emitter current, it will lose isolation and there will be interference in the IGBT gate signal.
For high performance and high power semiconductor modules, electrically isolated sensors are typically used. Pure Hall effect sensors without compensation current have poor performance in terms of error and temperature stability. Sensors can be used in user-specified modules because the requirements in these modules are clearly defined. Sensors with high linearity and low temperature drift operate with the compensation current. This current cancels the magnetic field of the current measured in the sensor core. The control signal for the compensation current amplifier is provided by a Hall effect, magnetic field or magnetoresistive probe.
For intelligent power modules (IPMs) such as the SEMIKRON SKiiP system, the use of high-precision sensors is most suitable due to the high performance requirements of the final application. In the final application, the sensor is integrated directly into the module's housing, wrapping around the main terminal to save space (Figure 1). The evaluation circuit for signal monitoring and conversion is part of the driver circuit. The specially designed ASIC chip guarantees high integration and high reliability, which is difficult to achieve in an external sensor solution.
Inside the IPM, the current monitoring circuit is directly connected to the driver circuit. It detects an external short circuit in the shortest time and shuts down the power semiconductor within 2 to 3 μs. In the future, this feature will become more and more important, because the new generation IGBT only allows a short circuit time of 6μs compared to the past IGBT allowing a short circuit time of 10μs.
The current sensor at the AC terminal of the voltage source inverter circuit cannot detect a short circuit in the inverter bridge. Here, by monitoring VCE(sat), the slope resistance of the semiconductor in the on state is used for protection purposes. This method is sufficient for short circuit protection, but it is not suitable for current measurement.
Figure 1 SKiiP power module with AC sensor integrated with AC sensor
Temperature Sensor
There are several temperature sensors available for device protection. These sensors have a negative temperature coefficient (NTC) or a positive temperature coefficient (PTC). The most used in standard industrial modules is the NTC sensor. SEMIKRON uses its own silicon chip sensor SKCS, which features PTC characteristics, high linearity and low error. In conjunction with suitable monitoring circuits, IPMs such as SKiiP provide an analog output signal for temperature measurement and protection with a failure rate below 5 °C.
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