Input Impedance Measurement for DC-DC Converters

J2121A High Power Line Injector Supports PSRR, Input Impedance and DC Biased Inductor Testing

Testing the input impedance of a switching converter is a necessary step in designing a stable input filter to counterbalance the converter’s negative input resistance. The J2121A High Power Line Injector makes this challenging measurement easy. The J2121A uses the vector network analyzer’s (VNA) oscillator signal to modulate the input voltage (output of the J2121A) while accommodating a wide range of voltage and current conditions. The input voltage at the converter and the input current taken from the J2121A’s current sense monitor are divided in the VNA, displaying input impedance. A simple through calibration, using the included 1-ohm calibration fixture, corrects for the scaling of the current monitor and the probe connections. The plot below shows the LM20143 buck converter being tested in the image, clearly showing the negative input resistance.

The J2121A is especially suited to this measurement and other high-power applications such as those associated with military and satellite busses. It can be used with input bus voltages up to 400V and supply up to 20A at 1MHz. The line injector allows the input DC supply voltage to be modulated by the network analyzer source signal (oscillator). The J2121A line injector output is DC regulated and provides a fixed voltage drop from the input bus voltage fed into the injector in addition to the AC modulation signal. The J2121A includes a current sense monitor output that can be connected to the VNA.