A Fast Voltage Clamp Circuit for the Accurate Measurement of the Dynamic ON-Resistance of Power Transistors

For determining the dynamic ON-resistance R-dyn,(on) of a power transistor, the voltage and current wave-forms have to be measured during the switching operation. The novel heterostructure wide-bandgap (e.g., AlGaN/GaN) transistors inherently suffer from the current collapse phenomenon, causing the dynamic ON-resistance to be different from the static. Measuring voltage waveforms using an oscilloscope distorts the characteristics of an amplifier inside the oscilloscope when the range of the measurement channel is not set wide enough to measure both ON-state and OFF-state voltage levels, resulting in failure to accurately measure the voltage waveforms. A novel voltage clamp circuit improving the accuracy of the transistor's ON-state voltage measurement is presented. Unlike the traditional clamping circuit, the presented voltage clamp circuit does not introduce delay caused by RC time constants, keeping the voltage waveform clear, even during state transitions of the device under test. The performance of the presented circuit is illustrated by measurements on a 2-MHz inverted buck converter.