In the last article we discussed Bode plots and how we can use the information displayed on a Bode plot in order to make an assessment of the stability of a power supply. At the end of that article we showed a Bode plot displaying real measurement data of the loop of a power supply.

In this article we are going to discuss how to physically make this measurement, what you will need to do to your power supply, the hardware required and how to connect it to your power supply. We will also cover how to measure the plant and compensator individually.

A typical power supply is shown in Figure 1. The power supply consists of a plant, which in turn can be sub-divided into a power stage, the PWM stage and a compensator. The output voltage is fed into our compensator, which is implemented using an op-amp and the appropriate selection of capacitors and resistors (much more on compensator design in later articles). The output of our compensator is fed into a comparator which generates our new value of duty cycle thus closing the loop. Of course the comparator and compensator op-amp are usually implemented inside our controller IC.

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