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The table below describles state space analysis in the field of power electronics. It is from a paper entitled "Signal Flow Graph in Loop Gain Analysis of DC–DC PWM CCM Switching Converters by Wing-Hung Ki, Member, IEEE."

As shown in the picture, the input vector u(t) has the current component \$i_2\$. Unfortunately, the paper doesn't explain what it is. Could anyone clarify that?
For dc-dc converters such as buck, boost, buck-boost, the only input component that I know is \$V_g\$. So I have no idea what the input current \$i_2\$ is.

enter image description here

emnha
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Generally in DC-DC circuit models, the converter load is modeled as a current source (i2) with a resistance in parallel. This is to generalize the model as much as possible and to take into account not only the passive loads, but also the active loads such as batteries.

You can see i2 as current source in load side in the aforementioned paper (Fig. 5).

Hazem
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  • Thanks, that make sense but let's assume the the load is battery. Why do we put a battery at output of a dc-dc converter as battery voltage is already almost constant? – emnha Aug 23 '17 at 13:32
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    Yes we put battery at the output of DC-DC converter when this converter is used as a charging circuit. The battery voltage IS NOT constant, and more importantly the battery withdraw different current values depends on its state of charge, terminals voltage, and the control mode (constant power mode, constant voltage mode or constant current mode). Moreover, as it is generalized theorem in the paper, the same converter could be used in discharging mode, bi-directional converter for example. In this case, the sign of i2 will be reversed. – Hazem Aug 23 '17 at 14:05
  • Thanks. I will wait for some days before giving the bounty. – emnha Aug 24 '17 at 02:05