What is Kelvin connection?
A Kelvin connection is a four-wire connection method for highly accurate measurements of resistance, voltage, etc. For 4-pin package MOSFETs such as TO-247-4L, using this method, it is possible to apply the gate-source voltage VGS without being affected by wiring resistance and inductance.
When measuring a resistor R whose resistance value is unknown, the resistance can be obtained by applying current I using a current source and measuring the voltage as shown in Fig. 1. At this time, if the wiring is long, the measurement accuracy may decrease due to the resistance of the wiring.
In such a case, by setting a measurement path separate from the current flow path as shown in Fig. 2, even if there is wiring resistance r' in the measurement path, current will not flow in the measurement path. As a result, the voltage across the resistor can be measured with high accuracy, enabling highly accurate resistance R measurement.
A MOSFET is a device controlled by the voltage between the gate and source. When the drain current is large, the same phenomenon as shown in Fig. 1 occurs when the voltage is applied between the gate and source. In particular, during turn-on/turn-off of the MOSFET, where the drain current changes greatly, the voltage change due to the wiring inductance becomes large and may not be negligible.
Consider the circuit in Fig. 3. A gate voltage VGS is applied externally between the gate terminal and GND.
Due to the change in the current (dID/dt) through these inductances during turn-on and turn-off, the source voltage VLS has a voltage of VLS = ( Lb + LW ) x dID/dt from GND, which greatly affects the switching characteristics.
As in the circuit in Fig. 4, when the path through which the drain current ID flows and the path through which the gate-source voltage VGS, which is the control signal, is applied are separated, VGS can reduce the effect of ID.
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