Power supply measurement tips (Part 5): Testing power stage switch characteristics

 

    Before you begin, make sure the startup, shutdown, duty cycle, and dead time of all switches, such as MOSFETs and IGBTs, are as expected. Tektronix oscilloscopes include a high-resolution mode that radically increases vertical resolution so startup and shutdown times can be calculated with the highest accuracy.

 

    Although almost all components of a power supply experience energy losses, the majority of losses occur when the switching transistor transitions from the off state to the on state (or vice versa). Using the trajectory plots of the start-up losses and turn-off losses for all switching cycles (available in the DPOPWR software), a more complete understanding of switching losses can be obtained, as shown in the figure below.

 

    Switch track map on/off.

 

    At this point, check all VGS signals for noise and glitches. This is an important step because any unexpected glitches on this terminal can cause unwanted startup and shoot-through. To ensure that shoot-through is not possible, check the dead time of the synchronous rectifier or H-bridge.

 

    The timing relationship between the gate drivers and related instrumentation is then verified to ensure it matches the calculations for the design.

 

    To safely measure signals that are not ground referenced, we recommend using a differential probe with the appropriate voltage rating. Be sure not to float the oscilloscope, as this can lead to poor results. You can consider the TDP1000, TDP0500, or P6251 high-voltage differential probe, depending on the application. Each probe implements high-speed, broadband acquisition and measurement capabilities, provides outstanding electrical performance, universal DUT connections, and is easy to use.

 

    It is undoubtedly difficult to measure floating gate signals. We recommend probing the signal at the gate driver input so that you can verify the dead time between the top FET and the bottom FET.

 

    Measuring current at the lowest voltage slew rate can also help you minimize crosstalk and improve accuracy.