Then, the error amplifier compensation can be designed to set the bandwidth or crossover frequency and the phase margin for the feedback loop. Two elements need to be considered: The feedback loop’s control to output transfer function has to be measured first when designing the feedback loop. Power supply stability is complex, and measuring the actual gain and phase of the circuit over a range of frequencies is often more effective and accurate than reliance on only simulation.īode plot analysis is needed to ensure loop stability in switching power supplies under all load and line conditions. Control loop measurements are needed to understand how a power supply design responds to changing load conditions. In a real power supply design, component variations and other factors can too easily result in an unstable feedback loop, poor regulation, and slow dynamic characteristics. (Image: Venable Instruments)Īn ideal power supply design would respond quickly to load changes and maintain a stable output voltage. Too much phase shift causes instability and oscillation as the feedback becomes positive. But at higher frequencies, reactive elements in the circuit, including parasitic elements and various time delays, cause phase shifts in the feedback loop. The difference between the two is inverted then amplified and used to control the plant. Depending on the design, an output variable, such as voltage, is measured and compared to a reference. It consists of a power processing block, also called the modulator or plant, and an error detecting block called the error amplifier or compensator. It gives designers important information about the speed of the control loop and the stability of the power supply output.Ī basic power supply feedback loop is pictured below. The converter loop transfer function defines the regulation characteristics of a power supply and can be illustrated with a Bode plot. It starts with a general overview of Bode plots as used with power supplies, then briefly reviews two methods of developing Bode plots: standard oscilloscopes and specially designed frequency response analyzers. This FAQ looks specifically at the use of Bode plots in the design of switching power supplies.

Bode plots have been extended to design servo systems, power supplies, and other systems with feedback controls. The graphical design of a Bode plot charts the gain margin and phase margin needed to ensure stable operation when dealing with circuit variations resulting from manufacturing or during use. He invented it while at Bell Labs to help design stable amplifiers with the feedback needed in telephone networks. It was developed by Hendrik Wade Bode in the 1930s and reflects the system’s frequency-domain behavior. A Bode plot represents the gain and phase of a feedback system as a function of frequency.

Bode plot simulated gain bjt multisim