Power Supply Instability (1 of 3)

The power supply can operate in different modes, such as the continuous and discontinuous current mode, presenting different control loop parameters in each mode.

The input voltage can appear in the gain of the feedback loop, changing the gain characteristics as the input voltage swings over the wide operating range typical of many applications.

Load variations affect the location of poles or zeroes associated with the output filter.

The effective value of components contributing to poles and zeros, such as inductors and the load resistance, can vary as a function of line and load variations.

The circuit can contain both real and complex right-half-plane zeroes that migrate as a function of line, load, and temperature.

Many of the components affecting circuit poles and zeroes are nonlinear, such as swinging inductors.

Many of the components affecting stability have large variations in tolerance as purchased and over operating temperatures and system life, such as electrolytic capacitors.

Long power source leads or added system filters, such as EMI filters can have a dramatic effect on system stability criteria. See Input Filter Interaction.

Added load capacitance, including high quality decoupling capacitors shorting the ESR of the power supply output capacitor, which may be contributing a stabilizing zero, can effect stability.

Minor loops inside the power supply, like those causing emitter follower oscillations or power MOSFET drive resistance instabilities, may go unstable over the range of purchasing tolerance or variations caused by manufacturing, temperature, and age. These can go unstable with little noticeable effect on output observables (except perhaps radiated EMI in the Megahertz range), but can alter the feedback loop by causing saturation of components or DC level shifts in interior states and can greatly degrade field reliability.

Things like the magnetizing current in the magnetics can affect stability.

Switching noise can affect stability and stability measurement. One of the industries first challenges with switching-mode power supplies was trying to measure small gain and phase signals in a noise environment much larger than the signal. See personal anecdote below.

Chaos can occur in these circuits. See Chaos and personal anecdote below.