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Modelling of IGBT-based power electronics for HEV

13 Sep 2013  | Shengnan Li

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The protection circuit senses the Vce and turns on the reaction circuit. One of the key factors for the protection to work properly is the IGBT module turn-off delay and fall time, which is why it's so important to have an accurate IGBT model. The other key factor is selecting components for use in the protection circuit to ensure the reaction time meets the requirement. In this case, the aim is to keep Vce below 550V. The red curve in figure 3 is the voltage overshoot during turn-off, without protection. The blue and green curves show the difference in behaviour by changing the time delay in the control loop and the gate driver. Using Saber or Spice models of the driver circuit ensures the accuracy of the overall simulation model.

Figure 3: Using simulation to understand the effects of changing device parameters within the protection circuitry.

Accurate Saber models help to verify the function of the designed circuit, select proper components, and tune the parameters. By using simulation in advance of implementing hardware, significant time has been saved and engineering costs reduced.

Conducted electromagnetic interference (EMI) prediction
Conducted emissions exist in every power circuit. They occur as a result of fast-changing switch currents or voltages, which are common in pulsating circuits. The frequency of interest for EMI usually ranges from 100kHz to 100MHz.

The pulsating current or voltage in a power electronics circuit looks like the square wave in figure 4a. The Fast Fourier Transform (FFT) analysis of the square wave is shown in figure 4b. The attenuation of the spectrum depends on two factors of the square wave: the first crossover frequency depends on pulse width, and the second corner frequency depends on the rise and fall time of the square wave. Hence we need to model the rise and fall time of the IGBT model because it affects the result.

Figure 4a: The signal is modelled as a trapezoidal waveform. The waveform duration (t0) defines the first corner frequency, the rise or fall time (Tr or Tf) defines the second corner frequency.

Figure 4b: Corresponding frequency spectrum.

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