Design of Current Sensors in IGBT's

The operation of current sensors (or pilots) in insulated-gate bipolar transistors (IGBTs) is described for the first time 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 to the sensor current decreases. Experimentally, the dependence of the normalized current-sensor (or pilot) voltage (V
_p
) on the main IGBT current of a 500 V, n-channel, asymmetric IGBT bas 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. Also, during IGBT turn-off, the current and voltage in the current sensor tends to lead, due to lesser RC delay, the main device. This could lead to latchup or loss of gate control in the current sensor path. To avoid this problem, a different type of current sensor is designed and characterized. The n+ source diffusion is removed from the current sensor area and hence the MOS channel current as well as the parasitic thyristor is removed. This current sensor only collects the minority carriers (holes here) injected from the p+ substrate. However, it has more non-linearities in its current ratio when compared to the conventional structure. The usage range of the current-sensor voltage can be obtained by examining I
_pE
vs V
_p
.