Bode diagram of a COT converter

Bode diagrams are normal only valid for LTI (Linear, Time Invariant) systems. Can you measure and get any benefit from a COT converter bode diagram?



The COT buck converter can be measured with a gain-phase analyzer and a bode plot generated. The results will likely not reflect the real performance of the COT system, however, as only the gain and phase margin results are useful, at a single switching frequency, but the bandwidth for COT will be much higher than measured.

The better way to evaluate the performance of the COT system is with transient response measurement. The incremental error voltage feedback with direct response can provide much quicker step response than the current mode systems, yielding a much higher bandwidth and better stability, for the given bandwidth.

For a complete treatment of the comparison between current mode and COT buck converter gain, phase and bandwidth, as well as load transient response, see application note #136, on the MPS website.

Just FYI, our NISM impedance based stability does work for COT converters.

It is available for most vector network analyzers and is also now available for the Keysight ADS simulator. We did a paper using NISM at DesignCon 2018 with Heidi Barnes from Keysight and Jack Carrel from XYLINX (now AMD).

This is much more precise than a step load and correlates well with step load, since they both measure Stability Margin and not phase margin/gain margin.

An even better way is using the Picotest NISM (Non-Invasive Stability Margin) tool. That uses closed loop measurement data (output impedance), which being a narrow band, frequency domain measuring with LOG displays has much better dynamic range and fidelity.