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Switchable Gate Resistance

Optimisation of dV/dt – Losses Trade Off using Switchable Gate Resistance

System-level effects in power converters are often caused or constrained by behaviour of the power semiconductor devices, for example high dV/dt from modern IGBTs. Bulky and lossy dV/dt filters are often required in high-power converters to limit the voltage stress on insulation of wound components, e.g. motors. By taking advantage of real-time bi-directional communication over the fibre-optic link, Amantys gate drives reduce the need for dV/dt filtering through intelligent selection of gate resistance, thus enhancing the overall system performance and power density.

By Dr Angus Bryant, Amantys Power Electronics Limited

Introduction

The rate of change of voltage, dV/dt, generated by the power semiconductors in an inverter bridge has a significant impact on any wound components connected to its output. In a motor drive, the interaction of cable impedance and motor winding capacitance causes over- voltages in the windings depending on the dV/dt generated in the inverter. Limiting the dV/dt is often a key requirement in power converter and motor drive design, which results in bulky and lossy dV/dt filters at the converter output or excessive demands on wound component insulation.

Related Posts:  Testing and Managing Converter Power Stacks

Switchable Gate Resistance on Amantys Gate Drives

Amantys gate drives feature Power Insight, which is a two way communications protocol between the converter controller and the gate drive. Data is superimposed on top of the PWM switching command and feedback / acknowledge signal. This allows online configurability of the gate drive; for example, the converter designer can modify gate resistor vales (Rg_on, Rg_off and Rg_soft_off) and gate-emitter capacitances (Cge)

without removing the gate drives from the inverter stack or interrupting device switching. This opens the door to selection of gate resistances appropriate to different converter operating conditions while the converter is switching.

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Conclusions

This article has explored the application of selectable (switchable) gate resistances to optimise the trade-off between dV/dt and switching loss, using real-time bi- directional communication over the fibre-optic link. The need for dV/dt filtering is reduced, thus enhancing the overall system performance and power density, and reducing stress on the electrical machine.



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