I’ve been reviewing the datasheet for the UJ4C075060K4S SiC FET, and I have a question regarding Figure 5, which shows the drain-source on-resistance (RDS(on)) versus drain current (ID) at different junction temperatures (TJ).
I understand that “TJ” refers to the junction temperature, but I would like to get more clarity on how exactly this parameter is measured during testing. Specifically, how is the graph for RDS(on) vs. ID created at different TJ values like -55°C, 25°C, and 175°C? What are the testing conditions and methods used to collect these measurements, is the current pulsed, and how is the junction temperature controlled during this process?
Any insights into the equipment or methodology used to generate such graphs would be greatly appreciated!
Excellent question. Yes, the output characteristic and other parameters are measured with narrow pulses to limit self-heating to a negligible amount. The datasheet shows the maximum pulse width for the output characteristic tests, which for the UJ4C075060K4S and other parts is 250 microseconds. Temperature is feedback-controlled with a ceramic heater attached to the device under test, or by a temperature controlled chamber.
Thank you for the great answer regarding the pulse testing conditions for measuring R_DS(on). I’m curious if you have a method for determining R_DS(on) with respect to a DC I_Drain and temperature under continuous conditions. Specifically, is there a way to calculate this parameter without relying on pulse measurements, accounting for possible thermal effects over time due to self-heating?
You can sense drain current knowing the chip temperature of a JFET. How to do this is described in the JFET User Guide. You sense temperature by injecting a small current into the JFET gate, and knowing (approximately) this current and measuring Vgs, you can accurately measure the JFET chip temperature. Knowing the temperature, you know its Rds(on), and then you can use a Vds measurement to sense the current. This technique would be impractical with a cascode, or other device types for that matter such as MOSFET and IGBT, due to the change in on-resistance with temperature.