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FRD risk assessment and testing methods


Using the IGBT double-pulse test circuit and changing the position of the voltage and current measurement probes, the related parameters of the freewheeling diode (ie FRD) connected in parallel with the IGBT can be measured and evaluated.

1. Risk assessment when working with FRD


The parallel FRD in the IGBT module is a very important component, but it is often overlooked. The risks at work are mainly reflected in the following two aspects:


1. When the IGBT is short-circuited or faulty, the IGBT driver can help protect it, but when the FRD chip is damaged, there is no other means of protection;


2. When the IGBT is turned on, it is actually the moment when the FRD is turned off. All power semiconductors, including IGBT chips and FRD chips, face far greater risks when they are turned off than when they are turned on;


2. FRD parameter measurement method


In the FRD parameter measurement circuit, the test circuit is the same as the double-pulse test. The specific probe connection and calculation are as follows:


1. Add the current probe to the collector of the upper tube IGBT;


2. Add the voltage probe between the C-E poles of the upper tube IGBT;


3. Taking the product of the instantaneous value of the detected voltage and current as a function, the instantaneous power of the diode can be calculated;


4. FRD will only have reverse recovery behavior when the IGBT is turned on for the second time. When capturing waveforms with an oscilloscope, you should pay attention to the time selection.


3. The relationship between FRD performance and external parameters


When the external parameters change, the risk of the diode also changes. Here, we give a few parameters that need special attention:


1. The size of the gate resistance of the drive. The size of the gate resistance driven by the IGBT directly affects the di/dt size of the front edge of the FRD. The smaller the gate resistance, the larger the di/dt of the FRD, the easier the FRD reverse recovery process is to oscillate, and the more easily the device is damaged;


2. Junction temperature. Since the on-voltage VF of FRD has a negative temperature coefficient, the lower the junction temperature, the faster the switching speed of the diode, the steeper the trailing edge of its reverse recovery current, and the higher the voltage spike generated;


3. The level of the bus voltage. The higher the bus voltage, the higher the pressure across the FRD.


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