Buck Converter with Stray IGBT Tail Current

Overview

This demonstration shows a simple unregulated buck converter that models the impacts of stray inductances and an IGBT with tail current.

Model

The schematic below shows a simple buck converter using the IGBT with Limited \(di/dt\) component. This IGBT model represents the collector current transients during switching, including a finite fall time during turn-off and limited \(di/dt\) during turn-on. Since the current is not turned off instantaneously, this IGBT model may be connected in series with a stray inductance, \(L_{\mathrm{2}}\) in this model.

The free-wheeling diode is also modeled as subsystem that represents the dynamics of the diode’s reverse recovery characteristic. A stray inductance, \(L_{\mathrm{3}}\) is modeled series with the diode as well.

../../_images/buck_converter_with_stray_igbt_tail_current_DIN.svg — Fig. 1 Buck Converter with Stray IGBT Tail Current

Simulation

The simulation shows the over-voltages that stem from the stray inductances and the reverse recovery of the free-wheeling diode. The \(di/dt\) limitation during turn-on determines the magnitude of the reverse recovery effect in the free-wheeling diode.

../../_images/scope_small.svg — Fig. 2 Voltage and current transients during IGBT turn-on

Conclusion

In contrast to the ideal IGBT model that switches instantaneously, this model represents the IGBT transients during switching. With this approach stray inductances may be connected in series with the device. The additional voltage stress due to the diode reverse recovery effect is also modeled.