Design of current sensors in IGBTs

The operation of current sensors (or pilots) in IGBTs (insulated-gate bipolar transistors) is described experimentally and with two-dimensional simulations. Two different sensor structures are compared. In the most conventional structure, a small IGBT, separated from the main device by metallization only, is used as a current sensor (or pilot) and the emitter of this small IGBT is connected to ground via a resistor. As the voltage at the emitter of the current sensor increases due to increasing current flow, the forward drop across the current sensor decreases and the ratio between the main current and the sensor current decreases. Experimentally, the dependence of the normalized current sensor (or pilot) voltage on the main IGBT current of a 500-V, n-channel, asymmetric IGBT has been shown to be fairly linear, except when a small main current or a large pilot resistor is used. The main-to-pilot current ratio increases with increasing carrier lifetime and increasing distance from the main IGBT.