Dynamic On-Resistance Measurement Technique for GaN Power Transistors

Dynamic On-Resistance  Measurement Technique for GaN Power Transistors

Dynamic on-state resistance (RDS(ON)) is critical for the reliable and stable operation of GaN power transistors. However, many engineers are struggling to evaluate dynamic RDS(ON) because of the difficulty in measuring it consistently with sufficient resolution. In this article, we discuss a measurement technique of dynamic RDS(ON) using a double pulse test system with a clamp circuit.

By Takamasa Arai – Keysight Solution Application Engineer, Ryo Takeda - Keysight Solution Architect, Bernhard Holzinger - Keysight Technical Architect, Michael Zimmerman - Keysight R&D Engineer, and Mike Hawes - Keysight Power Solution Consultant

“Current collapse” behavior of GaN Power Transistors

While GaN power transistors are becoming popular in power electronics applications because of their low energy loss and high power density capability, design engineers still have some concerns about their reliability. One of the key concerns about GaN power transistors is their dynamic on-state resistance (RDS(ON)) increase during switching operation, the phenomenon known as “current collapse”. Current collapse is the result of trapped electrons in the transistor structure when a high drain off-voltage is applied.  It takes time to clear out the trapped electrons during a switch on event, which is characterized by the dynamic RDS(ON) measurement.  Increased dynamic RDS(ON) degrades conduction loss of the GaN power transistors and leads to higher temperature, which affects reliability of the GaN power transistor and the system overall. Although many manufacturers provide “collapse-free” GaN power transistors, engineers are still concerned about the effect of the current collapse. Therefore, not only device manufacturers, but also power converter design engineers need to evaluate dynamic RDS(ON) of GaN power transistors accurately.

Challenges for dynamic ON-state resistance measurement

Many engineers are struggling to evaluate dynamic RDS(ON) accurately. There are two main reasons: 1) overdriving, and 2) the limitation in the oscilloscopes’ dynamic range.

When we measure dynamic RDS(ON), we would like to set the oscilloscope range just enough to monitor only on-state drain voltage (VDS(ON)) such as 1V/div, giving us the best resolution from the oscilloscope.  Unfortunately, the transistor is switching from high drain off-voltage (VDS(OFF)) such as 400 V.  The amplifiers in the oscilloscope distort the waveform if the measurement range is not wide enough to cover both VDS(OFF) and VDS(ON). This phenomenon is called “overdriving” of the oscilloscope [1] and results in saturated oscilloscope amplifiers and erroneous VDS(ON) measurements.

Therefore, we must set the oscilloscope range wide enough to capture both VDS(OFF) and VDS(ON) to avoid overdriving the input. However, the issue we come across this time is the limitation in the dynamic range of oscilloscopes. Even the high-end oscilloscopes, which have the highest vertical resolution in the market only have around nine effective number of bits (ENOB) at 20 MHz bandwidth (NOTE:  In most cases, ENOB is the more useful parameter to use than the raw number of bits of the ADC in the oscilloscope.  Often a few of the raw bits are below the noise floor of the amplifier, making them unusable). Therefore, the oscilloscope can only identify 1/29 = 1/512 of full scale. If VDS(OFF) is 400 V, the minimum resolution will be 400/512 = 0.78 V, which is completely unacceptable resolution for dynamic RDS(ON) measurements.