Phase Margin Degradation from Digital Filter Latency in Control Loops
Digital filter latency introduces pure time delay into feedback loops, eroding phase margin at crossover frequencies by 360 degrees per cycle of propagation delay.
Power Converter Bandwidth
Dynamic Response
Switching regulator control loops are defined by the frequency range over which they can actively track load and line changes. This capability is quantified by power converter bandwidth, which represents the unity-gain crossover frequency of the open-loop transfer function. It establishes the speed of the transient recovery and the limits of the output voltage regulation under dynamic conditions.
Loop Frequency
The control loop must be designed with a crossover frequency that is a fraction of the switching frequency to prevent instability. In high-performance systems, the power converter bandwidth is typically set to one-tenth or one-fifth of the switching frequency. This separation ensures that the high-frequency switching noise is attenuated before it can affect the feedback loop.
Measurement Procedure
Frequency response analyzers measure this parameter by injecting a small-signal AC perturbation into the feedback path. This measurement yields the bode plot of the loop gain, showing the phase and gain margins at the crossover point. Standard procedures require verifying these margins across the full temperature and load range of the unit.
Operating Limit
Component aging and output capacitor degradation reduce the effective margins of the converter. A decrease in these margins leads to ringing or oscillations, indicating a loss of loop stability.