​In the previous article we discussed power supply compensators in detail; we can now design our first compensator to stabilise a voltage mode, Forward type power stage. The design method presented here can be applied to all hard-switched “non-isolated” Forward type converterswith or without atransformer (i.e.Buck, Forward, Push-Pull, Half-Bridge, Full Bridge). We will discuss current mode control and opto-isolated power supply design in great detail in future articles.

But first let us have a quick word about voltage mode control. Voltage mode is one of the earliest forms of switch mode power supply control and its operation is very simple.

All we have to do is to look at the output voltage; if it goes up with respect to the desired value, we reduce the PWM duty and if it goes down we increase it. As such, all we are doing is regulating the output voltage with respect to our desired “reference” voltage.

Of course the manner with which we change this duty is dictated by the compensator that we design i.e. the position of its poles and zeros. The compensator should give us good transient performance, whilst at the same time make sure that we do not violate the stability criteria.
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Voltage Mode Compensator Design

For voltage mode control we almost always need a Type III compensator. The circuit for our Type III compensators is given in Figure 1.

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