or Buck/Boost

 

http://schmidt-walter.fbe.fh-darmstadt.de/snt/snt_eng/snte_pdf.html

 

This circuit simply reverses the polarity of the input voltage.

The output magnitude can be less than, equal to, or greater than that of the input.

Initial State (simplified circuit)

Step 1 – Close the switch.

When the switch closes:

• The inductor starts to store energy.
• The inductor current during the ON period is:

and rises to a maximum of:

• The diode does not conduct.

Step 2– Open the switch

When the switch opens:

• The voltage across the inductor reverses polarity.
• The diode starts to conduct.
• The inductor discharges its energy to the capacitor and load.

The inductor current during the OFF period is:

The slope of the falling inductor current is;

The output voltage is negative.

Step 3– Close the switch.

Principle Waveforms

The capacitor current can be approximated by using Kirchoff’s current law: the sum of all currents in a node is zero.

Deriving the Minimum Inductor Size

From the falling inductor current, we observe that:

Rearranging, we obtain: