Step by step for an optimised flyback design: Isolated close loop design - Part 2

January 19, 2012 // By Michele Sclocchi
Michele Sclocchi of Texas Instruments in this second part of a two part series continues with a step by step guide on how to approach optimising a flyback design.

This article is the second part of a two-part series. To read part 1 of the series click here.

The control close loop modelling and compensation stage of an isolated flyback power supply has always been considered the most challenging part of the design, with complicated AC modelling, and obscure compensation rules. Designers tend to avoid the pure academic mathematic approach to the problem and rely more on practical lab test results, like load and line transient response, or the switching waveform of the switching node of the converter.

This article demystifies close loop modelling of a flyback design with a simple mathematic approach, guiding the designer to a well-optimised design.

Close loop design

The best design approach is to have a simple and intuitive model of the control circuit, select the proper compensation network, and verify the results with a more precise close loop measurement with the network analyser. Following it is a description of a crude method that provides a good understanding of the problematic nature of the flyback close loop analysis with PWM peak current mode control, operating in continuous conduction mode.

At the beginning of a switching period (T), the switch is turned on, and the inductor current is sensed by   Rsense  and the current-sense amplifier. This current-sense signal is added to a corrective ramp, and when the sum of these two waveforms exceeds Vc, the comparator output goes low, turning off the output switch.

The power supply system can be represented by the simplified model in Figure 1:

  • Control-to-output stage that includes the modulator, transformer, and output filter
  • Compensation stage of the error amplifier, that is designed in order to make the power supply stable

Figure 1: Simplified close loop model of a flyback power supply with
PWM peak current mode control   
 

The control to output transfer function can be simplified as one low frequency pole, one zero,

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