Analysis of a Transistor-Based On-State Voltage Measurement Circuit for Condition Monitoring of Power Transistors

This paper presents the design and operation of an n-channel depletion mode MOSFET based ON-state voltage measurement circuit. A current limiter circuit is used as a voltage blocking stage and combined with a diode-based voltage clamp, enabling an arbitrary measurement range of the ON-state voltage by choosing an appropriate Zener diode. The proposed topology consists of a blocking stage realized with a commercial off- the-shelf 600 V MOSFET that can be cascaded in series, to increase the maximum allowed input voltage in positive and in negative direction. Experimental results exhibit a flat transient response of the circuit until a frequency of 60 MHz. Dynamic performance shows a settling time down to 50 ns after which the ON-state voltage measurement is accurate for a turn-on transient. By cascading multiple transistor stages, the settling time is limited to 500 ns due to the time needed to discharge the gate-source capacitance after blocking. This circuit combines a low-complexity setup with high dynamic performance, including a scalable approach to the blocking stage which allows for its applicability to a wide range of voltage classes.